diff --git "a/CosmoAstro04_24.json" "b/CosmoAstro04_24.json" new file mode 100644--- /dev/null +++ "b/CosmoAstro04_24.json" @@ -0,0 +1 @@ +[{"video_title": "Black holes Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "In the last video, we saw that if we started with a massive star, about 9 to 20 times the mass of the sun, and when it finally matures, the remnant of the star is roughly, or that remnant core of the star is roughly 1.5 to 3 times the solar mass, or the mass of the sun. Then this remnant right here, let me just be clear, this 9 to 20 times is the mass of that star when it's in its main sequence. This 1.5 to 3 times is the mass once it's shed off a lot of the, I guess, outer material of the star, and this is really the mass of the remnant of the star, kind of the core of the star. But that remnant, once it stops fusing, once it stops having outward pressure, once it has enough density, this, we saw in the last video, will cause a supernova, it will cause a shockwave to move out through the rest of the material and essentially cause it to blow up, and this will condense into a neutron star. Now in this video, what I want to talk about is what if we're starting with a star that has a mass more than, this is give or take, we don't know the actual firm boundaries here, but what if we have a star that is more than 20 times the mass of the sun, and this is kind of the original mass before the star burns itself out. Or, when that star has kind of reached this old age, once it has that iron core, it has more than, so I could say the remnant, the dense remnant has more than 3 to 4 times the mass of the sun. And remember, it's going to have 3 to 4 times the mass of the sun, but it's going to be far denser, it's just going to be a core, it's going to be an iron-nickel core that's no longer fusing."}, {"video_title": "Black holes Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But that remnant, once it stops fusing, once it stops having outward pressure, once it has enough density, this, we saw in the last video, will cause a supernova, it will cause a shockwave to move out through the rest of the material and essentially cause it to blow up, and this will condense into a neutron star. Now in this video, what I want to talk about is what if we're starting with a star that has a mass more than, this is give or take, we don't know the actual firm boundaries here, but what if we have a star that is more than 20 times the mass of the sun, and this is kind of the original mass before the star burns itself out. Or, when that star has kind of reached this old age, once it has that iron core, it has more than, so I could say the remnant, the dense remnant has more than 3 to 4 times the mass of the sun. And remember, it's going to have 3 to 4 times the mass of the sun, but it's going to be far denser, it's just going to be a core, it's going to be an iron-nickel core that's no longer fusing. So what happens to these stars? So it turns out that these are so massive that even the neutron degeneracy pressure will not be enough to keep the mass from imploding. And these stars, all of the mass in these stars, will just keep imploding."}, {"video_title": "Black holes Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And remember, it's going to have 3 to 4 times the mass of the sun, but it's going to be far denser, it's just going to be a core, it's going to be an iron-nickel core that's no longer fusing. So what happens to these stars? So it turns out that these are so massive that even the neutron degeneracy pressure will not be enough to keep the mass from imploding. And these stars, all of the mass in these stars, will just keep imploding. So in the neutron, so we imagine the first, in kind of sun-like stars, things would collapse into white dwarfs. So maybe I should draw that in white. So they would collapse into white dwarfs."}, {"video_title": "Black holes Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And these stars, all of the mass in these stars, will just keep imploding. So in the neutron, so we imagine the first, in kind of sun-like stars, things would collapse into white dwarfs. So maybe I should draw that in white. So they would collapse into white dwarfs. No, that's not white either. There you go. They would collapse into white dwarfs eventually."}, {"video_title": "Black holes Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So they would collapse into white dwarfs. No, that's not white either. There you go. They would collapse into white dwarfs eventually. So this is a white dwarf. And here, the pressure that's keeping this from collapsing further is electron degeneracy pressure. The atoms are squeezed so much that the electrons are essentially keeping them from squeezing anymore."}, {"video_title": "Black holes Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "They would collapse into white dwarfs eventually. So this is a white dwarf. And here, the pressure that's keeping this from collapsing further is electron degeneracy pressure. The atoms are squeezed so much that the electrons are essentially keeping them from squeezing anymore. But if the pressure gets large enough, then you have the neutron star. So you have even more mass and even a smaller, and I'm not drawing this to scale, neutron stars are tiny. White dwarf stars are on the scale of an Earth-like planet."}, {"video_title": "Black holes Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "The atoms are squeezed so much that the electrons are essentially keeping them from squeezing anymore. But if the pressure gets large enough, then you have the neutron star. So you have even more mass and even a smaller, and I'm not drawing this to scale, neutron stars are tiny. White dwarf stars are on the scale of an Earth-like planet. Neutron stars, we learned in the last video, are on the scale of a city. So these are super dense, super tiny, and this has more mass than this over here. In fact, maybe I should just draw it as a dot, just so you have a sense of how dense it is."}, {"video_title": "Black holes Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "White dwarf stars are on the scale of an Earth-like planet. Neutron stars, we learned in the last video, are on the scale of a city. So these are super dense, super tiny, and this has more mass than this over here. In fact, maybe I should just draw it as a dot, just so you have a sense of how dense it is. It's really just like one big atomic nucleus. Well, it's still small, but it's the size of a city. It's like a nucleus the size of a city."}, {"video_title": "Black holes Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "In fact, maybe I should just draw it as a dot, just so you have a sense of how dense it is. It's really just like one big atomic nucleus. Well, it's still small, but it's the size of a city. It's like a nucleus the size of a city. But this right here is a neutron star. And what's unintuitive about what I'm drawing is each of these smaller things have more mass. This overcame the electron degeneracy pressure to collapse even further."}, {"video_title": "Black holes Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It's like a nucleus the size of a city. But this right here is a neutron star. And what's unintuitive about what I'm drawing is each of these smaller things have more mass. This overcame the electron degeneracy pressure to collapse even further. But if the mass is large enough, and this is what we're talking about in this video, even the neutron degeneracy pressure will not be able to keep that mass from collapsing. And there's even theoretical quark stars where the quark degeneracy pressure, but if you get even beyond that, it all collapses into a single point, and I'm simplifying here, but it collapses into a single point of infinite density. Infinite mass density."}, {"video_title": "Black holes Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "This overcame the electron degeneracy pressure to collapse even further. But if the mass is large enough, and this is what we're talking about in this video, even the neutron degeneracy pressure will not be able to keep that mass from collapsing. And there's even theoretical quark stars where the quark degeneracy pressure, but if you get even beyond that, it all collapses into a single point, and I'm simplifying here, but it collapses into a single point of infinite density. Infinite mass density. And this is really the mass of a black hole. And I'm calling it the mass of a black hole because there's different ways how you can view where a black hole starts and ends or what exactly is the black hole. So this is all the mass of the black hole."}, {"video_title": "Black holes Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Infinite mass density. And this is really the mass of a black hole. And I'm calling it the mass of a black hole because there's different ways how you can view where a black hole starts and ends or what exactly is the black hole. So this is all the mass of the black hole. Or we could say of the original star. So when we're talking about that remnant being 3 to 4 solar masses, all of that mass is now being contained. Well, not all of it."}, {"video_title": "Black holes Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So this is all the mass of the black hole. Or we could say of the original star. So when we're talking about that remnant being 3 to 4 solar masses, all of that mass is now being contained. Well, not all of it. Some of it was released as energy during the supernova, and that was also true of the neutron star. But most of that mass is now being contained in this infinitely small point. And you'll hear physicists and mathematicians talk about singularities."}, {"video_title": "Black holes Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Well, not all of it. Some of it was released as energy during the supernova, and that was also true of the neutron star. But most of that mass is now being contained in this infinitely small point. And you'll hear physicists and mathematicians talk about singularities. And singularities are really points, even in mathematics, where everything breaks down, where nothing starts to make sense anymore, where the mathematical equations don't give you a defined answer. And this is a singularity because you have a ton of mass in an infinitely small space. You essentially have an infinite density right here."}, {"video_title": "Black holes Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And you'll hear physicists and mathematicians talk about singularities. And singularities are really points, even in mathematics, where everything breaks down, where nothing starts to make sense anymore, where the mathematical equations don't give you a defined answer. And this is a singularity because you have a ton of mass in an infinitely small space. You essentially have an infinite density right here. And this is hard to visualize, but you have kind of an infinite curvature in space-time right here, and I can't visualize that. Maybe we'll think about that in more videos. But the reason why I said that there's different ways to think about where a black hole is or where it starts and ends is that this is where the mass is."}, {"video_title": "Black holes Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "You essentially have an infinite density right here. And this is hard to visualize, but you have kind of an infinite curvature in space-time right here, and I can't visualize that. Maybe we'll think about that in more videos. But the reason why I said that there's different ways to think about where a black hole is or where it starts and ends is that this is where the mass is. And if there was any other mass that was over here, it would obviously be attracted to this mass and then become part of that singularity. It would add to that mass, that already huge mass that's in an infinitely small point in space. But the reason why the boundary is hard to define is because there's some point in space around that singularity at which no matter what that thing is, no matter how much energy that thing has, it will not be able to escape the gravitational influence of the black hole, of that ultra-dense mass."}, {"video_title": "Black holes Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But the reason why I said that there's different ways to think about where a black hole is or where it starts and ends is that this is where the mass is. And if there was any other mass that was over here, it would obviously be attracted to this mass and then become part of that singularity. It would add to that mass, that already huge mass that's in an infinitely small point in space. But the reason why the boundary is hard to define is because there's some point in space around that singularity at which no matter what that thing is, no matter how much energy that thing has, it will not be able to escape the gravitational influence of the black hole, of that ultra-dense mass. So even if it was electromagnetic radiation, even if it was light, and even if it's light that's shown away from the mass, it will eventually have to go back. It will not be able to escape the gravitational influence. And so the boundary, where if you're within that boundary, that's really a sphere, so that boundary around the singularity, where if you're within the boundary, no matter what you do, no matter if you're electromagnetic radiation, you're still going to, you're never going to be able to escape the black hole."}, {"video_title": "Black holes Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But the reason why the boundary is hard to define is because there's some point in space around that singularity at which no matter what that thing is, no matter how much energy that thing has, it will not be able to escape the gravitational influence of the black hole, of that ultra-dense mass. So even if it was electromagnetic radiation, even if it was light, and even if it's light that's shown away from the mass, it will eventually have to go back. It will not be able to escape the gravitational influence. And so the boundary, where if you're within that boundary, that's really a sphere, so that boundary around the singularity, where if you're within the boundary, no matter what you do, no matter if you're electromagnetic radiation, you're still going to, you're never going to be able to escape the black hole. If you are beyond that boundary, you might be able to escape the black hole. So this guy could escape. This guy over here, no matter what he does, is going to have to go back into the black hole."}, {"video_title": "Black holes Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And so the boundary, where if you're within that boundary, that's really a sphere, so that boundary around the singularity, where if you're within the boundary, no matter what you do, no matter if you're electromagnetic radiation, you're still going to, you're never going to be able to escape the black hole. If you are beyond that boundary, you might be able to escape the black hole. So this guy could escape. This guy over here, no matter what he does, is going to have to go back into the black hole. This boundary right here is called the event horizon. Another word used in a lot of science fiction movies, and for good reason, because it's fascinating. We'll actually learn in future videos, hopefully about Hawking radiation, we'll see that that is not radiation from the black hole itself."}, {"video_title": "Black holes Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "This guy over here, no matter what he does, is going to have to go back into the black hole. This boundary right here is called the event horizon. Another word used in a lot of science fiction movies, and for good reason, because it's fascinating. We'll actually learn in future videos, hopefully about Hawking radiation, we'll see that that is not radiation from the black hole itself. It's the byproduct of quantum effects that are occurring at the event horizon. But the event horizon is just this kind of point in space, or this sphere in space, or this boundary in space. Anything closer than or within the event horizon has to eventually end up in the singularity, contributing to that mass."}, {"video_title": "Black holes Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "We'll actually learn in future videos, hopefully about Hawking radiation, we'll see that that is not radiation from the black hole itself. It's the byproduct of quantum effects that are occurring at the event horizon. But the event horizon is just this kind of point in space, or this sphere in space, or this boundary in space. Anything closer than or within the event horizon has to eventually end up in the singularity, contributing to that mass. Anything on the outside has a chance of escaping. So what would a black hole look like? Well, not even light can escape from it, so it will be black."}, {"video_title": "Black holes Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Anything closer than or within the event horizon has to eventually end up in the singularity, contributing to that mass. Anything on the outside has a chance of escaping. So what would a black hole look like? Well, not even light can escape from it, so it will be black. It will be black in the purest sense. It will not emit any type of radiation from the black hole itself, from that mass. So here are some depictions I got from NASA of black holes."}, {"video_title": "Black holes Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Well, not even light can escape from it, so it will be black. It will be black in the purest sense. It will not emit any type of radiation from the black hole itself, from that mass. So here are some depictions I got from NASA of black holes. So just to be clear, what's happening here, what you're seeing here is black. You can view that as the black hole. When people talk about the black hole, that's often what they're talking about."}, {"video_title": "Black holes Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So here are some depictions I got from NASA of black holes. So just to be clear, what's happening here, what you're seeing here is black. You can view that as the black hole. When people talk about the black hole, that's often what they're talking about. But there's a point of infinite density at the center of this black sphere right here. And what you see is that black sphere, that really is the boundary of the event horizon. So this right here is the boundary of the event horizon."}, {"video_title": "Black holes Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "When people talk about the black hole, that's often what they're talking about. But there's a point of infinite density at the center of this black sphere right here. And what you see is that black sphere, that really is the boundary of the event horizon. So this right here is the boundary of the event horizon. And what we're seeing right here is the accretion disk around the black hole. As all of this matter gets closer and closer to it, it's being squeezed more and more, it's moving faster and faster, and getting hotter and hotter. And that's why the way this art is depicted, it looks like this stuff over here is redder and hotter than the stuff further out."}, {"video_title": "Black holes Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So this right here is the boundary of the event horizon. And what we're seeing right here is the accretion disk around the black hole. As all of this matter gets closer and closer to it, it's being squeezed more and more, it's moving faster and faster, and getting hotter and hotter. And that's why the way this art is depicted, it looks like this stuff over here is redder and hotter than the stuff further out. It's just accelerating as it approaches that event horizon. Once it's in the event horizon, we cannot even see the light that it's emitting, even though it's starting to become unbelievably energetic. Here's some other pictures."}, {"video_title": "Black holes Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And that's why the way this art is depicted, it looks like this stuff over here is redder and hotter than the stuff further out. It's just accelerating as it approaches that event horizon. Once it's in the event horizon, we cannot even see the light that it's emitting, even though it's starting to become unbelievably energetic. Here's some other pictures. This is a picture of a star being ripped apart. Not a picture, this is actually an artist's depiction. We never were able to get such good pictures of actual action occurring near black holes."}, {"video_title": "Black holes Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Here's some other pictures. This is a picture of a star being ripped apart. Not a picture, this is actually an artist's depiction. We never were able to get such good pictures of actual action occurring near black holes. These are artist's depictions. But this is a star being ripped apart by a black hole. So this star is getting pretty close to this black hole."}, {"video_title": "Black holes Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "We never were able to get such good pictures of actual action occurring near black holes. These are artist's depictions. But this is a star being ripped apart by a black hole. So this star is getting pretty close to this black hole. Already out here, where the star is, it's very strong gravitational attraction. So any mass that's being emitted from the star in that direction is slowly being pulled into the black hole. So the star is kind of being ripped apart by the black hole."}, {"video_title": "Black holes Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So this star is getting pretty close to this black hole. Already out here, where the star is, it's very strong gravitational attraction. So any mass that's being emitted from the star in that direction is slowly being pulled into the black hole. So the star is kind of being ripped apart by the black hole. This is maybe a better depiction of it. This is the star at first, and once it becomes under the influence of the black hole's gravitation, it starts to kind of elongate and gets ripped apart, and its matter starts spiraling in closer and closer to that black hole. Once it's in the event horizon, we won't even see it anymore, because even the light from that matter, that intensely hot matter that's entering into the black hole, cannot even escape the black hole itself."}, {"video_title": "Black holes Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So the star is kind of being ripped apart by the black hole. This is maybe a better depiction of it. This is the star at first, and once it becomes under the influence of the black hole's gravitation, it starts to kind of elongate and gets ripped apart, and its matter starts spiraling in closer and closer to that black hole. Once it's in the event horizon, we won't even see it anymore, because even the light from that matter, that intensely hot matter that's entering into the black hole, cannot even escape the black hole itself. Anyway, hopefully you found that interesting. And I want to be clear. We still don't understand a lot about black holes."}, {"video_title": "Black holes Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Once it's in the event horizon, we won't even see it anymore, because even the light from that matter, that intensely hot matter that's entering into the black hole, cannot even escape the black hole itself. Anyway, hopefully you found that interesting. And I want to be clear. We still don't understand a lot about black holes. In fact, this whole notion of a singularity, the fact that all the math and all the theory breaks down at the singularity is a pretty good sign that our theory isn't complete. Because if our theory is complete, we would maybe get something a little bit more sensical than just all of our equations not making sense at that infinitely dense point. Anyway, hopefully you found that interesting."}, {"video_title": "Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Probably, and I would say it ranks their top three, because it really enabled people like Hubble to start realizing that the universe is expanding. Or even being able to think about how to measure distances to objects in space well beyond the reach of our tools with parallax. We saw with parallax, you have to have extremely sensitive instruments just to even measure distances to stars relatively close to us. Very sensitive instruments to get to stars maybe further out into our galaxy. And we don't have the instruments even today to measure things beyond our galaxy. But because of Henrietta Swan Leavitt, we were able to approximate or get good senses of the distance to objects beyond our galaxy. So let's just think about what she did."}, {"video_title": "Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Very sensitive instruments to get to stars maybe further out into our galaxy. And we don't have the instruments even today to measure things beyond our galaxy. But because of Henrietta Swan Leavitt, we were able to approximate or get good senses of the distance to objects beyond our galaxy. So let's just think about what she did. So her job was literally to classify stars in the large Magellanic, I have trouble saying that, Magellanic cloud and the small Magellanic clouds. And this is what they look like from the southern hemisphere. This is the large right over here."}, {"video_title": "Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So let's just think about what she did. So her job was literally to classify stars in the large Magellanic, I have trouble saying that, Magellanic cloud and the small Magellanic clouds. And this is what they look like from the southern hemisphere. This is the large right over here. And this is the small right over here. And remember, this is before Hubble realized or showed the world that there are stars beyond our galaxy, that there are galaxies beyond our galaxy. So at this point in time, people didn't even fully appreciate that these were separate galaxies."}, {"video_title": "Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "This is the large right over here. And this is the small right over here. And remember, this is before Hubble realized or showed the world that there are stars beyond our galaxy, that there are galaxies beyond our galaxy. So at this point in time, people didn't even fully appreciate that these were separate galaxies. We just said, hey, these are kind of these blobs or these clusters of stars that we see in the southern hemisphere. And just to get a sense of where they are relative to our galaxy, the Milky Way galaxy, this is obviously not an actual picture. We can't take a picture from this vantage point."}, {"video_title": "Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So at this point in time, people didn't even fully appreciate that these were separate galaxies. We just said, hey, these are kind of these blobs or these clusters of stars that we see in the southern hemisphere. And just to get a sense of where they are relative to our galaxy, the Milky Way galaxy, this is obviously not an actual picture. We can't take a picture from this vantage point. This would have to be very, very far away. But this is the Milky Way right here. And this is the small Magellanic cloud."}, {"video_title": "Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "We can't take a picture from this vantage point. This would have to be very, very far away. But this is the Milky Way right here. And this is the small Magellanic cloud. And this is the large Magellanic cloud. I'm getting better at saying it. So her job was literally just to classify the different stars that she saw."}, {"video_title": "Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And this is the small Magellanic cloud. And this is the large Magellanic cloud. I'm getting better at saying it. So her job was literally just to classify the different stars that she saw. But while she was classifying, she looked at these things called variables. It turns out what she was looking at were a class of stars called Cepheid or Cepheid variable stars. And what's interesting about them is two things."}, {"video_title": "Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So her job was literally just to classify the different stars that she saw. But while she was classifying, she looked at these things called variables. It turns out what she was looking at were a class of stars called Cepheid or Cepheid variable stars. And what's interesting about them is two things. They're super duper bright. They're up to 30,000 times as luminous as the sun. And they're 5 to 20 times more massive than the sun."}, {"video_title": "Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And what's interesting about them is two things. They're super duper bright. They're up to 30,000 times as luminous as the sun. And they're 5 to 20 times more massive than the sun. 5 to 20 times the sun's mass. But what makes them interesting is one, they're really bright. So you can see them from really far away."}, {"video_title": "Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And they're 5 to 20 times more massive than the sun. 5 to 20 times the sun's mass. But what makes them interesting is one, they're really bright. So you can see them from really far away. You can see these Cepheid variable stars in other galaxies. In fact, we can see it well beyond even the small Magellanic cloud or the large Magellanic cloud. But you can see these stars in other galaxies."}, {"video_title": "Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So you can see them from really far away. You can see these Cepheid variable stars in other galaxies. In fact, we can see it well beyond even the small Magellanic cloud or the large Magellanic cloud. But you can see these stars in other galaxies. And what's even more interesting about them is that their intensity is variable, that they become brighter and dimmer with a well-defined period. So if you're looking at a Cepheid variable star, and this is just kind of a simulation, a very cheap simulation, it might look like this. And then over the course of the next three, four days, it might reduce in intensity to something like this."}, {"video_title": "Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But you can see these stars in other galaxies. And what's even more interesting about them is that their intensity is variable, that they become brighter and dimmer with a well-defined period. So if you're looking at a Cepheid variable star, and this is just kind of a simulation, a very cheap simulation, it might look like this. And then over the course of the next three, four days, it might reduce in intensity to something like this. And then after three, four days again, it will look like this. And then it'll look like this again. So its actual intensity is going up and down with a well-defined period."}, {"video_title": "Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And then over the course of the next three, four days, it might reduce in intensity to something like this. And then after three, four days again, it will look like this. And then it'll look like this again. So its actual intensity is going up and down with a well-defined period. So if this takes three days and this is another three days, then the period, one entire cycle of its going from low intensity back to high intensity is going to be six days. So this is a six-day period. And what Henrietta Leavitt saw, and this wasn't an obvious thing to do, she plotted, she assumed that things, everything in each of these clouds are roughly the same distance away."}, {"video_title": "Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So its actual intensity is going up and down with a well-defined period. So if this takes three days and this is another three days, then the period, one entire cycle of its going from low intensity back to high intensity is going to be six days. So this is a six-day period. And what Henrietta Leavitt saw, and this wasn't an obvious thing to do, she plotted, she assumed that things, everything in each of these clouds are roughly the same distance away. Everything in the large Magellanic cloud is roughly the same distance away. And it's obviously not exact. This is an entire galaxy, so you have obviously things further away in that galaxy and things closer up."}, {"video_title": "Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And what Henrietta Leavitt saw, and this wasn't an obvious thing to do, she plotted, she assumed that things, everything in each of these clouds are roughly the same distance away. Everything in the large Magellanic cloud is roughly the same distance away. And it's obviously not exact. This is an entire galaxy, so you have obviously things further away in that galaxy and things closer up. You have stars here and here, and their distance isn't going to be exactly the same to us, that we're sitting maybe over here someplace. But it's going to be close. It wasn't a bad approximation."}, {"video_title": "Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "This is an entire galaxy, so you have obviously things further away in that galaxy and things closer up. You have stars here and here, and their distance isn't going to be exactly the same to us, that we're sitting maybe over here someplace. But it's going to be close. It wasn't a bad approximation. And by making that assumption, she saw something pretty neat. So let me plot this right over here. So she plotted on the horizontal axis, she plotted the relative luminosity."}, {"video_title": "Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It wasn't a bad approximation. And by making that assumption, she saw something pretty neat. So let me plot this right over here. So she plotted on the horizontal axis, she plotted the relative luminosity. So really, the only way that she could measure this is just how bright did they look to her? And she's assuming that they're same distance. So obviously, if you have a brighter star, but it's much, much further away, it's going to look dimmer."}, {"video_title": "Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So she plotted on the horizontal axis, she plotted the relative luminosity. So really, the only way that she could measure this is just how bright did they look to her? And she's assuming that they're same distance. So obviously, if you have a brighter star, but it's much, much further away, it's going to look dimmer. So if you assume that they're all roughly the same distance, then how bright it is will tell you how bright it is at the actual star. So she plotted relative luminosity of a star on one axis, and on the other axis, she plotted the period of these variable stars. And what I'm going to do is I'm going to do this on a logarithmic scale."}, {"video_title": "Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So obviously, if you have a brighter star, but it's much, much further away, it's going to look dimmer. So if you assume that they're all roughly the same distance, then how bright it is will tell you how bright it is at the actual star. So she plotted relative luminosity of a star on one axis, and on the other axis, she plotted the period of these variable stars. And what I'm going to do is I'm going to do this on a logarithmic scale. So let's say that this is in days. So this is one day, this is 10 days, this is 100 days, right over here. It's a logarithmic scale because I'm going up in powers of 10."}, {"video_title": "Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And what I'm going to do is I'm going to do this on a logarithmic scale. So let's say that this is in days. So this is one day, this is 10 days, this is 100 days, right over here. It's a logarithmic scale because I'm going up in powers of 10. I could say that if we take the log of these, this would be 0, this would be 1, this would be 2. And so that's what I'm using as a scale. So I'm using the log of the period, or I'm just marking them as 1, 10, 100, but I'm giving each of these factors of 10 an equal spacing."}, {"video_title": "Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It's a logarithmic scale because I'm going up in powers of 10. I could say that if we take the log of these, this would be 0, this would be 1, this would be 2. And so that's what I'm using as a scale. So I'm using the log of the period, or I'm just marking them as 1, 10, 100, but I'm giving each of these factors of 10 an equal spacing. When you plot it on this scale, the relative luminosity versus the period, she got a plot that looks something like this. This is obviously not exact. She got a plot that looks something like this."}, {"video_title": "Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So I'm using the log of the period, or I'm just marking them as 1, 10, 100, but I'm giving each of these factors of 10 an equal spacing. When you plot it on this scale, the relative luminosity versus the period, she got a plot that looks something like this. This is obviously not exact. She got a plot that looks something like this. It was a fairly linear relationship when you plot the relative luminosity against the log of the period. So this is obviously a logarithmic scale over here. And so you could fit a line."}, {"video_title": "Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "She got a plot that looks something like this. It was a fairly linear relationship when you plot the relative luminosity against the log of the period. So this is obviously a logarithmic scale over here. And so you could fit a line. And why I'd argue, and I think most people would argue, this is one of the most important discoveries in astronomy, is if you know, because think about what the problem here is. We can look at all of these stars in space. Let's say you look at a fraction of the sky and you look at something that looks like that."}, {"video_title": "Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And so you could fit a line. And why I'd argue, and I think most people would argue, this is one of the most important discoveries in astronomy, is if you know, because think about what the problem here is. We can look at all of these stars in space. Let's say you look at a fraction of the sky and you look at something that looks like that. So it's really bright. And then you see something dim that looks like that. So if you have a very superficial understanding, you say, oh, this star is brighter."}, {"video_title": "Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Let's say you look at a fraction of the sky and you look at something that looks like that. So it's really bright. And then you see something dim that looks like that. So if you have a very superficial understanding, you say, oh, this star is brighter. You would say that this is a fundamentally brighter star. But how do you know that? Maybe instead of being brighter, maybe it's just a dimmer, closer star."}, {"video_title": "Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So if you have a very superficial understanding, you say, oh, this star is brighter. You would say that this is a fundamentally brighter star. But how do you know that? Maybe instead of being brighter, maybe it's just a dimmer, closer star. Maybe this is an entire galaxy, but it's so far away that you can't even tell. But all of a sudden, by the work that Henrietta Leavitt did, if you see one of these Cepheid variable stars in another galaxy, you know its relative brightness compared to other Cepheid variable stars. And so if you can place just one of these Cepheid variable stars, if you know exactly the distance to one of them, and then you know its absolute luminosity, you then know the absolute luminosity of any other Cepheid variable stars."}, {"video_title": "Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Maybe instead of being brighter, maybe it's just a dimmer, closer star. Maybe this is an entire galaxy, but it's so far away that you can't even tell. But all of a sudden, by the work that Henrietta Leavitt did, if you see one of these Cepheid variable stars in another galaxy, you know its relative brightness compared to other Cepheid variable stars. And so if you can place just one of these Cepheid variable stars, if you know exactly the distance to one of them, and then you know its absolute luminosity, you then know the absolute luminosity of any other Cepheid variable stars. So let's say using parallax, which is our other tool, we find, let's say there's some star in our galaxy. And let's say using parallax, we're able to come up with a pretty good measure that it is, I don't know, let's say it's 100 light years away. And this star is a Cepheid variable star."}, {"video_title": "Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And so if you can place just one of these Cepheid variable stars, if you know exactly the distance to one of them, and then you know its absolute luminosity, you then know the absolute luminosity of any other Cepheid variable stars. So let's say using parallax, which is our other tool, we find, let's say there's some star in our galaxy. And let's say using parallax, we're able to come up with a pretty good measure that it is, I don't know, let's say it's 100 light years away. And this star is a Cepheid variable star. And let's say its period is one day. So we now know something interesting. We know variable stars with a period of one day at 100 light years away will look like this."}, {"video_title": "Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And this star is a Cepheid variable star. And let's say its period is one day. So we now know something interesting. We know variable stars with a period of one day at 100 light years away will look like this. Will look like this drawing right over here. So if we later on see a Cepheid variable star with a period of one day, so it gets brighter and dim over the course of one day, and maybe it's redshifted as well, but maybe it looks a little bit dimmer, it looks like this. We now know that if it was 100 light years away, it would have this luminosity."}, {"video_title": "Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "We know variable stars with a period of one day at 100 light years away will look like this. Will look like this drawing right over here. So if we later on see a Cepheid variable star with a period of one day, so it gets brighter and dim over the course of one day, and maybe it's redshifted as well, but maybe it looks a little bit dimmer, it looks like this. We now know that if it was 100 light years away, it would have this luminosity. So based on how much dimmer it is, we can then figure out how much further away this Cepheid variable star is. If that confuses you a little bit, I'll do a little bit more details in the next few videos so we can get a closer sense of how the math would work. But this was a big discovery, just discovering this class of stars, this Cepheid variable class."}, {"video_title": "Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "We now know that if it was 100 light years away, it would have this luminosity. So based on how much dimmer it is, we can then figure out how much further away this Cepheid variable star is. If that confuses you a little bit, I'll do a little bit more details in the next few videos so we can get a closer sense of how the math would work. But this was a big discovery, just discovering this class of stars, this Cepheid variable class. She wasn't the one who discovered them. People knew before her that there were these stars that got brighter and dimmer. But what her big discovery was is seeing this linear relationship between the relative luminosity of these stars and their period."}, {"video_title": "Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But this was a big discovery, just discovering this class of stars, this Cepheid variable class. She wasn't the one who discovered them. People knew before her that there were these stars that got brighter and dimmer. But what her big discovery was is seeing this linear relationship between the relative luminosity of these stars and their period. Because then, if we see Cepheid variable stars in completely different galaxies or galactic clusters, by looking at their period, we know what their real relative luminosity is. And then we could guess how far those things really are. Or we could estimate how far those things really are."}, {"video_title": "Detectable civilizations in our galaxy 4 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So let's actually do that here. So let's just assume that there are 100 billion stars. So that's my first term right over there. Let's say that one-fourth will develop planets. And let's say of the solar systems that develop planets, on average, let's say that they develop an average of 0.1 planets capable of sustaining life. Or really, that you'll have one planet for every 10 of these solar systems with planets. That's just my assumption there."}, {"video_title": "Detectable civilizations in our galaxy 4 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Let's say that one-fourth will develop planets. And let's say of the solar systems that develop planets, on average, let's say that they develop an average of 0.1 planets capable of sustaining life. Or really, that you'll have one planet for every 10 of these solar systems with planets. That's just my assumption there. I don't know if that's right. Now let's multiply that times the fraction of these planets capable of sustaining life that actually will get life. And I don't know what that is, but I hinted in previous videos that life is one of those things that it seems like if you have all the right ingredients, it's so robust that you have life at these underwater volcanoes."}, {"video_title": "Detectable civilizations in our galaxy 4 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "That's just my assumption there. I don't know if that's right. Now let's multiply that times the fraction of these planets capable of sustaining life that actually will get life. And I don't know what that is, but I hinted in previous videos that life is one of those things that it seems like if you have all the right ingredients, it's so robust that you have life at these underwater volcanoes. You have bacteria that can process all sorts of weird things. So let's say that that probability is pretty high. Let's say that that is 50% or half of the planets that are capable of getting life actually do have life."}, {"video_title": "Detectable civilizations in our galaxy 4 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And I don't know what that is, but I hinted in previous videos that life is one of those things that it seems like if you have all the right ingredients, it's so robust that you have life at these underwater volcanoes. You have bacteria that can process all sorts of weird things. So let's say that that probability is pretty high. Let's say that that is 50% or half of the planets that are capable of getting life actually do have life. I would guess that that might even be higher. But once again, just a guess. Now we have to think about of the life, what fraction becomes intelligent?"}, {"video_title": "Detectable civilizations in our galaxy 4 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Let's say that that is 50% or half of the planets that are capable of getting life actually do have life. I would guess that that might even be higher. But once again, just a guess. Now we have to think about of the life, what fraction becomes intelligent? What becomes intelligent over some point in the history? Well, I'll say it's a tenth. Maybe if the asteroids didn't kill the dinosaurs, it wouldn't have happened on Earth."}, {"video_title": "Detectable civilizations in our galaxy 4 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Now we have to think about of the life, what fraction becomes intelligent? What becomes intelligent over some point in the history? Well, I'll say it's a tenth. Maybe if the asteroids didn't kill the dinosaurs, it wouldn't have happened on Earth. Who knows? Or maybe we just have some very intelligent dinosaurs around. We don't know."}, {"video_title": "Detectable civilizations in our galaxy 4 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Maybe if the asteroids didn't kill the dinosaurs, it wouldn't have happened on Earth. Who knows? Or maybe we just have some very intelligent dinosaurs around. We don't know. And maybe there's other forms of intelligent life even on our own planet that we haven't fully appreciated. Dolphins are a good candidate. Some people believe that octopuses, because they have such flexible arms, there's a theory that they could develop eventually the ability to kind of one day, if their brains mature and all of the rest make tools the same way primitive primates eventually were able to have larger brain sizes and actually manipulate things to make tools."}, {"video_title": "Detectable civilizations in our galaxy 4 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "We don't know. And maybe there's other forms of intelligent life even on our own planet that we haven't fully appreciated. Dolphins are a good candidate. Some people believe that octopuses, because they have such flexible arms, there's a theory that they could develop eventually the ability to kind of one day, if their brains mature and all of the rest make tools the same way primitive primates eventually were able to have larger brain sizes and actually manipulate things to make tools. So who knows? I don't want to get into all of that. So there's a 1 in 10 chance that you get intelligent life."}, {"video_title": "Detectable civilizations in our galaxy 4 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Some people believe that octopuses, because they have such flexible arms, there's a theory that they could develop eventually the ability to kind of one day, if their brains mature and all of the rest make tools the same way primitive primates eventually were able to have larger brain sizes and actually manipulate things to make tools. So who knows? I don't want to get into all of that. So there's a 1 in 10 chance that you get intelligent life. And then assuming that intelligent life shows up, what fraction is going to become detectable? I don't know. I don't know whether dolphins will ever communicate via radio or not."}, {"video_title": "Detectable civilizations in our galaxy 4 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So there's a 1 in 10 chance that you get intelligent life. And then assuming that intelligent life shows up, what fraction is going to become detectable? I don't know. I don't know whether dolphins will ever communicate via radio or not. So let's just say that that is, I don't know, let's say that that is another 1 in 10 chance or I'll say 0.1. And then we have to multiply it times the detectable life of the civilization on average. Once again, huge assumptions being here."}, {"video_title": "Detectable civilizations in our galaxy 4 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "I don't know whether dolphins will ever communicate via radio or not. So let's just say that that is, I don't know, let's say that that is another 1 in 10 chance or I'll say 0.1. And then we have to multiply it times the detectable life of the civilization on average. Once again, huge assumptions being here. But the detectable life of a civilization, let me just put it at 10,000 years. Either they destroy themselves or they get beyond that type of radio-type communication, electromagnetic-type communication. Maybe they start doing all sorts of weird, wacky things."}, {"video_title": "Detectable civilizations in our galaxy 4 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Once again, huge assumptions being here. But the detectable life of a civilization, let me just put it at 10,000 years. Either they destroy themselves or they get beyond that type of radio-type communication, electromagnetic-type communication. Maybe they start doing all sorts of weird, wacky things. Probably won't take you 10,000 years to even progress it. That might take you less time. But let's just do this just for the sake of fun."}, {"video_title": "Detectable civilizations in our galaxy 4 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Maybe they start doing all sorts of weird, wacky things. Probably won't take you 10,000 years to even progress it. That might take you less time. But let's just do this just for the sake of fun. And then the lifespan of your average star, that's probably one of the things that we have the best sense of. So on average, let's put it at 10 billion years. So let's calculate all of this."}, {"video_title": "Detectable civilizations in our galaxy 4 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But let's just do this just for the sake of fun. And then the lifespan of your average star, that's probably one of the things that we have the best sense of. So on average, let's put it at 10 billion years. So let's calculate all of this. Let's get my handy TI-85 out. And so we're going to have 100 billion. That's 1 times 10 to the 9th."}, {"video_title": "Detectable civilizations in our galaxy 4 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So let's calculate all of this. Let's get my handy TI-85 out. And so we're going to have 100 billion. That's 1 times 10 to the 9th. Sorry, that's 100 times 10 to the 9th. So let me clear it. Or you could have 1 times 10 to the 11th."}, {"video_title": "Detectable civilizations in our galaxy 4 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "That's 1 times 10 to the 9th. Sorry, that's 100 times 10 to the 9th. So let me clear it. Or you could have 1 times 10 to the 11th. That is 100 billion times 0.25 times 0.1 times 0.5 times 0.5 times 0.1 again times 0.1 times 0.1 again times 0.1 times 10,000 divided by 10 billion. So times 10,000 divided by 10 billion. So that's 1E10."}, {"video_title": "Detectable civilizations in our galaxy 4 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Or you could have 1 times 10 to the 11th. That is 100 billion times 0.25 times 0.1 times 0.5 times 0.5 times 0.1 again times 0.1 times 0.1 again times 0.1 times 10,000 divided by 10 billion. So times 10,000 divided by 10 billion. So that's 1E10. 1 times 10 to the 10th power. 1 with 10 zeros. So let's see what we get."}, {"video_title": "Detectable civilizations in our galaxy 4 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So that's 1E10. 1 times 10 to the 10th power. 1 with 10 zeros. So let's see what we get. We get 12.5, which is kind of a neat number. But these are heavily dependent on this. So we're saying, given these assumptions, there should be 12.5 detectable civilizations in our galaxy right now."}, {"video_title": "Detectable civilizations in our galaxy 4 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So let's see what we get. We get 12.5, which is kind of a neat number. But these are heavily dependent on this. So we're saying, given these assumptions, there should be 12.5 detectable civilizations in our galaxy right now. So the question is, why aren't we detecting it? Maybe their radio signals, maybe their electromagnetic waves are getting to us. But we can't differentiate it from noise right now."}, {"video_title": "Detectable civilizations in our galaxy 4 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So we're saying, given these assumptions, there should be 12.5 detectable civilizations in our galaxy right now. So the question is, why aren't we detecting it? Maybe their radio signals, maybe their electromagnetic waves are getting to us. But we can't differentiate it from noise right now. And that's what the whole SETI project is all about, trying to keep track of all of this information, all of these radio waves and electromagnetic waves that are coming from outer space towards Earth. And seeing if any of them actually have any non-noise signal that actually look like they're being generated by some type of intelligent civilization. So maybe we're getting them and we're just not detecting them."}, {"video_title": "Detectable civilizations in our galaxy 4 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But we can't differentiate it from noise right now. And that's what the whole SETI project is all about, trying to keep track of all of this information, all of these radio waves and electromagnetic waves that are coming from outer space towards Earth. And seeing if any of them actually have any non-noise signal that actually look like they're being generated by some type of intelligent civilization. So maybe we're getting them and we're just not detecting them. Or maybe something else is at play. Maybe we've overestimated one of these. Maybe there is a lot of life, but maybe they're not using electromagnetic waves to communicate."}, {"video_title": "Detectable civilizations in our galaxy 4 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So maybe we're getting them and we're just not detecting them. Or maybe something else is at play. Maybe we've overestimated one of these. Maybe there is a lot of life, but maybe they're not using electromagnetic waves to communicate. Maybe that's some type of primitive way of communicating. Maybe they start doing telepathy or something crazy. Or they start using some type of quantum thing that allows them to communicate more directly without having to wait for the speed of light."}, {"video_title": "Detectable civilizations in our galaxy 4 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Maybe there is a lot of life, but maybe they're not using electromagnetic waves to communicate. Maybe that's some type of primitive way of communicating. Maybe they start doing telepathy or something crazy. Or they start using some type of quantum thing that allows them to communicate more directly without having to wait for the speed of light. Maybe that is a very slow way to communicate. And it is a slow way, frankly, if you're trying to communicate across solar systems and stars and planets or even across galaxies, one could imagine. So maybe we're just kind of in a transition state of communication."}, {"video_title": "Detectable civilizations in our galaxy 4 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Or they start using some type of quantum thing that allows them to communicate more directly without having to wait for the speed of light. Maybe that is a very slow way to communicate. And it is a slow way, frankly, if you're trying to communicate across solar systems and stars and planets or even across galaxies, one could imagine. So maybe we're just kind of in a transition state of communication. That electromagnetic waves, radio, and all the rest is just a transition state. Maybe in 100 years we'll figure out another better way that's not detectable in our traditional ways. Maybe we're being bombarded with another type of communication mechanism that we're just not ready to perceive yet."}, {"video_title": "Introduction to light Electronic structure of atoms Chemistry Khan Academy.mp3", "Sentence": "What I want to do in this video is give ourselves a basic introduction to the phenomenon of light. And light is, at least to me, it is mysterious. Because on one level, it really defines our reality. It's maybe the most defining characteristic of our reality. Everything we see, how we perceive reality, is based on light bouncing off of objects, or bending around objects, or diffracting around objects, and then being sensed by our eyes, and then sending signals into our brain that create models of the world we see around us. So it really is almost the defining characteristic of our reality. But at the same time, when you really go down to experiment and observe with light, it starts to have a bunch of mysterious properties, and to a large degree, it is not fully understood yet."}, {"video_title": "Introduction to light Electronic structure of atoms Chemistry Khan Academy.mp3", "Sentence": "It's maybe the most defining characteristic of our reality. Everything we see, how we perceive reality, is based on light bouncing off of objects, or bending around objects, or diffracting around objects, and then being sensed by our eyes, and then sending signals into our brain that create models of the world we see around us. So it really is almost the defining characteristic of our reality. But at the same time, when you really go down to experiment and observe with light, it starts to have a bunch of mysterious properties, and to a large degree, it is not fully understood yet. And probably the most amazing thing about light, well, actually there's tons of amazing things about light, but one of the mysterious things is when you really get down to it, and this is actually not just true of light, this is actually true of almost anything once you get onto a small enough quantum mechanical level, but light behaves as both a wave and a particle. And this is probably not that intuitive to you, because it's not that intuitive to me. In my life, I'm used to certain things behaving as waves, like sound waves, or the waves of an ocean, and I'm used to certain things behaving like particles, like basketballs, or my coffee cup."}, {"video_title": "Introduction to light Electronic structure of atoms Chemistry Khan Academy.mp3", "Sentence": "But at the same time, when you really go down to experiment and observe with light, it starts to have a bunch of mysterious properties, and to a large degree, it is not fully understood yet. And probably the most amazing thing about light, well, actually there's tons of amazing things about light, but one of the mysterious things is when you really get down to it, and this is actually not just true of light, this is actually true of almost anything once you get onto a small enough quantum mechanical level, but light behaves as both a wave and a particle. And this is probably not that intuitive to you, because it's not that intuitive to me. In my life, I'm used to certain things behaving as waves, like sound waves, or the waves of an ocean, and I'm used to certain things behaving like particles, like basketballs, or my coffee cup. I'm not used to things behaving as both. And it really depends on what experiment you run and how you observe the light. So when you observe it as a particle, and this comes out of Einstein's work with the photoelectric effect, and I won't go into the details here, maybe in a future video when we start thinking about quantum mechanics, you can view light as a train of particles moving at the speed of light, which I'll talk about in a second."}, {"video_title": "Introduction to light Electronic structure of atoms Chemistry Khan Academy.mp3", "Sentence": "In my life, I'm used to certain things behaving as waves, like sound waves, or the waves of an ocean, and I'm used to certain things behaving like particles, like basketballs, or my coffee cup. I'm not used to things behaving as both. And it really depends on what experiment you run and how you observe the light. So when you observe it as a particle, and this comes out of Einstein's work with the photoelectric effect, and I won't go into the details here, maybe in a future video when we start thinking about quantum mechanics, you can view light as a train of particles moving at the speed of light, which I'll talk about in a second. We call these particles photons. If you view light in other ways, and you see it even when you see light being refracted by a prism here, it looks like it is a wave. And it has the properties of a wave."}, {"video_title": "Introduction to light Electronic structure of atoms Chemistry Khan Academy.mp3", "Sentence": "So when you observe it as a particle, and this comes out of Einstein's work with the photoelectric effect, and I won't go into the details here, maybe in a future video when we start thinking about quantum mechanics, you can view light as a train of particles moving at the speed of light, which I'll talk about in a second. We call these particles photons. If you view light in other ways, and you see it even when you see light being refracted by a prism here, it looks like it is a wave. And it has the properties of a wave. It has a frequency, and it has a wavelength. And like other waves, the velocity of that wave is the frequency times its wavelength. Now, even if you ignore this particle aspect of light, if you just look at the wave aspect of light, it's still fascinating, because most waves require a medium to travel through."}, {"video_title": "Introduction to light Electronic structure of atoms Chemistry Khan Academy.mp3", "Sentence": "And it has the properties of a wave. It has a frequency, and it has a wavelength. And like other waves, the velocity of that wave is the frequency times its wavelength. Now, even if you ignore this particle aspect of light, if you just look at the wave aspect of light, it's still fascinating, because most waves require a medium to travel through. So for example, if I think about how sound travels through air, so let me draw a bunch of air particles. I'll draw a sound wave traveling through the air particles. What happens in a sound wave is you compress some of the air particles, and those compress the ones next to them, and so you have points in the air that have higher pressure and points that have lower pressure."}, {"video_title": "Introduction to light Electronic structure of atoms Chemistry Khan Academy.mp3", "Sentence": "Now, even if you ignore this particle aspect of light, if you just look at the wave aspect of light, it's still fascinating, because most waves require a medium to travel through. So for example, if I think about how sound travels through air, so let me draw a bunch of air particles. I'll draw a sound wave traveling through the air particles. What happens in a sound wave is you compress some of the air particles, and those compress the ones next to them, and so you have points in the air that have higher pressure and points that have lower pressure. And you can plot that. So we have high pressure over here, high pressure, low pressure, high pressure, low pressure. And as these things bump into each other, and this wave essentially travels to the right, and if you were to plot that, you would see this waveform traveling to the right."}, {"video_title": "Introduction to light Electronic structure of atoms Chemistry Khan Academy.mp3", "Sentence": "What happens in a sound wave is you compress some of the air particles, and those compress the ones next to them, and so you have points in the air that have higher pressure and points that have lower pressure. And you can plot that. So we have high pressure over here, high pressure, low pressure, high pressure, low pressure. And as these things bump into each other, and this wave essentially travels to the right, and if you were to plot that, you would see this waveform traveling to the right. But this is all predicated, or this is all based on, this energy traveling through a medium. And I'm used to visualizing waves in that way. But light needs no medium."}, {"video_title": "Introduction to light Electronic structure of atoms Chemistry Khan Academy.mp3", "Sentence": "And as these things bump into each other, and this wave essentially travels to the right, and if you were to plot that, you would see this waveform traveling to the right. But this is all predicated, or this is all based on, this energy traveling through a medium. And I'm used to visualizing waves in that way. But light needs no medium. Light will actually travel fastest through nothing, through a vacuum. And it will travel at an unimaginably fast speed, 3 times 10 to the 8th meters per second. And just to give you a sense of this, this is 300 million meters per second."}, {"video_title": "Introduction to light Electronic structure of atoms Chemistry Khan Academy.mp3", "Sentence": "But light needs no medium. Light will actually travel fastest through nothing, through a vacuum. And it will travel at an unimaginably fast speed, 3 times 10 to the 8th meters per second. And just to give you a sense of this, this is 300 million meters per second. Or another way of thinking about it is it would take light less than a seventh of a second to travel around the Earth, or it would travel around the Earth more than seven times in one second. So unimaginably fast. And not only is this just a super fast rate, or a super fast speed, but once again, it tells us that light is something fundamental to our universe."}, {"video_title": "Introduction to light Electronic structure of atoms Chemistry Khan Academy.mp3", "Sentence": "And just to give you a sense of this, this is 300 million meters per second. Or another way of thinking about it is it would take light less than a seventh of a second to travel around the Earth, or it would travel around the Earth more than seven times in one second. So unimaginably fast. And not only is this just a super fast rate, or a super fast speed, but once again, it tells us that light is something fundamental to our universe. Because it's not just an unimaginable fast speed, it is the fastest speed not just known to physics, but possible in physics. So once again, something very unintuitive to us in our everyday realm. We always imagine that, okay, if something is going at some speed, maybe if there was an ant riding on top of that something, and it was moving in the same direction, it would be going even faster."}, {"video_title": "Introduction to light Electronic structure of atoms Chemistry Khan Academy.mp3", "Sentence": "And not only is this just a super fast rate, or a super fast speed, but once again, it tells us that light is something fundamental to our universe. Because it's not just an unimaginable fast speed, it is the fastest speed not just known to physics, but possible in physics. So once again, something very unintuitive to us in our everyday realm. We always imagine that, okay, if something is going at some speed, maybe if there was an ant riding on top of that something, and it was moving in the same direction, it would be going even faster. But nothing can go faster than the speed of light. It's absolutely impossible based on our current understanding of physics. So it's not just a fast speed, it is the fastest speed."}, {"video_title": "Introduction to light Electronic structure of atoms Chemistry Khan Academy.mp3", "Sentence": "We always imagine that, okay, if something is going at some speed, maybe if there was an ant riding on top of that something, and it was moving in the same direction, it would be going even faster. But nothing can go faster than the speed of light. It's absolutely impossible based on our current understanding of physics. So it's not just a fast speed, it is the fastest speed. It is the fastest speed possible. And this right here is an approximation. It's actually 2.99 something something times 10 to the 8th meters per second."}, {"video_title": "Introduction to light Electronic structure of atoms Chemistry Khan Academy.mp3", "Sentence": "So it's not just a fast speed, it is the fastest speed. It is the fastest speed possible. And this right here is an approximation. It's actually 2.99 something something times 10 to the 8th meters per second. 3 times 10 to the 8th meters per second is a pretty good approximation. Now, within the visible light spectrum, and I'll talk about what's beyond the visible light spectrum in a second, you're probably familiar with the colors, maybe you imagine them as the colors of the rainbow. And rainbows really happen because the light from the sun, the white light, is being refracted by these little water particles."}, {"video_title": "Introduction to light Electronic structure of atoms Chemistry Khan Academy.mp3", "Sentence": "It's actually 2.99 something something times 10 to the 8th meters per second. 3 times 10 to the 8th meters per second is a pretty good approximation. Now, within the visible light spectrum, and I'll talk about what's beyond the visible light spectrum in a second, you're probably familiar with the colors, maybe you imagine them as the colors of the rainbow. And rainbows really happen because the light from the sun, the white light, is being refracted by these little water particles. And you can see that in maybe a clearer way when you see light being refracted by a prism right over here. And the different wavelengths of light, so white light contains all of the visible wavelengths, but the different wavelengths get refracted differently by a prism. So in this case, the higher frequency wavelengths, the violet and the blue, gets refracted more, it gets bent, its direction gets bent more than the low frequency wavelengths, than the reds and the oranges right over here."}, {"video_title": "Introduction to light Electronic structure of atoms Chemistry Khan Academy.mp3", "Sentence": "And rainbows really happen because the light from the sun, the white light, is being refracted by these little water particles. And you can see that in maybe a clearer way when you see light being refracted by a prism right over here. And the different wavelengths of light, so white light contains all of the visible wavelengths, but the different wavelengths get refracted differently by a prism. So in this case, the higher frequency wavelengths, the violet and the blue, gets refracted more, it gets bent, its direction gets bent more than the low frequency wavelengths, than the reds and the oranges right over here. And if you want to look at the wavelength of light, they're a visible light, it's between 400 nanometers and 700 nanometers, and the higher the frequency, the higher the energy of that light. And that actually goes into when you start talking about the quantum mechanics of it. That the higher frequency means that each of these photons have higher energy."}, {"video_title": "Introduction to light Electronic structure of atoms Chemistry Khan Academy.mp3", "Sentence": "So in this case, the higher frequency wavelengths, the violet and the blue, gets refracted more, it gets bent, its direction gets bent more than the low frequency wavelengths, than the reds and the oranges right over here. And if you want to look at the wavelength of light, they're a visible light, it's between 400 nanometers and 700 nanometers, and the higher the frequency, the higher the energy of that light. And that actually goes into when you start talking about the quantum mechanics of it. That the higher frequency means that each of these photons have higher energy. They have a better ability to give kinetic energy to knock off electrons or whatever else they need to do. So higher frequency, let me write that down. Higher frequency means higher energy."}, {"video_title": "Introduction to light Electronic structure of atoms Chemistry Khan Academy.mp3", "Sentence": "That the higher frequency means that each of these photons have higher energy. They have a better ability to give kinetic energy to knock off electrons or whatever else they need to do. So higher frequency, let me write that down. Higher frequency means higher energy. Now, I keep referring to this idea of the visible light. And you might say, what is beyond visible light? And what you'll find is that light is just part of a much broader phenomenon, and it's just the part that we happen to observe."}, {"video_title": "Introduction to light Electronic structure of atoms Chemistry Khan Academy.mp3", "Sentence": "Higher frequency means higher energy. Now, I keep referring to this idea of the visible light. And you might say, what is beyond visible light? And what you'll find is that light is just part of a much broader phenomenon, and it's just the part that we happen to observe. And if we want to broaden the discussion a little bit, light is just, or I should say visible light, is just really part of the electromagnetic spectrum. So light is really just electromagnetic radiation. And everything that I told you about light just now, it has a wave property, and it has particle properties."}, {"video_title": "Introduction to light Electronic structure of atoms Chemistry Khan Academy.mp3", "Sentence": "And what you'll find is that light is just part of a much broader phenomenon, and it's just the part that we happen to observe. And if we want to broaden the discussion a little bit, light is just, or I should say visible light, is just really part of the electromagnetic spectrum. So light is really just electromagnetic radiation. And everything that I told you about light just now, it has a wave property, and it has particle properties. This is not just specific to visible light. This is true of all of electromagnetic radiation. So at very low frequencies or very long wavelengths, we're talking about things like radio waves, the things that allow a radio to reach your car, the things that allow your cell phone to communicate with cell towers."}, {"video_title": "Introduction to light Electronic structure of atoms Chemistry Khan Academy.mp3", "Sentence": "And everything that I told you about light just now, it has a wave property, and it has particle properties. This is not just specific to visible light. This is true of all of electromagnetic radiation. So at very low frequencies or very long wavelengths, we're talking about things like radio waves, the things that allow a radio to reach your car, the things that allow your cell phone to communicate with cell towers. Microwaves, the things that start vibrating water molecules in your food so that they heat up. Infrared, which is what our body releases, and that's why you can detect people through walls with infrared cameras. Visible light, ultraviolet light, the UV light coming from the sun that will give you sunburn."}, {"video_title": "Introduction to light Electronic structure of atoms Chemistry Khan Academy.mp3", "Sentence": "So at very low frequencies or very long wavelengths, we're talking about things like radio waves, the things that allow a radio to reach your car, the things that allow your cell phone to communicate with cell towers. Microwaves, the things that start vibrating water molecules in your food so that they heat up. Infrared, which is what our body releases, and that's why you can detect people through walls with infrared cameras. Visible light, ultraviolet light, the UV light coming from the sun that will give you sunburn. X-rays, the radiation that allows us to see through the soft material and just visualize the bones. Gamma rays, the super high energy that comes from quasars and other certain types of physical phenomena. These are all examples of the exact same thing."}, {"video_title": "Introduction to light Electronic structure of atoms Chemistry Khan Academy.mp3", "Sentence": "Visible light, ultraviolet light, the UV light coming from the sun that will give you sunburn. X-rays, the radiation that allows us to see through the soft material and just visualize the bones. Gamma rays, the super high energy that comes from quasars and other certain types of physical phenomena. These are all examples of the exact same thing. We just happen to perceive certain frequencies of this as visible light. You might say, hey Sal, how come we only perceive certain frequencies of this? How come we only see these frequencies?"}, {"video_title": "Introduction to light Electronic structure of atoms Chemistry Khan Academy.mp3", "Sentence": "These are all examples of the exact same thing. We just happen to perceive certain frequencies of this as visible light. You might say, hey Sal, how come we only perceive certain frequencies of this? How come we only see these frequencies? Literally, we can see those frequencies with our unaided eye. The reason, or at least my best guess of the reason of that, is that's the frequency where the sun dumps out a lot of electromagnetic radiation. It's inundating the earth, and if as a species you wanted to observe things based on reflected energy, a reflected electromagnetic energy, it is most useful to be able to perceive the things where there is the most electromagnetic radiation."}, {"video_title": "Introduction to light Electronic structure of atoms Chemistry Khan Academy.mp3", "Sentence": "How come we only see these frequencies? Literally, we can see those frequencies with our unaided eye. The reason, or at least my best guess of the reason of that, is that's the frequency where the sun dumps out a lot of electromagnetic radiation. It's inundating the earth, and if as a species you wanted to observe things based on reflected energy, a reflected electromagnetic energy, it is most useful to be able to perceive the things where there is the most electromagnetic radiation. It is possible that in other realities or other planets, there are species that perceive more in the ultraviolet range or in the infrared range, and even on earth there are some that perform better at either end of the range. But we see really well in the part of the spectrum where the sun just happens to dump a lot of radiation on us. I'll leave you there."}, {"video_title": "Introduction to light Electronic structure of atoms Chemistry Khan Academy.mp3", "Sentence": "It's inundating the earth, and if as a species you wanted to observe things based on reflected energy, a reflected electromagnetic energy, it is most useful to be able to perceive the things where there is the most electromagnetic radiation. It is possible that in other realities or other planets, there are species that perceive more in the ultraviolet range or in the infrared range, and even on earth there are some that perform better at either end of the range. But we see really well in the part of the spectrum where the sun just happens to dump a lot of radiation on us. I'll leave you there. I think that's a pretty good overview of light. If any of this stuff seems kind of unintuitive or daunting or really on some level confusing, this wave-particle duality, this idea of a transfer of energy through nothing, and it seems unintuitive, don't worry. It seems unintuitive even for the best of physicists."}, {"video_title": "Structure of the Earth.mp3", "Sentence": "So let me draw a cross-section of the Earth over here. And I'll try to do it, I won't be able to do it perfectly to scale, but I'll try to do a little bit better job at giving you a little bit of a sense of how thick these layers are. So let's say that this is the crust up here. And I'm going to make the continental crust a little bit thicker. So let's say that that is continental crust and this is continental crust. And then in between, let me put some oceanic crust, which is going to be thinner. So this right here is, actually let me do it in a different color."}, {"video_title": "Structure of the Earth.mp3", "Sentence": "And I'm going to make the continental crust a little bit thicker. So let's say that that is continental crust and this is continental crust. And then in between, let me put some oceanic crust, which is going to be thinner. So this right here is, actually let me do it in a different color. Let me do the oceanic crust in blue. But this isn't water, this is rock. I'll do it in purple, that's even better."}, {"video_title": "Structure of the Earth.mp3", "Sentence": "So this right here is, actually let me do it in a different color. Let me do the oceanic crust in blue. But this isn't water, this is rock. I'll do it in purple, that's even better. I don't want it to be that thick. So let me draw the oceanic crust is thinner than the continental crust, which I'm trying to depict right over here. So this right over here is oceanic crust."}, {"video_title": "Structure of the Earth.mp3", "Sentence": "I'll do it in purple, that's even better. I don't want it to be that thick. So let me draw the oceanic crust is thinner than the continental crust, which I'm trying to depict right over here. So this right over here is oceanic crust. And up here is continental crust. And the thickness, or how deep you can go and still be in crust, it depends on where you are. And we know that near hot spots, the oceanic crust can actually thin out a good bit."}, {"video_title": "Structure of the Earth.mp3", "Sentence": "So this right over here is oceanic crust. And up here is continental crust. And the thickness, or how deep you can go and still be in crust, it depends on where you are. And we know that near hot spots, the oceanic crust can actually thin out a good bit. But roughly, when we talk about the crust, we're talking about something that's 30 to 60 kilometers deep. So if you are on a continent, which I'm assuming you are, and you dig for 20 kilometers, you will still be in the crust. 30 kilometers, probably still in the crust."}, {"video_title": "Structure of the Earth.mp3", "Sentence": "And we know that near hot spots, the oceanic crust can actually thin out a good bit. But roughly, when we talk about the crust, we're talking about something that's 30 to 60 kilometers deep. So if you are on a continent, which I'm assuming you are, and you dig for 20 kilometers, you will still be in the crust. 30 kilometers, probably still in the crust. If you dig for 70 kilometers or 100 kilometers, you will probably reach the mantle. And remember, what we're describing here when we talk about the crust, the mantle, and the core, we're talking about the chemical makeup. Let me make this clear."}, {"video_title": "Structure of the Earth.mp3", "Sentence": "30 kilometers, probably still in the crust. If you dig for 70 kilometers or 100 kilometers, you will probably reach the mantle. And remember, what we're describing here when we talk about the crust, the mantle, and the core, we're talking about the chemical makeup. Let me make this clear. We're talking about the chemical makeup. The crust is fundamentally different than the mantle based on the molecules that it is made up of, based on its composition. So let's talk about the mantle now."}, {"video_title": "Structure of the Earth.mp3", "Sentence": "Let me make this clear. We're talking about the chemical makeup. The crust is fundamentally different than the mantle based on the molecules that it is made up of, based on its composition. So let's talk about the mantle now. So the mantle layer like this. And once again, this is not to scale, because the crust we're talking about 30 to 60 kilometers. The mantle we're talking about on the order of about 2,900 or 3,000 kilometers thick."}, {"video_title": "Structure of the Earth.mp3", "Sentence": "So let's talk about the mantle now. So the mantle layer like this. And once again, this is not to scale, because the crust we're talking about 30 to 60 kilometers. The mantle we're talking about on the order of about 2,900 or 3,000 kilometers thick. So this right here is the entire mantle. So that's the mantle. And this is 2,900 to 3,000 kilometers thick."}, {"video_title": "Structure of the Earth.mp3", "Sentence": "The mantle we're talking about on the order of about 2,900 or 3,000 kilometers thick. So this right here is the entire mantle. So that's the mantle. And this is 2,900 to 3,000 kilometers thick. So this isn't even 1 30th of that. So I would have to draw it even narrower than the way I've drawn it over here. And the mantle itself can be subdivided into the upper mantle and the lower mantle."}, {"video_title": "Structure of the Earth.mp3", "Sentence": "And this is 2,900 to 3,000 kilometers thick. So this isn't even 1 30th of that. So I would have to draw it even narrower than the way I've drawn it over here. And the mantle itself can be subdivided into the upper mantle and the lower mantle. So let me draw this division right over here. So this is the upper mantle. And there's different ways to define the boundary."}, {"video_title": "Structure of the Earth.mp3", "Sentence": "And the mantle itself can be subdivided into the upper mantle and the lower mantle. So let me draw this division right over here. So this is the upper mantle. And there's different ways to define the boundary. But the upper mantle is roughly about 700 kilometers down. So these are huge distances. I mean, this is going straight down."}, {"video_title": "Structure of the Earth.mp3", "Sentence": "And there's different ways to define the boundary. But the upper mantle is roughly about 700 kilometers down. So these are huge distances. I mean, this is going straight down. So this is the upper mantle. Let me write it on the actual mantle here. This is the upper mantle."}, {"video_title": "Structure of the Earth.mp3", "Sentence": "I mean, this is going straight down. So this is the upper mantle. Let me write it on the actual mantle here. This is the upper mantle. And this over here is the lower mantle. And just to be clear on things. So the crust is solid."}, {"video_title": "Structure of the Earth.mp3", "Sentence": "This is the upper mantle. And this over here is the lower mantle. And just to be clear on things. So the crust is solid. Now, when you go into the upper mantle, the upper part of the upper mantle, and we'll talk about that a little bit more, is cool enough to be solid. So there is a solid portion of the upper mantle. So all of this up here is solid because it's cool enough."}, {"video_title": "Structure of the Earth.mp3", "Sentence": "So the crust is solid. Now, when you go into the upper mantle, the upper part of the upper mantle, and we'll talk about that a little bit more, is cool enough to be solid. So there is a solid portion of the upper mantle. So all of this up here is solid because it's cool enough. It hasn't reached the melting point of those rocks just yet. And we learned in previous videos that the combination of the solid part of the upper mantle and the crust combined, we call that the lithosphere. And when we talk about the lithosphere, we're not talking about the mechanical makeup."}, {"video_title": "Structure of the Earth.mp3", "Sentence": "So all of this up here is solid because it's cool enough. It hasn't reached the melting point of those rocks just yet. And we learned in previous videos that the combination of the solid part of the upper mantle and the crust combined, we call that the lithosphere. And when we talk about the lithosphere, we're not talking about the mechanical makeup. We're not talking about what's solid and what's not solid. So this is the lithosphere. You go a little bit deeper, right below the lithosphere."}, {"video_title": "Structure of the Earth.mp3", "Sentence": "And when we talk about the lithosphere, we're not talking about the mechanical makeup. We're not talking about what's solid and what's not solid. So this is the lithosphere. You go a little bit deeper, right below the lithosphere. Now the temperatures are high enough for, and I use the word liquid, but that's not exactly right. You can kind of think of it as kind of a deformable solid or a plastic solid or a magma. And that's the asthenosphere."}, {"video_title": "Structure of the Earth.mp3", "Sentence": "You go a little bit deeper, right below the lithosphere. Now the temperatures are high enough for, and I use the word liquid, but that's not exactly right. You can kind of think of it as kind of a deformable solid or a plastic solid or a magma. And that's the asthenosphere. So this area right over here, the liquid part, actually I shouldn't use the word liquid, kind of the deformable, it deforms over long periods of time. But it is more fluid than what we normally associate with rock. Magma would be a good way to think about it."}, {"video_title": "Structure of the Earth.mp3", "Sentence": "And that's the asthenosphere. So this area right over here, the liquid part, actually I shouldn't use the word liquid, kind of the deformable, it deforms over long periods of time. But it is more fluid than what we normally associate with rock. Magma would be a good way to think about it. That's what we call the asthenosphere. It is fluid, just not as fluid as water. It is more viscous than something like water."}, {"video_title": "Structure of the Earth.mp3", "Sentence": "Magma would be a good way to think about it. That's what we call the asthenosphere. It is fluid, just not as fluid as water. It is more viscous than something like water. So this is the asthenosphere. Now the upper mantle is hot enough for the rock to melt and be fluid, and the pressure is low enough for it to still be able to kind of move past itself, to still be somewhat fluid. But then once you get even deeper into the lower mantle, you have higher pressure."}, {"video_title": "Structure of the Earth.mp3", "Sentence": "It is more viscous than something like water. So this is the asthenosphere. Now the upper mantle is hot enough for the rock to melt and be fluid, and the pressure is low enough for it to still be able to kind of move past itself, to still be somewhat fluid. But then once you get even deeper into the lower mantle, you have higher pressure. And so it's still fluid, but it's less fluid. It's kind of thicker, I guess is the best way to think about it, in the lower mantle. It's thicker."}, {"video_title": "Structure of the Earth.mp3", "Sentence": "But then once you get even deeper into the lower mantle, you have higher pressure. And so it's still fluid, but it's less fluid. It's kind of thicker, I guess is the best way to think about it, in the lower mantle. It's thicker. So this whole area over here, you can kind of think of it as melted rock. It's fluid, but the upper part of the melted rock is more fluid. It's able to move easier because there's less pressure."}, {"video_title": "Structure of the Earth.mp3", "Sentence": "It's thicker. So this whole area over here, you can kind of think of it as melted rock. It's fluid, but the upper part of the melted rock is more fluid. It's able to move easier because there's less pressure. And the pressure is just from all of the rock that's above it. Remember, gravity is pulling down on everything. Gravity, every molecule here wants to go downward because of gravity."}, {"video_title": "Structure of the Earth.mp3", "Sentence": "It's able to move easier because there's less pressure. And the pressure is just from all of the rock that's above it. Remember, gravity is pulling down on everything. Gravity, every molecule here wants to go downward because of gravity. So it's applying pressure downward. So the deeper you go, the more pressure you get. Now, when we get even deeper than that, we get to the core, and the core is divided between the outer core and the inner core."}, {"video_title": "Structure of the Earth.mp3", "Sentence": "Gravity, every molecule here wants to go downward because of gravity. So it's applying pressure downward. So the deeper you go, the more pressure you get. Now, when we get even deeper than that, we get to the core, and the core is divided between the outer core and the inner core. So you have the outer core, and then, of course, you have the inner core. And just so we have a sense for distances, the width or the thickness of the outer core is approximately 2,300 kilometers. So these are huge distances when you think about thickness."}, {"video_title": "Structure of the Earth.mp3", "Sentence": "Now, when we get even deeper than that, we get to the core, and the core is divided between the outer core and the inner core. So you have the outer core, and then, of course, you have the inner core. And just so we have a sense for distances, the width or the thickness of the outer core is approximately 2,300 kilometers. So these are huge distances when you think about thickness. You can go down another 2,300 kilometers, or once you go through the mantle, you go 2,300 kilometers to the outer core, and then you're in the inner core, and that essentially takes you to the rest. That's essentially the center of the Earth. And the inner core, and maybe I should draw the boundaries a little bit more to scale."}, {"video_title": "Structure of the Earth.mp3", "Sentence": "So these are huge distances when you think about thickness. You can go down another 2,300 kilometers, or once you go through the mantle, you go 2,300 kilometers to the outer core, and then you're in the inner core, and that essentially takes you to the rest. That's essentially the center of the Earth. And the inner core, and maybe I should draw the boundaries a little bit more to scale. Let me do it this way. It should actually look a little bit more like this because the outer core is thicker than the inner core. So the outer core is, as I said, let me rewrite it."}, {"video_title": "Structure of the Earth.mp3", "Sentence": "And the inner core, and maybe I should draw the boundaries a little bit more to scale. Let me do it this way. It should actually look a little bit more like this because the outer core is thicker than the inner core. So the outer core is, as I said, let me rewrite it. The outer core is on the order, it's about 2,300 kilometers thick, and then you have your inner core. I shouldn't do it in blue. I should do it in a hot color."}, {"video_title": "Structure of the Earth.mp3", "Sentence": "So the outer core is, as I said, let me rewrite it. The outer core is on the order, it's about 2,300 kilometers thick, and then you have your inner core. I shouldn't do it in blue. I should do it in a hot color. So the inner core right over here just kind of takes us to the center of the Earth, and that's a little over 1,000 kilometers thick. So this is the inner core. The number I have is about 1,200 kilometers thick."}, {"video_title": "Structure of the Earth.mp3", "Sentence": "I should do it in a hot color. So the inner core right over here just kind of takes us to the center of the Earth, and that's a little over 1,000 kilometers thick. So this is the inner core. The number I have is about 1,200 kilometers thick. And both the entire core, both the outer core and the inner core, is mainly nickel and iron. Think about when the Earth was forming. What happens is when this whole Earth was super hot and it was kind of in a fluid state, the heavier elements were allowed to sink down when everything was fluid."}, {"video_title": "Structure of the Earth.mp3", "Sentence": "The number I have is about 1,200 kilometers thick. And both the entire core, both the outer core and the inner core, is mainly nickel and iron. Think about when the Earth was forming. What happens is when this whole Earth was super hot and it was kind of in a fluid state, the heavier elements were allowed to sink down when everything was fluid. The things that are in between would kind of, or the things that were lighter would go up, and then the gases, things that would naturally be in the gaseous state, would kind of bubble up through that fluid, kind of the way actually carbon bubbles up in a soda. It would eventually bubble out of the fluid, and it would actually form the atmosphere. So that's why when you look at the composition of the Earth, you have the densest, the heaviest elements in the center, and then the lightest elements are forming the atmosphere."}, {"video_title": "Structure of the Earth.mp3", "Sentence": "What happens is when this whole Earth was super hot and it was kind of in a fluid state, the heavier elements were allowed to sink down when everything was fluid. The things that are in between would kind of, or the things that were lighter would go up, and then the gases, things that would naturally be in the gaseous state, would kind of bubble up through that fluid, kind of the way actually carbon bubbles up in a soda. It would eventually bubble out of the fluid, and it would actually form the atmosphere. So that's why when you look at the composition of the Earth, you have the densest, the heaviest elements in the center, and then the lightest elements are forming the atmosphere. And the outer core and the inner core, they are made up predominantly of nickel and iron. And their makeup is actually very similar. Chemically, they have a very similar composition."}, {"video_title": "Structure of the Earth.mp3", "Sentence": "So that's why when you look at the composition of the Earth, you have the densest, the heaviest elements in the center, and then the lightest elements are forming the atmosphere. And the outer core and the inner core, they are made up predominantly of nickel and iron. And their makeup is actually very similar. Chemically, they have a very similar composition. What's different about them is at the outer core, you have temperatures high enough that nickel and iron can melt, but the pressures are low enough that they can still be in a fluid state. So this is our liquid outer core, and this has a pretty low viscosity, especially even relative to the mantle. So that's why people kind of consider this in kind of a more traditional liquid state."}, {"video_title": "Structure of the Earth.mp3", "Sentence": "Chemically, they have a very similar composition. What's different about them is at the outer core, you have temperatures high enough that nickel and iron can melt, but the pressures are low enough that they can still be in a fluid state. So this is our liquid outer core, and this has a pretty low viscosity, especially even relative to the mantle. So that's why people kind of consider this in kind of a more traditional liquid state. But as you get deeper and deeper and deeper, the pressure becomes so huge as you get to the inner core. Remember, all of the weight of all of the rock above you, of these thousands of miles of rock above you, is all pushing down on the rock below it. So the inner core, even though the temperature is really, really, really hot, the pressure is so big that the molecules can't flow past each other."}, {"video_title": "Structure of the Earth.mp3", "Sentence": "So that's why people kind of consider this in kind of a more traditional liquid state. But as you get deeper and deeper and deeper, the pressure becomes so huge as you get to the inner core. Remember, all of the weight of all of the rock above you, of these thousands of miles of rock above you, is all pushing down on the rock below it. So the inner core, even though the temperature is really, really, really hot, the pressure is so big that the molecules can't flow past each other. They can't be liquid. They're kind of jammed packed. And so the inner core, because of the high pressure, despite the high temperature, is solid."}, {"video_title": "Structure of the Earth.mp3", "Sentence": "So the inner core, even though the temperature is really, really, really hot, the pressure is so big that the molecules can't flow past each other. They can't be liquid. They're kind of jammed packed. And so the inner core, because of the high pressure, despite the high temperature, is solid. So the difference here is actually a mechanical one between the outer core and the inner core. They're made up of the same things, roughly the same chemical makeup. It's just slightly lower pressure on the outside, so you can actually be in a fluid state."}, {"video_title": "Land productivity limiting human population Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So what I wanna do in this video is give us a little bit of a framework for thinking about how humans have been getting calories from the land and how that's placed an upper limit on the number of humans that can live in any given area or the population density of humans. So right over here, you have some gentlemen looking for food. They are hunter-gatherers. I'll say HG for short. And the H part, the hunter part, they might actually find some animals. I think these guys right over here are trying to trap some rabbits. And the gathering part, they're just literally looking for food."}, {"video_title": "Land productivity limiting human population Cosmology & Astronomy Khan Academy.mp3", "Sentence": "I'll say HG for short. And the H part, the hunter part, they might actually find some animals. I think these guys right over here are trying to trap some rabbits. And the gathering part, they're just literally looking for food. Maybe they find fruit of some sort or some nuts or maybe some roots that are edible by humans. So literally, they just walk around, either try to kill things or find things that they can consume. So I'll call this right over here stage one."}, {"video_title": "Land productivity limiting human population Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And the gathering part, they're just literally looking for food. Maybe they find fruit of some sort or some nuts or maybe some roots that are edible by humans. So literally, they just walk around, either try to kill things or find things that they can consume. So I'll call this right over here stage one. And actually, let me write it over here. So this is hunter-gatherers. And this is what most humans have done through most of human history."}, {"video_title": "Land productivity limiting human population Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So I'll call this right over here stage one. And actually, let me write it over here. So this is hunter-gatherers. And this is what most humans have done through most of human history. And just to give us a little bit of a framework for maybe how much they could get from the land, and I looked at some of our best sense of studying hunter-gatherer populations. In land like this, maybe they can get about 200 calories. And I'll make this whole column right over here."}, {"video_title": "Land productivity limiting human population Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And this is what most humans have done through most of human history. And just to give us a little bit of a framework for maybe how much they could get from the land, and I looked at some of our best sense of studying hunter-gatherer populations. In land like this, maybe they can get about 200 calories. And I'll make this whole column right over here. This whole column right over here is the amount that they could get in terms of calories per square kilometer per day. Now, it's obviously going to be hugely dependent on the number of animals that are there, the type of land that's there. If they're next to a stream where maybe fish are just jumping out of the stream, this number would be much higher."}, {"video_title": "Land productivity limiting human population Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And I'll make this whole column right over here. This whole column right over here is the amount that they could get in terms of calories per square kilometer per day. Now, it's obviously going to be hugely dependent on the number of animals that are there, the type of land that's there. If they're next to a stream where maybe fish are just jumping out of the stream, this number would be much higher. If they were in some type of a desert, this number would be much lower. But this is actually fairly in line with some of the studies of hunter-gatherer cultures. Now, if this is the number of calories that they can get from each square kilometer per day, how many humans can live in a square kilometer per day?"}, {"video_title": "Land productivity limiting human population Cosmology & Astronomy Khan Academy.mp3", "Sentence": "If they're next to a stream where maybe fish are just jumping out of the stream, this number would be much higher. If they were in some type of a desert, this number would be much lower. But this is actually fairly in line with some of the studies of hunter-gatherer cultures. Now, if this is the number of calories that they can get from each square kilometer per day, how many humans can live in a square kilometer per day? Or what is the density of humans? Well, to figure that out, we have to know on average how many calories does a human need to survive. And for the sake of this video, I'm going to make the assumption that a human being needs 2,000 calories per day to survive in a non-malnourished state."}, {"video_title": "Land productivity limiting human population Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Now, if this is the number of calories that they can get from each square kilometer per day, how many humans can live in a square kilometer per day? Or what is the density of humans? Well, to figure that out, we have to know on average how many calories does a human need to survive. And for the sake of this video, I'm going to make the assumption that a human being needs 2,000 calories per day to survive in a non-malnourished state. And obviously, it's hugely dependent on how active this person is or how large they are. And one other note, in this whole video, I'm going to be using calories with a capital C. And the calories with a capital C are the calories that people are used to referring to when you go to the gym and you run on the treadmill and it says how many calories you've burned or you look at the back of your candy bar and it says 200 calories. These are the calories I'm talking about."}, {"video_title": "Land productivity limiting human population Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And for the sake of this video, I'm going to make the assumption that a human being needs 2,000 calories per day to survive in a non-malnourished state. And obviously, it's hugely dependent on how active this person is or how large they are. And one other note, in this whole video, I'm going to be using calories with a capital C. And the calories with a capital C are the calories that people are used to referring to when you go to the gym and you run on the treadmill and it says how many calories you've burned or you look at the back of your candy bar and it says 200 calories. These are the calories I'm talking about. They are a slightly different notion than the calories that you encounter in chemistry class. Those calories are calorie with a lowercase c. And just so that you can be optimally confused, it turns out that one calorie with an uppercase C is equal to 1,000 calories with a lowercase c. 1,000 calories. And the lowercase c calories is the amount of energy needed to heat one gram of water one degrees Celsius."}, {"video_title": "Land productivity limiting human population Cosmology & Astronomy Khan Academy.mp3", "Sentence": "These are the calories I'm talking about. They are a slightly different notion than the calories that you encounter in chemistry class. Those calories are calorie with a lowercase c. And just so that you can be optimally confused, it turns out that one calorie with an uppercase C is equal to 1,000 calories with a lowercase c. 1,000 calories. And the lowercase c calories is the amount of energy needed to heat one gram of water one degrees Celsius. And so this is what you see in your chemistry class, but this is not what we're going to be talking about in this video. We're talking about the capital C, the calories that dietitians are always talking about. So with this assumption that the average human eats 2,000 calories a day to not get malnourished, and obviously men would need more, women would need less, children would need even less, but with this assumption, what is the density of humans that could be supported by this culture right over here?"}, {"video_title": "Land productivity limiting human population Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And the lowercase c calories is the amount of energy needed to heat one gram of water one degrees Celsius. And so this is what you see in your chemistry class, but this is not what we're going to be talking about in this video. We're talking about the capital C, the calories that dietitians are always talking about. So with this assumption that the average human eats 2,000 calories a day to not get malnourished, and obviously men would need more, women would need less, children would need even less, but with this assumption, what is the density of humans that could be supported by this culture right over here? Well, 200 calories is 1 10th of the average daily human requirement if you believe this assumption. So the population density, so the density in, let me write it, humans per square kilometer, you can only support 1 10th of a human with this calorie output. So you can only support 1 10th of a human per square kilometer."}, {"video_title": "Land productivity limiting human population Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So with this assumption that the average human eats 2,000 calories a day to not get malnourished, and obviously men would need more, women would need less, children would need even less, but with this assumption, what is the density of humans that could be supported by this culture right over here? Well, 200 calories is 1 10th of the average daily human requirement if you believe this assumption. So the population density, so the density in, let me write it, humans per square kilometer, you can only support 1 10th of a human with this calorie output. So you can only support 1 10th of a human per square kilometer. So one human would actually need 10 square kilometers of hunter-gatherer, to hunt from and gather from in order to support just themselves. They would need maybe 30 or 40 square kilometers to support an entire family so that they could wander around and kill the animals and find whatever they need to find on that land. Now let's go to kind of, you can view it as maybe the next stage, although it's not always the case that herding is going to be more productive than hunter-gatherers, especially in the case where the fish are jumping out of the water, but let's go to this scenario right over here."}, {"video_title": "Land productivity limiting human population Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So you can only support 1 10th of a human per square kilometer. So one human would actually need 10 square kilometers of hunter-gatherer, to hunt from and gather from in order to support just themselves. They would need maybe 30 or 40 square kilometers to support an entire family so that they could wander around and kill the animals and find whatever they need to find on that land. Now let's go to kind of, you can view it as maybe the next stage, although it's not always the case that herding is going to be more productive than hunter-gatherers, especially in the case where the fish are jumping out of the water, but let's go to this scenario right over here. So this is, we can call this a pastoral lifestyle. So this is two, I'll call it pastoral. And over here is the realization that, look, you have all of this vegetation that maybe humans can't consume, but there are other animals that can consume this vegetation, and they can turn those calories into calories that can be consumed by a human, and namely, the calories are themselves."}, {"video_title": "Land productivity limiting human population Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Now let's go to kind of, you can view it as maybe the next stage, although it's not always the case that herding is going to be more productive than hunter-gatherers, especially in the case where the fish are jumping out of the water, but let's go to this scenario right over here. So this is, we can call this a pastoral lifestyle. So this is two, I'll call it pastoral. And over here is the realization that, look, you have all of this vegetation that maybe humans can't consume, but there are other animals that can consume this vegetation, and they can turn those calories into calories that can be consumed by a human, and namely, the calories are themselves. So this gentleman right over here, after he gets these sheep to be nice and fat, he can either eat the sheep or he can drink their milk. So one way to think about it is these cattle or these sheep right over here, by herding them and letting them eat the grass, he's turning non-human consumable calories into human consumable calories. And so for the sake of our thought experiment, let's say we get a 10 times increase in the human consumable calories per square kilometer."}, {"video_title": "Land productivity limiting human population Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And over here is the realization that, look, you have all of this vegetation that maybe humans can't consume, but there are other animals that can consume this vegetation, and they can turn those calories into calories that can be consumed by a human, and namely, the calories are themselves. So this gentleman right over here, after he gets these sheep to be nice and fat, he can either eat the sheep or he can drink their milk. So one way to think about it is these cattle or these sheep right over here, by herding them and letting them eat the grass, he's turning non-human consumable calories into human consumable calories. And so for the sake of our thought experiment, let's say we get a 10 times increase in the human consumable calories per square kilometer. So now instead of 200, we're up to 2,000. And so instead of one human per square kilometer, we have enough calories per square kilometer per day to support one human. So instead of, sorry, instead of 0.1, we can now support one human."}, {"video_title": "Land productivity limiting human population Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And so for the sake of our thought experiment, let's say we get a 10 times increase in the human consumable calories per square kilometer. So now instead of 200, we're up to 2,000. And so instead of one human per square kilometer, we have enough calories per square kilometer per day to support one human. So instead of, sorry, instead of 0.1, we can now support one human. So in that 10 square kilometers, we can now support 10 people. In 100 square kilometers, we could now support 100 people. Now, the next stage, and I'm skipping a bunch of stages, so because you have things like subsistence agriculture and various forms, and they're not going to be equally productive, and it depends what the land is like, and it depends what the tools are at your disposal."}, {"video_title": "Land productivity limiting human population Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So instead of, sorry, instead of 0.1, we can now support one human. So in that 10 square kilometers, we can now support 10 people. In 100 square kilometers, we could now support 100 people. Now, the next stage, and I'm skipping a bunch of stages, so because you have things like subsistence agriculture and various forms, and they're not going to be equally productive, and it depends what the land is like, and it depends what the tools are at your disposal. But the next stage that I'll just kind of jump to, we can call traditional agriculture. So this right over here, let's call that traditional agriculture. And that's this one over here as well."}, {"video_title": "Land productivity limiting human population Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Now, the next stage, and I'm skipping a bunch of stages, so because you have things like subsistence agriculture and various forms, and they're not going to be equally productive, and it depends what the land is like, and it depends what the tools are at your disposal. But the next stage that I'll just kind of jump to, we can call traditional agriculture. So this right over here, let's call that traditional agriculture. And that's this one over here as well. So both of these, I'm gonna call traditional agriculture. And for the purposes of this video, the difference between traditional agriculture and modern agriculture, in traditional agriculture, you didn't have mechanization, so you didn't, or very primitive mechanization. You definitely did not have fossil fuel-based engines."}, {"video_title": "Land productivity limiting human population Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And that's this one over here as well. So both of these, I'm gonna call traditional agriculture. And for the purposes of this video, the difference between traditional agriculture and modern agriculture, in traditional agriculture, you didn't have mechanization, so you didn't, or very primitive mechanization. You definitely did not have fossil fuel-based engines. You didn't have modern pesticides. You did not have modern genetically engineered crops. But you did have some of the basic science of breeding crops and irrigating and using animals as tools."}, {"video_title": "Land productivity limiting human population Cosmology & Astronomy Khan Academy.mp3", "Sentence": "You definitely did not have fossil fuel-based engines. You didn't have modern pesticides. You did not have modern genetically engineered crops. But you did have some of the basic science of breeding crops and irrigating and using animals as tools. So in this stage right over here, and once again, it completely depends on where you are on the planet, how fertile the land is, how good your tools are, what crops you're actually producing. Let's assume that we got a hundredfold increase in productivity. And looking at some of the historical records, it looks like, depending on, once again, where you are, that's not out of the realm of possibility."}, {"video_title": "Land productivity limiting human population Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But you did have some of the basic science of breeding crops and irrigating and using animals as tools. So in this stage right over here, and once again, it completely depends on where you are on the planet, how fertile the land is, how good your tools are, what crops you're actually producing. Let's assume that we got a hundredfold increase in productivity. And looking at some of the historical records, it looks like, depending on, once again, where you are, that's not out of the realm of possibility. So you have a hundredfold increase. So instead of 2,000 calories per square kilometer per day, you can get 200,000. 200,000 calories per square kilometer per day."}, {"video_title": "Land productivity limiting human population Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And looking at some of the historical records, it looks like, depending on, once again, where you are, that's not out of the realm of possibility. So you have a hundredfold increase. So instead of 2,000 calories per square kilometer per day, you can get 200,000. 200,000 calories per square kilometer per day. And now you could support 100 humans per square kilometer, if you wanted to. So you might not have a hundred humans. One, not all the land you might be able to farm from."}, {"video_title": "Land productivity limiting human population Cosmology & Astronomy Khan Academy.mp3", "Sentence": "200,000 calories per square kilometer per day. And now you could support 100 humans per square kilometer, if you wanted to. So you might not have a hundred humans. One, not all the land you might be able to farm from. Or there are other limits on the population for whatever they might be. But the important thing to think about this upper bound, in this traditional, if you are able to get this type of productivity from your land, and you're able to, in theory, support a hundred people per square kilometer, that means if all of a sudden you have 200 people living, there may be everyone's migrated to this land, because it seems especially fertile, or all the really smart farmers live there, then all of a sudden, not everyone's going to be able to get 2,000 calories a day. Some people might get malnourished."}, {"video_title": "Land productivity limiting human population Cosmology & Astronomy Khan Academy.mp3", "Sentence": "One, not all the land you might be able to farm from. Or there are other limits on the population for whatever they might be. But the important thing to think about this upper bound, in this traditional, if you are able to get this type of productivity from your land, and you're able to, in theory, support a hundred people per square kilometer, that means if all of a sudden you have 200 people living, there may be everyone's migrated to this land, because it seems especially fertile, or all the really smart farmers live there, then all of a sudden, not everyone's going to be able to get 2,000 calories a day. Some people might get malnourished. Other people might actually starve. There's this upper bound on the actual number of people that can be there, based on how productive the land actually is. And now let's move over to modern agriculture."}, {"video_title": "Land productivity limiting human population Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Some people might get malnourished. Other people might actually starve. There's this upper bound on the actual number of people that can be there, based on how productive the land actually is. And now let's move over to modern agriculture. And we've already talked a little bit about what exactly is modern agriculture. You have machines like this combine over here that does a lot of the human labor. One human can, and I'll talk about the different dimensions because there's actually two dimensions here."}, {"video_title": "Land productivity limiting human population Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And now let's move over to modern agriculture. And we've already talked a little bit about what exactly is modern agriculture. You have machines like this combine over here that does a lot of the human labor. One human can, and I'll talk about the different dimensions because there's actually two dimensions here. How much calories can you get from the land? And how much energy can one human, how much labor can one human input into the land using tools at their disposal? So in this case, cattle, you need this ox pulling this plow, or in this case, this combine that's fueled by fossil fuels."}, {"video_title": "Land productivity limiting human population Cosmology & Astronomy Khan Academy.mp3", "Sentence": "One human can, and I'll talk about the different dimensions because there's actually two dimensions here. How much calories can you get from the land? And how much energy can one human, how much labor can one human input into the land using tools at their disposal? So in this case, cattle, you need this ox pulling this plow, or in this case, this combine that's fueled by fossil fuels. But in modern agriculture, because of all of the things, you have these amazing tools, you have genetically engineered crops, you have modern pesticides, and not everyone is a fan of all of these things, but they have hugely increased our productivity. So you have modern agriculture. And let's say that you get another factor of 10 from traditional agriculture."}, {"video_title": "Land productivity limiting human population Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So in this case, cattle, you need this ox pulling this plow, or in this case, this combine that's fueled by fossil fuels. But in modern agriculture, because of all of the things, you have these amazing tools, you have genetically engineered crops, you have modern pesticides, and not everyone is a fan of all of these things, but they have hugely increased our productivity. So you have modern agriculture. And let's say that you get another factor of 10 from traditional agriculture. So now you can get 2 million calories per square kilometer per day, or you can support 1,000 humans per square kilometers. And once again, this right over here, this right over here is an upper bound. And just to give a sense, and I picked these numbers just so that the numbers would be clean."}, {"video_title": "Land productivity limiting human population Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And let's say that you get another factor of 10 from traditional agriculture. So now you can get 2 million calories per square kilometer per day, or you can support 1,000 humans per square kilometers. And once again, this right over here, this right over here is an upper bound. And just to give a sense, and I picked these numbers just so that the numbers would be clean. I looked at some historical records. These aren't completely out of line with what it looks like humans have been able to do in the past. But to give you a sense of what human population densities look like right now and why this upper bound seems to be right about correct, in a place like the United States, so in the United States, the population density is 30 people per square kilometer."}, {"video_title": "Land productivity limiting human population Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And just to give a sense, and I picked these numbers just so that the numbers would be clean. I looked at some historical records. These aren't completely out of line with what it looks like humans have been able to do in the past. But to give you a sense of what human population densities look like right now and why this upper bound seems to be right about correct, in a place like the United States, so in the United States, the population density is 30 people per square kilometer. So this is 30 humans per square kilometer. In a more dense country or significantly more dense country like India, the population density is 300 humans per square kilometer. And in the most population dense country in the world, which is where I come from, or actually I was born in New Orleans, but where some of my ancestors came from, which is Bangladesh."}, {"video_title": "Land productivity limiting human population Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But to give you a sense of what human population densities look like right now and why this upper bound seems to be right about correct, in a place like the United States, so in the United States, the population density is 30 people per square kilometer. So this is 30 humans per square kilometer. In a more dense country or significantly more dense country like India, the population density is 300 humans per square kilometer. And in the most population dense country in the world, which is where I come from, or actually I was born in New Orleans, but where some of my ancestors came from, which is Bangladesh. So there's a lot of people like me, I guess. In Bangladesh, you have a population density of 900 humans per square kilometer. To some degree, this is a testament to the fertility of the land and whatever else."}, {"video_title": "Land productivity limiting human population Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And in the most population dense country in the world, which is where I come from, or actually I was born in New Orleans, but where some of my ancestors came from, which is Bangladesh. So there's a lot of people like me, I guess. In Bangladesh, you have a population density of 900 humans per square kilometer. To some degree, this is a testament to the fertility of the land and whatever else. But this is pretty near the limits, depending on agricultural productivity and whatnot in the land, of modern technology. So it really makes you think if you don't get population under control, you might end up with some of these kind of hitting the wall type of scenarios. And so the last thing I want you to think about, and this is what I refer to a little bit more, is just think about those two dimensions, because sometimes they get a little bit muddled."}, {"video_title": "Land productivity limiting human population Cosmology & Astronomy Khan Academy.mp3", "Sentence": "To some degree, this is a testament to the fertility of the land and whatever else. But this is pretty near the limits, depending on agricultural productivity and whatnot in the land, of modern technology. So it really makes you think if you don't get population under control, you might end up with some of these kind of hitting the wall type of scenarios. And so the last thing I want you to think about, and this is what I refer to a little bit more, is just think about those two dimensions, because sometimes they get a little bit muddled. One is the kind of the productivity of land, productivity of land, and then the other is the productivity of labor, productivity of labor. So right over here in a hunter-gatherer, they're not getting money calories from their land, so they're right over there. And the humans have to do all the labor."}, {"video_title": "Land productivity limiting human population Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And so the last thing I want you to think about, and this is what I refer to a little bit more, is just think about those two dimensions, because sometimes they get a little bit muddled. One is the kind of the productivity of land, productivity of land, and then the other is the productivity of labor, productivity of labor. So right over here in a hunter-gatherer, they're not getting money calories from their land, so they're right over there. And the humans have to do all the labor. They don't have animals helping them in any way. They definitely don't have robots or any type of engines helping them in any way. And so they have to spend a lot of human time and a lot of human labor doing the work, getting that productivity from the land."}, {"video_title": "Land productivity limiting human population Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And the humans have to do all the labor. They don't have animals helping them in any way. They definitely don't have robots or any type of engines helping them in any way. And so they have to spend a lot of human time and a lot of human labor doing the work, getting that productivity from the land. But as we progress, so as we progress with things that aid humans, so for example, if all of a sudden you have cattle helping you or you have other tools that help you, you get more human productivity. So less and less of human labor has to be used to get that productivity of the land, and so maybe other humans can go do other things like paint pictures or become blacksmiths or whatever. And in this direction, you get higher productivity per unit of land."}, {"video_title": "Land productivity limiting human population Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And so they have to spend a lot of human time and a lot of human labor doing the work, getting that productivity from the land. But as we progress, so as we progress with things that aid humans, so for example, if all of a sudden you have cattle helping you or you have other tools that help you, you get more human productivity. So less and less of human labor has to be used to get that productivity of the land, and so maybe other humans can go do other things like paint pictures or become blacksmiths or whatever. And in this direction, you get higher productivity per unit of land. And so that comes from moving from hunter-gatherer to a pastoral lifestyle, to traditional farming with irrigation, to modern farming. And so on this graph right over here, kind of tools for the individuals move us up, getting more productivity of the land move us to the right. Modern agriculture gets us right over here."}, {"video_title": "Land productivity limiting human population Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And in this direction, you get higher productivity per unit of land. And so that comes from moving from hunter-gatherer to a pastoral lifestyle, to traditional farming with irrigation, to modern farming. And so on this graph right over here, kind of tools for the individuals move us up, getting more productivity of the land move us to the right. Modern agriculture gets us right over here. So we're getting much, much more calories per unit land, and we're getting much, much more calories per unit labor. So you need a much smaller percentage of the human population actually involved in the farming. Anyway, I'll let you go there."}, {"video_title": "Scale of distance to closest stars Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And the reason why, at least in my mind, it's kind of mind-blowing is because at this scale, the sun is obviously still a huge object at this scale, the Earth would be roughly, and this is an approximation, roughly that big. And so for me at least, this is mind-blowing because it's this idea that our whole planet, everything could fit into one of these plasma flares coming off of the sun. And you can only imagine, we can't realistically be there, but if you were in some type of protected capsule, what it would be like to be in this type of an environment. So I just thought this was kind of a fascinating concept. But anyway, with that out of the way, let's just think about what it means to be at the boundary of the solar system. In the last video, we explored the Oort Belt, which was about, it started a little under one light year away from the sun. But depending on what you view as the boundary of the solar system, it could be something way farther in or it could be something as far out as something like the Oort Cloud."}, {"video_title": "Scale of distance to closest stars Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So I just thought this was kind of a fascinating concept. But anyway, with that out of the way, let's just think about what it means to be at the boundary of the solar system. In the last video, we explored the Oort Belt, which was about, it started a little under one light year away from the sun. But depending on what you view as the boundary of the solar system, it could be something way farther in or it could be something as far out as something like the Oort Cloud. So if the sun, we see these things being ejected, but even in unseen ways, or unseen particles, super high energy electrons and protons are also being ejected from the sun. It's super high velocities, 400 kilometers per second. Let me write that down."}, {"video_title": "Scale of distance to closest stars Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But depending on what you view as the boundary of the solar system, it could be something way farther in or it could be something as far out as something like the Oort Cloud. So if the sun, we see these things being ejected, but even in unseen ways, or unseen particles, super high energy electrons and protons are also being ejected from the sun. It's super high velocities, 400 kilometers per second. Let me write that down. 400 kilometers per second. And on Earth, we're protected from these highly energetic particles because of Earth's magnetic field. But if you're on the surface of the moon, when the sun is on top, and you're not on the dark side of the moon, you'll have direct contact with these."}, {"video_title": "Scale of distance to closest stars Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Let me write that down. 400 kilometers per second. And on Earth, we're protected from these highly energetic particles because of Earth's magnetic field. But if you're on the surface of the moon, when the sun is on top, and you're not on the dark side of the moon, you'll have direct contact with these. And as you can imagine, not the best thing to hang around in too long. But the whole reason why I'm even talking about these, these charged particles that are coming out at huge velocities from the surface of the sun, these are considered the solar wind. These are the solar wind."}, {"video_title": "Scale of distance to closest stars Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But if you're on the surface of the moon, when the sun is on top, and you're not on the dark side of the moon, you'll have direct contact with these. And as you can imagine, not the best thing to hang around in too long. But the whole reason why I'm even talking about these, these charged particles that are coming out at huge velocities from the surface of the sun, these are considered the solar wind. These are the solar wind. And I'll put wind in quotes because it's really very different than our traditional association of a nice breeze. These are just charged particles that are going out at super high velocities from the sun. I'm even going into the idea of the solar wind because to some degree, they can help us with one definition of maybe the limits of the solar system."}, {"video_title": "Scale of distance to closest stars Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "These are the solar wind. And I'll put wind in quotes because it's really very different than our traditional association of a nice breeze. These are just charged particles that are going out at super high velocities from the sun. I'm even going into the idea of the solar wind because to some degree, they can help us with one definition of maybe the limits of the solar system. And that's the limits of how far the solar wind is getting before it kind of comes in confrontation with the interstellar medium. This right here shows a depiction of that. So the Oort cloud, at least the edges of the dense part of it, is way outside of this."}, {"video_title": "Scale of distance to closest stars Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "I'm even going into the idea of the solar wind because to some degree, they can help us with one definition of maybe the limits of the solar system. And that's the limits of how far the solar wind is getting before it kind of comes in confrontation with the interstellar medium. This right here shows a depiction of that. So the Oort cloud, at least the edges of the dense part of it, is way outside of this. As we saw, this is just where Voyager 1, Voyager 2, if we wanted the orbit of Sedna, it would be something like, the close part would be something over here and then it would go out. But the Oort cloud is much, much further out. So if you look at this kind of view of the solar system as the extent of the solar wind, it's much smaller than the Oort cloud, but it's still fairly large."}, {"video_title": "Scale of distance to closest stars Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So the Oort cloud, at least the edges of the dense part of it, is way outside of this. As we saw, this is just where Voyager 1, Voyager 2, if we wanted the orbit of Sedna, it would be something like, the close part would be something over here and then it would go out. But the Oort cloud is much, much further out. So if you look at this kind of view of the solar system as the extent of the solar wind, it's much smaller than the Oort cloud, but it's still fairly large. So this is right here, this heliopause right here, and I got this from Wikipedia. This is essentially where the velocity and the forces of the solar wind are counteracted. The pressure is so diluted at this point that it's counteracted by mainly the hydrogen and the helium that's in the interstellar medium that's just kind of out there."}, {"video_title": "Scale of distance to closest stars Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So if you look at this kind of view of the solar system as the extent of the solar wind, it's much smaller than the Oort cloud, but it's still fairly large. So this is right here, this heliopause right here, and I got this from Wikipedia. This is essentially where the velocity and the forces of the solar wind are counteracted. The pressure is so diluted at this point that it's counteracted by mainly the hydrogen and the helium that's in the interstellar medium that's just kind of out there. After this point, it's not really being injected out anymore. There's this kind of pause, I guess you could say. Voyager 1 and Voyager 2 have essentially gotten pretty close to, people believe, that pause over there."}, {"video_title": "Scale of distance to closest stars Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "The pressure is so diluted at this point that it's counteracted by mainly the hydrogen and the helium that's in the interstellar medium that's just kind of out there. After this point, it's not really being injected out anymore. There's this kind of pause, I guess you could say. Voyager 1 and Voyager 2 have essentially gotten pretty close to, people believe, that pause over there. So that's one view of the edges of the solar system. There's never going to be any hard edge to it. Another view would be something like the Oort cloud, the area where you have the still objects out there."}, {"video_title": "Scale of distance to closest stars Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Voyager 1 and Voyager 2 have essentially gotten pretty close to, people believe, that pause over there. So that's one view of the edges of the solar system. There's never going to be any hard edge to it. Another view would be something like the Oort cloud, the area where you have the still objects out there. This is all actually, we haven't directly observed objects in the Oort cloud. We think that they are out there. Then maybe the most abstract definition would be a significant influence from the sun's gravitational pull."}, {"video_title": "Scale of distance to closest stars Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Another view would be something like the Oort cloud, the area where you have the still objects out there. This is all actually, we haven't directly observed objects in the Oort cloud. We think that they are out there. Then maybe the most abstract definition would be a significant influence from the sun's gravitational pull. All of those ways are to imagine the extent of the solar system, but they all kind of leave a gray area for what is and what is not in the solar system. My whole point here, what I want to do is start exploring a little bit outside of the solar system and just give you a sense of the scale as we just go to the closest star. If we go right over here, this shows our local neighborhood from a stellar point of view."}, {"video_title": "Scale of distance to closest stars Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Then maybe the most abstract definition would be a significant influence from the sun's gravitational pull. All of those ways are to imagine the extent of the solar system, but they all kind of leave a gray area for what is and what is not in the solar system. My whole point here, what I want to do is start exploring a little bit outside of the solar system and just give you a sense of the scale as we just go to the closest star. If we go right over here, this shows our local neighborhood from a stellar point of view. Even though these stars look pretty big, if you actually were to draw, this is our solar system right here. You might be saying, oh, maybe that's the sun. No, the sun, if you were to draw it here, it wouldn't even make up one pixel."}, {"video_title": "Scale of distance to closest stars Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "If we go right over here, this shows our local neighborhood from a stellar point of view. Even though these stars look pretty big, if you actually were to draw, this is our solar system right here. You might be saying, oh, maybe that's the sun. No, the sun, if you were to draw it here, it wouldn't even make up one pixel. In fact, the entire orbit of Pluto, everything inside of it, still would not make up one pixel on the screen right here. What we see right here, which is a radius of about, give or take, a light year, this is roughly maybe the radius of the Oort cloud. We saw in the last video how huge that was, especially relative to the radius of, say, Pluto's orbit, which is roughly like that."}, {"video_title": "Scale of distance to closest stars Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "No, the sun, if you were to draw it here, it wouldn't even make up one pixel. In fact, the entire orbit of Pluto, everything inside of it, still would not make up one pixel on the screen right here. What we see right here, which is a radius of about, give or take, a light year, this is roughly maybe the radius of the Oort cloud. We saw in the last video how huge that was, especially relative to the radius of, say, Pluto's orbit, which is roughly like that. That itself is a huge, huge diameter or a huge distance away from the sun. That wouldn't even make a pixel on this diagram right over here. Just to give you an idea of how far we are, we're a speck of a speck of a speck inside here, of a pixel of a pixel in the center here."}, {"video_title": "Scale of distance to closest stars Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "We saw in the last video how huge that was, especially relative to the radius of, say, Pluto's orbit, which is roughly like that. That itself is a huge, huge diameter or a huge distance away from the sun. That wouldn't even make a pixel on this diagram right over here. Just to give you an idea of how far we are, we're a speck of a speck of a speck inside here, of a pixel of a pixel in the center here. We're in the center here to make it from our solar system, or in particular from Earth maybe, to the nearest star, or maybe the nearest cluster of stars, the Alpha Centauri. They're the nearest cluster of stars. There's three stars, Alpha Centauri A, which is the largest, Alpha Centauri B, and then there's one that you can't observe with the naked eye, Alpha Proximus, or I think it's Proximus Centauri, I think is what it's called, not Alpha Proximus."}, {"video_title": "Scale of distance to closest stars Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Just to give you an idea of how far we are, we're a speck of a speck of a speck inside here, of a pixel of a pixel in the center here. We're in the center here to make it from our solar system, or in particular from Earth maybe, to the nearest star, or maybe the nearest cluster of stars, the Alpha Centauri. They're the nearest cluster of stars. There's three stars, Alpha Centauri A, which is the largest, Alpha Centauri B, and then there's one that you can't observe with the naked eye, Alpha Proximus, or I think it's Proximus Centauri, I think is what it's called, not Alpha Proximus. Proximus Centauri. That's a much smaller star, but that's the closest star. You can view it as this whole cluster of stars right here."}, {"video_title": "Scale of distance to closest stars Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "There's three stars, Alpha Centauri A, which is the largest, Alpha Centauri B, and then there's one that you can't observe with the naked eye, Alpha Proximus, or I think it's Proximus Centauri, I think is what it's called, not Alpha Proximus. Proximus Centauri. That's a much smaller star, but that's the closest star. You can view it as this whole cluster of stars right here. They're the closest. It's about 4.2 light years away. Or another way to think about it, if someone were to shine a light on one of these planets, and assuming that light could get to us, it would take 4.2 years to get to us."}, {"video_title": "Scale of distance to closest stars Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "You can view it as this whole cluster of stars right here. They're the closest. It's about 4.2 light years away. Or another way to think about it, if someone were to shine a light on one of these planets, and assuming that light could get to us, it would take 4.2 years to get to us. If these guys just disappeared or blew up, we wouldn't know it for 4.2 years. We'd say, hey, that's not too bad. We should take a trip over there and check them out, see if there are any other people there that we can meet and exchange technologies with or whatnot."}, {"video_title": "Scale of distance to closest stars Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Or another way to think about it, if someone were to shine a light on one of these planets, and assuming that light could get to us, it would take 4.2 years to get to us. If these guys just disappeared or blew up, we wouldn't know it for 4.2 years. We'd say, hey, that's not too bad. We should take a trip over there and check them out, see if there are any other people there that we can meet and exchange technologies with or whatnot. But this is a huge distance. Just this 4.2 light years is an unbelievably ridiculous distance. Just to give you a sense, the Voyager 1 and 2, we talked about in the last video, and we can even see how far they've gotten."}, {"video_title": "Scale of distance to closest stars Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "We should take a trip over there and check them out, see if there are any other people there that we can meet and exchange technologies with or whatnot. But this is a huge distance. Just this 4.2 light years is an unbelievably ridiculous distance. Just to give you a sense, the Voyager 1 and 2, we talked about in the last video, and we can even see how far they've gotten. They've gotten pretty much to the heliopause. These guys are traveling at 60,000 kilometers an hour, which is the same thing as 17 kilometers per second. If we were able to get up to those type of velocities, and these guys got up to those type of velocities by leveraging the gravitational pull of some of the larger planets to accelerate and keep accelerating."}, {"video_title": "Scale of distance to closest stars Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Just to give you a sense, the Voyager 1 and 2, we talked about in the last video, and we can even see how far they've gotten. They've gotten pretty much to the heliopause. These guys are traveling at 60,000 kilometers an hour, which is the same thing as 17 kilometers per second. If we were able to get up to those type of velocities, and these guys got up to those type of velocities by leveraging the gravitational pull of some of the larger planets to accelerate and keep accelerating. This is a pretty hard velocity to actually reach. But if you were able to reach that velocity and go straight in the direction of the Alpha Centauri system, the closest stars to Earth, it would take you 80,000 years traveling at the same velocity as Voyager 1, which is the fastest of the Voyagers. It's a ridiculously long time."}, {"video_title": "Why gravity gets so strong near dense objects Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "Obviously, the sun's gravity isn't so strong that it keeps light from escaping. So why would something, or even a star that's two or three solar masses, its gravity isn't so strong that it keeps light from escaping. Why would a black hole that has the same mass, why would that keep light from escaping? And to understand that, let's just think a little bit about, and I'll just do Newtonian classical physics right here. I won't get into the whole general relativity of things. And this really will just give us the intuition of why a smaller, denser thing of the same mass can exert a stronger gravitational pull. So let's imagine, so let's take two examples."}, {"video_title": "Why gravity gets so strong near dense objects Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "And to understand that, let's just think a little bit about, and I'll just do Newtonian classical physics right here. I won't get into the whole general relativity of things. And this really will just give us the intuition of why a smaller, denser thing of the same mass can exert a stronger gravitational pull. So let's imagine, so let's take two examples. Let's say I have some star here. Let's just call that mass m1. And let's say that its radius, let's just call this r. And let's say that I have some other mass right at the surface of this star."}, {"video_title": "Why gravity gets so strong near dense objects Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "So let's imagine, so let's take two examples. Let's say I have some star here. Let's just call that mass m1. And let's say that its radius, let's just call this r. And let's say that I have some other mass right at the surface of this star. It's somehow able to survive those surface temperatures. And this mass over here has a mass of m1. This mass over here has a mass of m2."}, {"video_title": "Why gravity gets so strong near dense objects Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "And let's say that its radius, let's just call this r. And let's say that I have some other mass right at the surface of this star. It's somehow able to survive those surface temperatures. And this mass over here has a mass of m1. This mass over here has a mass of m2. The universal law of gravitation tells us that the force between these two masses is going to be equal to the gravitational constant times the product of the masses. So m1 times m2, all of that over the square of the distance. Now let me be very clear."}, {"video_title": "Why gravity gets so strong near dense objects Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "This mass over here has a mass of m2. The universal law of gravitation tells us that the force between these two masses is going to be equal to the gravitational constant times the product of the masses. So m1 times m2, all of that over the square of the distance. Now let me be very clear. You might say, wait, this magenta mass right here is touching this larger mass. Isn't the distance 0? And you have to be very careful."}, {"video_title": "Why gravity gets so strong near dense objects Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "Now let me be very clear. You might say, wait, this magenta mass right here is touching this larger mass. Isn't the distance 0? And you have to be very careful. This is the distance between their center of masses. So the center of mass of this large mass over here is r away from this mass that's on the surface. Now with that said, let's take another example."}, {"video_title": "Why gravity gets so strong near dense objects Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "And you have to be very careful. This is the distance between their center of masses. So the center of mass of this large mass over here is r away from this mass that's on the surface. Now with that said, let's take another example. Let's say that this large, massive star, or whatever it might be, eventually condenses into something 1,000 times smaller. So let me draw it like this. And obviously I'm not drawing it to scale."}, {"video_title": "Why gravity gets so strong near dense objects Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "Now with that said, let's take another example. Let's say that this large, massive star, or whatever it might be, eventually condenses into something 1,000 times smaller. So let me draw it like this. And obviously I'm not drawing it to scale. So let's say we have another case like this. And I'm not drawing it to scale. So this object, maybe it's the same object, or maybe it's a different object, that has the exact same mass as this larger object, but now it has a much smaller radius."}, {"video_title": "Why gravity gets so strong near dense objects Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "And obviously I'm not drawing it to scale. So let's say we have another case like this. And I'm not drawing it to scale. So this object, maybe it's the same object, or maybe it's a different object, that has the exact same mass as this larger object, but now it has a much smaller radius. It now has a much smaller radius. So that radius now, the radius is 1 over, let's just say it's 1,000th of this radius over here. So it's 1, maybe I'll just call it r over 1,000."}, {"video_title": "Why gravity gets so strong near dense objects Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "So this object, maybe it's the same object, or maybe it's a different object, that has the exact same mass as this larger object, but now it has a much smaller radius. It now has a much smaller radius. So that radius now, the radius is 1 over, let's just say it's 1,000th of this radius over here. So it's 1, maybe I'll just call it r over 1,000. So if this had a million kilometer radius, so that would make it roughly about twice the radius of the sun. If this was a million kilometer radius right over here, this would be 1,000 kilometer radius. So maybe we're talking about something that's approaching a neutron star."}, {"video_title": "Why gravity gets so strong near dense objects Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "So it's 1, maybe I'll just call it r over 1,000. So if this had a million kilometer radius, so that would make it roughly about twice the radius of the sun. If this was a million kilometer radius right over here, this would be 1,000 kilometer radius. So maybe we're talking about something that's approaching a neutron star. But we don't have to think about what it actually is. Let's just think about the thought experiment here. So let's say I have this thing over here."}, {"video_title": "Why gravity gets so strong near dense objects Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "So maybe we're talking about something that's approaching a neutron star. But we don't have to think about what it actually is. Let's just think about the thought experiment here. So let's say I have this thing over here. And let's say I have something on the surface of this. So let's say I have that same mass, it's on the surface of this thing. So this is m2 right over here."}, {"video_title": "Why gravity gets so strong near dense objects Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "So let's say I have this thing over here. And let's say I have something on the surface of this. So let's say I have that same mass, it's on the surface of this thing. So this is m2 right over here. So what is going to be the force between these two masses? How strong are they going to want to, what's the force pulling them together? So let's just do the universal law of gravitation again."}, {"video_title": "Why gravity gets so strong near dense objects Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "So this is m2 right over here. So what is going to be the force between these two masses? How strong are they going to want to, what's the force pulling them together? So let's just do the universal law of gravitation again. The force, let's just call this force 1, and let's call this force 2. Once again, it's going to be the gravitational constant times the product of their masses. So the big m1 times the smaller mass, m2, all of that over this distance squared, this radius squared."}, {"video_title": "Why gravity gets so strong near dense objects Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "So let's just do the universal law of gravitation again. The force, let's just call this force 1, and let's call this force 2. Once again, it's going to be the gravitational constant times the product of their masses. So the big m1 times the smaller mass, m2, all of that over this distance squared, this radius squared. Remember, it's the distance to the center of masses. This center of mass here, we're considering m2 to kind of be just a point mass right over there. So what's the radius squared?"}, {"video_title": "Why gravity gets so strong near dense objects Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "So the big m1 times the smaller mass, m2, all of that over this distance squared, this radius squared. Remember, it's the distance to the center of masses. This center of mass here, we're considering m2 to kind of be just a point mass right over there. So what's the radius squared? It's going to be r over 1,000 squared. Or if we simplify this, what will this be? This is the same thing, and I'll just write it in one color just because it takes less time."}, {"video_title": "Why gravity gets so strong near dense objects Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "So what's the radius squared? It's going to be r over 1,000 squared. Or if we simplify this, what will this be? This is the same thing, and I'll just write it in one color just because it takes less time. Gravitational constant m1, m2 over r squared over 1,000 squared, or over 1 million. Over 1 million. That's just 1,000 squared."}, {"video_title": "Why gravity gets so strong near dense objects Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "This is the same thing, and I'll just write it in one color just because it takes less time. Gravitational constant m1, m2 over r squared over 1,000 squared, or over 1 million. Over 1 million. That's just 1,000 squared. Or we can multiply the numerator and the denominator by a million, and this is going to be equal to 1 million, I'm going to write it out, 1 million, let me scroll to the right a little bit, times the gravitational constant times m1, m2, all of that over r squared. Now what is this thing right over here? That's the same thing as this F1."}, {"video_title": "Why gravity gets so strong near dense objects Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "That's just 1,000 squared. Or we can multiply the numerator and the denominator by a million, and this is going to be equal to 1 million, I'm going to write it out, 1 million, let me scroll to the right a little bit, times the gravitational constant times m1, m2, all of that over r squared. Now what is this thing right over here? That's the same thing as this F1. So this is going to be 1 million times F1. So even though the masses involved are the same, this yellow object right here is the same mass as this larger object over here. It's able to exert a million times the gravitational force on this point mass, and actually vice versa."}, {"video_title": "Why gravity gets so strong near dense objects Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "That's the same thing as this F1. So this is going to be 1 million times F1. So even though the masses involved are the same, this yellow object right here is the same mass as this larger object over here. It's able to exert a million times the gravitational force on this point mass, and actually vice versa. They're both being attracted. They're both exerting this on each other. And the reality is, is because this thing is smaller, because this m1 on the right here, this one I'm coloring in, because this one is smaller and denser, this particle is able to get closer to its center of mass."}, {"video_title": "Why gravity gets so strong near dense objects Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "It's able to exert a million times the gravitational force on this point mass, and actually vice versa. They're both being attracted. They're both exerting this on each other. And the reality is, is because this thing is smaller, because this m1 on the right here, this one I'm coloring in, because this one is smaller and denser, this particle is able to get closer to its center of mass. Now you might be saying, OK, well I can buy that. This just comes straight from the universal law of gravitation. But wouldn't something closer to this center of mass experience that same thing?"}, {"video_title": "Why gravity gets so strong near dense objects Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "And the reality is, is because this thing is smaller, because this m1 on the right here, this one I'm coloring in, because this one is smaller and denser, this particle is able to get closer to its center of mass. Now you might be saying, OK, well I can buy that. This just comes straight from the universal law of gravitation. But wouldn't something closer to this center of mass experience that same thing? If this was a star, wouldn't photons that are over here, wouldn't this experience the same force? If this distance right here is r over 1,000, wouldn't some photon here, or atom here, or molecule, or whatever it's over here, wouldn't that experience the same force, this million times the force as this thing? And you've got to remember, all of a sudden when this thing is inside of this larger mass, what's happening?"}, {"video_title": "Why gravity gets so strong near dense objects Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "But wouldn't something closer to this center of mass experience that same thing? If this was a star, wouldn't photons that are over here, wouldn't this experience the same force? If this distance right here is r over 1,000, wouldn't some photon here, or atom here, or molecule, or whatever it's over here, wouldn't that experience the same force, this million times the force as this thing? And you've got to remember, all of a sudden when this thing is inside of this larger mass, what's happening? It no longer has the entire mass is no longer pulling on it in that direction. It's no longer pulling it in that inward direction. You now have all of this mass over here."}, {"video_title": "Why gravity gets so strong near dense objects Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "And you've got to remember, all of a sudden when this thing is inside of this larger mass, what's happening? It no longer has the entire mass is no longer pulling on it in that direction. It's no longer pulling it in that inward direction. You now have all of this mass over here. Let me think of the best way of doing it. So you could think of it all of this mass over here is pulling it in an outward direction. It's not telling you what that mass out there is doing, since that mass itself is being pulled inward."}, {"video_title": "Why gravity gets so strong near dense objects Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "You now have all of this mass over here. Let me think of the best way of doing it. So you could think of it all of this mass over here is pulling it in an outward direction. It's not telling you what that mass out there is doing, since that mass itself is being pulled inward. It is pushing down on this. It is exerting pressure on that point. But the actual gravitational force that that point is experiencing is actually going to be less."}, {"video_title": "Why gravity gets so strong near dense objects Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "It's not telling you what that mass out there is doing, since that mass itself is being pulled inward. It is pushing down on this. It is exerting pressure on that point. But the actual gravitational force that that point is experiencing is actually going to be less. It's actually going to be mitigated by the fact that there's so much mass over here pulling in the other direction. And so you can imagine if you were in the center of a really massive object, there would be no net gravitational force being pulled on you, because you're at its center of mass. The rest of the mass is outward."}, {"video_title": "Why gravity gets so strong near dense objects Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "But the actual gravitational force that that point is experiencing is actually going to be less. It's actually going to be mitigated by the fact that there's so much mass over here pulling in the other direction. And so you can imagine if you were in the center of a really massive object, there would be no net gravitational force being pulled on you, because you're at its center of mass. The rest of the mass is outward. So at every point, it will be pulling you outward. And so that's why if you were to enter the core of a star, if you were to get a lot closer to its center of mass, it's not going to be pulling on you with this type of force. And the only way you can get these type of forces is if the entire mass is contained in a very dense region, in a very small region."}, {"video_title": "Carbon 14 dating 2 Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "In the last video, we talked about the idea that if I dug up a bone someplace, and if I were to measure its carbon-14, and I found that it had half of the carbon-14 that I would expect to find in a living animal or plant, then I said, hey, maybe one half-life has gone by. Or roughly, for carbon-14, one half-life is 5,730 years. So I said, maybe it's 5,730 years since this bone was part of a living animal, or it's roughly that old. Now, when I did that, I made a pretty big assumption. And some of you all have touched on this in the comments on YouTube on the last video. It's how do I know that this estimate I made is based on the assumption that the amount of carbon-14 in the atmosphere would have been roughly constant from when this bone was living to now. And so the question is, is the amount of carbon-14 in the atmosphere and in the water and in living plants and animals, is it constant?"}, {"video_title": "Carbon 14 dating 2 Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Now, when I did that, I made a pretty big assumption. And some of you all have touched on this in the comments on YouTube on the last video. It's how do I know that this estimate I made is based on the assumption that the amount of carbon-14 in the atmosphere would have been roughly constant from when this bone was living to now. And so the question is, is the amount of carbon-14 in the atmosphere and in the water and in living plants and animals, is it constant? And if it isn't constant, how do you calibrate your measurement so you can actually figure out how much carbon-14 there is relative to living plants and animals at that time? And the way that you can make that calibration, because it turns out it isn't perfectly constant, the way that you can make that calibration, there's two ways. And I have pictures here of both of them."}, {"video_title": "Carbon 14 dating 2 Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And so the question is, is the amount of carbon-14 in the atmosphere and in the water and in living plants and animals, is it constant? And if it isn't constant, how do you calibrate your measurement so you can actually figure out how much carbon-14 there is relative to living plants and animals at that time? And the way that you can make that calibration, because it turns out it isn't perfectly constant, the way that you can make that calibration, there's two ways. And I have pictures here of both of them. One is to look at tree rings. And I'm told that this will work up to about 10,000 years old. I don't know of any 10,000-year-old trees."}, {"video_title": "Carbon 14 dating 2 Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And I have pictures here of both of them. One is to look at tree rings. And I'm told that this will work up to about 10,000 years old. I don't know of any 10,000-year-old trees. I don't think anyone does. But maybe there are some remains of old trees. And you can look at their tree rings."}, {"video_title": "Carbon 14 dating 2 Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "I don't know of any 10,000-year-old trees. I don't think anyone does. But maybe there are some remains of old trees. And you can look at their tree rings. And I think most of us are familiar with this idea that every year that a tree grows, it forms another layer of bark. And so you can look back to that layer of bark, adjust for the half-life of carbon-14, and then figure out how much carbon-14 was there in the atmosphere at that period in time. And so it's kind of a record of the atmosphere up to 10,000 years."}, {"video_title": "Carbon 14 dating 2 Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And you can look at their tree rings. And I think most of us are familiar with this idea that every year that a tree grows, it forms another layer of bark. And so you can look back to that layer of bark, adjust for the half-life of carbon-14, and then figure out how much carbon-14 was there in the atmosphere at that period in time. And so it's kind of a record of the atmosphere up to 10,000 years. If you want to go even further back, you can look at cave deposits. And the fancy word for these cave deposits are speleothems. You might be familiar with stalagmites."}, {"video_title": "Carbon 14 dating 2 Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And so it's kind of a record of the atmosphere up to 10,000 years. If you want to go even further back, you can look at cave deposits. And the fancy word for these cave deposits are speleothems. You might be familiar with stalagmites. Those are those speleothems that are kind of coming out of the bottom of the cave. Or stalactites, those are the speleothems that are coming from the top of the cave. But the reason why these are useful is these are formed by calcium carbonate."}, {"video_title": "Carbon 14 dating 2 Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "You might be familiar with stalagmites. Those are those speleothems that are kind of coming out of the bottom of the cave. Or stalactites, those are the speleothems that are coming from the top of the cave. But the reason why these are useful is these are formed by calcium carbonate. So they have carbon in them. And slowly, over really tens of thousands of years, the water in the cave deposits that calcium carbonate. So it's a record of the fraction of carbon-14 in some of those years."}, {"video_title": "Carbon 14 dating 2 Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But the reason why these are useful is these are formed by calcium carbonate. So they have carbon in them. And slowly, over really tens of thousands of years, the water in the cave deposits that calcium carbonate. So it's a record of the fraction of carbon-14 in some of those years. And you can go down to resolutions of as small as 10 years. And so this will give us pretty good estimates over tens of thousands of years, up to 50,000 years. And frankly, carbon-14 isn't even useful beyond really 50,000 or 60,000 years."}, {"video_title": "Carbon 14 dating 2 Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So it's a record of the fraction of carbon-14 in some of those years. And you can go down to resolutions of as small as 10 years. And so this will give us pretty good estimates over tens of thousands of years, up to 50,000 years. And frankly, carbon-14 isn't even useful beyond really 50,000 or 60,000 years. So this gives us a pretty good record of carbon-14 in the atmosphere, assuming that it's fairly uniform throughout the atmosphere. And all evidence suggests that. And that uniformity through the atmosphere also goes into the water supply and into living plants and animals."}, {"video_title": "Carbon 14 dating 2 Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And frankly, carbon-14 isn't even useful beyond really 50,000 or 60,000 years. So this gives us a pretty good record of carbon-14 in the atmosphere, assuming that it's fairly uniform throughout the atmosphere. And all evidence suggests that. And that uniformity through the atmosphere also goes into the water supply and into living plants and animals. Now the other thing, and I looked into this a little bit, it actually turns out because we are spewing so much fossil fuel right now, we are changing the amount or the proportion of carbon-14 much, much faster than has happened in other time periods. So just to answer the question, it's actually probably in the last 50 years where the fossil fuel use has really exploded that we've really been changing the proportion of carbon-14 relative to the other isotopes of carbon. But anyway, hopefully that rests some of your worries about the assumption that I made in the last video about carbon-14 being relatively constant."}, {"video_title": "White and black dwarfs Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "In the last video, we started with a star in its main sequence, like the sun. And inside the core of that star, you had hydrogen fusion going on. So that is hydrogen fusion. And then outside of the core, you just had hydrogen. You just had hydrogen plasma. And when we say plasma, it's the electrons and protons of the individual atoms have been disassociated because the temperatures and pressures are so high. So they're really just kind of like the soup of electrons and protons, as opposed to proper atoms that we associate with at lower temperatures."}, {"video_title": "White and black dwarfs Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And then outside of the core, you just had hydrogen. You just had hydrogen plasma. And when we say plasma, it's the electrons and protons of the individual atoms have been disassociated because the temperatures and pressures are so high. So they're really just kind of like the soup of electrons and protons, as opposed to proper atoms that we associate with at lower temperatures. So this is a main sequence star right over here. This is a main sequence star right over here. And we saw in the last video that this hydrogen is fusing into helium."}, {"video_title": "White and black dwarfs Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So they're really just kind of like the soup of electrons and protons, as opposed to proper atoms that we associate with at lower temperatures. So this is a main sequence star right over here. This is a main sequence star right over here. And we saw in the last video that this hydrogen is fusing into helium. So we start having more and more helium here. And as we have more and more helium, the core becomes more and more dense because helium is a more massive atom. It is able to pack more mass in a smaller volume."}, {"video_title": "White and black dwarfs Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And we saw in the last video that this hydrogen is fusing into helium. So we start having more and more helium here. And as we have more and more helium, the core becomes more and more dense because helium is a more massive atom. It is able to pack more mass in a smaller volume. So this gets more and more dense. So core becomes more dense until at some point. And so while the core is becoming more and more dense, that actually makes the fusion happen faster and faster because it's more dense, more gravitational pressure, more mass wanting to get to it, more pressure on the hydrogen that's fusing."}, {"video_title": "White and black dwarfs Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It is able to pack more mass in a smaller volume. So this gets more and more dense. So core becomes more dense until at some point. And so while the core is becoming more and more dense, that actually makes the fusion happen faster and faster because it's more dense, more gravitational pressure, more mass wanting to get to it, more pressure on the hydrogen that's fusing. So it starts to fuse hotter. So let me write this. So the fusion."}, {"video_title": "White and black dwarfs Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And so while the core is becoming more and more dense, that actually makes the fusion happen faster and faster because it's more dense, more gravitational pressure, more mass wanting to get to it, more pressure on the hydrogen that's fusing. So it starts to fuse hotter. So let me write this. So the fusion. So hydrogen fuses faster. And actually, we even see this in our sun. Our sun today is brighter and hotter."}, {"video_title": "White and black dwarfs Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So the fusion. So hydrogen fuses faster. And actually, we even see this in our sun. Our sun today is brighter and hotter. It's fusing faster than it was when it was born 4.5 or 4.6 billion years ago. But eventually, you're going to get to the point so that the core, you only have helium. So there's going to be some point where the entire core is all helium."}, {"video_title": "White and black dwarfs Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Our sun today is brighter and hotter. It's fusing faster than it was when it was born 4.5 or 4.6 billion years ago. But eventually, you're going to get to the point so that the core, you only have helium. So there's going to be some point where the entire core is all helium. And it's going to be way denser than this core over here. All of that mass over there has now been turned into helium. Not all of it."}, {"video_title": "White and black dwarfs Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So there's going to be some point where the entire core is all helium. And it's going to be way denser than this core over here. All of that mass over there has now been turned into helium. Not all of it. A lot of it has been turned into energy. But most of it is now in helium. And it's going to be in a much, much smaller volume."}, {"video_title": "White and black dwarfs Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Not all of it. A lot of it has been turned into energy. But most of it is now in helium. And it's going to be in a much, much smaller volume. And the whole time, the temperature is increasing. The fusion is getting faster and faster. And now that it's in this dense volume of helium that's not fusing, you do have, and we saw this in this video, a shell around it of hydrogen that is fusing."}, {"video_title": "White and black dwarfs Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And it's going to be in a much, much smaller volume. And the whole time, the temperature is increasing. The fusion is getting faster and faster. And now that it's in this dense volume of helium that's not fusing, you do have, and we saw this in this video, a shell around it of hydrogen that is fusing. So this right here is hydrogen fusion going on. And then this over here is just hydrogen plasma. Now the unintuitive thing, or at least this was unintuitive to me at first, was what's going on in the core is that the core is getting more and more dense."}, {"video_title": "White and black dwarfs Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And now that it's in this dense volume of helium that's not fusing, you do have, and we saw this in this video, a shell around it of hydrogen that is fusing. So this right here is hydrogen fusion going on. And then this over here is just hydrogen plasma. Now the unintuitive thing, or at least this was unintuitive to me at first, was what's going on in the core is that the core is getting more and more dense. It's fusing at a faster rate. And so it's getting hotter and hotter. So the core is hotter, fusing faster, getting more and more dense."}, {"video_title": "White and black dwarfs Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Now the unintuitive thing, or at least this was unintuitive to me at first, was what's going on in the core is that the core is getting more and more dense. It's fusing at a faster rate. And so it's getting hotter and hotter. So the core is hotter, fusing faster, getting more and more dense. I kind of imagine it's starting to collapse. Every time it collapses, it's getting hotter and more dense. But at the same time that's happening, the star itself is getting bigger."}, {"video_title": "White and black dwarfs Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So the core is hotter, fusing faster, getting more and more dense. I kind of imagine it's starting to collapse. Every time it collapses, it's getting hotter and more dense. But at the same time that's happening, the star itself is getting bigger. The star itself is getting bigger. And this is actually not drawn to scale. Red giants are much, much larger than main sequence stars, but the whole time that this is getting more dense, the rest of the stars, you can kind of view it as getting less dense."}, {"video_title": "White and black dwarfs Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But at the same time that's happening, the star itself is getting bigger. The star itself is getting bigger. And this is actually not drawn to scale. Red giants are much, much larger than main sequence stars, but the whole time that this is getting more dense, the rest of the stars, you can kind of view it as getting less dense. And that's because this is generating so much energy that it's able to more than offset, or better offset, the gravitational pull into it. So even though this is hotter, it's able to disperse the rest of the material in the sun over a larger volume. And so that volume is so big that the surface, and we saw this in the last video, the surface of the red giant is actually cooler than the surface of a main sequence star, this right here is hotter."}, {"video_title": "White and black dwarfs Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Red giants are much, much larger than main sequence stars, but the whole time that this is getting more dense, the rest of the stars, you can kind of view it as getting less dense. And that's because this is generating so much energy that it's able to more than offset, or better offset, the gravitational pull into it. So even though this is hotter, it's able to disperse the rest of the material in the sun over a larger volume. And so that volume is so big that the surface, and we saw this in the last video, the surface of the red giant is actually cooler than the surface of a main sequence star, this right here is hotter. And just to put things in perspective, when the sun becomes a red giant, and it will become a red giant, its diameter will be 100 times the diameter that it is today. Or another way to put it, it will have the same diameter as the Earth's orbit around the current sun. Or another way to view it is, where we are right now will be on the surface, or near the surface, or maybe even inside of that future sun."}, {"video_title": "White and black dwarfs Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And so that volume is so big that the surface, and we saw this in the last video, the surface of the red giant is actually cooler than the surface of a main sequence star, this right here is hotter. And just to put things in perspective, when the sun becomes a red giant, and it will become a red giant, its diameter will be 100 times the diameter that it is today. Or another way to put it, it will have the same diameter as the Earth's orbit around the current sun. Or another way to view it is, where we are right now will be on the surface, or near the surface, or maybe even inside of that future sun. Or another way to put it, when the sun becomes a red giant, the Earth's going to be not even a speck out here, and it will be liquefied and vaporized at that point in time, so this is super, super huge. And we've even thought about it. Just for light to reach the current sun to our point in orbit, it takes 8 minutes."}, {"video_title": "White and black dwarfs Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Or another way to view it is, where we are right now will be on the surface, or near the surface, or maybe even inside of that future sun. Or another way to put it, when the sun becomes a red giant, the Earth's going to be not even a speck out here, and it will be liquefied and vaporized at that point in time, so this is super, super huge. And we've even thought about it. Just for light to reach the current sun to our point in orbit, it takes 8 minutes. So that's how big one of these stars are. To get from one side of the star to another side of the star, it'll take 16 minutes for light to travel, if it was traveling that diameter. And even slightly longer, if it was to travel in a circumference."}, {"video_title": "White and black dwarfs Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Just for light to reach the current sun to our point in orbit, it takes 8 minutes. So that's how big one of these stars are. To get from one side of the star to another side of the star, it'll take 16 minutes for light to travel, if it was traveling that diameter. And even slightly longer, if it was to travel in a circumference. So these are huge, huge, huge stars. And we'll talk about other stars in the future that are even bigger than this when they become super giants. But anyway, we have the helium in the center."}, {"video_title": "White and black dwarfs Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And even slightly longer, if it was to travel in a circumference. So these are huge, huge, huge stars. And we'll talk about other stars in the future that are even bigger than this when they become super giants. But anyway, we have the helium in the center. Let me write this down. We have a helium core in the center. We're fusing faster and faster and faster."}, {"video_title": "White and black dwarfs Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But anyway, we have the helium in the center. Let me write this down. We have a helium core in the center. We're fusing faster and faster and faster. We're now a red giant. The core is getting hotter and hotter and hotter, until it gets to the temperature for ignition of helium. So until it gets to 100 million Kelvin."}, {"video_title": "White and black dwarfs Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "We're fusing faster and faster and faster. We're now a red giant. The core is getting hotter and hotter and hotter, until it gets to the temperature for ignition of helium. So until it gets to 100 million Kelvin. Remember, the ignition temperature for hydrogen was 10 million Kelvin. So now we're at 100 million Kelvin, factor of 10. And now, all of a sudden, in the core, you actually start to have helium fusion."}, {"video_title": "White and black dwarfs Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So until it gets to 100 million Kelvin. Remember, the ignition temperature for hydrogen was 10 million Kelvin. So now we're at 100 million Kelvin, factor of 10. And now, all of a sudden, in the core, you actually start to have helium fusion. And we touched on this in the last video, but the helium is fusing into heavier elements. And some of those heavier elements, and predominantly it will be carbon and oxygen. And you may suspect, this is how heavier and heavier elements form in the universe."}, {"video_title": "White and black dwarfs Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And now, all of a sudden, in the core, you actually start to have helium fusion. And we touched on this in the last video, but the helium is fusing into heavier elements. And some of those heavier elements, and predominantly it will be carbon and oxygen. And you may suspect, this is how heavier and heavier elements form in the universe. They form literally due to fusion in the core of stars. Especially when we're talking about elements up to iron. But anyway, the core is now experiencing helium fusion."}, {"video_title": "White and black dwarfs Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And you may suspect, this is how heavier and heavier elements form in the universe. They form literally due to fusion in the core of stars. Especially when we're talking about elements up to iron. But anyway, the core is now experiencing helium fusion. It has a shell around it of helium that is not quite there. Does not quite have the pressures and temperatures to fuse yet. So just regular helium."}, {"video_title": "White and black dwarfs Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But anyway, the core is now experiencing helium fusion. It has a shell around it of helium that is not quite there. Does not quite have the pressures and temperatures to fuse yet. So just regular helium. But then outside of that, we do have the pressures and temperatures for hydrogen to continue to fuse. So out here, you do have hydrogen fusion. And then outside over here, you just have the regular hydrogen plasma."}, {"video_title": "White and black dwarfs Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So just regular helium. But then outside of that, we do have the pressures and temperatures for hydrogen to continue to fuse. So out here, you do have hydrogen fusion. And then outside over here, you just have the regular hydrogen plasma. So what just happened here? When you have helium fusion all of a sudden, now this is once again providing some type of outward support for the core. So it's going to counteract the ever-increasing contraction of the core as it gets more and more dense."}, {"video_title": "White and black dwarfs Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And then outside over here, you just have the regular hydrogen plasma. So what just happened here? When you have helium fusion all of a sudden, now this is once again providing some type of outward support for the core. So it's going to counteract the ever-increasing contraction of the core as it gets more and more dense. Because now we have energy going outward. Energy pushing things outward. But at the same time that that is happening, more and more hydrogen in this layer is turning into helium."}, {"video_title": "White and black dwarfs Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So it's going to counteract the ever-increasing contraction of the core as it gets more and more dense. Because now we have energy going outward. Energy pushing things outward. But at the same time that that is happening, more and more hydrogen in this layer is turning into helium. Is fusing into helium. So it's making this inert part of the helium core even larger and larger denser. And even larger and larger."}, {"video_title": "White and black dwarfs Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But at the same time that that is happening, more and more hydrogen in this layer is turning into helium. Is fusing into helium. So it's making this inert part of the helium core even larger and larger denser. And even larger and larger. And putting even more pressure on this inside part. And so what's actually going to happen within a few moments from, I guess, especially from a cosmological point of view, this helium fusion is going to be burning super, I shouldn't use igniting or fusing, in a super hot level. But it's contained to do all of this pressure."}, {"video_title": "White and black dwarfs Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And even larger and larger. And putting even more pressure on this inside part. And so what's actually going to happen within a few moments from, I guess, especially from a cosmological point of view, this helium fusion is going to be burning super, I shouldn't use igniting or fusing, in a super hot level. But it's contained to do all of this pressure. But at some point, that pressure won't be able to contain it and the core is going to explode. But it's not going to be one of these catastrophic explosions where the star is going to be destroyed. It's just going to release a lot of energy all of a sudden into the star."}, {"video_title": "White and black dwarfs Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But it's contained to do all of this pressure. But at some point, that pressure won't be able to contain it and the core is going to explode. But it's not going to be one of these catastrophic explosions where the star is going to be destroyed. It's just going to release a lot of energy all of a sudden into the star. And that's called a helium flash. A helium flash. But once that happens all of a sudden, then now the star is going to be more stable."}, {"video_title": "White and black dwarfs Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It's just going to release a lot of energy all of a sudden into the star. And that's called a helium flash. A helium flash. But once that happens all of a sudden, then now the star is going to be more stable. And I'll use that in quotes without writing it down. Because red giants in general are already getting to be less stable than a main sequence star. But once that happens, you now will have a slightly larger volume."}, {"video_title": "White and black dwarfs Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But once that happens all of a sudden, then now the star is going to be more stable. And I'll use that in quotes without writing it down. Because red giants in general are already getting to be less stable than a main sequence star. But once that happens, you now will have a slightly larger volume. So it's not being contained in as small of a tight volume. That helium flash kind of took care of that. So now you have helium fusing into carbon and oxygen."}, {"video_title": "White and black dwarfs Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But once that happens, you now will have a slightly larger volume. So it's not being contained in as small of a tight volume. That helium flash kind of took care of that. So now you have helium fusing into carbon and oxygen. And there's all sorts of other combinations of things. Obviously, there's many elements in between helium and carbon and oxygen. But these are the ones that dominate."}, {"video_title": "White and black dwarfs Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So now you have helium fusing into carbon and oxygen. And there's all sorts of other combinations of things. Obviously, there's many elements in between helium and carbon and oxygen. But these are the ones that dominate. And then outside of that, you have helium forming. You have helium that is not fusing. And then outside of that, you have your fusing hydrogen."}, {"video_title": "White and black dwarfs Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But these are the ones that dominate. And then outside of that, you have helium forming. You have helium that is not fusing. And then outside of that, you have your fusing hydrogen. You have over here, you have hydrogen fusing into helium. And then out here in the rest of the radius of our super huge red giant, you just have your hydrogen plasma out here. Now what's going to happen as this star ages?"}, {"video_title": "White and black dwarfs Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And then outside of that, you have your fusing hydrogen. You have over here, you have hydrogen fusing into helium. And then out here in the rest of the radius of our super huge red giant, you just have your hydrogen plasma out here. Now what's going to happen as this star ages? Well, if we fast forward this a bunch. And remember, as the star gets denser and denser in the core and the reactions happen faster and faster in this core, it's expelling more and more energy outward. The star keeps growing."}, {"video_title": "White and black dwarfs Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Now what's going to happen as this star ages? Well, if we fast forward this a bunch. And remember, as the star gets denser and denser in the core and the reactions happen faster and faster in this core, it's expelling more and more energy outward. The star keeps growing. And the surface gets cooler and cooler. So if we fast forward a bunch, and this is what's going to happen to something the mass of our sun. If it's more massive, then at some point, the core of carbon and oxygen that's forming can start to fuse into even heavier elements."}, {"video_title": "White and black dwarfs Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "The star keeps growing. And the surface gets cooler and cooler. So if we fast forward a bunch, and this is what's going to happen to something the mass of our sun. If it's more massive, then at some point, the core of carbon and oxygen that's forming can start to fuse into even heavier elements. But in the case of the sun, it will never get to that 600 million Kelvin to actually fuse the carbon and the oxygen. And so eventually, you will have a core of carbon and oxygen, or mainly carbon and oxygen, surrounded by fusing helium, surrounded by non-fusing helium, surrounded by fusing hydrogen, which is surrounded by non-fusing hydrogen, or just a hydrogen plasma of the sun. But eventually, all of this fuel will run out."}, {"video_title": "White and black dwarfs Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "If it's more massive, then at some point, the core of carbon and oxygen that's forming can start to fuse into even heavier elements. But in the case of the sun, it will never get to that 600 million Kelvin to actually fuse the carbon and the oxygen. And so eventually, you will have a core of carbon and oxygen, or mainly carbon and oxygen, surrounded by fusing helium, surrounded by non-fusing helium, surrounded by fusing hydrogen, which is surrounded by non-fusing hydrogen, or just a hydrogen plasma of the sun. But eventually, all of this fuel will run out. All of the hydrogen will run out in the stars. All of this fusing hydrogen will run out. All of this fusing helium will run out."}, {"video_title": "White and black dwarfs Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But eventually, all of this fuel will run out. All of the hydrogen will run out in the stars. All of this fusing hydrogen will run out. All of this fusing helium will run out. All of this is the fusing hydrogen. This is the inert helium, which will run out. It'll be used in kind of this core being fused into the carbon and oxygen, until you get to a point where you literally just have a really hot core of carbon and oxygen."}, {"video_title": "White and black dwarfs Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "All of this fusing helium will run out. All of this is the fusing hydrogen. This is the inert helium, which will run out. It'll be used in kind of this core being fused into the carbon and oxygen, until you get to a point where you literally just have a really hot core of carbon and oxygen. And it's super dense. This whole time, it'll be getting more and more dense as heavier and heavier elements show up in the core. So it gets denser and denser and denser."}, {"video_title": "White and black dwarfs Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It'll be used in kind of this core being fused into the carbon and oxygen, until you get to a point where you literally just have a really hot core of carbon and oxygen. And it's super dense. This whole time, it'll be getting more and more dense as heavier and heavier elements show up in the core. So it gets denser and denser and denser. But this super dense thing will not, in the case of the sun, if it was a more massive star, it would get there. But in the case of the sun, it will not get hot enough for the carbon and the oxygen to form. So it really will just be this dense ball of carbon, super dense ball of carbon and oxygen."}, {"video_title": "White and black dwarfs Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So it gets denser and denser and denser. But this super dense thing will not, in the case of the sun, if it was a more massive star, it would get there. But in the case of the sun, it will not get hot enough for the carbon and the oxygen to form. So it really will just be this dense ball of carbon, super dense ball of carbon and oxygen. And all of the other material in the sun, remember, was super energetic. It was releasing tons and tons of energy. The more that we progressed down this, the more energy was releasing outward."}, {"video_title": "White and black dwarfs Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So it really will just be this dense ball of carbon, super dense ball of carbon and oxygen. And all of the other material in the sun, remember, was super energetic. It was releasing tons and tons of energy. The more that we progressed down this, the more energy was releasing outward. And the larger the radius of the star gained. And the cooler the outside of the star became. Until the outside just becomes this kind of cloud, this huge cloud of gas around what once was the star."}, {"video_title": "White and black dwarfs Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "The more that we progressed down this, the more energy was releasing outward. And the larger the radius of the star gained. And the cooler the outside of the star became. Until the outside just becomes this kind of cloud, this huge cloud of gas around what once was the star. And in the center, so I could just draw it as this huge, this is now way far away from the star, much even bigger than the radius or the diameter of a red giant. And all we'll have left is a mass, a super dense mass of, I would call it, inert carbon or oxygen. This is in the case of the sun."}, {"video_title": "White and black dwarfs Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Until the outside just becomes this kind of cloud, this huge cloud of gas around what once was the star. And in the center, so I could just draw it as this huge, this is now way far away from the star, much even bigger than the radius or the diameter of a red giant. And all we'll have left is a mass, a super dense mass of, I would call it, inert carbon or oxygen. This is in the case of the sun. And at first, when it's hot, and it will be releasing radiation because it's so hot, we'll call this a white dwarf. This right here is called a white dwarf. And it'll cool down over many, many, many, many, many, many, many years until it becomes, when it's no longer, when it's completely cooled down, lost all of its energy, it'll just be this super dense ball of carbon and oxygen."}, {"video_title": "White and black dwarfs Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "This is in the case of the sun. And at first, when it's hot, and it will be releasing radiation because it's so hot, we'll call this a white dwarf. This right here is called a white dwarf. And it'll cool down over many, many, many, many, many, many, many years until it becomes, when it's no longer, when it's completely cooled down, lost all of its energy, it'll just be this super dense ball of carbon and oxygen. At which point, we would call it a black dwarf. And these are obviously very hard to observe because they're not emitting light. And they don't have quite the mass of something like a black hole that isn't even emitting light, but you can see how it's affecting things around it."}, {"video_title": "White and black dwarfs Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And it'll cool down over many, many, many, many, many, many, many years until it becomes, when it's no longer, when it's completely cooled down, lost all of its energy, it'll just be this super dense ball of carbon and oxygen. At which point, we would call it a black dwarf. And these are obviously very hard to observe because they're not emitting light. And they don't have quite the mass of something like a black hole that isn't even emitting light, but you can see how it's affecting things around it. So that's what's going to happen to the sun. In the next few videos, we're going to talk about what would happen to things less massive than the sun and what would happen to things more massive than the sun. Although I think you can imagine the more massive, then there would be so much pressure on these things because you have so much mass around it that these would begin to fuse into heavier and heavier elements until we get to iron."}, {"video_title": "Hawaiian islands formation Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It is the dominant place or the most likely place to find mountains or volcanoes on the surface of the earth. But that's not the only place that mountains or volcanoes can form. And probably the biggest example of volcanic activity, or the most popular one, this might be a slightly American, Amerocentric point of view. But the most often cited example of volcanic activity away from a plate boundary is Hawaii. So this right here, these are the Hawaiian islands. This is the big island of Hawaii. And it is experiencing an active volcano."}, {"video_title": "Hawaiian islands formation Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But the most often cited example of volcanic activity away from a plate boundary is Hawaii. So this right here, these are the Hawaiian islands. This is the big island of Hawaii. And it is experiencing an active volcano. Lava or magma is flowing from underneath the ground. And once it surfaces, we call it lava. And that lava is actively making the island bigger."}, {"video_title": "Hawaiian islands formation Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And it is experiencing an active volcano. Lava or magma is flowing from underneath the ground. And once it surfaces, we call it lava. And that lava is actively making the island bigger. So where is that volcanic activity coming from? And then how can we think about that volcanic activity or that kind of heat rising from below the surface of the earth to explain some of the geological features we see around Hawaii? So what we think is happening, and once again, this is all theory right here, is that Hawaii is sitting on top of a hotspot."}, {"video_title": "Hawaiian islands formation Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And that lava is actively making the island bigger. So where is that volcanic activity coming from? And then how can we think about that volcanic activity or that kind of heat rising from below the surface of the earth to explain some of the geological features we see around Hawaii? So what we think is happening, and once again, this is all theory right here, is that Hawaii is sitting on top of a hotspot. In particular, the big island of Hawaii is sitting on top of the hotspot right now. And this hotspot, there's different theories on how it might emerge. But we think that at the mantle core boundary, and I don't know in this diagram whether they intended this white area to be the core, but let's just say that this is the outer core down here."}, {"video_title": "Hawaiian islands formation Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So what we think is happening, and once again, this is all theory right here, is that Hawaii is sitting on top of a hotspot. In particular, the big island of Hawaii is sitting on top of the hotspot right now. And this hotspot, there's different theories on how it might emerge. But we think that at the mantle core boundary, and I don't know in this diagram whether they intended this white area to be the core, but let's just say that this is the outer core down here. So let's just say that this is the outer core for the sake of explaining things. It's possible that plumes of very hot material can kind of, just based on the fluid dynamics of what's happening at that mantle outer core boundary, that plumes of really hot material can kind of rise up. Let me do this in a darker color."}, {"video_title": "Hawaiian islands formation Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But we think that at the mantle core boundary, and I don't know in this diagram whether they intended this white area to be the core, but let's just say that this is the outer core down here. So let's just say that this is the outer core for the sake of explaining things. It's possible that plumes of very hot material can kind of, just based on the fluid dynamics of what's happening at that mantle outer core boundary, that plumes of really hot material can kind of rise up. Let me do this in a darker color. Can rise up from the outer core. They would rise up from the outer core and then create hotspots underneath the moving lithospheric plates. Now, we don't know for sure whether the hotspots are being created by these mantle plumes, this material formed or heated up at the outer core mantle boundary."}, {"video_title": "Hawaiian islands formation Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Let me do this in a darker color. Can rise up from the outer core. They would rise up from the outer core and then create hotspots underneath the moving lithospheric plates. Now, we don't know for sure whether the hotspots are being created by these mantle plumes, this material formed or heated up at the outer core mantle boundary. But what we do feel pretty confident about is that there is this hotspot here. And it's independent of any of those convection patterns that we saw. I shouldn't say independent."}, {"video_title": "Hawaiian islands formation Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Now, we don't know for sure whether the hotspots are being created by these mantle plumes, this material formed or heated up at the outer core mantle boundary. But what we do feel pretty confident about is that there is this hotspot here. And it's independent of any of those convection patterns that we saw. I shouldn't say independent. It's obviously all related because we have all this fluidic motion going on in the mantle. But it's separate on some degree from all of those convection patterns that we talked about that would actually cause the plates to move. And to a large degree, or the way we think about it right now, this is stationary."}, {"video_title": "Hawaiian islands formation Cosmology & Astronomy Khan Academy.mp3", "Sentence": "I shouldn't say independent. It's obviously all related because we have all this fluidic motion going on in the mantle. But it's separate on some degree from all of those convection patterns that we talked about that would actually cause the plates to move. And to a large degree, or the way we think about it right now, this is stationary. This hotspot is stationary relative to the plates. And the reason why we feel pretty good about thinking that it's stationary relative to the plates is we see this notion right here. If you look at the volcanic rock in Kauai, which is one of the older inhabited Hawaiian islands, the oldest rock that we've observed there is 5.5 million years old."}, {"video_title": "Hawaiian islands formation Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And to a large degree, or the way we think about it right now, this is stationary. This hotspot is stationary relative to the plates. And the reason why we feel pretty good about thinking that it's stationary relative to the plates is we see this notion right here. If you look at the volcanic rock in Kauai, which is one of the older inhabited Hawaiian islands, the oldest rock that we've observed there is 5.5 million years old. And it's all volcanic rock. Now, the oldest rock we've observed on the big island is about 700,000 years old. We also know that the Pacific plate, you could look at this diagram right over here, is moving in this general direction."}, {"video_title": "Hawaiian islands formation Cosmology & Astronomy Khan Academy.mp3", "Sentence": "If you look at the volcanic rock in Kauai, which is one of the older inhabited Hawaiian islands, the oldest rock that we've observed there is 5.5 million years old. And it's all volcanic rock. Now, the oldest rock we've observed on the big island is about 700,000 years old. We also know that the Pacific plate, you could look at this diagram right over here, is moving in this general direction. In fact, we know it from GPS measurements. It's moving exactly in the direction that the Hawaiian islands are kind of distributed in. So frankly, the only good explanation for why we see this pattern, why we see newer land here, and then as we go further and further up the Hawaiian island chain, we see older and older land."}, {"video_title": "Hawaiian islands formation Cosmology & Astronomy Khan Academy.mp3", "Sentence": "We also know that the Pacific plate, you could look at this diagram right over here, is moving in this general direction. In fact, we know it from GPS measurements. It's moving exactly in the direction that the Hawaiian islands are kind of distributed in. So frankly, the only good explanation for why we see this pattern, why we see newer land here, and then as we go further and further up the Hawaiian island chain, we see older and older land. And actually, if we keep going, we have the Leeward Islands over here. And as we keep measuring the rock on the Leeward Islands, they get older and older as you go to the northwest. And then if you even look at what's below the ocean, this is the big island of Hawaii."}, {"video_title": "Hawaiian islands formation Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So frankly, the only good explanation for why we see this pattern, why we see newer land here, and then as we go further and further up the Hawaiian island chain, we see older and older land. And actually, if we keep going, we have the Leeward Islands over here. And as we keep measuring the rock on the Leeward Islands, they get older and older as you go to the northwest. And then if you even look at what's below the ocean, this is the big island of Hawaii. These are the main Hawaiian islands. These are the Leeward Islands. But you see, even beyond that, submerged under the Pacific Ocean, you continue to see a chain of islands."}, {"video_title": "Hawaiian islands formation Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And then if you even look at what's below the ocean, this is the big island of Hawaii. These are the main Hawaiian islands. These are the Leeward Islands. But you see, even beyond that, submerged under the Pacific Ocean, you continue to see a chain of islands. So the explanation for what's happening here is that you have a stationary hotspot that is right now underneath the big island of Hawaii. And I just want to be clear. The big island is called the island of Hawaii."}, {"video_title": "Hawaiian islands formation Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But you see, even beyond that, submerged under the Pacific Ocean, you continue to see a chain of islands. So the explanation for what's happening here is that you have a stationary hotspot that is right now underneath the big island of Hawaii. And I just want to be clear. The big island is called the island of Hawaii. It is one of the islands in the state of Hawaii. So I don't want to cause you confusion. I'll just call it the big island from here on out."}, {"video_title": "Hawaiian islands formation Cosmology & Astronomy Khan Academy.mp3", "Sentence": "The big island is called the island of Hawaii. It is one of the islands in the state of Hawaii. So I don't want to cause you confusion. I'll just call it the big island from here on out. So the hotspot is right under the big island. But if you were to rewind 5 million years ago, the entire Pacific plate was probably on the order of about 150, 200 miles, however far Kauai is from the big island. It was probably shifted that much to the southeast if you go back 5 million years ago."}, {"video_title": "Hawaiian islands formation Cosmology & Astronomy Khan Academy.mp3", "Sentence": "I'll just call it the big island from here on out. So the hotspot is right under the big island. But if you were to rewind 5 million years ago, the entire Pacific plate was probably on the order of about 150, 200 miles, however far Kauai is from the big island. It was probably shifted that much to the southeast if you go back 5 million years ago. So 5 million years ago, when all of this was shifted down and to the right, then Kauai was on top of the hotspot. And so this is how each of these islands are formed. If you rewind a ton of years, then maybe this area over here on the Pacific plate was over the hotspot."}, {"video_title": "Hawaiian islands formation Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It was probably shifted that much to the southeast if you go back 5 million years ago. So 5 million years ago, when all of this was shifted down and to the right, then Kauai was on top of the hotspot. And so this is how each of these islands are formed. If you rewind a ton of years, then maybe this area over here on the Pacific plate was over the hotspot. An island formed there. Then the Pacific plate kept moving to the northwest. And new islands, new volcanoes kept forming."}, {"video_title": "Hawaiian islands formation Cosmology & Astronomy Khan Academy.mp3", "Sentence": "If you rewind a ton of years, then maybe this area over here on the Pacific plate was over the hotspot. An island formed there. Then the Pacific plate kept moving to the northwest. And new islands, new volcanoes kept forming. Those volcanoes would release lava that would keep piling up, eventually go above the surface of the water and form this whole chain of islands. And as the whole Pacific plate kept moving to the northwest, it kept forming new islands. Now the one question you might ask is, well, how come the big island is bigger?"}, {"video_title": "Hawaiian islands formation Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And new islands, new volcanoes kept forming. Those volcanoes would release lava that would keep piling up, eventually go above the surface of the water and form this whole chain of islands. And as the whole Pacific plate kept moving to the northwest, it kept forming new islands. Now the one question you might ask is, well, how come the big island is bigger? Has the plate kind of paused over there? Is it spending more time over the hotspot so that more lava can kind of form there to form this? Essentially, it's an underwater mountain that's now also above the water."}, {"video_title": "Hawaiian islands formation Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Now the one question you might ask is, well, how come the big island is bigger? Has the plate kind of paused over there? Is it spending more time over the hotspot so that more lava can kind of form there to form this? Essentially, it's an underwater mountain that's now also above the water. And actually, if you go from the base of the Pacific Ocean to the top of the big island of Hawaii, it's actually 50% higher than Mount Everest. So you could really just view it as a big mountain. But the question is, this looks so much bigger than Kauai, and they keep getting smaller as you keep going to the northwest."}, {"video_title": "Hawaiian islands formation Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Essentially, it's an underwater mountain that's now also above the water. And actually, if you go from the base of the Pacific Ocean to the top of the big island of Hawaii, it's actually 50% higher than Mount Everest. So you could really just view it as a big mountain. But the question is, this looks so much bigger than Kauai, and they keep getting smaller as you keep going to the northwest. Is somehow the Pacific plate slowing? Is it spending more time here? And the answer is, it's probably not slowing."}, {"video_title": "Hawaiian islands formation Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But the question is, this looks so much bigger than Kauai, and they keep getting smaller as you keep going to the northwest. Is somehow the Pacific plate slowing? Is it spending more time here? And the answer is, it's probably not slowing. What's happening is, at one time, Kauai was also probably a relatively large island. If you rewind maybe 5 million years ago, Kauai also might have been about that big. But over 5 million years, it's just experienced a ton of erosion."}, {"video_title": "Hawaiian islands formation Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And the answer is, it's probably not slowing. What's happening is, at one time, Kauai was also probably a relatively large island. If you rewind maybe 5 million years ago, Kauai also might have been about that big. But over 5 million years, it's just experienced a ton of erosion. Remember, once it moved over the hotspot, new land wasn't being created. It's in the middle of the Pacific Ocean. It's experiencing weather."}, {"video_title": "Hawaiian islands formation Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But over 5 million years, it's just experienced a ton of erosion. Remember, once it moved over the hotspot, new land wasn't being created. It's in the middle of the Pacific Ocean. It's experiencing weather. 5 million years is a long period of time. And so it just got eroded over that time. So the older the island is, the more eroded it's going to be, and the smaller it's going to be."}, {"video_title": "Hawaiian islands formation Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It's experiencing weather. 5 million years is a long period of time. And so it just got eroded over that time. So the older the island is, the more eroded it's going to be, and the smaller it's going to be. So if you go to these underwater mountains up here that don't even surface above the ocean, at one time, they might have surfaced. But due to the ocean and weather and whatnot, they've just been eroded over time to become smaller and smaller kind of remnants of volcanoes. So anyway, I thought you would find that entertaining of how the Hawaiian Islands actually got formed and how we can actually have these hotspots and this volcanic activity and actually even earthquake activity outside of actually plate boundaries."}, {"video_title": "Hawaiian islands formation Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So the older the island is, the more eroded it's going to be, and the smaller it's going to be. So if you go to these underwater mountains up here that don't even surface above the ocean, at one time, they might have surfaced. But due to the ocean and weather and whatnot, they've just been eroded over time to become smaller and smaller kind of remnants of volcanoes. So anyway, I thought you would find that entertaining of how the Hawaiian Islands actually got formed and how we can actually have these hotspots and this volcanic activity and actually even earthquake activity outside of actually plate boundaries. Actually, while we're looking at this diagram, we talked about the trenches at plate boundaries. You can actually see it here because this shows the depth. And the really dark, dark, dark, dark blue is really deep parts of the ocean."}, {"video_title": "Hawaiian islands formation Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So anyway, I thought you would find that entertaining of how the Hawaiian Islands actually got formed and how we can actually have these hotspots and this volcanic activity and actually even earthquake activity outside of actually plate boundaries. Actually, while we're looking at this diagram, we talked about the trenches at plate boundaries. You can actually see it here because this shows the depth. And the really dark, dark, dark, dark blue is really deep parts of the ocean. So this right here is the Mariana Trench. And you can see over here the Pacific Plate just getting abducted, or not abducted, getting subducted into other plates underneath and forms these trenches here. Anyway, hopefully you found that entertaining."}, {"video_title": "Correction calendar notation Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But there is no year zero. Right after that you go from December 31, 1 BC to January 1, 1 AD. There's no year zero. And despite the fact that I emphasized that in the last video, I didn't take that into consideration when I calculated how many years there were between Plato's birth and Columbus discovering the New World. And the reason why I didn't take that into consideration is that the year 1492, whether you want to call it AD 1492, Anno Domini 1492, whether you want to call it that or whether you want to call it 1492 in the Common Era, it's not 1492 years since the theoretical birth of Jesus, which we know is not the actual birth. He was probably born before that. It is 1491 years since the birth of Jesus."}, {"video_title": "Correction calendar notation Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And despite the fact that I emphasized that in the last video, I didn't take that into consideration when I calculated how many years there were between Plato's birth and Columbus discovering the New World. And the reason why I didn't take that into consideration is that the year 1492, whether you want to call it AD 1492, Anno Domini 1492, whether you want to call it that or whether you want to call it 1492 in the Common Era, it's not 1492 years since the theoretical birth of Jesus, which we know is not the actual birth. He was probably born before that. It is 1491 years since the birth of Jesus. And to think about it this way, let's just assume, I'll keep emphasizing, it's a theoretical date that we're talking about, this theoretical event, this kind of birth of Jesus that our calendars revolve around. If we talk about January 1, let's think about it this way, so January 1, 1 in the Common Era, how long is that since the birth of Jesus? It's not one year."}, {"video_title": "Correction calendar notation Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It is 1491 years since the birth of Jesus. And to think about it this way, let's just assume, I'll keep emphasizing, it's a theoretical date that we're talking about, this theoretical event, this kind of birth of Jesus that our calendars revolve around. If we talk about January 1, let's think about it this way, so January 1, 1 in the Common Era, how long is that since the birth of Jesus? It's not one year. You wouldn't just look at this and say it's been one year, because this is theoretically the day that he was born, so this is zero years or almost zero years since that theoretical birth of Jesus. Another way to think about it is, how long after January 1, the year 1 before the Common Era, and I could have called this AD and I could have called this BC, what's the time difference between these two dates? The way I calculated it before, I said this is one year after that theoretical birth, that's wrong, this is during that theoretical birth."}, {"video_title": "Correction calendar notation Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It's not one year. You wouldn't just look at this and say it's been one year, because this is theoretically the day that he was born, so this is zero years or almost zero years since that theoretical birth of Jesus. Another way to think about it is, how long after January 1, the year 1 before the Common Era, and I could have called this AD and I could have called this BC, what's the time difference between these two dates? The way I calculated it before, I said this is one year after that theoretical birth, that's wrong, this is during that theoretical birth. But the way I did it in the last video, I said that's one year after, one year before, you add them together and you would get 2. But that's wrong, because there is no year zero, so January 1, 1 AD or 1 in the Common Era is right over here, and then January 1, 1 BCE is exactly one year before that, so there's only one year difference. The reason why the math is strange is because there is no year zero."}, {"video_title": "Correction calendar notation Cosmology & Astronomy Khan Academy.mp3", "Sentence": "The way I calculated it before, I said this is one year after that theoretical birth, that's wrong, this is during that theoretical birth. But the way I did it in the last video, I said that's one year after, one year before, you add them together and you would get 2. But that's wrong, because there is no year zero, so January 1, 1 AD or 1 in the Common Era is right over here, and then January 1, 1 BCE is exactly one year before that, so there's only one year difference. The reason why the math is strange is because there is no year zero. If there was a year zero, then my calculation in the last video was correct. So really, the way that you would calculate the time between Plato's birth at 428 BC and Columbus sailing across the Atlantic in 1492, you would say, okay, this is 428 years before that theoretical birth of Christ. But this isn't 1492 years after that theoretical birth, this is 1492 minus 1."}, {"video_title": "Correction calendar notation Cosmology & Astronomy Khan Academy.mp3", "Sentence": "The reason why the math is strange is because there is no year zero. If there was a year zero, then my calculation in the last video was correct. So really, the way that you would calculate the time between Plato's birth at 428 BC and Columbus sailing across the Atlantic in 1492, you would say, okay, this is 428 years before that theoretical birth of Christ. But this isn't 1492 years after that theoretical birth, this is 1492 minus 1. So what you would do is you would add these two numbers, this is 428 before, this is 1492 minus 1 years after, so you would add them and then subtract 1. So the correct answer, this is the correction part, it isn't 1920 years between Plato's birth and Columbus, we want to subtract 1 from that. It is 1920 minus 1 years, so that is 1919 years."}, {"video_title": "Correction calendar notation Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But this isn't 1492 years after that theoretical birth, this is 1492 minus 1. So what you would do is you would add these two numbers, this is 428 before, this is 1492 minus 1 years after, so you would add them and then subtract 1. So the correct answer, this is the correction part, it isn't 1920 years between Plato's birth and Columbus, we want to subtract 1 from that. It is 1920 minus 1 years, so that is 1919 years. The same way that the difference between 1 AD and 1 BCE, you would say, you could almost view it as positive and negative numbers, you would say, oh, this is positive 1 minus negative 1, that would give me 2, but there is no zero, so you subtract another 1. So this is exactly 1 year difference. So I just want to clarify that."}, {"video_title": "Becoming a red giant Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And when the pressure and the temperature got high enough, and so this is what we saw in the last video, when the pressure and temperature got high enough, we were able to get the hydrogen protons, the hydrogen nucleuses, close enough to each other, or hydrogen nuclei, close enough to each other for the strong force to take over and fusion to happen and release energy. And then that energy begins to offset the actual gravitational force so the whole star, what's now a star, does not collapse on itself. And once we're there, we're now in the main sequence of a star. What I want to do in this video is to take off from that starting point and think about what happens in the star next. So in the main sequence, we have the core of the star. So this is the star's core. And you have hydrogen fusing into helium."}, {"video_title": "Becoming a red giant Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "What I want to do in this video is to take off from that starting point and think about what happens in the star next. So in the main sequence, we have the core of the star. So this is the star's core. And you have hydrogen fusing into helium. And it's releasing just a ton of energy. And that energy is what keeps the core from imploding. It's kind of the outward force to offset the gravitational force that wants to implode everything, that wants to crush everything."}, {"video_title": "Becoming a red giant Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And you have hydrogen fusing into helium. And it's releasing just a ton of energy. And that energy is what keeps the core from imploding. It's kind of the outward force to offset the gravitational force that wants to implode everything, that wants to crush everything. And so you have the core of a star, a star like the sun. And that energy then heats up all of the other gas on the outside of the core to create that really bright object that we see as a star, or in our case, in our sun's case, the sun. Now, as the hydrogen is fusing into helium, you can imagine that more and more helium is forming in the core."}, {"video_title": "Becoming a red giant Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It's kind of the outward force to offset the gravitational force that wants to implode everything, that wants to crush everything. And so you have the core of a star, a star like the sun. And that energy then heats up all of the other gas on the outside of the core to create that really bright object that we see as a star, or in our case, in our sun's case, the sun. Now, as the hydrogen is fusing into helium, you can imagine that more and more helium is forming in the core. So I'll do the helium as green. So more and more helium forms in the core. The closer you get to the center, the higher the pressures will be and the faster that this fusion, this ignition, will happen."}, {"video_title": "Becoming a red giant Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Now, as the hydrogen is fusing into helium, you can imagine that more and more helium is forming in the core. So I'll do the helium as green. So more and more helium forms in the core. The closer you get to the center, the higher the pressures will be and the faster that this fusion, this ignition, will happen. In fact, the bigger the mass of the star, the more the pressure, the faster the fusion occurs. And so you have this helium building up inside of the core as this hydrogen in the core gets fused. Now, what's going to happen there?"}, {"video_title": "Becoming a red giant Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "The closer you get to the center, the higher the pressures will be and the faster that this fusion, this ignition, will happen. In fact, the bigger the mass of the star, the more the pressure, the faster the fusion occurs. And so you have this helium building up inside of the core as this hydrogen in the core gets fused. Now, what's going to happen there? Helium is a more dense atom. It's packing more mass in a smaller space. So as more and more of this hydrogen here turns into helium, what you're going to have is the core itself is going to shrink."}, {"video_title": "Becoming a red giant Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Now, what's going to happen there? Helium is a more dense atom. It's packing more mass in a smaller space. So as more and more of this hydrogen here turns into helium, what you're going to have is the core itself is going to shrink. So let me draw a smaller core here. So the core itself is going to shrink. And now it has a lot more helium in it."}, {"video_title": "Becoming a red giant Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So as more and more of this hydrogen here turns into helium, what you're going to have is the core itself is going to shrink. So let me draw a smaller core here. So the core itself is going to shrink. And now it has a lot more helium in it. And let's just stick it to the extreme point where it's all helium, where it's depleted. But it's been shrinking. It's much denser."}, {"video_title": "Becoming a red giant Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And now it has a lot more helium in it. And let's just stick it to the extreme point where it's all helium, where it's depleted. But it's been shrinking. It's much denser. That same amount of mass that was in this sphere is now in a denser sphere, in a helium sphere. So it's going to have just as much attraction to it, gravitational attraction. But things can get even closer to it."}, {"video_title": "Becoming a red giant Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It's much denser. That same amount of mass that was in this sphere is now in a denser sphere, in a helium sphere. So it's going to have just as much attraction to it, gravitational attraction. But things can get even closer to it. And we know that the closer you are to a mass, the stronger the pull of gravity. So then instead of having just the hydrogen fusion occurring at the core, you're now going to have hydrogen fusion in a shell around the core. So now you're going to have hydrogen fusing in a shell around the core."}, {"video_title": "Becoming a red giant Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But things can get even closer to it. And we know that the closer you are to a mass, the stronger the pull of gravity. So then instead of having just the hydrogen fusion occurring at the core, you're now going to have hydrogen fusion in a shell around the core. So now you're going to have hydrogen fusing in a shell around the core. Let me just be clear. This isn't just happens all of a sudden. It is a gradual process."}, {"video_title": "Becoming a red giant Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So now you're going to have hydrogen fusing in a shell around the core. Let me just be clear. This isn't just happens all of a sudden. It is a gradual process. As we have more and more helium in the core, the core gets denser and denser and denser. And so the pressures become even larger and larger near the core. Because you're able to get closer to a more massive core since it is now more dense."}, {"video_title": "Becoming a red giant Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It is a gradual process. As we have more and more helium in the core, the core gets denser and denser and denser. And so the pressures become even larger and larger near the core. Because you're able to get closer to a more massive core since it is now more dense. And as that pressure near the core increases even more and more, the fusion reaction happens faster and faster and faster. Until you get to this point. So here, let me be clear."}, {"video_title": "Becoming a red giant Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Because you're able to get closer to a more massive core since it is now more dense. And as that pressure near the core increases even more and more, the fusion reaction happens faster and faster and faster. Until you get to this point. So here, let me be clear. You have a helium core. All of the hydrogen in the core has been used up. And then you have the hydrogen right outside of the core is now under enormous pressure."}, {"video_title": "Becoming a red giant Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So here, let me be clear. You have a helium core. All of the hydrogen in the core has been used up. And then you have the hydrogen right outside of the core is now under enormous pressure. It's actually under more pressure than it was when it was just a pure hydrogen core. Because there's so much mass on the outside here trying to, I guess you could say, exerting downward or gravitational force. Down trying to get to that even denser helium core."}, {"video_title": "Becoming a red giant Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And then you have the hydrogen right outside of the core is now under enormous pressure. It's actually under more pressure than it was when it was just a pure hydrogen core. Because there's so much mass on the outside here trying to, I guess you could say, exerting downward or gravitational force. Down trying to get to that even denser helium core. Because everything is able to get closer in. And so now you have fusion occurring even faster. And it's occurring over a larger radius."}, {"video_title": "Becoming a red giant Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Down trying to get to that even denser helium core. Because everything is able to get closer in. And so now you have fusion occurring even faster. And it's occurring over a larger radius. So this faster fusion over a larger radius is then going to expel, the force is now going to expel, the energy that's released from this fusion is now going to expel these outer layers of the star even further. So the whole time, this gradual process, as the hydrogen turns into helium or fuses into helium in the core, what the hydrogen right outside of the core, right outside of that area, starts to burn faster and faster. It starts to, or I shouldn't say burn, it starts to fuse faster and faster."}, {"video_title": "Becoming a red giant Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And it's occurring over a larger radius. So this faster fusion over a larger radius is then going to expel, the force is now going to expel, the energy that's released from this fusion is now going to expel these outer layers of the star even further. So the whole time, this gradual process, as the hydrogen turns into helium or fuses into helium in the core, what the hydrogen right outside of the core, right outside of that area, starts to burn faster and faster. It starts to, or I shouldn't say burn, it starts to fuse faster and faster. And over a larger and larger radius. So the unintuitive thing is the fusion is happening faster over a larger radius. And the reason that is is because you have even a denser core that is causing even more gravitational pressure."}, {"video_title": "Becoming a red giant Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It starts to, or I shouldn't say burn, it starts to fuse faster and faster. And over a larger and larger radius. So the unintuitive thing is the fusion is happening faster over a larger radius. And the reason that is is because you have even a denser core that is causing even more gravitational pressure. And as that's happening, the star is getting brighter. And it's also the fusion reactions, since they're happening in a more intense way and over a larger radius, are able to expel the material of the star even larger. So the radius of the star itself is getting bigger and bigger and bigger."}, {"video_title": "Becoming a red giant Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And the reason that is is because you have even a denser core that is causing even more gravitational pressure. And as that's happening, the star is getting brighter. And it's also the fusion reactions, since they're happening in a more intense way and over a larger radius, are able to expel the material of the star even larger. So the radius of the star itself is getting bigger and bigger and bigger. So if this star would look like this, if this star, maybe let me draw it in white, this star looked like this. That's not white. This star looked like, what's happening to my color changer?"}, {"video_title": "Becoming a red giant Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So the radius of the star itself is getting bigger and bigger and bigger. So if this star would look like this, if this star, maybe let me draw it in white, this star looked like this. That's not white. This star looked like, what's happening to my color changer? There you go. OK, this star looked like this right over here. Now this star over here, since a faster fusion reaction is happening over a larger radius, is going to be far larger, and I'm not even drawing it to scale."}, {"video_title": "Becoming a red giant Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "This star looked like, what's happening to my color changer? There you go. OK, this star looked like this right over here. Now this star over here, since a faster fusion reaction is happening over a larger radius, is going to be far larger, and I'm not even drawing it to scale. In the case of our sun, when it gets to this point, it's going to be 100 times the diameter. And at this point, it is a red giant. And the reason why it's redder than this one over here is that even though the fusion is happening more furiously, that energy is being dissipated over a larger surface area."}, {"video_title": "Becoming a red giant Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Now this star over here, since a faster fusion reaction is happening over a larger radius, is going to be far larger, and I'm not even drawing it to scale. In the case of our sun, when it gets to this point, it's going to be 100 times the diameter. And at this point, it is a red giant. And the reason why it's redder than this one over here is that even though the fusion is happening more furiously, that energy is being dissipated over a larger surface area. So the actual surface temperature of the red giant at this point is actually going to be cooler. So it's going to emit a light at a larger wavelength, a redder wavelength than this thing over here. This thing, the core, was not burning as furiously as this thing over here, but that energy was being dissipated over a smaller volume."}, {"video_title": "Becoming a red giant Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And the reason why it's redder than this one over here is that even though the fusion is happening more furiously, that energy is being dissipated over a larger surface area. So the actual surface temperature of the red giant at this point is actually going to be cooler. So it's going to emit a light at a larger wavelength, a redder wavelength than this thing over here. This thing, the core, was not burning as furiously as this thing over here, but that energy was being dissipated over a smaller volume. So this had a higher surface temperature. This over here, the core is no longer burning. The core is now helium that's not burning."}, {"video_title": "Becoming a red giant Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "This thing, the core, was not burning as furiously as this thing over here, but that energy was being dissipated over a smaller volume. So this had a higher surface temperature. This over here, the core is no longer burning. The core is now helium that's not burning. It's getting denser and denser as the helium packs in on it itself, but the hydrogen fusion over here is occurring more intensely. It's occurring in a hotter way, but the surface here is less hot because it's just a larger surface area. So the increased heat is more than mitigated by how large the star has become."}, {"video_title": "Becoming a red giant Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "The core is now helium that's not burning. It's getting denser and denser as the helium packs in on it itself, but the hydrogen fusion over here is occurring more intensely. It's occurring in a hotter way, but the surface here is less hot because it's just a larger surface area. So the increased heat is more than mitigated by how large the star has become. Now this is going to keep happening, keep happening, and the pressures keep intensifying because more and more helium is getting produced. And this core keeps collapsing, and the temperature here keeps going up. So we said that the first ignition, the first fusion occurs at around 10 million Kelvin."}, {"video_title": "Becoming a red giant Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So the increased heat is more than mitigated by how large the star has become. Now this is going to keep happening, keep happening, and the pressures keep intensifying because more and more helium is getting produced. And this core keeps collapsing, and the temperature here keeps going up. So we said that the first ignition, the first fusion occurs at around 10 million Kelvin. This thing will keep heating up until it gets to 100 million Kelvin. And now I'm talking about a star that's about as massive as the sun. Some stars will never even be massive enough to condense the core so that its temperature reaches 100 million."}, {"video_title": "Becoming a red giant Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So we said that the first ignition, the first fusion occurs at around 10 million Kelvin. This thing will keep heating up until it gets to 100 million Kelvin. And now I'm talking about a star that's about as massive as the sun. Some stars will never even be massive enough to condense the core so that its temperature reaches 100 million. But let's just talk about the case in which it does. So eventually you'll get to a point, so we're still sitting in the red giant phase. So we're this huge star over here."}, {"video_title": "Becoming a red giant Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Some stars will never even be massive enough to condense the core so that its temperature reaches 100 million. But let's just talk about the case in which it does. So eventually you'll get to a point, so we're still sitting in the red giant phase. So we're this huge star over here. We have this helium core, and that helium core keeps getting condensed and condensed and condensed. And then we have a shell of hydrogen that keeps fusing into helium around it. So this is our hydrogen shell."}, {"video_title": "Becoming a red giant Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So we're this huge star over here. We have this helium core, and that helium core keeps getting condensed and condensed and condensed. And then we have a shell of hydrogen that keeps fusing into helium around it. So this is our hydrogen shell. Hydrogen fusion is occurring in this yellow shell over here that's causing the radius of the star to get bigger and bigger to expand. But when the temperature gets sufficiently hot, and now I think you're going to get a sense of how heavier and heavier elements form in the universe. And all of the heavy elements that you see around us, including the ones that are in you, were formed this way from initially hydrogen."}, {"video_title": "Becoming a red giant Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So this is our hydrogen shell. Hydrogen fusion is occurring in this yellow shell over here that's causing the radius of the star to get bigger and bigger to expand. But when the temperature gets sufficiently hot, and now I think you're going to get a sense of how heavier and heavier elements form in the universe. And all of the heavy elements that you see around us, including the ones that are in you, were formed this way from initially hydrogen. When it gets hot enough at 100 million Kelvin in this core, because of such enormous pressures, then the helium itself will start to fuse. So then we're going to have a core in here where the helium itself will start to fuse. And now we're talking about a situation, you have helium and you had hydrogen, and all sorts of combinations will form, but in general, the helium is mainly going to fuse into carbon and oxygen."}, {"video_title": "Becoming a red giant Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And all of the heavy elements that you see around us, including the ones that are in you, were formed this way from initially hydrogen. When it gets hot enough at 100 million Kelvin in this core, because of such enormous pressures, then the helium itself will start to fuse. So then we're going to have a core in here where the helium itself will start to fuse. And now we're talking about a situation, you have helium and you had hydrogen, and all sorts of combinations will form, but in general, the helium is mainly going to fuse into carbon and oxygen. And it will form into other things, and it becomes much more complicated, but I don't want to go into all of the details. But let me just show you a periodic table. I didn't have this in the last one."}, {"video_title": "Becoming a red giant Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And now we're talking about a situation, you have helium and you had hydrogen, and all sorts of combinations will form, but in general, the helium is mainly going to fuse into carbon and oxygen. And it will form into other things, and it becomes much more complicated, but I don't want to go into all of the details. But let me just show you a periodic table. I didn't have this in the last one. I somehow lost it. But we see hydrogen here has one proton. It actually has no neutrons."}, {"video_title": "Becoming a red giant Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "I didn't have this in the last one. I somehow lost it. But we see hydrogen here has one proton. It actually has no neutrons. It was getting fused in the main sequence into helium. Two protons, two neutrons. We would need four of these to get one of those, because this actually has an atomic mass of 4, if we're talking about helium-4."}, {"video_title": "Becoming a red giant Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It actually has no neutrons. It was getting fused in the main sequence into helium. Two protons, two neutrons. We would need four of these to get one of those, because this actually has an atomic mass of 4, if we're talking about helium-4. And then the helium, once we get to 100 million Kelvin, can start being fused. If you get roughly three of them, and there's all these other things that are coming and leaving the reactions, you can get to a carbon. You get four of them, at least as the starting raw material, you get to an oxygen."}, {"video_title": "Becoming a red giant Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "We would need four of these to get one of those, because this actually has an atomic mass of 4, if we're talking about helium-4. And then the helium, once we get to 100 million Kelvin, can start being fused. If you get roughly three of them, and there's all these other things that are coming and leaving the reactions, you can get to a carbon. You get four of them, at least as the starting raw material, you get to an oxygen. So we're starting to fuse heavier and heavier elements. So what happens here is this helium is fusing into carbon and oxygen, so you start building a carbon and oxygen core. So I'm going to leave you there."}, {"video_title": "Becoming a red giant Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "You get four of them, at least as the starting raw material, you get to an oxygen. So we're starting to fuse heavier and heavier elements. So what happens here is this helium is fusing into carbon and oxygen, so you start building a carbon and oxygen core. So I'm going to leave you there. I realize I'm already past my self-imposed limit of 10 minutes. But what I want you to think about is what is likely to happen. What is likely to happen here if this star will never have the mass to begin to fuse this carbon and oxygen?"}, {"video_title": "Cosmic background radiation 2 Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "In the last video, we learned that 380,000 years after the Big Bang, which is still roughly 13.7 billion years ago, every point, I shouldn't say every point, every atom in space that was kind of at this roughly 3,000 Kelvin temperature was emitting this electromagnetic radiation. Since every point in space was, there were points in space, or there was points in the universe, that that radiation is only just now reaching us. It has been traveling for 13.7 billion years. So when we look at radiation that's been traveling for that long, we can look in any direction and we'll see this uniform radiation. And that radiation has been redshifted into the microwave range from the higher frequencies that it was actually emitted at. Now, a question that might pop in your brain is, well, what happens if we wait a billion years? Because if we wait a billion years, if we have a billion 380,000 years after the beginning of the universe, this stuff won't just be atoms anymore."}, {"video_title": "Cosmic background radiation 2 Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So when we look at radiation that's been traveling for that long, we can look in any direction and we'll see this uniform radiation. And that radiation has been redshifted into the microwave range from the higher frequencies that it was actually emitted at. Now, a question that might pop in your brain is, well, what happens if we wait a billion years? Because if we wait a billion years, if we have a billion 380,000 years after the beginning of the universe, this stuff won't just be atoms anymore. It will have started to condense into actual stars. We would not, the universe at every point in space will no longer be this uniform. We'll actually start having condensation into stars."}, {"video_title": "Cosmic background radiation 2 Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Because if we wait a billion years, if we have a billion 380,000 years after the beginning of the universe, this stuff won't just be atoms anymore. It will have started to condense into actual stars. We would not, the universe at every point in space will no longer be this uniform. We'll actually start having condensation into stars. So if we go, the universe, if we move forward a little bit, the universe will expand. Maybe I'll just draw half of it since it's expanded. It's obviously expanded much more."}, {"video_title": "Cosmic background radiation 2 Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "We'll actually start having condensation into stars. So if we go, the universe, if we move forward a little bit, the universe will expand. Maybe I'll just draw half of it since it's expanded. It's obviously expanded much more. But now all of a sudden, we actually have stars. These are no longer just uniform atoms spread through the universe. We actually have condensation into stars."}, {"video_title": "Cosmic background radiation 2 Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It's obviously expanded much more. But now all of a sudden, we actually have stars. These are no longer just uniform atoms spread through the universe. We actually have condensation into stars. And so if you look at what is being emitted from the points in space from which we're only now getting this cosmic background radiation, if we wait a billion years, the light that we see from those points in space will not look like this uniform radiation. It'll start to look a little bit more like the more mature parts of the universe. It'll start to, we'll essentially be looking at the universe a billion years after the Big Bang when stars have formed, other structures have formed."}, {"video_title": "Cosmic background radiation 2 Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "We actually have condensation into stars. And so if you look at what is being emitted from the points in space from which we're only now getting this cosmic background radiation, if we wait a billion years, the light that we see from those points in space will not look like this uniform radiation. It'll start to look a little bit more like the more mature parts of the universe. It'll start to, we'll essentially be looking at the universe a billion years after the Big Bang when stars have formed, other structures have formed. So the question is, in a billion years, will this cosmic microwave background radiation disappear? And I'm using the billion just because that's just to arbitrarily use a number. But will it eventually disappear?"}, {"video_title": "Cosmic background radiation 2 Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It'll start to, we'll essentially be looking at the universe a billion years after the Big Bang when stars have formed, other structures have formed. So the question is, in a billion years, will this cosmic microwave background radiation disappear? And I'm using the billion just because that's just to arbitrarily use a number. But will it eventually disappear? And there's kind of, the answer to that is yes and no. So to think about it, it is true that this point in space will mature. It will mature in a billion years."}, {"video_title": "Cosmic background radiation 2 Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But will it eventually disappear? And there's kind of, the answer to that is yes and no. So to think about it, it is true that this point in space will mature. It will mature in a billion years. It will no longer be this uniform haze of hot hydrogen atoms. But what you have to think about is, there were further points in the universe. There were further points in the universe."}, {"video_title": "Cosmic background radiation 2 Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It will mature in a billion years. It will no longer be this uniform haze of hot hydrogen atoms. But what you have to think about is, there were further points in the universe. There were further points in the universe. At that same time, there were further points that were also emitting this radiation. And the original photons from those original points still haven't gotten to us. So from those further out points, right now the observable universe is, we can only see electromagnetic radiation that's been traveling for 13.7 billion years."}, {"video_title": "Cosmic background radiation 2 Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "There were further points in the universe. At that same time, there were further points that were also emitting this radiation. And the original photons from those original points still haven't gotten to us. So from those further out points, right now the observable universe is, we can only see electromagnetic radiation that's been traveling for 13.7 billion years. In another billion years, the universe will be a billion years older. And then there will be radiation that has been traveling for 14.7 billion years. And so we will start to observe that."}, {"video_title": "Cosmic background radiation 2 Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So from those further out points, right now the observable universe is, we can only see electromagnetic radiation that's been traveling for 13.7 billion years. In another billion years, the universe will be a billion years older. And then there will be radiation that has been traveling for 14.7 billion years. And so we will start to observe that. And we'll start to observe that radiation from the same time period in the universe. It will just be from further out. Now, what I want to make clear is that since those points were even further out, where that radiation was emitted, the stuff that we'll see in a billion years, it will be even more redshifted."}, {"video_title": "Cosmic background radiation 2 Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And so we will start to observe that. And we'll start to observe that radiation from the same time period in the universe. It will just be from further out. Now, what I want to make clear is that since those points were even further out, where that radiation was emitted, the stuff that we'll see in a billion years, it will be even more redshifted. So at that point, the cosmic background radiation we see will have longer wavelengths than the radio spectrum. It will be redder. And I should say redder, because we're already more."}, {"video_title": "Cosmic background radiation 2 Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Now, what I want to make clear is that since those points were even further out, where that radiation was emitted, the stuff that we'll see in a billion years, it will be even more redshifted. So at that point, the cosmic background radiation we see will have longer wavelengths than the radio spectrum. It will be redder. And I should say redder, because we're already more. Would redder have two D's? I've never written redder. Well, it would be more red than the microwave radiation."}, {"video_title": "Cosmic background radiation 2 Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And I should say redder, because we're already more. Would redder have two D's? I've never written redder. Well, it would be more red than the microwave radiation. And of course, that's a funny thing, because microwave radiation is already more red than actual visible red light. It has a longer wavelength. Now, this will keep happening."}, {"video_title": "Cosmic background radiation 2 Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Well, it would be more red than the microwave radiation. And of course, that's a funny thing, because microwave radiation is already more red than actual visible red light. It has a longer wavelength. Now, this will keep happening. And I don't know what, you know, it'll keep happening. We'll keep getting radiation as we go further and further into the future. We'll keep getting radiation from further out points in space."}, {"video_title": "Cosmic background radiation 2 Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Now, this will keep happening. And I don't know what, you know, it'll keep happening. We'll keep getting radiation as we go further and further into the future. We'll keep getting radiation from further out points in space. And it'll get more and more redshifted. The actual wavelengths of that electromagnetic light will be bigger and bigger and bigger, until we really aren't able to even see it as electromagnetic light, because it'll be redshifted to infinity. It'll have an infinite wavelength."}, {"video_title": "Cosmic background radiation 2 Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "We'll keep getting radiation from further out points in space. And it'll get more and more redshifted. The actual wavelengths of that electromagnetic light will be bigger and bigger and bigger, until we really aren't able to even see it as electromagnetic light, because it'll be redshifted to infinity. It'll have an infinite wavelength. And to make that point clear, I want to show you that there'll even be points, at some point, there'll be kind of a threshold where we can't even get radiation from further out. And let me show you. Let me draw a diagram of that."}, {"video_title": "Cosmic background radiation 2 Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It'll have an infinite wavelength. And to make that point clear, I want to show you that there'll even be points, at some point, there'll be kind of a threshold where we can't even get radiation from further out. And let me show you. Let me draw a diagram of that. So let's say that this is the universe 13.7 billion years ago, right when that radiation, what we now see as cosmic microwave background radiation, right when it started to be emitted. And let's say that this is the point in the universe where we are now. So this is us."}, {"video_title": "Cosmic background radiation 2 Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Let me draw a diagram of that. So let's say that this is the universe 13.7 billion years ago, right when that radiation, what we now see as cosmic microwave background radiation, right when it started to be emitted. And let's say that this is the point in the universe where we are now. So this is us. Let's say that this is the point in the universe that where we now observe the background radiation. Or this is one of the points. We obviously could form a circle around us."}, {"video_title": "Cosmic background radiation 2 Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So this is us. Let's say that this is the point in the universe that where we now observe the background radiation. Or this is one of the points. We obviously could form a circle around us. It could be any of these points over here. We're only where the photons, the electromagnetic radiation that were emitted from this point 380,000 years after the beginning of the universe, is only just now reaching us. So this is the point in the universe from which we are observing the cosmic background radiation."}, {"video_title": "Cosmic background radiation 2 Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "We obviously could form a circle around us. It could be any of these points over here. We're only where the photons, the electromagnetic radiation that were emitted from this point 380,000 years after the beginning of the universe, is only just now reaching us. So this is the point in the universe from which we are observing the cosmic background radiation. And let me be very clear. That point in the universe has now matured into things that look into stars and galaxies and planets. And if they were to look at our point in space, they are also going to see cosmic background radiation from us."}, {"video_title": "Cosmic background radiation 2 Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So this is the point in the universe from which we are observing the cosmic background radiation. And let me be very clear. That point in the universe has now matured into things that look into stars and galaxies and planets. And if they were to look at our point in space, they are also going to see cosmic background radiation from us. It's not like this is some type of permanently old place. It's just the light we're getting from them right now is old light, light that that point in space emitted way before it was able to mature into actual structures. So this is the point in space from which we are receiving cosmic background radiation right now."}, {"video_title": "Cosmic background radiation 2 Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And if they were to look at our point in space, they are also going to see cosmic background radiation from us. It's not like this is some type of permanently old place. It's just the light we're getting from them right now is old light, light that that point in space emitted way before it was able to mature into actual structures. So this is the point in space from which we are receiving cosmic background radiation right now. I don't want to write all that. It'll take me forever. Now, let's take another point in space that's whatever this distance is."}, {"video_title": "Cosmic background radiation 2 Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So this is the point in space from which we are receiving cosmic background radiation right now. I don't want to write all that. It'll take me forever. Now, let's take another point in space that's whatever this distance is. Well, it's actually estimated to be about now. It's estimated to be about 46 billion light years. At that time, when things were just beginning to be emitted, this was only about 36 million light years."}, {"video_title": "Cosmic background radiation 2 Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Now, let's take another point in space that's whatever this distance is. Well, it's actually estimated to be about now. It's estimated to be about 46 billion light years. At that time, when things were just beginning to be emitted, this was only about 36 million light years. And this is a very rough estimate. I shouldn't even write it down, because that's really based on how fast we assume the universe is expanding and all of that type of thing. But it was just a lot smaller than 46 billion light years."}, {"video_title": "Cosmic background radiation 2 Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "At that time, when things were just beginning to be emitted, this was only about 36 million light years. And this is a very rough estimate. I shouldn't even write it down, because that's really based on how fast we assume the universe is expanding and all of that type of thing. But it was just a lot smaller than 46 billion light years. Now, let's go that same distance again from this point in space. So let me make it clear. This is 380,000 years ago."}, {"video_title": "Cosmic background radiation 2 Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But it was just a lot smaller than 46 billion light years. Now, let's go that same distance again from this point in space. So let me make it clear. This is 380,000 years ago. Now, let's fast forward. Let's fast forward. Sorry, not 380,000 years ago."}, {"video_title": "Cosmic background radiation 2 Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "This is 380,000 years ago. Now, let's fast forward. Let's fast forward. Sorry, not 380,000 years ago. 380,000 years after the Big Bang, which is approximately, it's still 13.7 billion years ago. So that's then. Now, let's look at now."}, {"video_title": "Cosmic background radiation 2 Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Sorry, not 380,000 years ago. 380,000 years after the Big Bang, which is approximately, it's still 13.7 billion years ago. So that's then. Now, let's look at now. Now, I'll just draw it a little bit bigger. It's actually going to be much, much bigger now. Now, if we do it a little bit bigger, so when I draw it like this, this is where we are now."}, {"video_title": "Cosmic background radiation 2 Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Now, let's look at now. Now, I'll just draw it a little bit bigger. It's actually going to be much, much bigger now. Now, if we do it a little bit bigger, so when I draw it like this, this is where we are now. This point in space is this point in space from which we are only now receiving that cosmic background radiation is over here. And then this other point in space is going to be over here. And we saw in the video on the actual size of the observable universe, not just what it appears to be based on how long the light's been traveling."}, {"video_title": "Cosmic background radiation 2 Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Now, if we do it a little bit bigger, so when I draw it like this, this is where we are now. This point in space is this point in space from which we are only now receiving that cosmic background radiation is over here. And then this other point in space is going to be over here. And we saw in the video on the actual size of the observable universe, not just what it appears to be based on how long the light's been traveling. This is now on the order of 46 or 47 billion light years. And so this distance is also going to be 46 billion light years. Now, every point in space back then was emitting this radiation."}, {"video_title": "Cosmic background radiation 2 Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And we saw in the video on the actual size of the observable universe, not just what it appears to be based on how long the light's been traveling. This is now on the order of 46 or 47 billion light years. And so this distance is also going to be 46 billion light years. Now, every point in space back then was emitting this radiation. We have this uniform radiation. It was just hydrogen atoms everywhere, these hot hydrogen atoms. Now, maybe I should just do it in the color of the radiation."}, {"video_title": "Cosmic background radiation 2 Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Now, every point in space back then was emitting this radiation. We have this uniform radiation. It was just hydrogen atoms everywhere, these hot hydrogen atoms. Now, maybe I should just do it in the color of the radiation. So this guy's receiving. I'm just showing it's coming from this guy. We're only now, 13.7 billion years in the future, receiving photons from this guy."}, {"video_title": "Cosmic background radiation 2 Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Now, maybe I should just do it in the color of the radiation. So this guy's receiving. I'm just showing it's coming from this guy. We're only now, 13.7 billion years in the future, receiving photons from this guy. Only now are we receiving it. And frankly, this green guy only now is going to be receiving photons. When he looks at the points in space, or the things that he thinks are the points in space out there, he will see that uniform radiation."}, {"video_title": "Cosmic background radiation 2 Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "We're only now, 13.7 billion years in the future, receiving photons from this guy. Only now are we receiving it. And frankly, this green guy only now is going to be receiving photons. When he looks at the points in space, or the things that he thinks are the points in space out there, he will see that uniform radiation. And likewise, this guy over here will only now be receiving photons from the point in space for where we are now. He'll see the universe where we are now as it was 380,000 years after the Big Bang. And same thing from that point in space."}, {"video_title": "Cosmic background radiation 2 Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "When he looks at the points in space, or the things that he thinks are the points in space out there, he will see that uniform radiation. And likewise, this guy over here will only now be receiving photons from the point in space for where we are now. He'll see the universe where we are now as it was 380,000 years after the Big Bang. And same thing from that point in space. The photons will only just now reach. Now, let's think about it. It took this guy's photons."}, {"video_title": "Cosmic background radiation 2 Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And same thing from that point in space. The photons will only just now reach. Now, let's think about it. It took this guy's photons. Let me make it clear. It took him 13.7 billion years to reach this point over here, which is now 46 billion light years away from us. And the universe continues to expand."}, {"video_title": "Cosmic background radiation 2 Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It took this guy's photons. Let me make it clear. It took him 13.7 billion years to reach this point over here, which is now 46 billion light years away from us. And the universe continues to expand. Depending on if the universe expands fast enough, there's no way that that photon that got to this guy will eventually get to this. The universe is expanding faster that the light can never even catch up to us, and this light will never, ever, ever get to us. And so there is some threshold, some distance from which we will never get light during this time period, or actually from which we will never, ever get any electromagnetic radiation."}, {"video_title": "Cosmic background radiation 2 Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And the universe continues to expand. Depending on if the universe expands fast enough, there's no way that that photon that got to this guy will eventually get to this. The universe is expanding faster that the light can never even catch up to us, and this light will never, ever, ever get to us. And so there is some threshold, some distance from which we will never get light during this time period, or actually from which we will never, ever get any electromagnetic radiation. So the simple answer is the cosmic background radiation from this point, yes, it will start to mature. It won't be as uniform if we go fast forward 400 million years or a billion years, but we will get uniform radiation from further out, but it'll be even more redshifted. And the further forward we get into the future, the background radiation we get will be from further and further out, and it will be more and more and more redshifted until some point where it's going to be so redshifted that we won't even observe it as electromagnetic radiation."}, {"video_title": "Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "And she made, a little over 100 years ago, this is in the early 1900s, while working for Edward Charles Pickering, who was a Harvard astronomer, while working for his observatory, she made what is arguably, well, definitely one of the most important discoveries in all of astronomy. Probably, and I would say it ranks their top three, because it really enabled people like Hubble to start realizing that the universe is expanding. Or even being able to think about how to measure distances to objects in space well beyond the reach of our tools with parallax. We saw with parallax, you have to have extremely sensitive instruments just to even measure distances to stars relatively close to us. Very sensitive instruments to get to stars maybe further out into our galaxy. And we don't have the instruments even today to measure things beyond our galaxy. But because of Henrietta Swan Leavitt, we were able to approximate or get good senses of the distance to objects beyond our galaxy."}, {"video_title": "Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "We saw with parallax, you have to have extremely sensitive instruments just to even measure distances to stars relatively close to us. Very sensitive instruments to get to stars maybe further out into our galaxy. And we don't have the instruments even today to measure things beyond our galaxy. But because of Henrietta Swan Leavitt, we were able to approximate or get good senses of the distance to objects beyond our galaxy. So let's just think about what she did. So her job was literally to classify stars in the large Magellanic, I have trouble saying that, Magellanic cloud and the small Magellanic clouds. And this is what they look like from the southern hemisphere."}, {"video_title": "Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "But because of Henrietta Swan Leavitt, we were able to approximate or get good senses of the distance to objects beyond our galaxy. So let's just think about what she did. So her job was literally to classify stars in the large Magellanic, I have trouble saying that, Magellanic cloud and the small Magellanic clouds. And this is what they look like from the southern hemisphere. This is the large right over here. And this is the small right over here. And remember, this is before Hubble realized or showed the world that there are stars beyond our galaxy, that there are galaxies beyond our galaxy."}, {"video_title": "Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "And this is what they look like from the southern hemisphere. This is the large right over here. And this is the small right over here. And remember, this is before Hubble realized or showed the world that there are stars beyond our galaxy, that there are galaxies beyond our galaxy. So at this point in time, people didn't even fully appreciate that these were separate galaxies. We just said, hey, these are kind of these blobs or these clusters of stars that we see in the southern hemisphere. And just to get a sense of where they are relative to our galaxy, the Milky Way galaxy, this is obviously not an actual picture."}, {"video_title": "Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "And remember, this is before Hubble realized or showed the world that there are stars beyond our galaxy, that there are galaxies beyond our galaxy. So at this point in time, people didn't even fully appreciate that these were separate galaxies. We just said, hey, these are kind of these blobs or these clusters of stars that we see in the southern hemisphere. And just to get a sense of where they are relative to our galaxy, the Milky Way galaxy, this is obviously not an actual picture. We can't take a picture from this vantage point. This would have to be very, very far away. But this is the Milky Way right here."}, {"video_title": "Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "And just to get a sense of where they are relative to our galaxy, the Milky Way galaxy, this is obviously not an actual picture. We can't take a picture from this vantage point. This would have to be very, very far away. But this is the Milky Way right here. And this is the small Magellanic cloud. And this is the large Magellanic cloud. I'm getting better at saying it."}, {"video_title": "Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "But this is the Milky Way right here. And this is the small Magellanic cloud. And this is the large Magellanic cloud. I'm getting better at saying it. So her job was literally just to classify the different stars that she saw. But while she was classifying, she looked at these things called variables. It turns out what she was looking at were a class of stars called Cepheid or Cepheid variable stars."}, {"video_title": "Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "I'm getting better at saying it. So her job was literally just to classify the different stars that she saw. But while she was classifying, she looked at these things called variables. It turns out what she was looking at were a class of stars called Cepheid or Cepheid variable stars. And what's interesting about them is two things. They're super duper bright. They're up to 30,000 times as luminous as the sun."}, {"video_title": "Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "It turns out what she was looking at were a class of stars called Cepheid or Cepheid variable stars. And what's interesting about them is two things. They're super duper bright. They're up to 30,000 times as luminous as the sun. And they're 5 to 20 times more massive than the sun. 5 to 20 times the sun's mass. But what makes them interesting is one, they're really bright."}, {"video_title": "Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "They're up to 30,000 times as luminous as the sun. And they're 5 to 20 times more massive than the sun. 5 to 20 times the sun's mass. But what makes them interesting is one, they're really bright. So you can see them from really far away. You can see these Cepheid variable stars in other galaxies. In fact, we can see it well beyond even the small Magellanic cloud or the large Magellanic cloud."}, {"video_title": "Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "But what makes them interesting is one, they're really bright. So you can see them from really far away. You can see these Cepheid variable stars in other galaxies. In fact, we can see it well beyond even the small Magellanic cloud or the large Magellanic cloud. But you can see these stars in other galaxies. And what's even more interesting about them is that their intensity is variable, that they become brighter and dimmer with a well-defined period. So if you're looking at a Cepheid variable star, and this is just kind of a simulation, a very cheap simulation, it might look like this."}, {"video_title": "Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "In fact, we can see it well beyond even the small Magellanic cloud or the large Magellanic cloud. But you can see these stars in other galaxies. And what's even more interesting about them is that their intensity is variable, that they become brighter and dimmer with a well-defined period. So if you're looking at a Cepheid variable star, and this is just kind of a simulation, a very cheap simulation, it might look like this. And then over the course of the next three, four days, it might reduce in intensity to something like this. And then after three, four days again, it will look like this. And then it'll look like this again."}, {"video_title": "Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "So if you're looking at a Cepheid variable star, and this is just kind of a simulation, a very cheap simulation, it might look like this. And then over the course of the next three, four days, it might reduce in intensity to something like this. And then after three, four days again, it will look like this. And then it'll look like this again. So its actual intensity is going up and down with a well-defined period. So if this takes three days and this is another three days, then the period, one entire cycle of its going from low intensity back to high intensity is going to be six days. So this is a six-day period."}, {"video_title": "Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "And then it'll look like this again. So its actual intensity is going up and down with a well-defined period. So if this takes three days and this is another three days, then the period, one entire cycle of its going from low intensity back to high intensity is going to be six days. So this is a six-day period. And what Henrietta Leavitt saw, and this wasn't an obvious thing to do, she plotted, she assumed that things, everything in each of these clouds are roughly the same distance away. Everything in the large Magellanic cloud is roughly the same distance away. And it's obviously not exact."}, {"video_title": "Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "So this is a six-day period. And what Henrietta Leavitt saw, and this wasn't an obvious thing to do, she plotted, she assumed that things, everything in each of these clouds are roughly the same distance away. Everything in the large Magellanic cloud is roughly the same distance away. And it's obviously not exact. This is an entire galaxy, so you have obviously things further away in that galaxy and things closer up. You have stars here and here, and their distance isn't going to be exactly the same to us, that we're sitting maybe over here someplace. But it's going to be close."}, {"video_title": "Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "And it's obviously not exact. This is an entire galaxy, so you have obviously things further away in that galaxy and things closer up. You have stars here and here, and their distance isn't going to be exactly the same to us, that we're sitting maybe over here someplace. But it's going to be close. It wasn't a bad approximation. And by making that assumption, she saw something pretty neat. So let me plot this right over here."}, {"video_title": "Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "But it's going to be close. It wasn't a bad approximation. And by making that assumption, she saw something pretty neat. So let me plot this right over here. So she plotted on the horizontal axis, she plotted the relative luminosity. So really, the only way that she could measure this is just how bright did they look to her? And she's assuming that they're same distance."}, {"video_title": "Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "So let me plot this right over here. So she plotted on the horizontal axis, she plotted the relative luminosity. So really, the only way that she could measure this is just how bright did they look to her? And she's assuming that they're same distance. So obviously, if you have a brighter star, but it's much, much further away, it's going to look dimmer. So if you assume that they're all roughly the same distance, then how bright it is will tell you how bright it is at the actual star. So she plotted relative luminosity of a star on one axis, and on the other axis, she plotted the period of these variable stars."}, {"video_title": "Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "And she's assuming that they're same distance. So obviously, if you have a brighter star, but it's much, much further away, it's going to look dimmer. So if you assume that they're all roughly the same distance, then how bright it is will tell you how bright it is at the actual star. So she plotted relative luminosity of a star on one axis, and on the other axis, she plotted the period of these variable stars. And what I'm going to do is I'm going to do this on a logarithmic scale. So let's say that this is in days. So this is one day, this is 10 days, this is 100 days, right over here."}, {"video_title": "Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "So she plotted relative luminosity of a star on one axis, and on the other axis, she plotted the period of these variable stars. And what I'm going to do is I'm going to do this on a logarithmic scale. So let's say that this is in days. So this is one day, this is 10 days, this is 100 days, right over here. It's a logarithmic scale because I'm going up in powers of 10. I could say that if we take the log of these, this would be 0, this would be 1, this would be 2. And so that's what I'm using as a scale."}, {"video_title": "Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "So this is one day, this is 10 days, this is 100 days, right over here. It's a logarithmic scale because I'm going up in powers of 10. I could say that if we take the log of these, this would be 0, this would be 1, this would be 2. And so that's what I'm using as a scale. So I'm using the log of the period, or I'm just marking them as 1, 10, 100, but I'm giving each of these factors of 10 an equal spacing. When you plot it on this scale, the relative luminosity versus the period, she got a plot that looks something like this. This is obviously not exact."}, {"video_title": "Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "And so that's what I'm using as a scale. So I'm using the log of the period, or I'm just marking them as 1, 10, 100, but I'm giving each of these factors of 10 an equal spacing. When you plot it on this scale, the relative luminosity versus the period, she got a plot that looks something like this. This is obviously not exact. She got a plot that looks something like this. It was a fairly linear relationship when you plot the relative luminosity against the log of the period. So this is obviously a logarithmic scale over here."}, {"video_title": "Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "This is obviously not exact. She got a plot that looks something like this. It was a fairly linear relationship when you plot the relative luminosity against the log of the period. So this is obviously a logarithmic scale over here. And so you could fit a line. And why I'd argue, and I think most people would argue, this is one of the most important discoveries in astronomy, is if you know, because think about what the problem here is. We can look at all of these stars in space."}, {"video_title": "Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "So this is obviously a logarithmic scale over here. And so you could fit a line. And why I'd argue, and I think most people would argue, this is one of the most important discoveries in astronomy, is if you know, because think about what the problem here is. We can look at all of these stars in space. Let's say you look at a fraction of the sky and you look at something that looks like that. So it's really bright. And then you see something dim that looks like that."}, {"video_title": "Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "We can look at all of these stars in space. Let's say you look at a fraction of the sky and you look at something that looks like that. So it's really bright. And then you see something dim that looks like that. So if you have a very superficial understanding, you say, oh, this star is brighter. You would say that this is a fundamentally brighter star. But how do you know that?"}, {"video_title": "Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "And then you see something dim that looks like that. So if you have a very superficial understanding, you say, oh, this star is brighter. You would say that this is a fundamentally brighter star. But how do you know that? Maybe instead of being brighter, maybe it's just a dimmer, closer star. Maybe this is an entire galaxy, but it's so far away that you can't even tell. But all of a sudden, by the work that Henrietta Leavitt did, if you see one of these Cepheid variable stars in another galaxy, you know its relative brightness compared to other Cepheid variable stars."}, {"video_title": "Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "But how do you know that? Maybe instead of being brighter, maybe it's just a dimmer, closer star. Maybe this is an entire galaxy, but it's so far away that you can't even tell. But all of a sudden, by the work that Henrietta Leavitt did, if you see one of these Cepheid variable stars in another galaxy, you know its relative brightness compared to other Cepheid variable stars. And so if you can place just one of these Cepheid variable stars, if you know exactly the distance to one of them, and then you know its absolute luminosity, you then know the absolute luminosity of any other Cepheid variable stars. So let's say using parallax, which is our other tool, we find, let's say there's some star in our galaxy. And let's say using parallax, we're able to come up with a pretty good measure that it is, I don't know, let's say it's 100 light years away."}, {"video_title": "Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "But all of a sudden, by the work that Henrietta Leavitt did, if you see one of these Cepheid variable stars in another galaxy, you know its relative brightness compared to other Cepheid variable stars. And so if you can place just one of these Cepheid variable stars, if you know exactly the distance to one of them, and then you know its absolute luminosity, you then know the absolute luminosity of any other Cepheid variable stars. So let's say using parallax, which is our other tool, we find, let's say there's some star in our galaxy. And let's say using parallax, we're able to come up with a pretty good measure that it is, I don't know, let's say it's 100 light years away. And this star is a Cepheid variable star. And let's say its period is one day. So we now know something interesting."}, {"video_title": "Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "And let's say using parallax, we're able to come up with a pretty good measure that it is, I don't know, let's say it's 100 light years away. And this star is a Cepheid variable star. And let's say its period is one day. So we now know something interesting. We know variable stars with a period of one day at 100 light years away will look like this. Will look like this drawing right over here. So if we later on see a Cepheid variable star with a period of one day, so it gets brighter and dim over the course of one day, and maybe it's redshifted as well, but maybe it looks a little bit dimmer, it looks like this."}, {"video_title": "Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "So we now know something interesting. We know variable stars with a period of one day at 100 light years away will look like this. Will look like this drawing right over here. So if we later on see a Cepheid variable star with a period of one day, so it gets brighter and dim over the course of one day, and maybe it's redshifted as well, but maybe it looks a little bit dimmer, it looks like this. We now know that if it was 100 light years away, it would have this luminosity. So based on how much dimmer it is, we can then figure out how much further away this Cepheid variable star is. If that confuses you a little bit, I'll do a little bit more details in the next few videos so we can get a closer sense of how the math would work."}, {"video_title": "Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "So if we later on see a Cepheid variable star with a period of one day, so it gets brighter and dim over the course of one day, and maybe it's redshifted as well, but maybe it looks a little bit dimmer, it looks like this. We now know that if it was 100 light years away, it would have this luminosity. So based on how much dimmer it is, we can then figure out how much further away this Cepheid variable star is. If that confuses you a little bit, I'll do a little bit more details in the next few videos so we can get a closer sense of how the math would work. But this was a big discovery, just discovering this class of stars, this Cepheid variable class. She wasn't the one who discovered them. People knew before her that there were these stars that got brighter and dimmer."}, {"video_title": "Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "If that confuses you a little bit, I'll do a little bit more details in the next few videos so we can get a closer sense of how the math would work. But this was a big discovery, just discovering this class of stars, this Cepheid variable class. She wasn't the one who discovered them. People knew before her that there were these stars that got brighter and dimmer. But what her big discovery was is seeing this linear relationship between the relative luminosity of these stars and their period. Because then, if we see Cepheid variable stars in completely different galaxies or galactic clusters, by looking at their period, we know what their real relative luminosity is. And then we could guess how far those things really are."}, {"video_title": "Red shift Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "I'm going to do two scenarios. So I'm an observer over here. This is me. And then maybe even better, I should just draw my eyeball because we're going to be observing light. So I'm just going to draw my eyeball. So this is me in the first scenario, or this is one of my eyeballs, and then this is one of my eyeballs in the second scenario. Now in the first scenario, so in both scenarios we're going to have an object."}, {"video_title": "Red shift Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And then maybe even better, I should just draw my eyeball because we're going to be observing light. So I'm just going to draw my eyeball. So this is me in the first scenario, or this is one of my eyeballs, and then this is one of my eyeballs in the second scenario. Now in the first scenario, so in both scenarios we're going to have an object. We're going to have some type of source of light. But in the first scenario, relative to me, the source of light will not be moving. While in the second scenario, the source of light, just for the sake of discussion, just for fun, will be moving at half the speed of light."}, {"video_title": "Red shift Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Now in the first scenario, so in both scenarios we're going to have an object. We're going to have some type of source of light. But in the first scenario, relative to me, the source of light will not be moving. While in the second scenario, the source of light, just for the sake of discussion, just for fun, will be moving at half the speed of light. Unimaginably fast speed, but let's just assuming it is. So it's moving at, it has a velocity of 1 half the speed of light, 1 half light speed away from me, who is the observer. Now let's just imagine what would happen."}, {"video_title": "Red shift Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "While in the second scenario, the source of light, just for the sake of discussion, just for fun, will be moving at half the speed of light. Unimaginably fast speed, but let's just assuming it is. So it's moving at, it has a velocity of 1 half the speed of light, 1 half light speed away from me, who is the observer. Now let's just imagine what would happen. They're both emitting light. So they're both going to start emitting light at the exact same time. And when they start emitting light, they're both at the exact same distance from my eye."}, {"video_title": "Red shift Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Now let's just imagine what would happen. They're both emitting light. So they're both going to start emitting light at the exact same time. And when they start emitting light, they're both at the exact same distance from my eye. The only difference is that this is stationary, relative to me, while this is moving away from me at half the speed of light. So let's say that after some period of time, the light wave from this source reaches my eye. And then it looks something like this."}, {"video_title": "Red shift Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And when they start emitting light, they're both at the exact same distance from my eye. The only difference is that this is stationary, relative to me, while this is moving away from me at half the speed of light. So let's say that after some period of time, the light wave from this source reaches my eye. And then it looks something like this. I'll try my best to draw it. So let's say I have, I want to draw a couple of wavelengths here. So let's say that's half a wavelength."}, {"video_title": "Red shift Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And then it looks something like this. I'll try my best to draw it. So let's say I have, I want to draw a couple of wavelengths here. So let's say that's half a wavelength. That's a full wavelength. That's another half a full wavelength. Another half full wavelength."}, {"video_title": "Red shift Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So let's say that's half a wavelength. That's a full wavelength. That's another half a full wavelength. Another half full wavelength. And then a half, and then a full wavelength. So let me see if I can draw that. So it would look like full wavelength."}, {"video_title": "Red shift Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Another half full wavelength. And then a half, and then a full wavelength. So let me see if I can draw that. So it would look like full wavelength. This is not easy to do. And then you get another full wavelength. So it would look something like that, the actual waveform."}, {"video_title": "Red shift Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So it would look like full wavelength. This is not easy to do. And then you get another full wavelength. So it would look something like that, the actual waveform. And so this is just the front of the waveform is just getting to my eye. And then as the waveforms keep going past my eye, my eye will perceive some type of a wavelength or frequency and perceive it to be some type of color, assuming that we're in the visible part of the electromagnetic spectrum. Now let's think about what's going to happen with this source."}, {"video_title": "Red shift Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So it would look something like that, the actual waveform. And so this is just the front of the waveform is just getting to my eye. And then as the waveforms keep going past my eye, my eye will perceive some type of a wavelength or frequency and perceive it to be some type of color, assuming that we're in the visible part of the electromagnetic spectrum. Now let's think about what's going to happen with this source. So the first thing is that the front of the waveform is going to reach me at the exact same time. One of those neat and amazing things about light traveling in general, or especially in a vacuum, it doesn't matter that this is moving away from me at half the speed of light. The light will still move towards me at the speed of light."}, {"video_title": "Red shift Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Now let's think about what's going to happen with this source. So the first thing is that the front of the waveform is going to reach me at the exact same time. One of those neat and amazing things about light traveling in general, or especially in a vacuum, it doesn't matter that this is moving away from me at half the speed of light. The light will still move towards me at the speed of light. It's absolute. Doesn't matter if this is going away at 0.9 the speed of light, the light will still travel to me at the speed of light. And it's very unintuitive."}, {"video_title": "Red shift Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "The light will still move towards me at the speed of light. It's absolute. Doesn't matter if this is going away at 0.9 the speed of light, the light will still travel to me at the speed of light. And it's very unintuitive. Because in our everyday sense, if I'm moving away from you at half the speed of a bullet and I shoot a bullet, the bullet will only move towards you at that half of its velocity will be subtracted, and it'll only move towards me at half of its normal velocity relative to whether it was stationary. But not the case with light. So with that out of the way, let's think about what the waveform would look like."}, {"video_title": "Red shift Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And it's very unintuitive. Because in our everyday sense, if I'm moving away from you at half the speed of a bullet and I shoot a bullet, the bullet will only move towards you at that half of its velocity will be subtracted, and it'll only move towards me at half of its normal velocity relative to whether it was stationary. But not the case with light. So with that out of the way, let's think about what the waveform would look like. So by the time the light reached here, we need to think, let me actually redraw this over here. Let me redraw this eyeball right over here. So this is me again."}, {"video_title": "Red shift Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So with that out of the way, let's think about what the waveform would look like. So by the time the light reached here, we need to think, let me actually redraw this over here. Let me redraw this eyeball right over here. So this is me again. So by the time the light reaches my eye, so they both started emitting the light at the exact same time. This guy has traveled half this distance. If it took light a certain amount of time to get this far, this guy will get half as far in that same amount of time."}, {"video_title": "Red shift Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So this is me again. So by the time the light reaches my eye, so they both started emitting the light at the exact same time. This guy has traveled half this distance. If it took light a certain amount of time to get this far, this guy will get half as far in that same amount of time. So by the time the light reaches my eye, this guy will have traveled about half that distance. So he would have traveled about that far. But they started emitting the light at the same time."}, {"video_title": "Red shift Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "If it took light a certain amount of time to get this far, this guy will get half as far in that same amount of time. So by the time the light reaches my eye, this guy will have traveled about half that distance. So he would have traveled about that far. But they started emitting the light at the same time. So that very first photon, if you view light as a particle, will reach my eye at the very same time as the very first photon from this guy. So the waveform is going to essentially be stretched. So we're still going to have 1, 2, 3, 4 full wavelengths."}, {"video_title": "Red shift Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But they started emitting the light at the same time. So that very first photon, if you view light as a particle, will reach my eye at the very same time as the very first photon from this guy. So the waveform is going to essentially be stretched. So we're still going to have 1, 2, 3, 4 full wavelengths. But they'll now be stretched. Let me see if I can draw 4 full wavelengths. So let me cut this in half over here."}, {"video_title": "Red shift Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So we're still going to have 1, 2, 3, 4 full wavelengths. But they'll now be stretched. Let me see if I can draw 4 full wavelengths. So let me cut this in half over here. Let me cut each of those in half. So each of these are going to be a full wavelength. And then they're going to have a half wavelength in between."}, {"video_title": "Red shift Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So let me cut this in half over here. Let me cut each of those in half. So each of these are going to be a full wavelength. And then they're going to have a half wavelength in between. And so the waveform is going to look like this. Let me try my best to draw it. This is the hardest part, drawing this stretched out waveform."}, {"video_title": "Red shift Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And then they're going to have a half wavelength in between. And so the waveform is going to look like this. Let me try my best to draw it. This is the hardest part, drawing this stretched out waveform. And there you go. It's going to look like this. And so when it gets to my eye, my eye is going to perceive it as having a longer wavelength, even though from the perspective of each of these objects, if you're traveling with each of them, the frequency and the wavelength of the light emitted is the same."}, {"video_title": "Red shift Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "This is the hardest part, drawing this stretched out waveform. And there you go. It's going to look like this. And so when it gets to my eye, my eye is going to perceive it as having a longer wavelength, even though from the perspective of each of these objects, if you're traveling with each of them, the frequency and the wavelength of the light emitted is the same. The only difference is this guy is moving away from me, or I'm moving away from it, depending on how you want to view it, while I am stationary, or it is stationary, while in this first case, the observer and the source are both stationary. Now in this situation, what's my eye going to say? Well, my eye will get each of these successive pulses, or each of these successive wavetrains."}, {"video_title": "Red shift Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And so when it gets to my eye, my eye is going to perceive it as having a longer wavelength, even though from the perspective of each of these objects, if you're traveling with each of them, the frequency and the wavelength of the light emitted is the same. The only difference is this guy is moving away from me, or I'm moving away from it, depending on how you want to view it, while I am stationary, or it is stationary, while in this first case, the observer and the source are both stationary. Now in this situation, what's my eye going to say? Well, my eye will get each of these successive pulses, or each of these successive wavetrains. And it's going to say, hey, there's a longer wavelength a perceived longer wavelength. Let me write that. And also a perceived lower frequency."}, {"video_title": "Red shift Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Well, my eye will get each of these successive pulses, or each of these successive wavetrains. And it's going to say, hey, there's a longer wavelength a perceived longer wavelength. Let me write that. And also a perceived lower frequency. So what would that do to the perception of the light? Let's say that this is green light. So if we're stationary with the observer, it would be green light."}, {"video_title": "Red shift Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And also a perceived lower frequency. So what would that do to the perception of the light? Let's say that this is green light. So if we're stationary with the observer, it would be green light. So let's look at the electromagnetic spectrum. I got this off of Wikipedia. So if I was stationary with the observer, we'd be in the green light part of the spectrum."}, {"video_title": "Red shift Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So if we're stationary with the observer, it would be green light. So let's look at the electromagnetic spectrum. I got this off of Wikipedia. So if I was stationary with the observer, we'd be in the green light part of the spectrum. So a 500 nanometer wavelength. But if all of a sudden, because the object is moving away from me at this huge velocity, the perceived wavelength becomes wider. So from my perception, it's going to have a wider wavelength."}, {"video_title": "Red shift Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So if I was stationary with the observer, we'd be in the green light part of the spectrum. So a 500 nanometer wavelength. But if all of a sudden, because the object is moving away from me at this huge velocity, the perceived wavelength becomes wider. So from my perception, it's going to have a wider wavelength. And you can see what's happening. It will look redder. It will move towards the red part of the spectrum."}, {"video_title": "Red shift Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So from my perception, it's going to have a wider wavelength. And you can see what's happening. It will look redder. It will move towards the red part of the spectrum. And this phenomenon is called redshift. And I've done a bunch of videos in the physics playlist on the Doppler effect. And over there, I talk about sound waves and the perceived frequency of sound as something travels towards you versus away from you."}, {"video_title": "Red shift Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It will move towards the red part of the spectrum. And this phenomenon is called redshift. And I've done a bunch of videos in the physics playlist on the Doppler effect. And over there, I talk about sound waves and the perceived frequency of sound as something travels towards you versus away from you. It's the exact same idea. This is the Doppler effect applied to light. And the reason why the Doppler effect works for light traveling through space and for sound traveling through air is because a sound wave in air, regardless of whether the source is moving away or towards you, the sound wave is going to move at the speed of sound in air at a certain pressure and all of that."}, {"video_title": "Red shift Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And over there, I talk about sound waves and the perceived frequency of sound as something travels towards you versus away from you. It's the exact same idea. This is the Doppler effect applied to light. And the reason why the Doppler effect works for light traveling through space and for sound traveling through air is because a sound wave in air, regardless of whether the source is moving away or towards you, the sound wave is going to move at the speed of sound in air at a certain pressure and all of that. And light is the same thing. But in a vacuum, regardless of what the source is doing, the actual light wave itself will always travel at the same velocity. The only difference is that its perceived frequency and wavelength will change."}, {"video_title": "Red shift Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And the reason why the Doppler effect works for light traveling through space and for sound traveling through air is because a sound wave in air, regardless of whether the source is moving away or towards you, the sound wave is going to move at the speed of sound in air at a certain pressure and all of that. And light is the same thing. But in a vacuum, regardless of what the source is doing, the actual light wave itself will always travel at the same velocity. The only difference is that its perceived frequency and wavelength will change. And now the whole reason why I'm talking about this is you can use this property of light, that it gets redshift, to see whether things are traveling away or towards you. And people talk about redshift because frankly, most things are traveling away from us. And that's one of the reasons why we tend to believe in the Big Bang."}, {"video_title": "Red shift Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "The only difference is that its perceived frequency and wavelength will change. And now the whole reason why I'm talking about this is you can use this property of light, that it gets redshift, to see whether things are traveling away or towards you. And people talk about redshift because frankly, most things are traveling away from us. And that's one of the reasons why we tend to believe in the Big Bang. The opposite, if something is traveling towards me at super high velocities, then we would have something called, and you don't hear the word, it would be violetshift. The frequency would increase. So it would look bluer or more purple."}, {"video_title": "Red shift Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And that's one of the reasons why we tend to believe in the Big Bang. The opposite, if something is traveling towards me at super high velocities, then we would have something called, and you don't hear the word, it would be violetshift. The frequency would increase. So it would look bluer or more purple. Now the other thing I want to highlight is this redshift phenomena, this idea, it doesn't apply only to visible light. So it could even apply to things that we can't even see. So it would become redder, but it's not like you can even see."}, {"video_title": "Red shift Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So it would look bluer or more purple. Now the other thing I want to highlight is this redshift phenomena, this idea, it doesn't apply only to visible light. So it could even apply to things that we can't even see. So it would become redder, but it's not like you can even see. It could even apply to things that are even more red than red. So maybe it's a microwave that is being emitted, but because the source is moving away from us so fast, it could be perceived as an actual radio wave. And actually, I should have talked about this in the video on the microwave background radiation, is that we're perceiving it as microwaves."}, {"video_title": "Red shift Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So it would become redder, but it's not like you can even see. It could even apply to things that are even more red than red. So maybe it's a microwave that is being emitted, but because the source is moving away from us so fast, it could be perceived as an actual radio wave. And actually, I should have talked about this in the video on the microwave background radiation, is that we're perceiving it as microwaves. But the sources were moving away from us. They were being redshift. So they were not actually emitting microwave radiation."}, {"video_title": "Red shift Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And actually, I should have talked about this in the video on the microwave background radiation, is that we're perceiving it as microwaves. But the sources were moving away from us. They were being redshift. So they were not actually emitting microwave radiation. Just what we observe, and this is actually what would be predicted based on the Big Bang, is actually microwave radiation. So anyway, hopefully that gives you a sense of what redshift is, and now we can use this tool to explain why we think many, many things are moving away from us. And now let me just actually make sure you get that idea."}, {"video_title": "Red shift Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So they were not actually emitting microwave radiation. Just what we observe, and this is actually what would be predicted based on the Big Bang, is actually microwave radiation. So anyway, hopefully that gives you a sense of what redshift is, and now we can use this tool to explain why we think many, many things are moving away from us. And now let me just actually make sure you get that idea. If I have two objects, let's say that these are suns, or both galaxies, either way, and because of other properties, and I won't talk about them right now, we know that they are probably emitting light of the same color. Because we know other properties of that star or of that galaxy. Now, if what we actually perceive is that this one looks redder to us than this one, then we know it is traveling away from us."}, {"video_title": "Seasons aren't dictated by closeness to sun Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And what I want to do in this video is show you why that isn't the case. So the line of reasoning would go something like this. This is the sun at the center of our solar system and roughly at the center of Earth's orbit. And let me draw Earth's orbit over here. And so the line of reasoning is that there are certain points in Earth's orbit where we're closer to the sun. And let me draw a better job than that. So let's say this is the point where we're closer to the sun."}, {"video_title": "Seasons aren't dictated by closeness to sun Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And let me draw Earth's orbit over here. And so the line of reasoning is that there are certain points in Earth's orbit where we're closer to the sun. And let me draw a better job than that. So let's say this is the point where we're closer to the sun. We get a little further, then we get a lot further, and then we get a little bit closer, and then we get a little bit closer, and then this is the closest point. So maybe Earth's orbit looks something like this. So the argument would go, look, there are points in Earth's orbit where we're closer to the sun and points where we are further from the sun."}, {"video_title": "Seasons aren't dictated by closeness to sun Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So let's say this is the point where we're closer to the sun. We get a little further, then we get a lot further, and then we get a little bit closer, and then we get a little bit closer, and then this is the closest point. So maybe Earth's orbit looks something like this. So the argument would go, look, there are points in Earth's orbit where we're closer to the sun and points where we are further from the sun. And actually that part of the argument is true. Earth's orbit is not a perfect circle, and there are points in Earth's orbit where we are closer or further away from the sun. And actually when we are closest to the sun, so if Earth is right over here, there's a word for that."}, {"video_title": "Seasons aren't dictated by closeness to sun Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So the argument would go, look, there are points in Earth's orbit where we're closer to the sun and points where we are further from the sun. And actually that part of the argument is true. Earth's orbit is not a perfect circle, and there are points in Earth's orbit where we are closer or further away from the sun. And actually when we are closest to the sun, so if Earth is right over here, there's a word for that. It's called perihelion. It just means the closest point in orbit. Perihelion."}, {"video_title": "Seasons aren't dictated by closeness to sun Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And actually when we are closest to the sun, so if Earth is right over here, there's a word for that. It's called perihelion. It just means the closest point in orbit. Perihelion. Closest point in orbit to the sun. And there is a furthest point from the sun, and this is called aphelion or aphelion. I've sometimes seen it called aphelion, pronounced aphelion."}, {"video_title": "Seasons aren't dictated by closeness to sun Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Perihelion. Closest point in orbit to the sun. And there is a furthest point from the sun, and this is called aphelion or aphelion. I've sometimes seen it called aphelion, pronounced aphelion. Aphelion. So it is true that Earth's orbit is not a perfect circle around the sun, although it's pretty close, but it's not a perfect circle. It has a slightly elliptical shape, and because of that, there are times in the year where we are closest to the sun, and there are times in the year where we are furthest to the sun."}, {"video_title": "Seasons aren't dictated by closeness to sun Cosmology & Astronomy Khan Academy.mp3", "Sentence": "I've sometimes seen it called aphelion, pronounced aphelion. Aphelion. So it is true that Earth's orbit is not a perfect circle around the sun, although it's pretty close, but it's not a perfect circle. It has a slightly elliptical shape, and because of that, there are times in the year where we are closest to the sun, and there are times in the year where we are furthest to the sun. And the difference is about 3%, so it's not a huge difference in distance. I've really exaggerated the difference in this diagram right over here. But based on this reasoning, people would say, and this is the flawed part, that when we're close to the sun, this must be the summer."}, {"video_title": "Seasons aren't dictated by closeness to sun Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It has a slightly elliptical shape, and because of that, there are times in the year where we are closest to the sun, and there are times in the year where we are furthest to the sun. And the difference is about 3%, so it's not a huge difference in distance. I've really exaggerated the difference in this diagram right over here. But based on this reasoning, people would say, and this is the flawed part, that when we're close to the sun, this must be the summer. And when we are furthest away from the sun, this must be the winter. And the most obvious point of evidence why this is not the case is that when it is summer at one point in the planet, it is not summer throughout the planet at that time. In particular, when it is summer in the northern hemisphere, it is winter in the southern hemisphere."}, {"video_title": "Seasons aren't dictated by closeness to sun Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But based on this reasoning, people would say, and this is the flawed part, that when we're close to the sun, this must be the summer. And when we are furthest away from the sun, this must be the winter. And the most obvious point of evidence why this is not the case is that when it is summer at one point in the planet, it is not summer throughout the planet at that time. In particular, when it is summer in the northern hemisphere, it is winter in the southern hemisphere. And when it is summer in the southern hemisphere, it is winter in the northern hemisphere. So the entire planet does not experience the seasons at the same time. So that's probably, I guess you could say, the biggest point of data that we observe on our planet, why this by itself cannot explain the change in seasons."}, {"video_title": "Seasons aren't dictated by closeness to sun Cosmology & Astronomy Khan Academy.mp3", "Sentence": "In particular, when it is summer in the northern hemisphere, it is winter in the southern hemisphere. And when it is summer in the southern hemisphere, it is winter in the northern hemisphere. So the entire planet does not experience the seasons at the same time. So that's probably, I guess you could say, the biggest point of data that we observe on our planet, why this by itself cannot explain the change in seasons. And in particular, it really goes against what we experience in the northern hemisphere because our perihelion right now is occurring in January. It is occurring during the winter, the northern hemisphere winter. Perihelion right now is during the northern hemisphere winter."}, {"video_title": "Seasons aren't dictated by closeness to sun Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So that's probably, I guess you could say, the biggest point of data that we observe on our planet, why this by itself cannot explain the change in seasons. And in particular, it really goes against what we experience in the northern hemisphere because our perihelion right now is occurring in January. It is occurring during the winter, the northern hemisphere winter. Perihelion right now is during the northern hemisphere winter. And when we are furthest away from the sun, this is actually the northern hemisphere summer. Northern hemisphere summer. So although it might seem like a fairly intuitive idea, hey, if we are close to the sun, the whole planet is getting warmer, maybe that is summer."}, {"video_title": "Seasons aren't dictated by closeness to sun Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Perihelion right now is during the northern hemisphere winter. And when we are furthest away from the sun, this is actually the northern hemisphere summer. Northern hemisphere summer. So although it might seem like a fairly intuitive idea, hey, if we are close to the sun, the whole planet is getting warmer, maybe that is summer. When we are further away, the whole planet is getting a little less energy, that is winter. The evidence we see on earth goes directly against that. In particular, we don't have the same seasons in both the northern and southern hemispheres at the same time."}, {"video_title": "Seasons aren't dictated by closeness to sun Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So although it might seem like a fairly intuitive idea, hey, if we are close to the sun, the whole planet is getting warmer, maybe that is summer. When we are further away, the whole planet is getting a little less energy, that is winter. The evidence we see on earth goes directly against that. In particular, we don't have the same seasons in both the northern and southern hemispheres at the same time. And in particular, in the northern hemisphere, when we are closest to the sun, it is actually in January. It is actually in the middle of winter. So I will leave you there in this video."}, {"video_title": "Seasons aren't dictated by closeness to sun Cosmology & Astronomy Khan Academy.mp3", "Sentence": "In particular, we don't have the same seasons in both the northern and southern hemispheres at the same time. And in particular, in the northern hemisphere, when we are closest to the sun, it is actually in January. It is actually in the middle of winter. So I will leave you there in this video. I have left you just saying, okay, so the closeness to the sun does not dictate what season we are in. It is just saying, what is the reason? And what we will see in the next video, the reason is the tilt of the axis of the earth, the rotational axis of the earth."}, {"video_title": "Energy inputs for tilling a hectare of land Cosmology & Astronomy Khan Academy.mp3", "Sentence": "I don't want you to think that the most that a square kilometer can support today is exactly 1,000 people. It depends hugely on what the land is like, how much of the land you're actually using for agriculture, what crops you're planting, et cetera, et cetera, how the people are living, how many calories they need. The whole point of the last video was just to give you a framework that, wow, there is this upper bound based on how much productivity you actually get from the land. Now, what we want to think about in this video, that was kind of the last video was this axis right here, getting more and more out of the land. What I want to think about in this video is how did humans, through different technologies, how did we get by doing less and less of the labor for getting those calories out of the land? Obviously, you just don't have land and things spontaneously grow. Well, I guess that would happen in the wild."}, {"video_title": "Energy inputs for tilling a hectare of land Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Now, what we want to think about in this video, that was kind of the last video was this axis right here, getting more and more out of the land. What I want to think about in this video is how did humans, through different technologies, how did we get by doing less and less of the labor for getting those calories out of the land? Obviously, you just don't have land and things spontaneously grow. Well, I guess that would happen in the wild. But if you're doing agriculture, you need to put some energy into the land. You've got to work the land. And so what we have over here in this chart, and this chart is derived from information from this book right over here."}, {"video_title": "Energy inputs for tilling a hectare of land Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Well, I guess that would happen in the wild. But if you're doing agriculture, you need to put some energy into the land. You've got to work the land. And so what we have over here in this chart, and this chart is derived from information from this book right over here. This is Energy and Society. And what we want to think about here is the different ways that humans have gone about to till soil. So we're not even going to think about the total process or the total energy required to grow a certain crop."}, {"video_title": "Energy inputs for tilling a hectare of land Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And so what we have over here in this chart, and this chart is derived from information from this book right over here. This is Energy and Society. And what we want to think about here is the different ways that humans have gone about to till soil. So we're not even going to think about the total process or the total energy required to grow a certain crop. What we're just going to focus is one step of the agricultural process, and that is tilling the soil. And in case you're like me and you have never worked on a farm, which that's one thing I would like to change one day, is actually go through that process. But tilling the soil is kind of churning it up."}, {"video_title": "Energy inputs for tilling a hectare of land Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So we're not even going to think about the total process or the total energy required to grow a certain crop. What we're just going to focus is one step of the agricultural process, and that is tilling the soil. And in case you're like me and you have never worked on a farm, which that's one thing I would like to change one day, is actually go through that process. But tilling the soil is kind of churning it up. So you get the nutrients from the bottom layers to the surface. You bury all of the remains from the last harvest. You bury all of the weeds so that they die."}, {"video_title": "Energy inputs for tilling a hectare of land Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But tilling the soil is kind of churning it up. So you get the nutrients from the bottom layers to the surface. You bury all of the remains from the last harvest. You bury all of the weeds so that they die. And you also kind of get air in the soil. What it does is essentially it prepares the soil for the next agricultural cycle. So it's a process that humans have been doing since antiquity."}, {"video_title": "Energy inputs for tilling a hectare of land Cosmology & Astronomy Khan Academy.mp3", "Sentence": "You bury all of the weeds so that they die. And you also kind of get air in the soil. What it does is essentially it prepares the soil for the next agricultural cycle. So it's a process that humans have been doing since antiquity. And what I want to do in this video is think about the different ways to do it and how much energy is required to do it. And we're going to think about the energy in two ways. How much of that energy comes from humans and how much of that energy comes from things other than humans."}, {"video_title": "Energy inputs for tilling a hectare of land Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So it's a process that humans have been doing since antiquity. And what I want to do in this video is think about the different ways to do it and how much energy is required to do it. And we're going to think about the energy in two ways. How much of that energy comes from humans and how much of that energy comes from things other than humans. So just as an example, when we talk about human power, we're talking about someone literally hand plowing this field. So this woman right over here is literally she has this little cart that's digging up the soil behind her. When we talk about oxen power, we're talking about the oxen doing most of the work."}, {"video_title": "Energy inputs for tilling a hectare of land Cosmology & Astronomy Khan Academy.mp3", "Sentence": "How much of that energy comes from humans and how much of that energy comes from things other than humans. So just as an example, when we talk about human power, we're talking about someone literally hand plowing this field. So this woman right over here is literally she has this little cart that's digging up the soil behind her. When we talk about oxen power, we're talking about the oxen doing most of the work. They're the ones dragging this plow, which is digging up all of the soil. And this gentleman has to kind of be there to supervise. But this still is fairly intense labor that this gentleman is doing right over here."}, {"video_title": "Energy inputs for tilling a hectare of land Cosmology & Astronomy Khan Academy.mp3", "Sentence": "When we talk about oxen power, we're talking about the oxen doing most of the work. They're the ones dragging this plow, which is digging up all of the soil. And this gentleman has to kind of be there to supervise. But this still is fairly intense labor that this gentleman is doing right over here. And then when we talk about tractors, we're talking about a scenario like this, where the tractor is doing most of the work of actually digging up, dragging this plow behind it, and digging up all of the soil. And from this book right over here, that's where we got these numbers. I'll tell you which numbers I got from them and which numbers I kind of reasoned through because I wasn't fully comfortable with the numbers that they had."}, {"video_title": "Energy inputs for tilling a hectare of land Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But this still is fairly intense labor that this gentleman is doing right over here. And then when we talk about tractors, we're talking about a scenario like this, where the tractor is doing most of the work of actually digging up, dragging this plow behind it, and digging up all of the soil. And from this book right over here, that's where we got these numbers. I'll tell you which numbers I got from them and which numbers I kind of reasoned through because I wasn't fully comfortable with the numbers that they had. But these are their numbers, that if you're of human power to till one hectare of soil, it'll take 400 hours. Oxen power, and this should be a pair, not a parry. That should be a pair of oxen."}, {"video_title": "Energy inputs for tilling a hectare of land Cosmology & Astronomy Khan Academy.mp3", "Sentence": "I'll tell you which numbers I got from them and which numbers I kind of reasoned through because I wasn't fully comfortable with the numbers that they had. But these are their numbers, that if you're of human power to till one hectare of soil, it'll take 400 hours. Oxen power, and this should be a pair, not a parry. That should be a pair of oxen. 65 hours. A 6-horsepower tractor, 25 hours. A 50-horsepower tractor, 4 hours."}, {"video_title": "Energy inputs for tilling a hectare of land Cosmology & Astronomy Khan Academy.mp3", "Sentence": "That should be a pair of oxen. 65 hours. A 6-horsepower tractor, 25 hours. A 50-horsepower tractor, 4 hours. And in case y'all are wondering, what is a hectare of soil? It is literally a plot of land, a hectare of soil. Let me write it."}, {"video_title": "Energy inputs for tilling a hectare of land Cosmology & Astronomy Khan Academy.mp3", "Sentence": "A 50-horsepower tractor, 4 hours. And in case y'all are wondering, what is a hectare of soil? It is literally a plot of land, a hectare of soil. Let me write it. A hectare of land, I should say, is a plot of land that is 100 meters by 100 meters. And it's roughly equal to 2 and 1\u20442 acres. Not exactly 2 and 1\u20442 acres."}, {"video_title": "Energy inputs for tilling a hectare of land Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Let me write it. A hectare of land, I should say, is a plot of land that is 100 meters by 100 meters. And it's roughly equal to 2 and 1\u20442 acres. Not exactly 2 and 1\u20442 acres. It's like 2.4, I think 2.47 something, but roughly 2 and 1\u20442 acres. So we're just thinking about how many hours to essentially dig up all of the soil for a plot of land 100 by 100 meters. So what we have over here, so clearly human takes a lot longer."}, {"video_title": "Energy inputs for tilling a hectare of land Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Not exactly 2 and 1\u20442 acres. It's like 2.4, I think 2.47 something, but roughly 2 and 1\u20442 acres. So we're just thinking about how many hours to essentially dig up all of the soil for a plot of land 100 by 100 meters. So what we have over here, so clearly human takes a lot longer. Oxen, they can do a little bit faster. 6-horsepower tractor, even faster. 50-horsepower tractor, very powerful tractor, even faster than that."}, {"video_title": "Energy inputs for tilling a hectare of land Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So what we have over here, so clearly human takes a lot longer. Oxen, they can do a little bit faster. 6-horsepower tractor, even faster. 50-horsepower tractor, very powerful tractor, even faster than that. Now this column right over here is the amount of energy required to actually produce and maintain the machinery used. So this is a very unintuitive thing. Whenever you think about, like for example, whenever you think about the amount of energy to plow this land over here, you tend to think, OK, well this individual is going to have to expend a lot of her energy."}, {"video_title": "Energy inputs for tilling a hectare of land Cosmology & Astronomy Khan Academy.mp3", "Sentence": "50-horsepower tractor, very powerful tractor, even faster than that. Now this column right over here is the amount of energy required to actually produce and maintain the machinery used. So this is a very unintuitive thing. Whenever you think about, like for example, whenever you think about the amount of energy to plow this land over here, you tend to think, OK, well this individual is going to have to expend a lot of her energy. You don't think about the amount of energy required to actually maintain the tool, to one, build the tool that she's using, in this case a hand plow, and then to maintain that as she does it. And so this estimate, and I got these two from these fellows right over here. Actually, one of them might be a gal."}, {"video_title": "Energy inputs for tilling a hectare of land Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Whenever you think about, like for example, whenever you think about the amount of energy to plow this land over here, you tend to think, OK, well this individual is going to have to expend a lot of her energy. You don't think about the amount of energy required to actually maintain the tool, to one, build the tool that she's using, in this case a hand plow, and then to maintain that as she does it. And so this estimate, and I got these two from these fellows right over here. Actually, one of them might be a gal. But it's about 6,000 kilocalories. And this is kilocalories for lowercase c. And one thing I want to emphasize here, 1 kcal is the same thing as 1 calorie with a capital C, which is the same thing as 1,000 calories with a lowercase c. And we talked about this in the last video. But these, when people talk about food calories, they're really talking about a calorie with a capital C. Or you could say they're talking about kilocalories."}, {"video_title": "Energy inputs for tilling a hectare of land Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Actually, one of them might be a gal. But it's about 6,000 kilocalories. And this is kilocalories for lowercase c. And one thing I want to emphasize here, 1 kcal is the same thing as 1 calorie with a capital C, which is the same thing as 1,000 calories with a lowercase c. And we talked about this in the last video. But these, when people talk about food calories, they're really talking about a calorie with a capital C. Or you could say they're talking about kilocalories. So your candy bar, 200 calories, they're talking about this right over here. In chemistry class, when you talk about the amount of energy to raise a gram of water 1 degree Celsius, you're talking about these calories here. So in all of these numbers in this chart right over here, you can either view them as this unit, kcals, kilocalories, or calories with a capital C. They're essentially the same units that we used in the last video."}, {"video_title": "Energy inputs for tilling a hectare of land Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But these, when people talk about food calories, they're really talking about a calorie with a capital C. Or you could say they're talking about kilocalories. So your candy bar, 200 calories, they're talking about this right over here. In chemistry class, when you talk about the amount of energy to raise a gram of water 1 degree Celsius, you're talking about these calories here. So in all of these numbers in this chart right over here, you can either view them as this unit, kcals, kilocalories, or calories with a capital C. They're essentially the same units that we used in the last video. And these are the same numbers that you were used to from a dietary calorie point of view. So for example, 6,000 calories, that's about how much a typical male would expend in three days. So this is to maintain it over the course of these 400 hours in the case of the hand plow."}, {"video_title": "Energy inputs for tilling a hectare of land Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So in all of these numbers in this chart right over here, you can either view them as this unit, kcals, kilocalories, or calories with a capital C. They're essentially the same units that we used in the last video. And these are the same numbers that you were used to from a dietary calorie point of view. So for example, 6,000 calories, that's about how much a typical male would expend in three days. So this is to maintain it over the course of these 400 hours in the case of the hand plow. And the total amount of calories that were needed to make the plow divided by the total number of hours. So whatever fraction of the plow's life is being used here, you use that fraction right over here to put this 6,000 calories. But needless to say, for at least the plow, for either the human or the oxen scenario, this isn't a significant amount of the total calories."}, {"video_title": "Energy inputs for tilling a hectare of land Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So this is to maintain it over the course of these 400 hours in the case of the hand plow. And the total amount of calories that were needed to make the plow divided by the total number of hours. So whatever fraction of the plow's life is being used here, you use that fraction right over here to put this 6,000 calories. But needless to say, for at least the plow, for either the human or the oxen scenario, this isn't a significant amount of the total calories. So obviously, if you're doing it either with human power or oxen power, you're not using any gasoline. You're not using any petroleum. In all of these scenarios, we're going to assume that someone has 10 working hours in the day."}, {"video_title": "Energy inputs for tilling a hectare of land Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But needless to say, for at least the plow, for either the human or the oxen scenario, this isn't a significant amount of the total calories. So obviously, if you're doing it either with human power or oxen power, you're not using any gasoline. You're not using any petroleum. In all of these scenarios, we're going to assume that someone has 10 working hours in the day. And that right over here, this is kind of a measure of how hard that person's work is. And I kind of estimated these numbers here. They're slightly different than what the original numbers were in this book right over here."}, {"video_title": "Energy inputs for tilling a hectare of land Cosmology & Astronomy Khan Academy.mp3", "Sentence": "In all of these scenarios, we're going to assume that someone has 10 working hours in the day. And that right over here, this is kind of a measure of how hard that person's work is. And I kind of estimated these numbers here. They're slightly different than what the original numbers were in this book right over here. But we're saying, look, if you are actually walking along using this hand plow, that is actually very, very vigorous activity. So it's going to require about 400 calories per hour to do this type of activity. You do it over 10 hours, it's going to require 4,000 calories just to do that over 10 hours."}, {"video_title": "Energy inputs for tilling a hectare of land Cosmology & Astronomy Khan Academy.mp3", "Sentence": "They're slightly different than what the original numbers were in this book right over here. But we're saying, look, if you are actually walking along using this hand plow, that is actually very, very vigorous activity. So it's going to require about 400 calories per hour to do this type of activity. You do it over 10 hours, it's going to require 4,000 calories just to do that over 10 hours. And then we're assuming that the rest of the day, you're going to walk around, and maybe you're going to cook dinner, eat breakfast, you're going to sleep, some of it. We're assuming that the other 14 hours of the day are going to be at about 100 calories per hour. And so this is the total."}, {"video_title": "Energy inputs for tilling a hectare of land Cosmology & Astronomy Khan Academy.mp3", "Sentence": "You do it over 10 hours, it's going to require 4,000 calories just to do that over 10 hours. And then we're assuming that the rest of the day, you're going to walk around, and maybe you're going to cook dinner, eat breakfast, you're going to sleep, some of it. We're assuming that the other 14 hours of the day are going to be at about 100 calories per hour. And so this is the total. If someone were to, using this technique, work for a total of 10 hours, this is how many calories they would consume in the day. And you could see this is the most labor intensive. So it looks like that they would consume the most calories per day."}, {"video_title": "Energy inputs for tilling a hectare of land Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And so this is the total. If someone were to, using this technique, work for a total of 10 hours, this is how many calories they would consume in the day. And you could see this is the most labor intensive. So it looks like that they would consume the most calories per day. These two are the least labor intensive. You're sitting on a tractor, although that still requires more calories than sleeping or watching TV. And so that's the number of calories they would consume."}, {"video_title": "Energy inputs for tilling a hectare of land Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So it looks like that they would consume the most calories per day. These two are the least labor intensive. You're sitting on a tractor, although that still requires more calories than sleeping or watching TV. And so that's the number of calories they would consume. Now this right over here, and this is the interesting number, or one of the really interesting numbers. Based on all of these assumptions, this is the total human input in calories to do this task, to till this one hectare of soil. So over here, you're using 5,400 calories a day."}, {"video_title": "Energy inputs for tilling a hectare of land Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And so that's the number of calories they would consume. Now this right over here, and this is the interesting number, or one of the really interesting numbers. Based on all of these assumptions, this is the total human input in calories to do this task, to till this one hectare of soil. So over here, you're using 5,400 calories a day. If you're working 10 hours per day and it requires 400 total hours, you're going to be working 40 days, 400 divided by 10. 40 days times 5,400 calories per day, it's going to take a human, just the human part, not even thinking about the 6,000 calories necessary to maintain and make that plow. The human is going to spend 216,000 calories to till, to plow that one hectare of land."}, {"video_title": "Energy inputs for tilling a hectare of land Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So over here, you're using 5,400 calories a day. If you're working 10 hours per day and it requires 400 total hours, you're going to be working 40 days, 400 divided by 10. 40 days times 5,400 calories per day, it's going to take a human, just the human part, not even thinking about the 6,000 calories necessary to maintain and make that plow. The human is going to spend 216,000 calories to till, to plow that one hectare of land. And if you add the other 6,000 in for the actual plow, and you could debate what this number should be, but it's not a significant number compared to this, you get about 222,000 total calories. When you go to the oxen situation, you're acquiring fewer hours, and each hour it requires a little less calories. This is still labor intensive, but not as labor intensive as what this woman right over here is doing."}, {"video_title": "Energy inputs for tilling a hectare of land Cosmology & Astronomy Khan Academy.mp3", "Sentence": "The human is going to spend 216,000 calories to till, to plow that one hectare of land. And if you add the other 6,000 in for the actual plow, and you could debate what this number should be, but it's not a significant number compared to this, you get about 222,000 total calories. When you go to the oxen situation, you're acquiring fewer hours, and each hour it requires a little less calories. This is still labor intensive, but not as labor intensive as what this woman right over here is doing. So on a daily basis, you're using a little bit fewer calories, but since you're only doing 6 and 1 1 days of this, 65 hours divided by 10, you've significantly reduced the number of calories, the total number of calories, that the human needs to put into this task. Now, there still is other energy being done. Now all of a sudden, the oxen have gotten involved."}, {"video_title": "Energy inputs for tilling a hectare of land Cosmology & Astronomy Khan Academy.mp3", "Sentence": "This is still labor intensive, but not as labor intensive as what this woman right over here is doing. So on a daily basis, you're using a little bit fewer calories, but since you're only doing 6 and 1 1 days of this, 65 hours divided by 10, you've significantly reduced the number of calories, the total number of calories, that the human needs to put into this task. Now, there still is other energy being done. Now all of a sudden, the oxen have gotten involved. And if you assume that each oxen consumes about 20,000 calories a day, and you have two of them, so 40,000 calories per day just to feed the oxen, and you're going to do that for 6 and 1 1 days, 65 divided by 10, the oxen are going to consume 260,000 calories to do this task. So the total energy input here now has gone up. So this is an interesting phenomenon that is going on right over here."}, {"video_title": "Energy inputs for tilling a hectare of land Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Now all of a sudden, the oxen have gotten involved. And if you assume that each oxen consumes about 20,000 calories a day, and you have two of them, so 40,000 calories per day just to feed the oxen, and you're going to do that for 6 and 1 1 days, 65 divided by 10, the oxen are going to consume 260,000 calories to do this task. So the total energy input here now has gone up. So this is an interesting phenomenon that is going on right over here. What the human is putting in as we get better and better technology goes down substantially, 216,000 to 33,000. And we'll see with the tractor it goes down even more. But the total energy, if you include the amount of energy that the oxen have to put in, or if you include the amount of energy due to the gasoline that has to be used for the tractor, the total amount of energy is going up to plow that field."}, {"video_title": "Energy inputs for tilling a hectare of land Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So this is an interesting phenomenon that is going on right over here. What the human is putting in as we get better and better technology goes down substantially, 216,000 to 33,000. And we'll see with the tractor it goes down even more. But the total energy, if you include the amount of energy that the oxen have to put in, or if you include the amount of energy due to the gasoline that has to be used for the tractor, the total amount of energy is going up to plow that field. But the human energy goes down dramatically. Now, the last thing I want to highlight here, and these are where my numbers depart a little bit, or fairly significantly, from this original study right over here, this original estimate, is the machinery input on the tractors. So if you look this up, and you can Google search it, they have much larger numbers here."}, {"video_title": "Energy inputs for tilling a hectare of land Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But the total energy, if you include the amount of energy that the oxen have to put in, or if you include the amount of energy due to the gasoline that has to be used for the tractor, the total amount of energy is going up to plow that field. But the human energy goes down dramatically. Now, the last thing I want to highlight here, and these are where my numbers depart a little bit, or fairly significantly, from this original study right over here, this original estimate, is the machinery input on the tractors. So if you look this up, and you can Google search it, they have much larger numbers here. But I did a little research, and it looks like for most petroleum-based, combustion-based engines or vehicles, roughly 20% of the total energy that's used in fuel, 20% of that energy is used for the actual production and maintenance of that vehicle over its life. So what we did over here is we said, OK, for a 6-horsepower tractor, I used their numbers, where you're going to have to use 25 hours to do it, it's going to use this much petroleum, assuming that it uses 23.5 liters of gas or petroleum over 25 hours. And then I just took 20% of that number for saying, well, how much energy had to be used to maintain that vehicle over that amount of time?"}, {"video_title": "Energy inputs for tilling a hectare of land Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So if you look this up, and you can Google search it, they have much larger numbers here. But I did a little research, and it looks like for most petroleum-based, combustion-based engines or vehicles, roughly 20% of the total energy that's used in fuel, 20% of that energy is used for the actual production and maintenance of that vehicle over its life. So what we did over here is we said, OK, for a 6-horsepower tractor, I used their numbers, where you're going to have to use 25 hours to do it, it's going to use this much petroleum, assuming that it uses 23.5 liters of gas or petroleum over 25 hours. And then I just took 20% of that number for saying, well, how much energy had to be used to maintain that vehicle over that amount of time? And if you think about what fraction of this vehicle's life that 25 hours represents, that fraction times the total amount of energy required to produce that. So remember, these things are made out of metal. They had to be made in furnaces."}, {"video_title": "Energy inputs for tilling a hectare of land Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And then I just took 20% of that number for saying, well, how much energy had to be used to maintain that vehicle over that amount of time? And if you think about what fraction of this vehicle's life that 25 hours represents, that fraction times the total amount of energy required to produce that. So remember, these things are made out of metal. They had to be made in furnaces. So just producing a vehicle requires a lot of energy. And so this right over here is 20%. And I just use that rule of thumb for most petroleum-based or combustion-based vehicles."}, {"video_title": "Energy inputs for tilling a hectare of land Cosmology & Astronomy Khan Academy.mp3", "Sentence": "They had to be made in furnaces. So just producing a vehicle requires a lot of energy. And so this right over here is 20%. And I just use that rule of thumb for most petroleum-based or combustion-based vehicles. The 20% of the total energy expenditure over the course of that vehicle's life is roughly equal to the amount of energy used to produce that vehicle. But either way, you go all the way over here, the human has to spend less calories sitting on the vehicle, so they spend less calories per day. And then the total human input right over here for the 6-horsepower tractor is going to take them 2 and 1 half days, 25 hours at 10 hours a day, is going to be 8,500 calories."}, {"video_title": "Energy inputs for tilling a hectare of land Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And I just use that rule of thumb for most petroleum-based or combustion-based vehicles. The 20% of the total energy expenditure over the course of that vehicle's life is roughly equal to the amount of energy used to produce that vehicle. But either way, you go all the way over here, the human has to spend less calories sitting on the vehicle, so they spend less calories per day. And then the total human input right over here for the 6-horsepower tractor is going to take them 2 and 1 half days, 25 hours at 10 hours a day, is going to be 8,500 calories. But of course, you have the petroleum used and then some estimate of the amount of energy used to produce that tractor. And you're just taking the fraction over that 25 hours. You're not taking the entire life of that 6-horsepower tractor."}, {"video_title": "Energy inputs for tilling a hectare of land Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And then the total human input right over here for the 6-horsepower tractor is going to take them 2 and 1 half days, 25 hours at 10 hours a day, is going to be 8,500 calories. But of course, you have the petroleum used and then some estimate of the amount of energy used to produce that tractor. And you're just taking the fraction over that 25 hours. You're not taking the entire life of that 6-horsepower tractor. To produce a 6-horsepower tractor, this number would be much, much larger. If you talked about the total number of energy, we're just taking the small fraction of its life that we're using it right over here. Same thing for the 50-horsepower tractor."}, {"video_title": "Energy inputs for tilling a hectare of land Cosmology & Astronomy Khan Academy.mp3", "Sentence": "You're not taking the entire life of that 6-horsepower tractor. To produce a 6-horsepower tractor, this number would be much, much larger. If you talked about the total number of energy, we're just taking the small fraction of its life that we're using it right over here. Same thing for the 50-horsepower tractor. But either way you look at it, the human, and this is the really interesting thing, humans, by going from human power all the way to a 50-horsepower tractor, you're getting almost a factor of 200 improvement in terms of how little energy has to be put in by the human to till that land. But you actually get a total increase if you factor in things like the petroleum and then definitely the amount of energy to actually produce that machine. So anyway, hopefully you found that interesting."}, {"video_title": "Energy inputs for tilling a hectare of land Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Same thing for the 50-horsepower tractor. But either way you look at it, the human, and this is the really interesting thing, humans, by going from human power all the way to a 50-horsepower tractor, you're getting almost a factor of 200 improvement in terms of how little energy has to be put in by the human to till that land. But you actually get a total increase if you factor in things like the petroleum and then definitely the amount of energy to actually produce that machine. So anyway, hopefully you found that interesting. I find this kind of, it's something that you don't think a lot about. How much energy input has to be put in? And oftentimes, we only think about the human energy input."}, {"video_title": "Accreting mass due to gravity simulation Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And the whole point of the simulation is to get an intuition for how galaxies form, why they have the structures they have, how solar systems form, how they have the structures they have, and how gravity alone can kind of define that structure. And what's really interesting about the simulation, besides the fact that it's just mesmerizing and extremely cool, is it shows the particles collide. Once they get to a certain critical mass, you see that they get colored yellow, maybe to indicate that they are now a star. Fusion can now occur. And you can zoom in at different levels to really see how the different particles or the different masses are interacting. And then you can actually rotate that to see a little bit clearer. This is if I'm looking kind of right on top of it to see how they're interacting."}, {"video_title": "Accreting mass due to gravity simulation Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Fusion can now occur. And you can zoom in at different levels to really see how the different particles or the different masses are interacting. And then you can actually rotate that to see a little bit clearer. This is if I'm looking kind of right on top of it to see how they're interacting. And it's a three-dimensional simulation, so it's a very rich way of thinking about these. And what's exciting for me is it's highly dependent on what the initial conditions are. In an earlier version of Peter's simulation, he did not give a net angular momentum to the system."}, {"video_title": "Accreting mass due to gravity simulation Cosmology & Astronomy Khan Academy.mp3", "Sentence": "This is if I'm looking kind of right on top of it to see how they're interacting. And it's a three-dimensional simulation, so it's a very rich way of thinking about these. And what's exciting for me is it's highly dependent on what the initial conditions are. In an earlier version of Peter's simulation, he did not give a net angular momentum to the system. And so you did not have as much of kind of the planet satellite or as much of the disk structures forming, although right here we don't have too much of a disk structure, although there does seem to be a dominant plane in this scenario. And what's exciting is here we have a binary system. Sometimes you restart it."}, {"video_title": "Accreting mass due to gravity simulation Cosmology & Astronomy Khan Academy.mp3", "Sentence": "In an earlier version of Peter's simulation, he did not give a net angular momentum to the system. And so you did not have as much of kind of the planet satellite or as much of the disk structures forming, although right here we don't have too much of a disk structure, although there does seem to be a dominant plane in this scenario. And what's exciting is here we have a binary system. Sometimes you restart it. You might not have a binary system. Depending on the initial conditions, you might have something that starts to look like the Milky Way. Sometimes you might have something that looks very different than the Milky Way."}, {"video_title": "Accreting mass due to gravity simulation Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Sometimes you restart it. You might not have a binary system. Depending on the initial conditions, you might have something that starts to look like the Milky Way. Sometimes you might have something that looks very different than the Milky Way. And it really gives us clues of why we see such diversity, especially when we're looking at galaxies, the structure of galaxies, that it's highly dependent on initial conditions. One can argue that our own solar system did have some net initial angular momentum because the current theory, what really catalyzed it was a nearby supernova that sent a shock wave and allowed the dust that would form our solar system to reach a critical mass and start to condense into the sun and the planets. And so this isn't, at least in my mind, too unrealistic of a scenario."}, {"video_title": "Accreting mass due to gravity simulation Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Sometimes you might have something that looks very different than the Milky Way. And it really gives us clues of why we see such diversity, especially when we're looking at galaxies, the structure of galaxies, that it's highly dependent on initial conditions. One can argue that our own solar system did have some net initial angular momentum because the current theory, what really catalyzed it was a nearby supernova that sent a shock wave and allowed the dust that would form our solar system to reach a critical mass and start to condense into the sun and the planets. And so this isn't, at least in my mind, too unrealistic of a scenario. And it's really cool to look at. And it really gives you a sense of things. You already see you have a binary star."}, {"video_title": "Accreting mass due to gravity simulation Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And so this isn't, at least in my mind, too unrealistic of a scenario. And it's really cool to look at. And it really gives you a sense of things. You already see you have a binary star. They're kind of orbiting around each other or orbiting around the center of mass, which kind of looks like around each other. And then this star right over here has its own kind of captive planet that is just rotating around it. We can see it a little bit clearer."}, {"video_title": "Accreting mass due to gravity simulation Cosmology & Astronomy Khan Academy.mp3", "Sentence": "You already see you have a binary star. They're kind of orbiting around each other or orbiting around the center of mass, which kind of looks like around each other. And then this star right over here has its own kind of captive planet that is just rotating around it. We can see it a little bit clearer. If we had a very, at least from this perspective, a very close range, we can zoom in a little bit more to see it a little bit better. This has a satellite, but then they're also kind of dancing around each other. So it's a really fascinating simulation."}, {"video_title": "Accreting mass due to gravity simulation Cosmology & Astronomy Khan Academy.mp3", "Sentence": "We can see it a little bit clearer. If we had a very, at least from this perspective, a very close range, we can zoom in a little bit more to see it a little bit better. This has a satellite, but then they're also kind of dancing around each other. So it's a really fascinating simulation. I could really stare at this and play with it for days. And I encourage you to play with it, restart it, see how the initial conditions or what type of solar systems or galaxies you might end up with, whether they form disks, whether you have binary systems or not, whether you have planets with satellites. And then if you are more advanced, actually play with the code and see if you can really change the initial conditions, the starting velocities of things, the number of particles of things, the distribution of mass that you start off with, the angular momentum that you start off with, and see how that might change the structure of the universes that you create."}, {"video_title": "Accreting mass due to gravity simulation Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So it's a really fascinating simulation. I could really stare at this and play with it for days. And I encourage you to play with it, restart it, see how the initial conditions or what type of solar systems or galaxies you might end up with, whether they form disks, whether you have binary systems or not, whether you have planets with satellites. And then if you are more advanced, actually play with the code and see if you can really change the initial conditions, the starting velocities of things, the number of particles of things, the distribution of mass that you start off with, the angular momentum that you start off with, and see how that might change the structure of the universes that you create. And I'm going to add an annotation to this video that links directly to this simulation. And I'll also put the link inside of the description. So have fun."}, {"video_title": "Accreting mass due to gravity simulation Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And then if you are more advanced, actually play with the code and see if you can really change the initial conditions, the starting velocities of things, the number of particles of things, the distribution of mass that you start off with, the angular momentum that you start off with, and see how that might change the structure of the universes that you create. And I'm going to add an annotation to this video that links directly to this simulation. And I'll also put the link inside of the description. So have fun. I could literally spend hours with this. It's a fascinating, fascinating module that he's created. We zoom in and out."}, {"video_title": "How we know about the earth's core Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And the answer there comes from the same technique that we saw Mohorovichik use in 1909 to essentially see how the behavior, or when you measure the seismic waves, or whether you can even measure the seismic waves, the different distances from an earthquake. So if there's an earthquake right here, and we're calling that 0 degrees, let's remember a couple of things here. Let's remember that P waves can travel through anything. They can travel through solid or liquid, or air for that matter. So they can travel through anything. But S waves can only, S for secondary, these are the transverse waves, these can only travel through solids. So it turns out that if an earthquake happens at 0 degrees, and you have seismograph stations all over the world, and these are extremely sensitive in order to be able to measure earthquakes that are happening thousands of kilometers away, it turns out that there's something called an S shadow, an S wave shadow."}, {"video_title": "How we know about the earth's core Cosmology & Astronomy Khan Academy.mp3", "Sentence": "They can travel through solid or liquid, or air for that matter. So they can travel through anything. But S waves can only, S for secondary, these are the transverse waves, these can only travel through solids. So it turns out that if an earthquake happens at 0 degrees, and you have seismograph stations all over the world, and these are extremely sensitive in order to be able to measure earthquakes that are happening thousands of kilometers away, it turns out that there's something called an S shadow, an S wave shadow. You can measure, if these are S waves, you can measure them here. You can measure them here. They can go all the way over here."}, {"video_title": "How we know about the earth's core Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So it turns out that if an earthquake happens at 0 degrees, and you have seismograph stations all over the world, and these are extremely sensitive in order to be able to measure earthquakes that are happening thousands of kilometers away, it turns out that there's something called an S shadow, an S wave shadow. You can measure, if these are S waves, you can measure them here. You can measure them here. They can go all the way over here. They can go over here. They can go over there. You can measure them over here."}, {"video_title": "How we know about the earth's core Cosmology & Astronomy Khan Academy.mp3", "Sentence": "They can go all the way over here. They can go over here. They can go over there. You can measure them over here. So you can measure them at all of these points, but then all of a sudden, at 105 degrees, and so we're measuring 0 degrees here and we're going outwards like that, all of a sudden at 105 degrees and further, you stop measuring S waves. They don't get, for some reason, you would think that the S waves would get over here. Maybe they would be a little bit weaker, but they would be able to get all the way over here."}, {"video_title": "How we know about the earth's core Cosmology & Astronomy Khan Academy.mp3", "Sentence": "You can measure them over here. So you can measure them at all of these points, but then all of a sudden, at 105 degrees, and so we're measuring 0 degrees here and we're going outwards like that, all of a sudden at 105 degrees and further, you stop measuring S waves. They don't get, for some reason, you would think that the S waves would get over here. Maybe they would be a little bit weaker, but they would be able to get all the way over here. But they just abruptly stop. No more S waves. So in this whole area right over here, you get no S waves."}, {"video_title": "How we know about the earth's core Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Maybe they would be a little bit weaker, but they would be able to get all the way over here. But they just abruptly stop. No more S waves. So in this whole area right over here, you get no S waves. And obviously, I could flip this picture over and you'd see a symmetric thing on the other side of the globe that all of this area over here, you also would not see. You would also not see S waves. You'd only see them from 105 degrees in this direction and 105 degrees in that direction."}, {"video_title": "How we know about the earth's core Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So in this whole area right over here, you get no S waves. And obviously, I could flip this picture over and you'd see a symmetric thing on the other side of the globe that all of this area over here, you also would not see. You would also not see S waves. You'd only see them from 105 degrees in this direction and 105 degrees in that direction. And the only reasonable explanation that we can give is that there must be some material that an S wave cannot travel through, that it would have to travel through to get to these points beyond 105 degrees. And we know that S waves only travel in solids. So the assumption there is that at some point beyond 105 degrees, it's hitting liquid."}, {"video_title": "How we know about the earth's core Cosmology & Astronomy Khan Academy.mp3", "Sentence": "You'd only see them from 105 degrees in this direction and 105 degrees in that direction. And the only reasonable explanation that we can give is that there must be some material that an S wave cannot travel through, that it would have to travel through to get to these points beyond 105 degrees. And we know that S waves only travel in solids. So the assumption there is that at some point beyond 105 degrees, it's hitting liquid. So that's what tells us that this right here is probably a liquid. So it's hitting some layer that is liquid. So that tells us that there's a core, and at least the outer part of that core is liquid, enough to stop S waves."}, {"video_title": "How we know about the earth's core Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So the assumption there is that at some point beyond 105 degrees, it's hitting liquid. So that's what tells us that this right here is probably a liquid. So it's hitting some layer that is liquid. So that tells us that there's a core, and at least the outer part of that core is liquid, enough to stop S waves. So the S waves, because it only travels in solids, it leads to this S wave shadow. And this tells us that we have a core. And that core, at least the outer part, is liquid."}, {"video_title": "How we know about the earth's core Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So that tells us that there's a core, and at least the outer part of that core is liquid, enough to stop S waves. So the S waves, because it only travels in solids, it leads to this S wave shadow. And this tells us that we have a core. And that core, at least the outer part, is liquid. We don't know yet whether the inner part is liquid or solid. Now the next point of evidence is how do we know that there's an inner core? And we can use P waves for that."}, {"video_title": "How we know about the earth's core Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And that core, at least the outer part, is liquid. We don't know yet whether the inner part is liquid or solid. Now the next point of evidence is how do we know that there's an inner core? And we can use P waves for that. The P wave can travel through anything. But remember, as you get denser material, in general, for the same type of material, if you get denser material, it's going to move faster. So it's going to refract outwards like we've seen over here."}, {"video_title": "How we know about the earth's core Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And we can use P waves for that. The P wave can travel through anything. But remember, as you get denser material, in general, for the same type of material, if you get denser material, it's going to move faster. So it's going to refract outwards like we've seen over here. But if it goes into a liquid, in general, sound waves, or I should say P waves, seismic waves, move slower in liquids. And so the refraction patterns we get when we do measure from seismograph stations around the world is that it looks like the P waves are doing what you would expect in the mantle. But then they're getting refracted as if they're going into a slower medium as they go through the outer core."}, {"video_title": "How we know about the earth's core Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So it's going to refract outwards like we've seen over here. But if it goes into a liquid, in general, sound waves, or I should say P waves, seismic waves, move slower in liquids. And so the refraction patterns we get when we do measure from seismograph stations around the world is that it looks like the P waves are doing what you would expect in the mantle. But then they're getting refracted as if they're going into a slower medium as they go through the outer core. And we see that right over here. And then they get refracted again to get to some point on the other side. Now that is just what you would expect if it was all liquid."}, {"video_title": "How we know about the earth's core Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But then they're getting refracted as if they're going into a slower medium as they go through the outer core. And we see that right over here. And then they get refracted again to get to some point on the other side. Now that is just what you would expect if it was all liquid. But if you go to stations that are even further out, it looks like, if you just look at the refraction patterns, and you can now model this with fancy computers and get all the data points, but you could say, well, the only way that reality can fit the data that we get based on when things reach here is if the P waves are being first refracted through the outer core. But then they're refracted in a way that they're going through denser material, significantly denser material in the inner core. And then they're just continuing to refract the way you would expect."}, {"video_title": "How we know about the earth's core Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Now that is just what you would expect if it was all liquid. But if you go to stations that are even further out, it looks like, if you just look at the refraction patterns, and you can now model this with fancy computers and get all the data points, but you could say, well, the only way that reality can fit the data that we get based on when things reach here is if the P waves are being first refracted through the outer core. But then they're refracted in a way that they're going through denser material, significantly denser material in the inner core. And then they're just continuing to refract the way you would expect. So it's really the refraction pattern of the P waves. And frankly, the fact that there's this what you call a P wave shadow. The P wave shadow by itself, all that tells you is that kind of roughly crazy things are happening someplace in the core."}, {"video_title": "How we know about the earth's core Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And then they're just continuing to refract the way you would expect. So it's really the refraction pattern of the P waves. And frankly, the fact that there's this what you call a P wave shadow. The P wave shadow by itself, all that tells you is that kind of roughly crazy things are happening someplace in the core. But the real way to know that we have an inner core that's solid, as opposed to the whole thing being liquid, is that the P waves is the pattern of how the P waves reach essentially the other side of the globe. And then you can kind of, based on modeling how waves would travel through different densities and different types of mediums, you could say, well, there's got to be an inner core right over here. And obviously, it's a lot more math than I'm going into."}, {"video_title": "How we know about the earth's core Cosmology & Astronomy Khan Academy.mp3", "Sentence": "The P wave shadow by itself, all that tells you is that kind of roughly crazy things are happening someplace in the core. But the real way to know that we have an inner core that's solid, as opposed to the whole thing being liquid, is that the P waves is the pattern of how the P waves reach essentially the other side of the globe. And then you can kind of, based on modeling how waves would travel through different densities and different types of mediums, you could say, well, there's got to be an inner core right over here. And obviously, it's a lot more math than I'm going into. But if you do the math based on the shadow and you know the speed of the material and all of that type of thing, then you can figure out the depth at which these transitions occur. We know that we have a transition from mantle to outer core here, and then a transition from outer core to core there. So hopefully that satiates your questions about how do we know what the composition of the earth is without ever having to dig down there, because we've never even gotten below our crust."}, {"video_title": "The mohorovicic seismic discontinuity Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And he was a meteorologist and seismologist, and he was the first one to notice in 1909 when there was an earthquake in Croatia, a little bit southeast of Zagreb. So the earthquake was roughly over here, and lucky for him and lucky for us, before that earthquake there was actually a bunch of seismographic stations already in the area. And all these seismographic stations are, they essentially, instruments were installed so that if there was any essentially seismic waves passing, they would be able to measure it when the waves got there. And what was interesting about this, Andrija realized that if the entire Earth was just kind of a uniform material, so let's draw that scenario, it would get denser as you go down, and so you would have kind of this refraction, this continuous refraction or these curved paths happening. But he realized that, let's say we had an earthquake right over here, so this is the uniform case, uniform layer, only one layer, although it does get denser. Then the closer you are to the earthquake, so waves would get there first, then waves would get over there, then waves would get over there, and these are the body waves, these are the ones that are traveling through the Earth's crust. But in general, the further you are away from the earthquake, or the time it takes for the waves to get to a point, is going to be proportional to the distance that point is away from the earthquake."}, {"video_title": "The mohorovicic seismic discontinuity Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And what was interesting about this, Andrija realized that if the entire Earth was just kind of a uniform material, so let's draw that scenario, it would get denser as you go down, and so you would have kind of this refraction, this continuous refraction or these curved paths happening. But he realized that, let's say we had an earthquake right over here, so this is the uniform case, uniform layer, only one layer, although it does get denser. Then the closer you are to the earthquake, so waves would get there first, then waves would get over there, then waves would get over there, and these are the body waves, these are the ones that are traveling through the Earth's crust. But in general, the further you are away from the earthquake, or the time it takes for the waves to get to a point, is going to be proportional to the distance that point is away from the earthquake. So you would expect to see something like this. So if you were to plot on the horizontal axis, if you were to plot distance, and on the vertical axis, if you were to plot time, you should see something like this. You should see a straight line."}, {"video_title": "The mohorovicic seismic discontinuity Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But in general, the further you are away from the earthquake, or the time it takes for the waves to get to a point, is going to be proportional to the distance that point is away from the earthquake. So you would expect to see something like this. So if you were to plot on the horizontal axis, if you were to plot distance, and on the vertical axis, if you were to plot time, you should see something like this. You should see a straight line. And that's just because it's traveling roughly the same velocity along any of these arcs. It's maybe getting a little bit faster as it's getting deeper, but roughly the same velocity is traveling along these arcs, and the distance of these arcs are proportional to the distance along the surface. So essentially, they're all traveling roughly at the same velocity, and they're just traveling different distances, so the time it takes is just going to be proportional to the distance."}, {"video_title": "The mohorovicic seismic discontinuity Cosmology & Astronomy Khan Academy.mp3", "Sentence": "You should see a straight line. And that's just because it's traveling roughly the same velocity along any of these arcs. It's maybe getting a little bit faster as it's getting deeper, but roughly the same velocity is traveling along these arcs, and the distance of these arcs are proportional to the distance along the surface. So essentially, they're all traveling roughly at the same velocity, and they're just traveling different distances, so the time it takes is just going to be proportional to the distance. But he noticed something interesting. When he actually measured when the waves from that earthquake reached different seismographic stations, he saw something interesting. So this is the theoretical, if we had this uniform one-layered Earth."}, {"video_title": "The mohorovicic seismic discontinuity Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So essentially, they're all traveling roughly at the same velocity, and they're just traveling different distances, so the time it takes is just going to be proportional to the distance. But he noticed something interesting. When he actually measured when the waves from that earthquake reached different seismographic stations, he saw something interesting. So this is the theoretical, if we had this uniform one-layered Earth. But he saw something interesting. So once again, this is distance, and this right over here is time. And at 200 kilometers away from the earthquake, so until 200 kilometers, he saw exactly what you would expect from a uniform Earth."}, {"video_title": "The mohorovicic seismic discontinuity Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So this is the theoretical, if we had this uniform one-layered Earth. But he saw something interesting. So once again, this is distance, and this right over here is time. And at 200 kilometers away from the earthquake, so until 200 kilometers, he saw exactly what you would expect from a uniform Earth. It was just the time was proportional to the distance. But at 200 kilometers, he saw something interesting. All of a sudden, the waves were reaching there faster."}, {"video_title": "The mohorovicic seismic discontinuity Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And at 200 kilometers away from the earthquake, so until 200 kilometers, he saw exactly what you would expect from a uniform Earth. It was just the time was proportional to the distance. But at 200 kilometers, he saw something interesting. All of a sudden, the waves were reaching there faster. The slope of this line changed. It took less time for each incremental distance. So for some reason, the waves that were going at these farther stations, the stations that were more than 200 kilometers away, the stations, somehow they were accelerated."}, {"video_title": "The mohorovicic seismic discontinuity Cosmology & Astronomy Khan Academy.mp3", "Sentence": "All of a sudden, the waves were reaching there faster. The slope of this line changed. It took less time for each incremental distance. So for some reason, the waves that were going at these farther stations, the stations that were more than 200 kilometers away, the stations, somehow they were accelerated. Somehow they were able to move faster. And he's the one that realized that this was because the waves that were getting to these further stations must have traveled through a more dense layer of the Earth. So let's just think about it."}, {"video_title": "The mohorovicic seismic discontinuity Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So for some reason, the waves that were going at these farther stations, the stations that were more than 200 kilometers away, the stations, somehow they were accelerated. Somehow they were able to move faster. And he's the one that realized that this was because the waves that were getting to these further stations must have traveled through a more dense layer of the Earth. So let's just think about it. So if we have a more dense layer, it will fit this information right over here. So if we have a layer like this, which we now know to be the crust, and then you have a denser layer, which we now know to be the mantle, then what you would have is, so you have your earthquake right over here. Closer by, while you're still within the crust, it would be proportional."}, {"video_title": "The mohorovicic seismic discontinuity Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So let's just think about it. So if we have a more dense layer, it will fit this information right over here. So if we have a layer like this, which we now know to be the crust, and then you have a denser layer, which we now know to be the mantle, then what you would have is, so you have your earthquake right over here. Closer by, while you're still within the crust, it would be proportional. It would be proportional. And then let's say that this is exactly, this right here is 200 kilometers away. But then if you go any further, the waves would have to travel."}, {"video_title": "The mohorovicic seismic discontinuity Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Closer by, while you're still within the crust, it would be proportional. It would be proportional. And then let's say that this is exactly, this right here is 200 kilometers away. But then if you go any further, the waves would have to travel. So they would travel, so they would go like this. And then they would get refracted even harder. So they would get refracted even, so they would be a little bit curved ahead of time."}, {"video_title": "The mohorovicic seismic discontinuity Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But then if you go any further, the waves would have to travel. So they would travel, so they would go like this. And then they would get refracted even harder. So they would get refracted even, so they would be a little bit curved ahead of time. But then they're going into a much denser material, or it's not gradually dense. It's actually kind of a, all of a sudden, a considerably more dense material. So it would get refracted even more."}, {"video_title": "The mohorovicic seismic discontinuity Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So they would get refracted even, so they would be a little bit curved ahead of time. But then they're going into a much denser material, or it's not gradually dense. It's actually kind of a, all of a sudden, a considerably more dense material. So it would get refracted even more. And then it'll go over here. And since it was able to travel all of this distance in a denser material, it would have traveled faster along this path. And so it would get to this distance on the surface that's more than 200 kilometers away."}, {"video_title": "The mohorovicic seismic discontinuity Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So it would get refracted even more. And then it'll go over here. And since it was able to travel all of this distance in a denser material, it would have traveled faster along this path. And so it would get to this distance on the surface that's more than 200 kilometers away. It would get there faster. And so he said that there must be a denser layer that those waves are traveling through, which we now know to be the mantle. And the boundary between what we now know to be the crust and this denser layer, which we now know to be the mantle, is actually named after him."}, {"video_title": "The mohorovicic seismic discontinuity Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And so it would get to this distance on the surface that's more than 200 kilometers away. It would get there faster. And so he said that there must be a denser layer that those waves are traveling through, which we now know to be the mantle. And the boundary between what we now know to be the crust and this denser layer, which we now know to be the mantle, is actually named after him. It's called the Mohorovicic discontinuity. And sometimes it's just called the Moho for short. So that boundary between the crust and the mantle is now named for him."}, {"video_title": "The mohorovicic seismic discontinuity Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And the boundary between what we now know to be the crust and this denser layer, which we now know to be the mantle, is actually named after him. It's called the Mohorovicic discontinuity. And sometimes it's just called the Moho for short. So that boundary between the crust and the mantle is now named for him. But this was a huge discovery, because not only was he able to tell us, based on the data, based on kind of indirect data, just based on earthquakes happening and measuring when the earthquakes reach different points of the Earth, that there probably is a denser layer. And if you do the math, under continental crust, that denser layer is about 35 kilometers down. He was able to tell us that there is that layer."}, {"video_title": "The mohorovicic seismic discontinuity Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So that boundary between the crust and the mantle is now named for him. But this was a huge discovery, because not only was he able to tell us, based on the data, based on kind of indirect data, just based on earthquakes happening and measuring when the earthquakes reach different points of the Earth, that there probably is a denser layer. And if you do the math, under continental crust, that denser layer is about 35 kilometers down. He was able to tell us that there is that layer. But even more importantly, he was able to give the clue that just using information from earthquakes, we could essentially figure out the actual composition of the Earth, because no one has ever dug that deep. No one has ever dug into the mantle, much less the outer core or the inner core. In the next few videos, we're going to kind of take this insight, that we can use information from earthquakes, to actually think about how we know that there is an outer liquid core and that there's an inner core as well."}, {"video_title": "Understanding calendar notation Cosmology & Astronomy Khan Academy.mp3", "Sentence": "This right here is a fresco by Raphael of Plato hanging out with his best student Aristotle. And you may or may not know that these two fellows, along with Plato's teacher Socrates, are considered kind of the fathers of Western philosophy. But that's not what this video is about. I'm just, this is actually just a small little video about different dates. Or maybe a better way to think about it, different ways to specify dates or dating mechanisms. And so if you were to look up Plato's birth, well you might get either 428 or 427, but we'll go with 428. If you were to look up Plato's birth, you might see it written as 428 BC, or you might see it written as 428 BCE."}, {"video_title": "Understanding calendar notation Cosmology & Astronomy Khan Academy.mp3", "Sentence": "I'm just, this is actually just a small little video about different dates. Or maybe a better way to think about it, different ways to specify dates or dating mechanisms. And so if you were to look up Plato's birth, well you might get either 428 or 427, but we'll go with 428. If you were to look up Plato's birth, you might see it written as 428 BC, or you might see it written as 428 BCE. And the natural question is, well what's the difference here, they both have a BC, but this one has an E, it's the same year right now. And the answer is, is that these are referring to the exact same year in history, but the acronyms here do stand for different things. BC literally stands for before Christ."}, {"video_title": "Understanding calendar notation Cosmology & Astronomy Khan Academy.mp3", "Sentence": "If you were to look up Plato's birth, you might see it written as 428 BC, or you might see it written as 428 BCE. And the natural question is, well what's the difference here, they both have a BC, but this one has an E, it's the same year right now. And the answer is, is that these are referring to the exact same year in history, but the acronyms here do stand for different things. BC literally stands for before Christ. So if the date is written 428 BC, the implication is that this is 428 years before the birth of Christ. We'll see in a second that that's not exactly right, but that's what the implication is. If someone writes BCE, they're saying something very different."}, {"video_title": "Understanding calendar notation Cosmology & Astronomy Khan Academy.mp3", "Sentence": "BC literally stands for before Christ. So if the date is written 428 BC, the implication is that this is 428 years before the birth of Christ. We'll see in a second that that's not exactly right, but that's what the implication is. If someone writes BCE, they're saying something very different. The B still stands for before, but the C in CE does not stand for Christ anymore. It now stands for common. And so the CE part is common era."}, {"video_title": "Understanding calendar notation Cosmology & Astronomy Khan Academy.mp3", "Sentence": "If someone writes BCE, they're saying something very different. The B still stands for before, but the C in CE does not stand for Christ anymore. It now stands for common. And so the CE part is common era. Even though it's not referring to Christ anymore, and this kind of the intention here is so that it's I guess less religious than the term before Christ, it's still kind of putting an importance on Christ's birth. Because it's saying that the common era is the time period after the birth of Christ, which we'll see in a second isn't exactly right. But there's essentially the same exact dating scheme, one not directly referring to Christ, one that is directly referring to Christ."}, {"video_title": "Understanding calendar notation Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And so the CE part is common era. Even though it's not referring to Christ anymore, and this kind of the intention here is so that it's I guess less religious than the term before Christ, it's still kind of putting an importance on Christ's birth. Because it's saying that the common era is the time period after the birth of Christ, which we'll see in a second isn't exactly right. But there's essentially the same exact dating scheme, one not directly referring to Christ, one that is directly referring to Christ. Similarly, this right here is a painting of Christopher Columbus. And if you were to look up in history books, when was his first voyage and when did he first show up in the New World, finding an island in the Bahamas, you would see it written as either 1492 or AD 1492 or 1492 CE. And once again, these are all referring to the same year, just using different acronyms."}, {"video_title": "Understanding calendar notation Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But there's essentially the same exact dating scheme, one not directly referring to Christ, one that is directly referring to Christ. Similarly, this right here is a painting of Christopher Columbus. And if you were to look up in history books, when was his first voyage and when did he first show up in the New World, finding an island in the Bahamas, you would see it written as either 1492 or AD 1492 or 1492 CE. And once again, these are all referring to the same year, just using different acronyms. One of them's a little bit more religious or more directly refers to Christ, and one is a little less religious. So AD, some people think it refers to after death. It does not refer to after death."}, {"video_title": "Understanding calendar notation Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And once again, these are all referring to the same year, just using different acronyms. One of them's a little bit more religious or more directly refers to Christ, and one is a little less religious. So AD, some people think it refers to after death. It does not refer to after death. Because if you think about it, if you have years before the birth of Christ, and if you started numbering after his death, how would you number the years during his life? So AD does not stand for after death. It stands for Anno Domini, which literally means year, and domini means Lord or the Lord."}, {"video_title": "Understanding calendar notation Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It does not refer to after death. Because if you think about it, if you have years before the birth of Christ, and if you started numbering after his death, how would you number the years during his life? So AD does not stand for after death. It stands for Anno Domini, which literally means year, and domini means Lord or the Lord. So it's the year of the Lord or the year of our Lord. So it's years since. And one Anno Domini would be the year of Jesus Christ's birth."}, {"video_title": "Understanding calendar notation Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It stands for Anno Domini, which literally means year, and domini means Lord or the Lord. So it's the year of the Lord or the year of our Lord. So it's years since. And one Anno Domini would be the year of Jesus Christ's birth. So not after death. It stands for Anno Domini, but literally year of our Lord, so years since that Jesus was born, with year one being implicitly starting with his birth. And we'll see in a second that's not exactly right."}, {"video_title": "Understanding calendar notation Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And one Anno Domini would be the year of Jesus Christ's birth. So not after death. It stands for Anno Domini, but literally year of our Lord, so years since that Jesus was born, with year one being implicitly starting with his birth. And we'll see in a second that's not exactly right. CE stands for Common Era. Once again, 1 CE is the same thing as AD 1. Sometimes we now write, instead of writing AD 1492, we'll write 1492 AD, all referring to the exact same thing."}, {"video_title": "Understanding calendar notation Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And we'll see in a second that's not exactly right. CE stands for Common Era. Once again, 1 CE is the same thing as AD 1. Sometimes we now write, instead of writing AD 1492, we'll write 1492 AD, all referring to the exact same thing. Now, all of these things refer to, when we say 428 BC, it implies 428 years before the birth of Christ. 1492 AD, that's in the year of our Lord, 1492. It implies 1,492 years since the birth of Christ."}, {"video_title": "Understanding calendar notation Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Sometimes we now write, instead of writing AD 1492, we'll write 1492 AD, all referring to the exact same thing. Now, all of these things refer to, when we say 428 BC, it implies 428 years before the birth of Christ. 1492 AD, that's in the year of our Lord, 1492. It implies 1,492 years since the birth of Christ. But the reality is that we're not really quite sure when Christ was born. And so these aren't exactly. So Columbus didn't sail across the Atlantic exactly 1492 years after the birth of Christ."}, {"video_title": "Understanding calendar notation Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It implies 1,492 years since the birth of Christ. But the reality is that we're not really quite sure when Christ was born. And so these aren't exactly. So Columbus didn't sail across the Atlantic exactly 1492 years after the birth of Christ. Most historians put the birth of Christ at 7 to 2 BC or BCE, depending on how you want to view it. Remember, BC is before Christ, which is a little ironic because we're talking about the actual birth of Christ. BCE is before the Common Era."}, {"video_title": "Understanding calendar notation Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So Columbus didn't sail across the Atlantic exactly 1492 years after the birth of Christ. Most historians put the birth of Christ at 7 to 2 BC or BCE, depending on how you want to view it. Remember, BC is before Christ, which is a little ironic because we're talking about the actual birth of Christ. BCE is before the Common Era. And they put his death at 30 to 36 AD, which is 30 to 36 in the year of our Lord. Or, that's what this stands for, Anno Domini, or in the Common Era, CE. Now, some people, they obviously don't like BC."}, {"video_title": "Understanding calendar notation Cosmology & Astronomy Khan Academy.mp3", "Sentence": "BCE is before the Common Era. And they put his death at 30 to 36 AD, which is 30 to 36 in the year of our Lord. Or, that's what this stands for, Anno Domini, or in the Common Era, CE. Now, some people, they obviously don't like BC. They don't like the BC AD naming mechanism because it's explicitly referring to Christ. And every year, it makes Christ the central figure in all of history. So they'll say that this is clearly too Christian."}, {"video_title": "Understanding calendar notation Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Now, some people, they obviously don't like BC. They don't like the BC AD naming mechanism because it's explicitly referring to Christ. And every year, it makes Christ the central figure in all of history. So they'll say that this is clearly too Christian. And they would prefer the situation, they would prefer the less Christian naming scheme, where you use BCE and CE. But a lot of people would still say, hey, look, OK, you changed, well, first of all, some Christians wouldn't like this, that you removed the direct references to the birth of Christ or being in the years since Christ's birth. But even here, and they'll say, hey, you've removed it."}, {"video_title": "Understanding calendar notation Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So they'll say that this is clearly too Christian. And they would prefer the situation, they would prefer the less Christian naming scheme, where you use BCE and CE. But a lot of people would still say, hey, look, OK, you changed, well, first of all, some Christians wouldn't like this, that you removed the direct references to the birth of Christ or being in the years since Christ's birth. But even here, and they'll say, hey, you've removed it. But even here, some people would complain that although you've made the direct reference, that this is saying common before the Common Era and the Common Era, even though you've removed the direct reference, it still makes Christ's birth the central thing in all of history. But this is the convention, whether people like it or not. In order to have the same reference point, and it would be too logistically difficult to switch at this time, everyone has essentially settled on this."}, {"video_title": "Understanding calendar notation Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But even here, and they'll say, hey, you've removed it. But even here, some people would complain that although you've made the direct reference, that this is saying common before the Common Era and the Common Era, even though you've removed the direct reference, it still makes Christ's birth the central thing in all of history. But this is the convention, whether people like it or not. In order to have the same reference point, and it would be too logistically difficult to switch at this time, everyone has essentially settled on this. And it's really just a matter of letters of which naming scheme you pick. But the whole point of this video is that you don't get confused between BC and BCE. You don't think that AD stands for after death."}, {"video_title": "Understanding calendar notation Cosmology & Astronomy Khan Academy.mp3", "Sentence": "In order to have the same reference point, and it would be too logistically difficult to switch at this time, everyone has essentially settled on this. And it's really just a matter of letters of which naming scheme you pick. But the whole point of this video is that you don't get confused between BC and BCE. You don't think that AD stands for after death. It stands for Anno Domini, the year of the Lord or the year of our Lord. And CE stands for Common Era. But this and this are referring to essentially the same count after the birth of Christ, or this theoretical birth of Christ, which we don't really know when it actually happened."}, {"video_title": "Understanding calendar notation Cosmology & Astronomy Khan Academy.mp3", "Sentence": "You don't think that AD stands for after death. It stands for Anno Domini, the year of the Lord or the year of our Lord. And CE stands for Common Era. But this and this are referring to essentially the same count after the birth of Christ, or this theoretical birth of Christ, which we don't really know when it actually happened. It probably did not happen at the beginning of 1 AD or 1 CE. And these two things both also refer to the same direction in the timeline. One last thing I want to point out is that there is no year zero."}, {"video_title": "Understanding calendar notation Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But this and this are referring to essentially the same count after the birth of Christ, or this theoretical birth of Christ, which we don't really know when it actually happened. It probably did not happen at the beginning of 1 AD or 1 CE. And these two things both also refer to the same direction in the timeline. One last thing I want to point out is that there is no year zero. So if you take either of these naming schemes, you have, so let's go very close to the year one. So there's just some point, this theoretical birth of Christ, which was probably not the actual birth of Christ, but at that theoretical point, right at that, so New Year's, so you have kind of December 31 of the previous year. All of a sudden now, you are on January 1 of 1 AD or CE, depending on how you want to refer to it."}, {"video_title": "Understanding calendar notation Cosmology & Astronomy Khan Academy.mp3", "Sentence": "One last thing I want to point out is that there is no year zero. So if you take either of these naming schemes, you have, so let's go very close to the year one. So there's just some point, this theoretical birth of Christ, which was probably not the actual birth of Christ, but at that theoretical point, right at that, so New Year's, so you have kind of December 31 of the previous year. All of a sudden now, you are on January 1 of 1 AD or CE, depending on how you want to refer to it. And the year before that was 1 BC or BCE, depending on how you want to refer to it. So there is no year zero in this scheme. And then the last thing I want to emphasize, and it might be obvious to you, is the larger a number you have here, the further back you're going in time."}, {"video_title": "Understanding calendar notation Cosmology & Astronomy Khan Academy.mp3", "Sentence": "All of a sudden now, you are on January 1 of 1 AD or CE, depending on how you want to refer to it. And the year before that was 1 BC or BCE, depending on how you want to refer to it. So there is no year zero in this scheme. And then the last thing I want to emphasize, and it might be obvious to you, is the larger a number you have here, the further back you're going in time. Because this is saying how many years before this theoretical birth of Christ. And obviously, larger numbers you have here, this is the further you're going off into the future. And if you wanted to figure out how many years passed between Plato's birth and Columbus sailing across the Atlantic to find the New World, you would say, well, look, it took 428 years to go from Plato's birth to this theoretical birth of Christ."}, {"video_title": "Understanding calendar notation Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And then the last thing I want to emphasize, and it might be obvious to you, is the larger a number you have here, the further back you're going in time. Because this is saying how many years before this theoretical birth of Christ. And obviously, larger numbers you have here, this is the further you're going off into the future. And if you wanted to figure out how many years passed between Plato's birth and Columbus sailing across the Atlantic to find the New World, you would say, well, look, it took 428 years to go from Plato's birth to this theoretical birth of Christ. And then you have another 1,492 years to wait until Columbus gets his ship together. So the total number of years would be, I'll do it right over here, 428 years to get to Christ from Plato's birth. And then you have another 1,492 years to wait for Columbus, so let's see, 8 plus 2, that is 10."}, {"video_title": "Understanding calendar notation Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And if you wanted to figure out how many years passed between Plato's birth and Columbus sailing across the Atlantic to find the New World, you would say, well, look, it took 428 years to go from Plato's birth to this theoretical birth of Christ. And then you have another 1,492 years to wait until Columbus gets his ship together. So the total number of years would be, I'll do it right over here, 428 years to get to Christ from Plato's birth. And then you have another 1,492 years to wait for Columbus, so let's see, 8 plus 2, that is 10. As you can see, I just wanted to add a little arithmetic in this video. So 1 plus 2 plus 9 is 12. And then we have 9, 1 plus 4 plus 4."}, {"video_title": "Firestick farming Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And when we think of farming, we imagine a farmer planting seeds and later harvesting the crops, or maybe having cattle that they can allow to graze and then using that cattle for either meat or milk or wool. But there's actually a different type of farming that predates this association with kind of, I guess what we could call the traditional form of farming. And it predates it by several tens of thousands of years. And we believe that it started with the original inhabitants of Australia. And what they did is, and this is why we call it farming, because if you think about farming in the most general sense, it's really humans using technology to manipulate their environment so it becomes more suitable for humans, so it becomes more suitable for things that humans might want to eat or get milk from or whatever. And this type of farming is called fire stick farming. Fire stick farming."}, {"video_title": "Firestick farming Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And we believe that it started with the original inhabitants of Australia. And what they did is, and this is why we call it farming, because if you think about farming in the most general sense, it's really humans using technology to manipulate their environment so it becomes more suitable for humans, so it becomes more suitable for things that humans might want to eat or get milk from or whatever. And this type of farming is called fire stick farming. Fire stick farming. And I think you can already imagine what it might involve. It involves using fire, which is really a form of technology, or it can be a form of technology, using fire to make the environment more suitable for human activity. And so what the original Australians did, the indigenous Australians, or sometimes referred to as the Aboriginal Australians, Aboriginal Australians, and if you're wondering where the word Aboriginal comes from, you might recognize some parts of it."}, {"video_title": "Firestick farming Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Fire stick farming. And I think you can already imagine what it might involve. It involves using fire, which is really a form of technology, or it can be a form of technology, using fire to make the environment more suitable for human activity. And so what the original Australians did, the indigenous Australians, or sometimes referred to as the Aboriginal Australians, Aboriginal Australians, and if you're wondering where the word Aboriginal comes from, you might recognize some parts of it. Original, you know what that means, the first things, the things that were there from the beginning. And then you have ab, which is Latin for from. So this is literally from the beginning."}, {"video_title": "Firestick farming Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And so what the original Australians did, the indigenous Australians, or sometimes referred to as the Aboriginal Australians, Aboriginal Australians, and if you're wondering where the word Aboriginal comes from, you might recognize some parts of it. Original, you know what that means, the first things, the things that were there from the beginning. And then you have ab, which is Latin for from. So this is literally from the beginning. So when you say Aboriginal Australians, you're really kind of saying the Australians that were there from the beginning. And so what they would do is, is that we believe if you go back 50 or 60,000 years, before the first Aboriginal Australians settled Australia, Australia had much more forest. It still has forest."}, {"video_title": "Firestick farming Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So this is literally from the beginning. So when you say Aboriginal Australians, you're really kind of saying the Australians that were there from the beginning. And so what they would do is, is that we believe if you go back 50 or 60,000 years, before the first Aboriginal Australians settled Australia, Australia had much more forest. It still has forest. This is a modern picture, obviously, of an Australian forest. But what they did is that they set up controlled burns. And what these controlled burns did is that they cleared away a lot of the forest, they cleared away a lot of the brush that's at the bottom of, you know, that's over here, and it made it much more suitable for grassland to develop."}, {"video_title": "Firestick farming Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It still has forest. This is a modern picture, obviously, of an Australian forest. But what they did is that they set up controlled burns. And what these controlled burns did is that they cleared away a lot of the forest, they cleared away a lot of the brush that's at the bottom of, you know, that's over here, and it made it much more suitable for grassland to develop. And the reason why they liked grassland, so let's make a little cycle here of what they did. So they have controlled burns, controlled fires, those controlled fires helped promote grassland. And then once you have grassland, that made the environment more suitable for animals that the original human settlers could essentially live off of, that they could hunt, that they could potentially eat their meat."}, {"video_title": "Firestick farming Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And what these controlled burns did is that they cleared away a lot of the forest, they cleared away a lot of the brush that's at the bottom of, you know, that's over here, and it made it much more suitable for grassland to develop. And the reason why they liked grassland, so let's make a little cycle here of what they did. So they have controlled burns, controlled fires, those controlled fires helped promote grassland. And then once you have grassland, that made the environment more suitable for animals that the original human settlers could essentially live off of, that they could hunt, that they could potentially eat their meat. And so, for example, things like kangaroos, and these supported the human population, which obviously would then do the controlled burns. And you see here, so we could have started off with something like this. Someone provides a controlled burn."}, {"video_title": "Firestick farming Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And then once you have grassland, that made the environment more suitable for animals that the original human settlers could essentially live off of, that they could hunt, that they could potentially eat their meat. And so, for example, things like kangaroos, and these supported the human population, which obviously would then do the controlled burns. And you see here, so we could have started off with something like this. Someone provides a controlled burn. And they were actually pretty scientific about how they did it. They wouldn't just go at the end of summer when everything was hot and ready to just blow up and then start a fire that they couldn't control. They would often do these in seasons knowing that it had a certain level of moisture in the air, it wasn't too hot."}, {"video_title": "Firestick farming Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Someone provides a controlled burn. And they were actually pretty scientific about how they did it. They wouldn't just go at the end of summer when everything was hot and ready to just blow up and then start a fire that they couldn't control. They would often do these in seasons knowing that it had a certain level of moisture in the air, it wasn't too hot. And to a large degree, by doing these controlled burns, not only did it provide an environment, kind of do this fire stick farming, not only did it provide an environment that was suitable for things like kangaroos, some type of things that humans could eat, but it also prevented major fires. And you still see forest strangers doing this type of thing. And there's some reason to believe that what the original Australians did, on some level, was more nuanced and more fine-tuned than even what we do in a modern sense in controlled burns."}, {"video_title": "Firestick farming Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "They would often do these in seasons knowing that it had a certain level of moisture in the air, it wasn't too hot. And to a large degree, by doing these controlled burns, not only did it provide an environment, kind of do this fire stick farming, not only did it provide an environment that was suitable for things like kangaroos, some type of things that humans could eat, but it also prevented major fires. And you still see forest strangers doing this type of thing. And there's some reason to believe that what the original Australians did, on some level, was more nuanced and more fine-tuned than even what we do in a modern sense in controlled burns. So these controlled fires also prevented major uncontrollable fires. Because what happens is if you don't have these controlled fires, then you have brush building up year after year after year. You have stuff building up."}, {"video_title": "Firestick farming Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And there's some reason to believe that what the original Australians did, on some level, was more nuanced and more fine-tuned than even what we do in a modern sense in controlled burns. So these controlled fires also prevented major uncontrollable fires. Because what happens is if you don't have these controlled fires, then you have brush building up year after year after year. You have stuff building up. And then when the fires do occur, they're not going to occur, or they're less likely, the uncontrolled fires are less likely to be started during the winter when the air is cool or when there might be some moisture. They're more likely to occur in the dry season. So you have all this stuff build up, and then when the fire does happen, it happens in the driest season."}, {"video_title": "Firestick farming Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "You have stuff building up. And then when the fires do occur, they're not going to occur, or they're less likely, the uncontrolled fires are less likely to be started during the winter when the air is cool or when there might be some moisture. They're more likely to occur in the dry season. So you have all this stuff build up, and then when the fire does happen, it happens in the driest season. And then when it happens with all of the stuff build up in the dry season, it just becomes uncontrollable. One of the byproducts, or actually there's several byproducts, of this fire stick farming, we believe, is a lot of the grassland in Australia now might have been more forested before. And even when the first European settlers came in the late 1700s, they were kind of surprised when they went into what is now Sydney Harbor, and they said, wow, look at all the grassland here."}, {"video_title": "Firestick farming Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So you have all this stuff build up, and then when the fire does happen, it happens in the driest season. And then when it happens with all of the stuff build up in the dry season, it just becomes uncontrollable. One of the byproducts, or actually there's several byproducts, of this fire stick farming, we believe, is a lot of the grassland in Australia now might have been more forested before. And even when the first European settlers came in the late 1700s, they were kind of surprised when they went into what is now Sydney Harbor, and they said, wow, look at all the grassland here. It almost looks like park space. And then they would let their sheep graze there, and they were surprised, because they had driven out the original inhabitants, and then they were surprised when forests just started to grow up in that grassland. And it was because the original Australians were actually controlling that forest growth to make it more inhabitable for things like kangaroos."}, {"video_title": "Firestick farming Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And even when the first European settlers came in the late 1700s, they were kind of surprised when they went into what is now Sydney Harbor, and they said, wow, look at all the grassland here. It almost looks like park space. And then they would let their sheep graze there, and they were surprised, because they had driven out the original inhabitants, and then they were surprised when forests just started to grow up in that grassland. And it was because the original Australians were actually controlling that forest growth to make it more inhabitable for things like kangaroos. And then when the English settlers came, they started to have their sheep graze in those grasslands. And it also is responsible for the disappearance, we think, of many major, I guess for lack of a better word, megafauna. So really large animals that inhabited Australia for really millions of years until humans showed up."}, {"video_title": "Firestick farming Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And it was because the original Australians were actually controlling that forest growth to make it more inhabitable for things like kangaroos. And then when the English settlers came, they started to have their sheep graze in those grasslands. And it also is responsible for the disappearance, we think, of many major, I guess for lack of a better word, megafauna. So really large animals that inhabited Australia for really millions of years until humans showed up. And this is one of them. It's just neat to look at them. This is called Driptodon optimum, or another way to think of it, the giant wombat."}, {"video_title": "Firestick farming Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So really large animals that inhabited Australia for really millions of years until humans showed up. And this is one of them. It's just neat to look at them. This is called Driptodon optimum, or another way to think of it, the giant wombat. And there's fossils of the giant wombat around 40,000, 50,000 years ago. But they disappeared with humans showing up. And there's multiple ways that you could think about why they disappeared."}, {"video_title": "Firestick farming Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "This is called Driptodon optimum, or another way to think of it, the giant wombat. And there's fossils of the giant wombat around 40,000, 50,000 years ago. But they disappeared with humans showing up. And there's multiple ways that you could think about why they disappeared. They might have, and this is probably the case, they might have been more dependent on the forest habitat. Or this was a more favorable habitat for them than the grasslands, maybe because they ate leaves that were high up. Or another thing is once the forest habitat goes away, they were actually also easier to hunt down."}, {"video_title": "Firestick farming Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And there's multiple ways that you could think about why they disappeared. They might have, and this is probably the case, they might have been more dependent on the forest habitat. Or this was a more favorable habitat for them than the grasslands, maybe because they ate leaves that were high up. Or another thing is once the forest habitat goes away, they were actually also easier to hunt down. Or either way you think about it, they might have just been hunted by humans. But we do see that with humans coming to the Australian continent, you start to see the disappearance. And this isn't the only one, but there was several major species of megafauna, of super large animals that disappeared at that time period."}, {"video_title": "Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And what's neat about them is, one, because they're so large and so bright, you can see them really, really far away. And what's even neater about them is that they're variable, that they pulsate. And because their pulsations are related to their actual luminosity, you know if you see a Cepheid variable star in some distant galaxy, you know what its luminosity actually is if you were kind of at the star, because you can see how its period of pulsation. And so if you know its actual luminosity, and then you know, obviously, its apparent luminosity, you know how much it's gotten dim. And the more dim it's gotten from its actual state, you know the farther away it is. So that's the real value of them. What I want to do in this video is to try to explain why they're variable, why they pulsate."}, {"video_title": "Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And so if you know its actual luminosity, and then you know, obviously, its apparent luminosity, you know how much it's gotten dim. And the more dim it's gotten from its actual state, you know the farther away it is. So that's the real value of them. What I want to do in this video is to try to explain why they're variable, why they pulsate. And to do that, what we're going to think about is doubly and singly ionized helium. And just to review, helium, so neutral helium, let me draw neutral helium. Neutral helium's got two protons, two neutrons, and then two electrons."}, {"video_title": "Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "What I want to do in this video is to try to explain why they're variable, why they pulsate. And to do that, what we're going to think about is doubly and singly ionized helium. And just to review, helium, so neutral helium, let me draw neutral helium. Neutral helium's got two protons, two neutrons, and then two electrons. And obviously, this is not drawn to scale. So this is neutral helium right over here. Now, if you singly ionize helium, you knock off one of these electrons."}, {"video_title": "Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Neutral helium's got two protons, two neutrons, and then two electrons. And obviously, this is not drawn to scale. So this is neutral helium right over here. Now, if you singly ionize helium, you knock off one of these electrons. And these type of things happen in stars, when you have a lot of heat, easier to ionize things. So singly ionized helium will look like this. It'll have the same nucleus, two protons, two neutrons."}, {"video_title": "Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Now, if you singly ionize helium, you knock off one of these electrons. And these type of things happen in stars, when you have a lot of heat, easier to ionize things. So singly ionized helium will look like this. It'll have the same nucleus, two protons, two neutrons. One of the electrons gets knocked off, so now you only have one electron. And now you have a net positive charge. So here, let me do this in a different color."}, {"video_title": "Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It'll have the same nucleus, two protons, two neutrons. One of the electrons gets knocked off, so now you only have one electron. And now you have a net positive charge. So here, let me do this in a different color. This helium now has a net charge. We could write 1 plus here, but if you just write a plus, you implicitly mean a positive charge of 1. Now, you can also doubly ionize helium if the environment is hot enough."}, {"video_title": "Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So here, let me do this in a different color. This helium now has a net charge. We could write 1 plus here, but if you just write a plus, you implicitly mean a positive charge of 1. Now, you can also doubly ionize helium if the environment is hot enough. You can doubly ionize helium. And doubly ionizing helium is essentially knocking off both of the electrons. So then it's really just a helium nucleus like this."}, {"video_title": "Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Now, you can also doubly ionize helium if the environment is hot enough. You can doubly ionize helium. And doubly ionizing helium is essentially knocking off both of the electrons. So then it's really just a helium nucleus like this. This right here is doubly ionized helium. Now, I just said, in order to do this, you have to have a hotter environment. There has to be a hotter environment in order to be able to knock off both."}, {"video_title": "Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So then it's really just a helium nucleus like this. This right here is doubly ionized helium. Now, I just said, in order to do this, you have to have a hotter environment. There has to be a hotter environment in order to be able to knock off both. This electron really doesn't want to leave. To take an electron off of something that's already positive is difficult. You have to have a lot of, really, pressure and temperature."}, {"video_title": "Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "There has to be a hotter environment in order to be able to knock off both. This electron really doesn't want to leave. To take an electron off of something that's already positive is difficult. You have to have a lot of, really, pressure and temperature. This is cooler. And this is all relative. We're talking about the insides of stars."}, {"video_title": "Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "You have to have a lot of, really, pressure and temperature. This is cooler. And this is all relative. We're talking about the insides of stars. So this is a hotter part of the star versus a cooler part of the star, I guess, is the way you think about it. It's a very hot environment by our traditional everyday standards. Now, the other thing about the doubly ionized helium is that it is more opaque, which means it doesn't allow light to go through it."}, {"video_title": "Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "We're talking about the insides of stars. So this is a hotter part of the star versus a cooler part of the star, I guess, is the way you think about it. It's a very hot environment by our traditional everyday standards. Now, the other thing about the doubly ionized helium is that it is more opaque, which means it doesn't allow light to go through it. It actually absorbs light. It is more opaque. It absorbs light."}, {"video_title": "Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Now, the other thing about the doubly ionized helium is that it is more opaque, which means it doesn't allow light to go through it. It actually absorbs light. It is more opaque. It absorbs light. Or another way, it absorbs that light energy. That energy will make it even hotter. So that's just something to think about."}, {"video_title": "Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It absorbs light. Or another way, it absorbs that light energy. That energy will make it even hotter. So that's just something to think about. Now, the singly ionized helium is more transparent. It allows the light to pass through it. So it doesn't get heated as much by photons that are kind of going near it or through it or whatever."}, {"video_title": "Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So that's just something to think about. Now, the singly ionized helium is more transparent. It allows the light to pass through it. So it doesn't get heated as much by photons that are kind of going near it or through it or whatever. It allows them to go through it. Here, the photons are going to actually heat up the ion. So let's think about how this might cause a Cepheid variable to pulsate."}, {"video_title": "Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So it doesn't get heated as much by photons that are kind of going near it or through it or whatever. It allows them to go through it. Here, the photons are going to actually heat up the ion. So let's think about how this might cause a Cepheid variable to pulsate. So assuming that Cepheid variables have large enough quantities, I should say, of these ions, we can imagine that when a Cepheid variable is dim. So let me draw a dim Cepheid variable. So I'll draw that like I'll draw this in a dim color."}, {"video_title": "Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So let's think about how this might cause a Cepheid variable to pulsate. So assuming that Cepheid variables have large enough quantities, I should say, of these ions, we can imagine that when a Cepheid variable is dim. So let me draw a dim Cepheid variable. So I'll draw that like I'll draw this in a dim color. So this is a dim Cepheid variable right here. In its dim state, just like this, you have a lot of the doubly ionized helium in the star, at least kind of the outer surface of the star. And so this does not allow a lot of light to pass through."}, {"video_title": "Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So I'll draw that like I'll draw this in a dim color. So this is a dim Cepheid variable right here. In its dim state, just like this, you have a lot of the doubly ionized helium in the star, at least kind of the outer surface of the star. And so this does not allow a lot of light to pass through. So this is the dim part of the pulsation of the Cepheid variable. Now, because this doubly ionized helium is opaque, it is absorbing the light. It is getting heated."}, {"video_title": "Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And so this does not allow a lot of light to pass through. So this is the dim part of the pulsation of the Cepheid variable. Now, because this doubly ionized helium is opaque, it is absorbing the light. It is getting heated. It is getting heated. And because it's getting heated, it'll cause the star to expand. So because it's getting heated, it'll become more energetic, and the star will actually expand."}, {"video_title": "Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It is getting heated. It is getting heated. And because it's getting heated, it'll cause the star to expand. So because it's getting heated, it'll become more energetic, and the star will actually expand. Now, as the star expands, because this doubly ionized helium is getting heated, what's going to happen? The further away you are from the core of the star, the cooler it gets. So this expanded because it was getting heated, but then because it expanded, the outer layers of the star become cooler."}, {"video_title": "Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So because it's getting heated, it'll become more energetic, and the star will actually expand. Now, as the star expands, because this doubly ionized helium is getting heated, what's going to happen? The further away you are from the core of the star, the cooler it gets. So this expanded because it was getting heated, but then because it expanded, the outer layers of the star become cooler. And since they're cooler, helium won't be doubly ionized anymore. It'll get an electron from each helium atom. It can now get an electron from the plasma, I guess we can say, to become singly ionized helium."}, {"video_title": "Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So this expanded because it was getting heated, but then because it expanded, the outer layers of the star become cooler. And since they're cooler, helium won't be doubly ionized anymore. It'll get an electron from each helium atom. It can now get an electron from the plasma, I guess we can say, to become singly ionized helium. So now we have singly ionized helium. And now the star is going to be more transparent. It's going to allow more light to pass through it."}, {"video_title": "Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It can now get an electron from the plasma, I guess we can say, to become singly ionized helium. So now we have singly ionized helium. And now the star is going to be more transparent. It's going to allow more light to pass through it. So now this is the bright part of the pulsation. It's going to allow more light through. So now it is bright."}, {"video_title": "Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It's going to allow more light to pass through it. So now this is the bright part of the pulsation. It's going to allow more light through. So now it is bright. The star is bright. But what's happening now? Because the light is no longer, or it's not being absorbed as well by the helium when it was a doubly ionized helium, now it's letting most of the light, or a lot more of the light, get through."}, {"video_title": "Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So now it is bright. The star is bright. But what's happening now? Because the light is no longer, or it's not being absorbed as well by the helium when it was a doubly ionized helium, now it's letting most of the light, or a lot more of the light, get through. It's not going to get heated as much. And so it won't have the kinetic energy to kind of keep pushing out, to keep moving outward. And so it'll collapse back into the star."}, {"video_title": "Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Because the light is no longer, or it's not being absorbed as well by the helium when it was a doubly ionized helium, now it's letting most of the light, or a lot more of the light, get through. It's not going to get heated as much. And so it won't have the kinetic energy to kind of keep pushing out, to keep moving outward. And so it'll collapse back into the star. And so then this will cool down and collapse back in. And when it collapses back in, what's going to happen? When it collapses back in, when these helium atoms get closer to the center of the star, to the core of the star, they're going to be heated again, because they're closer now to the core."}, {"video_title": "Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And so it'll collapse back into the star. And so then this will cool down and collapse back in. And when it collapses back in, what's going to happen? When it collapses back in, when these helium atoms get closer to the center of the star, to the core of the star, they're going to be heated again, because they're closer now to the core. And when they get heated, they're going to become doubly ionized. So then we have doubly ionized helium again. And then the cycle will go again."}, {"video_title": "Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "When it collapses back in, when these helium atoms get closer to the center of the star, to the core of the star, they're going to be heated again, because they're closer now to the core. And when they get heated, they're going to become doubly ionized. So then we have doubly ionized helium again. And then the cycle will go again. It is now opaque. It will now absorb more energy. That'll cause it to have more kinetic energy to expand."}, {"video_title": "Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And then the cycle will go again. It is now opaque. It will now absorb more energy. That'll cause it to have more kinetic energy to expand. Once it expands, it'll get cool again, and transparent, and bright. And so this is the current best theory of why Cepheid variable stars are variable to begin with. It's this whole notion of having the doubly ionized helium versus the singly ionized helium in kind of the outer layers of the star itself."}, {"video_title": "Radius of observable universe (correction) Cosmology & Astronomy Khan Academy.mp3", "Sentence": "I said that you had this state 300,000 years. So we talk about the Big Bang happening 13.7 billion years ago. And then I talk about this state of affairs where we're maybe 30 million light years away from the edge of the observable universe, the current observable universe. And I said that this was about 300,000 years after the Big Bang. That's what I talked about in the last video. That was our starting point when the photon started leaving that point, and obviously the universe expanding. The photon, it kind of traversed more and more, but still had more and more to travel as the universe expanded, as all of space expanded."}, {"video_title": "Radius of observable universe (correction) Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And I said that this was about 300,000 years after the Big Bang. That's what I talked about in the last video. That was our starting point when the photon started leaving that point, and obviously the universe expanding. The photon, it kind of traversed more and more, but still had more and more to travel as the universe expanded, as all of space expanded. But this is 300,000 years after the Big Bang. Now my brain, because I was kind of not thinking hard enough about it, I said, hey, this was 13.4 billion years ago. That's what I incorrectly said in the last video."}, {"video_title": "Radius of observable universe (correction) Cosmology & Astronomy Khan Academy.mp3", "Sentence": "The photon, it kind of traversed more and more, but still had more and more to travel as the universe expanded, as all of space expanded. But this is 300,000 years after the Big Bang. Now my brain, because I was kind of not thinking hard enough about it, I said, hey, this was 13.4 billion years ago. That's what I incorrectly said in the last video. I said that this is 13.4 billion years ago. That's what I said in the last video, and that is wrong. Because if this was 13.4 billion years ago, this would have been 300 million years after the Big Bang."}, {"video_title": "Radius of observable universe (correction) Cosmology & Astronomy Khan Academy.mp3", "Sentence": "That's what I incorrectly said in the last video. I said that this is 13.4 billion years ago. That's what I said in the last video, and that is wrong. Because if this was 13.4 billion years ago, this would have been 300 million years after the Big Bang. We were talking about only 300,000 years after the Big Bang, so it wouldn't have taken that many decimal places off of something in the billions. The correct answer is this would have been only a little less than 13.7 billion years. It actually wouldn't have even made the significant digits."}, {"video_title": "Radius of observable universe (correction) Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Because if this was 13.4 billion years ago, this would have been 300 million years after the Big Bang. We were talking about only 300,000 years after the Big Bang, so it wouldn't have taken that many decimal places off of something in the billions. The correct answer is this would have been only a little less than 13.7 billion years. It actually wouldn't have even made the significant digits. This is still approximately 13.7 billion years ago. So I wanted to just make that correction. It was a slight error."}, {"video_title": "Radius of observable universe (correction) Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It actually wouldn't have even made the significant digits. This is still approximately 13.7 billion years ago. So I wanted to just make that correction. It was a slight error. I shouldn't have viewed this as 0.3 billion years. This is only 0.3 million years. It doesn't even basically change the precision on this number right over here."}, {"video_title": "Milankovitch cycles precession and obliquity Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So let me draw the Earth's tilt relative to that orbital plane right over here. So if this is the orbital plane right over here, so we're looking right directly sideways on this orbital plane, right sideways along this orbital plane that I've drawn in orange, and maybe at the point in Earth's orbit right now, maybe the sun is to the left, and so the rays from the sun are coming in this general direction, we've learned that Earth has a certain tilt. Earth has a tilt, and what I mean by that, it means if you think about the axis around which it's rotating, it's not straight up from the orbital plane, it is at an angle. Let me draw that. So if I were to draw an arrow that's coming out of the North Pole, it would look like that, and I'll draw an arrow coming out of the South Pole, and the Earth is rotating in that direction right over here, and you notice this axis that I've drawn this arrow on, it is not straight up and down, and right now it is an angle of, it is at an angle of 23.4 degrees with the vertical, with being straight up and down. And we've learned how this is what is the primary cause of our seasons, in that when the Northern Hemisphere is pointed towards the sun, it's getting a disproportionate amount of the solar radiation, whatever is going through the atmosphere has to go through less atmosphere, and the things in the Northern Hemisphere are getting more daylight. And when the Earth is on the other side of the sun, and the Northern Hemisphere is pointed away from the sun, then the opposite is going to happen, and the reverse is true for the Southern Hemisphere."}, {"video_title": "Milankovitch cycles precession and obliquity Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Let me draw that. So if I were to draw an arrow that's coming out of the North Pole, it would look like that, and I'll draw an arrow coming out of the South Pole, and the Earth is rotating in that direction right over here, and you notice this axis that I've drawn this arrow on, it is not straight up and down, and right now it is an angle of, it is at an angle of 23.4 degrees with the vertical, with being straight up and down. And we've learned how this is what is the primary cause of our seasons, in that when the Northern Hemisphere is pointed towards the sun, it's getting a disproportionate amount of the solar radiation, whatever is going through the atmosphere has to go through less atmosphere, and the things in the Northern Hemisphere are getting more daylight. And when the Earth is on the other side of the sun, and the Northern Hemisphere is pointed away from the sun, then the opposite is going to happen, and the reverse is true for the Southern Hemisphere. But in that video when we talk about how tilt can affect the seasons, I also kind of hinted a little bit that this is the current tilt right now, and over long periods of time that this tilt will change. And in particular, it will vary, and even the boundaries for this varying are different for the past million years than they will be for the next million years, but it varies roughly between 22.1 degrees and 24.5 degrees. And just to make it clear that it's not wobbling back and forth like this, and just to visualize 22.1 versus 24.5, it's not a huge difference."}, {"video_title": "Milankovitch cycles precession and obliquity Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And when the Earth is on the other side of the sun, and the Northern Hemisphere is pointed away from the sun, then the opposite is going to happen, and the reverse is true for the Southern Hemisphere. But in that video when we talk about how tilt can affect the seasons, I also kind of hinted a little bit that this is the current tilt right now, and over long periods of time that this tilt will change. And in particular, it will vary, and even the boundaries for this varying are different for the past million years than they will be for the next million years, but it varies roughly between 22.1 degrees and 24.5 degrees. And just to make it clear that it's not wobbling back and forth like this, and just to visualize 22.1 versus 24.5, it's not a huge difference. So if this is 23.4, and I'm not measuring exactly, maybe pointing in this direction, maybe pointing in that, maybe 22.1 would look something like that. In fact, I've exaggerated it. And maybe 24.5 would look something like that."}, {"video_title": "Milankovitch cycles precession and obliquity Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And just to make it clear that it's not wobbling back and forth like this, and just to visualize 22.1 versus 24.5, it's not a huge difference. So if this is 23.4, and I'm not measuring exactly, maybe pointing in this direction, maybe pointing in that, maybe 22.1 would look something like that. In fact, I've exaggerated it. And maybe 24.5 would look something like that. And so it's not a huge difference, but it is enough of a difference, so we believe, to actually have a significant impact on what the climate is like, or what the seasons are like, especially in terms of how much of a chance different parts of our planet have a chance to freeze over, or not freeze over, and all the rest, or how much sunlight they get, and all the rest. So it has some impact, but I want to make it clear that it takes a long period of time. It actually takes 41,000 years to go from a minimum tilt to a maximum tilt, and then back to a minimum tilt."}, {"video_title": "Milankovitch cycles precession and obliquity Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And maybe 24.5 would look something like that. And so it's not a huge difference, but it is enough of a difference, so we believe, to actually have a significant impact on what the climate is like, or what the seasons are like, especially in terms of how much of a chance different parts of our planet have a chance to freeze over, or not freeze over, and all the rest, or how much sunlight they get, and all the rest. So it has some impact, but I want to make it clear that it takes a long period of time. It actually takes 41,000 years to go from a minimum tilt to a maximum tilt, and then back to a minimum tilt. 41,000 years. And right now, at a tilt of 23.4 degrees, we're someplace right smack in between. And we think the last maximum was at 8,700 BCE, before the common era, or you could say before Christ, and that the next minimum, when our tilt has been minimized, the next time our tilt will be minimized, will be at 11,800."}, {"video_title": "Milankovitch cycles precession and obliquity Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It actually takes 41,000 years to go from a minimum tilt to a maximum tilt, and then back to a minimum tilt. 41,000 years. And right now, at a tilt of 23.4 degrees, we're someplace right smack in between. And we think the last maximum was at 8,700 BCE, before the common era, or you could say before Christ, and that the next minimum, when our tilt has been minimized, the next time our tilt will be minimized, will be at 11,800. So this isn't something that's happening overnight, but it is something that could affect our climate over long periods of time. And this is just one factor, and sometimes this changing of the tilt, a fancier word for tilt is sometimes given, is obliquity. But this is really just a fancy word for tilt."}, {"video_title": "Milankovitch cycles precession and obliquity Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And we think the last maximum was at 8,700 BCE, before the common era, or you could say before Christ, and that the next minimum, when our tilt has been minimized, the next time our tilt will be minimized, will be at 11,800. So this isn't something that's happening overnight, but it is something that could affect our climate over long periods of time. And this is just one factor, and sometimes this changing of the tilt, a fancier word for tilt is sometimes given, is obliquity. But this is really just a fancy word for tilt. This changing of the obliquity, or changing of the tilt, is one of these changes in Earth's rotation, or Earth's orbit around the sun, that might have long-term cycles or effects on Earth's climate, and maybe they do help cause certain ice ages when they act together with each other over certain cycles. And broadly, this entire class of cycles are called Milankovitch cycles. Milankovitch, he was a Serbian scientist who was the guy who theorized that these changes in Earth's orbit might be responsible for long-term climate change, or maybe some cycles where we enter ice ages and get out of ice ages, or we have more extreme or less extreme weather."}, {"video_title": "Milankovitch cycles precession and obliquity Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But this is really just a fancy word for tilt. This changing of the obliquity, or changing of the tilt, is one of these changes in Earth's rotation, or Earth's orbit around the sun, that might have long-term cycles or effects on Earth's climate, and maybe they do help cause certain ice ages when they act together with each other over certain cycles. And broadly, this entire class of cycles are called Milankovitch cycles. Milankovitch, he was a Serbian scientist who was the guy who theorized that these changes in Earth's orbit might be responsible for long-term climate change, or maybe some cycles where we enter ice ages and get out of ice ages, or we have more extreme or less extreme weather. So these are Milankovitch cycles. And changes in the tilt, or the obliquity, are just one of the possible factors playing into Milankovitch cycles. And what I want to do in this video and the next few is talk about all of the different factors, or at least summarize all of the different factors."}, {"video_title": "Milankovitch cycles precession and obliquity Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Milankovitch, he was a Serbian scientist who was the guy who theorized that these changes in Earth's orbit might be responsible for long-term climate change, or maybe some cycles where we enter ice ages and get out of ice ages, or we have more extreme or less extreme weather. So these are Milankovitch cycles. And changes in the tilt, or the obliquity, are just one of the possible factors playing into Milankovitch cycles. And what I want to do in this video and the next few is talk about all of the different factors, or at least summarize all of the different factors. Now another one, this one is pretty intuitive for me, that this tilt can change. One that's a little bit less intuitive when you first think about it is something called precession. And the idea behind precession, I guess the best analogy I can think of, is if you imagine a top, or maybe you can imagine Earth as a top right over here."}, {"video_title": "Milankovitch cycles precession and obliquity Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And what I want to do in this video and the next few is talk about all of the different factors, or at least summarize all of the different factors. Now another one, this one is pretty intuitive for me, that this tilt can change. One that's a little bit less intuitive when you first think about it is something called precession. And the idea behind precession, I guess the best analogy I can think of, is if you imagine a top, or maybe you can imagine Earth as a top right over here. The top is spinning in this direction. And obliquity tells you essentially how much it's wobbling. Well, actually let me think of it this way."}, {"video_title": "Milankovitch cycles precession and obliquity Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And the idea behind precession, I guess the best analogy I can think of, is if you imagine a top, or maybe you can imagine Earth as a top right over here. The top is spinning in this direction. And obliquity tells you essentially how much it's wobbling. Well, actually let me think of it this way. Imagine a wobbling top. So it's rotating like this, it's tilted, and then it's also, if you imagine that this was a pole up here that's coming out of the pole, if this was actually a physical arrow, that that arrow itself would be rotating. So the best way to think about it is a wobbling top."}, {"video_title": "Milankovitch cycles precession and obliquity Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Well, actually let me think of it this way. Imagine a wobbling top. So it's rotating like this, it's tilted, and then it's also, if you imagine that this was a pole up here that's coming out of the pole, if this was actually a physical arrow, that that arrow itself would be rotating. So the best way to think about it is a wobbling top. After some point of time, this thing would wobble, so it would look like this. So now the arrow is pointing that way. And if you wait a few more seconds, now maybe the arrow is pointing a little bit out of the page."}, {"video_title": "Milankovitch cycles precession and obliquity Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So the best way to think about it is a wobbling top. After some point of time, this thing would wobble, so it would look like this. So now the arrow is pointing that way. And if you wait a few more seconds, now maybe the arrow is pointing a little bit out of the page. And then you wait a few more seconds, then it's pointing in this direction, then it's pointing into the page. And so this whole time the obliquity isn't changing. The obliquity you can kind of view it as how far is that wobble?"}, {"video_title": "Milankovitch cycles precession and obliquity Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And if you wait a few more seconds, now maybe the arrow is pointing a little bit out of the page. And then you wait a few more seconds, then it's pointing in this direction, then it's pointing into the page. And so this whole time the obliquity isn't changing. The obliquity you can kind of view it as how far is that wobble? You can imagine how far from vertical is that wobble. And no matter where we are in that rotation, it hasn't changed. And you can imagine it as a procession as where we are in the wobble."}, {"video_title": "Milankovitch cycles precession and obliquity Cosmology & Astronomy Khan Academy.mp3", "Sentence": "The obliquity you can kind of view it as how far is that wobble? You can imagine how far from vertical is that wobble. And no matter where we are in that rotation, it hasn't changed. And you can imagine it as a procession as where we are in the wobble. And I want to, this is a little bit hard to visualize, and hopefully as we think about it in different ways and I draw different diagrams, it'll make it a little bit clearer. But I want to make it clear, just as it takes a long time for the inclination to change from a minimum value to a maximum value and back, it takes a huge amount of time for Earth's precession to change in a significant way. So for this top to kind of, if you imagined this arrow popping out, for this arrow to actually trace out an entire loop, it takes 26,000 years."}, {"video_title": "Milankovitch cycles precession and obliquity Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And you can imagine it as a procession as where we are in the wobble. And I want to, this is a little bit hard to visualize, and hopefully as we think about it in different ways and I draw different diagrams, it'll make it a little bit clearer. But I want to make it clear, just as it takes a long time for the inclination to change from a minimum value to a maximum value and back, it takes a huge amount of time for Earth's precession to change in a significant way. So for this top to kind of, if you imagined this arrow popping out, for this arrow to actually trace out an entire loop, it takes 26,000 years. So 26,000 years to have an entire cycle of precession. Now what I want to do is think about, given that this precession is occurring, I want to think about how that would affect our seasons or how it would actually affect how we think about the year, the calendar. So let's draw the orbit of Earth around the sun."}, {"video_title": "Milankovitch cycles precession and obliquity Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So for this top to kind of, if you imagined this arrow popping out, for this arrow to actually trace out an entire loop, it takes 26,000 years. So 26,000 years to have an entire cycle of precession. Now what I want to do is think about, given that this precession is occurring, I want to think about how that would affect our seasons or how it would actually affect how we think about the year, the calendar. So let's draw the orbit of Earth around the sun. So here is my sun right over here, and here is the orbit of Earth. And I'm not going to think too much. I'm going to assume that it's almost circular for the sake of this video."}, {"video_title": "Milankovitch cycles precession and obliquity Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So let's draw the orbit of Earth around the sun. So here is my sun right over here, and here is the orbit of Earth. And I'm not going to think too much. I'm going to assume that it's almost circular for the sake of this video. In future videos, we'll talk about how the eccentricity or how elliptical the orbit is can also affect the Milankovitch cycles or play into the Milankovitch cycles. But let's just draw the orbit of Earth around the sun over here. And so you can imagine this is at one point in time."}, {"video_title": "Milankovitch cycles precession and obliquity Cosmology & Astronomy Khan Academy.mp3", "Sentence": "I'm going to assume that it's almost circular for the sake of this video. In future videos, we'll talk about how the eccentricity or how elliptical the orbit is can also affect the Milankovitch cycles or play into the Milankovitch cycles. But let's just draw the orbit of Earth around the sun over here. And so you can imagine this is at one point in time. This is the Earth. Let's say it is tilted towards the sun right now. So in the northern hemisphere, and I'm assuming this arrow is coming out of the North Pole, this would be the summer in the northern hemisphere."}, {"video_title": "Milankovitch cycles precession and obliquity Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And so you can imagine this is at one point in time. This is the Earth. Let's say it is tilted towards the sun right now. So in the northern hemisphere, and I'm assuming this arrow is coming out of the North Pole, this would be the summer in the northern hemisphere. And then if you had no precession, absolutely no precession, when you go to this time of year, you still have the same direction of tilt. Let me do that in blue. You still have the same direction of tilt."}, {"video_title": "Milankovitch cycles precession and obliquity Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So in the northern hemisphere, and I'm assuming this arrow is coming out of the North Pole, this would be the summer in the northern hemisphere. And then if you had no precession, absolutely no precession, when you go to this time of year, you still have the same direction of tilt. Let me do that in blue. You still have the same direction of tilt. We're still pointing to the same part of the universe. We still have the same north star. At this time, we're still tilting in the same direction relative to the universe, but we're not tilting away from the sun."}, {"video_title": "Milankovitch cycles precession and obliquity Cosmology & Astronomy Khan Academy.mp3", "Sentence": "You still have the same direction of tilt. We're still pointing to the same part of the universe. We still have the same north star. At this time, we're still tilting in the same direction relative to the universe, but we're not tilting away from the sun. And now this would be the winter in the northern hemisphere. And we'd keep going around. If you had no precession, when you get back to this point over here, we'd be tilting in the exact same direction."}, {"video_title": "Milankovitch cycles precession and obliquity Cosmology & Astronomy Khan Academy.mp3", "Sentence": "At this time, we're still tilting in the same direction relative to the universe, but we're not tilting away from the sun. And now this would be the winter in the northern hemisphere. And we'd keep going around. If you had no precession, when you get back to this point over here, we'd be tilting in the exact same direction. If your obliquity or if your tilt changed a little bit, you might move up or down, away or towards the sun a little bit. But this is all assuming no precession. Now I'm going to think about what happens if you do have precession."}, {"video_title": "Milankovitch cycles precession and obliquity Cosmology & Astronomy Khan Academy.mp3", "Sentence": "If you had no precession, when you get back to this point over here, we'd be tilting in the exact same direction. If your obliquity or if your tilt changed a little bit, you might move up or down, away or towards the sun a little bit. But this is all assuming no precession. Now I'm going to think about what happens if you do have precession. So what's happening with precession is when you go around one time around the sun, by the time you get to this point again, you're not pointing at exactly the same direction. You're now pointing a little bit further. So this arrow, let me draw it a little bit bigger."}, {"video_title": "Milankovitch cycles precession and obliquity Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Now I'm going to think about what happens if you do have precession. So what's happening with precession is when you go around one time around the sun, by the time you get to this point again, you're not pointing at exactly the same direction. You're now pointing a little bit further. So this arrow, let me draw it a little bit bigger. So this is the earth, and this is that arrow. And this is hard to visualize, or at least it's hard for me to visualize. Well, once you get it, it's easier to visualize."}, {"video_title": "Milankovitch cycles precession and obliquity Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So this arrow, let me draw it a little bit bigger. So this is the earth, and this is that arrow. And this is hard to visualize, or at least it's hard for me to visualize. Well, once you get it, it's easier to visualize. But the first time I tried to understand it, it was hard for me to understand how precession was different than obliquity or different than tilt. Obliquity is how much we're going from vertical. And so if we had no precession, we would be exactly pointing in that same direction every year."}, {"video_title": "Milankovitch cycles precession and obliquity Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Well, once you get it, it's easier to visualize. But the first time I tried to understand it, it was hard for me to understand how precession was different than obliquity or different than tilt. Obliquity is how much we're going from vertical. And so if we had no precession, we would be exactly pointing in that same direction every year. Now with just precession alone, what happens is every year, this arrow is slowly tracing out a circle that goes like this. So I'm going to exaggerate how much it's happening, just so that you can visualize it. So maybe after several years, that arrow is not, when you're at that same point relative to the sun, that same point in the solar system, that arrow is no longer pointing in that direction."}, {"video_title": "Milankovitch cycles precession and obliquity Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And so if we had no precession, we would be exactly pointing in that same direction every year. Now with just precession alone, what happens is every year, this arrow is slowly tracing out a circle that goes like this. So I'm going to exaggerate how much it's happening, just so that you can visualize it. So maybe after several years, that arrow is not, when you're at that same point relative to the sun, that same point in the solar system, that arrow is no longer pointing in that direction. It is now traced out a little bit of that circle. So it is now pointing in this direction. So if it is now pointing in this direction, will that same point in the solar system, that same point relative to the sun, that same exact point in the orbit, will it still be the summer in the northern hemisphere?"}, {"video_title": "Milankovitch cycles precession and obliquity Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So maybe after several years, that arrow is not, when you're at that same point relative to the sun, that same point in the solar system, that arrow is no longer pointing in that direction. It is now traced out a little bit of that circle. So it is now pointing in this direction. So if it is now pointing in this direction, will that same point in the solar system, that same point relative to the sun, that same exact point in the orbit, will it still be the summer in the northern hemisphere? Well, it won't, because we're now not pointing directly, or we're not most inclined to the sun at that point. Now we would have been most inclined to the sun a little bit earlier in the year or a little bit earlier in the orbit. So we would have been most inclined to the sun maybe over here."}, {"video_title": "Milankovitch cycles precession and obliquity Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So if it is now pointing in this direction, will that same point in the solar system, that same point relative to the sun, that same exact point in the orbit, will it still be the summer in the northern hemisphere? Well, it won't, because we're now not pointing directly, or we're not most inclined to the sun at that point. Now we would have been most inclined to the sun a little bit earlier in the year or a little bit earlier in the orbit. So we would have been most inclined to the sun maybe over here. And it would take many, many, many actual thousands of years for the precession to change this much. But then over here, this is where, at this point in that year, when we would be pointed most towards the sun. So what the real effect of precession is doing to our seasons and doing to what our sense of what our year is, is that every year, relative to our orbit on Earth, because Earth is kind of a top that's slowly circling, slowly tracing out this circle with, I guess you could say, with its pole, what it's doing is it's making it tilt towards the sun or away from the sun a little bit earlier each year."}, {"video_title": "Milankovitch cycles precession and obliquity Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So we would have been most inclined to the sun maybe over here. And it would take many, many, many actual thousands of years for the precession to change this much. But then over here, this is where, at this point in that year, when we would be pointed most towards the sun. So what the real effect of precession is doing to our seasons and doing to what our sense of what our year is, is that every year, relative to our orbit on Earth, because Earth is kind of a top that's slowly circling, slowly tracing out this circle with, I guess you could say, with its pole, what it's doing is it's making it tilt towards the sun or away from the sun a little bit earlier each year. I know it's hard to visualize, but you could even take a top out and have a basketball as the sun. And if you play with it, you'll see how that works. And precession is another one of those factors that play into, I should say, the Milankovitch cycles."}, {"video_title": "Are southern hemisphere seasons more severe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And when it is closer to the sun, so let's say that this is the time of the orbit when it's closer to the sun, this is the perihelion. And when it's furthest from the sun, and I'm exaggerating the difference, this is aphelion. This is the aphelion in our orbit when we are furthest from the sun. Maybe our orbit looks something like this. And what I point out in the first video where we discussed this is that this is not the cause of the seasons. Even though we are 3% closer right now, the way our orbit is set up, and we'll see in future videos that the difference or the eccentricity or how elliptical the orbit is does change over time, how much it deviates from being circular, that's one way to think about eccentricity, that does change over time. But right now, when we are closest to the sun, we are 3% closer than when we are furthest from the sun."}, {"video_title": "Are southern hemisphere seasons more severe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Maybe our orbit looks something like this. And what I point out in the first video where we discussed this is that this is not the cause of the seasons. Even though we are 3% closer right now, the way our orbit is set up, and we'll see in future videos that the difference or the eccentricity or how elliptical the orbit is does change over time, how much it deviates from being circular, that's one way to think about eccentricity, that does change over time. But right now, when we are closest to the sun, we are 3% closer than when we are furthest from the sun. So 3% closer than at aphelion. And we point out in the first video when we discussed this, that this is not the cause of the seasons. And in particular, perihelion, when we are closest to the sun, when we actually have the most radiation from the sun, that's actually when we have the northern hemisphere winter."}, {"video_title": "Are southern hemisphere seasons more severe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But right now, when we are closest to the sun, we are 3% closer than when we are furthest from the sun. So 3% closer than at aphelion. And we point out in the first video when we discussed this, that this is not the cause of the seasons. And in particular, perihelion, when we are closest to the sun, when we actually have the most radiation from the sun, that's actually when we have the northern hemisphere winter. So this occurs right over here. This occurs in January. And aphelion occurs in July."}, {"video_title": "Are southern hemisphere seasons more severe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And in particular, perihelion, when we are closest to the sun, when we actually have the most radiation from the sun, that's actually when we have the northern hemisphere winter. So this occurs right over here. This occurs in January. And aphelion occurs in July. Now, based on this, this might lead to an interesting question. Because, so let's think about January when we're at perihelion, and let's think about July when we're at aphelion. And let me draw a quick globe right over here."}, {"video_title": "Are southern hemisphere seasons more severe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And aphelion occurs in July. Now, based on this, this might lead to an interesting question. Because, so let's think about January when we're at perihelion, and let's think about July when we're at aphelion. And let me draw a quick globe right over here. And let's make that the equator. And I'll draw it in both situations. So January is obviously when we have the northern hemisphere winter."}, {"video_title": "Are southern hemisphere seasons more severe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And let me draw a quick globe right over here. And let's make that the equator. And I'll draw it in both situations. So January is obviously when we have the northern hemisphere winter. So I'll paint it in blue right over here. It is winter. And July is when we have the northern hemisphere summer or the southern hemisphere winter."}, {"video_title": "Are southern hemisphere seasons more severe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So January is obviously when we have the northern hemisphere winter. So I'll paint it in blue right over here. It is winter. And July is when we have the northern hemisphere summer or the southern hemisphere winter. So then we have winter during July in the southern hemisphere. And let me put summer in a more summery color. I guess that orange is a pretty good color."}, {"video_title": "Are southern hemisphere seasons more severe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And July is when we have the northern hemisphere summer or the southern hemisphere winter. So then we have winter during July in the southern hemisphere. And let me put summer in a more summery color. I guess that orange is a pretty good color. That's not orange. Here's orange. All right."}, {"video_title": "Are southern hemisphere seasons more severe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "I guess that orange is a pretty good color. That's not orange. Here's orange. All right. That's orange. And that's orange. So these are summer."}, {"video_title": "Are southern hemisphere seasons more severe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "All right. That's orange. And that's orange. So these are summer. So that's the summer in the southern hemisphere, which occurs during the winter in the northern hemisphere, and vice versa. Summer in the northern hemisphere occurs during winter in the southern hemisphere. And so the question might be rising in your head."}, {"video_title": "Are southern hemisphere seasons more severe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So these are summer. So that's the summer in the southern hemisphere, which occurs during the winter in the northern hemisphere, and vice versa. Summer in the northern hemisphere occurs during winter in the southern hemisphere. And so the question might be rising in your head. And I did see a few comments on that first video asking this question. And it's a good one. If we are closer to the sun in January, or we are really closest to the sun in January, and so we're getting more solar radiation in January, does that moderate the winter in the northern hemisphere?"}, {"video_title": "Are southern hemisphere seasons more severe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And so the question might be rising in your head. And I did see a few comments on that first video asking this question. And it's a good one. If we are closer to the sun in January, or we are really closest to the sun in January, and so we're getting more solar radiation in January, does that moderate the winter in the northern hemisphere? Or I guess another way to think about it, does it make the summer in the southern hemisphere when we are closer to the sun, does it make it more extreme or hotter? And vice versa, in July, when we are furthest from the sun, does that moderate the northern hemisphere winter? Because it's hot up there, but hey, we're a little bit further from the sun."}, {"video_title": "Are southern hemisphere seasons more severe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "If we are closer to the sun in January, or we are really closest to the sun in January, and so we're getting more solar radiation in January, does that moderate the winter in the northern hemisphere? Or I guess another way to think about it, does it make the summer in the southern hemisphere when we are closer to the sun, does it make it more extreme or hotter? And vice versa, in July, when we are furthest from the sun, does that moderate the northern hemisphere winter? Because it's hot up there, but hey, we're a little bit further from the sun. And does it make the southern hemisphere winter colder? So once again, does it make this more extreme? Because it's already winter and we're further from the sun, so maybe we're also getting less radiation."}, {"video_title": "Are southern hemisphere seasons more severe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Because it's hot up there, but hey, we're a little bit further from the sun. And does it make the southern hemisphere winter colder? So once again, does it make this more extreme? Because it's already winter and we're further from the sun, so maybe we're also getting less radiation. And so there's a couple ways to think about it. One, it is true that when we're further, we are getting a little bit less radiation from the sun, or we're getting heated up a little bit less. But the one reality is that the southern hemisphere climate as a whole is not more extreme, despite getting heated up, getting more solar energy in the summer, and getting less solar energy in the winter."}, {"video_title": "Are southern hemisphere seasons more severe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Because it's already winter and we're further from the sun, so maybe we're also getting less radiation. And so there's a couple ways to think about it. One, it is true that when we're further, we are getting a little bit less radiation from the sun, or we're getting heated up a little bit less. But the one reality is that the southern hemisphere climate as a whole is not more extreme, despite getting heated up, getting more solar energy in the summer, and getting less solar energy in the winter. And the reason why it is not as extreme, let me draw the equator here, just so that we can separate our hemispheres. The main reason, it is believed, why it is not more extreme is that the southern hemisphere has a lot more water in it. So just if you look at the surface of the southern hemisphere, you're looking at a lot more water than the surface of the northern hemisphere."}, {"video_title": "Are southern hemisphere seasons more severe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But the one reality is that the southern hemisphere climate as a whole is not more extreme, despite getting heated up, getting more solar energy in the summer, and getting less solar energy in the winter. And the reason why it is not as extreme, let me draw the equator here, just so that we can separate our hemispheres. The main reason, it is believed, why it is not more extreme is that the southern hemisphere has a lot more water in it. So just if you look at the surface of the southern hemisphere, you're looking at a lot more water than the surface of the northern hemisphere. This is, of course, a Mercator projection, and so it distorts things so that things near the poles get really kind of built up to look really huge, even though they really aren't that big. Greenland really isn't larger than South America. It just spreads them out so that you can flatten out the map, so to speak."}, {"video_title": "Are southern hemisphere seasons more severe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So just if you look at the surface of the southern hemisphere, you're looking at a lot more water than the surface of the northern hemisphere. This is, of course, a Mercator projection, and so it distorts things so that things near the poles get really kind of built up to look really huge, even though they really aren't that big. Greenland really isn't larger than South America. It just spreads them out so that you can flatten out the map, so to speak. But the southern hemisphere has more water, and as you may have learned in chemistry class, water has a higher specific heat, higher specific energy. It takes more energy, more heat, to raise water a degree than it does to raise, say, land a degree. And so water can absorb more energy."}, {"video_title": "Are southern hemisphere seasons more severe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It just spreads them out so that you can flatten out the map, so to speak. But the southern hemisphere has more water, and as you may have learned in chemistry class, water has a higher specific heat, higher specific energy. It takes more energy, more heat, to raise water a degree than it does to raise, say, land a degree. And so water can absorb more energy. Or when there's less energy, water will release more energy without dropping as much of a temperature. So water has a moderating influence on the climate. So even though the summers in the southern hemisphere actually are getting more radiation than the summers in the northern hemisphere, it's moderated on the actual temperature because the water has the ability to absorb more of that heat without changing the temperature as dramatically."}, {"video_title": "Are southern hemisphere seasons more severe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And so water can absorb more energy. Or when there's less energy, water will release more energy without dropping as much of a temperature. So water has a moderating influence on the climate. So even though the summers in the southern hemisphere actually are getting more radiation than the summers in the northern hemisphere, it's moderated on the actual temperature because the water has the ability to absorb more of that heat without changing the temperature as dramatically. Now with that said, it is true that in general, Antarctica is colder than the North Pole. But the main reason why Antarctica is colder, besides the fact that it's on land as opposed to the North Pole being in the center of the Arctic Ocean, is that it's actually a huge, very high-altitude ice sheet. And so the altitude for most of Antarctica is around 8,000 feet."}, {"video_title": "Are southern hemisphere seasons more severe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So even though the summers in the southern hemisphere actually are getting more radiation than the summers in the northern hemisphere, it's moderated on the actual temperature because the water has the ability to absorb more of that heat without changing the temperature as dramatically. Now with that said, it is true that in general, Antarctica is colder than the North Pole. But the main reason why Antarctica is colder, besides the fact that it's on land as opposed to the North Pole being in the center of the Arctic Ocean, is that it's actually a huge, very high-altitude ice sheet. And so the altitude for most of Antarctica is around 8,000 feet. So it's kind of like an alpine altitude. So the main reason why it's colder is possibly being further away from the sun in winter might play some role there. But the main reason why it's colder is that it's just at a much higher altitude."}, {"video_title": "Are southern hemisphere seasons more severe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And so the altitude for most of Antarctica is around 8,000 feet. So it's kind of like an alpine altitude. So the main reason why it's colder is possibly being further away from the sun in winter might play some role there. But the main reason why it's colder is that it's just at a much higher altitude. And it's to some degree insulated from the water. Or I guess you could say it's on the land. So especially during the long winters, it's going to get that much colder."}, {"video_title": "Are southern hemisphere seasons more severe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But the main reason why it's colder is that it's just at a much higher altitude. And it's to some degree insulated from the water. Or I guess you could say it's on the land. So especially during the long winters, it's going to get that much colder. But I'll leave you there. And to some degree, and this is the other aspect of it, during the summers, and all of this stuff is super complicated, so you can't just throw out one rule of thumb and say this is the reason. But these are all influences."}, {"video_title": "Are southern hemisphere seasons more severe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So especially during the long winters, it's going to get that much colder. But I'll leave you there. And to some degree, and this is the other aspect of it, during the summers, and all of this stuff is super complicated, so you can't just throw out one rule of thumb and say this is the reason. But these are all influences. Is that if you have a large, super large ice sheet, it's also more likely to reflect more of the energy. Because it's white, as opposed to a darker color like the ocean or the land. And so you can think about all of those factors."}, {"video_title": "Hubble image of galaxies Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "The whole point of this video is really just to look at what, in my mind, is one of the coolest pictures ever taken by anything. And this was actually taken by the Hubble telescope. Let me get my pen tool in place. And what they did is they pointed the telescope at this area of our night sky. And obviously, the Hubble telescope, it's out in orbit, so it doesn't have to worry about all of the interference from our actual atmosphere. So it gets a nice, good look at things. But it's right over here, relative to the moon."}, {"video_title": "Hubble image of galaxies Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And what they did is they pointed the telescope at this area of our night sky. And obviously, the Hubble telescope, it's out in orbit, so it doesn't have to worry about all of the interference from our actual atmosphere. So it gets a nice, good look at things. But it's right over here, relative to the moon. Obviously, the moon's moving around. But on that day, it was here, relative to the moon. And they picked this location right over here because there weren't that many nearby stars there."}, {"video_title": "Hubble image of galaxies Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But it's right over here, relative to the moon. Obviously, the moon's moving around. But on that day, it was here, relative to the moon. And they picked this location right over here because there weren't that many nearby stars there. So it really allowed the telescope, because if there were nearby stars, that light would have outshone things that are behind it, further away, maybe perhaps galaxies. So just keep in mind, everything you're going to see is in this little patch of the night sky. And I think the main point for showing the moon here, obviously, the moon's moving around."}, {"video_title": "Hubble image of galaxies Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And they picked this location right over here because there weren't that many nearby stars there. So it really allowed the telescope, because if there were nearby stars, that light would have outshone things that are behind it, further away, maybe perhaps galaxies. So just keep in mind, everything you're going to see is in this little patch of the night sky. And I think the main point for showing the moon here, obviously, the moon's moving around. I'm not telling you which exact patch of sky this is. But to really give you an idea of how small of a patch of sky that really is. But you really could have done this with any patch of sky."}, {"video_title": "Hubble image of galaxies Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And I think the main point for showing the moon here, obviously, the moon's moving around. I'm not telling you which exact patch of sky this is. But to really give you an idea of how small of a patch of sky that really is. But you really could have done this with any patch of sky. But in other patches of sky, there would have been other nearby stars that would have blocked things. But the galaxies are there, beyond that, in the clusters of galaxies, in the super clusters of galaxies. So with that said, just remember, everything we're talking about in this video is in this little patch of sky right over here."}, {"video_title": "Hubble image of galaxies Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But you really could have done this with any patch of sky. But in other patches of sky, there would have been other nearby stars that would have blocked things. But the galaxies are there, beyond that, in the clusters of galaxies, in the super clusters of galaxies. So with that said, just remember, everything we're talking about in this video is in this little patch of sky right over here. And the whole point of this, once again, like all of these videos, is really to kind of just blow your mind. So this right here is what the Hubble telescope saw in that patch. Everything."}, {"video_title": "Hubble image of galaxies Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So with that said, just remember, everything we're talking about in this video is in this little patch of sky right over here. And the whole point of this, once again, like all of these videos, is really to kind of just blow your mind. So this right here is what the Hubble telescope saw in that patch. Everything. Everything that I'm showing you right here. I just want to be clear. Some of these things are nearby stars."}, {"video_title": "Hubble image of galaxies Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Everything. Everything that I'm showing you right here. I just want to be clear. Some of these things are nearby stars. But most of these things are galaxies. That's a galaxy. That is a galaxy."}, {"video_title": "Hubble image of galaxies Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Some of these things are nearby stars. But most of these things are galaxies. That's a galaxy. That is a galaxy. That is a galaxy. That's a galaxy. And that's a galaxy."}, {"video_title": "Hubble image of galaxies Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "That is a galaxy. That is a galaxy. That's a galaxy. And that's a galaxy. And the reason I wanted to do this, obviously, in the last video, I showed you our local group. I showed you the Virgo super cluster. I showed you the kind of clusters of clusters."}, {"video_title": "Hubble image of galaxies Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And that's a galaxy. And the reason I wanted to do this, obviously, in the last video, I showed you our local group. I showed you the Virgo super cluster. I showed you the kind of clusters of clusters. And I even showed you a depiction of the observable universe. But what just is amazing about this, this is an actual picture. This is actually an image of a galaxy."}, {"video_title": "Hubble image of galaxies Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "I showed you the kind of clusters of clusters. And I even showed you a depiction of the observable universe. But what just is amazing about this, this is an actual picture. This is actually an image of a galaxy. Hey, there's another galaxy. Oh look, there's another galaxy up here. So you could keep doing that forever."}, {"video_title": "Hubble image of galaxies Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "This is actually an image of a galaxy. Hey, there's another galaxy. Oh look, there's another galaxy up here. So you could keep doing that forever. And this is just in that little patch of sky. And this is obviously not all of the galaxies in the universe. These are just the ones that we could see."}, {"video_title": "Hubble image of galaxies Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So you could keep doing that forever. And this is just in that little patch of sky. And this is obviously not all of the galaxies in the universe. These are just the ones that we could see. There are ones that might be even further. Or there definitely are ones that are further back. And their light is just even more."}, {"video_title": "Hubble image of galaxies Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "These are just the ones that we could see. There are ones that might be even further. Or there definitely are ones that are further back. And their light is just even more. We could probably even focus even on a patch of sky like that and see that many galaxies again. So you could kind of keep, keep zooming in. But I just, this thing, and I encourage you."}, {"video_title": "Hubble image of galaxies Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And their light is just even more. We could probably even focus even on a patch of sky like that and see that many galaxies again. So you could kind of keep, keep zooming in. But I just, this thing, and I encourage you. I mean, there's so many unbelievable images. You could look up, they're all on the NASA website. A lot of these are on Wikipedia."}, {"video_title": "Hubble image of galaxies Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But I just, this thing, and I encourage you. I mean, there's so many unbelievable images. You could look up, they're all on the NASA website. A lot of these are on Wikipedia. But these images are just, I mean, unbelievable. I mean, you see a galaxy, another galaxy, another galaxy, another galaxy. I suspect some of this stuff might actually be clusters of galaxies."}, {"video_title": "Hubble image of galaxies Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "A lot of these are on Wikipedia. But these images are just, I mean, unbelievable. I mean, you see a galaxy, another galaxy, another galaxy, another galaxy. I suspect some of this stuff might actually be clusters of galaxies. A galaxy, another galaxy. And remember, where I'm just circling these galaxies just left and right, you kind of lose sight of what each galaxy actually is. These galaxies have hundreds of billions of stars."}, {"video_title": "Hubble image of galaxies Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "I suspect some of this stuff might actually be clusters of galaxies. A galaxy, another galaxy. And remember, where I'm just circling these galaxies just left and right, you kind of lose sight of what each galaxy actually is. These galaxies have hundreds of billions of stars. That even from one pixel on these galaxies is an unimaginable distance. Something that we could never, based with current technology or even current science, we could ever hope to traverse in the lifetime of humanity. Much less the lifetime of one individual."}, {"video_title": "Hubble image of galaxies Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "These galaxies have hundreds of billions of stars. That even from one pixel on these galaxies is an unimaginable distance. Something that we could never, based with current technology or even current science, we could ever hope to traverse in the lifetime of humanity. Much less the lifetime of one individual. So these are just enormous things. And they're just an infinite number of them. So I just wanted to highlight this with you."}, {"video_title": "Hubble image of galaxies Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Much less the lifetime of one individual. So these are just enormous things. And they're just an infinite number of them. So I just wanted to highlight this with you. And really just, I mean, just look at it. I mean, it's kind of breathtaking. There's a galaxy right over there."}, {"video_title": "Hubble image of galaxies Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So I just wanted to highlight this with you. And really just, I mean, just look at it. I mean, it's kind of breathtaking. There's a galaxy right over there. Another galaxy. I mean, I could keep doing it all day. But it's really, and I feel bad about drawing on it."}, {"video_title": "Hubble image of galaxies Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "There's a galaxy right over there. Another galaxy. I mean, I could keep doing it all day. But it's really, and I feel bad about drawing on it. It kind of ruins the picture. But look at that. That's a nice looking galaxy right there."}, {"video_title": "Hubble image of galaxies Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But it's really, and I feel bad about drawing on it. It kind of ruins the picture. But look at that. That's a nice looking galaxy right there. You got a bunch right over here. So I mean, it really is just, you know. This video, it's probably the least dense with actual instruction."}, {"video_title": "Hubble image of galaxies Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "That's a nice looking galaxy right there. You got a bunch right over here. So I mean, it really is just, you know. This video, it's probably the least dense with actual instruction. But hopefully it's one of the most dense with inspiration. Anyway, hopefully you enjoyed that. Let me just finish looking at the entire picture."}, {"video_title": "Why S-waves only travel in solids Cosmology & Astronomy Khan Academy.mp3", "Sentence": "In the last video, I gave a little bit of a hand-wavy explanation about why S waves don't travel in liquid or air. And what I want to do in this video is give you a little bit more intuitive understanding of that and really go down to the molecular level. So let's draw a solid. And it has nice covalent bonds, strong bonds between the different molecules. And the bonds are drawn by these lines in between. So if I were to hit this solid, I have this really small hammer where I just hit it at a molecular level. But if I were to hit these molecules hard enough that they move, but not so hard enough that it breaks the bonds, then essentially what it's going to look like is this kind of row of molecules are going to move to the left."}, {"video_title": "Why S-waves only travel in solids Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And it has nice covalent bonds, strong bonds between the different molecules. And the bonds are drawn by these lines in between. So if I were to hit this solid, I have this really small hammer where I just hit it at a molecular level. But if I were to hit these molecules hard enough that they move, but not so hard enough that it breaks the bonds, then essentially what it's going to look like is this kind of row of molecules are going to move to the left. So you're going to have that row of molecules moving to the left. And then the row above it won't fully move to the left just yet, but it will start to get pulled. So let me just draw all of the bonds."}, {"video_title": "Why S-waves only travel in solids Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But if I were to hit these molecules hard enough that they move, but not so hard enough that it breaks the bonds, then essentially what it's going to look like is this kind of row of molecules are going to move to the left. So you're going to have that row of molecules moving to the left. And then the row above it won't fully move to the left just yet, but it will start to get pulled. So let me just draw all of the bonds. I'm just drawing all of the same bonds. Because these are strong bonds that we have in a solid, actually they could be ionic bonds as well. Because they are strong bonds that we have in this solid, they'll essentially be pulled."}, {"video_title": "Why S-waves only travel in solids Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So let me just draw all of the bonds. I'm just drawing all of the same bonds. Because these are strong bonds that we have in a solid, actually they could be ionic bonds as well. Because they are strong bonds that we have in this solid, they'll essentially be pulled. They'll be pulled in the direction, the top row will be pulled in the direction of the bottom row. And so they'll start kind of moving in that direction. And then the bottom row will essentially recoil back."}, {"video_title": "Why S-waves only travel in solids Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Because they are strong bonds that we have in this solid, they'll essentially be pulled. They'll be pulled in the direction, the top row will be pulled in the direction of the bottom row. And so they'll start kind of moving in that direction. And then the bottom row will essentially recoil back. And then you fast forward a little bit. And so then the top row will have moved to the left. And now the bottom row will start to move back."}, {"video_title": "Why S-waves only travel in solids Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And then the bottom row will essentially recoil back. And then you fast forward a little bit. And so then the top row will have moved to the left. And now the bottom row will start to move back. And then the bottom row will start to kind of move back. Especially because remember, it's bonded to other things down here. It's bonded to more of the solid down here."}, {"video_title": "Why S-waves only travel in solids Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And now the bottom row will start to move back. And then the bottom row will start to kind of move back. Especially because remember, it's bonded to other things down here. It's bonded to more of the solid down here. So it'll move back. And you can see this transverse wave. You can see this S wave propagating."}, {"video_title": "Why S-waves only travel in solids Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It's bonded to more of the solid down here. So it'll move back. And you can see this transverse wave. You can see this S wave propagating. Essentially right over here, the kind of peak of the S wave is here. Now it has moved up. Now let's think about the exact same situation with the liquids."}, {"video_title": "Why S-waves only travel in solids Cosmology & Astronomy Khan Academy.mp3", "Sentence": "You can see this S wave propagating. Essentially right over here, the kind of peak of the S wave is here. Now it has moved up. Now let's think about the exact same situation with the liquids. In liquids, you don't have these strong ionic or covalent bonds between the different molecules. You just have these weak kind of bonds, usually formed due to polarity. So in a liquid, water is a good example."}, {"video_title": "Why S-waves only travel in solids Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Now let's think about the exact same situation with the liquids. In liquids, you don't have these strong ionic or covalent bonds between the different molecules. You just have these weak kind of bonds, usually formed due to polarity. So in a liquid, water is a good example. You just have these kind of weaker bonds formed because water is a polar molecule. So the kind of halfway polar sides or the halfway positive sides are somewhat attracted to the halfway negative sides. So they kind of flow past each other."}, {"video_title": "Why S-waves only travel in solids Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So in a liquid, water is a good example. You just have these kind of weaker bonds formed because water is a polar molecule. So the kind of halfway polar sides or the halfway positive sides are somewhat attracted to the halfway negative sides. So they kind of flow past each other. But if I were to hit these water molecules right here with my hammer, what would happen? Well, they're going to start moving to the left. And this one's going to bump into that one, which is going to bump into that one, which is going to bump into that one."}, {"video_title": "Why S-waves only travel in solids Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So they kind of flow past each other. But if I were to hit these water molecules right here with my hammer, what would happen? Well, they're going to start moving to the left. And this one's going to bump into that one, which is going to bump into that one, which is going to bump into that one. And they're going to move to the left. So they're going to move to the left. But these molecules aren't going to move with them."}, {"video_title": "Why S-waves only travel in solids Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And this one's going to bump into that one, which is going to bump into that one, which is going to bump into that one. And they're going to move to the left. So they're going to move to the left. But these molecules aren't going to move with them. You could view it as going to break that very weak bond due to polarity. They're going to move away from each other. Let me draw these top molecules in green."}, {"video_title": "Why S-waves only travel in solids Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But these molecules aren't going to move with them. You could view it as going to break that very weak bond due to polarity. They're going to move away from each other. Let me draw these top molecules in green. They're essentially just going to flow past each other. And this guy might have had also weak bonds with stuff below it, too. I should draw just dotted lines with stuff below it, too."}, {"video_title": "Why S-waves only travel in solids Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Let me draw these top molecules in green. They're essentially just going to flow past each other. And this guy might have had also weak bonds with stuff below it, too. I should draw just dotted lines with stuff below it, too. But because of the impact here, these guys are just going to flow. They're actually going to compress in this direction. You're going to have a P wave, a compression wave, go in this direction, where this one bumps into that one and then goes back."}, {"video_title": "Why S-waves only travel in solids Cosmology & Astronomy Khan Academy.mp3", "Sentence": "I should draw just dotted lines with stuff below it, too. But because of the impact here, these guys are just going to flow. They're actually going to compress in this direction. You're going to have a P wave, a compression wave, go in this direction, where this one bumps into that one and then goes back. And then this one bumps into that one and goes back. And then this one bumps into that one. But the bonds aren't strong enough."}, {"video_title": "Why S-waves only travel in solids Cosmology & Astronomy Khan Academy.mp3", "Sentence": "You're going to have a P wave, a compression wave, go in this direction, where this one bumps into that one and then goes back. And then this one bumps into that one and goes back. And then this one bumps into that one. But the bonds aren't strong enough. And it's even more the case with air. But the bonds aren't strong enough for these blue guys to take these green guys for a ride. And the bonds are also not strong enough for the adjacent molecules to kind of help these blue guys to retract to their original position."}, {"video_title": "Why S-waves only travel in solids Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But the bonds aren't strong enough. And it's even more the case with air. But the bonds aren't strong enough for these blue guys to take these green guys for a ride. And the bonds are also not strong enough for the adjacent molecules to kind of help these blue guys to retract to their original position. So when I talked about the elasticity in the last video, that's what I was talking about. The bonds aren't strong enough to cause the things that have deformed to kind of move back to where they were. And also, their bonds aren't strong enough to allow the things that are deformed to pull other things with it."}, {"video_title": "Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "In the last video, we learned that there are a class of stars called Cepheid variables. And these are these super giant stars, as much as 30,000 times as bright as the sun, as a mass 20 times the mass of the sun. And what's neat about them is, one, because they're so large and so bright, you can see them really, really far away. And what's even neater about them is that they're variable, that they pulsate. And because their pulsations are related to their actual luminosity, you know if you see a Cepheid variable star in some distant galaxy, you know what its luminosity actually is if you were kind of at the star, because you can see how its period of pulsation. And so if you know its actual luminosity, and then you know, obviously, its apparent luminosity, you know how much it's gotten dim. And the more dim it's gotten from its actual state, you know the farther away it is."}, {"video_title": "Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "And what's even neater about them is that they're variable, that they pulsate. And because their pulsations are related to their actual luminosity, you know if you see a Cepheid variable star in some distant galaxy, you know what its luminosity actually is if you were kind of at the star, because you can see how its period of pulsation. And so if you know its actual luminosity, and then you know, obviously, its apparent luminosity, you know how much it's gotten dim. And the more dim it's gotten from its actual state, you know the farther away it is. So that's the real value of them. What I want to do in this video is to try to explain why they're variable, why they pulsate. And to do that, what we're going to think about is doubly and singly ionized helium."}, {"video_title": "Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "And the more dim it's gotten from its actual state, you know the farther away it is. So that's the real value of them. What I want to do in this video is to try to explain why they're variable, why they pulsate. And to do that, what we're going to think about is doubly and singly ionized helium. And just to review, helium, so neutral helium, let me draw neutral helium. Neutral helium's got two protons, two neutrons, and then two electrons. And obviously, this is not drawn to scale."}, {"video_title": "Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "And to do that, what we're going to think about is doubly and singly ionized helium. And just to review, helium, so neutral helium, let me draw neutral helium. Neutral helium's got two protons, two neutrons, and then two electrons. And obviously, this is not drawn to scale. So this is neutral helium right over here. Now, if you singly ionize helium, you knock off one of these electrons. And these type of things happen in stars, when you have a lot of heat, easier to ionize things."}, {"video_title": "Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "And obviously, this is not drawn to scale. So this is neutral helium right over here. Now, if you singly ionize helium, you knock off one of these electrons. And these type of things happen in stars, when you have a lot of heat, easier to ionize things. So singly ionized helium will look like this. It'll have the same nucleus, two protons, two neutrons. One of the electrons gets knocked off, so now you only have one electron."}, {"video_title": "Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "And these type of things happen in stars, when you have a lot of heat, easier to ionize things. So singly ionized helium will look like this. It'll have the same nucleus, two protons, two neutrons. One of the electrons gets knocked off, so now you only have one electron. And now you have a net positive charge. So here, let me do this in a different color. This helium now has a net charge."}, {"video_title": "Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "One of the electrons gets knocked off, so now you only have one electron. And now you have a net positive charge. So here, let me do this in a different color. This helium now has a net charge. We could write 1 plus here, but if you just write a plus, you implicitly mean a positive charge of 1. Now, you can also doubly ionize helium if the environment is hot enough. You can doubly ionize helium."}, {"video_title": "Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "This helium now has a net charge. We could write 1 plus here, but if you just write a plus, you implicitly mean a positive charge of 1. Now, you can also doubly ionize helium if the environment is hot enough. You can doubly ionize helium. And doubly ionizing helium is essentially knocking off both of the electrons. So then it's really just a helium nucleus like this. This right here is doubly ionized helium."}, {"video_title": "Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "You can doubly ionize helium. And doubly ionizing helium is essentially knocking off both of the electrons. So then it's really just a helium nucleus like this. This right here is doubly ionized helium. Now, I just said, in order to do this, you have to have a hotter environment. There has to be a hotter environment in order to be able to knock off both. This electron really doesn't want to leave."}, {"video_title": "Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "This right here is doubly ionized helium. Now, I just said, in order to do this, you have to have a hotter environment. There has to be a hotter environment in order to be able to knock off both. This electron really doesn't want to leave. To take an electron off of something that's already positive is difficult. You have to have a lot of, really, pressure and temperature. This is cooler."}, {"video_title": "Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "This electron really doesn't want to leave. To take an electron off of something that's already positive is difficult. You have to have a lot of, really, pressure and temperature. This is cooler. And this is all relative. We're talking about the insides of stars. So this is a hotter part of the star versus a cooler part of the star, I guess, is the way you think about it."}, {"video_title": "Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "This is cooler. And this is all relative. We're talking about the insides of stars. So this is a hotter part of the star versus a cooler part of the star, I guess, is the way you think about it. It's a very hot environment by our traditional everyday standards. Now, the other thing about the doubly ionized helium is that it is more opaque, which means it doesn't allow light to go through it. It actually absorbs light."}, {"video_title": "Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "So this is a hotter part of the star versus a cooler part of the star, I guess, is the way you think about it. It's a very hot environment by our traditional everyday standards. Now, the other thing about the doubly ionized helium is that it is more opaque, which means it doesn't allow light to go through it. It actually absorbs light. It is more opaque. It absorbs light. Or another way, it absorbs that light energy."}, {"video_title": "Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "It actually absorbs light. It is more opaque. It absorbs light. Or another way, it absorbs that light energy. That energy will make it even hotter. So that's just something to think about. Now, the singly ionized helium is more transparent."}, {"video_title": "Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "Or another way, it absorbs that light energy. That energy will make it even hotter. So that's just something to think about. Now, the singly ionized helium is more transparent. It allows the light to pass through it. So it doesn't get heated as much by photons that are kind of going near it or through it or whatever. It allows them to go through it."}, {"video_title": "Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "Now, the singly ionized helium is more transparent. It allows the light to pass through it. So it doesn't get heated as much by photons that are kind of going near it or through it or whatever. It allows them to go through it. Here, the photons are going to actually heat up the ion. So let's think about how this might cause a Cepheid variable to pulsate. So assuming that Cepheid variables have large enough quantities, I should say, of these ions, we can imagine that when a Cepheid variable is dim."}, {"video_title": "Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "It allows them to go through it. Here, the photons are going to actually heat up the ion. So let's think about how this might cause a Cepheid variable to pulsate. So assuming that Cepheid variables have large enough quantities, I should say, of these ions, we can imagine that when a Cepheid variable is dim. So let me draw a dim Cepheid variable. So I'll draw that like I'll draw this in a dim color. So this is a dim Cepheid variable right here."}, {"video_title": "Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "So assuming that Cepheid variables have large enough quantities, I should say, of these ions, we can imagine that when a Cepheid variable is dim. So let me draw a dim Cepheid variable. So I'll draw that like I'll draw this in a dim color. So this is a dim Cepheid variable right here. In its dim state, just like this, you have a lot of the doubly ionized helium in the star, at least kind of the outer surface of the star. And so this does not allow a lot of light to pass through. So this is the dim part of the pulsation of the Cepheid variable."}, {"video_title": "Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "So this is a dim Cepheid variable right here. In its dim state, just like this, you have a lot of the doubly ionized helium in the star, at least kind of the outer surface of the star. And so this does not allow a lot of light to pass through. So this is the dim part of the pulsation of the Cepheid variable. Now, because this doubly ionized helium is opaque, it is absorbing the light. It is getting heated. It is getting heated."}, {"video_title": "Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "So this is the dim part of the pulsation of the Cepheid variable. Now, because this doubly ionized helium is opaque, it is absorbing the light. It is getting heated. It is getting heated. And because it's getting heated, it'll cause the star to expand. So because it's getting heated, it'll become more energetic, and the star will actually expand. Now, as the star expands, because this doubly ionized helium is getting heated, what's going to happen?"}, {"video_title": "Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "It is getting heated. And because it's getting heated, it'll cause the star to expand. So because it's getting heated, it'll become more energetic, and the star will actually expand. Now, as the star expands, because this doubly ionized helium is getting heated, what's going to happen? The further away you are from the core of the star, the cooler it gets. So this expanded because it was getting heated, but then because it expanded, the outer layers of the star become cooler. And since they're cooler, helium won't be doubly ionized anymore."}, {"video_title": "Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "Now, as the star expands, because this doubly ionized helium is getting heated, what's going to happen? The further away you are from the core of the star, the cooler it gets. So this expanded because it was getting heated, but then because it expanded, the outer layers of the star become cooler. And since they're cooler, helium won't be doubly ionized anymore. It'll get an electron from each helium atom. It can now get an electron from the plasma, I guess we can say, to become singly ionized helium. So now we have singly ionized helium."}, {"video_title": "Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "And since they're cooler, helium won't be doubly ionized anymore. It'll get an electron from each helium atom. It can now get an electron from the plasma, I guess we can say, to become singly ionized helium. So now we have singly ionized helium. And now the star is going to be more transparent. It's going to allow more light to pass through it. So now this is the bright part of the pulsation."}, {"video_title": "Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "So now we have singly ionized helium. And now the star is going to be more transparent. It's going to allow more light to pass through it. So now this is the bright part of the pulsation. It's going to allow more light through. So now it is bright. The star is bright."}, {"video_title": "Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "So now this is the bright part of the pulsation. It's going to allow more light through. So now it is bright. The star is bright. But what's happening now? Because the light is no longer, or it's not being absorbed as well by the helium when it was a doubly ionized helium, now it's letting most of the light, or a lot more of the light, get through. It's not going to get heated as much."}, {"video_title": "Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "The star is bright. But what's happening now? Because the light is no longer, or it's not being absorbed as well by the helium when it was a doubly ionized helium, now it's letting most of the light, or a lot more of the light, get through. It's not going to get heated as much. And so it won't have the kinetic energy to kind of keep pushing out, to keep moving outward. And so it'll collapse back into the star. And so then this will cool down and collapse back in."}, {"video_title": "Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "It's not going to get heated as much. And so it won't have the kinetic energy to kind of keep pushing out, to keep moving outward. And so it'll collapse back into the star. And so then this will cool down and collapse back in. And when it collapses back in, what's going to happen? When it collapses back in, when these helium atoms get closer to the center of the star, to the core of the star, they're going to be heated again, because they're closer now to the core. And when they get heated, they're going to become doubly ionized."}, {"video_title": "Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "And so then this will cool down and collapse back in. And when it collapses back in, what's going to happen? When it collapses back in, when these helium atoms get closer to the center of the star, to the core of the star, they're going to be heated again, because they're closer now to the core. And when they get heated, they're going to become doubly ionized. So then we have doubly ionized helium again. And then the cycle will go again. It is now opaque."}, {"video_title": "Why cepheids pulsate Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "And when they get heated, they're going to become doubly ionized. So then we have doubly ionized helium again. And then the cycle will go again. It is now opaque. It will now absorb more energy. That'll cause it to have more kinetic energy to expand. Once it expands, it'll get cool again, and transparent, and bright."}, {"video_title": "Carbon 14 dating 1 Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "What I want to do in this video is kind of introduce you to the idea of, one, how carbon-14 comes about and how it gets into all living things. And then, either later in this video or in future videos, we'll talk about how it's actually used to date things. How we use it to actually figure out that that bone is 12,000 years old, that that person died 18,000 years ago, whatever it might be. So let me draw the Earth. So let me just draw the surface of the Earth like that. It's just a little section of the surface of the Earth. And then we have the atmosphere of the Earth."}, {"video_title": "Carbon 14 dating 1 Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So let me draw the Earth. So let me just draw the surface of the Earth like that. It's just a little section of the surface of the Earth. And then we have the atmosphere of the Earth. I'll draw that in yellow. So then you have the Earth's atmosphere right over here. Let me write that down."}, {"video_title": "Carbon 14 dating 1 Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And then we have the atmosphere of the Earth. I'll draw that in yellow. So then you have the Earth's atmosphere right over here. Let me write that down. Atmosphere. And 78% the most abundant element in our atmosphere is nitrogen. It is 78% nitrogen."}, {"video_title": "Carbon 14 dating 1 Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Let me write that down. Atmosphere. And 78% the most abundant element in our atmosphere is nitrogen. It is 78% nitrogen. And if I write nitrogen, its symbol is just N. And it has 7 protons and it also has 7 neutrons. So it has an atomic mass of roughly 14. And this is the most typical isotope of nitrogen."}, {"video_title": "Carbon 14 dating 1 Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It is 78% nitrogen. And if I write nitrogen, its symbol is just N. And it has 7 protons and it also has 7 neutrons. So it has an atomic mass of roughly 14. And this is the most typical isotope of nitrogen. And we talk about the word isotope in the chemistry playlist. An isotope, the protons define what element it is. But this number up here can change depending on the number of neutrons you have."}, {"video_title": "Carbon 14 dating 1 Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And this is the most typical isotope of nitrogen. And we talk about the word isotope in the chemistry playlist. An isotope, the protons define what element it is. But this number up here can change depending on the number of neutrons you have. So the different versions of a given element, those are each called isotopes. I just view them in my head as versions of an element. So anyway, we have our atmosphere."}, {"video_title": "Carbon 14 dating 1 Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But this number up here can change depending on the number of neutrons you have. So the different versions of a given element, those are each called isotopes. I just view them in my head as versions of an element. So anyway, we have our atmosphere. And then coming from our sun, we have what's commonly called cosmic rays. But they're actually not rays. They're cosmic particles."}, {"video_title": "Carbon 14 dating 1 Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So anyway, we have our atmosphere. And then coming from our sun, we have what's commonly called cosmic rays. But they're actually not rays. They're cosmic particles. They're mainly, you can view them as just single protons, which is the same thing as a hydrogen nucleus. They can also be alpha particles, which is the same thing as a helium nucleus. And there's even a few electrons."}, {"video_title": "Carbon 14 dating 1 Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "They're cosmic particles. They're mainly, you can view them as just single protons, which is the same thing as a hydrogen nucleus. They can also be alpha particles, which is the same thing as a helium nucleus. And there's even a few electrons. And they're going to come in and they're going to bump into things in our atmosphere. And they're actually going to form neutrons. So they're actually going to form neutrons."}, {"video_title": "Carbon 14 dating 1 Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And there's even a few electrons. And they're going to come in and they're going to bump into things in our atmosphere. And they're actually going to form neutrons. So they're actually going to form neutrons. And we'll show a neutron with a lowercase n and a 1 for its mass number. And we don't write anything because it has no protons down here. Like we had for nitrogen, we had 7 protons."}, {"video_title": "Carbon 14 dating 1 Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So they're actually going to form neutrons. And we'll show a neutron with a lowercase n and a 1 for its mass number. And we don't write anything because it has no protons down here. Like we had for nitrogen, we had 7 protons. So it's not really an element. It is a subatomic particle. But you have these neutrons form."}, {"video_title": "Carbon 14 dating 1 Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Like we had for nitrogen, we had 7 protons. So it's not really an element. It is a subatomic particle. But you have these neutrons form. And every now and then, and let's just be clear, this isn't like a typical reaction. But every now and then, one of those neutrons will bump into one of the nitrogen-14s in just the right way. So that it bumps off one of the protons in the nitrogen."}, {"video_title": "Carbon 14 dating 1 Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But you have these neutrons form. And every now and then, and let's just be clear, this isn't like a typical reaction. But every now and then, one of those neutrons will bump into one of the nitrogen-14s in just the right way. So that it bumps off one of the protons in the nitrogen. And essentially replaces that proton with itself. So let me make it clear. So it bumps off one of the protons."}, {"video_title": "Carbon 14 dating 1 Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So that it bumps off one of the protons in the nitrogen. And essentially replaces that proton with itself. So let me make it clear. So it bumps off one of the protons. So instead of 7 protons, we now have 6 protons. But this number 14 doesn't go down to 13 because it replaces it with itself. So this still stays at 14."}, {"video_title": "Carbon 14 dating 1 Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So it bumps off one of the protons. So instead of 7 protons, we now have 6 protons. But this number 14 doesn't go down to 13 because it replaces it with itself. So this still stays at 14. And now since it now only has 6 protons, this is no longer nitrogen by definition. This is now carbon. And that proton that was bumped off just kind of gets emitted."}, {"video_title": "Carbon 14 dating 1 Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So this still stays at 14. And now since it now only has 6 protons, this is no longer nitrogen by definition. This is now carbon. And that proton that was bumped off just kind of gets emitted. So then let me just do that in another color. And a proton that's just flying around, you could call that hydrogen-1. And it can gain an electron some ways."}, {"video_title": "Carbon 14 dating 1 Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And that proton that was bumped off just kind of gets emitted. So then let me just do that in another color. And a proton that's just flying around, you could call that hydrogen-1. And it can gain an electron some ways. If it doesn't gain an electron, it's just a hydrogen ion, a positive ion. Either way, or a hydrogen nucleus. But this process, and once again, it's not a typical process."}, {"video_title": "Carbon 14 dating 1 Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And it can gain an electron some ways. If it doesn't gain an electron, it's just a hydrogen ion, a positive ion. Either way, or a hydrogen nucleus. But this process, and once again, it's not a typical process. But it happens every now and then. This is how carbon-14 forms. So this right here is carbon-14."}, {"video_title": "Carbon 14 dating 1 Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But this process, and once again, it's not a typical process. But it happens every now and then. This is how carbon-14 forms. So this right here is carbon-14. You can essentially view it as a nitrogen-14, and one of the protons is replaced with a neutron. And what's interesting about this is this is constantly being formed in our atmosphere. Not in huge quantities, but in reasonable quantities."}, {"video_title": "Carbon 14 dating 1 Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So this right here is carbon-14. You can essentially view it as a nitrogen-14, and one of the protons is replaced with a neutron. And what's interesting about this is this is constantly being formed in our atmosphere. Not in huge quantities, but in reasonable quantities. So let me write this down. Constantly being formed. Constant formation."}, {"video_title": "Carbon 14 dating 1 Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Not in huge quantities, but in reasonable quantities. So let me write this down. Constantly being formed. Constant formation. And what happens is, and let me be very clear. Let's look at the periodic table over here. Typical carbon."}, {"video_title": "Carbon 14 dating 1 Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Constant formation. And what happens is, and let me be very clear. Let's look at the periodic table over here. Typical carbon. So carbon by definition, if you have 6 protons, carbon by definition has 6 protons. But the typical isotope, the most common isotope of carbon, is carbon-12. So carbon-12 is the most common."}, {"video_title": "Carbon 14 dating 1 Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Typical carbon. So carbon by definition, if you have 6 protons, carbon by definition has 6 protons. But the typical isotope, the most common isotope of carbon, is carbon-12. So carbon-12 is the most common. So most of the carbon in your body is carbon-12. But what's interesting is that a small fraction of carbon-14 forms, and then this carbon-14 can then also combine with oxygen to form carbon dioxide. And then that carbon dioxide gets absorbed into the rest of the atmosphere, into our oceans."}, {"video_title": "Carbon 14 dating 1 Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So carbon-12 is the most common. So most of the carbon in your body is carbon-12. But what's interesting is that a small fraction of carbon-14 forms, and then this carbon-14 can then also combine with oxygen to form carbon dioxide. And then that carbon dioxide gets absorbed into the rest of the atmosphere, into our oceans. It can be fixed by plants. When people talk about carbon fixation, they're really talking about using mainly light energy from the sun to take gaseous carbon and turn it into actual kind of organic tissue. And so this carbon-14 makes its way, it's constantly being formed, it makes its way into oceans."}, {"video_title": "Carbon 14 dating 1 Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And then that carbon dioxide gets absorbed into the rest of the atmosphere, into our oceans. It can be fixed by plants. When people talk about carbon fixation, they're really talking about using mainly light energy from the sun to take gaseous carbon and turn it into actual kind of organic tissue. And so this carbon-14 makes its way, it's constantly being formed, it makes its way into oceans. It's already in the air, but it completely mixes through the whole atmosphere. Oceans and the air. And then it makes its way into plants."}, {"video_title": "Carbon 14 dating 1 Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And so this carbon-14 makes its way, it's constantly being formed, it makes its way into oceans. It's already in the air, but it completely mixes through the whole atmosphere. Oceans and the air. And then it makes its way into plants. And then it makes its way, and plants are really just made out of that fixed carbon, that carbon that was taken in gaseous form and put into, I guess you could say, into kind of solid form, put into living form. That's what wood pretty much is. It gets put into plants, and then it gets put into the things that eat the plants."}, {"video_title": "Carbon 14 dating 1 Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And then it makes its way into plants. And then it makes its way, and plants are really just made out of that fixed carbon, that carbon that was taken in gaseous form and put into, I guess you could say, into kind of solid form, put into living form. That's what wood pretty much is. It gets put into plants, and then it gets put into the things that eat the plants. So that could be us. Now why is this even interesting? I've just explained a mechanism where some of our body, even though carbon-12 is the most common isotope, some of our body, while we're living, gets made up of this carbon-14 thing."}, {"video_title": "Carbon 14 dating 1 Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It gets put into plants, and then it gets put into the things that eat the plants. So that could be us. Now why is this even interesting? I've just explained a mechanism where some of our body, even though carbon-12 is the most common isotope, some of our body, while we're living, gets made up of this carbon-14 thing. Well, the interesting thing is the only time you can take in this carbon-14 is while you're alive, while you're eating new things. Because as soon as you die and you get buried under the ground, there's no way for the carbon-14 to become part of your tissue anymore because you're not eating anything with the new carbon-14. And what's interesting here is once you die, you're not going to get any new carbon-14, and that carbon-14 that you did have at your death is going to decay via beta decay, and we learned about this, back into nitrogen-14."}, {"video_title": "Carbon 14 dating 1 Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "I've just explained a mechanism where some of our body, even though carbon-12 is the most common isotope, some of our body, while we're living, gets made up of this carbon-14 thing. Well, the interesting thing is the only time you can take in this carbon-14 is while you're alive, while you're eating new things. Because as soon as you die and you get buried under the ground, there's no way for the carbon-14 to become part of your tissue anymore because you're not eating anything with the new carbon-14. And what's interesting here is once you die, you're not going to get any new carbon-14, and that carbon-14 that you did have at your death is going to decay via beta decay, and we learned about this, back into nitrogen-14. So this process reverses. So it'll decay back into nitrogen-14, and beta decay, you emit an electron and an electron antineutrino. I won't go into the details of that."}, {"video_title": "Carbon 14 dating 1 Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And what's interesting here is once you die, you're not going to get any new carbon-14, and that carbon-14 that you did have at your death is going to decay via beta decay, and we learned about this, back into nitrogen-14. So this process reverses. So it'll decay back into nitrogen-14, and beta decay, you emit an electron and an electron antineutrino. I won't go into the details of that. But essentially what you have happening here is you have one of the neutrons is turning into a proton and emitting this stuff in the process. Now why is this interesting? So I just said while you're living, you have kind of straight-up carbon-14, and carbon-14 is constantly doing this decay thing."}, {"video_title": "Carbon 14 dating 1 Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "I won't go into the details of that. But essentially what you have happening here is you have one of the neutrons is turning into a proton and emitting this stuff in the process. Now why is this interesting? So I just said while you're living, you have kind of straight-up carbon-14, and carbon-14 is constantly doing this decay thing. But what's interesting is as soon as you die and you're not ingesting any more plants or breathing from the atmosphere, if you are a plant, or fixing from the atmosphere, and this even applies to plants, once a plant dies, it's no longer taking in carbon dioxide from the atmosphere and turning it into new tissue. The carbon-14 in that tissue gets frozen, and this carbon-14 does this decay at a specific rate. And then you can use that rate to actually determine how long ago that thing must have died."}, {"video_title": "Carbon 14 dating 1 Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So I just said while you're living, you have kind of straight-up carbon-14, and carbon-14 is constantly doing this decay thing. But what's interesting is as soon as you die and you're not ingesting any more plants or breathing from the atmosphere, if you are a plant, or fixing from the atmosphere, and this even applies to plants, once a plant dies, it's no longer taking in carbon dioxide from the atmosphere and turning it into new tissue. The carbon-14 in that tissue gets frozen, and this carbon-14 does this decay at a specific rate. And then you can use that rate to actually determine how long ago that thing must have died. So the rate at which this happens, so the rate of carbon-14 decay is essentially half disappears, half gone in roughly 5,730 years. And this is actually called a half-life. And we talk about it in other videos."}, {"video_title": "Carbon 14 dating 1 Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And then you can use that rate to actually determine how long ago that thing must have died. So the rate at which this happens, so the rate of carbon-14 decay is essentially half disappears, half gone in roughly 5,730 years. And this is actually called a half-life. And we talk about it in other videos. This is called a half-life. And I want to be clear here. You don't know which half of it's gone."}, {"video_title": "Carbon 14 dating 1 Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And we talk about it in other videos. This is called a half-life. And I want to be clear here. You don't know which half of it's gone. It's a probabilistic thing. You can't just say, oh, all of the carbon-14s on the left are going to decay and all the carbon-14s on the right aren't going to decay in that 5,730 years. What it's essentially saying is any given carbon-14 atom has a 50% chance of decaying into nitrogen-14 in 5,730 years."}, {"video_title": "Carbon 14 dating 1 Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "You don't know which half of it's gone. It's a probabilistic thing. You can't just say, oh, all of the carbon-14s on the left are going to decay and all the carbon-14s on the right aren't going to decay in that 5,730 years. What it's essentially saying is any given carbon-14 atom has a 50% chance of decaying into nitrogen-14 in 5,730 years. So over the course of 5,730 years, roughly half of them will have decayed. Now why is that interesting? Well, if you know that all living things have a certain proportion of carbon-14 in their tissue as kind of part of what makes them up, and then if you were to find some bone, let's just say find some bone right here."}, {"video_title": "Carbon 14 dating 1 Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "What it's essentially saying is any given carbon-14 atom has a 50% chance of decaying into nitrogen-14 in 5,730 years. So over the course of 5,730 years, roughly half of them will have decayed. Now why is that interesting? Well, if you know that all living things have a certain proportion of carbon-14 in their tissue as kind of part of what makes them up, and then if you were to find some bone, let's just say find some bone right here. You dig it up on some type of archaeology dig. And you say, hey, that bone has one half the carbon-14 of all the living things that you see right now. So it would be a pretty reasonable estimate to say, well, that thing must be 5,730 years old."}, {"video_title": "Carbon 14 dating 1 Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Well, if you know that all living things have a certain proportion of carbon-14 in their tissue as kind of part of what makes them up, and then if you were to find some bone, let's just say find some bone right here. You dig it up on some type of archaeology dig. And you say, hey, that bone has one half the carbon-14 of all the living things that you see right now. So it would be a pretty reasonable estimate to say, well, that thing must be 5,730 years old. Even better, maybe you dig a little deeper and you find another bone. Maybe you find another bone, maybe a couple of feet even deeper. And you say, wow, this thing right over here has 1 4th the carbon-14."}, {"video_title": "Carbon 14 dating 1 Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So it would be a pretty reasonable estimate to say, well, that thing must be 5,730 years old. Even better, maybe you dig a little deeper and you find another bone. Maybe you find another bone, maybe a couple of feet even deeper. And you say, wow, this thing right over here has 1 4th the carbon-14. This has 1 4th the carbon-14 that I would expect to find in something living. So how old is this? Well, if it only has 1 4th the carbon-14, it must have gone through two half-lives."}, {"video_title": "Carbon 14 dating 1 Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And you say, wow, this thing right over here has 1 4th the carbon-14. This has 1 4th the carbon-14 that I would expect to find in something living. So how old is this? Well, if it only has 1 4th the carbon-14, it must have gone through two half-lives. After one half-life, it would have one half the carbon. And then after another half-life, half of that also turns into nitrogen-14. And so this would involve two half-lives, which is the same thing as 2 times 5,730 years."}, {"video_title": "Why gravity gets so strong near dense objects Cosmology & Astronomy Khan Academy.mp3", "Sentence": "In the video on black holes, several people asked what is actually a pretty good question, which is if the mass of, say, a black hole is only two or three solar masses, why is the gravity so strong? Obviously, the sun's gravity isn't so strong that it keeps light from escaping. So why would something, or even a star that's two or three solar masses, its gravity isn't so strong that it keeps light from escaping. Why would a black hole that has the same mass, why would that keep light from escaping? And to understand that, let's just think a little bit about, and I'll just do Newtonian classical physics right here. I won't get into the whole general relativity of things. And this really will just give us the intuition of why a smaller, denser thing of the same mass can exert a stronger gravitational pull."}, {"video_title": "Why gravity gets so strong near dense objects Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Why would a black hole that has the same mass, why would that keep light from escaping? And to understand that, let's just think a little bit about, and I'll just do Newtonian classical physics right here. I won't get into the whole general relativity of things. And this really will just give us the intuition of why a smaller, denser thing of the same mass can exert a stronger gravitational pull. So let's imagine, so let's take two examples. Let's say I have some star here. Let's just call that mass m1."}, {"video_title": "Why gravity gets so strong near dense objects Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And this really will just give us the intuition of why a smaller, denser thing of the same mass can exert a stronger gravitational pull. So let's imagine, so let's take two examples. Let's say I have some star here. Let's just call that mass m1. And let's say that its radius, let's just call this r. And let's say that I have some other mass right at the surface of this star. It's somehow able to survive those surface temperatures. And this mass over here has a mass of m1."}, {"video_title": "Why gravity gets so strong near dense objects Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Let's just call that mass m1. And let's say that its radius, let's just call this r. And let's say that I have some other mass right at the surface of this star. It's somehow able to survive those surface temperatures. And this mass over here has a mass of m1. This mass over here has a mass of m2. The universal law of gravitation tells us that the force between these two masses is going to be equal to the gravitational constant times the product of the masses. So m1 times m2, all of that over the square of the distance."}, {"video_title": "Why gravity gets so strong near dense objects Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And this mass over here has a mass of m1. This mass over here has a mass of m2. The universal law of gravitation tells us that the force between these two masses is going to be equal to the gravitational constant times the product of the masses. So m1 times m2, all of that over the square of the distance. Now let me be very clear. You might say, wait, this magenta mass right here is touching this larger mass. Isn't the distance 0?"}, {"video_title": "Why gravity gets so strong near dense objects Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So m1 times m2, all of that over the square of the distance. Now let me be very clear. You might say, wait, this magenta mass right here is touching this larger mass. Isn't the distance 0? And you have to be very careful. This is the distance between their center of masses. So the center of mass of this large mass over here is r away from this mass that's on the surface."}, {"video_title": "Why gravity gets so strong near dense objects Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Isn't the distance 0? And you have to be very careful. This is the distance between their center of masses. So the center of mass of this large mass over here is r away from this mass that's on the surface. Now with that said, let's take another example. Let's say that this large, massive star, or whatever it might be, eventually condenses into something 1,000 times smaller. So let me draw it like this."}, {"video_title": "Why gravity gets so strong near dense objects Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So the center of mass of this large mass over here is r away from this mass that's on the surface. Now with that said, let's take another example. Let's say that this large, massive star, or whatever it might be, eventually condenses into something 1,000 times smaller. So let me draw it like this. And obviously I'm not drawing it to scale. So let's say we have another case like this. And I'm not drawing it to scale."}, {"video_title": "Why gravity gets so strong near dense objects Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So let me draw it like this. And obviously I'm not drawing it to scale. So let's say we have another case like this. And I'm not drawing it to scale. So this object, maybe it's the same object, or maybe it's a different object, that has the exact same mass as this larger object, but now it has a much smaller radius. It now has a much smaller radius. So that radius now, the radius is 1 over, let's just say it's 1,000th of this radius over here."}, {"video_title": "Why gravity gets so strong near dense objects Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And I'm not drawing it to scale. So this object, maybe it's the same object, or maybe it's a different object, that has the exact same mass as this larger object, but now it has a much smaller radius. It now has a much smaller radius. So that radius now, the radius is 1 over, let's just say it's 1,000th of this radius over here. So it's 1, maybe I'll just call it r over 1,000. So if this had a million kilometer radius, so that would make it roughly about twice the radius of the sun. If this was a million kilometer radius right over here, this would be 1,000 kilometer radius."}, {"video_title": "Why gravity gets so strong near dense objects Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So that radius now, the radius is 1 over, let's just say it's 1,000th of this radius over here. So it's 1, maybe I'll just call it r over 1,000. So if this had a million kilometer radius, so that would make it roughly about twice the radius of the sun. If this was a million kilometer radius right over here, this would be 1,000 kilometer radius. So maybe we're talking about something that's approaching a neutron star. But we don't have to think about what it actually is. Let's just think about the thought experiment here."}, {"video_title": "Why gravity gets so strong near dense objects Cosmology & Astronomy Khan Academy.mp3", "Sentence": "If this was a million kilometer radius right over here, this would be 1,000 kilometer radius. So maybe we're talking about something that's approaching a neutron star. But we don't have to think about what it actually is. Let's just think about the thought experiment here. So let's say I have this thing over here. And let's say I have something on the surface of this. So let's say I have that same mass, it's on the surface of this thing."}, {"video_title": "Why gravity gets so strong near dense objects Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Let's just think about the thought experiment here. So let's say I have this thing over here. And let's say I have something on the surface of this. So let's say I have that same mass, it's on the surface of this thing. So this is m2 right over here. So what is going to be the force between these two masses? How strong are they going to want to, what's the force pulling them together?"}, {"video_title": "Why gravity gets so strong near dense objects Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So let's say I have that same mass, it's on the surface of this thing. So this is m2 right over here. So what is going to be the force between these two masses? How strong are they going to want to, what's the force pulling them together? So let's just do the universal law of gravitation again. The force, let's just call this force 1, and let's call this force 2. Once again, it's going to be the gravitational constant times the product of their masses."}, {"video_title": "Why gravity gets so strong near dense objects Cosmology & Astronomy Khan Academy.mp3", "Sentence": "How strong are they going to want to, what's the force pulling them together? So let's just do the universal law of gravitation again. The force, let's just call this force 1, and let's call this force 2. Once again, it's going to be the gravitational constant times the product of their masses. So the big m1 times the smaller mass, m2, all of that over this distance squared, this radius squared. Remember, it's the distance to the center of masses. This center of mass here, we're considering m2 to kind of be just a point mass right over there."}, {"video_title": "Why gravity gets so strong near dense objects Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Once again, it's going to be the gravitational constant times the product of their masses. So the big m1 times the smaller mass, m2, all of that over this distance squared, this radius squared. Remember, it's the distance to the center of masses. This center of mass here, we're considering m2 to kind of be just a point mass right over there. So what's the radius squared? It's going to be r over 1,000 squared. Or if we simplify this, what will this be?"}, {"video_title": "Why gravity gets so strong near dense objects Cosmology & Astronomy Khan Academy.mp3", "Sentence": "This center of mass here, we're considering m2 to kind of be just a point mass right over there. So what's the radius squared? It's going to be r over 1,000 squared. Or if we simplify this, what will this be? This is the same thing, and I'll just write it in one color just because it takes less time. Gravitational constant m1, m2 over r squared over 1,000 squared, or over 1 million. Over 1 million."}, {"video_title": "Why gravity gets so strong near dense objects Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Or if we simplify this, what will this be? This is the same thing, and I'll just write it in one color just because it takes less time. Gravitational constant m1, m2 over r squared over 1,000 squared, or over 1 million. Over 1 million. That's just 1,000 squared. Or we can multiply the numerator and the denominator by a million, and this is going to be equal to 1 million, I'm going to write it out, 1 million, let me scroll to the right a little bit, times the gravitational constant times m1, m2, all of that over r squared. Now what is this thing right over here?"}, {"video_title": "Why gravity gets so strong near dense objects Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Over 1 million. That's just 1,000 squared. Or we can multiply the numerator and the denominator by a million, and this is going to be equal to 1 million, I'm going to write it out, 1 million, let me scroll to the right a little bit, times the gravitational constant times m1, m2, all of that over r squared. Now what is this thing right over here? That's the same thing as this F1. So this is going to be 1 million times F1. So even though the masses involved are the same, this yellow object right here is the same mass as this larger object over here."}, {"video_title": "Why gravity gets so strong near dense objects Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Now what is this thing right over here? That's the same thing as this F1. So this is going to be 1 million times F1. So even though the masses involved are the same, this yellow object right here is the same mass as this larger object over here. It's able to exert a million times the gravitational force on this point mass, and actually vice versa. They're both being attracted. They're both exerting this on each other."}, {"video_title": "Why gravity gets so strong near dense objects Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So even though the masses involved are the same, this yellow object right here is the same mass as this larger object over here. It's able to exert a million times the gravitational force on this point mass, and actually vice versa. They're both being attracted. They're both exerting this on each other. And the reality is, is because this thing is smaller, because this m1 on the right here, this one I'm coloring in, because this one is smaller and denser, this particle is able to get closer to its center of mass. Now you might be saying, OK, well I can buy that. This just comes straight from the universal law of gravitation."}, {"video_title": "Why gravity gets so strong near dense objects Cosmology & Astronomy Khan Academy.mp3", "Sentence": "They're both exerting this on each other. And the reality is, is because this thing is smaller, because this m1 on the right here, this one I'm coloring in, because this one is smaller and denser, this particle is able to get closer to its center of mass. Now you might be saying, OK, well I can buy that. This just comes straight from the universal law of gravitation. But wouldn't something closer to this center of mass experience that same thing? If this was a star, wouldn't photons that are over here, wouldn't this experience the same force? If this distance right here is r over 1,000, wouldn't some photon here, or atom here, or molecule, or whatever it's over here, wouldn't that experience the same force, this million times the force as this thing?"}, {"video_title": "Why gravity gets so strong near dense objects Cosmology & Astronomy Khan Academy.mp3", "Sentence": "This just comes straight from the universal law of gravitation. But wouldn't something closer to this center of mass experience that same thing? If this was a star, wouldn't photons that are over here, wouldn't this experience the same force? If this distance right here is r over 1,000, wouldn't some photon here, or atom here, or molecule, or whatever it's over here, wouldn't that experience the same force, this million times the force as this thing? And you've got to remember, all of a sudden when this thing is inside of this larger mass, what's happening? It no longer has the entire mass is no longer pulling on it in that direction. It's no longer pulling it in that inward direction."}, {"video_title": "Why gravity gets so strong near dense objects Cosmology & Astronomy Khan Academy.mp3", "Sentence": "If this distance right here is r over 1,000, wouldn't some photon here, or atom here, or molecule, or whatever it's over here, wouldn't that experience the same force, this million times the force as this thing? And you've got to remember, all of a sudden when this thing is inside of this larger mass, what's happening? It no longer has the entire mass is no longer pulling on it in that direction. It's no longer pulling it in that inward direction. You now have all of this mass over here. Let me think of the best way of doing it. So you could think of it all of this mass over here is pulling it in an outward direction."}, {"video_title": "Why gravity gets so strong near dense objects Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It's no longer pulling it in that inward direction. You now have all of this mass over here. Let me think of the best way of doing it. So you could think of it all of this mass over here is pulling it in an outward direction. It's not telling you what that mass out there is doing, since that mass itself is being pulled inward. It is pushing down on this. It is exerting pressure on that point."}, {"video_title": "Why gravity gets so strong near dense objects Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So you could think of it all of this mass over here is pulling it in an outward direction. It's not telling you what that mass out there is doing, since that mass itself is being pulled inward. It is pushing down on this. It is exerting pressure on that point. But the actual gravitational force that that point is experiencing is actually going to be less. It's actually going to be mitigated by the fact that there's so much mass over here pulling in the other direction. And so you can imagine if you were in the center of a really massive object, there would be no net gravitational force being pulled on you, because you're at its center of mass."}, {"video_title": "Why gravity gets so strong near dense objects Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It is exerting pressure on that point. But the actual gravitational force that that point is experiencing is actually going to be less. It's actually going to be mitigated by the fact that there's so much mass over here pulling in the other direction. And so you can imagine if you were in the center of a really massive object, there would be no net gravitational force being pulled on you, because you're at its center of mass. The rest of the mass is outward. So at every point, it will be pulling you outward. And so that's why if you were to enter the core of a star, if you were to get a lot closer to its center of mass, it's not going to be pulling on you with this type of force."}, {"video_title": "Why gravity gets so strong near dense objects Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And so you can imagine if you were in the center of a really massive object, there would be no net gravitational force being pulled on you, because you're at its center of mass. The rest of the mass is outward. So at every point, it will be pulling you outward. And so that's why if you were to enter the core of a star, if you were to get a lot closer to its center of mass, it's not going to be pulling on you with this type of force. And the only way you can get these type of forces is if the entire mass is contained in a very dense region, in a very small region. And that's why a black hole is able to exert such strong gravity that not even light can escape. Hopefully that clarifies things a little bit."}, {"video_title": "Scale of the small Scale of the universe Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "So we're all familiar with what a meter looks like. The average adult male is a little under 2 meters. If you were to divide a meter into a thousand units, you would get a millimeter. I think we probably know what a millimeter is if you've ever looked at a meter stick. It's the smallest measurement on that meter stick. So it's already pretty hard to look at. If you were to divide each of those millimeters into a thousand sections, you'd get a micrometer."}, {"video_title": "Scale of the small Scale of the universe Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "I think we probably know what a millimeter is if you've ever looked at a meter stick. It's the smallest measurement on that meter stick. So it's already pretty hard to look at. If you were to divide each of those millimeters into a thousand sections, you'd get a micrometer. Or another way to think about a micrometer is it's one millionth of a meter. So this is kind of beyond what we're capable of really perceiving. If you were to take each of those micrometers and divide them into a thousand sections, you would get a nanometer."}, {"video_title": "Scale of the small Scale of the universe Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "If you were to divide each of those millimeters into a thousand sections, you'd get a micrometer. Or another way to think about a micrometer is it's one millionth of a meter. So this is kind of beyond what we're capable of really perceiving. If you were to take each of those micrometers and divide them into a thousand sections, you would get a nanometer. So now we're at one billionth of a meter. You divide that by a thousand. You get a picometer."}, {"video_title": "Scale of the small Scale of the universe Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "If you were to take each of those micrometers and divide them into a thousand sections, you would get a nanometer. So now we're at one billionth of a meter. You divide that by a thousand. You get a picometer. So a picometer is one thousand billionth of a meter. Or you could say a trillionth of a meter. You divide one of those by a thousand and you would get a femtometer."}, {"video_title": "Scale of the small Scale of the universe Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "You get a picometer. So a picometer is one thousand billionth of a meter. Or you could say a trillionth of a meter. You divide one of those by a thousand and you would get a femtometer. So these are unimaginably small things. Now once you're familiar with the units, let's explore what types of things we can expect to find at these different scales. I'll start over here, and I've written them on the left as well, but it's more compelling when you see the pictures."}, {"video_title": "Scale of the small Scale of the universe Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "You divide one of those by a thousand and you would get a femtometer. So these are unimaginably small things. Now once you're familiar with the units, let's explore what types of things we can expect to find at these different scales. I'll start over here, and I've written them on the left as well, but it's more compelling when you see the pictures. We'll start over here with the b. I've arbitrarily picked something of this scale. There's many, many, many, almost an infinite number of things I could have picked at this scale. But the average b is about two centimeters long."}, {"video_title": "Scale of the small Scale of the universe Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "I'll start over here, and I've written them on the left as well, but it's more compelling when you see the pictures. We'll start over here with the b. I've arbitrarily picked something of this scale. There's many, many, many, almost an infinite number of things I could have picked at this scale. But the average b is about two centimeters long. This b right over here. It's about, give or take, one hundredth the length of the average adult human being. But once again, a honey bee, not too exciting, although it is pretty exciting to see it zoomed in like this."}, {"video_title": "Scale of the small Scale of the universe Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "But the average b is about two centimeters long. This b right over here. It's about, give or take, one hundredth the length of the average adult human being. But once again, a honey bee, not too exciting, although it is pretty exciting to see it zoomed in like this. But a honey bee is something that we can relate to. We've all seen honey bees. Now, what I want to do is zoom in or look at something that's fifty times smaller than a honey bee."}, {"video_title": "Scale of the small Scale of the universe Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "But once again, a honey bee, not too exciting, although it is pretty exciting to see it zoomed in like this. But a honey bee is something that we can relate to. We've all seen honey bees. Now, what I want to do is zoom in or look at something that's fifty times smaller than a honey bee. So something that if I were to kind of show how big it is relative to this honey bee, it would look something like this. I'm doing it very rough. And that is a dust mite."}, {"video_title": "Scale of the small Scale of the universe Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "Now, what I want to do is zoom in or look at something that's fifty times smaller than a honey bee. So something that if I were to kind of show how big it is relative to this honey bee, it would look something like this. I'm doing it very rough. And that is a dust mite. And this right here, these are both pictures of dust mites. Now, dust mites look like these strange and alien creatures. But what's amazing about them is that they are everywhere."}, {"video_title": "Scale of the small Scale of the universe Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "And that is a dust mite. And this right here, these are both pictures of dust mites. Now, dust mites look like these strange and alien creatures. But what's amazing about them is that they are everywhere. They're all around us. You probably have many of them lying on your skin or wherever right now, which is kind of a creepy idea. But we're talking about scale here."}, {"video_title": "Scale of the small Scale of the universe Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "But what's amazing about them is that they are everywhere. They're all around us. You probably have many of them lying on your skin or wherever right now, which is kind of a creepy idea. But we're talking about scale here. And the average dust mite, so we were talking about centimeters before. Now we'll talk about millimeters. The average dust mite is less than half of a millimeter."}, {"video_title": "Scale of the small Scale of the universe Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "But we're talking about scale here. And the average dust mite, so we were talking about centimeters before. Now we'll talk about millimeters. The average dust mite is less than half of a millimeter. Or if you want to talk in micrometers, it's about 400 micrometers long. So this length right over here is about 400 micrometers, so about 150th the length. Remember, this huge thing that I'm showing right here, this is a honey bee."}, {"video_title": "Scale of the small Scale of the universe Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "The average dust mite is less than half of a millimeter. Or if you want to talk in micrometers, it's about 400 micrometers long. So this length right over here is about 400 micrometers, so about 150th the length. Remember, this huge thing that I'm showing right here, this is a honey bee. It's about 150th the length of a honey bee. Or maybe to put it in other terms that you might be familiar with, this is a zoomed in picture of human hair. And you might say, oh my god, this person has horrible hair."}, {"video_title": "Scale of the small Scale of the universe Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "Remember, this huge thing that I'm showing right here, this is a honey bee. It's about 150th the length of a honey bee. Or maybe to put it in other terms that you might be familiar with, this is a zoomed in picture of human hair. And you might say, oh my god, this person has horrible hair. But no, if you were to look at your own hair under an electron microscope, you'd be lucky if it looked this good. This person actually I've seen pictures of more damaged hair than this. This is probably smooth and silky hair right here."}, {"video_title": "Scale of the small Scale of the universe Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "And you might say, oh my god, this person has horrible hair. But no, if you were to look at your own hair under an electron microscope, you'd be lucky if it looked this good. This person actually I've seen pictures of more damaged hair than this. This is probably smooth and silky hair right here. But the diameter of human hair, and this is on average. It depends on whose hair you're talking about. The diameter of human hair is about 100 micrometers thick."}, {"video_title": "Scale of the small Scale of the universe Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "This is probably smooth and silky hair right here. But the diameter of human hair, and this is on average. It depends on whose hair you're talking about. The diameter of human hair is about 100 micrometers thick. That's the diameter. So it's about a fourth the length of a dust mite. Or if I were to draw some human hair relative to this honey bee, it would look something like this."}, {"video_title": "Scale of the small Scale of the universe Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "The diameter of human hair is about 100 micrometers thick. That's the diameter. So it's about a fourth the length of a dust mite. Or if I were to draw some human hair relative to this honey bee, it would look something like this. And I'm drawing the whole hair, so its width would be the width of this thing that I just drew. Remember, we're looking at a honey bee here. It looks like some type of giant, but it is a honey bee."}, {"video_title": "Scale of the small Scale of the universe Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "Or if I were to draw some human hair relative to this honey bee, it would look something like this. And I'm drawing the whole hair, so its width would be the width of this thing that I just drew. Remember, we're looking at a honey bee here. It looks like some type of giant, but it is a honey bee. Let's zoom in even more. So we started with the honey bee. We zoomed in by 50 to get the dust mite."}, {"video_title": "Scale of the small Scale of the universe Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "It looks like some type of giant, but it is a honey bee. Let's zoom in even more. So we started with the honey bee. We zoomed in by 50 to get the dust mite. We zoomed in by another factor of 4 to get the width of human hair. If we zoom in, we're in the micrometer range now. If we zoom in by roughly another factor of 10, we get to the scale of cells."}, {"video_title": "Scale of the small Scale of the universe Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "We zoomed in by 50 to get the dust mite. We zoomed in by another factor of 4 to get the width of human hair. If we zoom in, we're in the micrometer range now. If we zoom in by roughly another factor of 10, we get to the scale of cells. And this right here is a red blood cell. I think this is a white blood cell right over here. About 6 to 8 micrometers."}, {"video_title": "Scale of the small Scale of the universe Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "If we zoom in by roughly another factor of 10, we get to the scale of cells. And this right here is a red blood cell. I think this is a white blood cell right over here. About 6 to 8 micrometers. So once again, if I were to draw a cell relative to this human hair, it would probably look something like this. Something on a similar scale that we can still kind of relate to is the width of spider silk. It's about 3 to 8 micrometers."}, {"video_title": "Scale of the small Scale of the universe Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "About 6 to 8 micrometers. So once again, if I were to draw a cell relative to this human hair, it would probably look something like this. Something on a similar scale that we can still kind of relate to is the width of spider silk. It's about 3 to 8 micrometers. So if I were to draw some spider silk on the same diagram, it would look something like this. This is an actual image of spider silk. So once again, something that we can kind of perceive."}, {"video_title": "Scale of the small Scale of the universe Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "It's about 3 to 8 micrometers. So if I were to draw some spider silk on the same diagram, it would look something like this. This is an actual image of spider silk. So once again, something that we can kind of perceive. You can bump into it. You can touch spider silk. You can see it if the sun is reflecting just right or if it has a little bit of moisture on it."}, {"video_title": "Scale of the small Scale of the universe Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "So once again, something that we can kind of perceive. You can bump into it. You can touch spider silk. You can see it if the sun is reflecting just right or if it has a little bit of moisture on it. But it's about the thinnest thing that humans can perceive. And this is in the ones of micrometer range. At that same range, you start to have some of your larger bacteria."}, {"video_title": "Scale of the small Scale of the universe Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "You can see it if the sun is reflecting just right or if it has a little bit of moisture on it. But it's about the thinnest thing that humans can perceive. And this is in the ones of micrometer range. At that same range, you start to have some of your larger bacteria. Bacteria can be anywhere from, and I'm speaking very roughly, 1 to 10 micrometers. So in general, they're smaller than cells. Most bacteria are smaller than most cells."}, {"video_title": "Scale of the small Scale of the universe Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "At that same range, you start to have some of your larger bacteria. Bacteria can be anywhere from, and I'm speaking very roughly, 1 to 10 micrometers. So in general, they're smaller than cells. Most bacteria are smaller than most cells. And just to figure out where we sit on our scale, have it over here. So we started off, I want to keep reminding ourselves, humans, you divide by 100, you get to the B. So each of these slashes right here are dividing by 10."}, {"video_title": "Scale of the small Scale of the universe Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "Most bacteria are smaller than most cells. And just to figure out where we sit on our scale, have it over here. So we started off, I want to keep reminding ourselves, humans, you divide by 100, you get to the B. So each of these slashes right here are dividing by 10. So this is divide by 10. Divide by 10 again, you're divided in size by 100. Divide by 10 again, you get to millimeter."}, {"video_title": "Scale of the small Scale of the universe Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "So each of these slashes right here are dividing by 10. So this is divide by 10. Divide by 10 again, you're divided in size by 100. Divide by 10 again, you get to millimeter. You've divided by 1,000. Divide by 10 again, you are doing tenths of millimeters, which is about the size of the human hair. You divide again by 10, you're going into tens of micrometers."}, {"video_title": "Scale of the small Scale of the universe Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "Divide by 10 again, you get to millimeter. You've divided by 1,000. Divide by 10 again, you are doing tenths of millimeters, which is about the size of the human hair. You divide again by 10, you're going into tens of micrometers. By 10 again, you get into the micrometer range. So now we're talking about cells, we're talking about bacteria. Now things are going to get really crazy."}, {"video_title": "Scale of the small Scale of the universe Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "You divide again by 10, you're going into tens of micrometers. By 10 again, you get into the micrometer range. So now we're talking about cells, we're talking about bacteria. Now things are going to get really crazy. This was in the ones of micrometer range. Now we're going to start getting into the hundreds of nanometer range. Just to get a sense of things."}, {"video_title": "Scale of the small Scale of the universe Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "Now things are going to get really crazy. This was in the ones of micrometer range. Now we're going to start getting into the hundreds of nanometer range. Just to get a sense of things. So remember, a nanometer is a thousandth of a micrometer. Or 100 nanometers would be a tenth of a micrometer. And this picture right here, this big, enormous planet or asteroid looking thing, this is a white blood cell."}, {"video_title": "Scale of the small Scale of the universe Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "Just to get a sense of things. So remember, a nanometer is a thousandth of a micrometer. Or 100 nanometers would be a tenth of a micrometer. And this picture right here, this big, enormous planet or asteroid looking thing, this is a white blood cell. The enormous blue thing in this picture. And so if I were to zoom out, it might look something like this right over here. But what's really fascinating about this picture for multiple reasons are these little green things that are emerging, that are essentially reproducing, emerging from the surface of this white blood cell."}, {"video_title": "Scale of the small Scale of the universe Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "And this picture right here, this big, enormous planet or asteroid looking thing, this is a white blood cell. The enormous blue thing in this picture. And so if I were to zoom out, it might look something like this right over here. But what's really fascinating about this picture for multiple reasons are these little green things that are emerging, that are essentially reproducing, emerging from the surface of this white blood cell. And these things right here, these are AIDS viruses. So now if we zoom in, roughly another factor of about 100 to 1000 from the size of a cell, you're now getting to the size of a virus. And all of the genetic material necessary to replicate that virus is right inside each of these little capsids, right inside each of these little green containers."}, {"video_title": "Scale of the small Scale of the universe Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "But what's really fascinating about this picture for multiple reasons are these little green things that are emerging, that are essentially reproducing, emerging from the surface of this white blood cell. And these things right here, these are AIDS viruses. So now if we zoom in, roughly another factor of about 100 to 1000 from the size of a cell, you're now getting to the size of a virus. And all of the genetic material necessary to replicate that virus is right inside each of these little capsids, right inside each of these little green containers. So now going back to our scale, we are down to the scale of a virus. So we're in the hundreds of nanometer range. If we divide by 10 and then divide by 10, you get to the nanometer range."}, {"video_title": "Scale of the small Scale of the universe Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "And all of the genetic material necessary to replicate that virus is right inside each of these little capsids, right inside each of these little green containers. So now going back to our scale, we are down to the scale of a virus. So we're in the hundreds of nanometer range. If we divide by 10 and then divide by 10, you get to the nanometer range. And right in the ones of nanometer range, you get to the width of the double helix of a DNA molecule. So this right here is, if you were to zoom in, and this is an artist's depiction of it, obviously this is not a picture, so to speak, of a DNA molecule. But the width of this double helix is about 2 nanometers, or another way to think about it, about 160th the diameter of one of these viral capsids."}, {"video_title": "Scale of the small Scale of the universe Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "If we divide by 10 and then divide by 10, you get to the nanometer range. And right in the ones of nanometer range, you get to the width of the double helix of a DNA molecule. So this right here is, if you were to zoom in, and this is an artist's depiction of it, obviously this is not a picture, so to speak, of a DNA molecule. But the width of this double helix is about 2 nanometers, or another way to think about it, about 160th the diameter of one of these viral capsids. It would have to be, because it's going to have to get all wound up and fit into one of these viral capsids. And DNA, just to make it clear, this is just the width of DNA. It's much, much, much, much, much, much longer, and we can talk about that in future videos."}, {"video_title": "Scale of the small Scale of the universe Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "But the width of this double helix is about 2 nanometers, or another way to think about it, about 160th the diameter of one of these viral capsids. It would have to be, because it's going to have to get all wound up and fit into one of these viral capsids. And DNA, just to make it clear, this is just the width of DNA. It's much, much, much, much, much, much longer, and we can talk about that in future videos. So once again, we're at a very, very small scale. If you want to think of it in terms of meters, we're at 2 billionths of a meter. You could put 500 million of these side by side to get to a meter."}, {"video_title": "Scale of the small Scale of the universe Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "It's much, much, much, much, much, much longer, and we can talk about that in future videos. So once again, we're at a very, very small scale. If you want to think of it in terms of meters, we're at 2 billionths of a meter. You could put 500 million of these side by side to get to a meter. Or you could even think of it this way. This is 2 millionths of a millimeter. So once again, super small."}, {"video_title": "Scale of the small Scale of the universe Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "You could put 500 million of these side by side to get to a meter. Or you could even think of it this way. This is 2 millionths of a millimeter. So once again, super small. You could put these side by side, one DNA, another DNA. If you made them touch, you could put 500,000 next to each other in a millimeter. So this is an unbelievably small amount of space."}, {"video_title": "Scale of the small Scale of the universe Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "So once again, super small. You could put these side by side, one DNA, another DNA. If you made them touch, you could put 500,000 next to each other in a millimeter. So this is an unbelievably small amount of space. It's going to fit into another unit that's not kind of in the conventional prefix followed by meters, and this is an angstrom. And 10 angstroms equal 1 nanometer. So the width of this DNA double helix would be 2 nanometers or 20 angstroms."}, {"video_title": "Scale of the small Scale of the universe Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "So this is an unbelievably small amount of space. It's going to fit into another unit that's not kind of in the conventional prefix followed by meters, and this is an angstrom. And 10 angstroms equal 1 nanometer. So the width of this DNA double helix would be 2 nanometers or 20 angstroms. Now, if we were to divide again by 10, you get to something that's 2 angstroms or 0.2 nanometers wide, and that is a water molecule. Maybe instead of using red, I should have used blue or something. But this right here is the oxygen, and it is bonded to the two hydrogens right over here."}, {"video_title": "Scale of the small Scale of the universe Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "So the width of this DNA double helix would be 2 nanometers or 20 angstroms. Now, if we were to divide again by 10, you get to something that's 2 angstroms or 0.2 nanometers wide, and that is a water molecule. Maybe instead of using red, I should have used blue or something. But this right here is the oxygen, and it is bonded to the two hydrogens right over here. So this is beyond, frankly, human perception, or even really stuff that we can conceptualize, not to even speak of perception. We're still imagining how small we're dealing with right over here. Remember, we're dealing with less than a fifth of a billionth of a meter, or a fifth of a millionth of a millimeter, something that I really can't fathom."}, {"video_title": "Scale of the small Scale of the universe Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "But this right here is the oxygen, and it is bonded to the two hydrogens right over here. So this is beyond, frankly, human perception, or even really stuff that we can conceptualize, not to even speak of perception. We're still imagining how small we're dealing with right over here. Remember, we're dealing with less than a fifth of a billionth of a meter, or a fifth of a millionth of a millimeter, something that I really can't fathom. But we're going to get even smaller than that. If we were to zoom in on one of these hydrogen atoms, and now things start to get kind of abstract, and we start dealing in the quantum realm, and it's hard to define where one thing ends and one thing begins, and what is real and what is not real, and all of that silliness. But if we try our best to do it, if we were to zoom in and we were to put some boundary on a hydrogen atom, because the electrons actually could jump around anywhere, but if we set some boundary of where the electrons are most likely to be found, the diameter of a hydrogen atom is roughly one angstrom, which makes sense from this diagram, too."}, {"video_title": "Scale of the small Scale of the universe Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "Remember, we're dealing with less than a fifth of a billionth of a meter, or a fifth of a millionth of a millimeter, something that I really can't fathom. But we're going to get even smaller than that. If we were to zoom in on one of these hydrogen atoms, and now things start to get kind of abstract, and we start dealing in the quantum realm, and it's hard to define where one thing ends and one thing begins, and what is real and what is not real, and all of that silliness. But if we try our best to do it, if we were to zoom in and we were to put some boundary on a hydrogen atom, because the electrons actually could jump around anywhere, but if we set some boundary of where the electrons are most likely to be found, the diameter of a hydrogen atom is roughly one angstrom, which makes sense from this diagram, too. It's about half of the diameter of this water molecule. What's extra crazy is, one, this atom is super, super, duper small, something that we can't, you know, this is one ten billionth of a meter, or one ten millionth of a millimeter, so something we really, really can't fathom. But what's crazier than that is that it's mostly free space."}, {"video_title": "Scale of the small Scale of the universe Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "But if we try our best to do it, if we were to zoom in and we were to put some boundary on a hydrogen atom, because the electrons actually could jump around anywhere, but if we set some boundary of where the electrons are most likely to be found, the diameter of a hydrogen atom is roughly one angstrom, which makes sense from this diagram, too. It's about half of the diameter of this water molecule. What's extra crazy is, one, this atom is super, super, duper small, something that we can't, you know, this is one ten billionth of a meter, or one ten millionth of a millimeter, so something we really, really can't fathom. But what's crazier than that is that it's mostly free space. We've gotten this small. We're trying to get to these fundamental units, and this thing right here is mostly free space. And that's because if you look at an electron, and when we say radius here, it's really hard to define where it starts and ends, and you have to do some things related to the charge, and we're not even thinking about quantum effects and all of that."}, {"video_title": "Scale of the small Scale of the universe Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "But what's crazier than that is that it's mostly free space. We've gotten this small. We're trying to get to these fundamental units, and this thing right here is mostly free space. And that's because if you look at an electron, and when we say radius here, it's really hard to define where it starts and ends, and you have to do some things related to the charge, and we're not even thinking about quantum effects and all of that. An electron has a radius of 3 times 10 to the negative 5th angstroms, and the nucleus of a hydrogen atom, which is really just a proton, has a radius a little bit, and, you know, don't even worry about this number right here. The general idea is it's the same order of magnitude. It's about one ten thousandth of an angstrom."}, {"video_title": "Scale of the small Scale of the universe Cosmology & Astronomy Khan Academy (2).mp3", "Sentence": "And that's because if you look at an electron, and when we say radius here, it's really hard to define where it starts and ends, and you have to do some things related to the charge, and we're not even thinking about quantum effects and all of that. An electron has a radius of 3 times 10 to the negative 5th angstroms, and the nucleus of a hydrogen atom, which is really just a proton, has a radius a little bit, and, you know, don't even worry about this number right here. The general idea is it's the same order of magnitude. It's about one ten thousandth of an angstrom. And just to give a sense of what it's like, if you view the entire atomic radius to be about an angstrom, kind of just have a conception for scale of the atom and how much free space there is in an atom, if we even want to think what is free space. Imagine a nucleus being maybe a marble at the center of a football stadium, of a domed football stadium, and imagine an electron being a honeybee just randomly jumping around random parts of that entire volume inside of that football stadium. And obviously it's a quantum honeybee, so it can jump around from spot to spot, and it's not easy to predict where it's going to go next and all of the rest."}, {"video_title": "Intergalactic scale Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "where we left off in the last video, where we were just kind of staring amazed at this Earth's view of the Milky Way galaxy, just making sure we understood how enormous and how many stars we were looking at. And even this, even if each of these dots were a star, this is a huge amount of stars, but a lot of these dots are thousands of stars, are thousands of stars. So this, you know, our mind was already blown, but what we're gonna see in this video is that in some ways this is kind of just the beginning. And to some degree, I'm gonna stop doing these particles of sand in a football field analogy, because at some point the particles of sand become so vast that our minds can't even grasp it to begin with. But let's just start with our Milky Way. And we saw in the last video, the Milky Way right here, we're sitting here about 25,000 light years from the center. It's roughly 100,000 light years in diameter."}, {"video_title": "Intergalactic scale Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And to some degree, I'm gonna stop doing these particles of sand in a football field analogy, because at some point the particles of sand become so vast that our minds can't even grasp it to begin with. But let's just start with our Milky Way. And we saw in the last video, the Milky Way right here, we're sitting here about 25,000 light years from the center. It's roughly 100,000 light years in diameter. And then let's put it in perspective of its local neighborhood. So let's look at the local group. And when we talk about local group, we're talking about the local group of galaxies."}, {"video_title": "Intergalactic scale Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It's roughly 100,000 light years in diameter. And then let's put it in perspective of its local neighborhood. So let's look at the local group. And when we talk about local group, we're talking about the local group of galaxies. Of galaxies. So this right here is the Milky Way's local group. That's us right there, sitting right over here, about 25,000 light years from the center of the Milky Way."}, {"video_title": "Intergalactic scale Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And when we talk about local group, we're talking about the local group of galaxies. Of galaxies. So this right here is the Milky Way's local group. That's us right there, sitting right over here, about 25,000 light years from the center of the Milky Way. You have some of these small, and I use the word, I use small in quotation marks, because these are also vast entities, also unimaginable entities. But we have these satellite galaxies around, under the gravitational influence, some of them, of the Milky Way. But the nearest large galaxy to us is Andromeda right over here."}, {"video_title": "Intergalactic scale Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "That's us right there, sitting right over here, about 25,000 light years from the center of the Milky Way. You have some of these small, and I use the word, I use small in quotation marks, because these are also vast entities, also unimaginable entities. But we have these satellite galaxies around, under the gravitational influence, some of them, of the Milky Way. But the nearest large galaxy to us is Andromeda right over here. And this distance right over here, and now we're gonna start talking about the millions of light years. So this distance right here is 2.5 million. 2.5 million light years."}, {"video_title": "Intergalactic scale Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But the nearest large galaxy to us is Andromeda right over here. And this distance right over here, and now we're gonna start talking about the millions of light years. So this distance right here is 2.5 million. 2.5 million light years. And just as a bit of a reference, if that's any reference at all, one light year is roughly the radius of the Oort Cloud. And the Oort Cloud was, or another way to think about it, the Oort Cloud, or one radius of the Oort Cloud, is about 50 or 60,000 atomic, not atomic, astronomical units. And that's the distance from the sun to the Earth."}, {"video_title": "Intergalactic scale Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "2.5 million light years. And just as a bit of a reference, if that's any reference at all, one light year is roughly the radius of the Oort Cloud. And the Oort Cloud was, or another way to think about it, the Oort Cloud, or one radius of the Oort Cloud, is about 50 or 60,000 atomic, not atomic, astronomical units. And that's the distance from the sun to the Earth. So you could view this as 2.5 million times 60,000 or so, times the distance from the sun to the Earth. So this is an unbelievably large distance we're talking about here, and that's to get to the next big galaxy over here. But even these things are huge things with many, I mean, just unfathomably many stars."}, {"video_title": "Intergalactic scale Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And that's the distance from the sun to the Earth. So you could view this as 2.5 million times 60,000 or so, times the distance from the sun to the Earth. So this is an unbelievably large distance we're talking about here, and that's to get to the next big galaxy over here. But even these things are huge things with many, I mean, just unfathomably many stars. But Andromeda in particular, you know, we said that the Milky Way has 200 to 400 billion stars. Andromeda, people believe, has on the order of one trillion. One trillion stars."}, {"video_title": "Intergalactic scale Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But even these things are huge things with many, I mean, just unfathomably many stars. But Andromeda in particular, you know, we said that the Milky Way has 200 to 400 billion stars. Andromeda, people believe, has on the order of one trillion. One trillion stars. So even, even, even, you know, these just start to become numbers. It's hard to grasp. But we're not gonna stop here."}, {"video_title": "Intergalactic scale Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "One trillion stars. So even, even, even, you know, these just start to become numbers. It's hard to grasp. But we're not gonna stop here. So in this over here, this whole diagram right here, it's about four light years across if you go from point to point. If you go from one side to the other side, this is about, not four light years, sorry, this is four million light years. Four million."}, {"video_title": "Intergalactic scale Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But we're not gonna stop here. So in this over here, this whole diagram right here, it's about four light years across if you go from point to point. If you go from one side to the other side, this is about, not four light years, sorry, this is four million light years. Four million. Four million light years. Four light years is just the distance from us to Alpha Centauri. So that was nothing."}, {"video_title": "Intergalactic scale Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Four million. Four million light years. Four light years is just the distance from us to Alpha Centauri. So that was nothing. That would only take a Voyager 1 80,000 years to get, this is four million light years. So four million times the distance to the nearest star. But let's, but even this, even this is, I mean, I'm starting to stumble on my words because there's really no words to describe it."}, {"video_title": "Intergalactic scale Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So that was nothing. That would only take a Voyager 1 80,000 years to get, this is four million light years. So four million times the distance to the nearest star. But let's, but even this, even this is, I mean, I'm starting to stumble on my words because there's really no words to describe it. Even this is small on a, on an intergalactic scale. Because when you zoom out more, you can see our local group, our local group is right over here, is right over here. And this right over here is the Virgo supercluster."}, {"video_title": "Intergalactic scale Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But let's, but even this, even this is, I mean, I'm starting to stumble on my words because there's really no words to describe it. Even this is small on a, on an intergalactic scale. Because when you zoom out more, you can see our local group, our local group is right over here, is right over here. And this right over here is the Virgo supercluster. And each dot here is at least one galaxy, but it might be more than one galaxy. And this, and more than one galaxies. And the diameter here, the diameter here is 150 million."}, {"video_title": "Intergalactic scale Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And this right over here is the Virgo supercluster. And each dot here is at least one galaxy, but it might be more than one galaxy. And this, and more than one galaxies. And the diameter here, the diameter here is 150 million. 150 million. 150 million light years. Light years."}, {"video_title": "Intergalactic scale Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And the diameter here, the diameter here is 150 million. 150 million. 150 million light years. Light years. So what we saw in the local group in the last diagram, the distance from the Milky Way to Andromeda, which was two and a half million light years, which would be just a little dot, just like that. That would be the distance between the Milky Way and Andromeda. And now we're looking at the Virgo supercluster that is 150 million light years."}, {"video_title": "Intergalactic scale Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Light years. So what we saw in the local group in the last diagram, the distance from the Milky Way to Andromeda, which was two and a half million light years, which would be just a little dot, just like that. That would be the distance between the Milky Way and Andromeda. And now we're looking at the Virgo supercluster that is 150 million light years. But we're not done yet. We can zoom out even more. We can zoom out even more."}, {"video_title": "Intergalactic scale Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And now we're looking at the Virgo supercluster that is 150 million light years. But we're not done yet. We can zoom out even more. We can zoom out even more. And over here, so you had your Virgo supercluster, 150 million light years was that last diagram. This diagram right over here, I want to keep both of them on the screen if I can. This diagram right here, 150 million light years across."}, {"video_title": "Intergalactic scale Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "We can zoom out even more. And over here, so you had your Virgo supercluster, 150 million light years was that last diagram. This diagram right over here, I want to keep both of them on the screen if I can. This diagram right here, 150 million light years across. That would fit right about here on this diagram. So this is all of the superclusters that are near us. And once again, near has to be used very, very, very loosely."}, {"video_title": "Intergalactic scale Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "This diagram right here, 150 million light years across. That would fit right about here on this diagram. So this is all of the superclusters that are near us. And once again, near has to be used very, very, very loosely. Here, this distance is about 150 million light years. A billion light years is two, three, four, five. A billion light years is about from here to there."}, {"video_title": "Intergalactic scale Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And once again, near has to be used very, very, very loosely. Here, this distance is about 150 million light years. A billion light years is two, three, four, five. A billion light years is about from here to there. So we're starting to talk on a fairly massive, I guess we've always been talking on a massive scale, but now it's an even more massive scale. But we're still not done because this whole diagram, now these dots that you're seeing now, I want to make it very clear, these aren't stars. These aren't even clusters of stars."}, {"video_title": "Intergalactic scale Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "A billion light years is about from here to there. So we're starting to talk on a fairly massive, I guess we've always been talking on a massive scale, but now it's an even more massive scale. But we're still not done because this whole diagram, now these dots that you're seeing now, I want to make it very clear, these aren't stars. These aren't even clusters of stars. These aren't even clusters of millions or even billions of stars. Each of these dots are clusters of galaxies. Each of those galaxies having hundreds of billions to trillions of stars."}, {"video_title": "Intergalactic scale Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "These aren't even clusters of stars. These aren't even clusters of millions or even billions of stars. Each of these dots are clusters of galaxies. Each of those galaxies having hundreds of billions to trillions of stars. So we're just on an unbelievably massive scale at this point, but we're still not done. This is roughly about a billion light years across. But right here is actually the best estimate of the visible universe."}, {"video_title": "Intergalactic scale Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Each of those galaxies having hundreds of billions to trillions of stars. So we're just on an unbelievably massive scale at this point, but we're still not done. This is roughly about a billion light years across. But right here is actually the best estimate of the visible universe. And in future videos, we're going to talk a lot more about what the visible universe means. So if you were to zoom out enough, this entire diagram right here, about a billion light years, would fit right over, would fit just like that. We're talking about a super, super small amount of this part right here."}, {"video_title": "Intergalactic scale Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But right here is actually the best estimate of the visible universe. And in future videos, we're going to talk a lot more about what the visible universe means. So if you were to zoom out enough, this entire diagram right here, about a billion light years, would fit right over, would fit just like that. We're talking about a super, super small amount of this part right here. And this is just the visible universe. I want to make it clear, this is not the entire universe. And we say it's the visible universe because think about what's happening."}, {"video_title": "Intergalactic scale Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "We're talking about a super, super small amount of this part right here. And this is just the visible universe. I want to make it clear, this is not the entire universe. And we say it's the visible universe because think about what's happening. When we think about a point out here, and we're observing it, and that's, let's say, 13 billion light years away. Let's say that point, 13 billion. We're going to talk more about this in future videos."}, {"video_title": "Intergalactic scale Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And we say it's the visible universe because think about what's happening. When we think about a point out here, and we're observing it, and that's, let's say, 13 billion light years away. Let's say that point, 13 billion. We're going to talk more about this in future videos. 13 billion light years. And I feel almost, it's almost a sacrilege to be writing on this because this complexity that we're seeing here is just mind boggling. But this 13 billion light year away object, what we're observing, the light is just getting to us."}, {"video_title": "Intergalactic scale Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "We're going to talk more about this in future videos. 13 billion light years. And I feel almost, it's almost a sacrilege to be writing on this because this complexity that we're seeing here is just mind boggling. But this 13 billion light year away object, what we're observing, the light is just getting to us. This light left some point 13 billion light years ago. So what we're actually doing is observing that object close to the beginning of the actual universe. And the reason why it's the visible universe is there might have been something a little bit further out."}, {"video_title": "Intergalactic scale Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But this 13 billion light year away object, what we're observing, the light is just getting to us. This light left some point 13 billion light years ago. So what we're actually doing is observing that object close to the beginning of the actual universe. And the reason why it's the visible universe is there might have been something a little bit further out. Maybe its light hasn't reached us yet. Or maybe the universe itself, and we'll talk more about this, it's expanding so fast that the light will never, ever reach us. So it's actually a huge question mark."}, {"video_title": "Intergalactic scale Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And the reason why it's the visible universe is there might have been something a little bit further out. Maybe its light hasn't reached us yet. Or maybe the universe itself, and we'll talk more about this, it's expanding so fast that the light will never, ever reach us. So it's actually a huge question mark. It's actually a huge question mark on how big the actual universe is. And then some people might say, well, does it even matter because this by itself is a huge distance. And I want to make it clear."}, {"video_title": "Intergalactic scale Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So it's actually a huge question mark. It's actually a huge question mark on how big the actual universe is. And then some people might say, well, does it even matter because this by itself is a huge distance. And I want to make it clear. You might say, OK, if this light over here, this is coming from 13 billion light years away, or if this is 13 billion light years away, then you could say, hey, so everything that we can observe or that we can even observe the past of, the radius is about 26 billion light years. But even there, we have to be careful. Because remember, the universe is expanding."}, {"video_title": "Intergalactic scale Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And I want to make it clear. You might say, OK, if this light over here, this is coming from 13 billion light years away, or if this is 13 billion light years away, then you could say, hey, so everything that we can observe or that we can even observe the past of, the radius is about 26 billion light years. But even there, we have to be careful. Because remember, the universe is expanding. When this light was emitted, and I'll do a whole video on this because the geometry of it is kind of hard to visualize. When this light was emitted, where we are in the Virgo supercluster inside of the Milky Way galaxy, where we are was much closer to that point. It was on the order of, and I want to make sure I get this right, 36 million light years."}, {"video_title": "Intergalactic scale Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Because remember, the universe is expanding. When this light was emitted, and I'll do a whole video on this because the geometry of it is kind of hard to visualize. When this light was emitted, where we are in the Virgo supercluster inside of the Milky Way galaxy, where we are was much closer to that point. It was on the order of, and I want to make sure I get this right, 36 million light years. So we were super close by, I guess, astronomical scales. We were super close, only 36 million light years, to this object when that light was released. But that light was coming to us, and the whole time the universe expanding."}, {"video_title": "Intergalactic scale Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It was on the order of, and I want to make sure I get this right, 36 million light years. So we were super close by, I guess, astronomical scales. We were super close, only 36 million light years, to this object when that light was released. But that light was coming to us, and the whole time the universe expanding. We were also moving away from it. If you just think about all of the spaces, everything is expanding away from each other. And only 13 billion years later did it finally catch up with us."}, {"video_title": "Intergalactic scale Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But that light was coming to us, and the whole time the universe expanding. We were also moving away from it. If you just think about all of the spaces, everything is expanding away from each other. And only 13 billion years later did it finally catch up with us. But the whole time that that was happening, this object has also been moving. This object has also been moving away from us. And so our best estimate of where this object is now, based on how space is expanding, is on the order of 40 or 45 billion light years away."}, {"video_title": "Intergalactic scale Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And only 13 billion years later did it finally catch up with us. But the whole time that that was happening, this object has also been moving. This object has also been moving away from us. And so our best estimate of where this object is now, based on how space is expanding, is on the order of 40 or 45 billion light years away. We're just observing where that light was emitted 13 billion years ago. And I want to be very clear, what we're observing, this light is coming from something very, very, very primitive. That object, or that area of space where that light was emitted from, has now condensed into way more, I guess, mature astronomical structures."}, {"video_title": "Intergalactic scale Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And so our best estimate of where this object is now, based on how space is expanding, is on the order of 40 or 45 billion light years away. We're just observing where that light was emitted 13 billion years ago. And I want to be very clear, what we're observing, this light is coming from something very, very, very primitive. That object, or that area of space where that light was emitted from, has now condensed into way more, I guess, mature astronomical structures. If you take it from the other point of view, people sitting in this point of space now, and they've now moved 46 billion light years out, when they observe our region of space, they're not going to see us. They're not going to see Earth as it is now. They're going to see the region of space where Earth is at a super primitive stage, shortly after the Big Bang."}, {"video_title": "Intergalactic scale Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "That object, or that area of space where that light was emitted from, has now condensed into way more, I guess, mature astronomical structures. If you take it from the other point of view, people sitting in this point of space now, and they've now moved 46 billion light years out, when they observe our region of space, they're not going to see us. They're not going to see Earth as it is now. They're going to see the region of space where Earth is at a super primitive stage, shortly after the Big Bang. When I use words like shortly, I use that also loosely. We're still talking about hundreds of thousands or even millions of years. So we'll talk more about that in a future video."}, {"video_title": "Intergalactic scale Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "They're going to see the region of space where Earth is at a super primitive stage, shortly after the Big Bang. When I use words like shortly, I use that also loosely. We're still talking about hundreds of thousands or even millions of years. So we'll talk more about that in a future video. But the whole point of this video is it's beyond mind numbing. I would say the last video about the Milky Way, that alone was mind numbing. But now we're in a reality where just the Milky Way becomes something that's almost unbelievably insignificant when you think about this picture right here."}, {"video_title": "Intergalactic scale Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So we'll talk more about that in a future video. But the whole point of this video is it's beyond mind numbing. I would say the last video about the Milky Way, that alone was mind numbing. But now we're in a reality where just the Milky Way becomes something that's almost unbelievably insignificant when you think about this picture right here. And the really mind numbing thing is if someone told me that this is the entire universe, this by itself would certainly put things in perspective. But it's unknown what's beyond it. There are some estimates that this might only be 1 times 10 to the 23rd of the entire universe."}, {"video_title": "Intergalactic scale Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But now we're in a reality where just the Milky Way becomes something that's almost unbelievably insignificant when you think about this picture right here. And the really mind numbing thing is if someone told me that this is the entire universe, this by itself would certainly put things in perspective. But it's unknown what's beyond it. There are some estimates that this might only be 1 times 10 to the 23rd of the entire universe. And we could even, we'll talk, it might even be the reality that the entire universe is smaller than this. And that's an interesting thing to think about. But I'll leave you there because I think no matter how you think about it, it's just, I don't know."}, {"video_title": "Detectable civilizations in our galaxy 2 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Some of you might be wondering, why are we even worried about this Drake equation? Or why are we even tempted to go through this thought experiment of the number of detectable civilizations in the galaxy? When we don't have a clue of some of these assumptions. We don't know what fraction of planets capable of sustaining life actually do generate life. We don't know of all of the planets that have life, what fraction of those planets go on to have intelligent life. What fractions of those civilizations go on to using electromagnetic radiation as a form of communication. We don't know these answers."}, {"video_title": "Detectable civilizations in our galaxy 2 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "We don't know what fraction of planets capable of sustaining life actually do generate life. We don't know of all of the planets that have life, what fraction of those planets go on to have intelligent life. What fractions of those civilizations go on to using electromagnetic radiation as a form of communication. We don't know these answers. In fact, we probably won't know some of these answers for some, some time. So what's the point of going through this exercise? And that is a valid point of view."}, {"video_title": "Detectable civilizations in our galaxy 2 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "We don't know these answers. In fact, we probably won't know some of these answers for some, some time. So what's the point of going through this exercise? And that is a valid point of view. The Drake equation, or even this little equation that we've set up here, it's not an equation in the traditional sense where we can immediately apply it to some engineering problem or some physical problem or anything like that. I view it more as a bit of a thought experiment. And what's interesting about it is it can structure our thought around the problem."}, {"video_title": "Detectable civilizations in our galaxy 2 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And that is a valid point of view. The Drake equation, or even this little equation that we've set up here, it's not an equation in the traditional sense where we can immediately apply it to some engineering problem or some physical problem or anything like that. I view it more as a bit of a thought experiment. And what's interesting about it is it can structure our thought around the problem. And I think that's where it has the most value. We'll probably not get a solid number on this any time soon. But it does lead us to thinking about these interesting problems of what does it mean, or what do we think has to happen for a planet to start getting life, even if it has all the right ingredients."}, {"video_title": "Detectable civilizations in our galaxy 2 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And what's interesting about it is it can structure our thought around the problem. And I think that's where it has the most value. We'll probably not get a solid number on this any time soon. But it does lead us to thinking about these interesting problems of what does it mean, or what do we think has to happen for a planet to start getting life, even if it has all the right ingredients. And then what does it mean for things to eventually get to the point that you have intelligent life? And in all fairness to this, is that probably 200 years ago, there would have been no way to even have a decent estimate of the number of stars in the galaxy. Now we're starting to do an okay job on that."}, {"video_title": "Detectable civilizations in our galaxy 2 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But it does lead us to thinking about these interesting problems of what does it mean, or what do we think has to happen for a planet to start getting life, even if it has all the right ingredients. And then what does it mean for things to eventually get to the point that you have intelligent life? And in all fairness to this, is that probably 200 years ago, there would have been no way to even have a decent estimate of the number of stars in the galaxy. Now we're starting to do an okay job on that. 20 or 30 years ago, it would have been viewed impossible to say the fraction of stars that have planets. But now we're finding exoplanets, we're seeing stars wobble, we're getting more and more accurate instruments so we can start to think about planets that are closer to the size of Earth. So we're making headway there."}, {"video_title": "Detectable civilizations in our galaxy 2 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Now we're starting to do an okay job on that. 20 or 30 years ago, it would have been viewed impossible to say the fraction of stars that have planets. But now we're finding exoplanets, we're seeing stars wobble, we're getting more and more accurate instruments so we can start to think about planets that are closer to the size of Earth. So we're making headway there. There's other indirect methods to think about, well, you know, some of these exoplanets look like they're in the right zone, and they look like they have the right chemical signature based on other information that we're getting. Maybe they are capable of sustaining life. So as time goes on and as technology improves, we might be able to get better and better and better at this."}, {"video_title": "Detectable civilizations in our galaxy 2 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So we're making headway there. There's other indirect methods to think about, well, you know, some of these exoplanets look like they're in the right zone, and they look like they have the right chemical signature based on other information that we're getting. Maybe they are capable of sustaining life. So as time goes on and as technology improves, we might be able to get better and better and better at this. But with that said, it's not going to happen any time soon. And the real value of all of this is really to structure our thought about a super, super interesting topic. Now the other thing I want to talk about is a slight clarification of what I talked about in the last video."}, {"video_title": "Detectable civilizations in our galaxy 2 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So as time goes on and as technology improves, we might be able to get better and better and better at this. But with that said, it's not going to happen any time soon. And the real value of all of this is really to structure our thought about a super, super interesting topic. Now the other thing I want to talk about is a slight clarification of what I talked about in the last video. In the last video for this L, I said it's the civilization's lifespan. But what's actually relevant is the lifespan of the civilization while it is detectable. So it doesn't matter if the civilization is around 100,000 years, but it's not releasing any type of thing that we can detect."}, {"video_title": "Detectable civilizations in our galaxy 2 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Now the other thing I want to talk about is a slight clarification of what I talked about in the last video. In the last video for this L, I said it's the civilization's lifespan. But what's actually relevant is the lifespan of the civilization while it is detectable. So it doesn't matter if the civilization is around 100,000 years, but it's not releasing any type of thing that we can detect. That's not what we care about. We care about the 5,000 years or the 10,000 years or the 100,000 years when they are actually using some type of communications or some type of electromagnetic radiation that we can eventually detect once those things reach us. Now the other thing I want to make it clear is we're talking about the number of detectable civilizations in the galaxy right now."}, {"video_title": "Detectable civilizations in our galaxy 2 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So it doesn't matter if the civilization is around 100,000 years, but it's not releasing any type of thing that we can detect. That's not what we care about. We care about the 5,000 years or the 10,000 years or the 100,000 years when they are actually using some type of communications or some type of electromagnetic radiation that we can eventually detect once those things reach us. Now the other thing I want to make it clear is we're talking about the number of detectable civilizations in the galaxy right now. And I'll write now in quotation marks. Because we're not talking about a civilization that is maybe even a pure civilization with us that developed radio communication on the order of 100 years ago. Because frankly, they would have to be no more than 100 light years away for us to be able to detect those signals now."}, {"video_title": "Detectable civilizations in our galaxy 2 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Now the other thing I want to make it clear is we're talking about the number of detectable civilizations in the galaxy right now. And I'll write now in quotation marks. Because we're not talking about a civilization that is maybe even a pure civilization with us that developed radio communication on the order of 100 years ago. Because frankly, they would have to be no more than 100 light years away for us to be able to detect those signals now. If they were on the other side of the galaxy, we wouldn't be able to detect their signals for tens of thousands of years. So when I talk about now, I'm saying that the signals are getting to us. Signals getting."}, {"video_title": "Detectable civilizations in our galaxy 2 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Because frankly, they would have to be no more than 100 light years away for us to be able to detect those signals now. If they were on the other side of the galaxy, we wouldn't be able to detect their signals for tens of thousands of years. So when I talk about now, I'm saying that the signals are getting to us. Signals getting. Signals received. The signals are being received right now. So you could have a civilization that developed radio 70,000 years ago, but they're 70,000 light years away."}, {"video_title": "Detectable civilizations in our galaxy 2 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Signals getting. Signals received. The signals are being received right now. So you could have a civilization that developed radio 70,000 years ago, but they're 70,000 light years away. And maybe they collapse 10,000 years later, but we're just receiving their first radio signal. So that would be a civilization that I would count in this equation we're setting up. And so just to make sure we understand it, and then we can play with some numbers, let's remind ourselves."}, {"video_title": "Detectable civilizations in our galaxy 2 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So you could have a civilization that developed radio 70,000 years ago, but they're 70,000 light years away. And maybe they collapse 10,000 years later, but we're just receiving their first radio signal. So that would be a civilization that I would count in this equation we're setting up. And so just to make sure we understand it, and then we can play with some numbers, let's remind ourselves. This is the number of stars, our estimate of the number of stars in the galaxy. Multiply by this, you now know the number of stars in the galaxy that have planets. You multiply by this N sub p, the average number of planets capable of sustaining life."}, {"video_title": "Detectable civilizations in our galaxy 2 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And so just to make sure we understand it, and then we can play with some numbers, let's remind ourselves. This is the number of stars, our estimate of the number of stars in the galaxy. Multiply by this, you now know the number of stars in the galaxy that have planets. You multiply by this N sub p, the average number of planets capable of sustaining life. And these first three terms will give you the average number of planets in the galaxy, or I should say the number, the total number of planets in the galaxy that have been capable of sustaining life at some point in their history. Multiply it by this. This is the number of planets in the galaxy that have sustained actual life, not just capability of it."}, {"video_title": "Detectable civilizations in our galaxy 2 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "You multiply by this N sub p, the average number of planets capable of sustaining life. And these first three terms will give you the average number of planets in the galaxy, or I should say the number, the total number of planets in the galaxy that have been capable of sustaining life at some point in their history. Multiply it by this. This is the number of planets in the galaxy that have sustained actual life, not just capability of it. They actually had life on them at some point in their history. Multiply it by this. This is the fraction that have developed intelligent life on these planets, the number of planets with intelligent life at some point in their history."}, {"video_title": "Detectable civilizations in our galaxy 2 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "This is the number of planets in the galaxy that have sustained actual life, not just capability of it. They actually had life on them at some point in their history. Multiply it by this. This is the fraction that have developed intelligent life on these planets, the number of planets with intelligent life at some point in their history. Multiply it by this fraction, all of these terms. You have the number of planets in the galaxy that have developed, that have had intelligent life, that became detectable, that started emitting some type of radio signature. We don't know."}, {"video_title": "Detectable civilizations in our galaxy 2 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "This is the fraction that have developed intelligent life on these planets, the number of planets with intelligent life at some point in their history. Multiply it by this fraction, all of these terms. You have the number of planets in the galaxy that have developed, that have had intelligent life, that became detectable, that started emitting some type of radio signature. We don't know. Some type of thing like that at some point in their history. So over here, all of these first six terms tell us the number of detectable civilizations that occurred at some point in the history of the stars, the solar systems, the planets that are out there right now. But we care about the ones that are detectable now."}, {"video_title": "Detectable civilizations in our galaxy 2 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "We don't know. Some type of thing like that at some point in their history. So over here, all of these first six terms tell us the number of detectable civilizations that occurred at some point in the history of the stars, the solar systems, the planets that are out there right now. But we care about the ones that are detectable now. We don't care about the ones that came and went, and their radio signature went past us while we were still living in caves, or we were hunter-gatherers. We care about the ones that their radio signatures are receiving us now. That's why we have this little term right over here."}, {"video_title": "Detectable civilizations in our galaxy 2 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But we care about the ones that are detectable now. We don't care about the ones that came and went, and their radio signature went past us while we were still living in caves, or we were hunter-gatherers. We care about the ones that their radio signatures are receiving us now. That's why we have this little term right over here. This is the civilization of, I guess you could say, this is the length of the detectable civilization. So while they were actually releasing a radio signature, divided by the life of that planet or that solar system or that star. For any given star or planet that meets all of these criterion, what's the probability that it's releasing its... At some point in the history, there was a detectable civilization or more that was releasing some type of a radio signature."}, {"video_title": "Detectable civilizations in our galaxy 2 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "That's why we have this little term right over here. This is the civilization of, I guess you could say, this is the length of the detectable civilization. So while they were actually releasing a radio signature, divided by the life of that planet or that solar system or that star. For any given star or planet that meets all of these criterion, what's the probability that it's releasing its... At some point in the history, there was a detectable civilization or more that was releasing some type of a radio signature. But what's the probability that it's doing it right now? That's the detectable lifespan of that civilization divided by the life of that solar system or of that planet. Because frankly, the star and the solar system and the planet, they're all going to essentially have, give or take, a few hundreds of thousands of years or even a few millions of years, because we're thinking in the billions here."}, {"video_title": "Detectable civilizations in our galaxy 2 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "For any given star or planet that meets all of these criterion, what's the probability that it's releasing its... At some point in the history, there was a detectable civilization or more that was releasing some type of a radio signature. But what's the probability that it's doing it right now? That's the detectable lifespan of that civilization divided by the life of that solar system or of that planet. Because frankly, the star and the solar system and the planet, they're all going to essentially have, give or take, a few hundreds of thousands of years or even a few millions of years, because we're thinking in the billions here. They're going to have roughly the same lifespan. And so if you have... Let's say, and just to make this a little bit more tangible, let's say that the sun has a lifespan, and let's say that with the Earth and our solar system, has a lifespan of approximately 10 billion years. And let's say that us as humans, let me be pretty optimistic about it, let's say that we are detectable as a civilization for 1 million years."}, {"video_title": "Detectable civilizations in our galaxy 2 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Because frankly, the star and the solar system and the planet, they're all going to essentially have, give or take, a few hundreds of thousands of years or even a few millions of years, because we're thinking in the billions here. They're going to have roughly the same lifespan. And so if you have... Let's say, and just to make this a little bit more tangible, let's say that the sun has a lifespan, and let's say that with the Earth and our solar system, has a lifespan of approximately 10 billion years. And let's say that us as humans, let me be pretty optimistic about it, let's say that we are detectable as a civilization for 1 million years. So we have our best days are ahead of us. So we are detectable for 1 million years. So this term right over here will be 1 million over 10 billion."}, {"video_title": "Detectable civilizations in our galaxy 2 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And let's say that us as humans, let me be pretty optimistic about it, let's say that we are detectable as a civilization for 1 million years. So we have our best days are ahead of us. So we are detectable for 1 million years. So this term right over here will be 1 million over 10 billion. So this will be 1 over 10,000. So even though we might be around sending out detectable signals for a million years, the odds relative to the entire span of the history of our... And I'm making some simplifying assumptions here. But relative to the entire span of the history of our planet and our sun, if someone is just randomly sampling our solar system at a random time in its history, in a random part of this 10 billion years, there's only a 1 in 10,000 chance that they'll be sampling us at a time that we are releasing signals, assuming that there weren't any other civilizations on Mars or Venus or whatever else, or that there weren't any other civilizations on Earth hundreds of thousands of years ago that were doing this, they'll definitely only have a 1 in 10,000 chance of detecting us, assuming that they're sampling."}, {"video_title": "Detectable civilizations in our galaxy 2 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So this term right over here will be 1 million over 10 billion. So this will be 1 over 10,000. So even though we might be around sending out detectable signals for a million years, the odds relative to the entire span of the history of our... And I'm making some simplifying assumptions here. But relative to the entire span of the history of our planet and our sun, if someone is just randomly sampling our solar system at a random time in its history, in a random part of this 10 billion years, there's only a 1 in 10,000 chance that they'll be sampling us at a time that we are releasing signals, assuming that there weren't any other civilizations on Mars or Venus or whatever else, or that there weren't any other civilizations on Earth hundreds of thousands of years ago that were doing this, they'll definitely only have a 1 in 10,000 chance of detecting us, assuming that they're sampling. There could have been a civilization that was around 3 million years ago, and they did this whole search for extraterrestrial life, maybe they're 20 or 100 or 1,000 light years away, and they pointed their radio telescopes at us. About a million or 2 million years ago, they would have pointed at the direction of our sun, and they would have not gotten any radio signals, and they're like, man, when is extraterrestrial life going to show up? Even though the sun and Earth does eventually develop us, they weren't able to observe us because when they sampled was outside of that 1 in 10,000 window."}, {"video_title": "Potassium-argon (K-Ar) dating Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Potassium has 19 protons, and we could write it like this. And this is a little bit redundant. We know that if it's potassium, that atom has 19 protons, and we know if an atom has 19 protons, it is going to be potassium. Now, we also know that not all members of, or not all of the atoms of a given element have the same number of neutrons. And when we talk about a given element, but we have different numbers of neutrons, we call them isotopes of that element. So, for example, potassium can come in a form that has exactly 20 neutrons, and we call that potassium 39. And the 39, this mass number, it's a count of the 19 protons, 19 protons plus 20 neutrons."}, {"video_title": "Potassium-argon (K-Ar) dating Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Now, we also know that not all members of, or not all of the atoms of a given element have the same number of neutrons. And when we talk about a given element, but we have different numbers of neutrons, we call them isotopes of that element. So, for example, potassium can come in a form that has exactly 20 neutrons, and we call that potassium 39. And the 39, this mass number, it's a count of the 19 protons, 19 protons plus 20 neutrons. And this is actually the most common isotope of potassium. It accounts for 93.3, I'm just rounding off. This is 93.3% of the potassium that you would find on Earth."}, {"video_title": "Potassium-argon (K-Ar) dating Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And the 39, this mass number, it's a count of the 19 protons, 19 protons plus 20 neutrons. And this is actually the most common isotope of potassium. It accounts for 93.3, I'm just rounding off. This is 93.3% of the potassium that you would find on Earth. Now, some of the other isotopes of potassium, you also have potassium, and once again, writing the K and the 19 are a little bit redundant. You also have potassium 41, so this would have 22 neutrons. 22 plus 19 is 41."}, {"video_title": "Potassium-argon (K-Ar) dating Cosmology & Astronomy Khan Academy.mp3", "Sentence": "This is 93.3% of the potassium that you would find on Earth. Now, some of the other isotopes of potassium, you also have potassium, and once again, writing the K and the 19 are a little bit redundant. You also have potassium 41, so this would have 22 neutrons. 22 plus 19 is 41. This accounts for about 6.7% of the potassium on the planet. And then you have a very scarce isotope of potassium called potassium 40. Potassium 40 currently has 21 neutrons, and it's very, very, very, very scarce."}, {"video_title": "Potassium-argon (K-Ar) dating Cosmology & Astronomy Khan Academy.mp3", "Sentence": "22 plus 19 is 41. This accounts for about 6.7% of the potassium on the planet. And then you have a very scarce isotope of potassium called potassium 40. Potassium 40 currently has 21 neutrons, and it's very, very, very, very scarce. It accounts for only 0.0117% of all the potassium. But this is also the isotope of potassium that's interesting to us from the point of view of dating old, old rock, and especially old volcanic rock. And as we'll see, when you can date old volcanic rock, it allows you to date other types of rock or other types of fossils that might be sandwiched in between old volcanic rock."}, {"video_title": "Potassium-argon (K-Ar) dating Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Potassium 40 currently has 21 neutrons, and it's very, very, very, very scarce. It accounts for only 0.0117% of all the potassium. But this is also the isotope of potassium that's interesting to us from the point of view of dating old, old rock, and especially old volcanic rock. And as we'll see, when you can date old volcanic rock, it allows you to date other types of rock or other types of fossils that might be sandwiched in between old volcanic rock. And so what's really interesting about potassium 40 here is that it has a half-life of 1.25 billion years. So the good thing about that, as opposed to something like carbon-14, it can be used to date really, really, really old things. And every 1.25 billion years, let me write it like this, that's its half-life, so 50% of any given sample will have decayed."}, {"video_title": "Potassium-argon (K-Ar) dating Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And as we'll see, when you can date old volcanic rock, it allows you to date other types of rock or other types of fossils that might be sandwiched in between old volcanic rock. And so what's really interesting about potassium 40 here is that it has a half-life of 1.25 billion years. So the good thing about that, as opposed to something like carbon-14, it can be used to date really, really, really old things. And every 1.25 billion years, let me write it like this, that's its half-life, so 50% of any given sample will have decayed. And 11% will have decayed into argon-40. So argon is right over here. It has 18 protons."}, {"video_title": "Potassium-argon (K-Ar) dating Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And every 1.25 billion years, let me write it like this, that's its half-life, so 50% of any given sample will have decayed. And 11% will have decayed into argon-40. So argon is right over here. It has 18 protons. So when you think about it decaying into argon-40, what you see is that it lost a proton, but it has the same mass number. So one of the protons must have somehow turned into a neutron. It actually captures one of the inner electrons, and then emits other things, I won't go into all the quantum physics of it, but turns into argon-40, and 89% turn into calcium-40."}, {"video_title": "Potassium-argon (K-Ar) dating Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It has 18 protons. So when you think about it decaying into argon-40, what you see is that it lost a proton, but it has the same mass number. So one of the protons must have somehow turned into a neutron. It actually captures one of the inner electrons, and then emits other things, I won't go into all the quantum physics of it, but turns into argon-40, and 89% turn into calcium-40. And you see calcium on the periodic table right over here is 20 protons. So this is a situation where one of the neutrons turns into a proton. This is a situation where one of the protons turns into a neutron."}, {"video_title": "Potassium-argon (K-Ar) dating Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It actually captures one of the inner electrons, and then emits other things, I won't go into all the quantum physics of it, but turns into argon-40, and 89% turn into calcium-40. And you see calcium on the periodic table right over here is 20 protons. So this is a situation where one of the neutrons turns into a proton. This is a situation where one of the protons turns into a neutron. And what's really interesting to us is this part right over here. Because what's cool about argon, and we studied this a little bit in the chemistry playlist, it is a noble gas, it is unreactive. And so when it is embedded in something that's in a liquid state, it'll kind of just bubble out."}, {"video_title": "Potassium-argon (K-Ar) dating Cosmology & Astronomy Khan Academy.mp3", "Sentence": "This is a situation where one of the protons turns into a neutron. And what's really interesting to us is this part right over here. Because what's cool about argon, and we studied this a little bit in the chemistry playlist, it is a noble gas, it is unreactive. And so when it is embedded in something that's in a liquid state, it'll kind of just bubble out. It's not bonded to anything, and so it'll just bubble out and go out into the atmosphere. So what's interesting about this whole situation is you can imagine what happens during a volcanic eruption. Let me draw a volcano here."}, {"video_title": "Potassium-argon (K-Ar) dating Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And so when it is embedded in something that's in a liquid state, it'll kind of just bubble out. It's not bonded to anything, and so it'll just bubble out and go out into the atmosphere. So what's interesting about this whole situation is you can imagine what happens during a volcanic eruption. Let me draw a volcano here. So let's say that this is our volcano. And it erupts at some time in the past. So it erupts, and you have all of this lava flowing."}, {"video_title": "Potassium-argon (K-Ar) dating Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Let me draw a volcano here. So let's say that this is our volcano. And it erupts at some time in the past. So it erupts, and you have all of this lava flowing. And while the lava, that lava will contain some amount of potassium-40, and actually it'll already contain some amount of argon-40. It'll already contain some amount of argon-40. But what's neat about argon-40 is that while it's lava, while it's in this liquid state, so let's imagine this lava right over here."}, {"video_title": "Potassium-argon (K-Ar) dating Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So it erupts, and you have all of this lava flowing. And while the lava, that lava will contain some amount of potassium-40, and actually it'll already contain some amount of argon-40. It'll already contain some amount of argon-40. But what's neat about argon-40 is that while it's lava, while it's in this liquid state, so let's imagine this lava right over here. It's a bunch of stuff right over here. But in that stuff, it is going to have, I'll do the potassium-40 in, let me do it in a color that I haven't used yet, I'll do the potassium-40 in magenta. It'll have some potassium-40 in it."}, {"video_title": "Potassium-argon (K-Ar) dating Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But what's neat about argon-40 is that while it's lava, while it's in this liquid state, so let's imagine this lava right over here. It's a bunch of stuff right over here. But in that stuff, it is going to have, I'll do the potassium-40 in, let me do it in a color that I haven't used yet, I'll do the potassium-40 in magenta. It'll have some potassium-40 in it. I may be overdoing it, it's a very scarce isotope. But it'll have some potassium-40 in it, and it might already have some argon-40 in it. So it might already have some argon-40 in it."}, {"video_title": "Potassium-argon (K-Ar) dating Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It'll have some potassium-40 in it. I may be overdoing it, it's a very scarce isotope. But it'll have some potassium-40 in it, and it might already have some argon-40 in it. So it might already have some argon-40 in it. Just like that. But argon-40 is a noble gas, it's not going to bond to anything, and while this lava is in a liquid state, it's going to be able to bubble out. It'll just float to the top, it has no bonds, and it'll just evaporate, I should say evaporate, it'll just bubble out, essentially, because it's not bonded to anything, and so it'll just seep out while we are in a liquid state."}, {"video_title": "Potassium-argon (K-Ar) dating Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So it might already have some argon-40 in it. Just like that. But argon-40 is a noble gas, it's not going to bond to anything, and while this lava is in a liquid state, it's going to be able to bubble out. It'll just float to the top, it has no bonds, and it'll just evaporate, I should say evaporate, it'll just bubble out, essentially, because it's not bonded to anything, and so it'll just seep out while we are in a liquid state. And what's really interesting about that is that when you have these volcanic eruptions, and because this argon-40 is seeping out, by the time this lava has hardened into volcanic rock, and I'll do that, let me do that volcanic rock in a different color, by the time it has hardened into volcanic rock, all of the argon-40 will be gone. It won't be there anymore. And so what's neat is, this volcanic event, the fact that this rock has become liquid, it kind of resets the amount of argon-40 there, so then you're only going to be left with potassium-40 here."}, {"video_title": "Potassium-argon (K-Ar) dating Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It'll just float to the top, it has no bonds, and it'll just evaporate, I should say evaporate, it'll just bubble out, essentially, because it's not bonded to anything, and so it'll just seep out while we are in a liquid state. And what's really interesting about that is that when you have these volcanic eruptions, and because this argon-40 is seeping out, by the time this lava has hardened into volcanic rock, and I'll do that, let me do that volcanic rock in a different color, by the time it has hardened into volcanic rock, all of the argon-40 will be gone. It won't be there anymore. And so what's neat is, this volcanic event, the fact that this rock has become liquid, it kind of resets the amount of argon-40 there, so then you're only going to be left with potassium-40 here. You're going to be left with potassium-40. And that's why the argon-40 is more interesting, because the calcium-40 won't necessarily have seeped out, and there might have already been calcium-40 here, so it won't necessarily seep out, but the argon-40 will seep out, so it kind of resets it. The volcanic event resets the amount of argon-40."}, {"video_title": "Potassium-argon (K-Ar) dating Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And so what's neat is, this volcanic event, the fact that this rock has become liquid, it kind of resets the amount of argon-40 there, so then you're only going to be left with potassium-40 here. You're going to be left with potassium-40. And that's why the argon-40 is more interesting, because the calcium-40 won't necessarily have seeped out, and there might have already been calcium-40 here, so it won't necessarily seep out, but the argon-40 will seep out, so it kind of resets it. The volcanic event resets the amount of argon-40. So at future date, so right when the event happened, you shouldn't have any argon-40, right when that lava actually becomes solid. And so if you fast-forward to some future date, and if you look at the sample, let me copy and paste it. So let me copy and let me paste it."}, {"video_title": "Potassium-argon (K-Ar) dating Cosmology & Astronomy Khan Academy.mp3", "Sentence": "The volcanic event resets the amount of argon-40. So at future date, so right when the event happened, you shouldn't have any argon-40, right when that lava actually becomes solid. And so if you fast-forward to some future date, and if you look at the sample, let me copy and paste it. So let me copy and let me paste it. So if you fast-forward to some future date, and you see that there is some argon-40 there, you see that there is some argon-40 in that sample, you know this is volcanic rock, you know that it was due to some previous volcanic event, you know that this argon-40 is from decayed potassium-40. And you know that it has decayed since that volcanic event, because if it was there before, it would have seeped out. So the only way that this would have been able to get trapped is if, while it was liquid, it would seep out, but once it's solid, it can get trapped inside the rock."}, {"video_title": "Potassium-argon (K-Ar) dating Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So let me copy and let me paste it. So if you fast-forward to some future date, and you see that there is some argon-40 there, you see that there is some argon-40 in that sample, you know this is volcanic rock, you know that it was due to some previous volcanic event, you know that this argon-40 is from decayed potassium-40. And you know that it has decayed since that volcanic event, because if it was there before, it would have seeped out. So the only way that this would have been able to get trapped is if, while it was liquid, it would seep out, but once it's solid, it can get trapped inside the rock. And so you know that the only way this argon-40 can exist there is by decay from that potassium-40. So you can look at the ratio. So you know for every one of these argon-40s, because it's only 11% of the decay products are argon-40, for every one of those, you must have on the order of about 9 calcium-40s that also decayed."}, {"video_title": "Potassium-argon (K-Ar) dating Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So the only way that this would have been able to get trapped is if, while it was liquid, it would seep out, but once it's solid, it can get trapped inside the rock. And so you know that the only way this argon-40 can exist there is by decay from that potassium-40. So you can look at the ratio. So you know for every one of these argon-40s, because it's only 11% of the decay products are argon-40, for every one of those, you must have on the order of about 9 calcium-40s that also decayed. And so for every one of these argon-40s, you know that there must have been 10 original potassium-40s. And so what you can do is you can look at the ratio of the number of potassium-40s there are today to the number that there must have been, based on this evidence right over here, to actually date it. And in the next video, I'll actually go through the mathematical calculation to show you that you can actually date it."}, {"video_title": "Potassium-argon (K-Ar) dating Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So you know for every one of these argon-40s, because it's only 11% of the decay products are argon-40, for every one of those, you must have on the order of about 9 calcium-40s that also decayed. And so for every one of these argon-40s, you know that there must have been 10 original potassium-40s. And so what you can do is you can look at the ratio of the number of potassium-40s there are today to the number that there must have been, based on this evidence right over here, to actually date it. And in the next video, I'll actually go through the mathematical calculation to show you that you can actually date it. And the reason this is really useful is you can look at those ratios, and volcanic eruptions aren't happening every day, but if you start looking over millions and millions of years, on that timescale, they're actually happening reasonably frequent. And so if you dig in the ground, and so let's dig in the ground. So let's say this is the ground right over here, and you dig enough, and you see a volcanic eruption."}, {"video_title": "Potassium-argon (K-Ar) dating Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And in the next video, I'll actually go through the mathematical calculation to show you that you can actually date it. And the reason this is really useful is you can look at those ratios, and volcanic eruptions aren't happening every day, but if you start looking over millions and millions of years, on that timescale, they're actually happening reasonably frequent. And so if you dig in the ground, and so let's dig in the ground. So let's say this is the ground right over here, and you dig enough, and you see a volcanic eruption. You see some volcanic rock right over there, and then you dig even more. There's another layer of volcanic rock right over there. So this is another layer of volcanic rock."}, {"video_title": "Potassium-argon (K-Ar) dating Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So let's say this is the ground right over here, and you dig enough, and you see a volcanic eruption. You see some volcanic rock right over there, and then you dig even more. There's another layer of volcanic rock right over there. So this is another layer of volcanic rock. And let's say that this one over here, so they're all going to have a certain amount of potassium-40 in it. This is going to have some amount of potassium-40 in it. And then let's say this one over here has more argon-40."}, {"video_title": "Potassium-argon (K-Ar) dating Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So this is another layer of volcanic rock. And let's say that this one over here, so they're all going to have a certain amount of potassium-40 in it. This is going to have some amount of potassium-40 in it. And then let's say this one over here has more argon-40. This one has a little bit less. And using the math that we're going to do in the next video, let's say you're able to say that this is, using the half-life and using the ratio of argon-40 that's left, versus what you, or using the ratio of the potassium-40 left to what you know was there before, you say that this must have solidified 100 million years ago, 100 million years before the present. And you know that this layer right over here solidified, let's say you know it solidified 150 million years before the present."}, {"video_title": "Potassium-argon (K-Ar) dating Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And then let's say this one over here has more argon-40. This one has a little bit less. And using the math that we're going to do in the next video, let's say you're able to say that this is, using the half-life and using the ratio of argon-40 that's left, versus what you, or using the ratio of the potassium-40 left to what you know was there before, you say that this must have solidified 100 million years ago, 100 million years before the present. And you know that this layer right over here solidified, let's say you know it solidified 150 million years before the present. And let's say you feel pretty good that this soil hasn't been dug up and mixed or anything like that. It looks like it's been pretty untouched when you look at these soil samples right over here. And let's say you see some fossils in here."}, {"video_title": "Potassium-argon (K-Ar) dating Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And you know that this layer right over here solidified, let's say you know it solidified 150 million years before the present. And let's say you feel pretty good that this soil hasn't been dug up and mixed or anything like that. It looks like it's been pretty untouched when you look at these soil samples right over here. And let's say you see some fossils in here. Then, even though carbon-14 dating is kind of useless, really when you get beyond 50,000 years, you see these fossils in between these two periods. It's a pretty good indicator, if you can assume that this soil hasn't been dug around and mixed, that this fossil is between 100 million and 150 million years old. This event happened, then you have these fossils got deposited, these animals died, or they lived and they died, and then you had this other volcanic event."}, {"video_title": "Birth of stars Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Let's imagine we have a huge cloud of hydrogen atoms floating in space. When I say huge cloud, huge both in distance and in mass. If you were to combine all of the hydrogen atoms, it would just be this really, really massive thing. So you have this huge cloud. Well, we know that gravity would make the atoms actually attracted to each other. We normally don't think about the gravity of atoms, but it would slowly affect these atoms. And they would slowly draw close to each other."}, {"video_title": "Birth of stars Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So you have this huge cloud. Well, we know that gravity would make the atoms actually attracted to each other. We normally don't think about the gravity of atoms, but it would slowly affect these atoms. And they would slowly draw close to each other. It would slowly condense. They'd slowly move towards the center of mass of all of the atoms. They'd slowly move in."}, {"video_title": "Birth of stars Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And they would slowly draw close to each other. It would slowly condense. They'd slowly move towards the center of mass of all of the atoms. They'd slowly move in. So if we fast forward, this cloud's going to get denser and denser. The hydrogen atoms are going to start bumping into each other and rubbing up against each other and interacting with each other. So it's going to get denser and denser and denser."}, {"video_title": "Birth of stars Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "They'd slowly move in. So if we fast forward, this cloud's going to get denser and denser. The hydrogen atoms are going to start bumping into each other and rubbing up against each other and interacting with each other. So it's going to get denser and denser and denser. Remember, it was a huge mass of hydrogen atoms. So the temperature is going up. And they'll keep condensing."}, {"video_title": "Birth of stars Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So it's going to get denser and denser and denser. Remember, it was a huge mass of hydrogen atoms. So the temperature is going up. And they'll keep condensing. They'll just keep condensing and condensing until something really interesting happens. So let's imagine that they've gotten really dense here in the center. And there's a bunch of hydrogen atoms all over."}, {"video_title": "Birth of stars Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And they'll keep condensing. They'll just keep condensing and condensing until something really interesting happens. So let's imagine that they've gotten really dense here in the center. And there's a bunch of hydrogen atoms all over. It's really dense. I could never draw the actual number of atoms here. This is really giving you an idea."}, {"video_title": "Birth of stars Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And there's a bunch of hydrogen atoms all over. It's really dense. I could never draw the actual number of atoms here. This is really giving you an idea. There's a huge amount of inward pressure from gravity, from everything that wants to get to that center of mass of our entire cloud. The temperature here is approaching 10 million Kelvin. And at that point, something neat happens."}, {"video_title": "Birth of stars Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "This is really giving you an idea. There's a huge amount of inward pressure from gravity, from everything that wants to get to that center of mass of our entire cloud. The temperature here is approaching 10 million Kelvin. And at that point, something neat happens. To kind of realize the neat thing that's happening, let's remember what a hydrogen atom looks like. And even more, I'm just going to focus on the hydrogen nucleus. So the hydrogen nucleus is a proton."}, {"video_title": "Birth of stars Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And at that point, something neat happens. To kind of realize the neat thing that's happening, let's remember what a hydrogen atom looks like. And even more, I'm just going to focus on the hydrogen nucleus. So the hydrogen nucleus is a proton. If you want to think about a hydrogen atom, it also has an electron orbiting around or floating around. And let's draw another hydrogen atom over here. And obviously this distance isn't to scale."}, {"video_title": "Birth of stars Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So the hydrogen nucleus is a proton. If you want to think about a hydrogen atom, it also has an electron orbiting around or floating around. And let's draw another hydrogen atom over here. And obviously this distance isn't to scale. This distance is also not to scale. The nucleus of atoms are actually much, much, much, much smaller than the actual radius of an atom. And so is the electron."}, {"video_title": "Birth of stars Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And obviously this distance isn't to scale. This distance is also not to scale. The nucleus of atoms are actually much, much, much, much smaller than the actual radius of an atom. And so is the electron. But anyway, this just gives you an idea. So we know from the Coulomb forces, from electromagnetic forces, that these two positively charged nucleuses will not want to get anywhere near each other. But we do know from what we learned about the four forces that if they did get close enough to each other, that if somehow under huge temperatures and huge pressures, you were able to get these two protons close enough to each other, then all of a sudden the strong force will overtake."}, {"video_title": "Birth of stars Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And so is the electron. But anyway, this just gives you an idea. So we know from the Coulomb forces, from electromagnetic forces, that these two positively charged nucleuses will not want to get anywhere near each other. But we do know from what we learned about the four forces that if they did get close enough to each other, that if somehow under huge temperatures and huge pressures, you were able to get these two protons close enough to each other, then all of a sudden the strong force will overtake. It's much stronger than the Coulomb force, and that these two hydrogens will actually, these nucleuses would actually fuse, or is it nuclei? Well anyway, they would actually fuse together. And so that is what actually happens once this gets hot and dense enough."}, {"video_title": "Birth of stars Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But we do know from what we learned about the four forces that if they did get close enough to each other, that if somehow under huge temperatures and huge pressures, you were able to get these two protons close enough to each other, then all of a sudden the strong force will overtake. It's much stronger than the Coulomb force, and that these two hydrogens will actually, these nucleuses would actually fuse, or is it nuclei? Well anyway, they would actually fuse together. And so that is what actually happens once this gets hot and dense enough. You now have enough pressure and enough temperature to overcome the Coulomb force and bring these protons close enough to each other for fusion to occur. For fusion. Fusion."}, {"video_title": "Birth of stars Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And so that is what actually happens once this gets hot and dense enough. You now have enough pressure and enough temperature to overcome the Coulomb force and bring these protons close enough to each other for fusion to occur. For fusion. Fusion. Ignition. And the reason why is, and I want to be very careful, it's not ignition, it's not combustion in the traditional sense. It's not like you're burning a carbon molecule in the presence of oxygen."}, {"video_title": "Birth of stars Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Fusion. Ignition. And the reason why is, and I want to be very careful, it's not ignition, it's not combustion in the traditional sense. It's not like you're burning a carbon molecule in the presence of oxygen. It's not combustion, it's ignition. And the reason why it's called ignition is because when two of these protons, or two of the nucleuses fuse, the resulting nucleus has a slightly smaller mass. And so in the first stage of this, you actually have two protons under enough pressure."}, {"video_title": "Birth of stars Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It's not like you're burning a carbon molecule in the presence of oxygen. It's not combustion, it's ignition. And the reason why it's called ignition is because when two of these protons, or two of the nucleuses fuse, the resulting nucleus has a slightly smaller mass. And so in the first stage of this, you actually have two protons under enough pressure. Obviously this would not happen with just the Coulomb forces. With enough pressure, they're close enough, and then the strong interaction actually keeps them together. One of these guys degrades into a neutron."}, {"video_title": "Birth of stars Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And so in the first stage of this, you actually have two protons under enough pressure. Obviously this would not happen with just the Coulomb forces. With enough pressure, they're close enough, and then the strong interaction actually keeps them together. One of these guys degrades into a neutron. And the resulting mass of the combined protons is lower than the mass of each of the original, by a little bit. But that little bit of mass results in a lot of energy. Plus energy."}, {"video_title": "Birth of stars Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "One of these guys degrades into a neutron. And the resulting mass of the combined protons is lower than the mass of each of the original, by a little bit. But that little bit of mass results in a lot of energy. Plus energy. And this energy is why we call it ignition. So what this energy does is it provides a little bit of outward pressure so that this thing doesn't keep collapsing. So once you get pressure enough, the fusion occurs, and then that energy provides outward pressure to balance what is now a star."}, {"video_title": "Birth of stars Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Plus energy. And this energy is why we call it ignition. So what this energy does is it provides a little bit of outward pressure so that this thing doesn't keep collapsing. So once you get pressure enough, the fusion occurs, and then that energy provides outward pressure to balance what is now a star. So now we actually have the ignition at the center, and we still have all of the other molecules trying to get in, providing the pressure for this fusion ignition. Now, what is the hydrogen being fused into? Well, in the first step of the reaction, and I'm just kind of doing the most basic type of fusion that happens in stars, the hydrogen gets fused into deuterium, which is another way of calling heavy hydrogen."}, {"video_title": "Birth of stars Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So once you get pressure enough, the fusion occurs, and then that energy provides outward pressure to balance what is now a star. So now we actually have the ignition at the center, and we still have all of the other molecules trying to get in, providing the pressure for this fusion ignition. Now, what is the hydrogen being fused into? Well, in the first step of the reaction, and I'm just kind of doing the most basic type of fusion that happens in stars, the hydrogen gets fused into deuterium, which is another way of calling heavy hydrogen. This is still hydrogen because it has one proton and one neutron now. It does not have helium yet. It does not have two protons."}, {"video_title": "Birth of stars Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Well, in the first step of the reaction, and I'm just kind of doing the most basic type of fusion that happens in stars, the hydrogen gets fused into deuterium, which is another way of calling heavy hydrogen. This is still hydrogen because it has one proton and one neutron now. It does not have helium yet. It does not have two protons. But then the deuterium keeps fusing, and then we eventually end up with helium. And we can even see that on the periodic table. I lost my periodic table."}, {"video_title": "Birth of stars Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It does not have two protons. But then the deuterium keeps fusing, and then we eventually end up with helium. And we can even see that on the periodic table. I lost my periodic table. Well, I'll show you in the next video. But we know hydrogen in its atomic state has an atomic number of one, and it also has a mass of one. It only has one nucleon in its nucleus."}, {"video_title": "Birth of stars Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "I lost my periodic table. Well, I'll show you in the next video. But we know hydrogen in its atomic state has an atomic number of one, and it also has a mass of one. It only has one nucleon in its nucleus. But it's being fused. It goes to hydrogen two, which is deuterium, which is one neutron, one proton in its nucleus, two nucleons. And then that eventually gets fused, and I'm not going into the detail of the reaction, into helium."}, {"video_title": "Birth of stars Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It only has one nucleon in its nucleus. But it's being fused. It goes to hydrogen two, which is deuterium, which is one neutron, one proton in its nucleus, two nucleons. And then that eventually gets fused, and I'm not going into the detail of the reaction, into helium. And by definition, helium has two protons and two neutrons. We're talking about helium-4 in particular, that isotope of helium. It has an atomic mass of four."}, {"video_title": "Birth of stars Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And then that eventually gets fused, and I'm not going into the detail of the reaction, into helium. And by definition, helium has two protons and two neutrons. We're talking about helium-4 in particular, that isotope of helium. It has an atomic mass of four. And this process releases a ton of energy because the atomic mass of the helium that gets produced is slightly lower than four times the atomic mass of each of the constituent hydrogens. So all of this energy from the fusion, but it needs super high pressure, super high temperatures to happen, keeps the star from collapsing. And once a star is in this stage, once it is using hydrogen, it is fusing hydrogen in its core where the pressure and the temperature is the most to form helium, it is now in its main sequence."}, {"video_title": "Birth of stars Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It has an atomic mass of four. And this process releases a ton of energy because the atomic mass of the helium that gets produced is slightly lower than four times the atomic mass of each of the constituent hydrogens. So all of this energy from the fusion, but it needs super high pressure, super high temperatures to happen, keeps the star from collapsing. And once a star is in this stage, once it is using hydrogen, it is fusing hydrogen in its core where the pressure and the temperature is the most to form helium, it is now in its main sequence. This is now a main sequence star. And that's actually where the sun is right now. Now, there's questions of, well, what if there just wasn't enough mass to get to this level over here?"}, {"video_title": "Birth of stars Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And once a star is in this stage, once it is using hydrogen, it is fusing hydrogen in its core where the pressure and the temperature is the most to form helium, it is now in its main sequence. This is now a main sequence star. And that's actually where the sun is right now. Now, there's questions of, well, what if there just wasn't enough mass to get to this level over here? And there actually are things that never get to quite that threshold to fuse all the way into helium. There are a few things that don't quite make the threshold of stars that only fuse to this level, so they are generating some of their heat. Or there are even smaller objects that just get to the point there's a huge temperature and pressure, but fusion is not actually occurring inside of the core."}, {"video_title": "Birth of stars Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Now, there's questions of, well, what if there just wasn't enough mass to get to this level over here? And there actually are things that never get to quite that threshold to fuse all the way into helium. There are a few things that don't quite make the threshold of stars that only fuse to this level, so they are generating some of their heat. Or there are even smaller objects that just get to the point there's a huge temperature and pressure, but fusion is not actually occurring inside of the core. And something like Jupiter would be an example. And you can go several masses above Jupiter where you get something like that. So you have to reach a certain threshold where the mass, where the pressure, and the temperature due to the heavy mass gets so large that you start this fusion."}, {"video_title": "Birth of stars Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Or there are even smaller objects that just get to the point there's a huge temperature and pressure, but fusion is not actually occurring inside of the core. And something like Jupiter would be an example. And you can go several masses above Jupiter where you get something like that. So you have to reach a certain threshold where the mass, where the pressure, and the temperature due to the heavy mass gets so large that you start this fusion. But the smaller you are above that threshold, the slower the fusion will occur. But if you're supermassive, the fusion will occur really, really fast. So that's a general idea of just how stars get formed and why they don't collapse on themselves and why they are these kind of little balls of fusion reactions existing in the universe."}, {"video_title": "K-Ar dating calculation Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "In this video, I want to go through a concrete example. It'll get a little bit math-y, usually involving a little bit of algebra or a little bit of exponential decay but to really show you how you can actually figure out the age of some volcanic rock using this technique, using a little bit of mathematics. So we know that anything that is experiencing radioactive decay, it's experiencing exponential decay, and we know that we can, there's a generalized way to describe that and we go into more depth and kind of prove it in other Khan Academy videos. But we know that the amount as a function of time, so if we say n is the amount of a radioactive sample we have at some time, we know that's equal to the initial amount we have, we'll call that n sub zero, times e to the negative kt, where this constant is particular to that thing's half-life. And to figure it out, and we're gonna do this for the example of potassium-40, we know that after, when time is 1.25 billion years, that the amount we have left is half of our initial amount. So let's write it that way. So let's say when we start with n naught, we start with n naught, whatever that might be, it might be one gram, kilogram, five grams, whatever it might be, whatever we start with, we take e to the negative k times 1.25 billion years, that's the half-life of potassium-40, so 1.25 billion years, we know after that long that half of the sample will be left."}, {"video_title": "K-Ar dating calculation Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But we know that the amount as a function of time, so if we say n is the amount of a radioactive sample we have at some time, we know that's equal to the initial amount we have, we'll call that n sub zero, times e to the negative kt, where this constant is particular to that thing's half-life. And to figure it out, and we're gonna do this for the example of potassium-40, we know that after, when time is 1.25 billion years, that the amount we have left is half of our initial amount. So let's write it that way. So let's say when we start with n naught, we start with n naught, whatever that might be, it might be one gram, kilogram, five grams, whatever it might be, whatever we start with, we take e to the negative k times 1.25 billion years, that's the half-life of potassium-40, so 1.25 billion years, we know after that long that half of the sample will be left. So we will have one half, one half n naught left. Whatever we started with, we're going to have half left after 1.25 billion years. Divide both sides by n naught, divide both sides by n naught, and then to solve for k, we can take the natural log of both sides, so you get the natural log of one half, we don't have that n naught there anymore, is equal to the natural log of this thing."}, {"video_title": "K-Ar dating calculation Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So let's say when we start with n naught, we start with n naught, whatever that might be, it might be one gram, kilogram, five grams, whatever it might be, whatever we start with, we take e to the negative k times 1.25 billion years, that's the half-life of potassium-40, so 1.25 billion years, we know after that long that half of the sample will be left. So we will have one half, one half n naught left. Whatever we started with, we're going to have half left after 1.25 billion years. Divide both sides by n naught, divide both sides by n naught, and then to solve for k, we can take the natural log of both sides, so you get the natural log of one half, we don't have that n naught there anymore, is equal to the natural log of this thing. The natural log is just saying, to what power do I have to raise e to get e to the negative k times 1.25 billion? So the natural log of this, the power that I have to raise e to to get to e to the negative k times 1.25 billion is just negative k times 1.25 billion. I could write it as negative 1.25, let me write it times 10 to the ninth, times 10 to the ninth k. That's the same thing as 1.25 billion, we have our negative sign and we have our k. And then to solve for k, we can divide both sides by negative 1.25 billion, and so we get k, and I'll just flip the sides here, k is equal to the natural log of one half times negative 1.25 times 10 to the ninth power."}, {"video_title": "K-Ar dating calculation Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Divide both sides by n naught, divide both sides by n naught, and then to solve for k, we can take the natural log of both sides, so you get the natural log of one half, we don't have that n naught there anymore, is equal to the natural log of this thing. The natural log is just saying, to what power do I have to raise e to get e to the negative k times 1.25 billion? So the natural log of this, the power that I have to raise e to to get to e to the negative k times 1.25 billion is just negative k times 1.25 billion. I could write it as negative 1.25, let me write it times 10 to the ninth, times 10 to the ninth k. That's the same thing as 1.25 billion, we have our negative sign and we have our k. And then to solve for k, we can divide both sides by negative 1.25 billion, and so we get k, and I'll just flip the sides here, k is equal to the natural log of one half times negative 1.25 times 10 to the ninth power. And what we can do is we can multiply the negative times the top, or you can view it as multiplying the numerator and the denominator by a negative so that the negative shows up at the top. And so we could make this as over 1.25 times 10 to the ninth, this is 1.25 billion, and negative, let me write it over here in a different color. Negative, the negative natural log, well I could just write it this way."}, {"video_title": "K-Ar dating calculation Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "I could write it as negative 1.25, let me write it times 10 to the ninth, times 10 to the ninth k. That's the same thing as 1.25 billion, we have our negative sign and we have our k. And then to solve for k, we can divide both sides by negative 1.25 billion, and so we get k, and I'll just flip the sides here, k is equal to the natural log of one half times negative 1.25 times 10 to the ninth power. And what we can do is we can multiply the negative times the top, or you can view it as multiplying the numerator and the denominator by a negative so that the negative shows up at the top. And so we could make this as over 1.25 times 10 to the ninth, this is 1.25 billion, and negative, let me write it over here in a different color. Negative, the negative natural log, well I could just write it this way. If I have a, a natural log of b, we know from our logarithm properties this is the same thing as the natural log of b to the a power, so the negative natural log of one half is the same thing as the natural log of one half to the negative one power, and so this is the same thing. Anything to the negative one power is just this multiplicative inverse, so this is just the natural log of two. So negative natural log of one half is just the natural log of two over here."}, {"video_title": "K-Ar dating calculation Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Negative, the negative natural log, well I could just write it this way. If I have a, a natural log of b, we know from our logarithm properties this is the same thing as the natural log of b to the a power, so the negative natural log of one half is the same thing as the natural log of one half to the negative one power, and so this is the same thing. Anything to the negative one power is just this multiplicative inverse, so this is just the natural log of two. So negative natural log of one half is just the natural log of two over here. So we were able to figure out our k, it's essentially the natural log of two over the half-life of this substance, so we could actually generalize this if we were talking about some other radioactive substance. And now let's think about a situation, now that we've figured out a k, let's think about a situation where we find in some sample, so let's say the potassium that we find, let's say it is one milligram, I'm just gonna make up these numbers, and let's say, and usually these aren't measured directly and you really care about the relative amounts, but let's say you're able to figure out the potassium is one milligram, and let's say that the argon, actually let me say the potassium 40 found, let's say the argon 40 found, let's say it is 0.01, 0.01 milligram. So how can we use this information, what we just figured out here, which is derived from the half-life, to figure out how old this sample right over here, how do we figure out how old this sample is right over there?"}, {"video_title": "K-Ar dating calculation Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So negative natural log of one half is just the natural log of two over here. So we were able to figure out our k, it's essentially the natural log of two over the half-life of this substance, so we could actually generalize this if we were talking about some other radioactive substance. And now let's think about a situation, now that we've figured out a k, let's think about a situation where we find in some sample, so let's say the potassium that we find, let's say it is one milligram, I'm just gonna make up these numbers, and let's say, and usually these aren't measured directly and you really care about the relative amounts, but let's say you're able to figure out the potassium is one milligram, and let's say that the argon, actually let me say the potassium 40 found, let's say the argon 40 found, let's say it is 0.01, 0.01 milligram. So how can we use this information, what we just figured out here, which is derived from the half-life, to figure out how old this sample right over here, how do we figure out how old this sample is right over there? Well, what we need to figure out, we know that n, the amount we were left with, is this thing right over here. So we know that we're left with one milligram, so we know that what we have left is one milligram, and that's going to be equal to some initial amount, it's going to be some initial amount, we'll use both of this information to figure that initial amount out, times e to the negative kT, and we know what k is and we'll figure it out later, so k is this thing right over here, so we need to figure out what our initial amount is, we know what k is and then we can solve for T, how old is this sample? And to figure out our initial amount, we just have to remember that for every argon-40 we see, that must have decayed from, when you have potassium-40, 11% decays, when it decays, 11% decays into argon-40, and the rest, 89%, decays into calcium-40, we saw that in the last video."}, {"video_title": "K-Ar dating calculation Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So how can we use this information, what we just figured out here, which is derived from the half-life, to figure out how old this sample right over here, how do we figure out how old this sample is right over there? Well, what we need to figure out, we know that n, the amount we were left with, is this thing right over here. So we know that we're left with one milligram, so we know that what we have left is one milligram, and that's going to be equal to some initial amount, it's going to be some initial amount, we'll use both of this information to figure that initial amount out, times e to the negative kT, and we know what k is and we'll figure it out later, so k is this thing right over here, so we need to figure out what our initial amount is, we know what k is and then we can solve for T, how old is this sample? And to figure out our initial amount, we just have to remember that for every argon-40 we see, that must have decayed from, when you have potassium-40, 11% decays, when it decays, 11% decays into argon-40, and the rest, 89%, decays into calcium-40, we saw that in the last video. So however much argon-40, that is 11% of the decay product, so if you want to think about the total number of potassium-40s that have decayed since this was kind of stuck in the lava and we learned that anything that was there before, any argon-40 that was there before would have been able to get out of the liquid lava before it froze or before it hardened, so to figure out how much potassium-40 this is derived from, we just derive it, we divide it by 11%, so maybe I could say k initial, the potassium-40 initial is going to be equal to the amount of potassium-40 we have today, one milligram, plus the amount of potassium-40 we needed to get this amount of argon-40. So we have this amount of argon-40,.01 milligrams, and that, the number of milligrams there, it's really just 11% of the original potassium-40 that it had to come from, the rest of it turned into calcium-40, so we divide it by 11%, or 0.11, and I'm doing, this isn't the exact number, but it'll get the general idea. And so our initial, which is really this thing right over here, I could call this N naught, I could call this N naught, this is going to be equal to, and I won't do any of the math, so we have one milligram we have left, is equal to one milligram, which is what we found, plus 0.01 milligram over 0.11, and then all of that times e to the negative kT."}, {"video_title": "K-Ar dating calculation Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And to figure out our initial amount, we just have to remember that for every argon-40 we see, that must have decayed from, when you have potassium-40, 11% decays, when it decays, 11% decays into argon-40, and the rest, 89%, decays into calcium-40, we saw that in the last video. So however much argon-40, that is 11% of the decay product, so if you want to think about the total number of potassium-40s that have decayed since this was kind of stuck in the lava and we learned that anything that was there before, any argon-40 that was there before would have been able to get out of the liquid lava before it froze or before it hardened, so to figure out how much potassium-40 this is derived from, we just derive it, we divide it by 11%, so maybe I could say k initial, the potassium-40 initial is going to be equal to the amount of potassium-40 we have today, one milligram, plus the amount of potassium-40 we needed to get this amount of argon-40. So we have this amount of argon-40,.01 milligrams, and that, the number of milligrams there, it's really just 11% of the original potassium-40 that it had to come from, the rest of it turned into calcium-40, so we divide it by 11%, or 0.11, and I'm doing, this isn't the exact number, but it'll get the general idea. And so our initial, which is really this thing right over here, I could call this N naught, I could call this N naught, this is going to be equal to, and I won't do any of the math, so we have one milligram we have left, is equal to one milligram, which is what we found, plus 0.01 milligram over 0.11, and then all of that times e to the negative kT. And what you see here is, when we want to solve for T, assuming we know k, and we do know k now, that it really, the absolute amount doesn't matter, what actually matters is the ratio, because if we're solving for T, you want to divide both sides of this equation by this quantity right over here, so you get this side, the left-hand side, divide both sides, you get one milligram over this quantity, I'll write it in blue, over this quantity is going to be one plus, I'm just going to assume, actually, that the units here are milligrams. So you get one over this quantity, which is one plus 0.01 over the 11%, that is equal to e to the negative kT, and then you want to take, if you want to solve for T, you want to take the natural log of both sides, so then you get, so this is equal right over here, you want to take the natural log of both sides, so you get the natural log of one over one plus 0.01 over 0.11, or 11%, is equal to negative kT, and then to solve for T, you divide both sides by negative k, so I'll write it over here. You can see this is a little bit cumbersome mathematically, but we're getting to the answer."}, {"video_title": "K-Ar dating calculation Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And so our initial, which is really this thing right over here, I could call this N naught, I could call this N naught, this is going to be equal to, and I won't do any of the math, so we have one milligram we have left, is equal to one milligram, which is what we found, plus 0.01 milligram over 0.11, and then all of that times e to the negative kT. And what you see here is, when we want to solve for T, assuming we know k, and we do know k now, that it really, the absolute amount doesn't matter, what actually matters is the ratio, because if we're solving for T, you want to divide both sides of this equation by this quantity right over here, so you get this side, the left-hand side, divide both sides, you get one milligram over this quantity, I'll write it in blue, over this quantity is going to be one plus, I'm just going to assume, actually, that the units here are milligrams. So you get one over this quantity, which is one plus 0.01 over the 11%, that is equal to e to the negative kT, and then you want to take, if you want to solve for T, you want to take the natural log of both sides, so then you get, so this is equal right over here, you want to take the natural log of both sides, so you get the natural log of one over one plus 0.01 over 0.11, or 11%, is equal to negative kT, and then to solve for T, you divide both sides by negative k, so I'll write it over here. You can see this is a little bit cumbersome mathematically, but we're getting to the answer. So we got the natural log of one over one plus 0.01 over 0.11 over negative k. Well, what is negative k? We're just dividing both sides of this equation by negative k. Negative k is the negative of this over the negative natural log of two over 1.25 times 10 to the ninth. And now we can get our calculator out and just solve for what this time is, and it's going to be in years, because that's how we figured out this constant."}, {"video_title": "K-Ar dating calculation Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "You can see this is a little bit cumbersome mathematically, but we're getting to the answer. So we got the natural log of one over one plus 0.01 over 0.11 over negative k. Well, what is negative k? We're just dividing both sides of this equation by negative k. Negative k is the negative of this over the negative natural log of two over 1.25 times 10 to the ninth. And now we can get our calculator out and just solve for what this time is, and it's going to be in years, because that's how we figured out this constant. So let's get out my handy TI-85. And so first I'll do this part. So this is one divided by one plus 0.01 divided by 0.11."}, {"video_title": "K-Ar dating calculation Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And now we can get our calculator out and just solve for what this time is, and it's going to be in years, because that's how we figured out this constant. So let's get out my handy TI-85. And so first I'll do this part. So this is one divided by one plus 0.01 divided by 0.11. So that's this part right over here. That gives us that number. And then we want to take the natural log of that."}, {"video_title": "K-Ar dating calculation Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So this is one divided by one plus 0.01 divided by 0.11. So that's this part right over here. That gives us that number. And then we want to take the natural log of that. So let's take the natural log of our, this is just our previous answer, so natural log of 0.9166667 gives us negative 0.087. So that's this numerator over here. And we're going to divide that."}, {"video_title": "K-Ar dating calculation Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And then we want to take the natural log of that. So let's take the natural log of our, this is just our previous answer, so natural log of 0.9166667 gives us negative 0.087. So that's this numerator over here. And we're going to divide that. So this number is our numerator right over here. We're going to divide that by the negative, let me make, I'll use parentheses carefully, the negative natural log of two, the negative natural log of two, that's that there, divided by 1.25 times 10 to the ninth. So divided by, so it's negative natural log of two divided by 1.25."}, {"video_title": "K-Ar dating calculation Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And we're going to divide that. So this number is our numerator right over here. We're going to divide that by the negative, let me make, I'll use parentheses carefully, the negative natural log of two, the negative natural log of two, that's that there, divided by 1.25 times 10 to the ninth. So divided by, so it's negative natural log of two divided by 1.25. E nine means times 10 to the ninth. And I closed both parentheses. And now we need our drum roll."}, {"video_title": "K-Ar dating calculation Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So divided by, so it's negative natural log of two divided by 1.25. E nine means times 10 to the ninth. And I closed both parentheses. And now we need our drum roll. So this should give us our T in years. And we get, let's see how many, this is thousand, so it's 3,000. So we get 156 million, or 156.9 million years if we round."}, {"video_title": "K-Ar dating calculation Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And now we need our drum roll. So this should give us our T in years. And we get, let's see how many, this is thousand, so it's 3,000. So we get 156 million, or 156.9 million years if we round. So this is approximately, or I could just say approximately 157 million years old sample. So the whole point of this, I know the math was a little bit involved, but it's something that you would actually see in kind of a pre-calculus class or an algebra two class when you're studying exponential growth and decay. But the whole point I wanted to do this is to show you that it's not some crazy voodoo here."}, {"video_title": "K-Ar dating calculation Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So we get 156 million, or 156.9 million years if we round. So this is approximately, or I could just say approximately 157 million years old sample. So the whole point of this, I know the math was a little bit involved, but it's something that you would actually see in kind of a pre-calculus class or an algebra two class when you're studying exponential growth and decay. But the whole point I wanted to do this is to show you that it's not some crazy voodoo here. And you know, Sal gave this very high level explanation. And then you say, oh, well, you know, there must be some super difficult mathematics after that. The mathematics really is something that you would see in high school."}, {"video_title": "Pangaea Earth geological and climatic history Cosmology & Astronomy Khan Academy.mp3", "Sentence": "We know that new plate material is being formed and these lithospheric plates on the surface of the earth are moving around. And that might raise the question in your brain, what happens if we kind of reverse things? We know the direction they're moving in, what does that tell us about where they came from? So let's just do the thought experiment. Right now South America and Africa are moving away from each other because of new plate material being created at the mid-Atlantic rift. Let's rewind it, let's bring them back together. We know that India is jamming into the Eurasian plate right now, causing the Himalayas to get higher and higher."}, {"video_title": "Pangaea Earth geological and climatic history Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So let's just do the thought experiment. Right now South America and Africa are moving away from each other because of new plate material being created at the mid-Atlantic rift. Let's rewind it, let's bring them back together. We know that India is jamming into the Eurasian plate right now, causing the Himalayas to get higher and higher. What if we rewind that? Let's bring India back down towards Antarctica. Same thing with Australia."}, {"video_title": "Pangaea Earth geological and climatic history Cosmology & Astronomy Khan Academy.mp3", "Sentence": "We know that India is jamming into the Eurasian plate right now, causing the Himalayas to get higher and higher. What if we rewind that? Let's bring India back down towards Antarctica. Same thing with Australia. We have new plate material being formed between Australia and Antarctica that's making the continents move apart. Let's bring them back together. Let's rewind the clock."}, {"video_title": "Pangaea Earth geological and climatic history Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Same thing with Australia. We have new plate material being formed between Australia and Antarctica that's making the continents move apart. Let's bring them back together. Let's rewind the clock. Even North America, it's not as obvious from this diagram, but if you actually look at the GPS data, it becomes pretty obvious. That North America right now is kind of moving in a counter-clockwise rotation. So let's rewind it."}, {"video_title": "Pangaea Earth geological and climatic history Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Let's rewind the clock. Even North America, it's not as obvious from this diagram, but if you actually look at the GPS data, it becomes pretty obvious. That North America right now is kind of moving in a counter-clockwise rotation. So let's rewind it. Let's go back, moving it in a clockwise direction. Instead of Eurasia going further away from North America, let's bring it back together. So what you can imagine is a reality where India and Australia are jammed down into Antarctica, South America and Africa are jammed together, North America is jammed in there, and essentially Eurasia is also jammed in there."}, {"video_title": "Pangaea Earth geological and climatic history Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So let's rewind it. Let's go back, moving it in a clockwise direction. Instead of Eurasia going further away from North America, let's bring it back together. So what you can imagine is a reality where India and Australia are jammed down into Antarctica, South America and Africa are jammed together, North America is jammed in there, and essentially Eurasia is also jammed in there. So it looks like they all would clump together if you go back a few hundred million years. Based on just that thought experiment, you can imagine at one point all of the continents on the world were merged into one supercontinent. That supercontinent is called Pangea."}, {"video_title": "Pangaea Earth geological and climatic history Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So what you can imagine is a reality where India and Australia are jammed down into Antarctica, South America and Africa are jammed together, North America is jammed in there, and essentially Eurasia is also jammed in there. So it looks like they all would clump together if you go back a few hundred million years. Based on just that thought experiment, you can imagine at one point all of the continents on the world were merged into one supercontinent. That supercontinent is called Pangea. Pan for entire or whole, and Gea for coming from Gea, for the world. It turns out that all of the evidence we've seen actually does make us believe that there was a supercontinent called Pangea. Obviously there probably weren't things on the planet calling it anything back then."}, {"video_title": "Pangaea Earth geological and climatic history Cosmology & Astronomy Khan Academy.mp3", "Sentence": "That supercontinent is called Pangea. Pan for entire or whole, and Gea for coming from Gea, for the world. It turns out that all of the evidence we've seen actually does make us believe that there was a supercontinent called Pangea. Obviously there probably weren't things on the planet calling it anything back then. Well, there were things back then, but not things that would actually go and try to label continents that we know of. But all of the evidence tells us that Pangea existed about 200 to 300 million years ago, roughly 250 million, give or take, years ago. I want to be clear, this was not the first supercontinent."}, {"video_title": "Pangaea Earth geological and climatic history Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Obviously there probably weren't things on the planet calling it anything back then. Well, there were things back then, but not things that would actually go and try to label continents that we know of. But all of the evidence tells us that Pangea existed about 200 to 300 million years ago, roughly 250 million, give or take, years ago. I want to be clear, this was not the first supercontinent. To a large degree, it's kind of the most recent supercontinent that's easiest for us to construct because it was the most recent one. But we believe that there were other supercontinents before this. If you rewind even more, you would have to break up Pangea and it would reform, but we're not going back in time."}, {"video_title": "Pangaea Earth geological and climatic history Cosmology & Astronomy Khan Academy.mp3", "Sentence": "I want to be clear, this was not the first supercontinent. To a large degree, it's kind of the most recent supercontinent that's easiest for us to construct because it was the most recent one. But we believe that there were other supercontinents before this. If you rewind even more, you would have to break up Pangea and it would reform, but we're not going back in time. There were several supercontinents in the past that broke up, reformed, broke up, reformed, and the last time that we had a supercontinent was Pangea, about 250 million years ago. Now it's broken up into our current day geography. I won't go into all of the detail why we believe that there was a Pangea about 250 million years ago, or this diagram tells us about 225 million years ago, give or take."}, {"video_title": "Pangaea Earth geological and climatic history Cosmology & Astronomy Khan Academy.mp3", "Sentence": "If you rewind even more, you would have to break up Pangea and it would reform, but we're not going back in time. There were several supercontinents in the past that broke up, reformed, broke up, reformed, and the last time that we had a supercontinent was Pangea, about 250 million years ago. Now it's broken up into our current day geography. I won't go into all of the detail why we believe that there was a Pangea about 250 million years ago, or this diagram tells us about 225 million years ago, give or take. But I'll go into some of the interesting evidence. On a very high level, you have a lot of rock commonalities between things that would have had to combine during Pangea. Probably the most interesting thing is the fossil evidence."}, {"video_title": "Pangaea Earth geological and climatic history Cosmology & Astronomy Khan Academy.mp3", "Sentence": "I won't go into all of the detail why we believe that there was a Pangea about 250 million years ago, or this diagram tells us about 225 million years ago, give or take. But I'll go into some of the interesting evidence. On a very high level, you have a lot of rock commonalities between things that would have had to combine during Pangea. Probably the most interesting thing is the fossil evidence. There's a whole bunch of fossils, and here are examples of it, from species that were around between 200 and 300 million years ago. Their fossils are found in a very specific place, this animal right here, Sinognathus. This animal's fossils are only found in this area of South America, a nice clean band here, and this part of Africa."}, {"video_title": "Pangaea Earth geological and climatic history Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Probably the most interesting thing is the fossil evidence. There's a whole bunch of fossils, and here are examples of it, from species that were around between 200 and 300 million years ago. Their fossils are found in a very specific place, this animal right here, Sinognathus. This animal's fossils are only found in this area of South America, a nice clean band here, and this part of Africa. Not only does South America look like it fits very nicely into Africa, but the fossil evidence also makes it look like there was a nice clean band where this animal lived and where we find the fossils. It really makes it seem like these were connected at least when this animal lived, maybe on the order of 250 million years ago. This species right over here, its fossils are found in this area."}, {"video_title": "Pangaea Earth geological and climatic history Cosmology & Astronomy Khan Academy.mp3", "Sentence": "This animal's fossils are only found in this area of South America, a nice clean band here, and this part of Africa. Not only does South America look like it fits very nicely into Africa, but the fossil evidence also makes it look like there was a nice clean band where this animal lived and where we find the fossils. It really makes it seem like these were connected at least when this animal lived, maybe on the order of 250 million years ago. This species right over here, its fossils are found in this area. Let me do it in a color that has more contrast. In this area right over here. This plant, its fossils."}, {"video_title": "Pangaea Earth geological and climatic history Cosmology & Astronomy Khan Academy.mp3", "Sentence": "This species right over here, its fossils are found in this area. Let me do it in a color that has more contrast. In this area right over here. This plant, its fossils. Now this starts to connect a lot of dots between a lot of content. Its fossils are found in this entire area, across South America, Africa, Antarctica, India, and Australia. Not only does it look like the continents kind of fit together in a puzzle piece, not only do we get it to a configuration like this, we essentially just rewind the movement that we're seeing now, but the fossil evidence also kind of confirms that they fit together in this way."}, {"video_title": "Pangaea Earth geological and climatic history Cosmology & Astronomy Khan Academy.mp3", "Sentence": "This plant, its fossils. Now this starts to connect a lot of dots between a lot of content. Its fossils are found in this entire area, across South America, Africa, Antarctica, India, and Australia. Not only does it look like the continents kind of fit together in a puzzle piece, not only do we get it to a configuration like this, we essentially just rewind the movement that we're seeing now, but the fossil evidence also kind of confirms that they fit together in this way. This animal right here, we find fossils on this nice stripe that goes from Africa through India all the way to Antarctica. Now this only gives us evidence of kind of the more southern hemisphere of Pangea, but there is other evidence. We find kind of continuing mountain chains between North America and Europe."}, {"video_title": "Pangaea Earth geological and climatic history Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Not only does it look like the continents kind of fit together in a puzzle piece, not only do we get it to a configuration like this, we essentially just rewind the movement that we're seeing now, but the fossil evidence also kind of confirms that they fit together in this way. This animal right here, we find fossils on this nice stripe that goes from Africa through India all the way to Antarctica. Now this only gives us evidence of kind of the more southern hemisphere of Pangea, but there is other evidence. We find kind of continuing mountain chains between North America and Europe. We find rock evidence where just the way we see the fossils kind of line up nicely, there's a common rock that lines up nicely between South America and Africa and other continents that were at once connected. So all of the evidence, as far as we can tell now, does make us think that there at one time was a Pangea. And for all we know, all the continents are going to keep moving and maybe in a few hundred million years we'll have another supercontinent."}, {"video_title": "Refraction of seismic waves Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And our focus here is going to be on seismic waves. But the principles, how things refract when they go from a fast to a slow medium or slow to a fast medium, it's actually the same as you would see when you're studying light waves or actually any type of wave. So let's think about it a little bit. So let's say I have a slow medium right over here. And let's say I have a fast medium right over here. Let's say, just so that we can travel through both solid and liquid, let's think about maybe P waves. And a slow medium could be maybe some type of liquid."}, {"video_title": "Refraction of seismic waves Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So let's say I have a slow medium right over here. And let's say I have a fast medium right over here. Let's say, just so that we can travel through both solid and liquid, let's think about maybe P waves. And a slow medium could be maybe some type of liquid. And our fast medium could be some type of solid. So let me draw the boundary right over here. And let's say, so if I have something that comes out P wave, let's say it's going through the water and it's going right perpendicular to the boundary, it will then just continue to travel in the faster medium in the same direction."}, {"video_title": "Refraction of seismic waves Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And a slow medium could be maybe some type of liquid. And our fast medium could be some type of solid. So let me draw the boundary right over here. And let's say, so if I have something that comes out P wave, let's say it's going through the water and it's going right perpendicular to the boundary, it will then just continue to travel in the faster medium in the same direction. If it's going right, if it goes right at the boundary, it'll just travel faster in the faster medium. And that's because that faster medium is going to be more dense. And the molecules are going to bump into each other faster."}, {"video_title": "Refraction of seismic waves Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And let's say, so if I have something that comes out P wave, let's say it's going through the water and it's going right perpendicular to the boundary, it will then just continue to travel in the faster medium in the same direction. If it's going right, if it goes right at the boundary, it'll just travel faster in the faster medium. And that's because that faster medium is going to be more dense. And the molecules are going to bump into each other faster. In the same amount of time, more molecules, kind of the chain reaction, is going to be able to travel further because they're more closely packed and they rebound faster than it would in the slow medium. So that's obviously no refraction is going on. It has not been deflected."}, {"video_title": "Refraction of seismic waves Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And the molecules are going to bump into each other faster. In the same amount of time, more molecules, kind of the chain reaction, is going to be able to travel further because they're more closely packed and they rebound faster than it would in the slow medium. So that's obviously no refraction is going on. It has not been deflected. And just as a bit of a reminder, in general, refraction is when a wave gets deflected. Reflection is when it bounces back. Refraction is when it gets deflected a little bit."}, {"video_title": "Refraction of seismic waves Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It has not been deflected. And just as a bit of a reminder, in general, refraction is when a wave gets deflected. Reflection is when it bounces back. Refraction is when it gets deflected a little bit. Let me just make that clear. So if I have some type of boundary here and I have a wave that bounces off, that's reflection. But if the wave goes through the boundary and just gets bent a little bit, its direction changes, that is refraction."}, {"video_title": "Refraction of seismic waves Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Refraction is when it gets deflected a little bit. Let me just make that clear. So if I have some type of boundary here and I have a wave that bounces off, that's reflection. But if the wave goes through the boundary and just gets bent a little bit, its direction changes, that is refraction. And that's what we're talking about. So clearly, so far, this P wave has not been refracted. But if this P wave comes in at an angle, so let's make this P wave come in at an angle, what's going to happen is, and the way you should think about it, it's the easiest way to think about which direction will be refracted, or at least the way I think about it, is literally I imagine some type of vehicle with wheels on it."}, {"video_title": "Refraction of seismic waves Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But if the wave goes through the boundary and just gets bent a little bit, its direction changes, that is refraction. And that's what we're talking about. So clearly, so far, this P wave has not been refracted. But if this P wave comes in at an angle, so let's make this P wave come in at an angle, what's going to happen is, and the way you should think about it, it's the easiest way to think about which direction will be refracted, or at least the way I think about it, is literally I imagine some type of vehicle with wheels on it. So this is the top view of my vehicle. So if I have some type of vehicle, and the wheels will be able to move slowly in this medium. You can view it as on mud so it doesn't get good traction."}, {"video_title": "Refraction of seismic waves Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But if this P wave comes in at an angle, so let's make this P wave come in at an angle, what's going to happen is, and the way you should think about it, it's the easiest way to think about which direction will be refracted, or at least the way I think about it, is literally I imagine some type of vehicle with wheels on it. So this is the top view of my vehicle. So if I have some type of vehicle, and the wheels will be able to move slowly in this medium. You can view it as on mud so it doesn't get good traction. And then the fast medium, maybe it's a road, so it gets good traction and it can move faster. So what's going to happen when the vehicle gets to the boundary? Well, this bottom right wheel is going to go on the fast medium before any of the other wheels do."}, {"video_title": "Refraction of seismic waves Cosmology & Astronomy Khan Academy.mp3", "Sentence": "You can view it as on mud so it doesn't get good traction. And then the fast medium, maybe it's a road, so it gets good traction and it can move faster. So what's going to happen when the vehicle gets to the boundary? Well, this bottom right wheel is going to go on the fast medium before any of the other wheels do. So it's going to get the traction first. These wheels on the left side of the vehicle, these wheels right here, these are still going to be stuck in the mud. So what's going to happen is, this wheel right over here is moving faster, so it's essentially going to be able to turn the vehicle."}, {"video_title": "Refraction of seismic waves Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Well, this bottom right wheel is going to go on the fast medium before any of the other wheels do. So it's going to get the traction first. These wheels on the left side of the vehicle, these wheels right here, these are still going to be stuck in the mud. So what's going to happen is, this wheel right over here is moving faster, so it's essentially going to be able to turn the vehicle. These guys are still stuck in the mud. And so you fast forward a little bit, the direction of the vehicle will change. And so the vehicle will now move in a direction something like this."}, {"video_title": "Refraction of seismic waves Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So what's going to happen is, this wheel right over here is moving faster, so it's essentially going to be able to turn the vehicle. These guys are still stuck in the mud. And so you fast forward a little bit, the direction of the vehicle will change. And so the vehicle will now move in a direction something like this. The same thing would happen in a wave. If the P wave is approaching the boundary like this, and something analogous to this is happening at the molecular level. You can kind of view it as even billiard balls, and maybe they're kind of hitting each other."}, {"video_title": "Refraction of seismic waves Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And so the vehicle will now move in a direction something like this. The same thing would happen in a wave. If the P wave is approaching the boundary like this, and something analogous to this is happening at the molecular level. You can kind of view it as even billiard balls, and maybe they're kind of hitting each other. I won't go into that, because that can kind of get a little confusing, depending on the different cases and the different boundaries. But this is the easiest way to think about in which direction it will refract, and hopefully it makes a little bit of intuitive sense. And so when you go from a slow to a fast medium, our P wave, its angle would accentuate in that direction."}, {"video_title": "Refraction of seismic waves Cosmology & Astronomy Khan Academy.mp3", "Sentence": "You can kind of view it as even billiard balls, and maybe they're kind of hitting each other. I won't go into that, because that can kind of get a little confusing, depending on the different cases and the different boundaries. But this is the easiest way to think about in which direction it will refract, and hopefully it makes a little bit of intuitive sense. And so when you go from a slow to a fast medium, our P wave, its angle would accentuate in that direction. If you went from the fast medium to the slow medium, once again, you can just go through the same thought experiment. So let's say you have our wave coming in like that. Draw the car."}, {"video_title": "Refraction of seismic waves Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And so when you go from a slow to a fast medium, our P wave, its angle would accentuate in that direction. If you went from the fast medium to the slow medium, once again, you can just go through the same thought experiment. So let's say you have our wave coming in like that. Draw the car. Visualize the car here. Visualize the car right here. And you say, well, look, this tire is going to get stuck in the mud, because now we're going from, it was on the road, now this top left, this top tire right over here is getting stuck in the mud first, so it's going to be moving slower."}, {"video_title": "Refraction of seismic waves Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Draw the car. Visualize the car here. Visualize the car right here. And you say, well, look, this tire is going to get stuck in the mud, because now we're going from, it was on the road, now this top left, this top tire right over here is getting stuck in the mud first, so it's going to be moving slower. So these tires are going to be able to move faster, so the vehicle is going to turn. So you'll be refracted in a direction like that when you're going from the fast to the slow medium. So that's just a primer on refraction generally."}, {"video_title": "Refraction of seismic waves Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And you say, well, look, this tire is going to get stuck in the mud, because now we're going from, it was on the road, now this top left, this top tire right over here is getting stuck in the mud first, so it's going to be moving slower. So these tires are going to be able to move faster, so the vehicle is going to turn. So you'll be refracted in a direction like that when you're going from the fast to the slow medium. So that's just a primer on refraction generally. Now let's think about what would happen when sound waves are traveling through the Earth. And this will help inform us of, essentially, how do we figure out what the actual structure of the Earth is? So if the Earth was just made up of some uniform material, and you had an earthquake right here on Earth, maybe a little bit below the surface."}, {"video_title": "Refraction of seismic waves Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So that's just a primer on refraction generally. Now let's think about what would happen when sound waves are traveling through the Earth. And this will help inform us of, essentially, how do we figure out what the actual structure of the Earth is? So if the Earth was just made up of some uniform material, and you had an earthquake right here on Earth, maybe a little bit below the surface. So it's happening in the crust, but a little bit below the surface of the Earth. If Earth was of uniform density, if it was all the same material, how would those, let's just think about the P waves, because P waves can travel in anything. Let's think about how those P waves would travel."}, {"video_title": "Refraction of seismic waves Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So if the Earth was just made up of some uniform material, and you had an earthquake right here on Earth, maybe a little bit below the surface. So it's happening in the crust, but a little bit below the surface of the Earth. If Earth was of uniform density, if it was all the same material, how would those, let's just think about the P waves, because P waves can travel in anything. Let's think about how those P waves would travel. Well, they would just go in straight lines. There's nothing that would refract the P waves. It would just go in straight lines, radially outward, from where the earthquake occurred."}, {"video_title": "Refraction of seismic waves Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Let's think about how those P waves would travel. Well, they would just go in straight lines. There's nothing that would refract the P waves. It would just go in straight lines, radially outward, from where the earthquake occurred. Now, at a first approximation, we know that as we go deeper and deeper into Earth, there's more and more rock above that. The weight of that rock is kind of compressing the rock below it. So you get higher and higher pressures, and higher and higher densities."}, {"video_title": "Refraction of seismic waves Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It would just go in straight lines, radially outward, from where the earthquake occurred. Now, at a first approximation, we know that as we go deeper and deeper into Earth, there's more and more rock above that. The weight of that rock is kind of compressing the rock below it. So you get higher and higher pressures, and higher and higher densities. So this is a uniform Earth. So this is a uniform. But let's imagine an Earth that's made up of uniform material, that's all solid, a completely solid Earth, but one where the density is constantly increasing as you go down."}, {"video_title": "Refraction of seismic waves Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So you get higher and higher pressures, and higher and higher densities. So this is a uniform Earth. So this is a uniform. But let's imagine an Earth that's made up of uniform material, that's all solid, a completely solid Earth, but one where the density is constantly increasing as you go down. So let's just think about it in, before we go into the continuous case, because we're talking about the density as you go deeper, it's just getting continuously more dense. Let's think about the discrete case, where we have the least dense layer. So let me draw it right over here."}, {"video_title": "Refraction of seismic waves Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But let's imagine an Earth that's made up of uniform material, that's all solid, a completely solid Earth, but one where the density is constantly increasing as you go down. So let's just think about it in, before we go into the continuous case, because we're talking about the density as you go deeper, it's just getting continuously more dense. Let's think about the discrete case, where we have the least dense layer. So let me draw it right over here. Let's say this is the surface of the Earth, and this is least dense. Then let's say you have another layer over here that is more dense. So this is more dense."}, {"video_title": "Refraction of seismic waves Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So let me draw it right over here. Let's say this is the surface of the Earth, and this is least dense. Then let's say you have another layer over here that is more dense. So this is more dense. Let's say you have another layer that's even more dense. So you have another layer over here that's even more dense. And then let's do one more layer."}, {"video_title": "Refraction of seismic waves Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So this is more dense. Let's say you have another layer that's even more dense. So you have another layer over here that's even more dense. And then let's do one more layer. Let's do this layer here, that this is the densest layer. So in general, your P wave, your seismic wave, is going to travel faster and denser material. So it's going to travel the fastest here, then here, then here."}, {"video_title": "Refraction of seismic waves Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And then let's do one more layer. Let's do this layer here, that this is the densest layer. So in general, your P wave, your seismic wave, is going to travel faster and denser material. So it's going to travel the fastest here, then here, then here. It's going to travel the slowest in this least dense material. So if you're coming in at an angle, let's think about what's going to happen. So let's say you have your P wave coming in at an angle like this."}, {"video_title": "Refraction of seismic waves Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So it's going to travel the fastest here, then here, then here. It's going to travel the slowest in this least dense material. So if you're coming in at an angle, let's think about what's going to happen. So let's say you have your P wave coming in at an angle like this. So it's going straight through the least dense material. What's going to happen when it goes into the slightly shallower angle? So let's say it's like that."}, {"video_title": "Refraction of seismic waves Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So let's say you have your P wave coming in at an angle like this. So it's going straight through the least dense material. What's going to happen when it goes into the slightly shallower angle? So let's say it's like that. What's going to happen when it goes into the more dense material? So once again, let's imagine our little car. So this tire is going to be able to go faster before the tires on the other side."}, {"video_title": "Refraction of seismic waves Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So let's say it's like that. What's going to happen when it goes into the more dense material? So once again, let's imagine our little car. So this tire is going to be able to go faster before the tires on the other side. So the car is going to be deflected to the left, to the down left. So now it's going to travel like this. So it's now going to travel something like this."}, {"video_title": "Refraction of seismic waves Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So this tire is going to be able to go faster before the tires on the other side. So the car is going to be deflected to the left, to the down left. So now it's going to travel like this. So it's now going to travel something like this. Now what's going to happen at this boundary? Once again, imagine the car. This tire right here is going to be able to travel faster before the other tire, so it'll be deflected even more in that direction."}, {"video_title": "Refraction of seismic waves Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So it's now going to travel something like this. Now what's going to happen at this boundary? Once again, imagine the car. This tire right here is going to be able to travel faster before the other tire, so it'll be deflected even more in that direction. Then we go into the densest material. Once again, the tire is on kind of the bottom side. When we look at it this way, we're going to be able to move faster before the other tire, so we're going to get deflected even more."}, {"video_title": "Refraction of seismic waves Cosmology & Astronomy Khan Academy.mp3", "Sentence": "This tire right here is going to be able to travel faster before the other tire, so it'll be deflected even more in that direction. Then we go into the densest material. Once again, the tire is on kind of the bottom side. When we look at it this way, we're going to be able to move faster before the other tire, so we're going to get deflected even more. So you see as you go from least dense material to more dense material, you're kind of curving outward. So if this was continuous, if you had a continuous structure where as you go down, it just gets more and more dense. As you go, so this is less dense, and then it just continuously gets more dense."}, {"video_title": "Refraction of seismic waves Cosmology & Astronomy Khan Academy.mp3", "Sentence": "When we look at it this way, we're going to be able to move faster before the other tire, so we're going to get deflected even more. So you see as you go from least dense material to more dense material, you're kind of curving outward. So if this was continuous, if you had a continuous structure where as you go down, it just gets more and more dense. As you go, so this is less dense, and then it just continuously gets more dense. So this is the most dense down here. How would the refraction look? Well, then it would just be a continuous curve."}, {"video_title": "Refraction of seismic waves Cosmology & Astronomy Khan Academy.mp3", "Sentence": "As you go, so this is less dense, and then it just continuously gets more dense. So this is the most dense down here. How would the refraction look? Well, then it would just be a continuous curve. It would look like this. Your P wave would constantly be refracted out like that. It would curve outwards."}, {"video_title": "Refraction of seismic waves Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Well, then it would just be a continuous curve. It would look like this. Your P wave would constantly be refracted out like that. It would curve outwards. So if Earth, so this was the simplest example, where Earth is uniform. And that's pretty easy to dismiss, that obviously things will get denser because of more pressure down. So let's say we assume another thing."}, {"video_title": "Refraction of seismic waves Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It would curve outwards. So if Earth, so this was the simplest example, where Earth is uniform. And that's pretty easy to dismiss, that obviously things will get denser because of more pressure down. So let's say we assume another thing. We have a uniform Earth in terms of composition, but let's say it gets denser. So denser at the center. Then how would the P waves travel?"}, {"video_title": "Refraction of seismic waves Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So let's say we assume another thing. We have a uniform Earth in terms of composition, but let's say it gets denser. So denser at the center. Then how would the P waves travel? Then how would the P waves travel? Or how would any seismic waves travel? Well, then if you have your earthquake right over here, the ones that are going straight down still would go straight down, because we know that we won't get refracted if we're kind of going perpendicular to the change in medium or the change in boundaries."}, {"video_title": "Refraction of seismic waves Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Then how would the P waves travel? Then how would the P waves travel? Or how would any seismic waves travel? Well, then if you have your earthquake right over here, the ones that are going straight down still would go straight down, because we know that we won't get refracted if we're kind of going perpendicular to the change in medium or the change in boundaries. But things that are coming at a slight angle, as they get deeper, they're going to get deflected more and more and more, and they're going to be refracted outward. Just like we saw in this example here. If they go in this angle, they're going to be refracted outward like that."}, {"video_title": "Refraction of seismic waves Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Well, then if you have your earthquake right over here, the ones that are going straight down still would go straight down, because we know that we won't get refracted if we're kind of going perpendicular to the change in medium or the change in boundaries. But things that are coming at a slight angle, as they get deeper, they're going to get deflected more and more and more, and they're going to be refracted outward. Just like we saw in this example here. If they go in this angle, they're going to be refracted outward like that. If they go here, they're going to be refracted outward like that. They're going to be refracted outward like that. If you're here, you're going to be refracted outward like that."}, {"video_title": "Refraction of seismic waves Cosmology & Astronomy Khan Academy.mp3", "Sentence": "If they go in this angle, they're going to be refracted outward like that. If they go here, they're going to be refracted outward like that. They're going to be refracted outward like that. If you're here, you're going to be refracted outward like that. If you're here, you're going to be refracted outward like this. Now, what we're going to do in the next few videos is use what we just learned about refraction in the case of seismic waves, and hopefully we learned it in this video, and how it would refract as we're going through ever increasing denser material. We're going to use that information to essentially try to figure out the composition of the Earth based on what we've actually observed."}, {"video_title": "Plates moving due to convection in mantle Cosmology & Astronomy Khan Academy.mp3", "Sentence": "This is just kind of the current thinking, the leading thinking on why plates are actually moving. Although we haven't seen the definitive evidence yet. And it's probably a combination of a bunch of things. Now before we even talk about plates, let's just talk about convection. And you might already be familiar with the term, but just in case you're not, let's do a little bit of review of convection. So let's say I have a pot over here. So that is my pot."}, {"video_title": "Plates moving due to convection in mantle Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Now before we even talk about plates, let's just talk about convection. And you might already be familiar with the term, but just in case you're not, let's do a little bit of review of convection. So let's say I have a pot over here. So that is my pot. And it contains some water. So I have water in my pot. And let's say I only heat one end of the pot."}, {"video_title": "Plates moving due to convection in mantle Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So that is my pot. And it contains some water. So I have water in my pot. And let's say I only heat one end of the pot. So I put a flame right over at that end of the pot. So what's going to happen? Well the water that's right over the flame is going to be warmed up more than any of the other water."}, {"video_title": "Plates moving due to convection in mantle Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And let's say I only heat one end of the pot. So I put a flame right over at that end of the pot. So what's going to happen? Well the water that's right over the flame is going to be warmed up more than any of the other water. So this water is going to get warm. But when it gets warm, it also becomes less dense. When you have a fluid, if you warm it up, the molecules are vibrating more."}, {"video_title": "Plates moving due to convection in mantle Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Well the water that's right over the flame is going to be warmed up more than any of the other water. So this water is going to get warm. But when it gets warm, it also becomes less dense. When you have a fluid, if you warm it up, the molecules are vibrating more. They have more kinetic energy. They're going to bounce further distances away from each other. They will become less dense."}, {"video_title": "Plates moving due to convection in mantle Cosmology & Astronomy Khan Academy.mp3", "Sentence": "When you have a fluid, if you warm it up, the molecules are vibrating more. They have more kinetic energy. They're going to bounce further distances away from each other. They will become less dense. And if you have something that's less dense and it's surrounded by things that are more dense, and we're dealing in kind of a fluid state right here, that warm, less dense water is going to move upwards. Well when it moves upwards, something has to replace it. So you're going to have cooler water from this side of the container kind of replacing where that water was."}, {"video_title": "Plates moving due to convection in mantle Cosmology & Astronomy Khan Academy.mp3", "Sentence": "They will become less dense. And if you have something that's less dense and it's surrounded by things that are more dense, and we're dealing in kind of a fluid state right here, that warm, less dense water is going to move upwards. Well when it moves upwards, something has to replace it. So you're going to have cooler water from this side of the container kind of replacing where that water was. Now this water, as it rises, what's going to happen to it? Well it's going to cool down. It's going to get further from the flame."}, {"video_title": "Plates moving due to convection in mantle Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So you're going to have cooler water from this side of the container kind of replacing where that water was. Now this water, as it rises, what's going to happen to it? Well it's going to cool down. It's going to get further from the flame. It's going to mix with maybe some of the other water, or transfer some of its kinetic energy to the neighboring water. So it'll cool down. But once it cools down, what's it going to want to do?"}, {"video_title": "Plates moving due to convection in mantle Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It's going to get further from the flame. It's going to mix with maybe some of the other water, or transfer some of its kinetic energy to the neighboring water. So it'll cool down. But once it cools down, what's it going to want to do? Remember, in general, the closer you are to the flame. So the water closer to the flame in general is going to be warmer. So all of this stuff is going to be warmer."}, {"video_title": "Plates moving due to convection in mantle Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But once it cools down, what's it going to want to do? Remember, in general, the closer you are to the flame. So the water closer to the flame in general is going to be warmer. So all of this stuff is going to be warmer. And all of this stuff up here, like the coldest water, is always going to be furthest from the flame. And so the coldest water is going to be over here. But remember, the coldest water is also the densest water."}, {"video_title": "Plates moving due to convection in mantle Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So all of this stuff is going to be warmer. And all of this stuff up here, like the coldest water, is always going to be furthest from the flame. And so the coldest water is going to be over here. But remember, the coldest water is also the densest water. So this water over here is dense. And so it will sink. It's denser than the water around it."}, {"video_title": "Plates moving due to convection in mantle Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But remember, the coldest water is also the densest water. So this water over here is dense. And so it will sink. It's denser than the water around it. And it also helps replace the water that's going here to get warmed up again. And so what you do is you have this cycle here. Warm water rises, moves over to the right down here, and then goes back down as it cools down."}, {"video_title": "Plates moving due to convection in mantle Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It's denser than the water around it. And it also helps replace the water that's going here to get warmed up again. And so what you do is you have this cycle here. Warm water rises, moves over to the right down here, and then goes back down as it cools down. And it's dense, and then it gets warmed up again. And so this process, essentially what it's doing is it's transferring the heat. It's allowing the heat to be transferred from this one spot throughout the fluid."}, {"video_title": "Plates moving due to convection in mantle Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Warm water rises, moves over to the right down here, and then goes back down as it cools down. And it's dense, and then it gets warmed up again. And so this process, essentially what it's doing is it's transferring the heat. It's allowing the heat to be transferred from this one spot throughout the fluid. And so we call this process, this is convection. Now, the reason why we think the plates are moving is because we think that there are similar types of convection currents in the asthenosphere, in the mantle, in the more fluid part of the mantle. Remember, most of the mantle is kind of this mushy, spongy, not quite liquid, but not quite solid state, kind of plastic."}, {"video_title": "Plates moving due to convection in mantle Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It's allowing the heat to be transferred from this one spot throughout the fluid. And so we call this process, this is convection. Now, the reason why we think the plates are moving is because we think that there are similar types of convection currents in the asthenosphere, in the mantle, in the more fluid part of the mantle. Remember, most of the mantle is kind of this mushy, spongy, not quite liquid, but not quite solid state, kind of plastic. It can kind of mush past. It can kind of flow like super, super, super thick, I guess you could call them, like super viscous fluid. So not quite solid, not quite liquid."}, {"video_title": "Plates moving due to convection in mantle Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Remember, most of the mantle is kind of this mushy, spongy, not quite liquid, but not quite solid state, kind of plastic. It can kind of mush past. It can kind of flow like super, super, super thick, I guess you could call them, like super viscous fluid. So not quite solid, not quite liquid. But the same thing could be happening. You have certain areas in the mantle that are hotter than others, and it's particular in the asthenosphere. And those areas, that's where you're going to have the material in the mantle move up, because it's hotter, it's less dense, and it will move up."}, {"video_title": "Plates moving due to convection in mantle Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So not quite solid, not quite liquid. But the same thing could be happening. You have certain areas in the mantle that are hotter than others, and it's particular in the asthenosphere. And those areas, that's where you're going to have the material in the mantle move up, because it's hotter, it's less dense, and it will move up. And maybe it'll cause one of these divergent rifts where kind of plate material and crustal material is forming. And then as it moves up, it cools down and eventually sinks, only to get heated up again. So it has these kind of circular motions, just like we saw with the boiling water."}, {"video_title": "Plates moving due to convection in mantle Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And those areas, that's where you're going to have the material in the mantle move up, because it's hotter, it's less dense, and it will move up. And maybe it'll cause one of these divergent rifts where kind of plate material and crustal material is forming. And then as it moves up, it cools down and eventually sinks, only to get heated up again. So it has these kind of circular motions, just like we saw with the boiling water. And so that process, remember, this isn't completely liquid, so it is rocky. So it's going to potentially be able to take other things with it. It could maybe drag the crust along with it, and that would cause that."}, {"video_title": "Plates moving due to convection in mantle Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So it has these kind of circular motions, just like we saw with the boiling water. And so that process, remember, this isn't completely liquid, so it is rocky. So it's going to potentially be able to take other things with it. It could maybe drag the crust along with it, and that would cause that. Or I should say it could drag the lithosphere along with it, not just the crust. It could drag all this rigid rock up here along with it, causing it to move in that general direction. So you have the drag there."}, {"video_title": "Plates moving due to convection in mantle Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It could maybe drag the crust along with it, and that would cause that. Or I should say it could drag the lithosphere along with it, not just the crust. It could drag all this rigid rock up here along with it, causing it to move in that general direction. So you have the drag there. You could also imagine that there's kind of a suction effect, where if you view it as a fluid, you have a bunch of fluid coming down here. So it would kind of pull the lithosphere down at those points, and it would kind of push the lithosphere up at those points. So you have these convection currents that are essentially driving."}, {"video_title": "Plates moving due to convection in mantle Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So you have the drag there. You could also imagine that there's kind of a suction effect, where if you view it as a fluid, you have a bunch of fluid coming down here. So it would kind of pull the lithosphere down at those points, and it would kind of push the lithosphere up at those points. So you have these convection currents that are essentially driving. And these aren't going to be super fast-moving fluid convection currents like you would expect with boiling water or with heated water. These would be slow-moving convection currents, but they're moving enough, and they're able to kind of put enough drag on the lithosphere to take the lithosphere along with it. And so that's kind of, at a high level, the dominant theory as to why they're moving."}, {"video_title": "Plates moving due to convection in mantle Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So you have these convection currents that are essentially driving. And these aren't going to be super fast-moving fluid convection currents like you would expect with boiling water or with heated water. These would be slow-moving convection currents, but they're moving enough, and they're able to kind of put enough drag on the lithosphere to take the lithosphere along with it. And so that's kind of, at a high level, the dominant theory as to why they're moving. There's other ones that talk about maybe the lithospheric plates. They thicken as they move further away from this area where the ridge is forming. And if we look at this oceanic crust right over here."}, {"video_title": "Plates moving due to convection in mantle Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And so that's kind of, at a high level, the dominant theory as to why they're moving. There's other ones that talk about maybe the lithospheric plates. They thicken as they move further away from this area where the ridge is forming. And if we look at this oceanic crust right over here. And so over time, they're denser over here because there's more cool-down material at these points. And it's already a little bit lower because this is where a lot of the land is being created, and so there's maybe some gravitational effects. But the dominant effect, we think, is due to this convection in the upper mantle."}, {"video_title": "Detectable civilizations in our galaxy 5 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And we got that there should be 12.5 detectable civilizations in the Milky Way galaxy. And I talked about a bunch of reasons about why we aren't detecting them. But I left out one of the most obvious reasons that we're not detecting them. And it was rightfully pointed out in the comments below that video. And that's just the signal might be too weak. If there's 12.5, if there's on the order of 10, 11, 12 detectable civilizations in our galaxy, they could be quite far from us. This isn't the Milky Way, but this is a galaxy that probably doesn't look too different from our Milky Way."}, {"video_title": "Detectable civilizations in our galaxy 5 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And it was rightfully pointed out in the comments below that video. And that's just the signal might be too weak. If there's 12.5, if there's on the order of 10, 11, 12 detectable civilizations in our galaxy, they could be quite far from us. This isn't the Milky Way, but this is a galaxy that probably doesn't look too different from our Milky Way. We could obviously never get this vantage point of our galaxy, at least not for a while, not unless we can travel quite far away from it. But let's say that we're over here. You could imagine if the 10 civilizations or the 12 civilizations are here."}, {"video_title": "Detectable civilizations in our galaxy 5 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "This isn't the Milky Way, but this is a galaxy that probably doesn't look too different from our Milky Way. We could obviously never get this vantage point of our galaxy, at least not for a while, not unless we can travel quite far away from it. But let's say that we're over here. You could imagine if the 10 civilizations or the 12 civilizations are here. 1, 2, 3, 4. There's probably a lot more in the center, actually, because that's where our density is higher. So let me put it here."}, {"video_title": "Detectable civilizations in our galaxy 5 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "You could imagine if the 10 civilizations or the 12 civilizations are here. 1, 2, 3, 4. There's probably a lot more in the center, actually, because that's where our density is higher. So let me put it here. 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12. The closest of them might be tens of thousands of light years away from us. And there might be a lot of stuff in between, all sorts of crazy things happening, stars exploding, all sorts of signals that we're receiving."}, {"video_title": "Detectable civilizations in our galaxy 5 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So let me put it here. 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12. The closest of them might be tens of thousands of light years away from us. And there might be a lot of stuff in between, all sorts of crazy things happening, stars exploding, all sorts of signals that we're receiving. And it might just be that the signals from those civilizations are too weak to reach us, or that there's somehow too much interference from all of the other craziness that's happening around the galaxy. There's also these other reasons that I talked about in the last video. Maybe they've gone beyond using radio as a form of communication, and that's why they never use it to begin with."}, {"video_title": "Detectable civilizations in our galaxy 5 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And there might be a lot of stuff in between, all sorts of crazy things happening, stars exploding, all sorts of signals that we're receiving. And it might just be that the signals from those civilizations are too weak to reach us, or that there's somehow too much interference from all of the other craziness that's happening around the galaxy. There's also these other reasons that I talked about in the last video. Maybe they've gone beyond using radio as a form of communication, and that's why they never use it to begin with. And that's why we don't even see them ever using it. Or they use it for a very short period, kind of a transition period, and maybe in 100 years we'll discover the next best thing. The other idea behind why we're probably, or maybe why we might not be able to detect civilizations is that, well, there might be a lot fewer than 10."}, {"video_title": "Detectable civilizations in our galaxy 5 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Maybe they've gone beyond using radio as a form of communication, and that's why they never use it to begin with. And that's why we don't even see them ever using it. Or they use it for a very short period, kind of a transition period, and maybe in 100 years we'll discover the next best thing. The other idea behind why we're probably, or maybe why we might not be able to detect civilizations is that, well, there might be a lot fewer than 10. When I did the Drake equation right over here, I just made a bunch of assumptions. None of these seemed crazy. But I kind of assumed a reality where you didn't have these kind of cataclysmic events in the galaxy at regular intervals."}, {"video_title": "Detectable civilizations in our galaxy 5 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "The other idea behind why we're probably, or maybe why we might not be able to detect civilizations is that, well, there might be a lot fewer than 10. When I did the Drake equation right over here, I just made a bunch of assumptions. None of these seemed crazy. But I kind of assumed a reality where you didn't have these kind of cataclysmic events in the galaxy at regular intervals. But we know that there are cataclysmic events that happen in our galaxy, in other galaxies. The one that we know the most about, although there are probably all sorts of things that we don't know much about, are gamma ray bursts. And these are still kind of trying to be understood."}, {"video_title": "Detectable civilizations in our galaxy 5 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But I kind of assumed a reality where you didn't have these kind of cataclysmic events in the galaxy at regular intervals. But we know that there are cataclysmic events that happen in our galaxy, in other galaxies. The one that we know the most about, although there are probably all sorts of things that we don't know much about, are gamma ray bursts. And these are still kind of trying to be understood. And you could watch the video on quasars. Those are essentially a lot of highly energetic rays being released when all of this material is being absorbed into supermassive black holes at the centers of galaxies that tend to be very, very, very often billions of light years away. And gamma rays are one of the things that get emitted from those."}, {"video_title": "Detectable civilizations in our galaxy 5 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And these are still kind of trying to be understood. And you could watch the video on quasars. Those are essentially a lot of highly energetic rays being released when all of this material is being absorbed into supermassive black holes at the centers of galaxies that tend to be very, very, very often billions of light years away. And gamma rays are one of the things that get emitted from those. But you can also have gamma ray bursts within galaxies. We believe maybe certain types of stars, when they collapse into black holes, you have this burst of gamma rays. There might be certain types of neutron stars with the right properties that might every now and then release gamma rays."}, {"video_title": "Detectable civilizations in our galaxy 5 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And gamma rays are one of the things that get emitted from those. But you can also have gamma ray bursts within galaxies. We believe maybe certain types of stars, when they collapse into black holes, you have this burst of gamma rays. There might be certain types of neutron stars with the right properties that might every now and then release gamma rays. And the view is that if there is a civilization that is within a few thousand light years near one of these gamma ray bursts, and it's in the wrong place, it's kind of in the path of the burst, then it's a good chance that those civilizations will be completely wiped out, that those planets will be sterilized, because there's so much radiation coming out from that gamma ray. And there's even some theories that some of the extinction events that have happened in Earth's history, so we're not talking about the dinosaurs, we're talking about billions of years ago, maybe a billion years ago or two billion years ago, that these might have been caused by relatively local gamma ray bursts. The theory is that these might hit Earth on the order of once every billion years."}, {"video_title": "Detectable civilizations in our galaxy 5 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "There might be certain types of neutron stars with the right properties that might every now and then release gamma rays. And the view is that if there is a civilization that is within a few thousand light years near one of these gamma ray bursts, and it's in the wrong place, it's kind of in the path of the burst, then it's a good chance that those civilizations will be completely wiped out, that those planets will be sterilized, because there's so much radiation coming out from that gamma ray. And there's even some theories that some of the extinction events that have happened in Earth's history, so we're not talking about the dinosaurs, we're talking about billions of years ago, maybe a billion years ago or two billion years ago, that these might have been caused by relatively local gamma ray bursts. The theory is that these might hit Earth on the order of once every billion years. And if you think about the galaxy as a whole, we're kind of in where our solar system is orbiting the galaxy. It's kind of a nice distance from the center of the galaxy. The closer you get into the center, the higher densities of stars you have."}, {"video_title": "Detectable civilizations in our galaxy 5 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "The theory is that these might hit Earth on the order of once every billion years. And if you think about the galaxy as a whole, we're kind of in where our solar system is orbiting the galaxy. It's kind of a nice distance from the center of the galaxy. The closer you get into the center, the higher densities of stars you have. So you can imagine if Earth gets hit with one of these gamma ray bursts every couple of billion years, you could imagine something closer to the center of the galaxy gets hit with these gamma ray bursts much, much, much more frequently, just because there's more activity there. There's more stars that are closer by, more stars that are aging, more stars that might be collapsing into black holes. So the simple answer is we don't know."}, {"video_title": "Detectable civilizations in our galaxy 5 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "The closer you get into the center, the higher densities of stars you have. So you can imagine if Earth gets hit with one of these gamma ray bursts every couple of billion years, you could imagine something closer to the center of the galaxy gets hit with these gamma ray bursts much, much, much more frequently, just because there's more activity there. There's more stars that are closer by, more stars that are aging, more stars that might be collapsing into black holes. So the simple answer is we don't know. There could be 1,000 civilizations out there. We're just not sophisticated enough to notice them just yet. Or there might be very, very few because of all of this craziness that happens in the galaxy."}, {"video_title": "Scale of the galaxy Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "In the last video, I hinted that things were about to get wacky, and they are. So if we start where we left off in the last video, we started right over here, looking at the distance to the nearest star. And just as a reminder, in this drawing right here, this depiction right here, this circle right here, this solar system circle, it's not the size of the sun, it's not the size of the orbits of the Earth or Pluto or the Kuiper Belt. This is close to the size of the Oort Cloud. And the actual orbit of Earth is about 1... Well, the diameter of the orbit of Earth is about 1 50,000th of this. So you wouldn't even see it on this. It would not even make up a pixel on this screen right here, much less the actual size of the sun or something much smaller."}, {"video_title": "Scale of the galaxy Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "This is close to the size of the Oort Cloud. And the actual orbit of Earth is about 1... Well, the diameter of the orbit of Earth is about 1 50,000th of this. So you wouldn't even see it on this. It would not even make up a pixel on this screen right here, much less the actual size of the sun or something much smaller. And just remember, that orbit of the Earth, that was that huge distance. It took 8 minutes for light to get from the sun to the Earth. This super long distance."}, {"video_title": "Scale of the galaxy Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It would not even make up a pixel on this screen right here, much less the actual size of the sun or something much smaller. And just remember, that orbit of the Earth, that was that huge distance. It took 8 minutes for light to get from the sun to the Earth. This super long distance. If you shot a bullet at the sun from Earth, it would take you that 17 years to actually get to the sun. So once again, this huge distance wouldn't even show up on this picture. Now, what we saw in the last video is if you travel at unimaginably fast speeds, if you travel at 60,000 kilometers per hour, you would have picked that speed because that's how fast Voyager 1 actually is traveling."}, {"video_title": "Scale of the galaxy Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "This super long distance. If you shot a bullet at the sun from Earth, it would take you that 17 years to actually get to the sun. So once again, this huge distance wouldn't even show up on this picture. Now, what we saw in the last video is if you travel at unimaginably fast speeds, if you travel at 60,000 kilometers per hour, you would have picked that speed because that's how fast Voyager 1 actually is traveling. That's one of the, I think, the fastest object we have out there in space right here. And it's actually kind of leaving the solar system as we speak. But even if you were able to get that fast, it would still take 80,000 years, 75 or 80,000 years, to travel the 4.2 light years to the Alpha Centauri cluster of stars, to the nearest star."}, {"video_title": "Scale of the galaxy Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Now, what we saw in the last video is if you travel at unimaginably fast speeds, if you travel at 60,000 kilometers per hour, you would have picked that speed because that's how fast Voyager 1 actually is traveling. That's one of the, I think, the fastest object we have out there in space right here. And it's actually kind of leaving the solar system as we speak. But even if you were able to get that fast, it would still take 80,000 years, 75 or 80,000 years, to travel the 4.2 light years to the Alpha Centauri cluster of stars, to the nearest star. It would take 80,000 years. And that scale of time is already amount of time that I have trouble comprehending. As you can imagine, all of modern civilization has occurred, definitely in the last 10,000 years, but most of recorded history is in the last 4 or 5,000 years."}, {"video_title": "Scale of the galaxy Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But even if you were able to get that fast, it would still take 80,000 years, 75 or 80,000 years, to travel the 4.2 light years to the Alpha Centauri cluster of stars, to the nearest star. It would take 80,000 years. And that scale of time is already amount of time that I have trouble comprehending. As you can imagine, all of modern civilization has occurred, definitely in the last 10,000 years, but most of recorded history is in the last 4 or 5,000 years. So this is 80,000 years to travel to the nearest star. So it's a huge distance. Another way to think about it is if the sun, if the sun were the size of a basketball and you put that basketball in London, if you wanted to do it in scale, the next closest star, which is actually a smaller basketball, right over here, Proxima Centauri, that smaller basketball, you would have to put in Kiev, Ukraine, in order to have a similar scale."}, {"video_title": "Scale of the galaxy Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "As you can imagine, all of modern civilization has occurred, definitely in the last 10,000 years, but most of recorded history is in the last 4 or 5,000 years. So this is 80,000 years to travel to the nearest star. So it's a huge distance. Another way to think about it is if the sun, if the sun were the size of a basketball and you put that basketball in London, if you wanted to do it in scale, the next closest star, which is actually a smaller basketball, right over here, Proxima Centauri, that smaller basketball, you would have to put in Kiev, Ukraine, in order to have a similar scale. So these are basketballs sitting in these cities and you would have to travel about 1,200 miles to place the next basketball. And these basketballs are representing these super huge things that we saw in the first video. The sun, if you actually made the Earth relative to these basketballs, these would be little grains of sand."}, {"video_title": "Scale of the galaxy Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Another way to think about it is if the sun, if the sun were the size of a basketball and you put that basketball in London, if you wanted to do it in scale, the next closest star, which is actually a smaller basketball, right over here, Proxima Centauri, that smaller basketball, you would have to put in Kiev, Ukraine, in order to have a similar scale. So these are basketballs sitting in these cities and you would have to travel about 1,200 miles to place the next basketball. And these basketballs are representing these super huge things that we saw in the first video. The sun, if you actually made the Earth relative to these basketballs, these would be little grains of sand. So if there are any little small planets over here, they would have to be grains of sand in Kiev, Ukraine, versus the grain of sand in London. So this is a massive, massive distance, already, at least in my mind, unimaginable. And when it gets really racky is when you start realizing that even this is a super, super small distance relative to the galactic scale."}, {"video_title": "Scale of the galaxy Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "The sun, if you actually made the Earth relative to these basketballs, these would be little grains of sand. So if there are any little small planets over here, they would have to be grains of sand in Kiev, Ukraine, versus the grain of sand in London. So this is a massive, massive distance, already, at least in my mind, unimaginable. And when it gets really racky is when you start realizing that even this is a super, super small distance relative to the galactic scale. So this whole depiction of kind of our neighborhood of stars, this thing over here is about, give or take, and we're doing rough estimates right here, it's about 30 light years. I'll just do LY for short. So that's about 30 light years."}, {"video_title": "Scale of the galaxy Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And when it gets really racky is when you start realizing that even this is a super, super small distance relative to the galactic scale. So this whole depiction of kind of our neighborhood of stars, this thing over here is about, give or take, and we're doing rough estimates right here, it's about 30 light years. I'll just do LY for short. So that's about 30 light years. And once again, you can take pictures of our galaxy from our point of view, but you actually can't take a picture of the whole galaxy from above it, so these are going to be artist depictions. But if this is 30 light years, this drawing right here of kind of our local neighborhood of the galaxy, this right here is roughly, these are all approximations, this is about, let me do this in a darker color, this is about 1,000 light years. And this is the 1,000 light years of our sun's neighborhood, if you can even call it a neighborhood anymore."}, {"video_title": "Scale of the galaxy Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So that's about 30 light years. And once again, you can take pictures of our galaxy from our point of view, but you actually can't take a picture of the whole galaxy from above it, so these are going to be artist depictions. But if this is 30 light years, this drawing right here of kind of our local neighborhood of the galaxy, this right here is roughly, these are all approximations, this is about, let me do this in a darker color, this is about 1,000 light years. And this is the 1,000 light years of our sun's neighborhood, if you can even call it a neighborhood anymore. Even this isn't really a neighborhood if it takes you 80,000 years to get to your nearest neighbor. But this whole drawing over here, and it would take forever to get anywhere over here, it would be 1 30th of this, so it would be about that big, this whole drawing. And what's really going to blow your mind is this would be roughly a little bit more than a pixel on this drawing right here that spans 1,000 light years."}, {"video_title": "Scale of the galaxy Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And this is the 1,000 light years of our sun's neighborhood, if you can even call it a neighborhood anymore. Even this isn't really a neighborhood if it takes you 80,000 years to get to your nearest neighbor. But this whole drawing over here, and it would take forever to get anywhere over here, it would be 1 30th of this, so it would be about that big, this whole drawing. And what's really going to blow your mind is this would be roughly a little bit more than a pixel on this drawing right here that spans 1,000 light years. But then when you start to really put it into perspective, so now let's zoom out a little bit, so this drawing right here, this 1,000 light years is now this 1,000 light years over here. So this is the local vicinity of the sun, and once again the word local is used in a very liberal way at this point. So this right here is 1,000 light years."}, {"video_title": "Scale of the galaxy Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And what's really going to blow your mind is this would be roughly a little bit more than a pixel on this drawing right here that spans 1,000 light years. But then when you start to really put it into perspective, so now let's zoom out a little bit, so this drawing right here, this 1,000 light years is now this 1,000 light years over here. So this is the local vicinity of the sun, and once again the word local is used in a very liberal way at this point. So this right here is 1,000 light years. If you're sitting here and you're looking at an object that's sitting, let me do this in a darker color, if we're sitting here on earth and we're looking at an object out here that's 500 light years away, we're looking at it as it was 500 years ago, because the light that is reaching our eyeballs right now, or our telescopes right now, left this guy over here 500 years ago. In fact, he's not going to even be there anymore. He probably has moved around a little bit."}, {"video_title": "Scale of the galaxy Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So this right here is 1,000 light years. If you're sitting here and you're looking at an object that's sitting, let me do this in a darker color, if we're sitting here on earth and we're looking at an object out here that's 500 light years away, we're looking at it as it was 500 years ago, because the light that is reaching our eyeballs right now, or our telescopes right now, left this guy over here 500 years ago. In fact, he's not going to even be there anymore. He probably has moved around a little bit. So just even on this scale, we're already talking about these unimaginably huge distances. And then when we zoom out, this is kind of our local part of the galaxy right over here. This piece right here, this is called the Orion Spur."}, {"video_title": "Scale of the galaxy Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "He probably has moved around a little bit. So just even on this scale, we're already talking about these unimaginably huge distances. And then when we zoom out, this is kind of our local part of the galaxy right over here. This piece right here, this is called the Orion Spur. And people are still trying to work out exactly what the actual details of the actual shape of the Milky Way galaxy, the galaxy that we're in. But we're pretty sure, we're very sure we have these spiral arms and we have these spurs off of them, but it's actually very hard to come up with the actual shape, especially because you can't see a lot of the galaxy because it's kind of on the other side, on the other side of the center. But really just to get a sense of something that at least, I mean it blows my mind if you really think about what it's saying."}, {"video_title": "Scale of the galaxy Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "This piece right here, this is called the Orion Spur. And people are still trying to work out exactly what the actual details of the actual shape of the Milky Way galaxy, the galaxy that we're in. But we're pretty sure, we're very sure we have these spiral arms and we have these spurs off of them, but it's actually very hard to come up with the actual shape, especially because you can't see a lot of the galaxy because it's kind of on the other side, on the other side of the center. But really just to get a sense of something that at least, I mean it blows my mind if you really think about what it's saying. These unbelievable distances show up as a little dot here. This whole drawing shows up as a dot here. Now when we zoom out, over here that dot would no longer even show up."}, {"video_title": "Scale of the galaxy Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But really just to get a sense of something that at least, I mean it blows my mind if you really think about what it's saying. These unbelievable distances show up as a little dot here. This whole drawing shows up as a dot here. Now when we zoom out, over here that dot would no longer even show up. It wouldn't even register a pixel on this drawing right over here. And then this whole drawing, this whole thing right over here, this whole picture is this grid right over here. It is this right over here."}, {"video_title": "Scale of the galaxy Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Now when we zoom out, over here that dot would no longer even show up. It wouldn't even register a pixel on this drawing right over here. And then this whole drawing, this whole thing right over here, this whole picture is this grid right over here. It is this right over here. So hopefully that gives you a sense of how small, how small even our local neighborhood is relative to the galaxy as a whole. And the galaxy as a whole, just to give you a sense, has 200 to 400 billion, billion stars. Billion stars, or maybe I should say solar systems, just to give you a sense that when we saw the solar system, it's not just the sun, there's all this neat dynamic stuff in there, planets and asteroids and solar winds."}, {"video_title": "Scale of the galaxy Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It is this right over here. So hopefully that gives you a sense of how small, how small even our local neighborhood is relative to the galaxy as a whole. And the galaxy as a whole, just to give you a sense, has 200 to 400 billion, billion stars. Billion stars, or maybe I should say solar systems, just to give you a sense that when we saw the solar system, it's not just the sun, there's all this neat dynamic stuff in there, planets and asteroids and solar winds. So there's 200 to 400 billion stars and for the most part, 200 to 400 billion solar systems. So it's an unimaginably, I guess, complex or huge place. And just to make it clear, even when we zoom in to this picture right here, and I think it was obvious based on telling you about this, that these little white pockets right here, this isn't one star, this isn't two stars, these are thousands of stars here."}, {"video_title": "Scale of the galaxy Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Billion stars, or maybe I should say solar systems, just to give you a sense that when we saw the solar system, it's not just the sun, there's all this neat dynamic stuff in there, planets and asteroids and solar winds. So there's 200 to 400 billion stars and for the most part, 200 to 400 billion solar systems. So it's an unimaginably, I guess, complex or huge place. And just to make it clear, even when we zoom in to this picture right here, and I think it was obvious based on telling you about this, that these little white pockets right here, this isn't one star, this isn't two stars, these are thousands of stars here. So when you go over here, each little blotch of white that you see, that's not a star, that's not a thousand stars, we're starting to talk in the millions of stars when you look at certain blotches here and there. Maybe it might be one star that's closer to you, or it might be a million stars that are far apart and that are just relatively close together and everything has to be used in kind of loose terms here. And we'll talk more about other galaxies, but even this isn't the upper bound of galaxies."}, {"video_title": "Scale of the galaxy Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And just to make it clear, even when we zoom in to this picture right here, and I think it was obvious based on telling you about this, that these little white pockets right here, this isn't one star, this isn't two stars, these are thousands of stars here. So when you go over here, each little blotch of white that you see, that's not a star, that's not a thousand stars, we're starting to talk in the millions of stars when you look at certain blotches here and there. Maybe it might be one star that's closer to you, or it might be a million stars that are far apart and that are just relatively close together and everything has to be used in kind of loose terms here. And we'll talk more about other galaxies, but even this isn't the upper bound of galaxies. People believe the Andromeda Galaxy has a trillion stars in it, a trillion solar systems. We're talking about these huge, huge, immense distances. And so just to give you a sense of where we fit in the picture, this is a rough location of our sun."}, {"video_title": "Scale of the galaxy Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And we'll talk more about other galaxies, but even this isn't the upper bound of galaxies. People believe the Andromeda Galaxy has a trillion stars in it, a trillion solar systems. We're talking about these huge, huge, immense distances. And so just to give you a sense of where we fit in the picture, this is a rough location of our sun. And remember, that little dot I drew just now is including millions of stars, millions of solar systems, already unimaginable distances. But if you really want to get the sense relative to the whole galaxy, this is an artist's depiction. Again, we could never obviously get this perspective on the galaxy."}, {"video_title": "Scale of the galaxy Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And so just to give you a sense of where we fit in the picture, this is a rough location of our sun. And remember, that little dot I drew just now is including millions of stars, millions of solar systems, already unimaginable distances. But if you really want to get the sense relative to the whole galaxy, this is an artist's depiction. Again, we could never obviously get this perspective on the galaxy. It would take us forever to travel this far so that you could see the galaxy from above. But this is our best guess looking at things from our vantage point. We actually can't even see this whole area over here because it's on the other side of the center of the galaxy, which is super, super dense and super bright."}, {"video_title": "Scale of the galaxy Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Again, we could never obviously get this perspective on the galaxy. It would take us forever to travel this far so that you could see the galaxy from above. But this is our best guess looking at things from our vantage point. We actually can't even see this whole area over here because it's on the other side of the center of the galaxy, which is super, super dense and super bright. And so it's very hard to see things on the other side. We think, or actually there's a super massive black hole at the center of the galaxy, and we think that they're at the center of all or most galaxies. But the whole point of this video, actually this whole series of videos, this is just kind of, I don't know, put you in awe a little bit of just how huge this is."}, {"video_title": "Scale of the galaxy Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "We actually can't even see this whole area over here because it's on the other side of the center of the galaxy, which is super, super dense and super bright. And so it's very hard to see things on the other side. We think, or actually there's a super massive black hole at the center of the galaxy, and we think that they're at the center of all or most galaxies. But the whole point of this video, actually this whole series of videos, this is just kind of, I don't know, put you in awe a little bit of just how huge this is. Because when you really think about the scales, it's, I don't know, no words can really describe it. But just to give you a sense, we're about 25,000 light years from the center of the galaxy. So even when we look at things in the center of the galaxy, that's as they were 25,000 years ago."}, {"video_title": "Scale of the galaxy Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But the whole point of this video, actually this whole series of videos, this is just kind of, I don't know, put you in awe a little bit of just how huge this is. Because when you really think about the scales, it's, I don't know, no words can really describe it. But just to give you a sense, we're about 25,000 light years from the center of the galaxy. So even when we look at things in the center of the galaxy, that's as they were 25,000 years ago. It took 25,000 years for that light to get to us. I mean, when that light left the center of the galaxy, I won't even guess to think what humanity was like at that point in time. So it's these huge distances, and the whole galaxy over here, and once again, like solar systems, it's hard to say the edge of the galaxy because there's always going to be a few more stars and other things orbiting around the galaxy as you go further and further out."}, {"video_title": "Scale of the galaxy Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So even when we look at things in the center of the galaxy, that's as they were 25,000 years ago. It took 25,000 years for that light to get to us. I mean, when that light left the center of the galaxy, I won't even guess to think what humanity was like at that point in time. So it's these huge distances, and the whole galaxy over here, and once again, like solar systems, it's hard to say the edge of the galaxy because there's always going to be a few more stars and other things orbiting around the galaxy as you go further and further out. But it gets less dense with stars. But the main density, the main disk, is about 100,000 light years. Is the diameter, roughly, of the main part of the galaxy."}, {"video_title": "Scale of the galaxy Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So it's these huge distances, and the whole galaxy over here, and once again, like solar systems, it's hard to say the edge of the galaxy because there's always going to be a few more stars and other things orbiting around the galaxy as you go further and further out. But it gets less dense with stars. But the main density, the main disk, is about 100,000 light years. Is the diameter, roughly, of the main part of the galaxy. And it's about 1,000 light years thick. So you kind of imagine it as this disk, this thing that's fairly flat. But it's 1,000 light years thick."}, {"video_title": "Scale of the galaxy Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Is the diameter, roughly, of the main part of the galaxy. And it's about 1,000 light years thick. So you kind of imagine it as this disk, this thing that's fairly flat. But it's 1,000 light years thick. You would have to do this distance 250 times just to go from the top part of the galaxy to the bottom part, much less going across the galaxy. So it might seem relatively flat, but it's still immensely, immensely thick. And just as another way to visualize it, if this thing right over here that includes the Oort cloud, roughly a light year in diameter, is a grain of sand, a millimeter in diameter grain of sand, then the universe as a whole is going to be the diameter of a football field."}, {"video_title": "Scale of the galaxy Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But it's 1,000 light years thick. You would have to do this distance 250 times just to go from the top part of the galaxy to the bottom part, much less going across the galaxy. So it might seem relatively flat, but it's still immensely, immensely thick. And just as another way to visualize it, if this thing right over here that includes the Oort cloud, roughly a light year in diameter, is a grain of sand, a millimeter in diameter grain of sand, then the universe as a whole is going to be the diameter of a football field. And that might say, okay, those are two tractable things. I can imagine a grain of sand, a millimeter wide grain of sand in a football field. But remember, remember that grain of sand is still 50,000 or 60,000 times the diameter of Earth's orbit."}, {"video_title": "Scale of the galaxy Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And just as another way to visualize it, if this thing right over here that includes the Oort cloud, roughly a light year in diameter, is a grain of sand, a millimeter in diameter grain of sand, then the universe as a whole is going to be the diameter of a football field. And that might say, okay, those are two tractable things. I can imagine a grain of sand, a millimeter wide grain of sand in a football field. But remember, remember that grain of sand is still 50,000 or 60,000 times the diameter of Earth's orbit. And Earth's orbit, it would still take a bullet or something traveling as fast as a jet plane 15 hours to just go half of that, or sorry, 15 years or 17 years. I forgot the exact number, but 15, 16, 17 years to even cover half of that distance, 30 years to cover an entire diameter of Earth. So 30 years just to cover the diameter of Earth's orbit, that's 1 60,000th of our little grain of sand in the football field."}, {"video_title": "Scale of the galaxy Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But remember, remember that grain of sand is still 50,000 or 60,000 times the diameter of Earth's orbit. And Earth's orbit, it would still take a bullet or something traveling as fast as a jet plane 15 hours to just go half of that, or sorry, 15 years or 17 years. I forgot the exact number, but 15, 16, 17 years to even cover half of that distance, 30 years to cover an entire diameter of Earth. So 30 years just to cover the diameter of Earth's orbit, that's 1 60,000th of our little grain of sand in the football field. And just to kind of really, I don't know, have an appreciation for how mind-blowing this really is, this is actually a picture of the Milky Way galaxy, our galaxy, from our vantage point. As you can see, we're in the galaxy, so we're seeing, and this is looking towards the center. And even this picture, you start to appreciate the complexity of what 100 billion stars are."}, {"video_title": "Scale of the galaxy Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So 30 years just to cover the diameter of Earth's orbit, that's 1 60,000th of our little grain of sand in the football field. And just to kind of really, I don't know, have an appreciation for how mind-blowing this really is, this is actually a picture of the Milky Way galaxy, our galaxy, from our vantage point. As you can see, we're in the galaxy, so we're seeing, and this is looking towards the center. And even this picture, you start to appreciate the complexity of what 100 billion stars are. But what I really want to point out is even in this picture, when you're looking at these things, some of these things that look like stars, those aren't stars, those are thousands of stars or millions of stars. Maybe it could be one star closer up, but when we're starting to approach the center of the galaxy, these are thousands and thousands and millions of stars or solar systems that we're actually looking at. So it really starts to boggle the mind to imagine what might actually be going on over there."}, {"video_title": "Scale of earth and sun Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But we'll at least give our best shot. So I think most of us watching this video know that this right here is Earth. And just to get a sense of scale here, I think probably the largest distance that we can somehow relate to is about 100 miles. You can get into a car for an hour, hour and a half and go about 100 miles. And on the Earth, that would be about this far. It would be a speck that would look something like that. That is 100 miles."}, {"video_title": "Scale of earth and sun Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "You can get into a car for an hour, hour and a half and go about 100 miles. And on the Earth, that would be about this far. It would be a speck that would look something like that. That is 100 miles. And also to get a bit of scale, let's think about a speed that at least we can kind of comprehend. And that would be maybe the speed of a bullet. Maybe we can't comprehend it, but I'll say this is the fastest thing that we can maybe kind of comprehend."}, {"video_title": "Scale of earth and sun Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "That is 100 miles. And also to get a bit of scale, let's think about a speed that at least we can kind of comprehend. And that would be maybe the speed of a bullet. Maybe we can't comprehend it, but I'll say this is the fastest thing that we can maybe kind of comprehend. It goes about, and there are different types of bullets depending on the type of gun and all of that, about 280 meters per second, which is about 1,000 kilometers per hour. And this is also roughly the speed of a jet. So just to give a sense of scale here, the Earth's circumference, so if you were to go around the planet, the Earth's circumference, just like that, is about 40,000 kilometers."}, {"video_title": "Scale of earth and sun Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Maybe we can't comprehend it, but I'll say this is the fastest thing that we can maybe kind of comprehend. It goes about, and there are different types of bullets depending on the type of gun and all of that, about 280 meters per second, which is about 1,000 kilometers per hour. And this is also roughly the speed of a jet. So just to give a sense of scale here, the Earth's circumference, so if you were to go around the planet, the Earth's circumference, just like that, is about 40,000 kilometers. It is 40,000 kilometers. So if you were to travel at the speed of a bullet or the speed of a jetliner, at 1,000 kilometers an hour, it would take you 40 hours to circumnavigate the Earth. 40 hours to go around the Earth."}, {"video_title": "Scale of earth and sun Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So just to give a sense of scale here, the Earth's circumference, so if you were to go around the planet, the Earth's circumference, just like that, is about 40,000 kilometers. It is 40,000 kilometers. So if you were to travel at the speed of a bullet or the speed of a jetliner, at 1,000 kilometers an hour, it would take you 40 hours to circumnavigate the Earth. 40 hours to go around the Earth. And I think none of this information is too surprising. You might have taken 12 or 15 hour flights that get you not all the way around the Earth, but get you pretty far, San Francisco to Australia or something like that. So right now these aren't at scales that are too crazy, although, you know, even for me, even the Earth itself is a pretty mind-blowingly large object."}, {"video_title": "Scale of earth and sun Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "40 hours to go around the Earth. And I think none of this information is too surprising. You might have taken 12 or 15 hour flights that get you not all the way around the Earth, but get you pretty far, San Francisco to Australia or something like that. So right now these aren't at scales that are too crazy, although, you know, even for me, even the Earth itself is a pretty mind-blowingly large object. Now, with that out of the way, let's think about the Sun, because the Sun starts to approach something far huger. So this obviously here is the Sun. And I think most people appreciate that the Sun is larger, that it's much larger than the Earth, and that it's pretty far away from the Earth."}, {"video_title": "Scale of earth and sun Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So right now these aren't at scales that are too crazy, although, you know, even for me, even the Earth itself is a pretty mind-blowingly large object. Now, with that out of the way, let's think about the Sun, because the Sun starts to approach something far huger. So this obviously here is the Sun. And I think most people appreciate that the Sun is larger, that it's much larger than the Earth, and that it's pretty far away from the Earth. But I don't think most people, including myself, fully appreciate how large the Sun is or how far it is away from the Earth. So just to give you a sense, the Sun is 109 times the circumference of the Earth. So if we do that same thought exercise there, if we said, OK, if I'm traveling at the speed of a bullet or the speed of a jetliner, it would take me 40 hours to go around the Earth."}, {"video_title": "Scale of earth and sun Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And I think most people appreciate that the Sun is larger, that it's much larger than the Earth, and that it's pretty far away from the Earth. But I don't think most people, including myself, fully appreciate how large the Sun is or how far it is away from the Earth. So just to give you a sense, the Sun is 109 times the circumference of the Earth. So if we do that same thought exercise there, if we said, OK, if I'm traveling at the speed of a bullet or the speed of a jetliner, it would take me 40 hours to go around the Earth. Well, how long would it take to go around the Sun? So if you were to get on a jet plane and try to go around the Sun, or if you were to somehow ride a bullet and try to go around the Sun, do a complete circumnavigation of the Sun, it's going to take you 109 times as long as it would have taken you to do the Earth. So it would be 100 times, I could do 109, but just for approximate."}, {"video_title": "Scale of earth and sun Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So if we do that same thought exercise there, if we said, OK, if I'm traveling at the speed of a bullet or the speed of a jetliner, it would take me 40 hours to go around the Earth. Well, how long would it take to go around the Sun? So if you were to get on a jet plane and try to go around the Sun, or if you were to somehow ride a bullet and try to go around the Sun, do a complete circumnavigation of the Sun, it's going to take you 109 times as long as it would have taken you to do the Earth. So it would be 100 times, I could do 109, but just for approximate. It's roughly 100 times the circumference of the Earth. So 109 times 40 is equal to 4,000 hours. And just to get a sense of what 4,000 is, actually, since I have the calculator out, let's do the exact calculation."}, {"video_title": "Scale of earth and sun Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So it would be 100 times, I could do 109, but just for approximate. It's roughly 100 times the circumference of the Earth. So 109 times 40 is equal to 4,000 hours. And just to get a sense of what 4,000 is, actually, since I have the calculator out, let's do the exact calculation. It's 109 times the circumference of the Earth times 40 hours. That's what it would take to do the circumference of the Earth. So it's 4,360 hours to circumnavigate the Sun going at the speed of a bullet or a jetliner."}, {"video_title": "Scale of earth and sun Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And just to get a sense of what 4,000 is, actually, since I have the calculator out, let's do the exact calculation. It's 109 times the circumference of the Earth times 40 hours. That's what it would take to do the circumference of the Earth. So it's 4,360 hours to circumnavigate the Sun going at the speed of a bullet or a jetliner. And so that is 24 hours of the day. That is 181 days. It would take you roughly half a year."}, {"video_title": "Scale of earth and sun Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So it's 4,360 hours to circumnavigate the Sun going at the speed of a bullet or a jetliner. And so that is 24 hours of the day. That is 181 days. It would take you roughly half a year. It would take half a year to go around the Sun at the speed of a jetliner. Let me write this down. Half a year."}, {"video_title": "Scale of earth and sun Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It would take you roughly half a year. It would take half a year to go around the Sun at the speed of a jetliner. Let me write this down. Half a year. The Sun is huge. That by itself may or may not be... Actually, let me give you a sense of scale here because I have this other diagram of a Sun. We'll talk more about the rest of the solar system in the next video."}, {"video_title": "Scale of earth and sun Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Half a year. The Sun is huge. That by itself may or may not be... Actually, let me give you a sense of scale here because I have this other diagram of a Sun. We'll talk more about the rest of the solar system in the next video. But over here, at this scale, at the Sun, at least on my screen, if I were to complete it, it would probably be about 20 inches in diameter. The Earth is just this little thing over here, smaller than a raindrop. It's just a small little thing over here."}, {"video_title": "Scale of earth and sun Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "We'll talk more about the rest of the solar system in the next video. But over here, at this scale, at the Sun, at least on my screen, if I were to complete it, it would probably be about 20 inches in diameter. The Earth is just this little thing over here, smaller than a raindrop. It's just a small little thing over here. If I were to draw it on this scale where the Sun is even smaller, the Earth would be right about that big. Now, what isn't obvious, because we've all done our science projects in third and fourth grade or we always see these diagrams of the solar system that look something like this, is that these planets are way further away. Even though these are depicted to scale, they're way further away from the Sun than this makes it look."}, {"video_title": "Scale of earth and sun Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It's just a small little thing over here. If I were to draw it on this scale where the Sun is even smaller, the Earth would be right about that big. Now, what isn't obvious, because we've all done our science projects in third and fourth grade or we always see these diagrams of the solar system that look something like this, is that these planets are way further away. Even though these are depicted to scale, they're way further away from the Sun than this makes it look. The Earth is 150 million kilometers from the Sun. If this is the Sun right here, at this scale, you wouldn't even be able to see the Earth. It wouldn't even be a pixel, but it would be 150 million kilometers from the Earth."}, {"video_title": "Scale of earth and sun Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Even though these are depicted to scale, they're way further away from the Sun than this makes it look. The Earth is 150 million kilometers from the Sun. If this is the Sun right here, at this scale, you wouldn't even be able to see the Earth. It wouldn't even be a pixel, but it would be 150 million kilometers from the Earth. This distance right here is called an astronomical unit. We'll be using that term in the next few videos, just because it's an easier way to think about distance. Sometimes abbreviated AU, astronomical unit."}, {"video_title": "Scale of earth and sun Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It wouldn't even be a pixel, but it would be 150 million kilometers from the Earth. This distance right here is called an astronomical unit. We'll be using that term in the next few videos, just because it's an easier way to think about distance. Sometimes abbreviated AU, astronomical unit. Just to give a sense of how far this is, light, which is something that we think is almost infinitely fast, something that looks instantaneous, that takes eight minutes to travel from the Sun to the Earth. If the Sun were to disappear, it would take eight minutes for that light for us to know that it disappeared on Earth. Or another way, just to put it in the sense of this jet airplane, let's get the calculator back out."}, {"video_title": "Scale of earth and sun Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Sometimes abbreviated AU, astronomical unit. Just to give a sense of how far this is, light, which is something that we think is almost infinitely fast, something that looks instantaneous, that takes eight minutes to travel from the Sun to the Earth. If the Sun were to disappear, it would take eight minutes for that light for us to know that it disappeared on Earth. Or another way, just to put it in the sense of this jet airplane, let's get the calculator back out. We're talking about 150 million kilometers. If we're going at 1,000 kilometers an hour, it would take us 150,000 hours at the speed of a bullet or at the speed of a jet plane to get to the Sun. Just to put that in perspective, if we wanted it in days, there's 24 hours per day."}, {"video_title": "Scale of earth and sun Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Or another way, just to put it in the sense of this jet airplane, let's get the calculator back out. We're talking about 150 million kilometers. If we're going at 1,000 kilometers an hour, it would take us 150,000 hours at the speed of a bullet or at the speed of a jet plane to get to the Sun. Just to put that in perspective, if we wanted it in days, there's 24 hours per day. This would be 6,250 days, or if we divide by 365, roughly 17 years. If you were to shoot a bullet straight at the Sun, it would take 17 years to get there, if it could maintain its velocity somehow. This would take a bullet or a jet plane 17 years to get to the Sun."}, {"video_title": "Scale of earth and sun Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Just to put that in perspective, if we wanted it in days, there's 24 hours per day. This would be 6,250 days, or if we divide by 365, roughly 17 years. If you were to shoot a bullet straight at the Sun, it would take 17 years to get there, if it could maintain its velocity somehow. This would take a bullet or a jet plane 17 years to get to the Sun. 17 years. Or another way to visualize it, this Sun right over here, it looks on my screen, it has about a 5 or 6 inch diameter. If I were to actually do it as scale, this little dot right here, this little dot which is the Earth, this speck, if I actually wanted to draw this distance at scale, I would have to put this speck about 50 feet away from the Sun."}, {"video_title": "Scale of earth and sun Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "This would take a bullet or a jet plane 17 years to get to the Sun. 17 years. Or another way to visualize it, this Sun right over here, it looks on my screen, it has about a 5 or 6 inch diameter. If I were to actually do it as scale, this little dot right here, this little dot which is the Earth, this speck, if I actually wanted to draw this distance at scale, I would have to put this speck about 50 feet away from the Sun. 50 or 60 feet away from the Sun. If you were to look at the solar system, and obviously there's other things in the solar system, we'll talk more about them in the next video, you wouldn't even notice this speck. This is a little dust thing flying around this Sun."}, {"video_title": "Scale of earth and sun Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "If I were to actually do it as scale, this little dot right here, this little dot which is the Earth, this speck, if I actually wanted to draw this distance at scale, I would have to put this speck about 50 feet away from the Sun. 50 or 60 feet away from the Sun. If you were to look at the solar system, and obviously there's other things in the solar system, we'll talk more about them in the next video, you wouldn't even notice this speck. This is a little dust thing flying around this Sun. As we go further and further out of this solar system, you're going to see even this distance starts to become ridiculously small. Or another way to think about it, if the Sun was about this size, then this speck, then the Earth at this scale would be about 200 feet away from it. So you can imagine, if you had a football field, these are the end zones, one end zone, another end zone, and if you were to stick something maybe the size of a medicine ball, a little bit bigger than a basketball at one end zone, this little speck would be about 60 yards away, or roughly 60 meters away."}, {"video_title": "Scale of earth and sun Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "This is a little dust thing flying around this Sun. As we go further and further out of this solar system, you're going to see even this distance starts to become ridiculously small. Or another way to think about it, if the Sun was about this size, then this speck, then the Earth at this scale would be about 200 feet away from it. So you can imagine, if you had a football field, these are the end zones, one end zone, another end zone, and if you were to stick something maybe the size of a medicine ball, a little bit bigger than a basketball at one end zone, this little speck would be about 60 yards away, or roughly 60 meters away. So it's a little speck, you wouldn't even notice it on the scale of a football field, something this size. Anyway, I'm going to leave you there. Hopefully that gives you just, starts to blow your mind when you think about just the scale of the Sun, the Earth, and how far the Earth is away from the Sun."}, {"video_title": "Supermassive black holes Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And we called these points black holes. And we learned there's an event horizon around these black holes, and if anything gets closer or goes within the boundary of that event horizon, there's no way they can ever escape from the black hole. All it can do is get closer and closer to the black hole, and that includes light, and that's why it's called a black hole. So even though all of the mass is at the central point, this entire area, or the entire surface of the event horizon, I'll do it in purple, because it's supposed to be black, this entire thing will appear black. It will emit no light. Now, these type of black holes that we described, we call those stellar black holes. And that's because they're formed from collapsing massive stars."}, {"video_title": "Supermassive black holes Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So even though all of the mass is at the central point, this entire area, or the entire surface of the event horizon, I'll do it in purple, because it's supposed to be black, this entire thing will appear black. It will emit no light. Now, these type of black holes that we described, we call those stellar black holes. And that's because they're formed from collapsing massive stars. And the largest stellar black holes that we've observed are on the order of 33 solar masses, give or take. So very massive to begin with. Let's just be clear, and this is what the remnant of the star has to be."}, {"video_title": "Supermassive black holes Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And that's because they're formed from collapsing massive stars. And the largest stellar black holes that we've observed are on the order of 33 solar masses, give or take. So very massive to begin with. Let's just be clear, and this is what the remnant of the star has to be. So a lot more of the original star's mass might have been pushed off in supernovae, plural of supernova. Now, there's another class of black holes here, and these are somewhat mysterious. And they're called supermassive black holes."}, {"video_title": "Supermassive black holes Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Let's just be clear, and this is what the remnant of the star has to be. So a lot more of the original star's mass might have been pushed off in supernovae, plural of supernova. Now, there's another class of black holes here, and these are somewhat mysterious. And they're called supermassive black holes. Supermassive black holes. And to some degree, the word super isn't big enough. Supermassive black holes."}, {"video_title": "Supermassive black holes Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And they're called supermassive black holes. Supermassive black holes. And to some degree, the word super isn't big enough. Supermassive black holes. Because they're not just a little bit more massive than stellar black holes, they're a lot more massive. They're on the order of hundreds of thousands to billions of solar masses. 100,000s to billions times the mass of our sun."}, {"video_title": "Supermassive black holes Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Supermassive black holes. Because they're not just a little bit more massive than stellar black holes, they're a lot more massive. They're on the order of hundreds of thousands to billions of solar masses. 100,000s to billions times the mass of our sun. And what's interesting about these, other than the fact that they're super huge, is that there doesn't seem to be black holes in between. Or at least we haven't observed black holes in between. The largest stellar black hole's 33 solar masses."}, {"video_title": "Supermassive black holes Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "100,000s to billions times the mass of our sun. And what's interesting about these, other than the fact that they're super huge, is that there doesn't seem to be black holes in between. Or at least we haven't observed black holes in between. The largest stellar black hole's 33 solar masses. And then there are these supermassive black holes that we think exist. And we think they mainly exist in the centers of galaxies. And we think most, if not all, centers of galaxies actually have one of these supermassive black holes."}, {"video_title": "Supermassive black holes Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "The largest stellar black hole's 33 solar masses. And then there are these supermassive black holes that we think exist. And we think they mainly exist in the centers of galaxies. And we think most, if not all, centers of galaxies actually have one of these supermassive black holes. But it's kind of an interesting question. If all black holes were formed from collapsing stars, wouldn't we see things in between? So one theory of how these really massive black holes form is that you have a regular stellar black hole in an area that has a lot of matter that it can accrete around it."}, {"video_title": "Supermassive black holes Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And we think most, if not all, centers of galaxies actually have one of these supermassive black holes. But it's kind of an interesting question. If all black holes were formed from collapsing stars, wouldn't we see things in between? So one theory of how these really massive black holes form is that you have a regular stellar black hole in an area that has a lot of matter that it can accrete around it. So let's imagine you have a regular, so I'll draw the, this is the event horizon around it. The actual black hole's going to be in the center of that, or the mass of the black hole will be in the center of it. And then over time, you have just more and more mass just falling into this black hole."}, {"video_title": "Supermassive black holes Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So one theory of how these really massive black holes form is that you have a regular stellar black hole in an area that has a lot of matter that it can accrete around it. So let's imagine you have a regular, so I'll draw the, this is the event horizon around it. The actual black hole's going to be in the center of that, or the mass of the black hole will be in the center of it. And then over time, you have just more and more mass just falling into this black hole. Just more and more stuff just keeps falling into this black hole, and then it just keeps growing. And so this could be a plausible reason, or at least the mass in the center keeps growing, and so the event horizon will also keep growing in radius. Now, this is a plausible explanation based on our current understanding."}, {"video_title": "Supermassive black holes Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And then over time, you have just more and more mass just falling into this black hole. Just more and more stuff just keeps falling into this black hole, and then it just keeps growing. And so this could be a plausible reason, or at least the mass in the center keeps growing, and so the event horizon will also keep growing in radius. Now, this is a plausible explanation based on our current understanding. But the reason why this one doesn't gel that well is if this was the explanation for supermassive black holes, you expect to see more black holes in between, maybe black holes with 100 solar masses, or 1,000 solar masses, or 10,000 solar masses. But we're not seeing those right now. We just see the stellar black holes, and we see the supermassive black holes."}, {"video_title": "Supermassive black holes Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Now, this is a plausible explanation based on our current understanding. But the reason why this one doesn't gel that well is if this was the explanation for supermassive black holes, you expect to see more black holes in between, maybe black holes with 100 solar masses, or 1,000 solar masses, or 10,000 solar masses. But we're not seeing those right now. We just see the stellar black holes, and we see the supermassive black holes. So another possible explanation, this is where my inclinations lean towards this one because it kind of explains the gap, is that these supermassive black holes actually formed shortly after the Big Bang. These are primordial black holes. These started near the beginning of our universe."}, {"video_title": "Supermassive black holes Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "We just see the stellar black holes, and we see the supermassive black holes. So another possible explanation, this is where my inclinations lean towards this one because it kind of explains the gap, is that these supermassive black holes actually formed shortly after the Big Bang. These are primordial black holes. These started near the beginning of our universe. Now remember, what do you need to have a black hole? You need to have an amazingly dense amount of matter or a dense amount of mass. If you have a lot of mass in a very small volume, then their gravitational pull will pull them closer and closer together, and they'll be able to overcome all of the electron degeneracy pressures, and the neutron degeneracy pressures, and the quark degeneracy pressures to really collapse into what we think is a single point."}, {"video_title": "Supermassive black holes Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "These started near the beginning of our universe. Now remember, what do you need to have a black hole? You need to have an amazingly dense amount of matter or a dense amount of mass. If you have a lot of mass in a very small volume, then their gravitational pull will pull them closer and closer together, and they'll be able to overcome all of the electron degeneracy pressures, and the neutron degeneracy pressures, and the quark degeneracy pressures to really collapse into what we think is a single point. I want to be clear here, too. We don't know it's a single point. We've never gone into the center of a black hole."}, {"video_title": "Supermassive black holes Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "If you have a lot of mass in a very small volume, then their gravitational pull will pull them closer and closer together, and they'll be able to overcome all of the electron degeneracy pressures, and the neutron degeneracy pressures, and the quark degeneracy pressures to really collapse into what we think is a single point. I want to be clear here, too. We don't know it's a single point. We've never gone into the center of a black hole. Just the mathematics of the black holes, or at least as we understand it right now, have everything colliding into a single point where the math starts to break down. So we're really not sure what happens at that very small center point. But needless to say, it will be an unbelievably, maybe infinite, maybe almost infinitely dense point in space, or dense amount of matter."}, {"video_title": "Supermassive black holes Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "We've never gone into the center of a black hole. Just the mathematics of the black holes, or at least as we understand it right now, have everything colliding into a single point where the math starts to break down. So we're really not sure what happens at that very small center point. But needless to say, it will be an unbelievably, maybe infinite, maybe almost infinitely dense point in space, or dense amount of matter. And the reason why I kind of favor this primordial black hole and why this would make sense is right after the formation of the universe, all of the matter was in the universe, was in a much denser space, because the universe was smaller. So let's say that this is right after the Big Bang, some period of time after the Big Bang. Now, what we've talked about before when we talked about cosmic background radiation is at that point, the universe was relatively uniform."}, {"video_title": "Supermassive black holes Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But needless to say, it will be an unbelievably, maybe infinite, maybe almost infinitely dense point in space, or dense amount of matter. And the reason why I kind of favor this primordial black hole and why this would make sense is right after the formation of the universe, all of the matter was in the universe, was in a much denser space, because the universe was smaller. So let's say that this is right after the Big Bang, some period of time after the Big Bang. Now, what we've talked about before when we talked about cosmic background radiation is at that point, the universe was relatively uniform. It was super, super dense, but it was relatively uniform. So a universe like this, there's no reason why anything would collapse into black holes, because if you look at a point here, sure, there's a ton of mass very close to it, but it's very close to it in every direction. So it would be pulled, the gravitational force would be the same in every direction, if it was completely uniform."}, {"video_title": "Supermassive black holes Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Now, what we've talked about before when we talked about cosmic background radiation is at that point, the universe was relatively uniform. It was super, super dense, but it was relatively uniform. So a universe like this, there's no reason why anything would collapse into black holes, because if you look at a point here, sure, there's a ton of mass very close to it, but it's very close to it in every direction. So it would be pulled, the gravitational force would be the same in every direction, if it was completely uniform. But if you go shortly after the Big Bang, maybe because of slight quantum fluctuation effects, it becomes slightly non-uniform. So let's say it becomes slightly non-uniform, but it still is unbelievably dense. So let's say it looks something like this, where you have areas that are denser, but it's slightly non-uniform."}, {"video_title": "Supermassive black holes Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So it would be pulled, the gravitational force would be the same in every direction, if it was completely uniform. But if you go shortly after the Big Bang, maybe because of slight quantum fluctuation effects, it becomes slightly non-uniform. So let's say it becomes slightly non-uniform, but it still is unbelievably dense. So let's say it looks something like this, where you have areas that are denser, but it's slightly non-uniform. But extremely dense. So here, all of a sudden, you have the type of densities necessary for a black hole. And where you have higher densities, where it's less uniform, here, all of a sudden, you will have inward force."}, {"video_title": "Supermassive black holes Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So let's say it looks something like this, where you have areas that are denser, but it's slightly non-uniform. But extremely dense. So here, all of a sudden, you have the type of densities necessary for a black hole. And where you have higher densities, where it's less uniform, here, all of a sudden, you will have inward force. The gravitational pull from things outside of this area are going to be less than the gravitational pull towards those areas. And the more things get pulled towards it, the less uniform it's going to get. So you can imagine, in that primordial universe, that very shortly after the Big Bang, when things were very dense and closely packed together, we may have had the conditions where these supermassive black holes could have formed."}, {"video_title": "Supermassive black holes Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And where you have higher densities, where it's less uniform, here, all of a sudden, you will have inward force. The gravitational pull from things outside of this area are going to be less than the gravitational pull towards those areas. And the more things get pulled towards it, the less uniform it's going to get. So you can imagine, in that primordial universe, that very shortly after the Big Bang, when things were very dense and closely packed together, we may have had the conditions where these supermassive black holes could have formed. Where we had so much mass in such a small volume, and it was just not uniform enough so that you could have this snowballing effect. So that more and more mass would collect into these supermassive black holes that are hundreds of thousands to billions of times the mass of the sun. And this is maybe the even more interesting part."}, {"video_title": "Supermassive black holes Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So you can imagine, in that primordial universe, that very shortly after the Big Bang, when things were very dense and closely packed together, we may have had the conditions where these supermassive black holes could have formed. Where we had so much mass in such a small volume, and it was just not uniform enough so that you could have this snowballing effect. So that more and more mass would collect into these supermassive black holes that are hundreds of thousands to billions of times the mass of the sun. And this is maybe the even more interesting part. Those black holes would become the centers of future galaxies. So you have these black holes forming, these supermassive black holes forming. And then not everything would go into a black hole."}, {"video_title": "Supermassive black holes Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And this is maybe the even more interesting part. Those black holes would become the centers of future galaxies. So you have these black holes forming, these supermassive black holes forming. And then not everything would go into a black hole. Only if it didn't have a lot of angular velocity, then it might go into the black hole. But if it's going past it fast enough, it'll just start going in orbit around the black hole. And so you can imagine that this is how the early galaxies, or even our galaxy, formed."}, {"video_title": "Supermassive black holes Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And then not everything would go into a black hole. Only if it didn't have a lot of angular velocity, then it might go into the black hole. But if it's going past it fast enough, it'll just start going in orbit around the black hole. And so you can imagine that this is how the early galaxies, or even our galaxy, formed. And so you might be wondering, well, what about the galaxy at the center of the Milky Way? Or sorry, what about the black hole at the center of the Milky Way? And we think there is one."}, {"video_title": "Supermassive black holes Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And so you can imagine that this is how the early galaxies, or even our galaxy, formed. And so you might be wondering, well, what about the galaxy at the center of the Milky Way? Or sorry, what about the black hole at the center of the Milky Way? And we think there is one. Because we've observed stars orbiting very quickly around something at the center of the universe. I'm sorry, at the center of our Milky Way. I want to be very clear."}, {"video_title": "Supermassive black holes Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And we think there is one. Because we've observed stars orbiting very quickly around something at the center of the universe. I'm sorry, at the center of our Milky Way. I want to be very clear. Not at the center of the universe. And the only plausible explanation for it orbiting so quickly around something is that it has to have a density of either a black hole or something that will eventually turn into a black hole. And when you do the math for the middle of our galaxy, the center of the Milky Way, our supermassive black hole is on the order of 4 million times the mass of the sun."}, {"video_title": "Supermassive black holes Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "I want to be very clear. Not at the center of the universe. And the only plausible explanation for it orbiting so quickly around something is that it has to have a density of either a black hole or something that will eventually turn into a black hole. And when you do the math for the middle of our galaxy, the center of the Milky Way, our supermassive black hole is on the order of 4 million times the mass of the sun. So hopefully that gives you a little bit of food for thought. There aren't just only stellar collapsed black holes. Or maybe there are, and somehow they grow into supermassive black holes."}, {"video_title": "Supermassive black holes Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And when you do the math for the middle of our galaxy, the center of the Milky Way, our supermassive black hole is on the order of 4 million times the mass of the sun. So hopefully that gives you a little bit of food for thought. There aren't just only stellar collapsed black holes. Or maybe there are, and somehow they grow into supermassive black holes. And that everything in between we just can't observe. Or that there really are a different class of black holes that are actually formed different ways. Maybe they formed near the beginning of the actual universe, when the density of things was a little ununiform, things condensed into each other."}, {"video_title": "Plate tectonics Geological features of divergent plate boundaries Khan Academy.mp3", "Sentence": "Before we go into possible theories as to why plates actually move, what I want to do in this video is think a little bit about why we see the geological features we do see at plate boundaries. And in particular, I want to focus on the features we see at divergent plate boundaries, where the plates are moving away from each other, where new land is being created, like we saw in the mid-oceanic ridges, where we see new land being created right in the center and moving outwards from them. So to do that, let's think about the different layers. And actually, I want to make one quick correction on the last video. Over here, I had drawn these arrows going in that direction, and based on how I defined them, they should be going into the page, and so they should have had these Xs there. Now, with that out of the way, let's draw a little diagram of what happens at the early stages of these divergent plate boundaries. So you might have your crust, and maybe it's continental crust."}, {"video_title": "Plate tectonics Geological features of divergent plate boundaries Khan Academy.mp3", "Sentence": "And actually, I want to make one quick correction on the last video. Over here, I had drawn these arrows going in that direction, and based on how I defined them, they should be going into the page, and so they should have had these Xs there. Now, with that out of the way, let's draw a little diagram of what happens at the early stages of these divergent plate boundaries. So you might have your crust, and maybe it's continental crust. So this right here is the Earth's crust. And then you have the solid part of the mantle, and the combination of them is the lithosphere. And then you have the liquid part, or the super-hot part of the mantle."}, {"video_title": "Plate tectonics Geological features of divergent plate boundaries Khan Academy.mp3", "Sentence": "So you might have your crust, and maybe it's continental crust. So this right here is the Earth's crust. And then you have the solid part of the mantle, and the combination of them is the lithosphere. And then you have the liquid part, or the super-hot part of the mantle. So this down here is magma. It hasn't solidified. It's hot enough to be in the liquid state."}, {"video_title": "Plate tectonics Geological features of divergent plate boundaries Khan Academy.mp3", "Sentence": "And then you have the liquid part, or the super-hot part of the mantle. So this down here is magma. It hasn't solidified. It's hot enough to be in the liquid state. And all of this combined, so this right here, we consider the mantle. Now, there's some debate, and we'll talk about this in the next video, of how hot spots actually form. It could be these mantle plumes that start at the border between the mantle and the core."}, {"video_title": "Plate tectonics Geological features of divergent plate boundaries Khan Academy.mp3", "Sentence": "It's hot enough to be in the liquid state. And all of this combined, so this right here, we consider the mantle. Now, there's some debate, and we'll talk about this in the next video, of how hot spots actually form. It could be these mantle plumes that start at the border between the mantle and the core. It could be some type of convection currents in the actual mantle. We'll talk more about that in the next video, or maybe a few videos from now. But let's take it for granted that hot spots form in the mantle."}, {"video_title": "Plate tectonics Geological features of divergent plate boundaries Khan Academy.mp3", "Sentence": "It could be these mantle plumes that start at the border between the mantle and the core. It could be some type of convection currents in the actual mantle. We'll talk more about that in the next video, or maybe a few videos from now. But let's take it for granted that hot spots form in the mantle. So let's say we have an area of magma right here that is particularly hot. Let me draw it. Let me do this in another color, blue and pink."}, {"video_title": "Plate tectonics Geological features of divergent plate boundaries Khan Academy.mp3", "Sentence": "But let's take it for granted that hot spots form in the mantle. So let's say we have an area of magma right here that is particularly hot. Let me draw it. Let me do this in another color, blue and pink. So this is particularly hot magma here. And we know, or maybe we don't know, or you learn known right now, if you take the same material and you make it hotter, it's going to become less dense, because the particles essentially are going to bump into each other with more kinetic energy and have more space in between them. And so this really hot part of the magma, or this really hot part of the mantle, it is going to move upwards because it is less dense."}, {"video_title": "Plate tectonics Geological features of divergent plate boundaries Khan Academy.mp3", "Sentence": "Let me do this in another color, blue and pink. So this is particularly hot magma here. And we know, or maybe we don't know, or you learn known right now, if you take the same material and you make it hotter, it's going to become less dense, because the particles essentially are going to bump into each other with more kinetic energy and have more space in between them. And so this really hot part of the magma, or this really hot part of the mantle, it is going to move upwards because it is less dense. It will have buoyancy. And as it moves upwards, it will heat up the things around it, and it will eventually make its way into the lithosphere. And it will kind of be able to break through the lithosphere, because it's so hot, it can kind of melt its way through."}, {"video_title": "Plate tectonics Geological features of divergent plate boundaries Khan Academy.mp3", "Sentence": "And so this really hot part of the magma, or this really hot part of the mantle, it is going to move upwards because it is less dense. It will have buoyancy. And as it moves upwards, it will heat up the things around it, and it will eventually make its way into the lithosphere. And it will kind of be able to break through the lithosphere, because it's so hot, it can kind of melt its way through. So let's fast forward this a little bit. So this is step one up here. Now step two, this hot magma is rising now through the lithosphere, and so it's going to create a hot spot."}, {"video_title": "Plate tectonics Geological features of divergent plate boundaries Khan Academy.mp3", "Sentence": "And it will kind of be able to break through the lithosphere, because it's so hot, it can kind of melt its way through. So let's fast forward this a little bit. So this is step one up here. Now step two, this hot magma is rising now through the lithosphere, and so it's going to create a hot spot. It's going to create a dome in the lithosphere and actually on the crust. And so it might look like this. So the crust is now going to have a dome in it."}, {"video_title": "Plate tectonics Geological features of divergent plate boundaries Khan Academy.mp3", "Sentence": "Now step two, this hot magma is rising now through the lithosphere, and so it's going to create a hot spot. It's going to create a dome in the lithosphere and actually on the crust. And so it might look like this. So the crust is now going to have a dome in it. And this is the original lithosphere. And it's now kind of been broken in two by this hot spot. So the lithosphere is now broken in two, or it's about to be broken in two, by this hot spot."}, {"video_title": "Plate tectonics Geological features of divergent plate boundaries Khan Academy.mp3", "Sentence": "So the crust is now going to have a dome in it. And this is the original lithosphere. And it's now kind of been broken in two by this hot spot. So the lithosphere is now broken in two, or it's about to be broken in two, by this hot spot. So all of this is still the lithosphere. I'll just write litho for short. This up here is the crust."}, {"video_title": "Plate tectonics Geological features of divergent plate boundaries Khan Academy.mp3", "Sentence": "So the lithosphere is now broken in two, or it's about to be broken in two, by this hot spot. So all of this is still the lithosphere. I'll just write litho for short. This up here is the crust. And if you take any rigid material, and the crust and the lithosphere, for that matter, they're rigid, and you push outward on it, it won't stretch nicely like a nice elastic balloon. It'll start to crack and have to be pulled apart in order to kind of take the pushing from below. So this crust is going to start to crack."}, {"video_title": "Plate tectonics Geological features of divergent plate boundaries Khan Academy.mp3", "Sentence": "This up here is the crust. And if you take any rigid material, and the crust and the lithosphere, for that matter, they're rigid, and you push outward on it, it won't stretch nicely like a nice elastic balloon. It'll start to crack and have to be pulled apart in order to kind of take the pushing from below. So this crust is going to start to crack. And actually the best example where you see this is actually in like sourdough bread that has really hard shells around it. You see sourdough bread. Let me see if I can draw a roll of sourdough bread."}, {"video_title": "Plate tectonics Geological features of divergent plate boundaries Khan Academy.mp3", "Sentence": "So this crust is going to start to crack. And actually the best example where you see this is actually in like sourdough bread that has really hard shells around it. You see sourdough bread. Let me see if I can draw a roll of sourdough bread. It has all of these cracks in the surface, and that's because the outer layer, the outer shell of the bread is really rigid. And so when the inside heats up and the surface area has to expand, these kind of rifts form in the bread to allow that kind of rigid shell to actually expand. And that exact same thing would happen to the crust, or actually the entire lithosphere."}, {"video_title": "Plate tectonics Geological features of divergent plate boundaries Khan Academy.mp3", "Sentence": "Let me see if I can draw a roll of sourdough bread. It has all of these cracks in the surface, and that's because the outer layer, the outer shell of the bread is really rigid. And so when the inside heats up and the surface area has to expand, these kind of rifts form in the bread to allow that kind of rigid shell to actually expand. And that exact same thing would happen to the crust, or actually the entire lithosphere. So let me draw this hot spot again. So now the hot spot, I want to do it in that pink color. Now the hot spot has gotten this far."}, {"video_title": "Plate tectonics Geological features of divergent plate boundaries Khan Academy.mp3", "Sentence": "And that exact same thing would happen to the crust, or actually the entire lithosphere. So let me draw this hot spot again. So now the hot spot, I want to do it in that pink color. Now the hot spot has gotten this far. This is the hot magma right over here. And if we fast forward a little bit, if we fast forward even a little bit more, then you could actually have the crust start to be fully pulled apart. So you fast forward a little bit more."}, {"video_title": "Plate tectonics Geological features of divergent plate boundaries Khan Academy.mp3", "Sentence": "Now the hot spot has gotten this far. This is the hot magma right over here. And if we fast forward a little bit, if we fast forward even a little bit more, then you could actually have the crust start to be fully pulled apart. So you fast forward a little bit more. The bottom boundary of the lithosphere maybe now starts to look something like this. The magma has kind of broken through the hardened part, the rigid part of the mantle. So maybe it looks like this."}, {"video_title": "Plate tectonics Geological features of divergent plate boundaries Khan Academy.mp3", "Sentence": "So you fast forward a little bit more. The bottom boundary of the lithosphere maybe now starts to look something like this. The magma has kind of broken through the hardened part, the rigid part of the mantle. So maybe it looks like this. Maybe it looks like this right now. You have the hot spot right over here. It's gotten that far now."}, {"video_title": "Plate tectonics Geological features of divergent plate boundaries Khan Academy.mp3", "Sentence": "So maybe it looks like this. Maybe it looks like this right now. You have the hot spot right over here. It's gotten that far now. And the crust on top, what we normally see, has now been pulled apart to kind of have to cover this new surface area. So now it kind of looks something like this. So it's been pushed apart."}, {"video_title": "Plate tectonics Geological features of divergent plate boundaries Khan Academy.mp3", "Sentence": "It's gotten that far now. And the crust on top, what we normally see, has now been pulled apart to kind of have to cover this new surface area. So now it kind of looks something like this. So it's been pushed apart. Let me see how well I can draw this. So now it's been pushed apart. And as it gets pushed apart, it kind of thins out a little bit, as you can imagine it doing."}, {"video_title": "Plate tectonics Geological features of divergent plate boundaries Khan Academy.mp3", "Sentence": "So it's been pushed apart. Let me see how well I can draw this. So now it's been pushed apart. And as it gets pushed apart, it kind of thins out a little bit, as you can imagine it doing. It's almost exactly as the bread analogy. When you look at bread like this, the rift, the parts, the depressions where it was expanding most vigorously, those parts of the bread are actually thinner. Like these parts of the bread are actually thinner, and they're not as hard as the parts that moved away."}, {"video_title": "Plate tectonics Geological features of divergent plate boundaries Khan Academy.mp3", "Sentence": "And as it gets pushed apart, it kind of thins out a little bit, as you can imagine it doing. It's almost exactly as the bread analogy. When you look at bread like this, the rift, the parts, the depressions where it was expanding most vigorously, those parts of the bread are actually thinner. Like these parts of the bread are actually thinner, and they're not as hard as the parts that moved away. And you see that exact same thing happening with the land. And all of this stuff is going to start getting pushed. All of this stuff is continuously getting pushed outward, essentially to kind of make space for this hot spot."}, {"video_title": "Plate tectonics Geological features of divergent plate boundaries Khan Academy.mp3", "Sentence": "Like these parts of the bread are actually thinner, and they're not as hard as the parts that moved away. And you see that exact same thing happening with the land. And all of this stuff is going to start getting pushed. All of this stuff is continuously getting pushed outward, essentially to kind of make space for this hot spot. Now this step right over here, you might have a volcano or two, but more important, you're going to have what's called a rift valley. Right now we're assuming that we're not at a below sea level yet, or we're assuming that this kind of depression in the land that you see here hasn't come in contact with another body of water, and so it'll just kind of become a little valley in between higher land. And you actually see that on Earth."}, {"video_title": "Plate tectonics Geological features of divergent plate boundaries Khan Academy.mp3", "Sentence": "All of this stuff is continuously getting pushed outward, essentially to kind of make space for this hot spot. Now this step right over here, you might have a volcano or two, but more important, you're going to have what's called a rift valley. Right now we're assuming that we're not at a below sea level yet, or we're assuming that this kind of depression in the land that you see here hasn't come in contact with another body of water, and so it'll just kind of become a little valley in between higher land. And you actually see that on Earth. And the most famous is the African rift valley that's right about this region here. I actually have a better diagram that depicts the African rift valley right over here. It's this whole region of Africa is actually kind of a big valley created by a hot spot right over there."}, {"video_title": "Plate tectonics Geological features of divergent plate boundaries Khan Academy.mp3", "Sentence": "And you actually see that on Earth. And the most famous is the African rift valley that's right about this region here. I actually have a better diagram that depicts the African rift valley right over here. It's this whole region of Africa is actually kind of a big valley created by a hot spot right over there. Now as the hot spot kind of keeps maturing, eventually some of the rift will become so depressed that it'll actually be below sea level. Remember, all of this land up here is being stretched apart. So let me go to the next step."}, {"video_title": "Plate tectonics Geological features of divergent plate boundaries Khan Academy.mp3", "Sentence": "It's this whole region of Africa is actually kind of a big valley created by a hot spot right over there. Now as the hot spot kind of keeps maturing, eventually some of the rift will become so depressed that it'll actually be below sea level. Remember, all of this land up here is being stretched apart. So let me go to the next step. The next step will be right over here. The land on top is now maybe below sea level, this next step, and it comes in contact with maybe an ocean or a sea. And so now it might look like this."}, {"video_title": "Plate tectonics Geological features of divergent plate boundaries Khan Academy.mp3", "Sentence": "So let me go to the next step. The next step will be right over here. The land on top is now maybe below sea level, this next step, and it comes in contact with maybe an ocean or a sea. And so now it might look like this. Now the land is super thin on top. I'll do my best to draw it. So it's super thin on top, and remember, it kind of keeps getting pulled apart."}, {"video_title": "Plate tectonics Geological features of divergent plate boundaries Khan Academy.mp3", "Sentence": "And so now it might look like this. Now the land is super thin on top. I'll do my best to draw it. So it's super thin on top, and remember, it kind of keeps getting pulled apart. It keeps getting pulled apart from this bubble of hot magma that's essentially coming up from below. Let me draw it like this. This is all solid rock here."}, {"video_title": "Plate tectonics Geological features of divergent plate boundaries Khan Academy.mp3", "Sentence": "So it's super thin on top, and remember, it kind of keeps getting pulled apart. It keeps getting pulled apart from this bubble of hot magma that's essentially coming up from below. Let me draw it like this. This is all solid rock here. What I drew in orange is the crust. This is kind of the rocky part. Actually, I shouldn't draw it."}, {"video_title": "Plate tectonics Geological features of divergent plate boundaries Khan Academy.mp3", "Sentence": "This is all solid rock here. What I drew in orange is the crust. This is kind of the rocky part. Actually, I shouldn't draw it. This is the rocky part of the mantle, so the combination is the lithosphere. And now you have the hot magma coming up like this. And it might peek through every now and then and create a volcano there."}, {"video_title": "Plate tectonics Geological features of divergent plate boundaries Khan Academy.mp3", "Sentence": "Actually, I shouldn't draw it. This is the rocky part of the mantle, so the combination is the lithosphere. And now you have the hot magma coming up like this. And it might peek through every now and then and create a volcano there. Maybe it'll peek through and create a volcano there. But in general, it's going to keep pushing the land up and outwards. And so this land, even though you're saying, hey, it's being pushed up, because of the outward motion, this land over here is going to be lower than the land around it, like the loaf of bread."}, {"video_title": "Plate tectonics Geological features of divergent plate boundaries Khan Academy.mp3", "Sentence": "And it might peek through every now and then and create a volcano there. Maybe it'll peek through and create a volcano there. But in general, it's going to keep pushing the land up and outwards. And so this land, even though you're saying, hey, it's being pushed up, because of the outward motion, this land over here is going to be lower than the land around it, like the loaf of bread. If it gets low enough and below sea level, actually, and it comes in contact with the body of water, or even if it doesn't, actually, water will start to gather over there. And once again, we actually see that in the rift forming between the African and the Arabian plates. The Red Sea is actually an example of exactly that."}, {"video_title": "Plate tectonics Geological features of divergent plate boundaries Khan Academy.mp3", "Sentence": "And so this land, even though you're saying, hey, it's being pushed up, because of the outward motion, this land over here is going to be lower than the land around it, like the loaf of bread. If it gets low enough and below sea level, actually, and it comes in contact with the body of water, or even if it doesn't, actually, water will start to gather over there. And once again, we actually see that in the rift forming between the African and the Arabian plates. The Red Sea is actually an example of exactly that. The Arabian plate is moving away from the African plate because of this hot spot. This is pushing all of the land up and out right over here. And so this is going out, that is going out."}, {"video_title": "Plate tectonics Geological features of divergent plate boundaries Khan Academy.mp3", "Sentence": "The Red Sea is actually an example of exactly that. The Arabian plate is moving away from the African plate because of this hot spot. This is pushing all of the land up and out right over here. And so this is going out, that is going out. It's moving outward in every direction. And so it creates these depressions where water can flow inwards. The rift valley hasn't had water flow into it the way the Red Sea has just yet."}, {"video_title": "Plate tectonics Geological features of divergent plate boundaries Khan Academy.mp3", "Sentence": "And so this is going out, that is going out. It's moving outward in every direction. And so it creates these depressions where water can flow inwards. The rift valley hasn't had water flow into it the way the Red Sea has just yet. But if it kept happening, eventually, it's going to get low enough so that the water will flow into it. So the Red Sea is exactly that. You essentially have the Indian Ocean flowing into this rift that formed from this hot spot."}, {"video_title": "Plate tectonics Geological features of divergent plate boundaries Khan Academy.mp3", "Sentence": "The rift valley hasn't had water flow into it the way the Red Sea has just yet. But if it kept happening, eventually, it's going to get low enough so that the water will flow into it. So the Red Sea is exactly that. You essentially have the Indian Ocean flowing into this rift that formed from this hot spot. And then if you fast forward a bunch, so that finally the magma can kind of surface. So let's fast forward from even this point even more. So let's fast forward even more."}, {"video_title": "Plate tectonics Geological features of divergent plate boundaries Khan Academy.mp3", "Sentence": "You essentially have the Indian Ocean flowing into this rift that formed from this hot spot. And then if you fast forward a bunch, so that finally the magma can kind of surface. So let's fast forward from even this point even more. So let's fast forward even more. And let's say now the land has been pushed a good bit apart. Now the hot spot has actually surfaced. Now the crust might look something like this."}, {"video_title": "Plate tectonics Geological features of divergent plate boundaries Khan Academy.mp3", "Sentence": "So let's fast forward even more. And let's say now the land has been pushed a good bit apart. Now the hot spot has actually surfaced. Now the crust might look something like this. The crust might look something like this. So it's been pushed apart a good bit at this point. Now we're talking about in the order of hundreds of thousands of years or tens of thousands of years."}, {"video_title": "Plate tectonics Geological features of divergent plate boundaries Khan Academy.mp3", "Sentence": "Now the crust might look something like this. The crust might look something like this. So it's been pushed apart a good bit at this point. Now we're talking about in the order of hundreds of thousands of years or tens of thousands of years. So the land, for example, the land that was here, this part of the land might now be out here. And this part of the land might now be out here. What's going to happen is that this hot spot is going to continue to fuel, and we're assuming everything's underwater at this point, since this depression that was created is now so low."}, {"video_title": "Plate tectonics Geological features of divergent plate boundaries Khan Academy.mp3", "Sentence": "Now we're talking about in the order of hundreds of thousands of years or tens of thousands of years. So the land, for example, the land that was here, this part of the land might now be out here. And this part of the land might now be out here. What's going to happen is that this hot spot is going to continue to fuel, and we're assuming everything's underwater at this point, since this depression that was created is now so low. The crust was stretched thin. We can assume that all of this is underwater. The hot spot is essentially going to come out of underwater volcanoes and start creating what's now, this body of water's gotten large enough that we can call it a mid-oceanic ridge."}, {"video_title": "Plate tectonics Geological features of divergent plate boundaries Khan Academy.mp3", "Sentence": "What's going to happen is that this hot spot is going to continue to fuel, and we're assuming everything's underwater at this point, since this depression that was created is now so low. The crust was stretched thin. We can assume that all of this is underwater. The hot spot is essentially going to come out of underwater volcanoes and start creating what's now, this body of water's gotten large enough that we can call it a mid-oceanic ridge. And so it'll actually start creating, let me do this in a different color, it'll start creating an actual ridge. It'll actually create a ridge with volcanoes in the center. And so that's why, one, we see things like the Rift Valley in Africa."}, {"video_title": "Plate tectonics Geological features of divergent plate boundaries Khan Academy.mp3", "Sentence": "The hot spot is essentially going to come out of underwater volcanoes and start creating what's now, this body of water's gotten large enough that we can call it a mid-oceanic ridge. And so it'll actually start creating, let me do this in a different color, it'll start creating an actual ridge. It'll actually create a ridge with volcanoes in the center. And so that's why, one, we see things like the Rift Valley in Africa. We see things like the Red Sea. And maybe even more importantly, that's why we see something like the mid-Atlantic Rift in the middle of the Atlantic Ocean, where you have all of this depressed land that was essentially analogous to that Rift Valley, but it's at a much later stage, and that's why it's able to collect water, because when the land was pushed out and stretched thin, water was able to flow into it. Going back to the bread analogy, it's essentially when this bread was baking and this part of the crust pushed outwards, you had this rift form, and then if there was some water on the bread, if it was raining or if it was connected to a body of water, water would have eventually flowed in here."}, {"video_title": "Plate tectonics Geological features of divergent plate boundaries Khan Academy.mp3", "Sentence": "And so that's why, one, we see things like the Rift Valley in Africa. We see things like the Red Sea. And maybe even more importantly, that's why we see something like the mid-Atlantic Rift in the middle of the Atlantic Ocean, where you have all of this depressed land that was essentially analogous to that Rift Valley, but it's at a much later stage, and that's why it's able to collect water, because when the land was pushed out and stretched thin, water was able to flow into it. Going back to the bread analogy, it's essentially when this bread was baking and this part of the crust pushed outwards, you had this rift form, and then if there was some water on the bread, if it was raining or if it was connected to a body of water, water would have eventually flowed in here. And if that bread kept growing, this rift would have kept growing, eventually to the size of the Atlantic Ocean in our theoretical bread. And so that's why you have this huge depressed area where an ocean can form, but in the middle of it you have this actual submerged mountain chain, this submerged chain of volcanoes, this submerged ridge where the land actually does go up a little bit because of all that magma flowing directly out of it. So hopefully that clears up a little bit."}, {"video_title": "Compositional and mechanical layers of the earth Cosmology & Astronomy Khan Academy.mp3", "Sentence": "What I want to do in this video is try to get a better understanding of the structure of the Earth. And we're actually going to think about it in two different ways. So let me just draw half of the Earth over here. That's my best shot at drawing half of a circle. And what we're going to do is think about it in two ways. And on the left-hand side, we're going to think about it as the compositional layers, or the chemical layers. So over here we're going to think about the chemical structure, or the composition."}, {"video_title": "Compositional and mechanical layers of the earth Cosmology & Astronomy Khan Academy.mp3", "Sentence": "That's my best shot at drawing half of a circle. And what we're going to do is think about it in two ways. And on the left-hand side, we're going to think about it as the compositional layers, or the chemical layers. So over here we're going to think about the chemical structure, or the composition. And on the right-hand side, we're going to think about the mechanical properties of the layer. And when I say the mechanical properties, I'm really just saying, is that layer kind of a solid, rigid layer? Is it kind of a liquid layer?"}, {"video_title": "Compositional and mechanical layers of the earth Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So over here we're going to think about the chemical structure, or the composition. And on the right-hand side, we're going to think about the mechanical properties of the layer. And when I say the mechanical properties, I'm really just saying, is that layer kind of a solid, rigid layer? Is it kind of a liquid layer? Or is it something in between? A kind of a putty-like, non-rigid, solid layer? So let's think about it on the chemical or the compositional side first."}, {"video_title": "Compositional and mechanical layers of the earth Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Is it kind of a liquid layer? Or is it something in between? A kind of a putty-like, non-rigid, solid layer? So let's think about it on the chemical or the compositional side first. Because to some degree, this is simpler. So the outermost layer is the crust. That's the layer that we're sitting on right here, right now, I'm assuming."}, {"video_title": "Compositional and mechanical layers of the earth Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So let's think about it on the chemical or the compositional side first. Because to some degree, this is simpler. So the outermost layer is the crust. That's the layer that we're sitting on right here, right now, I'm assuming. Assuming you're on the planet. So this right here is the crust. It is the outermost."}, {"video_title": "Compositional and mechanical layers of the earth Cosmology & Astronomy Khan Academy.mp3", "Sentence": "That's the layer that we're sitting on right here, right now, I'm assuming. Assuming you're on the planet. So this right here is the crust. It is the outermost. It's obviously solid. We'll think about that when we talk about the mechanical side of things. And it's also the thinnest layer."}, {"video_title": "Compositional and mechanical layers of the earth Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It is the outermost. It's obviously solid. We'll think about that when we talk about the mechanical side of things. And it's also the thinnest layer. And crust is not uniform. There is both oceanic crust and continental crust. Let me draw the crust on this side as well."}, {"video_title": "Compositional and mechanical layers of the earth Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And it's also the thinnest layer. And crust is not uniform. There is both oceanic crust and continental crust. Let me draw the crust on this side as well. So let me draw some crust over here. I've got some crust right over there. And there is both oceanic crust and continental crust."}, {"video_title": "Compositional and mechanical layers of the earth Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Let me draw the crust on this side as well. So let me draw some crust over here. I've got some crust right over there. And there is both oceanic crust and continental crust. So oceanic crust is thinner crust. So let's say that this part right here, let me draw some thicker crust. Let me draw some thicker crust right over here."}, {"video_title": "Compositional and mechanical layers of the earth Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And there is both oceanic crust and continental crust. So oceanic crust is thinner crust. So let's say that this part right here, let me draw some thicker crust. Let me draw some thicker crust right over here. We'll call the thicker stuff the continental crust. Which is thicker and less dense than the oceanic crust. So what I'm doing in this light green color, this is continental."}, {"video_title": "Compositional and mechanical layers of the earth Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Let me draw some thicker crust right over here. We'll call the thicker stuff the continental crust. Which is thicker and less dense than the oceanic crust. So what I'm doing in this light green color, this is continental. So this right here is continental. And then in this kind of more fluorescent green, this is oceanic. Oceanic crust."}, {"video_title": "Compositional and mechanical layers of the earth Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So what I'm doing in this light green color, this is continental. So this right here is continental. And then in this kind of more fluorescent green, this is oceanic. Oceanic crust. And the oceanic crust is pretty thin. It's on the order of about 5 or 10 kilometers. So let's just call this approximately 5 to 10 kilometers thick."}, {"video_title": "Compositional and mechanical layers of the earth Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Oceanic crust. And the oceanic crust is pretty thin. It's on the order of about 5 or 10 kilometers. So let's just call this approximately 5 to 10 kilometers thick. And when I talk about oceanic crust, I'm not talking about the oceans. I'm not talking about the liquid part, the water. I'm talking about the rock that kind of holds the water."}, {"video_title": "Compositional and mechanical layers of the earth Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So let's just call this approximately 5 to 10 kilometers thick. And when I talk about oceanic crust, I'm not talking about the oceans. I'm not talking about the liquid part, the water. I'm talking about the rock that kind of holds the water. The rock underneath the oceans. And so this is 5 to 10 kilometers thick. If you were to go to the bottom of the ocean and you were to kind of sit on the rock and then drill, you'd have to drill about 5 to 10 kilometers to get through that layer."}, {"video_title": "Compositional and mechanical layers of the earth Cosmology & Astronomy Khan Academy.mp3", "Sentence": "I'm talking about the rock that kind of holds the water. The rock underneath the oceans. And so this is 5 to 10 kilometers thick. If you were to go to the bottom of the ocean and you were to kind of sit on the rock and then drill, you'd have to drill about 5 to 10 kilometers to get through that layer. This compositional layer. So this is 5 to 10 kilometers. And the continental crust is about 10 to 70 kilometers thick."}, {"video_title": "Compositional and mechanical layers of the earth Cosmology & Astronomy Khan Academy.mp3", "Sentence": "If you were to go to the bottom of the ocean and you were to kind of sit on the rock and then drill, you'd have to drill about 5 to 10 kilometers to get through that layer. This compositional layer. So this is 5 to 10 kilometers. And the continental crust is about 10 to 70 kilometers thick. And obviously they are both rigid. They are both solid, solid rock. Now below, when you think about composition or what the layers are made up of, the next layer below that, and this is actually the biggest layer of the earth by volume, is the mantle."}, {"video_title": "Compositional and mechanical layers of the earth Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And the continental crust is about 10 to 70 kilometers thick. And obviously they are both rigid. They are both solid, solid rock. Now below, when you think about composition or what the layers are made up of, the next layer below that, and this is actually the biggest layer of the earth by volume, is the mantle. Let me draw it like that. I always have trouble drawing the right-hand side of this circle. So let me draw."}, {"video_title": "Compositional and mechanical layers of the earth Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Now below, when you think about composition or what the layers are made up of, the next layer below that, and this is actually the biggest layer of the earth by volume, is the mantle. Let me draw it like that. I always have trouble drawing the right-hand side of this circle. So let me draw. So this is the mantle right over here. Let me write down. This is all the mantle."}, {"video_title": "Compositional and mechanical layers of the earth Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So let me draw. So this is the mantle right over here. Let me write down. This is all the mantle. And once again, we differentiate it from the crust because it's composed of different types of rock. Now you go even deeper, and let me give you the depths here. So the mantle starts right below the crust, right below the oceanic and the continental crust, oceanic and continental, and it goes about 2,900 kilometers deeper."}, {"video_title": "Compositional and mechanical layers of the earth Cosmology & Astronomy Khan Academy.mp3", "Sentence": "This is all the mantle. And once again, we differentiate it from the crust because it's composed of different types of rock. Now you go even deeper, and let me give you the depths here. So the mantle starts right below the crust, right below the oceanic and the continental crust, oceanic and continental, and it goes about 2,900 kilometers deeper. So it's much, much, much thicker than the crust. The crust is on the order of 5 to maybe 70 kilometers thick. This is much, much thicker."}, {"video_title": "Compositional and mechanical layers of the earth Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So the mantle starts right below the crust, right below the oceanic and the continental crust, oceanic and continental, and it goes about 2,900 kilometers deeper. So it's much, much, much thicker than the crust. The crust is on the order of 5 to maybe 70 kilometers thick. This is much, much thicker. So even though I've drawn the crust fairly thin, I didn't draw it thin enough relative to how thick I've drawn the mantle. This isn't drawn to scale. Now you go even deeper than that, you get kind of the densest part of the earth, and that is the core."}, {"video_title": "Compositional and mechanical layers of the earth Cosmology & Astronomy Khan Academy.mp3", "Sentence": "This is much, much thicker. So even though I've drawn the crust fairly thin, I didn't draw it thin enough relative to how thick I've drawn the mantle. This isn't drawn to scale. Now you go even deeper than that, you get kind of the densest part of the earth, and that is the core. And there's going to be a couple of themes here, especially when we think about the mechanical properties of the earth, is the deeper you get, you're going to get denser elements and you're going to have more heat and more pressure. And the reason why you're going to have denser elements is when earth was first forming and it was kind of in its molten state, the denser elements just kind of sunk to the bottom and the lighter elements would kind of rise to the top. They would have this buoyancy because they're less dense than everything around it."}, {"video_title": "Compositional and mechanical layers of the earth Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Now you go even deeper than that, you get kind of the densest part of the earth, and that is the core. And there's going to be a couple of themes here, especially when we think about the mechanical properties of the earth, is the deeper you get, you're going to get denser elements and you're going to have more heat and more pressure. And the reason why you're going to have denser elements is when earth was first forming and it was kind of in its molten state, the denser elements just kind of sunk to the bottom and the lighter elements would kind of rise to the top. They would have this buoyancy because they're less dense than everything around it. And really, even the gases would kind of bubble up, would essentially bubble up and form our atmosphere. So that's why in general, the densest things are in the center and the least dense things are on the outside. They're in our atmosphere."}, {"video_title": "Compositional and mechanical layers of the earth Cosmology & Astronomy Khan Academy.mp3", "Sentence": "They would have this buoyancy because they're less dense than everything around it. And really, even the gases would kind of bubble up, would essentially bubble up and form our atmosphere. So that's why in general, the densest things are in the center and the least dense things are on the outside. They're in our atmosphere. And the core, once again, its composition is fundamentally different than the mantle and the crust. We believe that it's mainly metals, and in particular, iron and nickel. So that's the structure, the layers of the earth from a composition point of view, from a chemical point of view."}, {"video_title": "Compositional and mechanical layers of the earth Cosmology & Astronomy Khan Academy.mp3", "Sentence": "They're in our atmosphere. And the core, once again, its composition is fundamentally different than the mantle and the crust. We believe that it's mainly metals, and in particular, iron and nickel. So that's the structure, the layers of the earth from a composition point of view, from a chemical point of view. Now let's kind of think about the same layers, but we're going to think more in terms of what's liquid, what's rigid and solid, and what's in between. So the outermost rigid layer of the earth is made up of the crust, both the continental and the oceanic crust, and the coolest top layer of the mantle. So let me draw that in pink."}, {"video_title": "Compositional and mechanical layers of the earth Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So that's the structure, the layers of the earth from a composition point of view, from a chemical point of view. Now let's kind of think about the same layers, but we're going to think more in terms of what's liquid, what's rigid and solid, and what's in between. So the outermost rigid layer of the earth is made up of the crust, both the continental and the oceanic crust, and the coolest top layer of the mantle. So let me draw that in pink. So this layer right over here. So what I'm drawing in pink is the cool, rigid, solid part of the mantle. So this is the cool, so it is solid rock, the part of the mantle that's solid rock."}, {"video_title": "Compositional and mechanical layers of the earth Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So let me draw that in pink. So this layer right over here. So what I'm drawing in pink is the cool, rigid, solid part of the mantle. So this is the cool, so it is solid rock, the part of the mantle that's solid rock. Its composition is different than, say, the continental crust, but they are both rigid. So if you combine this topmost layer of the mantle with the crust, then you're talking about the lithosphere. So this is the lithosphere."}, {"video_title": "Compositional and mechanical layers of the earth Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So this is the cool, so it is solid rock, the part of the mantle that's solid rock. Its composition is different than, say, the continental crust, but they are both rigid. So if you combine this topmost layer of the mantle with the crust, then you're talking about the lithosphere. So this is the lithosphere. And this essentially gets you about the lithosphere, depending on where you are on the surface of the earth, is 10 to 200 kilometers thick. And most of the time it's closer to the high end of this range. The 10 is kind of where you have hot spots in the mantle, and it's essentially been able to kind of dissolve parts of the lithosphere and essentially create new, well we'll talk more about that when we talk about the actual plate tectonics of it all."}, {"video_title": "Compositional and mechanical layers of the earth Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So this is the lithosphere. And this essentially gets you about the lithosphere, depending on where you are on the surface of the earth, is 10 to 200 kilometers thick. And most of the time it's closer to the high end of this range. The 10 is kind of where you have hot spots in the mantle, and it's essentially been able to kind of dissolve parts of the lithosphere and essentially create new, well we'll talk more about that when we talk about the actual plate tectonics of it all. And when we talk about plate tectonics, the plates are actually lithospheric plates. It's actually the lithosphere that's moving on top of the lower layer, the lower layers of the mantle. So the lithosphere, it is rigid, it is solid, it's made up of the crust and the upper layer of the mantle, the uppermost layer of the mantle."}, {"video_title": "Compositional and mechanical layers of the earth Cosmology & Astronomy Khan Academy.mp3", "Sentence": "The 10 is kind of where you have hot spots in the mantle, and it's essentially been able to kind of dissolve parts of the lithosphere and essentially create new, well we'll talk more about that when we talk about the actual plate tectonics of it all. And when we talk about plate tectonics, the plates are actually lithospheric plates. It's actually the lithosphere that's moving on top of the lower layer, the lower layers of the mantle. So the lithosphere, it is rigid, it is solid, it's made up of the crust and the upper layer of the mantle, the uppermost layer of the mantle. Now you go a little bit deeper, the temperatures and the pressures increase, but now the temperatures have increased enough, you have the same composition as the uppermost, the rigid part of the mantle, but the temperatures have now gone up enough that it now turns into not quite a liquid, we won't call it a liquid, it actually still transmits the type of waves that liquids would not transmit. It's more of like a putty type texture, something, it has fluid properties, it can flow, it's way more viscous than what we would associate with most fluids, so it's not rigid and solid, it can have convection going on in it, but it's not a liquid, it still will transmit certain types of waves that liquids won't. And this is called the asthenosphere, kind of this jelly putty layer."}, {"video_title": "Compositional and mechanical layers of the earth Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So the lithosphere, it is rigid, it is solid, it's made up of the crust and the upper layer of the mantle, the uppermost layer of the mantle. Now you go a little bit deeper, the temperatures and the pressures increase, but now the temperatures have increased enough, you have the same composition as the uppermost, the rigid part of the mantle, but the temperatures have now gone up enough that it now turns into not quite a liquid, we won't call it a liquid, it actually still transmits the type of waves that liquids would not transmit. It's more of like a putty type texture, something, it has fluid properties, it can flow, it's way more viscous than what we would associate with most fluids, so it's not rigid and solid, it can have convection going on in it, but it's not a liquid, it still will transmit certain types of waves that liquids won't. And this is called the asthenosphere, kind of this jelly putty layer. And it's like that because it's so hot that the rock is somewhat melted. So this is, this layer right here in magenta is the asthenosphere. I've seen some spellings where there's an E after the A, I think that's maybe the European spelling."}, {"video_title": "Compositional and mechanical layers of the earth Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And this is called the asthenosphere, kind of this jelly putty layer. And it's like that because it's so hot that the rock is somewhat melted. So this is, this layer right here in magenta is the asthenosphere. I've seen some spellings where there's an E after the A, I think that's maybe the European spelling. And the asthenosphere obviously starts right below the lithosphere, it's what the lithospheric plates, when we talk about plate tectonics, are riding on top of. It's kind of the gummy material that allows it to actually move, that allows the rigid layer to actually move on top. And it goes, so it starts below the lithosphere and it ends at around 660 kilometers deep."}, {"video_title": "Compositional and mechanical layers of the earth Cosmology & Astronomy Khan Academy.mp3", "Sentence": "I've seen some spellings where there's an E after the A, I think that's maybe the European spelling. And the asthenosphere obviously starts right below the lithosphere, it's what the lithospheric plates, when we talk about plate tectonics, are riding on top of. It's kind of the gummy material that allows it to actually move, that allows the rigid layer to actually move on top. And it goes, so it starts below the lithosphere and it ends at around 660 kilometers deep. So this right here is 660 kilometers deep. And then you go even deeper than that, and now the pressures are so big that even though the temperatures are even higher, even though the temperatures are even higher, the pressures are so big that the same material can't have fluid motion anymore. It's essentially been jammed together."}, {"video_title": "Compositional and mechanical layers of the earth Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And it goes, so it starts below the lithosphere and it ends at around 660 kilometers deep. So this right here is 660 kilometers deep. And then you go even deeper than that, and now the pressures are so big that even though the temperatures are even higher, even though the temperatures are even higher, the pressures are so big that the same material can't have fluid motion anymore. It's essentially been jammed together. So you can imagine, if you have things that are somewhat fluid, that means that the molecules can kind of slide past each other, maybe very slowly. But if you increase the pressure enough, they'll be jammed into each other. And that's essentially what happens in the next layer of the mantle."}, {"video_title": "Compositional and mechanical layers of the earth Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It's essentially been jammed together. So you can imagine, if you have things that are somewhat fluid, that means that the molecules can kind of slide past each other, maybe very slowly. But if you increase the pressure enough, they'll be jammed into each other. And that's essentially what happens in the next layer of the mantle. All of these layers of the mantle are made up of the same thing, it's just a difference of temperature and pressure. And so that next layer of the mantle is called the mesosphere. This is called the mesosphere, but there's also a layer of our atmosphere, the layer right above the stratosphere, that's called the mesosphere."}, {"video_title": "Compositional and mechanical layers of the earth Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And that's essentially what happens in the next layer of the mantle. All of these layers of the mantle are made up of the same thing, it's just a difference of temperature and pressure. And so that next layer of the mantle is called the mesosphere. This is called the mesosphere, but there's also a layer of our atmosphere, the layer right above the stratosphere, that's called the mesosphere. And so don't get confused here, these are two different mesospheres. And this layer, the pressure is so big that now we are rigid again. We are kind of definitely solid."}, {"video_title": "Compositional and mechanical layers of the earth Cosmology & Astronomy Khan Academy.mp3", "Sentence": "This is called the mesosphere, but there's also a layer of our atmosphere, the layer right above the stratosphere, that's called the mesosphere. And so don't get confused here, these are two different mesospheres. And this layer, the pressure is so big that now we are rigid again. We are kind of definitely solid. None of this debate about a little bit of fluid motion because the pressures are so big. Now you go a little bit deeper. We are now in the core, the metallic core, and the temperatures are so high that even though the pressures are high, because we have a compositional change, we're at pressures where this type of mesospheric rock is rigid, but metals at these temperatures actually can be fluid, can actually be liquid."}, {"video_title": "Compositional and mechanical layers of the earth Cosmology & Astronomy Khan Academy.mp3", "Sentence": "We are kind of definitely solid. None of this debate about a little bit of fluid motion because the pressures are so big. Now you go a little bit deeper. We are now in the core, the metallic core, and the temperatures are so high that even though the pressures are high, because we have a compositional change, we're at pressures where this type of mesospheric rock is rigid, but metals at these temperatures actually can be fluid, can actually be liquid. And so we actually have a liquid outer core. The entire core, as far as we know, is made up of the same stuff, just the outer part of the core, the temperatures are high enough to melt the metal, but the pressures aren't so high enough to make them solid. The pressures are definitely high enough to make more rocky material solid, but not the metals."}, {"video_title": "Compositional and mechanical layers of the earth Cosmology & Astronomy Khan Academy.mp3", "Sentence": "We are now in the core, the metallic core, and the temperatures are so high that even though the pressures are high, because we have a compositional change, we're at pressures where this type of mesospheric rock is rigid, but metals at these temperatures actually can be fluid, can actually be liquid. And so we actually have a liquid outer core. The entire core, as far as we know, is made up of the same stuff, just the outer part of the core, the temperatures are high enough to melt the metal, but the pressures aren't so high enough to make them solid. The pressures are definitely high enough to make more rocky material solid, but not the metals. And then you go even deeper, now the pressure, even though the temperature keeps going up, the pressure is so strong that even the metals are solid. So this is the solid inner core. So when you think about the mechanical properties, the innermost, and just so you know the total distances we're talking about, the outer core starts at, I actually didn't tell you where the mesosphere ends, the mantle ends at about 2,900 kilometers deep, so that's clearly where the mesosphere ends as well, because the mesosphere is kind of the lower mantle."}, {"video_title": "Compositional and mechanical layers of the earth Cosmology & Astronomy Khan Academy.mp3", "Sentence": "The pressures are definitely high enough to make more rocky material solid, but not the metals. And then you go even deeper, now the pressure, even though the temperature keeps going up, the pressure is so strong that even the metals are solid. So this is the solid inner core. So when you think about the mechanical properties, the innermost, and just so you know the total distances we're talking about, the outer core starts at, I actually didn't tell you where the mesosphere ends, the mantle ends at about 2,900 kilometers deep, so that's clearly where the mesosphere ends as well, because the mesosphere is kind of the lower mantle. So this is 2,900 kilometers deep. Then you go even deeper, you're in the liquid outer core, and that extends from about 2,900 kilometers deep to about 5,100 kilometers deep. So I really should, I frankly should make the liquid core in my drawing even wider."}, {"video_title": "Compositional and mechanical layers of the earth Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So when you think about the mechanical properties, the innermost, and just so you know the total distances we're talking about, the outer core starts at, I actually didn't tell you where the mesosphere ends, the mantle ends at about 2,900 kilometers deep, so that's clearly where the mesosphere ends as well, because the mesosphere is kind of the lower mantle. So this is 2,900 kilometers deep. Then you go even deeper, you're in the liquid outer core, and that extends from about 2,900 kilometers deep to about 5,100 kilometers deep. So I really should, I frankly should make the liquid core in my drawing even wider. So this extends to about, this depth right over here is about 5,100 kilometers deep. And then obviously then you have the center of the earth, and the entire radius of the earth is about 6,400 kilometers. So hopefully that clarifies things when you hear people talking about the lithosphere or the mantle, that they're really talking about mechanical versus composition."}, {"video_title": "Compositional and mechanical layers of the earth Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So I really should, I frankly should make the liquid core in my drawing even wider. So this extends to about, this depth right over here is about 5,100 kilometers deep. And then obviously then you have the center of the earth, and the entire radius of the earth is about 6,400 kilometers. So hopefully that clarifies things when you hear people talking about the lithosphere or the mantle, that they're really talking about mechanical versus composition. When we talk about mechanical, solid inner core, liquid outer core, essentially solid mesosphere, it's rigid. Then you have something kind of a spongy, somewhat fluid, not solid, not liquid, a stenosphere that the lithospheric plates can ride on top of. And then you have your actual rigid, solid lithosphere made up of the uppermost part of the mantle and the crust."}, {"video_title": "Ozone layer and eukaryotes show up in the Proterozoic eon Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It's possible that maybe life first started to exist at the end of the Hadean Eon. And of course, this boundary is vague. And the Archean Eon is also the first eon where we still have rocks from that time. So we are able to find rocks that we can date to be roughly 3.8 billion years. Now, the other really interesting thing that happened in the Archean Eon, it really has pretty profound effects once we get into the Proterozoic Eon, is that you started to have cyanobacteria produce oxygen. And we said in the last video that they were producing oxygen, but most of that oxygen was being absorbed by iron in the oceans. But what happens is we enter into the Proterozoic Eon."}, {"video_title": "Ozone layer and eukaryotes show up in the Proterozoic eon Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So we are able to find rocks that we can date to be roughly 3.8 billion years. Now, the other really interesting thing that happened in the Archean Eon, it really has pretty profound effects once we get into the Proterozoic Eon, is that you started to have cyanobacteria produce oxygen. And we said in the last video that they were producing oxygen, but most of that oxygen was being absorbed by iron in the oceans. But what happens is we enter into the Proterozoic Eon. And Proterozoic Eon is right over here. I don't know if you can see that. I'll rewrite it."}, {"video_title": "Ozone layer and eukaryotes show up in the Proterozoic eon Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But what happens is we enter into the Proterozoic Eon. And Proterozoic Eon is right over here. I don't know if you can see that. I'll rewrite it. Proterozoic. We're now in the Proterozoic Eon that starts up about 2.5 billion years ago. And Proterozoic comes from the Greek for earlier life."}, {"video_title": "Ozone layer and eukaryotes show up in the Proterozoic eon Cosmology & Astronomy Khan Academy.mp3", "Sentence": "I'll rewrite it. Proterozoic. We're now in the Proterozoic Eon that starts up about 2.5 billion years ago. And Proterozoic comes from the Greek for earlier life. And I'm not a Greek scholar, so any of you Greeks out there, forgive me if I'm not getting the translation exactly right. But what's really interesting about the Proterozoic Eon is that that oxygen that was being produced by the cyanobacteria at some point begins to saturate the iron and any other molecule that could have absorbed it before. And once it saturates, it starts to get released into the atmosphere."}, {"video_title": "Ozone layer and eukaryotes show up in the Proterozoic eon Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And Proterozoic comes from the Greek for earlier life. And I'm not a Greek scholar, so any of you Greeks out there, forgive me if I'm not getting the translation exactly right. But what's really interesting about the Proterozoic Eon is that that oxygen that was being produced by the cyanobacteria at some point begins to saturate the iron and any other molecule that could have absorbed it before. And once it saturates, it starts to get released into the atmosphere. So the oxygen starts to get released and accumulate in the atmosphere. And we think this started to happen about 2.4 billion years ago. So 2.4 billion years ago, oxygen begins to accumulate in the atmosphere."}, {"video_title": "Ozone layer and eukaryotes show up in the Proterozoic eon Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And once it saturates, it starts to get released into the atmosphere. So the oxygen starts to get released and accumulate in the atmosphere. And we think this started to happen about 2.4 billion years ago. So 2.4 billion years ago, oxygen begins to accumulate in the atmosphere. And of course, these dates, they might be moved around a few hundred million years if you get more and more data. But this is the current understanding of when things happen. And maybe we'll look at the geological record or the fossil record, and we'll move these things around in the future."}, {"video_title": "Ozone layer and eukaryotes show up in the Proterozoic eon Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So 2.4 billion years ago, oxygen begins to accumulate in the atmosphere. And of course, these dates, they might be moved around a few hundred million years if you get more and more data. But this is the current understanding of when things happen. And maybe we'll look at the geological record or the fossil record, and we'll move these things around in the future. I could only imagine that 50 years from now or 100 years from now, if someone's still watching this video, a lot of this might say, hey, we found out later that oxygen started to accumulate in the atmosphere earlier or later or that eukaryotes occurred earlier or later. But this, as far as I can tell, is our best current understanding. But 2.4 billion years ago, oxygen begins to accumulate in the atmosphere."}, {"video_title": "Ozone layer and eukaryotes show up in the Proterozoic eon Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And maybe we'll look at the geological record or the fossil record, and we'll move these things around in the future. I could only imagine that 50 years from now or 100 years from now, if someone's still watching this video, a lot of this might say, hey, we found out later that oxygen started to accumulate in the atmosphere earlier or later or that eukaryotes occurred earlier or later. But this, as far as I can tell, is our best current understanding. But 2.4 billion years ago, oxygen begins to accumulate in the atmosphere. And what's interesting about this is once it accumulates and once it gets a critical amount of oxygen in the atmosphere, and I touched on this in the last video, about 2.3 billion years ago, we have something called the Great Oxygenation Event, sometimes called the Oxygen Catastrophe. And this is right here. They mark it on this right here, 2.3 billion years ago or 2,300 million years ago."}, {"video_title": "Ozone layer and eukaryotes show up in the Proterozoic eon Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But 2.4 billion years ago, oxygen begins to accumulate in the atmosphere. And what's interesting about this is once it accumulates and once it gets a critical amount of oxygen in the atmosphere, and I touched on this in the last video, about 2.3 billion years ago, we have something called the Great Oxygenation Event, sometimes called the Oxygen Catastrophe. And this is right here. They mark it on this right here, 2.3 billion years ago or 2,300 million years ago. Atmosphere becomes oxygen rich. And it's not as oxygen rich as our current atmosphere, but it becomes oxygen rich enough that at least the environment becomes suitable for eukaryotic organisms or eukaryotic cells. Now, the other interesting thing, and we might not care so much about it because we needed the oxygen, is that we think that this was actually the greatest extinction event in the history of Earth."}, {"video_title": "Ozone layer and eukaryotes show up in the Proterozoic eon Cosmology & Astronomy Khan Academy.mp3", "Sentence": "They mark it on this right here, 2.3 billion years ago or 2,300 million years ago. Atmosphere becomes oxygen rich. And it's not as oxygen rich as our current atmosphere, but it becomes oxygen rich enough that at least the environment becomes suitable for eukaryotic organisms or eukaryotic cells. Now, the other interesting thing, and we might not care so much about it because we needed the oxygen, is that we think that this was actually the greatest extinction event in the history of Earth. That's why it's called the Oxygen Catastrophe. So this right over here, 2.3 billion years ago, I shouldn't giggle about it. This is a serious matter."}, {"video_title": "Ozone layer and eukaryotes show up in the Proterozoic eon Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Now, the other interesting thing, and we might not care so much about it because we needed the oxygen, is that we think that this was actually the greatest extinction event in the history of Earth. That's why it's called the Oxygen Catastrophe. So this right over here, 2.3 billion years ago, I shouldn't giggle about it. This is a serious matter. This is the greatest extinction event in Earth's history. And I'll do history with a capital H. In Earth's history of the Earth. And that's because the cyanobacteria is producing all this oxygen, eventually saturates the iron, it accumulates in the air."}, {"video_title": "Ozone layer and eukaryotes show up in the Proterozoic eon Cosmology & Astronomy Khan Academy.mp3", "Sentence": "This is a serious matter. This is the greatest extinction event in Earth's history. And I'll do history with a capital H. In Earth's history of the Earth. And that's because the cyanobacteria is producing all this oxygen, eventually saturates the iron, it accumulates in the air. Once it gets to enough concentration, it begins to actually suffocate. It's poisonous. It's poisonous to most of the other organisms on the planet that were anaerobic, that did not need oxygen, that actually found oxygen poisonous."}, {"video_title": "Ozone layer and eukaryotes show up in the Proterozoic eon Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And that's because the cyanobacteria is producing all this oxygen, eventually saturates the iron, it accumulates in the air. Once it gets to enough concentration, it begins to actually suffocate. It's poisonous. It's poisonous to most of the other organisms on the planet that were anaerobic, that did not need oxygen, that actually found oxygen poisonous. Now, but since we have oxygen, there's two interesting things that happened once that oxygen accumulated, other than causing this mass extinction event. Actually, three interesting things. Two of them are crucial to us eventually showing up on this planet."}, {"video_title": "Ozone layer and eukaryotes show up in the Proterozoic eon Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It's poisonous to most of the other organisms on the planet that were anaerobic, that did not need oxygen, that actually found oxygen poisonous. Now, but since we have oxygen, there's two interesting things that happened once that oxygen accumulated, other than causing this mass extinction event. Actually, three interesting things. Two of them are crucial to us eventually showing up on this planet. The first is that it became suitable now for eukaryotes to exist. Eukaryotic organisms, remember, these are organisms that have nuclear membranes around their DNA. Most eukaryotes have other organelles, like mitochondria."}, {"video_title": "Ozone layer and eukaryotes show up in the Proterozoic eon Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Two of them are crucial to us eventually showing up on this planet. The first is that it became suitable now for eukaryotes to exist. Eukaryotic organisms, remember, these are organisms that have nuclear membranes around their DNA. Most eukaryotes have other organelles, like mitochondria. They need oxygen. You can go to the biology playlist. We actually talk about respiration that occurs in the mitochondria."}, {"video_title": "Ozone layer and eukaryotes show up in the Proterozoic eon Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Most eukaryotes have other organelles, like mitochondria. They need oxygen. You can go to the biology playlist. We actually talk about respiration that occurs in the mitochondria. And that's obviously a process that needs oxygen. So one, now that oxygen is in the atmosphere, we're starting to have an environment where eukaryotes could at least exist. And based on the fossil records, and when we look at how the DNA has changed over time, and we'll do multiple videos of that, we think that the first eukaryotes showed up about 2.2 billion years ago."}, {"video_title": "Ozone layer and eukaryotes show up in the Proterozoic eon Cosmology & Astronomy Khan Academy.mp3", "Sentence": "We actually talk about respiration that occurs in the mitochondria. And that's obviously a process that needs oxygen. So one, now that oxygen is in the atmosphere, we're starting to have an environment where eukaryotes could at least exist. And based on the fossil records, and when we look at how the DNA has changed over time, and we'll do multiple videos of that, we think that the first eukaryotes showed up about 2.2 billion years ago. Although there's some debate here. There's some evidence it might have been a little earlier, some evidence it might have been a little later. I'm sure that number will be refined."}, {"video_title": "Ozone layer and eukaryotes show up in the Proterozoic eon Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And based on the fossil records, and when we look at how the DNA has changed over time, and we'll do multiple videos of that, we think that the first eukaryotes showed up about 2.2 billion years ago. Although there's some debate here. There's some evidence it might have been a little earlier, some evidence it might have been a little later. I'm sure that number will be refined. But about, give or take a few hundred millions of years, one prokaryote got engulfed by another prokaryote and said, hey, we do pretty well living together. The current theory is that mitochondria is actually descended from kind of an ancient prokaryotic cell, an ancient bacteria. It actually has its own DNA."}, {"video_title": "Ozone layer and eukaryotes show up in the Proterozoic eon Cosmology & Astronomy Khan Academy.mp3", "Sentence": "I'm sure that number will be refined. But about, give or take a few hundred millions of years, one prokaryote got engulfed by another prokaryote and said, hey, we do pretty well living together. The current theory is that mitochondria is actually descended from kind of an ancient prokaryotic cell, an ancient bacteria. It actually has its own DNA. And actually, your mitochondrial DNA is passed down from your mother, and your mother's mother, and your mother's mother, so on and so forth. So it's kind of like another little animal living inside of a larger cell. And we are eukaryotes."}, {"video_title": "Ozone layer and eukaryotes show up in the Proterozoic eon Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It actually has its own DNA. And actually, your mitochondrial DNA is passed down from your mother, and your mother's mother, and your mother's mother, so on and so forth. So it's kind of like another little animal living inside of a larger cell. And we are eukaryotes. We needed this to happen. The human body, we're not just one eukaryotic cell. We're made up of trillions."}, {"video_title": "Ozone layer and eukaryotes show up in the Proterozoic eon Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And we are eukaryotes. We needed this to happen. The human body, we're not just one eukaryotic cell. We're made up of trillions. Estimates are 50 to 100 trillion eukaryotic cells. So these are our ancestors that had to come into being at that time. And once again, all of this is happening inside of the oceans."}, {"video_title": "Ozone layer and eukaryotes show up in the Proterozoic eon Cosmology & Astronomy Khan Academy.mp3", "Sentence": "We're made up of trillions. Estimates are 50 to 100 trillion eukaryotic cells. So these are our ancestors that had to come into being at that time. And once again, all of this is happening inside of the oceans. Now, the other interesting thing that happened. Remember, we're being bombarded with UV radiation from the sun. So if you're on the land, let me draw the land and the ocean."}, {"video_title": "Ozone layer and eukaryotes show up in the Proterozoic eon Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And once again, all of this is happening inside of the oceans. Now, the other interesting thing that happened. Remember, we're being bombarded with UV radiation from the sun. So if you're on the land, let me draw the land and the ocean. So here is the ocean, and then here is the land, right over there in yellow, constantly being bombarded with UV radiation. And UV stands for ultraviolet, so I'm drawing it in purple. But it's even more violet than purple."}, {"video_title": "Ozone layer and eukaryotes show up in the Proterozoic eon Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So if you're on the land, let me draw the land and the ocean. So here is the ocean, and then here is the land, right over there in yellow, constantly being bombarded with UV radiation. And UV stands for ultraviolet, so I'm drawing it in purple. But it's even more violet than purple. So it's constantly being bombarded with ultraviolet radiation from the sun, which is very inhospitable to DNA and to life. And so the only life at this point could occur in the ocean, where it was protected to some degree from the ultraviolet radiation. The land was just open to it."}, {"video_title": "Ozone layer and eukaryotes show up in the Proterozoic eon Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But it's even more violet than purple. So it's constantly being bombarded with ultraviolet radiation from the sun, which is very inhospitable to DNA and to life. And so the only life at this point could occur in the ocean, where it was protected to some degree from the ultraviolet radiation. The land was just open to it. Anything on the land would have just gotten irradiated. Its DNA would get mutated. It just would not be able to live."}, {"video_title": "Ozone layer and eukaryotes show up in the Proterozoic eon Cosmology & Astronomy Khan Academy.mp3", "Sentence": "The land was just open to it. Anything on the land would have just gotten irradiated. Its DNA would get mutated. It just would not be able to live. So what happened, and what I guess has to happen, and the reason why we are able to live on land now is that we have an ozone layer. We have an ozone layer up in the upper atmosphere that helps absorb, that blocks most of the UV radiation from the sun. And now that oxygen began to accumulate, we have the oxygen catastrophe."}, {"video_title": "Ozone layer and eukaryotes show up in the Proterozoic eon Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It just would not be able to live. So what happened, and what I guess has to happen, and the reason why we are able to live on land now is that we have an ozone layer. We have an ozone layer up in the upper atmosphere that helps absorb, that blocks most of the UV radiation from the sun. And now that oxygen began to accumulate, we have the oxygen catastrophe. Oxygen accumulates in the atmosphere. Some of that oxygen goes into the upper atmosphere. So we're now in this time period right over here."}, {"video_title": "Ozone layer and eukaryotes show up in the Proterozoic eon Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And now that oxygen began to accumulate, we have the oxygen catastrophe. Oxygen accumulates in the atmosphere. Some of that oxygen goes into the upper atmosphere. So we're now in this time period right over here. It goes into the upper atmosphere. It actually reacts with the UV light to turn into ozone, which then can help actually block the UV light. And I'll do another video maybe on the ozone oxygen cycle."}, {"video_title": "Ozone layer and eukaryotes show up in the Proterozoic eon Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So we're now in this time period right over here. It goes into the upper atmosphere. It actually reacts with the UV light to turn into ozone, which then can help actually block the UV light. And I'll do another video maybe on the ozone oxygen cycle. So this oxygen production, it's crucial, one, to having an ozone layer so that eventually life can exist on the land. And it's also crucial because eukaryotic organisms need that oxygen. Now the third thing that happened, this is also a pretty significant event, we believe that that oxygen that started to accumulate in the atmosphere reacted with methane in the atmosphere."}, {"video_title": "Ozone layer and eukaryotes show up in the Proterozoic eon Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And I'll do another video maybe on the ozone oxygen cycle. So this oxygen production, it's crucial, one, to having an ozone layer so that eventually life can exist on the land. And it's also crucial because eukaryotic organisms need that oxygen. Now the third thing that happened, this is also a pretty significant event, we believe that that oxygen that started to accumulate in the atmosphere reacted with methane in the atmosphere. So it reacted with methane. And methane is a greenhouse gas. It helps retain heat in the atmosphere."}, {"video_title": "Ozone layer and eukaryotes show up in the Proterozoic eon Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Now the third thing that happened, this is also a pretty significant event, we believe that that oxygen that started to accumulate in the atmosphere reacted with methane in the atmosphere. So it reacted with methane. And methane is a greenhouse gas. It helps retain heat in the atmosphere. And once it reacts with the oxygen and starts dropping out of the atmosphere as methane, we believe the Earth cooled down. And it entered its first, and some people believe its longest snowball period. So that's what they talk about right here on this diagram."}, {"video_title": "Ozone layer and eukaryotes show up in the Proterozoic eon Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It helps retain heat in the atmosphere. And once it reacts with the oxygen and starts dropping out of the atmosphere as methane, we believe the Earth cooled down. And it entered its first, and some people believe its longest snowball period. So that's what they talk about right here on this diagram. The first snowball Earth. It's sometimes called the Huronic glaciation. And that happened because we weren't able to retain our heat, if that theory is correct."}, {"video_title": "Ozone layer and eukaryotes show up in the Proterozoic eon Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So that's what they talk about right here on this diagram. The first snowball Earth. It's sometimes called the Huronic glaciation. And that happened because we weren't able to retain our heat, if that theory is correct. And so as the theory goes, the whole Earth essentially just iced over. So as we go through the Proterozoic eon, I guess the big markers of it is it's the first time that we now have an oxygen-rich atmosphere. It's the first time that eukaryotes can now come into existence."}, {"video_title": "Ozone layer and eukaryotes show up in the Proterozoic eon Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And that happened because we weren't able to retain our heat, if that theory is correct. And so as the theory goes, the whole Earth essentially just iced over. So as we go through the Proterozoic eon, I guess the big markers of it is it's the first time that we now have an oxygen-rich atmosphere. It's the first time that eukaryotes can now come into existence. Because they now have oxygen to, I guess we could say, breathe. And the other big thing is now this is where the ozone forms. So this kind of sets the stage in the next eon for animals or living things to eventually get onto the land."}, {"video_title": "Biodiversity flourishes in Phanerozoic eon Cosmology & Astronomy Khan Academy.mp3", "Sentence": "In the last video, we saw during the Proterozoic Eon, oxygen began to accumulate in the atmosphere. This actually caused this first snowball Earth and this mass extinction of all the anaerobic species. But it made conditions suitable for eukaryotic cells. And maybe even more important, these eukaryotic cells were able to form multicellular organisms. And we see where that starts right here on this chart, on this time clark. Multicellular life starts right over here. And I want to be clear."}, {"video_title": "Biodiversity flourishes in Phanerozoic eon Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And maybe even more important, these eukaryotic cells were able to form multicellular organisms. And we see where that starts right here on this chart, on this time clark. Multicellular life starts right over here. And I want to be clear. All of these things are a bit moving targets. As we discover more things in the geological record and we get more tools at our disposal, these numbers get tweaked. But they do give you a good sense, based on our current understanding, of when these things start to appear."}, {"video_title": "Biodiversity flourishes in Phanerozoic eon Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And I want to be clear. All of these things are a bit moving targets. As we discover more things in the geological record and we get more tools at our disposal, these numbers get tweaked. But they do give you a good sense, based on our current understanding, of when these things start to appear. And coinciding with multicellular life, and this is interesting in its own right because it has its own meta-level effect on evolution, you actually start also having sexual reproduction. And what's interesting about this, why this has such a big impact on evolution, and we talk about it a lot in the biology playlist, is before evolution, variation in DNA had to be completely dependent really on mutations and just random movement around within DNA or maybe some viruses. Now with sexual reproduction, you had kind of a systematic mixing of DNA so that you got more variation in the gene pool, which allowed more selection for, or I guess you had more variants to select for."}, {"video_title": "Biodiversity flourishes in Phanerozoic eon Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But they do give you a good sense, based on our current understanding, of when these things start to appear. And coinciding with multicellular life, and this is interesting in its own right because it has its own meta-level effect on evolution, you actually start also having sexual reproduction. And what's interesting about this, why this has such a big impact on evolution, and we talk about it a lot in the biology playlist, is before evolution, variation in DNA had to be completely dependent really on mutations and just random movement around within DNA or maybe some viruses. Now with sexual reproduction, you had kind of a systematic mixing of DNA so that you got more variation in the gene pool, which allowed more selection for, or I guess you had more variants to select for. And so you kind of had an acceleration in the actual pace of evolution. So that's what we're talking. I've looked at a bunch of sources from, they say, 1.2 billion, 1.5 billion, a little bit over a billion if you score a little bit several hundred million years ago."}, {"video_title": "Biodiversity flourishes in Phanerozoic eon Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Now with sexual reproduction, you had kind of a systematic mixing of DNA so that you got more variation in the gene pool, which allowed more selection for, or I guess you had more variants to select for. And so you kind of had an acceleration in the actual pace of evolution. So that's what we're talking. I've looked at a bunch of sources from, they say, 1.2 billion, 1.5 billion, a little bit over a billion if you score a little bit several hundred million years ago. You start having these multicellular life forms and sexual reproduction. The other thing that we talked about in the proterozoic eon is the accumulation of oxygen allowed the ozone layer to build up. Ozone is just three oxygen atoms."}, {"video_title": "Biodiversity flourishes in Phanerozoic eon Cosmology & Astronomy Khan Academy.mp3", "Sentence": "I've looked at a bunch of sources from, they say, 1.2 billion, 1.5 billion, a little bit over a billion if you score a little bit several hundred million years ago. You start having these multicellular life forms and sexual reproduction. The other thing that we talked about in the proterozoic eon is the accumulation of oxygen allowed the ozone layer to build up. Ozone is just three oxygen atoms. It is O3. And by the end of the proterozoic eon, so we're talking, I don't know, maybe 550 million years ago, give or take, tens of or maybe even 100 million years. These are all moving targets."}, {"video_title": "Biodiversity flourishes in Phanerozoic eon Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Ozone is just three oxygen atoms. It is O3. And by the end of the proterozoic eon, so we're talking, I don't know, maybe 550 million years ago, give or take, tens of or maybe even 100 million years. These are all moving targets. The ozone layer was dense enough to protect the land from UV rays. We talked about that in the last video. The Earth is being bombarded with UV rays."}, {"video_title": "Biodiversity flourishes in Phanerozoic eon Cosmology & Astronomy Khan Academy.mp3", "Sentence": "These are all moving targets. The ozone layer was dense enough to protect the land from UV rays. We talked about that in the last video. The Earth is being bombarded with UV rays. And the ozone layer is the only thing that really keeps us from being seriously irradiated by the sun and allows land animals to actually live. And so coinciding with that time period, around 550 million years ago, you start to have life colonizing, especially significant life colonizing land. So life colonizes land."}, {"video_title": "Biodiversity flourishes in Phanerozoic eon Cosmology & Astronomy Khan Academy.mp3", "Sentence": "The Earth is being bombarded with UV rays. And the ozone layer is the only thing that really keeps us from being seriously irradiated by the sun and allows land animals to actually live. And so coinciding with that time period, around 550 million years ago, you start to have life colonizing, especially significant life colonizing land. So life colonizes land. And this was kind of an interesting, when I first learned it, it was kind of an aha moment. You always assume that trees and grasses are kind of part of the background. They come part and parcel with land."}, {"video_title": "Biodiversity flourishes in Phanerozoic eon Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So life colonizes land. And this was kind of an interesting, when I first learned it, it was kind of an aha moment. You always assume that trees and grasses are kind of part of the background. They come part and parcel with land. But it turns out that animals colonized land before plants did. Plants didn't come into the picture until about 450 million years ago, give or take a few tens of millions of years. And so we're now entering the end of the Proterozoic Eon."}, {"video_title": "Biodiversity flourishes in Phanerozoic eon Cosmology & Astronomy Khan Academy.mp3", "Sentence": "They come part and parcel with land. But it turns out that animals colonized land before plants did. Plants didn't come into the picture until about 450 million years ago, give or take a few tens of millions of years. And so we're now entering the end of the Proterozoic Eon. Life has started to colonize land. We now have an ozone layer. And what happens, and actually there's another snowball glaciation or snowball Earth near the end of the Proterozoic Eon, I should say."}, {"video_title": "Biodiversity flourishes in Phanerozoic eon Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And so we're now entering the end of the Proterozoic Eon. Life has started to colonize land. We now have an ozone layer. And what happens, and actually there's another snowball glaciation or snowball Earth near the end of the Proterozoic Eon, I should say. And there's a bunch of theories about why it came about and then why it disappeared. Maybe there were volcanoes, greenhouse gases, who knows. But as we enter the end of that, we start seeing life begin to flourish."}, {"video_title": "Biodiversity flourishes in Phanerozoic eon Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And what happens, and actually there's another snowball glaciation or snowball Earth near the end of the Proterozoic Eon, I should say. And there's a bunch of theories about why it came about and then why it disappeared. Maybe there were volcanoes, greenhouse gases, who knows. But as we enter the end of that, we start seeing life begin to flourish. And it starts to really flourish as we enter the Phanerozoic Eon. And it's not even labeled here. The Phanerozoic Eon is this chunk of time right over here."}, {"video_title": "Biodiversity flourishes in Phanerozoic eon Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But as we enter the end of that, we start seeing life begin to flourish. And it starts to really flourish as we enter the Phanerozoic Eon. And it's not even labeled here. The Phanerozoic Eon is this chunk of time right over here. And let me write it out. This right over here is the Phanerozoic Eon. And so this chart, these divisions right here are eons."}, {"video_title": "Biodiversity flourishes in Phanerozoic eon Cosmology & Astronomy Khan Academy.mp3", "Sentence": "The Phanerozoic Eon is this chunk of time right over here. And let me write it out. This right over here is the Phanerozoic Eon. And so this chart, these divisions right here are eons. And then they jump into, instead of doing eons here, they then break into eras. Eras are subsets of eons. They're hundreds of millions of years."}, {"video_title": "Biodiversity flourishes in Phanerozoic eon Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And so this chart, these divisions right here are eons. And then they jump into, instead of doing eons here, they then break into eras. Eras are subsets of eons. They're hundreds of millions of years. So this is the Paleozoic Era, the Mesozoic Era, and the Cenozoic Era. And that's actually our current era. But perhaps the most interesting, well, I don't want to pick favorites here, but it's one of the most interesting times in the geologic era, is the first period in the Paleozoic Era, which is the first era in the Phanerozoic Eon."}, {"video_title": "Biodiversity flourishes in Phanerozoic eon Cosmology & Astronomy Khan Academy.mp3", "Sentence": "They're hundreds of millions of years. So this is the Paleozoic Era, the Mesozoic Era, and the Cenozoic Era. And that's actually our current era. But perhaps the most interesting, well, I don't want to pick favorites here, but it's one of the most interesting times in the geologic era, is the first period in the Paleozoic Era, which is the first era in the Phanerozoic Eon. And that's the Cambrian Period. You might have heard of it before. The Cambrian Period."}, {"video_title": "Biodiversity flourishes in Phanerozoic eon Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But perhaps the most interesting, well, I don't want to pick favorites here, but it's one of the most interesting times in the geologic era, is the first period in the Paleozoic Era, which is the first era in the Phanerozoic Eon. And that's the Cambrian Period. You might have heard of it before. The Cambrian Period. That's about this period of time right over here. And during this period of time, the Earth experiences what we call the Cambrian Explosion. And that's because there's just this explosion in the number of species and genera that existed, the biodiversity, on the planet."}, {"video_title": "Biodiversity flourishes in Phanerozoic eon Cosmology & Astronomy Khan Academy.mp3", "Sentence": "The Cambrian Period. That's about this period of time right over here. And during this period of time, the Earth experiences what we call the Cambrian Explosion. And that's because there's just this explosion in the number of species and genera that existed, the biodiversity, on the planet. It might just be that we had the ozone layer protecting us. Things were colonizing land. It was an oxygen-rich environment."}, {"video_title": "Biodiversity flourishes in Phanerozoic eon Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And that's because there's just this explosion in the number of species and genera that existed, the biodiversity, on the planet. It might just be that we had the ozone layer protecting us. Things were colonizing land. It was an oxygen-rich environment. We start seeing complex, multicellular organisms. It's about that time, if you fast forward maybe a few tens of millions of years, you start seeing the first fish, the first kind of pre-amphibians or proto-amphibians. You fast forward a little bit, as we get out of the Cambrian Period, we start seeing plants."}, {"video_title": "Biodiversity flourishes in Phanerozoic eon Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It was an oxygen-rich environment. We start seeing complex, multicellular organisms. It's about that time, if you fast forward maybe a few tens of millions of years, you start seeing the first fish, the first kind of pre-amphibians or proto-amphibians. You fast forward a little bit, as we get out of the Cambrian Period, we start seeing plants. So they actually draw it right over here on this land plants, or at this point right over here. And of course, these are moving targets, depending on what we discover in the fossil record. And for me, the big aha moment here is so many of these things that you consider fundamental to what Earth is are relatively recent phenomena."}, {"video_title": "Biodiversity flourishes in Phanerozoic eon Cosmology & Astronomy Khan Academy.mp3", "Sentence": "You fast forward a little bit, as we get out of the Cambrian Period, we start seeing plants. So they actually draw it right over here on this land plants, or at this point right over here. And of course, these are moving targets, depending on what we discover in the fossil record. And for me, the big aha moment here is so many of these things that you consider fundamental to what Earth is are relatively recent phenomena. Plants weren't on land until about 450 million years ago. Insects weren't on land, or did not even exist, until about 400 million years ago. Reptiles didn't exist until about 300 million years ago."}, {"video_title": "Biodiversity flourishes in Phanerozoic eon Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And for me, the big aha moment here is so many of these things that you consider fundamental to what Earth is are relatively recent phenomena. Plants weren't on land until about 450 million years ago. Insects weren't on land, or did not even exist, until about 400 million years ago. Reptiles didn't exist until about 300 million years ago. So we're about right over here now. Mammals didn't exist until about 200 million years ago. Birds didn't exist until about 150 million years ago."}, {"video_title": "Biodiversity flourishes in Phanerozoic eon Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Reptiles didn't exist until about 300 million years ago. So we're about right over here now. Mammals didn't exist until about 200 million years ago. Birds didn't exist until about 150 million years ago. The whole dinosaur age, which we kind of consider in our distant past, that's essentially the Mesozoic Era right here. So this is the age of the dinosaurs right over here. When you look at your time clock, you can see it's a relatively recent time period."}, {"video_title": "Biodiversity flourishes in Phanerozoic eon Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Birds didn't exist until about 150 million years ago. The whole dinosaur age, which we kind of consider in our distant past, that's essentially the Mesozoic Era right here. So this is the age of the dinosaurs right over here. When you look at your time clock, you can see it's a relatively recent time period. And it actually ends with, we currently believe, a huge rock, a six mile in diameter rock, colliding with what is now the Yucatan Peninsula in Mexico, or right off the coast of the Yucatan Peninsula. And it destroyed all of the large land life forms, especially the dinosaurs. And to put all of this in perspective, and actually the thing that really was an aha moment for me, plants are 450 million years ago."}, {"video_title": "Biodiversity flourishes in Phanerozoic eon Cosmology & Astronomy Khan Academy.mp3", "Sentence": "When you look at your time clock, you can see it's a relatively recent time period. And it actually ends with, we currently believe, a huge rock, a six mile in diameter rock, colliding with what is now the Yucatan Peninsula in Mexico, or right off the coast of the Yucatan Peninsula. And it destroyed all of the large land life forms, especially the dinosaurs. And to put all of this in perspective, and actually the thing that really was an aha moment for me, plants are 450 million years ago. Grass, I kind of use this fundamental thing in nature. But grass has only been around for about, I've seen multiple estimates, 40 to 70 million years. Grass is a relatively new thing on the planet."}, {"video_title": "Biodiversity flourishes in Phanerozoic eon Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And to put all of this in perspective, and actually the thing that really was an aha moment for me, plants are 450 million years ago. Grass, I kind of use this fundamental thing in nature. But grass has only been around for about, I've seen multiple estimates, 40 to 70 million years. Grass is a relatively new thing on the planet. Flowers have only been around for 130 million years. So there was a time where you had dinosaurs, but you did not have flowers, and you did not have grass. And so you fast forward all the way."}, {"video_title": "Biodiversity flourishes in Phanerozoic eon Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Grass is a relatively new thing on the planet. Flowers have only been around for 130 million years. So there was a time where you had dinosaurs, but you did not have flowers, and you did not have grass. And so you fast forward all the way. And so when you look at this scale, it's kind of funny to look at. They say, OK, this is the time period where the dinosaurs showed up. This whole brown line is where the mammals showed up."}, {"video_title": "Biodiversity flourishes in Phanerozoic eon Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And so you fast forward all the way. And so when you look at this scale, it's kind of funny to look at. They say, OK, this is the time period where the dinosaurs showed up. This whole brown line is where the mammals showed up. So the dinosaurs started to show up along with the mammals. And then, of course, the dinosaurs died out here. Our ancestors, when the giant rock hit the Earth, must have been burrowed in holes or able to stash some food away or who knows what, and didn't get fully affected."}, {"video_title": "Biodiversity flourishes in Phanerozoic eon Cosmology & Astronomy Khan Academy.mp3", "Sentence": "This whole brown line is where the mammals showed up. So the dinosaurs started to show up along with the mammals. And then, of course, the dinosaurs died out here. Our ancestors, when the giant rock hit the Earth, must have been burrowed in holes or able to stash some food away or who knows what, and didn't get fully affected. I'm sure most of the large mammals were destroyed. But what's almost humbling or almost humorous or almost ridiculous when you look at this chart is they put a little dot. You can't even see it here."}, {"video_title": "Biodiversity flourishes in Phanerozoic eon Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Our ancestors, when the giant rock hit the Earth, must have been burrowed in holes or able to stash some food away or who knows what, and didn't get fully affected. I'm sure most of the large mammals were destroyed. But what's almost humbling or almost humorous or almost ridiculous when you look at this chart is they put a little dot. You can't even see it here. They say 2 million years ago, the first humans. And even this is being pretty generous when they say first humans. These are really the first pre-humans, the first humans that are the same as us."}, {"video_title": "Biodiversity flourishes in Phanerozoic eon Cosmology & Astronomy Khan Academy.mp3", "Sentence": "You can't even see it here. They say 2 million years ago, the first humans. And even this is being pretty generous when they say first humans. These are really the first pre-humans, the first humans that are the same as us. If you took one of those babies and you brought them up in the suburbs and gave them haircuts and stuff, they would be the same thing as we are. Those didn't exist until 200,000 years ago, give or take. 200,000 to 400,000 years ago, I've seen estimates."}, {"video_title": "Biodiversity flourishes in Phanerozoic eon Cosmology & Astronomy Khan Academy.mp3", "Sentence": "These are really the first pre-humans, the first humans that are the same as us. If you took one of those babies and you brought them up in the suburbs and gave them haircuts and stuff, they would be the same thing as we are. Those didn't exist until 200,000 years ago, give or take. 200,000 to 400,000 years ago, I've seen estimates. So this is actually a very generous period of time to say first humans. It's actually 200,000 years ago. And just to give you an idea of how new we are and how new our evolution is, it was only 5 million years ago."}, {"video_title": "Biodiversity flourishes in Phanerozoic eon Cosmology & Astronomy Khan Academy.mp3", "Sentence": "200,000 to 400,000 years ago, I've seen estimates. So this is actually a very generous period of time to say first humans. It's actually 200,000 years ago. And just to give you an idea of how new we are and how new our evolution is, it was only 5 million years ago. And I mentioned this in a previous video. It was only 5 million years ago. So this is just to get a sense."}, {"video_title": "Biodiversity flourishes in Phanerozoic eon Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And just to give you an idea of how new we are and how new our evolution is, it was only 5 million years ago. And I mentioned this in a previous video. It was only 5 million years ago. So this is just to get a sense. This is zero years. Homo sapiens sapien, only around for 200,000 years. The Neanderthals, they were cousin species."}, {"video_title": "Biodiversity flourishes in Phanerozoic eon Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So this is just to get a sense. This is zero years. Homo sapiens sapien, only around for 200,000 years. The Neanderthals, they were cousin species. They weren't our ancestors. Many people think they were. They were cousin species."}, {"video_title": "Biodiversity flourishes in Phanerozoic eon Cosmology & Astronomy Khan Academy.mp3", "Sentence": "The Neanderthals, they were cousin species. They weren't our ancestors. Many people think they were. They were cousin species. We come from the same root. Although there are now theories that they might have remixed in with homo sapiens. So maybe some of us have some Neanderthal DNA."}, {"video_title": "Biodiversity flourishes in Phanerozoic eon Cosmology & Astronomy Khan Academy.mp3", "Sentence": "They were cousin species. We come from the same root. Although there are now theories that they might have remixed in with homo sapiens. So maybe some of us have some Neanderthal DNA. And it shouldn't be viewed as an insult. They had big brains. Well, they didn't necessarily have big brains."}, {"video_title": "Biodiversity flourishes in Phanerozoic eon Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So maybe some of us have some Neanderthal DNA. And it shouldn't be viewed as an insult. They had big brains. Well, they didn't necessarily have big brains. They had big heads. But that seems to imply a big brain. But who knows?"}, {"video_title": "Biodiversity flourishes in Phanerozoic eon Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Well, they didn't necessarily have big brains. They had big heads. But that seems to imply a big brain. But who knows? We always tend to portray them as somehow inferior. But I don't want to get into the political correctness of how to portray Neanderthals. But anyway, this is a very small period of time."}, {"video_title": "Biodiversity flourishes in Phanerozoic eon Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But who knows? We always tend to portray them as somehow inferior. But I don't want to get into the political correctness of how to portray Neanderthals. But anyway, this is a very small period of time. If you go 2 million years, then you get to the pre-human ancestors. And our family tree only diverged from the chimpanzees 5 million years ago. If you draw that on this clock right here, it would be like 2 pixels, or maybe not even 2 pixels, is when we diverged from the chimpanzees."}, {"video_title": "A Universe Smaller than the Observable.mp3", "Sentence": "Every video until now, we've been working from the assumption that the observable universe is smaller than the entire universe. And if you go by the cosmic inflation theory, and it was come up, or it was founded by Alan Guth, and I have an almost personal connection to Alan Guth. When I was at MIT, I always used to go to this Chinese food truck, and I always used to show up at the food truck like two seconds before Alan Guth. Like he was always one or two people behind me in line. But anyway, he was the founder of the cosmic inflation theory, which is basically this idea that in the very early moments, or the very early period after the Big Bang, we went through kind of this major inflation in the expansion of space. But anyway, if based on the theory of cosmic inflation, then the observable universe is on the order of, or maybe another way to say it, the entire universe is on the order of 10 to the 23 times the size of the observable universe. So that would mean, this is just a tiny, tiny fraction."}, {"video_title": "A Universe Smaller than the Observable.mp3", "Sentence": "Like he was always one or two people behind me in line. But anyway, he was the founder of the cosmic inflation theory, which is basically this idea that in the very early moments, or the very early period after the Big Bang, we went through kind of this major inflation in the expansion of space. But anyway, if based on the theory of cosmic inflation, then the observable universe is on the order of, or maybe another way to say it, the entire universe is on the order of 10 to the 23 times the size of the observable universe. So that would mean, this is just a tiny, tiny fraction. I mean, this is an unimaginably large number. In fact, it is unimaginable. So already everything we've talked about, this itself is a huge, this is an incomprehensible amount of space, but this is an incomprehensible multiple of this incomprehensible amount of space."}, {"video_title": "A Universe Smaller than the Observable.mp3", "Sentence": "So that would mean, this is just a tiny, tiny fraction. I mean, this is an unimaginably large number. In fact, it is unimaginable. So already everything we've talked about, this itself is a huge, this is an incomprehensible amount of space, but this is an incomprehensible multiple of this incomprehensible amount of space. And that's just based on that theory. But it is possible, we cannot rule out even the idea that the actual universe is smaller than the observable universe. And that one is in some ways even more mind blowing than the idea that the universe is this big."}, {"video_title": "A Universe Smaller than the Observable.mp3", "Sentence": "So already everything we've talked about, this itself is a huge, this is an incomprehensible amount of space, but this is an incomprehensible multiple of this incomprehensible amount of space. And that's just based on that theory. But it is possible, we cannot rule out even the idea that the actual universe is smaller than the observable universe. And that one is in some ways even more mind blowing than the idea that the universe is this big. The fact that what we're observing is actually larger than the actual universe. And so you might say, well, Sal, that's impossible. But just think about it a little bit."}, {"video_title": "A Universe Smaller than the Observable.mp3", "Sentence": "And that one is in some ways even more mind blowing than the idea that the universe is this big. The fact that what we're observing is actually larger than the actual universe. And so you might say, well, Sal, that's impossible. But just think about it a little bit. This is the observable universe. And the way we've depicted it is based on how long the light has taken to reach us. We've already covered before that this point in space is now 46 billion light years away, not 13.7 the way it looks right over here."}, {"video_title": "A Universe Smaller than the Observable.mp3", "Sentence": "But just think about it a little bit. This is the observable universe. And the way we've depicted it is based on how long the light has taken to reach us. We've already covered before that this point in space is now 46 billion light years away, not 13.7 the way it looks right over here. It just took 13.7 billion years to reach us. If there's any photon that would take longer than 13.7 billion years to reach us, it hasn't reached us yet. Because it could have only started 13.7 billion years ago, so they're on their way."}, {"video_title": "A Universe Smaller than the Observable.mp3", "Sentence": "We've already covered before that this point in space is now 46 billion light years away, not 13.7 the way it looks right over here. It just took 13.7 billion years to reach us. If there's any photon that would take longer than 13.7 billion years to reach us, it hasn't reached us yet. Because it could have only started 13.7 billion years ago, so they're on their way. And they started at some point outside of our observable universe, so our observable universe will grow over time. But with that said, let's imagine that the actual universe is a subset of this observable universe. Let's say it's roughly half the diameter."}, {"video_title": "A Universe Smaller than the Observable.mp3", "Sentence": "Because it could have only started 13.7 billion years ago, so they're on their way. And they started at some point outside of our observable universe, so our observable universe will grow over time. But with that said, let's imagine that the actual universe is a subset of this observable universe. Let's say it's roughly half the diameter. So let's say it looks like this. Maybe I'll make it a little bit of an oval. Maybe the actual universe, and this is just to be a little bit provocative, and it's not impossible."}, {"video_title": "A Universe Smaller than the Observable.mp3", "Sentence": "Let's say it's roughly half the diameter. So let's say it looks like this. Maybe I'll make it a little bit of an oval. Maybe the actual universe, and this is just to be a little bit provocative, and it's not impossible. Let's say that this is the actual universe. And the way I drew it, it makes it look like Earth is the center, that we're the center of it. But remember, this is very likely to be the surface, or it is curved."}, {"video_title": "A Universe Smaller than the Observable.mp3", "Sentence": "Maybe the actual universe, and this is just to be a little bit provocative, and it's not impossible. Let's say that this is the actual universe. And the way I drew it, it makes it look like Earth is the center, that we're the center of it. But remember, this is very likely to be the surface, or it is curved. It has a slight curvature, but it could very well be the surface of a four-dimensional object. And maybe the simplest one to visualize is a four-dimensional sphere. So if you really wanted to visualize this right, this whole volume, and remember, this whole picture, it keeps looking two-dimensional, but it has depth."}, {"video_title": "A Universe Smaller than the Observable.mp3", "Sentence": "But remember, this is very likely to be the surface, or it is curved. It has a slight curvature, but it could very well be the surface of a four-dimensional object. And maybe the simplest one to visualize is a four-dimensional sphere. So if you really wanted to visualize this right, this whole volume, and remember, this whole picture, it keeps looking two-dimensional, but it has depth. It is a volume of space, an incredibly vast volume of space. And so what I've done here is, this is an ellipsoid right here. It's an elliptical volume of space that I've bubbled out right over here."}, {"video_title": "A Universe Smaller than the Observable.mp3", "Sentence": "So if you really wanted to visualize this right, this whole volume, and remember, this whole picture, it keeps looking two-dimensional, but it has depth. It is a volume of space, an incredibly vast volume of space. And so what I've done here is, this is an ellipsoid right here. It's an elliptical volume of space that I've bubbled out right over here. But if this was really the entire universe, and if the entire universe really were the surface of a four-dimensional sphere, then the reality is that this entire space could be represented like this. It could be represented as the surface. If this was a four-dimensional sphere, obviously the way I can only draw three-dimensional spheres, but let me show you that it has some, that it's not just a circle, that it actually has some depth to it."}, {"video_title": "A Universe Smaller than the Observable.mp3", "Sentence": "It's an elliptical volume of space that I've bubbled out right over here. But if this was really the entire universe, and if the entire universe really were the surface of a four-dimensional sphere, then the reality is that this entire space could be represented like this. It could be represented as the surface. If this was a four-dimensional sphere, obviously the way I can only draw three-dimensional spheres, but let me show you that it has some, that it's not just a circle, that it actually has some depth to it. And I can even shade it right over here. And so you can imagine that this point over here is actually the same thing as that point over there, that they have wrapped around, that they're connected right at the back over here. Let me show the draw, go behind."}, {"video_title": "A Universe Smaller than the Observable.mp3", "Sentence": "If this was a four-dimensional sphere, obviously the way I can only draw three-dimensional spheres, but let me show you that it has some, that it's not just a circle, that it actually has some depth to it. And I can even shade it right over here. And so you can imagine that this point over here is actually the same thing as that point over there, that they have wrapped around, that they're connected right at the back over here. Let me show the draw, go behind. That they're connected right over there. And that this point and this point are actually the same point, that they've wrapped around. Maybe they've wrapped around, actually the way I've drawn it right here, they would actually all wrap around right back there at that point, if I'm visualizing properly."}, {"video_title": "A Universe Smaller than the Observable.mp3", "Sentence": "Let me show the draw, go behind. That they're connected right over there. And that this point and this point are actually the same point, that they've wrapped around. Maybe they've wrapped around, actually the way I've drawn it right here, they would actually all wrap around right back there at that point, if I'm visualizing properly. But if you go in any one direction, you would come back on the other side of the surface. If you go, let's say that Earth is right here. The way we've depicted it, Earth is the center."}, {"video_title": "A Universe Smaller than the Observable.mp3", "Sentence": "Maybe they've wrapped around, actually the way I've drawn it right here, they would actually all wrap around right back there at that point, if I'm visualizing properly. But if you go in any one direction, you would come back on the other side of the surface. If you go, let's say that Earth is right here. The way we've depicted it, Earth is the center. But we see that when you look at it like this, there is no center to the surface of a sphere, even a four-dimensional sphere. So in this sense, if you go in any one direction, you'll come back out the other side. So if you start from Earth and you go in that direction, once you get there, you're really here again."}, {"video_title": "A Universe Smaller than the Observable.mp3", "Sentence": "The way we've depicted it, Earth is the center. But we see that when you look at it like this, there is no center to the surface of a sphere, even a four-dimensional sphere. So in this sense, if you go in any one direction, you'll come back out the other side. So if you start from Earth and you go in that direction, once you get there, you're really here again. And then you would come back to Earth. And so if this were the case, if the actual volume of the true universe was smaller than what it looks like, the observable universe, then what's all this stuff on the outside? And to think about it, think about what would happen."}, {"video_title": "A Universe Smaller than the Observable.mp3", "Sentence": "So if you start from Earth and you go in that direction, once you get there, you're really here again. And then you would come back to Earth. And so if this were the case, if the actual volume of the true universe was smaller than what it looks like, the observable universe, then what's all this stuff on the outside? And to think about it, think about what would happen. If 13.7 billion years ago, when we were in that primitive state, where that background radiation, those photons, that electromagnetic waves are being released. Let's say they get released. And those photons, on their first pass, and I think you know where this is going, on their first pass, they would get to us in about, this looks like a distance of about, I don't know, this looks like about 6 billion years."}, {"video_title": "A Universe Smaller than the Observable.mp3", "Sentence": "And to think about it, think about what would happen. If 13.7 billion years ago, when we were in that primitive state, where that background radiation, those photons, that electromagnetic waves are being released. Let's say they get released. And those photons, on their first pass, and I think you know where this is going, on their first pass, they would get to us in about, this looks like a distance of about, I don't know, this looks like about 6 billion years. Then they would pass us up, and then they would get back to this point again in another 6 billion years, and then they would come back here. And so that very first pass photon are going to be right over here. And from our point of view, we're not going to see them for a couple of billion years."}, {"video_title": "A Universe Smaller than the Observable.mp3", "Sentence": "And those photons, on their first pass, and I think you know where this is going, on their first pass, they would get to us in about, this looks like a distance of about, I don't know, this looks like about 6 billion years. Then they would pass us up, and then they would get back to this point again in another 6 billion years, and then they would come back here. And so that very first pass photon are going to be right over here. And from our point of view, we're not going to see them for a couple of billion years. And so when we do see them, we're going to perceive them, we're going to say, wow, it took 15, 16 billion years for that photon to get to me. That must be from something out here. But the reality is, it's a photon from something within a smaller physical universe, within a smaller actual universe, that's just taken several passes by us."}, {"video_title": "A Universe Smaller than the Observable.mp3", "Sentence": "And from our point of view, we're not going to see them for a couple of billion years. And so when we do see them, we're going to perceive them, we're going to say, wow, it took 15, 16 billion years for that photon to get to me. That must be from something out here. But the reality is, it's a photon from something within a smaller physical universe, within a smaller actual universe, that's just taken several passes by us. And we're just seeing a pass after 14 billion years. We just think it's from something further out. Now the other thing is, if this was the case, if we could just go in one direction of the universe and then come out of the other side, and if all of that was within the observable universe, wouldn't we be able to tell?"}, {"video_title": "A Universe Smaller than the Observable.mp3", "Sentence": "But the reality is, it's a photon from something within a smaller physical universe, within a smaller actual universe, that's just taken several passes by us. And we're just seeing a pass after 14 billion years. We just think it's from something further out. Now the other thing is, if this was the case, if we could just go in one direction of the universe and then come out of the other side, and if all of that was within the observable universe, wouldn't we be able to tell? Wouldn't we be able to look in two directions and see the same thing from a different perspective? And the answer there is to think about what happens. Or actually, wouldn't we even be able to see ourselves?"}, {"video_title": "A Universe Smaller than the Observable.mp3", "Sentence": "Now the other thing is, if this was the case, if we could just go in one direction of the universe and then come out of the other side, and if all of that was within the observable universe, wouldn't we be able to tell? Wouldn't we be able to look in two directions and see the same thing from a different perspective? And the answer there is to think about what happens. Or actually, wouldn't we even be able to see ourselves? Because if we emit some light, and it would take maybe, I don't know how far that is, let's say that's 6 or 7 billion light years to get right over here, which would be right over there. And then it would take another 6 or 7 billion light years to get over there. So maybe that background radiation we're seeing is actually background radiation emitted from that exact point in space that we are right now, or from a very similar point in space to where we are right now."}, {"video_title": "A Universe Smaller than the Observable.mp3", "Sentence": "Or actually, wouldn't we even be able to see ourselves? Because if we emit some light, and it would take maybe, I don't know how far that is, let's say that's 6 or 7 billion light years to get right over here, which would be right over there. And then it would take another 6 or 7 billion light years to get over there. So maybe that background radiation we're seeing is actually background radiation emitted from that exact point in space that we are right now, or from a very similar point in space to where we are right now. Or part of the background radiation is from a similar point in space that we are right now. So how come we can't just see ourselves? Well, I kind of just answered the question."}, {"video_title": "A Universe Smaller than the Observable.mp3", "Sentence": "So maybe that background radiation we're seeing is actually background radiation emitted from that exact point in space that we are right now, or from a very similar point in space to where we are right now. Or part of the background radiation is from a similar point in space that we are right now. So how come we can't just see ourselves? Well, I kind of just answered the question. That second path, if you're observing the same point in space, if you're observing light from the same point in space on a previous path, that light was emitted a long, long, long time ago. Maybe 13 billion years ago. And so it would be unrecognizable."}, {"video_title": "A Universe Smaller than the Observable.mp3", "Sentence": "Well, I kind of just answered the question. That second path, if you're observing the same point in space, if you're observing light from the same point in space on a previous path, that light was emitted a long, long, long time ago. Maybe 13 billion years ago. And so it would be unrecognizable. The region, this region of space, the region of space that we are in right now, if we saw the same region of space 13 billion years ago, we just wouldn't recognize it. Now there are people attempting to see if there's some patterns, see if you can model how the universe would change. And if you see patterns, and maybe the actual universe is a subset of the observable."}, {"video_title": "A Universe Smaller than the Observable.mp3", "Sentence": "And so it would be unrecognizable. The region, this region of space, the region of space that we are in right now, if we saw the same region of space 13 billion years ago, we just wouldn't recognize it. Now there are people attempting to see if there's some patterns, see if you can model how the universe would change. And if you see patterns, and maybe the actual universe is a subset of the observable. We just haven't seen it yet. But it's completely a possibility. Hopefully I didn't confuse you."}, {"video_title": "A Universe Smaller than the Observable.mp3", "Sentence": "And if you see patterns, and maybe the actual universe is a subset of the observable. We just haven't seen it yet. But it's completely a possibility. Hopefully I didn't confuse you. I actually find this kind of an interesting idea. That these things that we think are, this light that has taken 13, let's say the light that's taken us 8 billion years to reach us, we think it's from something based on this scale, 8 billion light years out. It's actually further because the universe is expanding."}, {"video_title": "A Universe Smaller than the Observable.mp3", "Sentence": "Hopefully I didn't confuse you. I actually find this kind of an interesting idea. That these things that we think are, this light that has taken 13, let's say the light that's taken us 8 billion years to reach us, we think it's from something based on this scale, 8 billion light years out. It's actually further because the universe is expanding. So it would have actually traversed more space than that, but we think it's from something like that. But it could have been something further in if the actual universe is smaller and it's just on its second path. It's actually coming back again."}, {"video_title": "A Universe Smaller than the Observable.mp3", "Sentence": "It's actually further because the universe is expanding. So it would have actually traversed more space than that, but we think it's from something like that. But it could have been something further in if the actual universe is smaller and it's just on its second path. It's actually coming back again. And that's why it took 8 billion years to reach us. And we don't even recognize it because it looks very different than that region of space right now. Or that region of space after 4 billion years."}, {"video_title": "A Universe Smaller than the Observable.mp3", "Sentence": "It's actually coming back again. And that's why it took 8 billion years to reach us. And we don't even recognize it because it looks very different than that region of space right now. Or that region of space after 4 billion years. Looks completely different than it did when it first released. Anyway, hopefully I didn't confuse you too much. But I think this is a fascinating topic."}, {"video_title": "Chronometric revolution Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Chrono, relating to time, metri, time measurement. And we take many, many things for granted these days. We assume that we know what happened the last 50 years, the last 100 years, and now we're starting to assume we know what happened 10,000 years ago, or what happened to our planet 100 million years ago, or a billion years ago. But these are all very, very, very new phenomena, this ability to kind of shine a light on the past. And even the traditional notions of history, the traditional stories of what led to what, the political nations that formed, the migrations of people, and when they happened, that traditional notion of history is even fairly new when you think about just the scope of how long we think humans have now been on this planet. And that first, that traditional notion of history, you can kind of use the first chronometric revolution. And that first chronometric revolution that gives us this kind of traditional notion of history really just comes out of humanity's ability to write."}, {"video_title": "Chronometric revolution Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But these are all very, very, very new phenomena, this ability to kind of shine a light on the past. And even the traditional notions of history, the traditional stories of what led to what, the political nations that formed, the migrations of people, and when they happened, that traditional notion of history is even fairly new when you think about just the scope of how long we think humans have now been on this planet. And that first, that traditional notion of history, you can kind of use the first chronometric revolution. And that first chronometric revolution that gives us this kind of traditional notion of history really just comes out of humanity's ability to write. So the first, so writing, writing gives us our first chronometric revolution. Because this was the first time, even though we think humans or human-like creatures have been around for hundreds of thousands of years at this point, they weren't able to keep their stories in a very exact way. They might have had an oral tradition."}, {"video_title": "Chronometric revolution Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And that first chronometric revolution that gives us this kind of traditional notion of history really just comes out of humanity's ability to write. So the first, so writing, writing gives us our first chronometric revolution. Because this was the first time, even though we think humans or human-like creatures have been around for hundreds of thousands of years at this point, they weren't able to keep their stories in a very exact way. They might have had an oral tradition. It might have gone from one generation to the other. But with those oral traditions, things would get lost. And the most important information would get lost is how long ago did these stories start up?"}, {"video_title": "Chronometric revolution Cosmology & Astronomy Khan Academy.mp3", "Sentence": "They might have had an oral tradition. It might have gone from one generation to the other. But with those oral traditions, things would get lost. And the most important information would get lost is how long ago did these stories start up? And we weren't able, as a species, to really have a firm understanding of when things happened and how long ago things happened until writing became mainstream and until writing was done in a way that it became permanent. And our best sense of when this happened the first time was by the Sumerians with cuneiform. And this happened right around the third millennia BC, so around 5,000 years before the present time."}, {"video_title": "Chronometric revolution Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And the most important information would get lost is how long ago did these stories start up? And we weren't able, as a species, to really have a firm understanding of when things happened and how long ago things happened until writing became mainstream and until writing was done in a way that it became permanent. And our best sense of when this happened the first time was by the Sumerians with cuneiform. And this happened right around the third millennia BC, so around 5,000 years before the present time. And this is what some of that earliest writing looked like. This is actually a letter from, I believe, a king. And you can see it's just highly symbolic carvings."}, {"video_title": "Chronometric revolution Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And this happened right around the third millennia BC, so around 5,000 years before the present time. And this is what some of that earliest writing looked like. This is actually a letter from, I believe, a king. And you can see it's just highly symbolic carvings. This is what we more traditionally associate with cuneiform. And it was symbolic-based, as opposed to now most of our languages are based on phonetics, so you have fewer symbols that can represent more meanings. But this was a huge technological revolution, I could say, for humanity."}, {"video_title": "Chronometric revolution Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And you can see it's just highly symbolic carvings. This is what we more traditionally associate with cuneiform. And it was symbolic-based, as opposed to now most of our languages are based on phonetics, so you have fewer symbols that can represent more meanings. But this was a huge technological revolution, I could say, for humanity. Because now, with the advent of cuneiform, you now had permanent writing that someone could look at 1,000 years later, 2,000 years later, and if they can decipher the cuneiform, they can get a written testimony of what was happening at that time. And they didn't have to rely on an oral tradition or even guess when that oral story might have started. But writing, since it only happened about 5,000 years ago, so this is 5,000 years before the present, or you could say 3,000 years BC, give or take, that was a start."}, {"video_title": "Chronometric revolution Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But this was a huge technological revolution, I could say, for humanity. Because now, with the advent of cuneiform, you now had permanent writing that someone could look at 1,000 years later, 2,000 years later, and if they can decipher the cuneiform, they can get a written testimony of what was happening at that time. And they didn't have to rely on an oral tradition or even guess when that oral story might have started. But writing, since it only happened about 5,000 years ago, so this is 5,000 years before the present, or you could say 3,000 years BC, give or take, that was a start. But this only gave us stories of about 5,000 years old. And even then, it was a very spotty historical record. We didn't really get really deep history, depending on where you are in the world, until really the last few thousand years."}, {"video_title": "Chronometric revolution Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But writing, since it only happened about 5,000 years ago, so this is 5,000 years before the present, or you could say 3,000 years BC, give or take, that was a start. But this only gave us stories of about 5,000 years old. And even then, it was a very spotty historical record. We didn't really get really deep history, depending on where you are in the world, until really the last few thousand years. But it was a start. This was the first chronometric revolution. But what you may or may not realize is that we are, frankly, I believe, at the very early stages of another chronometric revolution that has really just begun to accelerate in the last 50, 60, 70 years."}, {"video_title": "Chronometric revolution Cosmology & Astronomy Khan Academy.mp3", "Sentence": "We didn't really get really deep history, depending on where you are in the world, until really the last few thousand years. But it was a start. This was the first chronometric revolution. But what you may or may not realize is that we are, frankly, I believe, at the very early stages of another chronometric revolution that has really just begun to accelerate in the last 50, 60, 70 years. And this second chronometric revolution, I should write revolution up here too, this was a revolution. It allowed us to keep time in a permanent way, to understand things, to not have to talk to the people to whom something happened. We can see their written testimony of it."}, {"video_title": "Chronometric revolution Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But what you may or may not realize is that we are, frankly, I believe, at the very early stages of another chronometric revolution that has really just begun to accelerate in the last 50, 60, 70 years. And this second chronometric revolution, I should write revolution up here too, this was a revolution. It allowed us to keep time in a permanent way, to understand things, to not have to talk to the people to whom something happened. We can see their written testimony of it. But the second revolution really comes out of the advent of a lot of our understanding of modern science. So in the late 1800s, radioactivity gets discovered by Marie and Pierre Curie. So this is 1900 right here."}, {"video_title": "Chronometric revolution Cosmology & Astronomy Khan Academy.mp3", "Sentence": "We can see their written testimony of it. But the second revolution really comes out of the advent of a lot of our understanding of modern science. So in the late 1800s, radioactivity gets discovered by Marie and Pierre Curie. So this is 1900 right here. So this is relatively recent. Remember, we're talking about a species that has been around for several hundreds of thousands of years. And proto-humans have been around for millions of years."}, {"video_title": "Chronometric revolution Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So this is 1900 right here. So this is relatively recent. Remember, we're talking about a species that has been around for several hundreds of thousands of years. And proto-humans have been around for millions of years. And now, only 5,000 years ago, at least as far as we know, was the first writing. And then only a little over 100 years ago was a discovery of radioactivity. So radioactivity."}, {"video_title": "Chronometric revolution Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And proto-humans have been around for millions of years. And now, only 5,000 years ago, at least as far as we know, was the first writing. And then only a little over 100 years ago was a discovery of radioactivity. So radioactivity. And then the ability to use radioactivity. So radioactivity is interesting. It's this idea that, essentially, elements can change from one variation to another of an element over long periods of time, so through radioactivity."}, {"video_title": "Chronometric revolution Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So radioactivity. And then the ability to use radioactivity. So radioactivity is interesting. It's this idea that, essentially, elements can change from one variation to another of an element over long periods of time, so through radioactivity. So they become kind of this natural clock. No one had to go there and set up a timepiece for it. Luckily, there are these things that decay at a very predictable rate."}, {"video_title": "Chronometric revolution Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It's this idea that, essentially, elements can change from one variation to another of an element over long periods of time, so through radioactivity. So they become kind of this natural clock. No one had to go there and set up a timepiece for it. Luckily, there are these things that decay at a very predictable rate. So we discover radioactivity a little over 100 years ago. And then over the course of the 20th century, we got better and better, more sophisticated at really understanding radioactivity to be able to use it to measure the times of things. And if you fast-forward to the second half of the 20th century, so now, say, we're at 1950, this is where the second chronometric revolution really took hold."}, {"video_title": "Chronometric revolution Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Luckily, there are these things that decay at a very predictable rate. So we discover radioactivity a little over 100 years ago. And then over the course of the 20th century, we got better and better, more sophisticated at really understanding radioactivity to be able to use it to measure the times of things. And if you fast-forward to the second half of the 20th century, so now, say, we're at 1950, this is where the second chronometric revolution really took hold. This is where it really took hold, where we started to understand carbon-14 dating. We started to understand some of the other techniques that we talk about, where we can start to date older and older things. And I want to be clear."}, {"video_title": "Chronometric revolution Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And if you fast-forward to the second half of the 20th century, so now, say, we're at 1950, this is where the second chronometric revolution really took hold. This is where it really took hold, where we started to understand carbon-14 dating. We started to understand some of the other techniques that we talk about, where we can start to date older and older things. And I want to be clear. The understanding of radioactivity was just the beginning of this second chronometric revolution. The second chronometric revolution, which, frankly, we are still a part of, isn't just radioactivity. It's also understanding the expansion of the universe, the constancy, kind of the speed limit of light that now lets us figure out, wow, that background radiation we're getting, that must have been traveling for 13.7 billion years ago."}, {"video_title": "Chronometric revolution Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And I want to be clear. The understanding of radioactivity was just the beginning of this second chronometric revolution. The second chronometric revolution, which, frankly, we are still a part of, isn't just radioactivity. It's also understanding the expansion of the universe, the constancy, kind of the speed limit of light that now lets us figure out, wow, that background radiation we're getting, that must have been traveling for 13.7 billion years ago. So we can now look at evidence from our environment. And our environment is not just the Earth itself. It's radiation bombarding us from space that gives us clues as to not just the age of us, of humanity, the age of species, the age of the planet, but the age of the universe itself."}, {"video_title": "Chronometric revolution Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It's also understanding the expansion of the universe, the constancy, kind of the speed limit of light that now lets us figure out, wow, that background radiation we're getting, that must have been traveling for 13.7 billion years ago. So we can now look at evidence from our environment. And our environment is not just the Earth itself. It's radiation bombarding us from space that gives us clues as to not just the age of us, of humanity, the age of species, the age of the planet, but the age of the universe itself. So it isn't just about radioactivity. Radioactivity is a big part of our chronometric revolution. This is what allowed us for the first time, if we have layers on the Earth, people have known for a long time that if we assume that these layers haven't been jostled, that something at a lower layer down here is probably going to be older than at the upper layer because year after year you have deposits if it hasn't been messed up in some way."}, {"video_title": "Chronometric revolution Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It's radiation bombarding us from space that gives us clues as to not just the age of us, of humanity, the age of species, the age of the planet, but the age of the universe itself. So it isn't just about radioactivity. Radioactivity is a big part of our chronometric revolution. This is what allowed us for the first time, if we have layers on the Earth, people have known for a long time that if we assume that these layers haven't been jostled, that something at a lower layer down here is probably going to be older than at the upper layer because year after year you have deposits if it hasn't been messed up in some way. But no one knew. They said, okay, well, this is relative dating. This is older, this is younger, but we had no way of knowing that, hey, is this 1,000 years old, or is this a million years old, or is this a billion years old?"}, {"video_title": "Chronometric revolution Cosmology & Astronomy Khan Academy.mp3", "Sentence": "This is what allowed us for the first time, if we have layers on the Earth, people have known for a long time that if we assume that these layers haven't been jostled, that something at a lower layer down here is probably going to be older than at the upper layer because year after year you have deposits if it hasn't been messed up in some way. But no one knew. They said, okay, well, this is relative dating. This is older, this is younger, but we had no way of knowing that, hey, is this 1,000 years old, or is this a million years old, or is this a billion years old? But now with radioactivity, now we could start to say, hey, we can date some of the rocks here that are 150 million years old, and some of the rocks here are about 100 million years old. So maybe this fossil of a fish that we're finding or this primitive fish-like creature right over here, this would be between 100 and 150 million years old. And the only way we were able to do this was with being able to date things using radioactivity."}, {"video_title": "Chronometric revolution Cosmology & Astronomy Khan Academy.mp3", "Sentence": "This is older, this is younger, but we had no way of knowing that, hey, is this 1,000 years old, or is this a million years old, or is this a billion years old? But now with radioactivity, now we could start to say, hey, we can date some of the rocks here that are 150 million years old, and some of the rocks here are about 100 million years old. So maybe this fossil of a fish that we're finding or this primitive fish-like creature right over here, this would be between 100 and 150 million years old. And the only way we were able to do this was with being able to date things using radioactivity. But radioactivity is just a start. As I mentioned, we're getting better and better understanding of cosmology. We're getting better measurements of the universe itself."}, {"video_title": "Chronometric revolution Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And the only way we were able to do this was with being able to date things using radioactivity. But radioactivity is just a start. As I mentioned, we're getting better and better understanding of cosmology. We're getting better measurements of the universe itself. We're understanding physics at a deeper level. Now we can start to look at the genome and think about how the genome diverges from one species to another and how quickly it changes. So all of these things are just allowing us to get better and better refinements on the chronology."}, {"video_title": "Chronometric revolution Cosmology & Astronomy Khan Academy.mp3", "Sentence": "We're getting better measurements of the universe itself. We're understanding physics at a deeper level. Now we can start to look at the genome and think about how the genome diverges from one species to another and how quickly it changes. So all of these things are just allowing us to get better and better refinements on the chronology. Obviously, this is a start, but you still don't know, plus or minus 50 million years, how old this is and how this relates to other things that you might find. So I just wanted to point this out, that what we take for granted now, the age of the universe, the age of Earth at 4.5 million years old, humans being around for several hundreds of thousands of years, this understanding is a very, very, very new phenomenon. It's due to the second chronometric revolution that I think we are still a part of."}, {"video_title": "Chronometric revolution Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So all of these things are just allowing us to get better and better refinements on the chronology. Obviously, this is a start, but you still don't know, plus or minus 50 million years, how old this is and how this relates to other things that you might find. So I just wanted to point this out, that what we take for granted now, the age of the universe, the age of Earth at 4.5 million years old, humans being around for several hundreds of thousands of years, this understanding is a very, very, very new phenomenon. It's due to the second chronometric revolution that I think we are still a part of. And even the first chronometric revolution, this version of history, and I want to be clear, history was limited by this first chronometric revolution. It was limited by whatever was documented. But now maybe we can expand our notion of history."}, {"video_title": "Chronometric revolution Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It's due to the second chronometric revolution that I think we are still a part of. And even the first chronometric revolution, this version of history, and I want to be clear, history was limited by this first chronometric revolution. It was limited by whatever was documented. But now maybe we can expand our notion of history. And a lot of the videos that I've been working on have been for this big history project, which says, hey, before history was limited by the first chronometric revolution, to what was written, by what was testified by people and was made permanent in some way, now we have chronometry has taken us so that we can understand things into our deep past, before even the Earth has existed. So why not redefine history in a big way for it to encompass everything, for it to be big history? Anyway, I'll leave you there."}, {"video_title": "Chronometric revolution Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But now maybe we can expand our notion of history. And a lot of the videos that I've been working on have been for this big history project, which says, hey, before history was limited by the first chronometric revolution, to what was written, by what was testified by people and was made permanent in some way, now we have chronometry has taken us so that we can understand things into our deep past, before even the Earth has existed. So why not redefine history in a big way for it to encompass everything, for it to be big history? Anyway, I'll leave you there. And actually, I want to also emphasize that the second chronometric revolution is a big deal. It allows us to transform even our understandings of history. But even this first chronometric revolution, right over here, 5,000 years is still not very long in the entire scope of even human civilization."}, {"video_title": "Earth formation Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And our best sense of what actually happened is that there was a supernova in our vicinity of the galaxy. And this right here is a picture of a supernova remnant, actually the remnant for Kepler's supernova. The supernova in this picture actually happened 400 years ago in 1604. So right at the center, a star essentially exploded and for a few weeks was the brightest object in the night sky. And it was observed by Kepler and other people in 1604. And this is what it looks like now. So this is what we see is kind of the shock wave that's been traveling out for the past 400 years."}, {"video_title": "Earth formation Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So right at the center, a star essentially exploded and for a few weeks was the brightest object in the night sky. And it was observed by Kepler and other people in 1604. And this is what it looks like now. So this is what we see is kind of the shock wave that's been traveling out for the past 400 years. And so now it must be many light years across. It wasn't obviously, matter wasn't traveling at the speed of light, but it must have been traveling pretty, pretty fast, at least relativistic speeds where a reasonable fraction of the speed of light. So this has traveled a good bit out now."}, {"video_title": "Earth formation Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So this is what we see is kind of the shock wave that's been traveling out for the past 400 years. And so now it must be many light years across. It wasn't obviously, matter wasn't traveling at the speed of light, but it must have been traveling pretty, pretty fast, at least relativistic speeds where a reasonable fraction of the speed of light. So this has traveled a good bit out now. But what you can imagine is when you have the shock wave traveling out from a supernova, let's say you had a cloud of molecules, a cloud of gas, that before the shock wave came by, it just wasn't dense enough. It wasn't dense enough for gravity to take over and for it to accrete essentially into a solar system. But when the shock wave passes by, it compresses all of this gas and all of this material and all of these molecules."}, {"video_title": "Earth formation Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So this has traveled a good bit out now. But what you can imagine is when you have the shock wave traveling out from a supernova, let's say you had a cloud of molecules, a cloud of gas, that before the shock wave came by, it just wasn't dense enough. It wasn't dense enough for gravity to take over and for it to accrete essentially into a solar system. But when the shock wave passes by, it compresses all of this gas and all of this material and all of these molecules. So it now does have that critical density to form, to accrete into a star and a solar system. And so we think that's what's happening. The reason why we feel pretty strongly that it must have been caused by a supernova is that the only way that the really heavy elements can form or the only way that we know that they can form is in kind of the heat of a supernova."}, {"video_title": "Earth formation Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But when the shock wave passes by, it compresses all of this gas and all of this material and all of these molecules. So it now does have that critical density to form, to accrete into a star and a solar system. And so we think that's what's happening. The reason why we feel pretty strongly that it must have been caused by a supernova is that the only way that the really heavy elements can form or the only way that we know that they can form is in kind of the heat of a supernova. And our uranium, the uranium that seems to be in our solar system on Earth, seems to have formed roughly at the time of the formation of Earth, at about 4 and 1 half billion years ago. And we'll talk in a little bit more depth in future videos on exactly how people figure that out. But since the uranium seems about the same age as our solar system, it must have been formed at around the same time."}, {"video_title": "Earth formation Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "The reason why we feel pretty strongly that it must have been caused by a supernova is that the only way that the really heavy elements can form or the only way that we know that they can form is in kind of the heat of a supernova. And our uranium, the uranium that seems to be in our solar system on Earth, seems to have formed roughly at the time of the formation of Earth, at about 4 and 1 half billion years ago. And we'll talk in a little bit more depth in future videos on exactly how people figure that out. But since the uranium seems about the same age as our solar system, it must have been formed at around the same time. And so it must have been formed by a supernova. And it must be coming from a supernova. So a supernova shock wave must have passed through our part of the universe."}, {"video_title": "Earth formation Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But since the uranium seems about the same age as our solar system, it must have been formed at around the same time. And so it must have been formed by a supernova. And it must be coming from a supernova. So a supernova shock wave must have passed through our part of the universe. And that's a good reason for gas to get compressed and begin to accrete. So you fast forward a few million years ago that gas would have accreted into something like this. It would have reached the critical temperature, critical density, and pressure at the center for ignition to occur, for fusion to start to happen, for hydrogen to start fusing into helium."}, {"video_title": "Earth formation Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So a supernova shock wave must have passed through our part of the universe. And that's a good reason for gas to get compressed and begin to accrete. So you fast forward a few million years ago that gas would have accreted into something like this. It would have reached the critical temperature, critical density, and pressure at the center for ignition to occur, for fusion to start to happen, for hydrogen to start fusing into helium. This right here is our early sun. Around the sun, you have all of the gases and particles and molecules that had enough angular velocity to not fall into the sun, to go into orbit around the sun. They were actually supported by a little bit of pressure, too."}, {"video_title": "Earth formation Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It would have reached the critical temperature, critical density, and pressure at the center for ignition to occur, for fusion to start to happen, for hydrogen to start fusing into helium. This right here is our early sun. Around the sun, you have all of the gases and particles and molecules that had enough angular velocity to not fall into the sun, to go into orbit around the sun. They were actually supported by a little bit of pressure, too. Because you can kind of view this as kind of a big cloud of gas. So they're always bumping into each other. But for the most part, it was their angular velocity."}, {"video_title": "Earth formation Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "They were actually supported by a little bit of pressure, too. Because you can kind of view this as kind of a big cloud of gas. So they're always bumping into each other. But for the most part, it was their angular velocity. And over the next tens of millions of years, they'll slowly bump into each other and clump into each other. Even small particles have gravity. And they're going to slowly become rocks and asteroids and eventually what we'd call planetesimals, which are really kind of view them as seeds of planets or early planets."}, {"video_title": "Earth formation Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But for the most part, it was their angular velocity. And over the next tens of millions of years, they'll slowly bump into each other and clump into each other. Even small particles have gravity. And they're going to slowly become rocks and asteroids and eventually what we'd call planetesimals, which are really kind of view them as seeds of planets or early planets. And then those would have a reasonable amount of gravity. And other things would be attracted to them and slowly clump up to them. But this wasn't like a simple process."}, {"video_title": "Earth formation Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And they're going to slowly become rocks and asteroids and eventually what we'd call planetesimals, which are really kind of view them as seeds of planets or early planets. And then those would have a reasonable amount of gravity. And other things would be attracted to them and slowly clump up to them. But this wasn't like a simple process. You could imagine you might have one planetesimal form. And maybe there's another planetesimal form. And instead of having a nice, gentle, those two guys accreting into each other, they might have huge relative velocities and ram into each other and then just shatter."}, {"video_title": "Earth formation Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But this wasn't like a simple process. You could imagine you might have one planetesimal form. And maybe there's another planetesimal form. And instead of having a nice, gentle, those two guys accreting into each other, they might have huge relative velocities and ram into each other and then just shatter. So this wasn't just a nice, gentle process of constant accretion. It would actually have been a very violent process. It actually happened early in Earth's history."}, {"video_title": "Earth formation Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And instead of having a nice, gentle, those two guys accreting into each other, they might have huge relative velocities and ram into each other and then just shatter. So this wasn't just a nice, gentle process of constant accretion. It would actually have been a very violent process. It actually happened early in Earth's history. And we actually think this is why the moon formed. So at some point, you fast forward a little bit from this. Earth would have formed, or I should say the mass that eventually becomes our modern Earth would have been forming."}, {"video_title": "Earth formation Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It actually happened early in Earth's history. And we actually think this is why the moon formed. So at some point, you fast forward a little bit from this. Earth would have formed, or I should say the mass that eventually becomes our modern Earth would have been forming. Let me draw it over here. So let's say that that is our modern Earth. And what we think happened is that another protoplanet, or it was actually a planet because it was roughly the size of Mars, ran into what is eventually going to become our Earth."}, {"video_title": "Earth formation Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Earth would have formed, or I should say the mass that eventually becomes our modern Earth would have been forming. Let me draw it over here. So let's say that that is our modern Earth. And what we think happened is that another protoplanet, or it was actually a planet because it was roughly the size of Mars, ran into what is eventually going to become our Earth. And this is actually a picture of it. This is an artist's depiction of that collision, where this planet right here is the size of Mars. And it ran into what eventually would become Earth."}, {"video_title": "Earth formation Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And what we think happened is that another protoplanet, or it was actually a planet because it was roughly the size of Mars, ran into what is eventually going to become our Earth. And this is actually a picture of it. This is an artist's depiction of that collision, where this planet right here is the size of Mars. And it ran into what eventually would become Earth. And this we call Theia. This is Theia. And what we believe happened, and if you go onto the internet, you'll see some simulations that talk about this, is that we think it was a glancing blow, that it wasn't a direct hit that would have just kind of shattered each of them and turned them into one big molten ball."}, {"video_title": "Earth formation Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And it ran into what eventually would become Earth. And this we call Theia. This is Theia. And what we believe happened, and if you go onto the internet, you'll see some simulations that talk about this, is that we think it was a glancing blow, that it wasn't a direct hit that would have just kind of shattered each of them and turned them into one big molten ball. We think it was a glancing blow, something like this. So if this was essentially Earth, obviously Earth got changed dramatically once Theia ran into it. But Theia is right over here."}, {"video_title": "Earth formation Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And what we believe happened, and if you go onto the internet, you'll see some simulations that talk about this, is that we think it was a glancing blow, that it wasn't a direct hit that would have just kind of shattered each of them and turned them into one big molten ball. We think it was a glancing blow, something like this. So if this was essentially Earth, obviously Earth got changed dramatically once Theia ran into it. But Theia is right over here. And we think it was a glancing blow, where it came and it hit Earth at kind of an angle. And then obviously the combined energies from that interaction would have made both of them molten. And frankly, they probably already were molten, because you had a bunch of smaller collisions and accretion events and little things hitting the surface of probably both of them during this entire period."}, {"video_title": "Earth formation Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But Theia is right over here. And we think it was a glancing blow, where it came and it hit Earth at kind of an angle. And then obviously the combined energies from that interaction would have made both of them molten. And frankly, they probably already were molten, because you had a bunch of smaller collisions and accretion events and little things hitting the surface of probably both of them during this entire period. But this would have had a glancing blow on Earth and essentially splashed a bunch of molten material out into orbit. So it would have just come in, had a glancing blow on Earth, and then splashed a bunch of molten material. Some of it would have been captured by Earth."}, {"video_title": "Earth formation Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And frankly, they probably already were molten, because you had a bunch of smaller collisions and accretion events and little things hitting the surface of probably both of them during this entire period. But this would have had a glancing blow on Earth and essentially splashed a bunch of molten material out into orbit. So it would have just come in, had a glancing blow on Earth, and then splashed a bunch of molten material. Some of it would have been captured by Earth. So this is the before, and then the after. You could imagine Earth is kind of this molten, super hot ball. And some of it just gets splashed into orbit from the collision."}, {"video_title": "Earth formation Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Some of it would have been captured by Earth. So this is the before, and then the after. You could imagine Earth is kind of this molten, super hot ball. And some of it just gets splashed into orbit from the collision. And let me see if I can draw Theia here. So Theia has collided. And it's also molten now, because huge energies."}, {"video_title": "Earth formation Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And some of it just gets splashed into orbit from the collision. And let me see if I can draw Theia here. So Theia has collided. And it's also molten now, because huge energies. And it splashes some of it into orbit. And if we fast forward a little bit, this stuff that got splashed into orbit, it's going in that direction. That becomes our moon."}, {"video_title": "Earth formation Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And it's also molten now, because huge energies. And it splashes some of it into orbit. And if we fast forward a little bit, this stuff that got splashed into orbit, it's going in that direction. That becomes our moon. And then the rest of this material eventually kind of condenses back into a spherical shape and is what we now call our Earth. So that's how we actually think right now that the moon actually formed. And even after this happened, the Earth still had a lot more, I guess, violence to experience."}, {"video_title": "Earth formation Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "That becomes our moon. And then the rest of this material eventually kind of condenses back into a spherical shape and is what we now call our Earth. So that's how we actually think right now that the moon actually formed. And even after this happened, the Earth still had a lot more, I guess, violence to experience. So just to get a sense of where we are in the history of Earth, we're going to refer to this time clock a lot over the next few videos. This time clock starts right here at the formation of our solar system 4.6 billion years ago, probably coinciding with some type of supernova. And as we go clockwise on this diagram, we're moving forward in time."}, {"video_title": "Earth formation Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And even after this happened, the Earth still had a lot more, I guess, violence to experience. So just to get a sense of where we are in the history of Earth, we're going to refer to this time clock a lot over the next few videos. This time clock starts right here at the formation of our solar system 4.6 billion years ago, probably coinciding with some type of supernova. And as we go clockwise on this diagram, we're moving forward in time. And we're going to go all the way forward to the present period. And just so you understand some of the terminology, GA means billions of years ago, G for giga. MA means millions of years ago, M for mega."}, {"video_title": "Earth formation Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And as we go clockwise on this diagram, we're moving forward in time. And we're going to go all the way forward to the present period. And just so you understand some of the terminology, GA means billions of years ago, G for giga. MA means millions of years ago, M for mega. So where we are right now, the moon has formed. We're in what we call the Hadean period. Or actually, I shouldn't say period."}, {"video_title": "Earth formation Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "MA means millions of years ago, M for mega. So where we are right now, the moon has formed. We're in what we call the Hadean period. Or actually, I shouldn't say period. It's the Hadean eon of Earth. Period is actually another time period. So let me make this very clear."}, {"video_title": "Earth formation Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Or actually, I shouldn't say period. It's the Hadean eon of Earth. Period is actually another time period. So let me make this very clear. It's the Hadean. We are in the Hadean eon. And an eon is kind of the largest period of time that we talk about, especially relative to Earth."}, {"video_title": "Earth formation Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So let me make this very clear. It's the Hadean. We are in the Hadean eon. And an eon is kind of the largest period of time that we talk about, especially relative to Earth. And it's roughly 500 million to a billion years is an eon. And what makes the Hadean eon distinctive, well, from a geological point of view, what makes it distinctive is it's really we don't have any rocks from the Hadean period. We don't have any kind of macroscopic scale rocks from the Hadean period."}, {"video_title": "Earth formation Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And an eon is kind of the largest period of time that we talk about, especially relative to Earth. And it's roughly 500 million to a billion years is an eon. And what makes the Hadean eon distinctive, well, from a geological point of view, what makes it distinctive is it's really we don't have any rocks from the Hadean period. We don't have any kind of macroscopic scale rocks from the Hadean period. And that's because at that time, we believe, the Earth was just this molten ball of kind of magma and lava. And it was molten because it was the product of all of these accretion events and all of these collisions and all this kinetic energy turning into heat. So if you were to look at the surface of the Earth, if you were to be on the surface of the Earth during the Hadean eon, which you probably wouldn't want to be because you might get hit by a falling meteorite or probably burned by some magma or whatever, it would look like this."}, {"video_title": "Earth formation Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "We don't have any kind of macroscopic scale rocks from the Hadean period. And that's because at that time, we believe, the Earth was just this molten ball of kind of magma and lava. And it was molten because it was the product of all of these accretion events and all of these collisions and all this kinetic energy turning into heat. So if you were to look at the surface of the Earth, if you were to be on the surface of the Earth during the Hadean eon, which you probably wouldn't want to be because you might get hit by a falling meteorite or probably burned by some magma or whatever, it would look like this. And you wouldn't be able to breathe anyway. This is what the surface of the Earth would look like. It would look like a big magma pool."}, {"video_title": "Earth formation Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So if you were to look at the surface of the Earth, if you were to be on the surface of the Earth during the Hadean eon, which you probably wouldn't want to be because you might get hit by a falling meteorite or probably burned by some magma or whatever, it would look like this. And you wouldn't be able to breathe anyway. This is what the surface of the Earth would look like. It would look like a big magma pool. And that's why we don't have any rocks from there. Because the rocks were just constantly being recycled, being dissolved, and churned inside of this giant molten ball. And frankly, the Earth still is a giant molten ball."}, {"video_title": "Earth formation Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It would look like a big magma pool. And that's why we don't have any rocks from there. Because the rocks were just constantly being recycled, being dissolved, and churned inside of this giant molten ball. And frankly, the Earth still is a giant molten ball. It's just we live on the super thin, cooled crust of that molten ball. If you go right below that crust, and we'll talk a little bit more about that in future videos, you will get magma. And if you go dig deeper, you'll have liquid iron."}, {"video_title": "Earth formation Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And frankly, the Earth still is a giant molten ball. It's just we live on the super thin, cooled crust of that molten ball. If you go right below that crust, and we'll talk a little bit more about that in future videos, you will get magma. And if you go dig deeper, you'll have liquid iron. So I mean, it still is a molten ball. And this whole period is just a violent, not only was Earth itself volcanic, molten ball, it began to harden as you get to the late Hadean eon. But we also had stuff falling from the sky and constantly colliding with Earth and really just continuing to add to the heat of this molten ball."}, {"video_title": "Earth formation Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And if you go dig deeper, you'll have liquid iron. So I mean, it still is a molten ball. And this whole period is just a violent, not only was Earth itself volcanic, molten ball, it began to harden as you get to the late Hadean eon. But we also had stuff falling from the sky and constantly colliding with Earth and really just continuing to add to the heat of this molten ball. Anyway, I'll leave you there. And as you can imagine, at this point, as far as we can tell, there was no life on Earth. Some people believe that maybe some life could have formed in the late Hadean eon."}, {"video_title": "Earth formation Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But we also had stuff falling from the sky and constantly colliding with Earth and really just continuing to add to the heat of this molten ball. Anyway, I'll leave you there. And as you can imagine, at this point, as far as we can tell, there was no life on Earth. Some people believe that maybe some life could have formed in the late Hadean eon. But for the most part, this is completely inhospitable for any life forming. So I'll leave you there. And where we take up the next video, we'll talk a little bit about the Archean eon."}, {"video_title": "Hubble's law Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "What I want to do in this video is to put a few numbers behind it, or even better conceptualize what we've been talking about. So one way to think about it is that if at an early stage in the universe, I were to pick some points. So that's one point, another point, another point, another point. Let me just pick 9 points so that I have a proper grid. So this is at an early stage in the universe. If we fast forward a few billion years, and I'm clearly not drawing it to scale, all of these points have all moved away from each other. So this point is over here."}, {"video_title": "Hubble's law Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Let me just pick 9 points so that I have a proper grid. So this is at an early stage in the universe. If we fast forward a few billion years, and I'm clearly not drawing it to scale, all of these points have all moved away from each other. So this point is over here. Actually, let me draw another row, or actually another column, just to make it clear. So if we fast forward a few billion years, the universe has expanded, and so everything has moved away from everything. Let me color code it a little bit."}, {"video_title": "Hubble's law Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So this point is over here. Actually, let me draw another row, or actually another column, just to make it clear. So if we fast forward a few billion years, the universe has expanded, and so everything has moved away from everything. Let me color code it a little bit. Let me make this point magenta. So this point, the magenta point, is now here. This green point has now moved away from the magenta point."}, {"video_title": "Hubble's law Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Let me color code it a little bit. Let me make this point magenta. So this point, the magenta point, is now here. This green point has now moved away from the magenta point. And now this blue point has now moved away from the magenta point in that direction. And we could keep going. This yellow point is maybe over here now."}, {"video_title": "Hubble's law Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "This green point has now moved away from the magenta point. And now this blue point has now moved away from the magenta point in that direction. And we could keep going. This yellow point is maybe over here now. I think you get the general idea. And I'll just draw the other yellow points. So they've all moved away from each other."}, {"video_title": "Hubble's law Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "This yellow point is maybe over here now. I think you get the general idea. And I'll just draw the other yellow points. So they've all moved away from each other. So there's no center here. Everything is just expanding away from things next to it. And what you could see here is not only did this thing expand away from this, but this thing expanded away from this even further, because it had this expansion plus this expansion."}, {"video_title": "Hubble's law Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So they've all moved away from each other. So there's no center here. Everything is just expanding away from things next to it. And what you could see here is not only did this thing expand away from this, but this thing expanded away from this even further, because it had this expansion plus this expansion. Or another way to think about it is the apparent velocity with which something is expanding is going to be proportional to how far it is, because every point in between is also expanding away. And just to review a little bit of the visualization of this, one way to think of this, if you think of the universe as an infinite flat sheet, you can imagine that we're just taking a sheet of, I don't know, some type of sheet of stretching material and just stretching it out. We're just stretching it out."}, {"video_title": "Hubble's law Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And what you could see here is not only did this thing expand away from this, but this thing expanded away from this even further, because it had this expansion plus this expansion. Or another way to think about it is the apparent velocity with which something is expanding is going to be proportional to how far it is, because every point in between is also expanding away. And just to review a little bit of the visualization of this, one way to think of this, if you think of the universe as an infinite flat sheet, you can imagine that we're just taking a sheet of, I don't know, some type of sheet of stretching material and just stretching it out. We're just stretching it out. That's if we kind of imagine a more infinite universe that just goes off in every direction. We're just stretching that infinite sheet out so it has no boundaries, but we're still stretching it out. Another way to visualize it, and this is what we did earlier on, is you could imagine that the universe is the three-dimensional surface of a four-dimensional sphere or the three-dimensional surface of a hypersphere."}, {"video_title": "Hubble's law Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "We're just stretching it out. That's if we kind of imagine a more infinite universe that just goes off in every direction. We're just stretching that infinite sheet out so it has no boundaries, but we're still stretching it out. Another way to visualize it, and this is what we did earlier on, is you could imagine that the universe is the three-dimensional surface of a four-dimensional sphere or the three-dimensional surface of a hypersphere. So at an early stage in the universe, the sphere looked like this. And these points here, that magenta point is right over here. The green point is right over there."}, {"video_title": "Hubble's law Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Another way to visualize it, and this is what we did earlier on, is you could imagine that the universe is the three-dimensional surface of a four-dimensional sphere or the three-dimensional surface of a hypersphere. So at an early stage in the universe, the sphere looked like this. And these points here, that magenta point is right over here. The green point is right over there. Then we had the blue point up here. And then let me just draw the rest of the yellow points. And the yellow points are here."}, {"video_title": "Hubble's law Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "The green point is right over there. Then we had the blue point up here. And then let me just draw the rest of the yellow points. And the yellow points are here. They're all on the surface of this sphere. Obviously, I'm only dealing with two dimensions right now. It's nearly impossible, or maybe impossible, to imagine a three-dimensional surface of a four-dimensional sphere."}, {"video_title": "Hubble's law Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And the yellow points are here. They're all on the surface of this sphere. Obviously, I'm only dealing with two dimensions right now. It's nearly impossible, or maybe impossible, to imagine a three-dimensional surface of a four-dimensional sphere. But the analogy holds. If this is the surface of a balloon or the surface of a bubble, if the bubble were to expand over a few billion years, and once again, not drawn to scale, so now we have a bigger bubble here, this part of the surface is all going to expand. So once again, you have your magenta, you have your blue dot, you have your green dot right over here."}, {"video_title": "Hubble's law Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It's nearly impossible, or maybe impossible, to imagine a three-dimensional surface of a four-dimensional sphere. But the analogy holds. If this is the surface of a balloon or the surface of a bubble, if the bubble were to expand over a few billion years, and once again, not drawn to scale, so now we have a bigger bubble here, this part of the surface is all going to expand. So once again, you have your magenta, you have your blue dot, you have your green dot right over here. And then let me just draw the rest in yellow. So they will have all expanded away from each other on the surface of this sphere. And just to make it clear that this is a sphere, let me draw some contour lines."}, {"video_title": "Hubble's law Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So once again, you have your magenta, you have your blue dot, you have your green dot right over here. And then let me just draw the rest in yellow. So they will have all expanded away from each other on the surface of this sphere. And just to make it clear that this is a sphere, let me draw some contour lines. So this is a contour line. Just to make it clear that we are on the surface of an actual sphere. Now with that out of the way, let's think about how fast, or what is the apparent velocity with which things are moving away."}, {"video_title": "Hubble's law Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And just to make it clear that this is a sphere, let me draw some contour lines. So this is a contour line. Just to make it clear that we are on the surface of an actual sphere. Now with that out of the way, let's think about how fast, or what is the apparent velocity with which things are moving away. And remember, we're going to have to say not only how far things are moving away, but we're going to say how far they're moving away from, if the observer is us, depending on how far they already are. So what we're going to do, what we could say is, let me write this down, all objects moving away from each other, and the velocity, and the apparent velocity, the apparent relative velocity is proportional to distance. And what I've just written down here, this is why I wrote it down, this is a rephrasing of essentially Hubble's Law."}, {"video_title": "Hubble's law Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Now with that out of the way, let's think about how fast, or what is the apparent velocity with which things are moving away. And remember, we're going to have to say not only how far things are moving away, but we're going to say how far they're moving away from, if the observer is us, depending on how far they already are. So what we're going to do, what we could say is, let me write this down, all objects moving away from each other, and the velocity, and the apparent velocity, the apparent relative velocity is proportional to distance. And what I've just written down here, this is why I wrote it down, this is a rephrasing of essentially Hubble's Law. And he came up with this by just observing that when he looks, especially the further out he looks, the more red-shifted objects are. And not only were they moving faster and faster away from Earth, but they seemed to be moving faster and faster away from each other. So this is just a restating of Hubble's Law."}, {"video_title": "Hubble's law Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And what I've just written down here, this is why I wrote it down, this is a rephrasing of essentially Hubble's Law. And he came up with this by just observing that when he looks, especially the further out he looks, the more red-shifted objects are. And not only were they moving faster and faster away from Earth, but they seemed to be moving faster and faster away from each other. So this is just a restating of Hubble's Law. Or another way to say it is, from any point, let's say from the Earth, the velocity that something appears to be moving is going to be some constant times the distance that it is away from the observer. In this case, we are the observer. And we put this little zero, so this h here is called Hubble's Constant."}, {"video_title": "Hubble's law Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So this is just a restating of Hubble's Law. Or another way to say it is, from any point, let's say from the Earth, the velocity that something appears to be moving is going to be some constant times the distance that it is away from the observer. In this case, we are the observer. And we put this little zero, so this h here is called Hubble's Constant. And it's a very non-constant constant, because this constant will change depending on where we are in the evolution of the universe. So we put this little zero here, this little sub-zero right over here, to show that this is Hubble's Constant right now. And when we talk about distance, we're talking about the proper distance right now."}, {"video_title": "Hubble's law Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And we put this little zero, so this h here is called Hubble's Constant. And it's a very non-constant constant, because this constant will change depending on where we are in the evolution of the universe. So we put this little zero here, this little sub-zero right over here, to show that this is Hubble's Constant right now. And when we talk about distance, we're talking about the proper distance right now. And this has to be very important, because that proper distance is constantly changing as the universe expands. So the now will actually change slightly from the beginning of this video to the end of this video. But we can roughly say in our current period of time."}, {"video_title": "Hubble's law Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And when we talk about distance, we're talking about the proper distance right now. And this has to be very important, because that proper distance is constantly changing as the universe expands. So the now will actually change slightly from the beginning of this video to the end of this video. But we can roughly say in our current period of time. And when we say proper distance, we're talking about if you actually had rulers, and if you were to just lay them down instantaneously. Obviously we can't do something like that, but we can imagine doing something like that. So that's what we're talking about."}, {"video_title": "Hubble's law Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But we can roughly say in our current period of time. And when we say proper distance, we're talking about if you actually had rulers, and if you were to just lay them down instantaneously. Obviously we can't do something like that, but we can imagine doing something like that. So that's what we're talking about. So just to give a sense of, or do a little bit of math of how fast things are actually moving apart, the current Hubble Constant is 70.6 plus or minus 3.1. So we have observed some variation here. There is, I guess, some error to our actual measurements."}, {"video_title": "Hubble's law Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So that's what we're talking about. So just to give a sense of, or do a little bit of math of how fast things are actually moving apart, the current Hubble Constant is 70.6 plus or minus 3.1. So we have observed some variation here. There is, I guess, some error to our actual measurements. So that's kilometers per second per megaparsec. And remember, a parsec is roughly 3.2, 3.3 light years. So another way to think about it is, if this is where we are in the universe right now, and if this object right over here, if this distance right over here is 1 megaparsec, so 1 million parsecs, or 3.26 million light years from Earth, so this is roughly, just so we have a sense, this is 3.26 roughly, 3.26 million light years from Earth, then this object will appear to be moving away, although it's not moving in space."}, {"video_title": "Hubble's law Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "There is, I guess, some error to our actual measurements. So that's kilometers per second per megaparsec. And remember, a parsec is roughly 3.2, 3.3 light years. So another way to think about it is, if this is where we are in the universe right now, and if this object right over here, if this distance right over here is 1 megaparsec, so 1 million parsecs, or 3.26 million light years from Earth, so this is roughly, just so we have a sense, this is 3.26 roughly, 3.26 million light years from Earth, then this object will appear to be moving away, although it's not moving in space. Just the space that it's in is stretching in such a way that it looks to be moving at 70, it looks like it's moving at 70, based on its redshift, 70.6 kilometers per second away from us. So this is a huge velocity. 70.6 kilometers per second."}, {"video_title": "Hubble's law Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So another way to think about it is, if this is where we are in the universe right now, and if this object right over here, if this distance right over here is 1 megaparsec, so 1 million parsecs, or 3.26 million light years from Earth, so this is roughly, just so we have a sense, this is 3.26 roughly, 3.26 million light years from Earth, then this object will appear to be moving away, although it's not moving in space. Just the space that it's in is stretching in such a way that it looks to be moving at 70, it looks like it's moving at 70, based on its redshift, 70.6 kilometers per second away from us. So this is a huge velocity. 70.6 kilometers per second. This is a pretty fast velocity, but you have to remember, this is over 1 megaparsec. The Andromeda Galaxy is not even a megaparsec away. It was about 2.5 million light years, so it's about, I don't know, 0.7 or 0.8 of a megaparsec."}, {"video_title": "Hubble's law Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "70.6 kilometers per second. This is a pretty fast velocity, but you have to remember, this is over 1 megaparsec. The Andromeda Galaxy is not even a megaparsec away. It was about 2.5 million light years, so it's about, I don't know, 0.7 or 0.8 of a megaparsec. So if you look at a point in space a little bit further than the Andromeda Galaxy, it will look to be right now receding at about 70.6 kilometers per second. But what if you were to go twice that distance? If you were to look at something that's almost 7 light years away, 2 megaparsecs away."}, {"video_title": "Hubble's law Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It was about 2.5 million light years, so it's about, I don't know, 0.7 or 0.8 of a megaparsec. So if you look at a point in space a little bit further than the Andromeda Galaxy, it will look to be right now receding at about 70.6 kilometers per second. But what if you were to go twice that distance? If you were to look at something that's almost 7 light years away, 2 megaparsecs away. So if you were to look at this object over here, how fast would that be receding? Well, if you just look at it over here, it's 2 megaparsecs away, so it's going to be twice this. You're just going to multiply its distance, 2 megaparsecs times this."}, {"video_title": "Hubble's law Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "If you were to look at something that's almost 7 light years away, 2 megaparsecs away. So if you were to look at this object over here, how fast would that be receding? Well, if you just look at it over here, it's 2 megaparsecs away, so it's going to be twice this. You're just going to multiply its distance, 2 megaparsecs times this. The megaparsecs cancel out. So 70.6 times 2 is, so it's going to be moving, or it's going to look to be moving. It's not moving in space."}, {"video_title": "Hubble's law Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "You're just going to multiply its distance, 2 megaparsecs times this. The megaparsecs cancel out. So 70.6 times 2 is, so it's going to be moving, or it's going to look to be moving. It's not moving in space. Remember, space is just stretching. So its velocity, its apparent velocity, will be 70.6 times 2, so that's 141.2 kilometers per second. And this is, you know, one question."}, {"video_title": "Hubble's law Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It's not moving in space. Remember, space is just stretching. So its velocity, its apparent velocity, will be 70.6 times 2, so that's 141.2 kilometers per second. And this is, you know, one question. You say, well, how did Hubble know? You know, it's easy to, you could observe the redshift of objects moving away from us, but how did he know that they were moving away from each other? Well, if you were to look at the redshift of this object and say, wow, that's moving away at 70.6 kilometers per second, and then you were to look at the redshift of this and say, wow, that's moving away from us at 141.2 kilometers per second, then you also know that these two objects are moving away from each other at 70.6 kilometers per second."}, {"video_title": "Hubble's law Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And this is, you know, one question. You say, well, how did Hubble know? You know, it's easy to, you could observe the redshift of objects moving away from us, but how did he know that they were moving away from each other? Well, if you were to look at the redshift of this object and say, wow, that's moving away at 70.6 kilometers per second, and then you were to look at the redshift of this and say, wow, that's moving away from us at 141.2 kilometers per second, then you also know that these two objects are moving away from each other at 70.6 kilometers per second. And we could keep doing this over different distances, but hopefully this gives you a little bit more, a little bit bigger sense of things. And just remember, even though I said this is a huge distance, a megaparsec is further than it is to the Andromeda galaxy. The Andromeda galaxy is the nearest large galaxy to us."}, {"video_title": "Hubble's law Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Well, if you were to look at the redshift of this object and say, wow, that's moving away at 70.6 kilometers per second, and then you were to look at the redshift of this and say, wow, that's moving away from us at 141.2 kilometers per second, then you also know that these two objects are moving away from each other at 70.6 kilometers per second. And we could keep doing this over different distances, but hopefully this gives you a little bit more, a little bit bigger sense of things. And just remember, even though I said this is a huge distance, a megaparsec is further than it is to the Andromeda galaxy. The Andromeda galaxy is the nearest large galaxy to us. There are some smaller galaxies that are closer to us that are kind of satellite galaxies around the Milky Way, but the Andromeda is the nearest large galaxy to us. And we also know that we're talking about hundreds of billions of galaxies in just the observable universe. So very quickly, as you go near the edge of the observable universe, these velocities, the apparent distance at which things are moving away from us, start to become pretty, pretty significant."}, {"video_title": "Plate tectonics Difference between crust and lithosphere Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And it's really just the idea that the surface of the earth is made up of a bunch of these rigid plates. So it's broken up into a bunch of rigid plates. And these rigid plates move relative to each other. They move relative to each other and take everything that's on them for a ride. And the things that are on them include the continents. So it literally is talking about the movement of these plates. And over here, I have a picture I got off of Wikipedia of the actual plates."}, {"video_title": "Plate tectonics Difference between crust and lithosphere Cosmology & Astronomy Khan Academy.mp3", "Sentence": "They move relative to each other and take everything that's on them for a ride. And the things that are on them include the continents. So it literally is talking about the movement of these plates. And over here, I have a picture I got off of Wikipedia of the actual plates. And over here, you have the Pacific plate. Let me do that in a darker color. You have a Pacific plate."}, {"video_title": "Plate tectonics Difference between crust and lithosphere Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And over here, I have a picture I got off of Wikipedia of the actual plates. And over here, you have the Pacific plate. Let me do that in a darker color. You have a Pacific plate. You have a Nazca plate. You have a South American plate. I could keep going on."}, {"video_title": "Plate tectonics Difference between crust and lithosphere Cosmology & Astronomy Khan Academy.mp3", "Sentence": "You have a Pacific plate. You have a Nazca plate. You have a South American plate. I could keep going on. You have an Antarctic plate. It's actually, obviously, whenever you do a projection onto two dimensions of a surface of a sphere, the stuff at the bottom and the top look much bigger than they actually are. Antarctica isn't this big relative to, say, North America or South America."}, {"video_title": "Plate tectonics Difference between crust and lithosphere Cosmology & Astronomy Khan Academy.mp3", "Sentence": "I could keep going on. You have an Antarctic plate. It's actually, obviously, whenever you do a projection onto two dimensions of a surface of a sphere, the stuff at the bottom and the top look much bigger than they actually are. Antarctica isn't this big relative to, say, North America or South America. It's just that we've had to stretch it out to fill up the rectangle. But that's the Antarctic plate, North American plate. And you can see that they're actually moving relative to each other."}, {"video_title": "Plate tectonics Difference between crust and lithosphere Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Antarctica isn't this big relative to, say, North America or South America. It's just that we've had to stretch it out to fill up the rectangle. But that's the Antarctic plate, North American plate. And you can see that they're actually moving relative to each other. And that's what these arrows are depicting. You see right over here, the Nazca plate and the Pacific plate are moving away from each other. New land is forming here."}, {"video_title": "Plate tectonics Difference between crust and lithosphere Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And you can see that they're actually moving relative to each other. And that's what these arrows are depicting. You see right over here, the Nazca plate and the Pacific plate are moving away from each other. New land is forming here. We'll talk more about that in other videos. You see right over here, in the middle of the Atlantic Ocean, the African plate and the South American plate meet each other. And they're moving away from each other, which means that new land, more plate material, I guess you could say, is somehow being created right here."}, {"video_title": "Plate tectonics Difference between crust and lithosphere Cosmology & Astronomy Khan Academy.mp3", "Sentence": "New land is forming here. We'll talk more about that in other videos. You see right over here, in the middle of the Atlantic Ocean, the African plate and the South American plate meet each other. And they're moving away from each other, which means that new land, more plate material, I guess you could say, is somehow being created right here. And we'll talk about that in future videos in pushing these two plates apart. Now, before we go into the evidence for plate tonics, or even some of the more details about how plates are created, and some theories as to why the plates might move, what I want to do is get a little bit of the terminology of plate tectonics out of the way. Because sometimes people call them crustal plates, and that's not exactly right."}, {"video_title": "Plate tectonics Difference between crust and lithosphere Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And they're moving away from each other, which means that new land, more plate material, I guess you could say, is somehow being created right here. And we'll talk about that in future videos in pushing these two plates apart. Now, before we go into the evidence for plate tonics, or even some of the more details about how plates are created, and some theories as to why the plates might move, what I want to do is get a little bit of the terminology of plate tectonics out of the way. Because sometimes people call them crustal plates, and that's not exactly right. And to show you the difference, what I want to do is show you two different ways of classifying the different layers of the Earth, and then think about how they might relate to each other. So what you traditionally see, and actually I've made a video that goes into a lot more detail of this, is a breakdown of the chemical layers of the Earth. When I talk about chemical layers, I'm talking about what are the constituents of the different layers."}, {"video_title": "Plate tectonics Difference between crust and lithosphere Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Because sometimes people call them crustal plates, and that's not exactly right. And to show you the difference, what I want to do is show you two different ways of classifying the different layers of the Earth, and then think about how they might relate to each other. So what you traditionally see, and actually I've made a video that goes into a lot more detail of this, is a breakdown of the chemical layers of the Earth. When I talk about chemical layers, I'm talking about what are the constituents of the different layers. So when you talk of it in this term, the topmost layer, which is the thinnest layer, is the crust. Then below that is the mantle. Actually, let me show you the whole Earth, although I'm not going to draw it to scale."}, {"video_title": "Plate tectonics Difference between crust and lithosphere Cosmology & Astronomy Khan Academy.mp3", "Sentence": "When I talk about chemical layers, I'm talking about what are the constituents of the different layers. So when you talk of it in this term, the topmost layer, which is the thinnest layer, is the crust. Then below that is the mantle. Actually, let me show you the whole Earth, although I'm not going to draw it to scale. So if I were to draw the crust, the crust is the outer, the thinnest outer layer of the Earth. You can imagine the blue line itself is the crust. Then below that you have the mantle."}, {"video_title": "Plate tectonics Difference between crust and lithosphere Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Actually, let me show you the whole Earth, although I'm not going to draw it to scale. So if I were to draw the crust, the crust is the outer, the thinnest outer layer of the Earth. You can imagine the blue line itself is the crust. Then below that you have the mantle. So everything between the blue and the orange line is this over here is the mantle. Let me label the crust. The crust you can literally view as the actual blue pixels over here."}, {"video_title": "Plate tectonics Difference between crust and lithosphere Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Then below that you have the mantle. So everything between the blue and the orange line is this over here is the mantle. Let me label the crust. The crust you can literally view as the actual blue pixels over here. And then inside of the mantle you have the core. And when you do this very high level division, these are chemical divisions. This is saying that the crust is made up of different types of elements."}, {"video_title": "Plate tectonics Difference between crust and lithosphere Cosmology & Astronomy Khan Academy.mp3", "Sentence": "The crust you can literally view as the actual blue pixels over here. And then inside of the mantle you have the core. And when you do this very high level division, these are chemical divisions. This is saying that the crust is made up of different types of elements. Its makeup is different than the stuff that's in the mantle, which is made up of different things than what's inside of the core. It's not describing the mechanical properties of it. And when I talk about mechanical properties, I'm talking about whether something is."}, {"video_title": "Plate tectonics Difference between crust and lithosphere Cosmology & Astronomy Khan Academy.mp3", "Sentence": "This is saying that the crust is made up of different types of elements. Its makeup is different than the stuff that's in the mantle, which is made up of different things than what's inside of the core. It's not describing the mechanical properties of it. And when I talk about mechanical properties, I'm talking about whether something is. So mechanical properties are whether something is solid and rigid. Or maybe it's so hot and melted, it's kind of a magma or kind of a plastic solid. So then this would be the most brittle stuff."}, {"video_title": "Plate tectonics Difference between crust and lithosphere Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And when I talk about mechanical properties, I'm talking about whether something is. So mechanical properties are whether something is solid and rigid. Or maybe it's so hot and melted, it's kind of a magma or kind of a plastic solid. So then this would be the most brittle stuff. If it gets warmed up, if rock starts to melt a little bit, then you have something like a magma, or you can view it as a deformable or a plastic solid. When we talk about plastic, I'm not talking about the stuff that the case of your cell phone is made up. I'm talking about it's deformable."}, {"video_title": "Plate tectonics Difference between crust and lithosphere Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So then this would be the most brittle stuff. If it gets warmed up, if rock starts to melt a little bit, then you have something like a magma, or you can view it as a deformable or a plastic solid. When we talk about plastic, I'm not talking about the stuff that the case of your cell phone is made up. I'm talking about it's deformable. This rock is deformable because it's so hot and it's somewhat melted. It has somewhat, it kind of behaves like a fluid. It actually does behave like a fluid, but it's much more viscous."}, {"video_title": "Plate tectonics Difference between crust and lithosphere Cosmology & Astronomy Khan Academy.mp3", "Sentence": "I'm talking about it's deformable. This rock is deformable because it's so hot and it's somewhat melted. It has somewhat, it kind of behaves like a fluid. It actually does behave like a fluid, but it's much more viscous. It's much thicker and slower moving than what we would normally associate with a fluid like water. So this is viscous. This is a viscous fluid."}, {"video_title": "Plate tectonics Difference between crust and lithosphere Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It actually does behave like a fluid, but it's much more viscous. It's much thicker and slower moving than what we would normally associate with a fluid like water. So this is viscous. This is a viscous fluid. And then the most fluid would, of course, be the liquid state. This is what we mean when we talk about the mechanical properties. And when you look at this division over here, the crust is solid."}, {"video_title": "Plate tectonics Difference between crust and lithosphere Cosmology & Astronomy Khan Academy.mp3", "Sentence": "This is a viscous fluid. And then the most fluid would, of course, be the liquid state. This is what we mean when we talk about the mechanical properties. And when you look at this division over here, the crust is solid. The mantle actually has some parts of it that are solid. So the uppermost part of the mantle is solid. Then below that, it has a kind of, the rest of the mantle is kind of in this magma, this deformable, somewhat fluid state."}, {"video_title": "Plate tectonics Difference between crust and lithosphere Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And when you look at this division over here, the crust is solid. The mantle actually has some parts of it that are solid. So the uppermost part of the mantle is solid. Then below that, it has a kind of, the rest of the mantle is kind of in this magma, this deformable, somewhat fluid state. And depending on what depth you go into the mantle, there are kind of different levels of fluidity. And then the core, the outer layer of the core, the outer core is liquid because the temperature is so high. The inner core is made up of the same things and the temperature is even higher."}, {"video_title": "Plate tectonics Difference between crust and lithosphere Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Then below that, it has a kind of, the rest of the mantle is kind of in this magma, this deformable, somewhat fluid state. And depending on what depth you go into the mantle, there are kind of different levels of fluidity. And then the core, the outer layer of the core, the outer core is liquid because the temperature is so high. The inner core is made up of the same things and the temperature is even higher. But since the pressure is so high, it's actually solid. So that's why the mantle, crust, and core differentiations don't tell you about mechanical, whether it's solid, whether it's magma, or whether it's really a liquid. It just really tells you what the makeup is."}, {"video_title": "Plate tectonics Difference between crust and lithosphere Cosmology & Astronomy Khan Academy.mp3", "Sentence": "The inner core is made up of the same things and the temperature is even higher. But since the pressure is so high, it's actually solid. So that's why the mantle, crust, and core differentiations don't tell you about mechanical, whether it's solid, whether it's magma, or whether it's really a liquid. It just really tells you what the makeup is. Now to think about the makeup, and this is important for plate tectonics. Because when we talk about these plates, we're not talking about just the crust. We're talking about the outer rigid layer."}, {"video_title": "Plate tectonics Difference between crust and lithosphere Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It just really tells you what the makeup is. Now to think about the makeup, and this is important for plate tectonics. Because when we talk about these plates, we're not talking about just the crust. We're talking about the outer rigid layer. When we talk about that, let me just zoom in a little bit. Let me just zoom in. Let's say we zoomed in right over there."}, {"video_title": "Plate tectonics Difference between crust and lithosphere Cosmology & Astronomy Khan Academy.mp3", "Sentence": "We're talking about the outer rigid layer. When we talk about that, let me just zoom in a little bit. Let me just zoom in. Let's say we zoomed in right over there. So now we have the crust zoomed in. This right here is the crust. And then everything below here, we're actually talking about the upper mantle."}, {"video_title": "Plate tectonics Difference between crust and lithosphere Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Let's say we zoomed in right over there. So now we have the crust zoomed in. This right here is the crust. And then everything below here, we're actually talking about the upper mantle. So we're talking about, we haven't gotten too deep in the mantle right here. So that's why we call it the upper mantle. Now right below the crust, the mantle is cool enough that it is also in real solid form."}, {"video_title": "Plate tectonics Difference between crust and lithosphere Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And then everything below here, we're actually talking about the upper mantle. So we're talking about, we haven't gotten too deep in the mantle right here. So that's why we call it the upper mantle. Now right below the crust, the mantle is cool enough that it is also in real solid form. So this right here is solid mantle. And when we talk about the plates, we're actually talking about the outer solid layer. So that includes both the crust and the solid part of the mantle."}, {"video_title": "Plate tectonics Difference between crust and lithosphere Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Now right below the crust, the mantle is cool enough that it is also in real solid form. So this right here is solid mantle. And when we talk about the plates, we're actually talking about the outer solid layer. So that includes both the crust and the solid part of the mantle. And we call that the lithosphere. When people talk about plate tectonics, they shouldn't say crustal plates. They should call these lithospheric plates."}, {"video_title": "Plate tectonics Difference between crust and lithosphere Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So that includes both the crust and the solid part of the mantle. And we call that the lithosphere. When people talk about plate tectonics, they shouldn't say crustal plates. They should call these lithospheric plates. Lithospheric. And then below the lithosphere, you have the least viscous part of the mantle. Because the temperature is high enough for the rock to melt, but the pressure isn't so large as what will happen when you go into the lower part of the mantle that the fluid can actually kind of move past each other."}, {"video_title": "Plate tectonics Difference between crust and lithosphere Cosmology & Astronomy Khan Academy.mp3", "Sentence": "They should call these lithospheric plates. Lithospheric. And then below the lithosphere, you have the least viscous part of the mantle. Because the temperature is high enough for the rock to melt, but the pressure isn't so large as what will happen when you go into the lower part of the mantle that the fluid can actually kind of move past each other. Although still pretty viscous. It's still a magma. So this is still kind of in its magma state."}, {"video_title": "Plate tectonics Difference between crust and lithosphere Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Because the temperature is high enough for the rock to melt, but the pressure isn't so large as what will happen when you go into the lower part of the mantle that the fluid can actually kind of move past each other. Although still pretty viscous. It's still a magma. So this is still kind of in its magma state. And this fluid part of the mantle, we can't quite call it a liquid yet, but over large periods of time, it does have fluid properties. Essentially, the lithosphere is kind of riding on top of. We call this the asthenosphere."}, {"video_title": "Plate tectonics Difference between crust and lithosphere Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So this is still kind of in its magma state. And this fluid part of the mantle, we can't quite call it a liquid yet, but over large periods of time, it does have fluid properties. Essentially, the lithosphere is kind of riding on top of. We call this the asthenosphere. So when we talk about the lithosphere and asthenosphere, we're really talking about mechanical layers. The outer layer, the solid layer is the lithosphere. The more fluid layer right below that's the asthenosphere."}, {"video_title": "Radius of observable universe Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Right now, the best estimate of when the Big Bang occurred. And once again, I don't like the term that much, because it kind of implies some type of explosion. But what it really is is kind of an expansion of space, when space started to really start to expand from a singularity. But our best estimate of when this occurred is 13.7 billion years ago. And even though we're used to dealing with numbers in the billions, especially when we talk about large amounts of money and whatnot, this is an unbelievable amount of time. It seems like something that's tractable, but it really isn't. And in future videos, I'm actually going to talk about the timescale so we can really appreciate how long, or even start to appreciate, or appreciate that we can't appreciate how long 13.7 billion years is."}, {"video_title": "Radius of observable universe Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But our best estimate of when this occurred is 13.7 billion years ago. And even though we're used to dealing with numbers in the billions, especially when we talk about large amounts of money and whatnot, this is an unbelievable amount of time. It seems like something that's tractable, but it really isn't. And in future videos, I'm actually going to talk about the timescale so we can really appreciate how long, or even start to appreciate, or appreciate that we can't appreciate how long 13.7 billion years is. And I also want to emphasize that this is the current best estimate, even in my lifetime. Even in my lifetime that I actually knew about the Big Bang and that I would pay attention to what the best estimate was, this number has been moving around. So I suspect that in the future, this number might become more accurate or might move around some."}, {"video_title": "Radius of observable universe Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And in future videos, I'm actually going to talk about the timescale so we can really appreciate how long, or even start to appreciate, or appreciate that we can't appreciate how long 13.7 billion years is. And I also want to emphasize that this is the current best estimate, even in my lifetime. Even in my lifetime that I actually knew about the Big Bang and that I would pay attention to what the best estimate was, this number has been moving around. So I suspect that in the future, this number might become more accurate or might move around some. But this is our best guess. Now with that said, I want to think about what this tells us about the size of the observable universe. So if all of the expansion started 13.7 billion years ago, all of everything we know in our three-dimensional universe was in a single point, the longest that any photon of light could be traveling that's reaching us right now."}, {"video_title": "Radius of observable universe Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So I suspect that in the future, this number might become more accurate or might move around some. But this is our best guess. Now with that said, I want to think about what this tells us about the size of the observable universe. So if all of the expansion started 13.7 billion years ago, all of everything we know in our three-dimensional universe was in a single point, the longest that any photon of light could be traveling that's reaching us right now. So our eye is right, so let's say that that is my eye right over here, that's my eyelashes, just like that. The longest that some photon of light is just getting to my eye, or maybe it's just getting to the lens of a telescope. The longest that that could have been traveling is 13.7 billion years."}, {"video_title": "Radius of observable universe Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So if all of the expansion started 13.7 billion years ago, all of everything we know in our three-dimensional universe was in a single point, the longest that any photon of light could be traveling that's reaching us right now. So our eye is right, so let's say that that is my eye right over here, that's my eyelashes, just like that. The longest that some photon of light is just getting to my eye, or maybe it's just getting to the lens of a telescope. The longest that that could have been traveling is 13.7 billion years. So it could be traveling 13.7 billion years. So when we looked at that depiction, this I think was two or three videos ago of the observable universe, I drew this circle. And when we see light coming from these remote objects, that light is getting to us right here."}, {"video_title": "Radius of observable universe Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "The longest that that could have been traveling is 13.7 billion years. So it could be traveling 13.7 billion years. So when we looked at that depiction, this I think was two or three videos ago of the observable universe, I drew this circle. And when we see light coming from these remote objects, that light is getting to us right here. This is where we are. This is where, I guess, in the depiction the remote object was, but the light from that remote object is just now getting to us. And that light took 13.7 billion years to get to us."}, {"video_title": "Radius of observable universe Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And when we see light coming from these remote objects, that light is getting to us right here. This is where we are. This is where, I guess, in the depiction the remote object was, but the light from that remote object is just now getting to us. And that light took 13.7 billion years to get to us. Now what I'm going to hesitate to do, because we're talking over such large distances, and we're talking on such large timescales, and timescales over which space itself is expanding, we're going to see in this video that you cannot say that this object over here, this is not necessarily 13.7 billion light years away. If we're talking about smaller timescales, or I guess smaller distances, you could say approximately that. The expansion of the universe itself would not make as much of a difference."}, {"video_title": "Radius of observable universe Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And that light took 13.7 billion years to get to us. Now what I'm going to hesitate to do, because we're talking over such large distances, and we're talking on such large timescales, and timescales over which space itself is expanding, we're going to see in this video that you cannot say that this object over here, this is not necessarily 13.7 billion light years away. If we're talking about smaller timescales, or I guess smaller distances, you could say approximately that. The expansion of the universe itself would not make as much of a difference. And let me make it even more clear. I'm talking about an object over there. We could even talk about that coordinate in space."}, {"video_title": "Radius of observable universe Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "The expansion of the universe itself would not make as much of a difference. And let me make it even more clear. I'm talking about an object over there. We could even talk about that coordinate in space. And that coordinate in, and actually I should say that coordinate in space-time, because we're viewing it at a certain instant as well. But that coordinate is not 13.7 billion light years away from our current coordinate. And there's a couple of reasons to think about it."}, {"video_title": "Radius of observable universe Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "We could even talk about that coordinate in space. And that coordinate in, and actually I should say that coordinate in space-time, because we're viewing it at a certain instant as well. But that coordinate is not 13.7 billion light years away from our current coordinate. And there's a couple of reasons to think about it. First of all, think about it. That light was emitted 13.7 billion years ago. When that light was emitted, we were much closer to that coordinate."}, {"video_title": "Radius of observable universe Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And there's a couple of reasons to think about it. First of all, think about it. That light was emitted 13.7 billion years ago. When that light was emitted, we were much closer to that coordinate. This coordinate was much closer to that. Where we are in the universe right now was much closer to that point of the universe. The other thing to think about is as this, let me actually draw it."}, {"video_title": "Radius of observable universe Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "When that light was emitted, we were much closer to that coordinate. This coordinate was much closer to that. Where we are in the universe right now was much closer to that point of the universe. The other thing to think about is as this, let me actually draw it. So let's say that, let's go 300,000 years after that initial expansion of that singularity. So we're just 300,000 years into the universe's history right now. So this is roughly 300,000 years into the universe's life, I guess we could view it that way."}, {"video_title": "Radius of observable universe Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "The other thing to think about is as this, let me actually draw it. So let's say that, let's go 300,000 years after that initial expansion of that singularity. So we're just 300,000 years into the universe's history right now. So this is roughly 300,000 years into the universe's life, I guess we could view it that way. And let's say at that point, well first of all, at that point, things haven't differentiated in a meaningful way yet right now. I mean, we'll talk more about this when we talk about the cosmic microwave background radiation. But at this point of the universe, it was kind of this almost uniform white hot plasma of hydrogen."}, {"video_title": "Radius of observable universe Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So this is roughly 300,000 years into the universe's life, I guess we could view it that way. And let's say at that point, well first of all, at that point, things haven't differentiated in a meaningful way yet right now. I mean, we'll talk more about this when we talk about the cosmic microwave background radiation. But at this point of the universe, it was kind of this almost uniform white hot plasma of hydrogen. And we're going to talk about it was emitting microwave radiation. And we'll talk more about that in a future video. But let's just think about two points in this early universe."}, {"video_title": "Radius of observable universe Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But at this point of the universe, it was kind of this almost uniform white hot plasma of hydrogen. And we're going to talk about it was emitting microwave radiation. And we'll talk more about that in a future video. But let's just think about two points in this early universe. So in this early universe, let's say you have that point. And let's say you have the coordinate where we are right now. You have the coordinate where we are right now."}, {"video_title": "Radius of observable universe Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But let's just think about two points in this early universe. So in this early universe, let's say you have that point. And let's say you have the coordinate where we are right now. You have the coordinate where we are right now. In fact, I'll just make that roughly, I won't make it the center, just because I think it makes it easier to visualize if it's not the center. And let's say at that very early stage in the universe, if you were able to just take some rulers instantaneously and measure that, you would measure this distance to be 30 million light years. And let's just say right at that point, this object over here, I'll do it in magenta, this object over here emits a photon, maybe in the microwave frequency range."}, {"video_title": "Radius of observable universe Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "You have the coordinate where we are right now. In fact, I'll just make that roughly, I won't make it the center, just because I think it makes it easier to visualize if it's not the center. And let's say at that very early stage in the universe, if you were able to just take some rulers instantaneously and measure that, you would measure this distance to be 30 million light years. And let's just say right at that point, this object over here, I'll do it in magenta, this object over here emits a photon, maybe in the microwave frequency range. And we'll see that that was the range that it was emitting in. But it emits a photon. So right, and that photon is traveling at the speed of light."}, {"video_title": "Radius of observable universe Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And let's just say right at that point, this object over here, I'll do it in magenta, this object over here emits a photon, maybe in the microwave frequency range. And we'll see that that was the range that it was emitting in. But it emits a photon. So right, and that photon is traveling at the speed of light. It is light. And so that photon says, oh, you know what? I only got 30 million light years to travel."}, {"video_title": "Radius of observable universe Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So right, and that photon is traveling at the speed of light. It is light. And so that photon says, oh, you know what? I only got 30 million light years to travel. That's not too bad. I'm going to get there in 30 million years. And so I'm going to do it discrete."}, {"video_title": "Radius of observable universe Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "I only got 30 million light years to travel. That's not too bad. I'm going to get there in 30 million years. And so I'm going to do it discrete. The math is more complicated than what I'm doing here. But I really just want to give you the idea of what's going on here. So let's just say, well, that photon says, in about 10 million years, I should be right about at that coordinate."}, {"video_title": "Radius of observable universe Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And so I'm going to do it discrete. The math is more complicated than what I'm doing here. But I really just want to give you the idea of what's going on here. So let's just say, well, that photon says, in about 10 million years, I should be right about at that coordinate. I should be about 1 third of the distance. But what happens over the course of those 10 million years? Well, over the course of those 10 million years, the universe has expanded some."}, {"video_title": "Radius of observable universe Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So let's just say, well, that photon says, in about 10 million years, I should be right about at that coordinate. I should be about 1 third of the distance. But what happens over the course of those 10 million years? Well, over the course of those 10 million years, the universe has expanded some. The universe has expanded maybe a good deal. So let me draw the expanded universe. So after 10 million years, the universe might look like this."}, {"video_title": "Radius of observable universe Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Well, over the course of those 10 million years, the universe has expanded some. The universe has expanded maybe a good deal. So let me draw the expanded universe. So after 10 million years, the universe might look like this. Actually, it might even be bigger than that. Let me draw it like this. After 10 million years, the universe might have expanded a good bit."}, {"video_title": "Radius of observable universe Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So after 10 million years, the universe might look like this. Actually, it might even be bigger than that. Let me draw it like this. After 10 million years, the universe might have expanded a good bit. So this is 10 million years into the future. Still, on a cosmological time scale, still almost at the kind of the infancy of the universe, because we're talking about 13.7 billion years. So let's say 10 million years go by."}, {"video_title": "Radius of observable universe Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "After 10 million years, the universe might have expanded a good bit. So this is 10 million years into the future. Still, on a cosmological time scale, still almost at the kind of the infancy of the universe, because we're talking about 13.7 billion years. So let's say 10 million years go by. The universe has expanded. This coordinate, where we're sitting today at the present time, is now all the way over here. That coordinate where the photon was originally emitted is now going to be sitting right over here."}, {"video_title": "Radius of observable universe Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So let's say 10 million years go by. The universe has expanded. This coordinate, where we're sitting today at the present time, is now all the way over here. That coordinate where the photon was originally emitted is now going to be sitting right over here. And that photon, it said, OK, after 10 million light years, I'm going to get over there. And I'm approximating. And I'm doing it in a very discrete way."}, {"video_title": "Radius of observable universe Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "That coordinate where the photon was originally emitted is now going to be sitting right over here. And that photon, it said, OK, after 10 million light years, I'm going to get over there. And I'm approximating. And I'm doing it in a very discrete way. But I really just want to give you the idea. So that coordinate, where the photon roughly gets in 10 million light years, is about right over here. The whole universe has expanded."}, {"video_title": "Radius of observable universe Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And I'm doing it in a very discrete way. But I really just want to give you the idea. So that coordinate, where the photon roughly gets in 10 million light years, is about right over here. The whole universe has expanded. All the coordinates have gotten further away from each other. Now what just happened here? The universe has expanded."}, {"video_title": "Radius of observable universe Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "The whole universe has expanded. All the coordinates have gotten further away from each other. Now what just happened here? The universe has expanded. This distance that was 30 million light years, now, and I'm just making rough numbers here. I don't know the actual numbers here. Now it is actually, this is really just for the sake of giving you the idea of why, well, giving you the intuition of what's going on."}, {"video_title": "Radius of observable universe Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "The universe has expanded. This distance that was 30 million light years, now, and I'm just making rough numbers here. I don't know the actual numbers here. Now it is actually, this is really just for the sake of giving you the idea of why, well, giving you the intuition of what's going on. This distance now is no longer 30 million light years. It could be, maybe it's 100 million. So this is now 100 million light years away from each other."}, {"video_title": "Radius of observable universe Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Now it is actually, this is really just for the sake of giving you the idea of why, well, giving you the intuition of what's going on. This distance now is no longer 30 million light years. It could be, maybe it's 100 million. So this is now 100 million light years away from each other. The universe is expanding. These coordinates, the space is actually spreading out. You can imagine it's kind of a trampoline or the surface of a balloon getting stretched thin."}, {"video_title": "Radius of observable universe Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So this is now 100 million light years away from each other. The universe is expanding. These coordinates, the space is actually spreading out. You can imagine it's kind of a trampoline or the surface of a balloon getting stretched thin. And so this coordinate where the light happens to be after 10 million years, it has been traveling for 10 million years, but it's gone a much larger distance. It has now gone, that distance now might be on the order of, maybe it's on the order of 30 million light years. And the math isn't exact here."}, {"video_title": "Radius of observable universe Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "You can imagine it's kind of a trampoline or the surface of a balloon getting stretched thin. And so this coordinate where the light happens to be after 10 million years, it has been traveling for 10 million years, but it's gone a much larger distance. It has now gone, that distance now might be on the order of, maybe it's on the order of 30 million light years. And the math isn't exact here. I haven't done the math to figure it out. But the point here, so it's done 30 million light years, and I want to make it, actually I shouldn't even make it the same proportion, because the distance it's gone, the distance it has to go, because of the stretching, it's not going to be completely linear. At least when I'm thinking about it in my head, it shouldn't be, I think."}, {"video_title": "Radius of observable universe Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And the math isn't exact here. I haven't done the math to figure it out. But the point here, so it's done 30 million light years, and I want to make it, actually I shouldn't even make it the same proportion, because the distance it's gone, the distance it has to go, because of the stretching, it's not going to be completely linear. At least when I'm thinking about it in my head, it shouldn't be, I think. But I'm not going to make a hard statement about that. But the distance that it traversed, maybe this distance right here is now 20 million light years, because it got there, every time it moved some distance, the space that it had traversed is now stretched. So even though it's traveled for 10 million years, the space that it traversed is no longer just 10 million light years."}, {"video_title": "Radius of observable universe Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "At least when I'm thinking about it in my head, it shouldn't be, I think. But I'm not going to make a hard statement about that. But the distance that it traversed, maybe this distance right here is now 20 million light years, because it got there, every time it moved some distance, the space that it had traversed is now stretched. So even though it's traveled for 10 million years, the space that it traversed is no longer just 10 million light years. It's now stretched to 20 million light years. And the space that it has left to traverse is no longer only 20 million light years. It might now be 80 million light years."}, {"video_title": "Radius of observable universe Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So even though it's traveled for 10 million years, the space that it traversed is no longer just 10 million light years. It's now stretched to 20 million light years. And the space that it has left to traverse is no longer only 20 million light years. It might now be 80 million light years. It is now 80 million light years. And so this photon might be getting frustrated. There's an optimistic way of viewing it."}, {"video_title": "Radius of observable universe Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It might now be 80 million light years. It is now 80 million light years. And so this photon might be getting frustrated. There's an optimistic way of viewing it. It's like, wow, I was able to cover 20 million light years in only 10 million years. It looks like I'm moving faster than the speed of light. The reality is it's not, because the space coordinates themselves are spreading out."}, {"video_title": "Radius of observable universe Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "There's an optimistic way of viewing it. It's like, wow, I was able to cover 20 million light years in only 10 million years. It looks like I'm moving faster than the speed of light. The reality is it's not, because the space coordinates themselves are spreading out. Those are getting thin. So the photon is just moving at the speed of light. But the distance that it actually traversed in 10 million years is more than 10 million light years."}, {"video_title": "Radius of observable universe Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "The reality is it's not, because the space coordinates themselves are spreading out. Those are getting thin. So the photon is just moving at the speed of light. But the distance that it actually traversed in 10 million years is more than 10 million light years. It's 20 million light years. So you can't just multiply a rate times time on these cosmological scales here. Especially when the coordinates themselves, the distance coordinates are actually moving away from each other."}, {"video_title": "Radius of observable universe Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But the distance that it actually traversed in 10 million years is more than 10 million light years. It's 20 million light years. So you can't just multiply a rate times time on these cosmological scales here. Especially when the coordinates themselves, the distance coordinates are actually moving away from each other. But I think you see, or maybe you might see, where this is going. Now this says, OK, this photon says, oh, well, you know, in another, let me write this, this is 80 million light years. In another 40 million light years, maybe I'm going to get over here."}, {"video_title": "Radius of observable universe Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Especially when the coordinates themselves, the distance coordinates are actually moving away from each other. But I think you see, or maybe you might see, where this is going. Now this says, OK, this photon says, oh, well, you know, in another, let me write this, this is 80 million light years. In another 40 million light years, maybe I'm going to get over here. But the reality is over that next 40 million light years, sorry, over in 40 million years, I might get right over here, because this is 80 million light years. But the reality is after 40 million years, so for another 40 million years go by, now all of a sudden the universe has expanded even more. I won't even draw the whole bubble, but the place where the photon was emitted from might be over here."}, {"video_title": "Radius of observable universe Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "In another 40 million light years, maybe I'm going to get over here. But the reality is over that next 40 million light years, sorry, over in 40 million years, I might get right over here, because this is 80 million light years. But the reality is after 40 million years, so for another 40 million years go by, now all of a sudden the universe has expanded even more. I won't even draw the whole bubble, but the place where the photon was emitted from might be over here. And now our current position is over here, where the light got after 10 million years is now over here. And now where the light is after 40 million years is maybe it's over here. So now this distance, this distance between these two points, when we started it was 10 million light years, then it became 20 million light years."}, {"video_title": "Radius of observable universe Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "I won't even draw the whole bubble, but the place where the photon was emitted from might be over here. And now our current position is over here, where the light got after 10 million years is now over here. And now where the light is after 40 million years is maybe it's over here. So now this distance, this distance between these two points, when we started it was 10 million light years, then it became 20 million light years. Maybe now it's on the order of, I don't know, maybe it's a billion light years. And maybe this distance over here, and I'm just making up these numbers, in fact that's probably too big for that point, maybe this is now 100 million light years. And now this distance maybe is, maybe this distance right here is, I don't know, 500 million light years."}, {"video_title": "Radius of observable universe Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So now this distance, this distance between these two points, when we started it was 10 million light years, then it became 20 million light years. Maybe now it's on the order of, I don't know, maybe it's a billion light years. And maybe this distance over here, and I'm just making up these numbers, in fact that's probably too big for that point, maybe this is now 100 million light years. And now this distance maybe is, maybe this distance right here is, I don't know, 500 million light years. And maybe now the total distance between the two points is a billion light years. So as you can see, the photon might be getting frustrated as it covers more and more distance. It looks back and says, wow, you know, in only 50 million years I've been able to cover 600 million light years."}, {"video_title": "Radius of observable universe Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And now this distance maybe is, maybe this distance right here is, I don't know, 500 million light years. And maybe now the total distance between the two points is a billion light years. So as you can see, the photon might be getting frustrated as it covers more and more distance. It looks back and says, wow, you know, in only 50 million years I've been able to cover 600 million light years. That's pretty good. But it's frustrated because what it thought was it only had to cover 30 million light years in distance. That keeps stretching out because space itself is stretching."}, {"video_title": "Radius of observable universe Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It looks back and says, wow, you know, in only 50 million years I've been able to cover 600 million light years. That's pretty good. But it's frustrated because what it thought was it only had to cover 30 million light years in distance. That keeps stretching out because space itself is stretching. So the reality, just going to the original idea, this photon that is just reaching us, that's been traveling for, let's say, 13.4 billion years. So it's reaching us just now. So let me just fast forward 13.4 billion years from this point now to get to the present day."}, {"video_title": "Radius of observable universe Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "That keeps stretching out because space itself is stretching. So the reality, just going to the original idea, this photon that is just reaching us, that's been traveling for, let's say, 13.4 billion years. So it's reaching us just now. So let me just fast forward 13.4 billion years from this point now to get to the present day. So if I draw the whole visible universe right over here, this point right over here is going to be where it was emitted from, is right over there. We are sitting right over there. And actually, let me make something clear."}, {"video_title": "Radius of observable universe Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So let me just fast forward 13.4 billion years from this point now to get to the present day. So if I draw the whole visible universe right over here, this point right over here is going to be where it was emitted from, is right over there. We are sitting right over there. And actually, let me make something clear. If I'm drawing the whole observable universe, the center actually should be where we are because we can observe an equal distance. If things aren't really strange, we can observe an equal distance in any direction. So actually, maybe we should put us at the center."}, {"video_title": "Radius of observable universe Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And actually, let me make something clear. If I'm drawing the whole observable universe, the center actually should be where we are because we can observe an equal distance. If things aren't really strange, we can observe an equal distance in any direction. So actually, maybe we should put us at the center. So this was the entire observable universe. And the photon was emitted from here 13.4 billion years ago. So 300,000 years after that initial Big Bang."}, {"video_title": "Radius of observable universe Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So actually, maybe we should put us at the center. So this was the entire observable universe. And the photon was emitted from here 13.4 billion years ago. So 300,000 years after that initial Big Bang. And it's just getting to us. It is true that the photon has been traveling for 13.7 billion years. But what's kind of nutty about it is this object, since we've been expanding away from each other, this object is now, in our best estimates, this object is going to be about 46 billion light years away from us."}, {"video_title": "Radius of observable universe Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So 300,000 years after that initial Big Bang. And it's just getting to us. It is true that the photon has been traveling for 13.7 billion years. But what's kind of nutty about it is this object, since we've been expanding away from each other, this object is now, in our best estimates, this object is going to be about 46 billion light years away from us. And I want to make it very clear. This object is now 46 billion light years away from us. When we just use light to observe it, it looks like, just based on light years, hey, this light's been traveling 13.7 billion years to reach us."}, {"video_title": "Radius of observable universe Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But what's kind of nutty about it is this object, since we've been expanding away from each other, this object is now, in our best estimates, this object is going to be about 46 billion light years away from us. And I want to make it very clear. This object is now 46 billion light years away from us. When we just use light to observe it, it looks like, just based on light years, hey, this light's been traveling 13.7 billion years to reach us. That's our only way with light to think about the distance. So maybe it's 13.4 billion light years away. But the reality is, if you had a ruler today, light year rulers, the space here has stretched so much that this is now 46 billion light years."}, {"video_title": "Radius of observable universe Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "When we just use light to observe it, it looks like, just based on light years, hey, this light's been traveling 13.7 billion years to reach us. That's our only way with light to think about the distance. So maybe it's 13.4 billion light years away. But the reality is, if you had a ruler today, light year rulers, the space here has stretched so much that this is now 46 billion light years. And just to give you a hint of when we talk about the cosmic microwave background radiation, what will this point in space look like? This thing that's actually 46 billion light years away, but the photon only took 13.7 billion years to reach us. What will this look like?"}, {"video_title": "Radius of observable universe Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But the reality is, if you had a ruler today, light year rulers, the space here has stretched so much that this is now 46 billion light years. And just to give you a hint of when we talk about the cosmic microwave background radiation, what will this point in space look like? This thing that's actually 46 billion light years away, but the photon only took 13.7 billion years to reach us. What will this look like? Well, when we say look like, it's based on the photons that are reaching us right now. Those photons left 13.4 billion years ago. So those photons are the photons being emitted from this primitive structure, from this white hot haze of hydrogen plasma."}, {"video_title": "Radius of observable universe Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "What will this look like? Well, when we say look like, it's based on the photons that are reaching us right now. Those photons left 13.4 billion years ago. So those photons are the photons being emitted from this primitive structure, from this white hot haze of hydrogen plasma. So what we're going to see is this white hot haze, this kind of white hot plasma. White hot, undifferentiated, not differentiated into proper stable atoms, much less stars and galaxies. But we're going to see this white hot plasma."}, {"video_title": "Radius of observable universe Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So those photons are the photons being emitted from this primitive structure, from this white hot haze of hydrogen plasma. So what we're going to see is this white hot haze, this kind of white hot plasma. White hot, undifferentiated, not differentiated into proper stable atoms, much less stars and galaxies. But we're going to see this white hot plasma. The reality today is that that point in space that's 46 billion years from now, it's probably differentiated into stable atoms and stars and planets and galaxies. And frankly, if that person, if there is a civilization there right now, and if they're sitting right there, and if they're observing photons being emitted from our coordinate, from our point in space right now, they're not going to see us. They're going to see us 13.4 billion years ago."}, {"video_title": "Radius of observable universe Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But we're going to see this white hot plasma. The reality today is that that point in space that's 46 billion years from now, it's probably differentiated into stable atoms and stars and planets and galaxies. And frankly, if that person, if there is a civilization there right now, and if they're sitting right there, and if they're observing photons being emitted from our coordinate, from our point in space right now, they're not going to see us. They're going to see us 13.4 billion years ago. They're going to see the super primitive state of our region of space, when it really was just a white hot plasma. And we're going to talk more about this in the next video, but think about it. Any photon that's coming from that period in time, so from any direction that's been traveling for 13.4 billion years, from any direction, it's going to come from that primitive state."}, {"video_title": "Scale of the small Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "What I want to do in this video is explore what happens when we get to really, really, really small scales. Before we even think about it, I want to familiarize ourselves with the units here. So we're all familiar with what a meter looks like. The average adult male is a little under 2 meters. If you were to divide a meter into a thousand units, you would get a millimeter. I think we probably know what a millimeter is if you've ever looked at a meter stick. It's the smallest measurement on that meter stick."}, {"video_title": "Scale of the small Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "The average adult male is a little under 2 meters. If you were to divide a meter into a thousand units, you would get a millimeter. I think we probably know what a millimeter is if you've ever looked at a meter stick. It's the smallest measurement on that meter stick. So it's already pretty hard to look at. If you were to divide each of those millimeters into a thousand sections, you'd get a micrometer. Or another way to think about a micrometer is it's one millionth of a meter."}, {"video_title": "Scale of the small Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It's the smallest measurement on that meter stick. So it's already pretty hard to look at. If you were to divide each of those millimeters into a thousand sections, you'd get a micrometer. Or another way to think about a micrometer is it's one millionth of a meter. So this is kind of beyond what we're capable of really perceiving. If you were to take each of those micrometers and divide them into a thousand sections, you would get a nanometer. So now we're at one billionth of a meter."}, {"video_title": "Scale of the small Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Or another way to think about a micrometer is it's one millionth of a meter. So this is kind of beyond what we're capable of really perceiving. If you were to take each of those micrometers and divide them into a thousand sections, you would get a nanometer. So now we're at one billionth of a meter. You divide that by a thousand. You get a picometer. So a picometer is one thousand billionth of a meter."}, {"video_title": "Scale of the small Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So now we're at one billionth of a meter. You divide that by a thousand. You get a picometer. So a picometer is one thousand billionth of a meter. Or you could say a trillionth of a meter. You divide one of those by a thousand and you would get a femtometer. So these are unimaginably small things."}, {"video_title": "Scale of the small Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So a picometer is one thousand billionth of a meter. Or you could say a trillionth of a meter. You divide one of those by a thousand and you would get a femtometer. So these are unimaginably small things. Now once you're familiar with the units, let's explore what types of things we can expect to find at these different scales. I'll start over here, and I've written them on the left as well, but it's more compelling when you see the pictures. We'll start over here with the b. I've arbitrarily picked something of this scale."}, {"video_title": "Scale of the small Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So these are unimaginably small things. Now once you're familiar with the units, let's explore what types of things we can expect to find at these different scales. I'll start over here, and I've written them on the left as well, but it's more compelling when you see the pictures. We'll start over here with the b. I've arbitrarily picked something of this scale. There's many, many, many, almost an infinite number of things I could have picked at this scale. But the average b is about two centimeters long. This b right over here."}, {"video_title": "Scale of the small Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "We'll start over here with the b. I've arbitrarily picked something of this scale. There's many, many, many, almost an infinite number of things I could have picked at this scale. But the average b is about two centimeters long. This b right over here. It's about, give or take, one hundredth the length of the average adult human being. But once again, a honey bee, not too exciting, although it is pretty exciting to see it zoomed in like this. But a honey bee is something that we can relate to."}, {"video_title": "Scale of the small Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "This b right over here. It's about, give or take, one hundredth the length of the average adult human being. But once again, a honey bee, not too exciting, although it is pretty exciting to see it zoomed in like this. But a honey bee is something that we can relate to. We've all seen honey bees. Now, what I want to do is zoom in or look at something that's fifty times smaller than a honey bee. So something that if I were to kind of show how big it is relative to this honey bee, it would look something like this."}, {"video_title": "Scale of the small Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But a honey bee is something that we can relate to. We've all seen honey bees. Now, what I want to do is zoom in or look at something that's fifty times smaller than a honey bee. So something that if I were to kind of show how big it is relative to this honey bee, it would look something like this. I'm doing it very rough. And that is a dust mite. And this right here, these are both pictures of dust mites."}, {"video_title": "Scale of the small Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So something that if I were to kind of show how big it is relative to this honey bee, it would look something like this. I'm doing it very rough. And that is a dust mite. And this right here, these are both pictures of dust mites. Now, dust mites look like these strange and alien creatures. But what's amazing about them is that they are everywhere. They're all around us."}, {"video_title": "Scale of the small Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And this right here, these are both pictures of dust mites. Now, dust mites look like these strange and alien creatures. But what's amazing about them is that they are everywhere. They're all around us. You probably have many of them lying on your skin or wherever right now, which is kind of a creepy idea. But we're talking about scale here. And the average dust mite, so we were talking about centimeters before."}, {"video_title": "Scale of the small Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "They're all around us. You probably have many of them lying on your skin or wherever right now, which is kind of a creepy idea. But we're talking about scale here. And the average dust mite, so we were talking about centimeters before. Now we'll talk about millimeters. The average dust mite is less than half of a millimeter. Or if you want to talk in micrometers, it's about 400 micrometers long."}, {"video_title": "Scale of the small Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And the average dust mite, so we were talking about centimeters before. Now we'll talk about millimeters. The average dust mite is less than half of a millimeter. Or if you want to talk in micrometers, it's about 400 micrometers long. So this length right over here is about 400 micrometers, so about 150th the length. Remember, this huge thing that I'm showing right here, this is a honey bee. It's about 150th the length of a honey bee."}, {"video_title": "Scale of the small Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Or if you want to talk in micrometers, it's about 400 micrometers long. So this length right over here is about 400 micrometers, so about 150th the length. Remember, this huge thing that I'm showing right here, this is a honey bee. It's about 150th the length of a honey bee. Or maybe to put it in other terms that you might be familiar with, this is a zoomed in picture of human hair. And you might say, oh my god, this person has horrible hair. But no, if you were to look at your own hair under an electron microscope, you'd be lucky if it looked this good."}, {"video_title": "Scale of the small Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It's about 150th the length of a honey bee. Or maybe to put it in other terms that you might be familiar with, this is a zoomed in picture of human hair. And you might say, oh my god, this person has horrible hair. But no, if you were to look at your own hair under an electron microscope, you'd be lucky if it looked this good. This person actually I've seen pictures of more damaged hair than this. This is probably smooth and silky hair right here. But the diameter of human hair, and this is on average."}, {"video_title": "Scale of the small Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But no, if you were to look at your own hair under an electron microscope, you'd be lucky if it looked this good. This person actually I've seen pictures of more damaged hair than this. This is probably smooth and silky hair right here. But the diameter of human hair, and this is on average. It depends on whose hair you're talking about. The diameter of human hair is about 100 micrometers thick. That's the diameter."}, {"video_title": "Scale of the small Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But the diameter of human hair, and this is on average. It depends on whose hair you're talking about. The diameter of human hair is about 100 micrometers thick. That's the diameter. So it's about a fourth the length of a dust mite. Or if I were to draw some human hair relative to this honey bee, it would look something like this. And I'm drawing the whole hair, so its width would be the width of this thing that I just drew."}, {"video_title": "Scale of the small Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "That's the diameter. So it's about a fourth the length of a dust mite. Or if I were to draw some human hair relative to this honey bee, it would look something like this. And I'm drawing the whole hair, so its width would be the width of this thing that I just drew. Remember, we're looking at a honey bee here. It looks like some type of giant, but it is a honey bee. Let's zoom in even more."}, {"video_title": "Scale of the small Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And I'm drawing the whole hair, so its width would be the width of this thing that I just drew. Remember, we're looking at a honey bee here. It looks like some type of giant, but it is a honey bee. Let's zoom in even more. So we started with the honey bee. We zoomed in by 50 to get the dust mite. We zoomed in by another factor of 4 to get the width of human hair."}, {"video_title": "Scale of the small Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Let's zoom in even more. So we started with the honey bee. We zoomed in by 50 to get the dust mite. We zoomed in by another factor of 4 to get the width of human hair. If we zoom in, we're in the micrometer range now. If we zoom in by roughly another factor of 10, we get to the scale of cells. And this right here is a red blood cell."}, {"video_title": "Scale of the small Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "We zoomed in by another factor of 4 to get the width of human hair. If we zoom in, we're in the micrometer range now. If we zoom in by roughly another factor of 10, we get to the scale of cells. And this right here is a red blood cell. I think this is a white blood cell right over here. About 6 to 8 micrometers. So once again, if I were to draw a cell relative to this human hair, it would probably look something like this."}, {"video_title": "Scale of the small Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And this right here is a red blood cell. I think this is a white blood cell right over here. About 6 to 8 micrometers. So once again, if I were to draw a cell relative to this human hair, it would probably look something like this. Something on a similar scale that we can still kind of relate to is the width of spider silk. It's about 3 to 8 micrometers. So if I were to draw some spider silk on the same diagram, it would look something like this."}, {"video_title": "Scale of the small Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So once again, if I were to draw a cell relative to this human hair, it would probably look something like this. Something on a similar scale that we can still kind of relate to is the width of spider silk. It's about 3 to 8 micrometers. So if I were to draw some spider silk on the same diagram, it would look something like this. This is an actual image of spider silk. So once again, something that we can kind of perceive. You can bump into it."}, {"video_title": "Scale of the small Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So if I were to draw some spider silk on the same diagram, it would look something like this. This is an actual image of spider silk. So once again, something that we can kind of perceive. You can bump into it. You can touch spider silk. You can see it if the sun is reflecting just right or if it has a little bit of moisture on it. But it's about the thinnest thing that humans can perceive."}, {"video_title": "Scale of the small Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "You can bump into it. You can touch spider silk. You can see it if the sun is reflecting just right or if it has a little bit of moisture on it. But it's about the thinnest thing that humans can perceive. And this is in the ones of micrometer range. At that same range, you start to have some of your larger bacteria. Bacteria can be anywhere from, and I'm speaking very roughly, 1 to 10 micrometers."}, {"video_title": "Scale of the small Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But it's about the thinnest thing that humans can perceive. And this is in the ones of micrometer range. At that same range, you start to have some of your larger bacteria. Bacteria can be anywhere from, and I'm speaking very roughly, 1 to 10 micrometers. So in general, they're smaller than cells. Most bacteria are smaller than most cells. And just to figure out where we sit on our scale, have it over here."}, {"video_title": "Scale of the small Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Bacteria can be anywhere from, and I'm speaking very roughly, 1 to 10 micrometers. So in general, they're smaller than cells. Most bacteria are smaller than most cells. And just to figure out where we sit on our scale, have it over here. So we started off, I want to keep reminding ourselves, humans, you divide by 100, you get to the B. So each of these slashes right here are dividing by 10. So this is divide by 10."}, {"video_title": "Scale of the small Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And just to figure out where we sit on our scale, have it over here. So we started off, I want to keep reminding ourselves, humans, you divide by 100, you get to the B. So each of these slashes right here are dividing by 10. So this is divide by 10. Divide by 10 again, you're divided in size by 100. Divide by 10 again, you get to millimeter. You've divided by 1,000."}, {"video_title": "Scale of the small Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So this is divide by 10. Divide by 10 again, you're divided in size by 100. Divide by 10 again, you get to millimeter. You've divided by 1,000. Divide by 10 again, you are doing tenths of millimeters, which is about the size of the human hair. You divide again by 10, you're going into tens of micrometers. By 10 again, you get into the micrometer range."}, {"video_title": "Scale of the small Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "You've divided by 1,000. Divide by 10 again, you are doing tenths of millimeters, which is about the size of the human hair. You divide again by 10, you're going into tens of micrometers. By 10 again, you get into the micrometer range. So now we're talking about cells, we're talking about bacteria. Now things are going to get really crazy. This was in the ones of micrometer range."}, {"video_title": "Scale of the small Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "By 10 again, you get into the micrometer range. So now we're talking about cells, we're talking about bacteria. Now things are going to get really crazy. This was in the ones of micrometer range. Now we're going to start getting into the hundreds of nanometer range. Just to get a sense of things. So remember, a nanometer is a thousandth of a micrometer."}, {"video_title": "Scale of the small Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "This was in the ones of micrometer range. Now we're going to start getting into the hundreds of nanometer range. Just to get a sense of things. So remember, a nanometer is a thousandth of a micrometer. Or 100 nanometers would be a tenth of a micrometer. And this picture right here, this big, enormous planet or asteroid looking thing, this is a white blood cell. The enormous blue thing in this picture."}, {"video_title": "Scale of the small Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So remember, a nanometer is a thousandth of a micrometer. Or 100 nanometers would be a tenth of a micrometer. And this picture right here, this big, enormous planet or asteroid looking thing, this is a white blood cell. The enormous blue thing in this picture. And so if I were to zoom out, it might look something like this right over here. But what's really fascinating about this picture for multiple reasons are these little green things that are emerging, that are essentially reproducing, emerging from the surface of this white blood cell. And these things right here, these are AIDS viruses."}, {"video_title": "Scale of the small Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "The enormous blue thing in this picture. And so if I were to zoom out, it might look something like this right over here. But what's really fascinating about this picture for multiple reasons are these little green things that are emerging, that are essentially reproducing, emerging from the surface of this white blood cell. And these things right here, these are AIDS viruses. So now if we zoom in, roughly another factor of about 100 to 1000 from the size of a cell, you're now getting to the size of a virus. And all of the genetic material necessary to replicate that virus is right inside each of these little capsids, right inside each of these little green containers. So now going back to our scale, we are down to the scale of a virus."}, {"video_title": "Scale of the small Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And these things right here, these are AIDS viruses. So now if we zoom in, roughly another factor of about 100 to 1000 from the size of a cell, you're now getting to the size of a virus. And all of the genetic material necessary to replicate that virus is right inside each of these little capsids, right inside each of these little green containers. So now going back to our scale, we are down to the scale of a virus. So we're in the hundreds of nanometer range. If we divide by 10 and then divide by 10, you get to the nanometer range. And right in the ones of nanometer range, you get to the width of the double helix of a DNA molecule."}, {"video_title": "Scale of the small Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So now going back to our scale, we are down to the scale of a virus. So we're in the hundreds of nanometer range. If we divide by 10 and then divide by 10, you get to the nanometer range. And right in the ones of nanometer range, you get to the width of the double helix of a DNA molecule. So this right here is, if you were to zoom in, and this is an artist's depiction of it, obviously this is not a picture, so to speak, of a DNA molecule. But the width of this double helix is about 2 nanometers, or another way to think about it, about 160th the diameter of one of these viral capsids. It would have to be, because it's going to have to get all wound up and fit into one of these viral capsids."}, {"video_title": "Scale of the small Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And right in the ones of nanometer range, you get to the width of the double helix of a DNA molecule. So this right here is, if you were to zoom in, and this is an artist's depiction of it, obviously this is not a picture, so to speak, of a DNA molecule. But the width of this double helix is about 2 nanometers, or another way to think about it, about 160th the diameter of one of these viral capsids. It would have to be, because it's going to have to get all wound up and fit into one of these viral capsids. And DNA, just to make it clear, this is just the width of DNA. It's much, much, much, much, much, much longer, and we can talk about that in future videos. So once again, we're at a very, very small scale."}, {"video_title": "Scale of the small Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It would have to be, because it's going to have to get all wound up and fit into one of these viral capsids. And DNA, just to make it clear, this is just the width of DNA. It's much, much, much, much, much, much longer, and we can talk about that in future videos. So once again, we're at a very, very small scale. If you want to think of it in terms of meters, we're at 2 billionths of a meter. You could put 500 million of these side by side to get to a meter. Or you could even think of it this way."}, {"video_title": "Scale of the small Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So once again, we're at a very, very small scale. If you want to think of it in terms of meters, we're at 2 billionths of a meter. You could put 500 million of these side by side to get to a meter. Or you could even think of it this way. This is 2 millionths of a millimeter. So once again, super small. You could put these side by side, one DNA, another DNA."}, {"video_title": "Scale of the small Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Or you could even think of it this way. This is 2 millionths of a millimeter. So once again, super small. You could put these side by side, one DNA, another DNA. If you made them touch, you could put 500,000 next to each other in a millimeter. So this is an unbelievably small amount of space. It's going to fit into another unit that's not kind of in the conventional prefix followed by meters, and this is an angstrom."}, {"video_title": "Scale of the small Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "You could put these side by side, one DNA, another DNA. If you made them touch, you could put 500,000 next to each other in a millimeter. So this is an unbelievably small amount of space. It's going to fit into another unit that's not kind of in the conventional prefix followed by meters, and this is an angstrom. And 10 angstroms equal 1 nanometer. So the width of this DNA double helix would be 2 nanometers or 20 angstroms. Now, if we were to divide again by 10, you get to something that's 2 angstroms or 0.2 nanometers wide, and that is a water molecule."}, {"video_title": "Scale of the small Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It's going to fit into another unit that's not kind of in the conventional prefix followed by meters, and this is an angstrom. And 10 angstroms equal 1 nanometer. So the width of this DNA double helix would be 2 nanometers or 20 angstroms. Now, if we were to divide again by 10, you get to something that's 2 angstroms or 0.2 nanometers wide, and that is a water molecule. Maybe instead of using red, I should have used blue or something. But this right here is the oxygen, and it is bonded to the two hydrogens right over here. So this is beyond, frankly, human perception, or even really stuff that we can conceptualize, not to even speak of perception."}, {"video_title": "Scale of the small Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Now, if we were to divide again by 10, you get to something that's 2 angstroms or 0.2 nanometers wide, and that is a water molecule. Maybe instead of using red, I should have used blue or something. But this right here is the oxygen, and it is bonded to the two hydrogens right over here. So this is beyond, frankly, human perception, or even really stuff that we can conceptualize, not to even speak of perception. We're still imagining how small we're dealing with right over here. Remember, we're dealing with less than a fifth of a billionth of a meter, or a fifth of a millionth of a millimeter, something that I really can't fathom. But we're going to get even smaller than that."}, {"video_title": "Scale of the small Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So this is beyond, frankly, human perception, or even really stuff that we can conceptualize, not to even speak of perception. We're still imagining how small we're dealing with right over here. Remember, we're dealing with less than a fifth of a billionth of a meter, or a fifth of a millionth of a millimeter, something that I really can't fathom. But we're going to get even smaller than that. If we were to zoom in on one of these hydrogen atoms, and now things start to get kind of abstract, and we start dealing in the quantum realm, and it's hard to define where one thing ends and one thing begins, and what is real and what is not real, and all of that silliness. But if we try our best to do it, if we were to zoom in and we were to put some boundary on a hydrogen atom, because the electrons actually could jump around anywhere, but if we set some boundary of where the electrons are most likely to be found, the diameter of a hydrogen atom is roughly one angstrom, which makes sense from this diagram, too. It's about half of the diameter of this water molecule."}, {"video_title": "Scale of the small Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But we're going to get even smaller than that. If we were to zoom in on one of these hydrogen atoms, and now things start to get kind of abstract, and we start dealing in the quantum realm, and it's hard to define where one thing ends and one thing begins, and what is real and what is not real, and all of that silliness. But if we try our best to do it, if we were to zoom in and we were to put some boundary on a hydrogen atom, because the electrons actually could jump around anywhere, but if we set some boundary of where the electrons are most likely to be found, the diameter of a hydrogen atom is roughly one angstrom, which makes sense from this diagram, too. It's about half of the diameter of this water molecule. What's extra crazy is, one, this atom is super, super, duper small, something that we can't, you know, this is one ten billionth of a meter, or one ten millionth of a millimeter, so something we really, really can't fathom. But what's crazier than that is that it's mostly free space. We've gotten this small."}, {"video_title": "Scale of the small Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It's about half of the diameter of this water molecule. What's extra crazy is, one, this atom is super, super, duper small, something that we can't, you know, this is one ten billionth of a meter, or one ten millionth of a millimeter, so something we really, really can't fathom. But what's crazier than that is that it's mostly free space. We've gotten this small. We're trying to get to these fundamental units, and this thing right here is mostly free space. And that's because if you look at an electron, and when we say radius here, it's really hard to define where it starts and ends, and you have to do some things related to the charge, and we're not even thinking about quantum effects and all of that. An electron has a radius of 3 times 10 to the negative 5th angstroms, and the nucleus of a hydrogen atom, which is really just a proton, has a radius a little bit, and, you know, don't even worry about this number right here."}, {"video_title": "Scale of the small Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "We've gotten this small. We're trying to get to these fundamental units, and this thing right here is mostly free space. And that's because if you look at an electron, and when we say radius here, it's really hard to define where it starts and ends, and you have to do some things related to the charge, and we're not even thinking about quantum effects and all of that. An electron has a radius of 3 times 10 to the negative 5th angstroms, and the nucleus of a hydrogen atom, which is really just a proton, has a radius a little bit, and, you know, don't even worry about this number right here. The general idea is it's the same order of magnitude. It's about one ten thousandth of an angstrom. And just to give a sense of what it's like, if you view the entire atomic radius to be about an angstrom, kind of just have a conception for scale of the atom and how much free space there is in an atom, if we even want to think what is free space."}, {"video_title": "Scale of the small Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "An electron has a radius of 3 times 10 to the negative 5th angstroms, and the nucleus of a hydrogen atom, which is really just a proton, has a radius a little bit, and, you know, don't even worry about this number right here. The general idea is it's the same order of magnitude. It's about one ten thousandth of an angstrom. And just to give a sense of what it's like, if you view the entire atomic radius to be about an angstrom, kind of just have a conception for scale of the atom and how much free space there is in an atom, if we even want to think what is free space. Imagine a nucleus being maybe a marble at the center of a football stadium, of a domed football stadium, and imagine an electron being a honeybee just randomly jumping around random parts of that entire volume inside of that football stadium. And obviously it's a quantum honeybee, so it can jump around from spot to spot, and it's not easy to predict where it's going to go next and all of the rest. But that will give you a sense of the scale of the electron and the proton relative to the atom as a whole."}, {"video_title": "Four fundamental forces Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And I'm going to start with gravity. And it might surprise some of you that gravity is actually the weakest of the four fundamental forces. And that's surprising because you say, wow, that's what keeps us glued. Not glued, but it keeps us from jumping off the planet. It's what keeps the moon in orbit around the Earth. The Earth in orbit around the sun. The sun in orbit around the center of the Milky Way galaxy."}, {"video_title": "Four fundamental forces Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Not glued, but it keeps us from jumping off the planet. It's what keeps the moon in orbit around the Earth. The Earth in orbit around the sun. The sun in orbit around the center of the Milky Way galaxy. So it's a little bit surprising that it's actually the weakest of the forces. And that starts to make sense when you actually think about things on maybe more of a human scale, or a molecular scale, or even an atomic scale. Even on a human scale, your computer monitor and you have some type of gravitational attraction."}, {"video_title": "Four fundamental forces Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "The sun in orbit around the center of the Milky Way galaxy. So it's a little bit surprising that it's actually the weakest of the forces. And that starts to make sense when you actually think about things on maybe more of a human scale, or a molecular scale, or even an atomic scale. Even on a human scale, your computer monitor and you have some type of gravitational attraction. But you don't notice it. Or your cell phone and your wallet. There's gravitational attraction, but you don't see them being drawn to each other the way you might see two magnets drawn to each other or repelled from each other."}, {"video_title": "Four fundamental forces Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Even on a human scale, your computer monitor and you have some type of gravitational attraction. But you don't notice it. Or your cell phone and your wallet. There's gravitational attraction, but you don't see them being drawn to each other the way you might see two magnets drawn to each other or repelled from each other. And if you go to even a smaller scale, you'll see that it matters even less. We never even talk about gravity in chemistry. Although the gravity is there."}, {"video_title": "Four fundamental forces Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "There's gravitational attraction, but you don't see them being drawn to each other the way you might see two magnets drawn to each other or repelled from each other. And if you go to even a smaller scale, you'll see that it matters even less. We never even talk about gravity in chemistry. Although the gravity is there. But at those scales, the other forces really, really, really start to dominate. So gravities are weakest. So if we move up a little bit from that, we get, and this is maybe the hardest force for us to visualize, or at least the least intuitive force for me, is actually the weak force."}, {"video_title": "Four fundamental forces Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Although the gravity is there. But at those scales, the other forces really, really, really start to dominate. So gravities are weakest. So if we move up a little bit from that, we get, and this is maybe the hardest force for us to visualize, or at least the least intuitive force for me, is actually the weak force. The weak, sometimes called the weak interaction. And it's what's responsible for radioactive decay. In particular, beta minus and beta plus decay."}, {"video_title": "Four fundamental forces Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So if we move up a little bit from that, we get, and this is maybe the hardest force for us to visualize, or at least the least intuitive force for me, is actually the weak force. The weak, sometimes called the weak interaction. And it's what's responsible for radioactive decay. In particular, beta minus and beta plus decay. And just to give you an example of the actual weak interaction, if I had some cesium, 137, 137 means it has 137 nucleons. A nucleon is either a proton or a neutron. You add up the protons and neutrons of cesium, you get 137."}, {"video_title": "Four fundamental forces Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "In particular, beta minus and beta plus decay. And just to give you an example of the actual weak interaction, if I had some cesium, 137, 137 means it has 137 nucleons. A nucleon is either a proton or a neutron. You add up the protons and neutrons of cesium, you get 137. And it is cesium because it has exactly 55 protons. Now, the weak interaction is what's responsible for one of the neutrons, essentially one of its quarks flipping and turning into a proton. And I'm not going to go into detail of what a quark is and all of that, and the math can get pretty hairy."}, {"video_title": "Four fundamental forces Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "You add up the protons and neutrons of cesium, you get 137. And it is cesium because it has exactly 55 protons. Now, the weak interaction is what's responsible for one of the neutrons, essentially one of its quarks flipping and turning into a proton. And I'm not going to go into detail of what a quark is and all of that, and the math can get pretty hairy. But I just want to give you an example of what the weak interaction does. So if one of these neutrons turns into a proton, then we're going to have one extra proton, but we're going to have the same number of nucleons. Instead of an extra neutron here, you now have an extra proton here."}, {"video_title": "Four fundamental forces Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And I'm not going to go into detail of what a quark is and all of that, and the math can get pretty hairy. But I just want to give you an example of what the weak interaction does. So if one of these neutrons turns into a proton, then we're going to have one extra proton, but we're going to have the same number of nucleons. Instead of an extra neutron here, you now have an extra proton here. And so now this is a different atom. It is now barium. And in that flipping, it will actually emit an electron and an anti-electron neutrino."}, {"video_title": "Four fundamental forces Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Instead of an extra neutron here, you now have an extra proton here. And so now this is a different atom. It is now barium. And in that flipping, it will actually emit an electron and an anti-electron neutrino. And I'm not going to go into the details of what an anti-electron neutrino is. These are fundamental particles. But this is just what the weak interaction is."}, {"video_title": "Four fundamental forces Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And in that flipping, it will actually emit an electron and an anti-electron neutrino. And I'm not going to go into the details of what an anti-electron neutrino is. These are fundamental particles. But this is just what the weak interaction is. It's not something that's completely obvious to us. It's not kind of this traditional thing pulling or pushing away from each other like we associate with the other forces. Now the next strongest force, and just to give a sense of how weak gravity is even relative to the weak interaction, the weak interaction is 10 to the 25th times the strength of gravity."}, {"video_title": "Four fundamental forces Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But this is just what the weak interaction is. It's not something that's completely obvious to us. It's not kind of this traditional thing pulling or pushing away from each other like we associate with the other forces. Now the next strongest force, and just to give a sense of how weak gravity is even relative to the weak interaction, the weak interaction is 10 to the 25th times the strength of gravity. And you might be thinking, if this is so strong, how come this doesn't operate on planets or us relative to the Earth, or why doesn't this apply to intergalactic distances the way gravity does? And the reason is the weak interaction really applies to very small distances. So it can be much stronger than gravity, but only over very, very, and it really only applies on the subatomic scale."}, {"video_title": "Four fundamental forces Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Now the next strongest force, and just to give a sense of how weak gravity is even relative to the weak interaction, the weak interaction is 10 to the 25th times the strength of gravity. And you might be thinking, if this is so strong, how come this doesn't operate on planets or us relative to the Earth, or why doesn't this apply to intergalactic distances the way gravity does? And the reason is the weak interaction really applies to very small distances. So it can be much stronger than gravity, but only over very, very, and it really only applies on the subatomic scale. You go anything beyond that, it kind of disappears as an actual force, as an actual interaction. Now the next force up the hierarchy, which is one that we are more familiar with, it is something, it's what actually dominates most of the chemistry that we deal with, and electromagnetism that we deal with, and that's the electromagnetic force. Let me write it in magenta."}, {"video_title": "Four fundamental forces Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So it can be much stronger than gravity, but only over very, very, and it really only applies on the subatomic scale. You go anything beyond that, it kind of disappears as an actual force, as an actual interaction. Now the next force up the hierarchy, which is one that we are more familiar with, it is something, it's what actually dominates most of the chemistry that we deal with, and electromagnetism that we deal with, and that's the electromagnetic force. Let me write it in magenta. Electromagnetic force. And just to give a sense, this is 10 to the 36th times the strength of gravity, so it kind of puts the weak force in its place. It's 10 to the 12th times stronger than the weak force."}, {"video_title": "Four fundamental forces Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Let me write it in magenta. Electromagnetic force. And just to give a sense, this is 10 to the 36th times the strength of gravity, so it kind of puts the weak force in its place. It's 10 to the 12th times stronger than the weak force. These are huge numbers that we're talking about, either this relative to that, or even this relative to gravity. And so you might be saying, well, the electromagnetic force, that's unbelievably strong. Why doesn't that apply over these kind of macro scales like gravity?"}, {"video_title": "Four fundamental forces Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It's 10 to the 12th times stronger than the weak force. These are huge numbers that we're talking about, either this relative to that, or even this relative to gravity. And so you might be saying, well, the electromagnetic force, that's unbelievably strong. Why doesn't that apply over these kind of macro scales like gravity? Let me write there, macro scales. Why doesn't it apply to macro scales? And actually, there's nothing about the electromagnetic force why it can't, or it actually does, apply over large distances."}, {"video_title": "Four fundamental forces Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Why doesn't that apply over these kind of macro scales like gravity? Let me write there, macro scales. Why doesn't it apply to macro scales? And actually, there's nothing about the electromagnetic force why it can't, or it actually does, apply over large distances. The reality, though, is you don't have these huge concentrations of either Coulomb charges or magnetism, the way you do mass. So the mass, since you have such huge concentrations, it can operate over huge, huge distances, even though it's way, way, way weaker than the electromagnetic force. The electromagnetic force, what happens is because it's both attractive and repulsive, it tends to kind of sort itself out so you don't have these huge, huge, huge concentrations of charge."}, {"video_title": "Four fundamental forces Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And actually, there's nothing about the electromagnetic force why it can't, or it actually does, apply over large distances. The reality, though, is you don't have these huge concentrations of either Coulomb charges or magnetism, the way you do mass. So the mass, since you have such huge concentrations, it can operate over huge, huge distances, even though it's way, way, way weaker than the electromagnetic force. The electromagnetic force, what happens is because it's both attractive and repulsive, it tends to kind of sort itself out so you don't have these huge, huge, huge concentrations of charge. Now, the other thing you might be wondering about is why is it called the electromagnetic force? In our everyday life, there's things like the Coulomb force or the electrostatic force, which we're familiar with. Positive charges or like charges want to repel."}, {"video_title": "Four fundamental forces Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "The electromagnetic force, what happens is because it's both attractive and repulsive, it tends to kind of sort itself out so you don't have these huge, huge, huge concentrations of charge. Now, the other thing you might be wondering about is why is it called the electromagnetic force? In our everyday life, there's things like the Coulomb force or the electrostatic force, which we're familiar with. Positive charges or like charges want to repel. If both of these were negative, the same thing would be happening. And different charges like to attract. We've seen this multiple times."}, {"video_title": "Four fundamental forces Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Positive charges or like charges want to repel. If both of these were negative, the same thing would be happening. And different charges like to attract. We've seen this multiple times. This is the Coulomb force or the electrostatic force. And then on the other side of the word, I guess, you have the magnetic part. And magnets, you've played with magnets on your fridge."}, {"video_title": "Four fundamental forces Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "We've seen this multiple times. This is the Coulomb force or the electrostatic force. And then on the other side of the word, I guess, you have the magnetic part. And magnets, you've played with magnets on your fridge. If they're the same side of the magnet, they're going to repel each other. If they're the opposite sides, opposite poles, they're going to attract each other. So why is it called one force?"}, {"video_title": "Four fundamental forces Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And magnets, you've played with magnets on your fridge. If they're the same side of the magnet, they're going to repel each other. If they're the opposite sides, opposite poles, they're going to attract each other. So why is it called one force? And it's called one force. And once again, I'm not going to go into detail here. It's called one force because it turns out that the Coulomb force, the electrostatic force, and the magnetic force are actually the same thing viewed in different frames of references."}, {"video_title": "Four fundamental forces Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So why is it called one force? And it's called one force. And once again, I'm not going to go into detail here. It's called one force because it turns out that the Coulomb force, the electrostatic force, and the magnetic force are actually the same thing viewed in different frames of references. So I won't go into a lot of detail, but just keep that in the back of your mind, that they are connected. In a future video, I'll go more into the intuition of how they are connected. It's more apparent when the charges are moving at relativistic frames."}, {"video_title": "Four fundamental forces Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It's called one force because it turns out that the Coulomb force, the electrostatic force, and the magnetic force are actually the same thing viewed in different frames of references. So I won't go into a lot of detail, but just keep that in the back of your mind, that they are connected. In a future video, I'll go more into the intuition of how they are connected. It's more apparent when the charges are moving at relativistic frames. Well, I won't go into a lot of detail there. But just keep in mind that they really are the same force just viewed from different frames of reference. Now, the strongest of the force is probably the best named of them all."}, {"video_title": "Four fundamental forces Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It's more apparent when the charges are moving at relativistic frames. Well, I won't go into a lot of detail there. But just keep in mind that they really are the same force just viewed from different frames of reference. Now, the strongest of the force is probably the best named of them all. And that's the strong force. And although you probably haven't seen this yet in chemistry classes, it actually applies very strongly in chemistry. Because from the get-go, when you first learn about atoms, let me draw a helium atom."}, {"video_title": "Four fundamental forces Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Now, the strongest of the force is probably the best named of them all. And that's the strong force. And although you probably haven't seen this yet in chemistry classes, it actually applies very strongly in chemistry. Because from the get-go, when you first learn about atoms, let me draw a helium atom. A helium atom has two protons in its nucleus, and it has two neutrons, and then it also has two electrons circulating around. So it has an electron. I could draw the electrons as much smaller."}, {"video_title": "Four fundamental forces Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Because from the get-go, when you first learn about atoms, let me draw a helium atom. A helium atom has two protons in its nucleus, and it has two neutrons, and then it also has two electrons circulating around. So it has an electron. I could draw the electrons as much smaller. Well, I won't try to do anything in relative size, but it has two electrons floating around. And one question that may or may not have jumped into your mind when you first saw this model of an atom is like, well, I see why the electrons are attracted to the nucleus. It has a negative Coulomb charge."}, {"video_title": "Four fundamental forces Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "I could draw the electrons as much smaller. Well, I won't try to do anything in relative size, but it has two electrons floating around. And one question that may or may not have jumped into your mind when you first saw this model of an atom is like, well, I see why the electrons are attracted to the nucleus. It has a negative Coulomb charge. The nucleus has a net positive Coulomb charge. But what's not so obvious, and what tends not to sometimes be explained in chemistry class, is these two positive charges are sitting right next to each other. If the electromagnetic force was the only force in play, if the Coulomb force was the only thing happening, these guys would just run away from each other."}, {"video_title": "Four fundamental forces Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It has a negative Coulomb charge. The nucleus has a net positive Coulomb charge. But what's not so obvious, and what tends not to sometimes be explained in chemistry class, is these two positive charges are sitting right next to each other. If the electromagnetic force was the only force in play, if the Coulomb force was the only thing happening, these guys would just run away from each other. They would repel each other. And so the only reason why they're able to stick to each other is that there's an even stronger force than the electromagnetic force operating at these very, very, very small distances. So if you get two of these protons close enough together, and the strong force only applies over very, very, very small distances, subatomic, or I should even say subnucleic distances, then the strong interaction comes into play."}, {"video_title": "Four fundamental forces Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "If the electromagnetic force was the only force in play, if the Coulomb force was the only thing happening, these guys would just run away from each other. They would repel each other. And so the only reason why they're able to stick to each other is that there's an even stronger force than the electromagnetic force operating at these very, very, very small distances. So if you get two of these protons close enough together, and the strong force only applies over very, very, very small distances, subatomic, or I should even say subnucleic distances, then the strong interaction comes into play. So then you have the strong interaction actually keeping these charges together. And once again, just to keep it in mind relative to gravity, it is 10 to the 38th times the strength of gravity. Or it's about 100 times stronger than the electromagnetic force."}, {"video_title": "Four fundamental forces Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So if you get two of these protons close enough together, and the strong force only applies over very, very, very small distances, subatomic, or I should even say subnucleic distances, then the strong interaction comes into play. So then you have the strong interaction actually keeping these charges together. And once again, just to keep it in mind relative to gravity, it is 10 to the 38th times the strength of gravity. Or it's about 100 times stronger than the electromagnetic force. So once again, the reason why you don't see the strong force, which is the strongest of all the forces, or the weak interaction applying over huge scales, is that their strength dies off super, super fast, even when you start going to large radius nucleuses of atoms, the strength starts to die off, especially for the strong force. The reason why you don't see the electromagnetic force operating over large distances, even though in theory it can, like gravity, is that you don't see the type of charge concentrations the way you see mass concentrations in the universe. Because the charge concentrations tend to sort them out."}, {"video_title": "Four fundamental forces Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Or it's about 100 times stronger than the electromagnetic force. So once again, the reason why you don't see the strong force, which is the strongest of all the forces, or the weak interaction applying over huge scales, is that their strength dies off super, super fast, even when you start going to large radius nucleuses of atoms, the strength starts to die off, especially for the strong force. The reason why you don't see the electromagnetic force operating over large distances, even though in theory it can, like gravity, is that you don't see the type of charge concentrations the way you see mass concentrations in the universe. Because the charge concentrations tend to sort them out. They start to equalize. If I have some positive, a huge positive charge there and a huge negative charge there, they will attract each other and then become essentially a big lump of neutral charge. And once they're a big lump of neutral charge, they won't interact with anything else."}, {"video_title": "Four fundamental forces Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Because the charge concentrations tend to sort them out. They start to equalize. If I have some positive, a huge positive charge there and a huge negative charge there, they will attract each other and then become essentially a big lump of neutral charge. And once they're a big lump of neutral charge, they won't interact with anything else. In gravity, if you have one mass and another mass and they attract each other, then you have another mass that's even better at attracting each other, at other masses. And so it'll keep attracting things to it. So it kind of snowballs the process."}, {"video_title": "Human evolution overview Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And this is a very nice picture here of a dinosaur enjoying a sunset at the beach. But unfortunately for the dinosaurs, about 65 million years ago, we believe that a huge meteorite struck the Earth and essentially wiped out the dinosaurs. And they probably wiped out a bunch of other species with it. Because you can imagine, the shockwave itself would just exterminate tons of species. Then you would have the tsunami of unimaginable size that would just envelop the continents for some period of time. And then you would have all of the soot that would go into the air and maybe make it impossible for most of the plant species to live because it would be blocking out all of the sunlight. And so in an environment like that, we could imagine that an animal like this would be well suited to survive."}, {"video_title": "Human evolution overview Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Because you can imagine, the shockwave itself would just exterminate tons of species. Then you would have the tsunami of unimaginable size that would just envelop the continents for some period of time. And then you would have all of the soot that would go into the air and maybe make it impossible for most of the plant species to live because it would be blocking out all of the sunlight. And so in an environment like that, we could imagine that an animal like this would be well suited to survive. It's sitting there underground. Maybe it can hibernate in some way so it doesn't need food for long periods of time. Maybe it has its own food stash under there someplace."}, {"video_title": "Human evolution overview Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And so in an environment like that, we could imagine that an animal like this would be well suited to survive. It's sitting there underground. Maybe it can hibernate in some way so it doesn't need food for long periods of time. Maybe it has its own food stash under there someplace. And so we believe that our ancient ancestors, after this mass extinction event, might have been something like this, kind of a mole-looking rodent animal that was protected from all of this craziness that was happening on the surface because they like to hang out underground and have all their food nearby them. And maybe they could hibernate in some way. So you can imagine once everything settled down, and now we're talking hundreds of years, thousands of years, even millions of years, some of this guy's descendants start to poke their head out of the ground."}, {"video_title": "Human evolution overview Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Maybe it has its own food stash under there someplace. And so we believe that our ancient ancestors, after this mass extinction event, might have been something like this, kind of a mole-looking rodent animal that was protected from all of this craziness that was happening on the surface because they like to hang out underground and have all their food nearby them. And maybe they could hibernate in some way. So you can imagine once everything settled down, and now we're talking hundreds of years, thousands of years, even millions of years, some of this guy's descendants start to poke their head out of the ground. They're like, you know what? There's food in trees and there's no one else in the trees. And trees are a good place to maybe get away from some of the other predators that have managed to survive this mass extinction event."}, {"video_title": "Human evolution overview Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So you can imagine once everything settled down, and now we're talking hundreds of years, thousands of years, even millions of years, some of this guy's descendants start to poke their head out of the ground. They're like, you know what? There's food in trees and there's no one else in the trees. And trees are a good place to maybe get away from some of the other predators that have managed to survive this mass extinction event. And some of its ancestors, or some of its descendants, I should say, that were good at climbing trees decide, hey, let's try this tree thing out. And so you started to have some selection for the descendants of this rodent that could climb trees well. They were able to find food where their ancestors couldn't."}, {"video_title": "Human evolution overview Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And trees are a good place to maybe get away from some of the other predators that have managed to survive this mass extinction event. And some of its ancestors, or some of its descendants, I should say, that were good at climbing trees decide, hey, let's try this tree thing out. And so you started to have some selection for the descendants of this rodent that could climb trees well. They were able to find food where their ancestors couldn't. They could find protection in the trees where their ancestors couldn't. And so you could imagine that some subset of this guy's descendants evolved into something that might have looked like this guy. And all the pictures I'm showing you, these are of modern animals, except for, of course, the dinosaur."}, {"video_title": "Human evolution overview Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "They were able to find food where their ancestors couldn't. They could find protection in the trees where their ancestors couldn't. And so you could imagine that some subset of this guy's descendants evolved into something that might have looked like this guy. And all the pictures I'm showing you, these are of modern animals, except for, of course, the dinosaur. I'm sure this was kind of photoshopped in in some way. This is a modern bush baby. And I like this picture because it could have been what some of these primitive primates looked like."}, {"video_title": "Human evolution overview Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And all the pictures I'm showing you, these are of modern animals, except for, of course, the dinosaur. I'm sure this was kind of photoshopped in in some way. This is a modern bush baby. And I like this picture because it could have been what some of these primitive primates looked like. Because a bush baby, it kind of climbs trees. It kind of looks like it's starting to get a hand here to start climbing the trees. But it also has rodent-like qualities."}, {"video_title": "Human evolution overview Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And I like this picture because it could have been what some of these primitive primates looked like. Because a bush baby, it kind of climbs trees. It kind of looks like it's starting to get a hand here to start climbing the trees. But it also has rodent-like qualities. But this is, of course, a modern version of it. So this bush baby's ancient, ancient, ancient ancestor might have been that primitive primate, or that species of primitive primate that was a descendant of rodents that started to say, hey, let's see if we can climb these trees and find some food. And some of its descendants might have had just the right adaptations, found their own little niche in the right ecosystems, and they would have evolved into monkeys."}, {"video_title": "Human evolution overview Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But it also has rodent-like qualities. But this is, of course, a modern version of it. So this bush baby's ancient, ancient, ancient ancestor might have been that primitive primate, or that species of primitive primate that was a descendant of rodents that started to say, hey, let's see if we can climb these trees and find some food. And some of its descendants might have had just the right adaptations, found their own little niche in the right ecosystems, and they would have evolved into monkeys. Once again, this is a modern monkey, but you could imagine some type of primitive monkey. And then some of those primitive monkeys' descendants, they turn into these modern monkeys eventually. But some of them, they grow larger in size, they spend more time outside of trees, they lose their tail, they don't need it as much for balance."}, {"video_title": "Human evolution overview Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And some of its descendants might have had just the right adaptations, found their own little niche in the right ecosystems, and they would have evolved into monkeys. Once again, this is a modern monkey, but you could imagine some type of primitive monkey. And then some of those primitive monkeys' descendants, they turn into these modern monkeys eventually. But some of them, they grow larger in size, they spend more time outside of trees, they lose their tail, they don't need it as much for balance. Maybe it's actually a bad thing to have because someone else could grab it when you're in a fight or something like that. And they evolve into apes, and in particular, the great apes. So one of the great apes, the great apes involve gorillas and chimpanzees, and chimpanzees, and the ancestor, or really, the great apes also include humanity."}, {"video_title": "Human evolution overview Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But some of them, they grow larger in size, they spend more time outside of trees, they lose their tail, they don't need it as much for balance. Maybe it's actually a bad thing to have because someone else could grab it when you're in a fight or something like that. And they evolve into apes, and in particular, the great apes. So one of the great apes, the great apes involve gorillas and chimpanzees, and chimpanzees, and the ancestor, or really, the great apes also include humanity. So let me just review back on this timeline just so that we don't get confused. Let me just review what we just talked about. So before this mass extinction event, 65 million years ago, you had all these types of species here."}, {"video_title": "Human evolution overview Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So one of the great apes, the great apes involve gorillas and chimpanzees, and chimpanzees, and the ancestor, or really, the great apes also include humanity. So let me just review back on this timeline just so that we don't get confused. Let me just review what we just talked about. So before this mass extinction event, 65 million years ago, you had all these types of species here. Maybe this right up here, maybe this was, actually if I'm talking about species, maybe this was Tyrannosaurus rex, because the dinosaurs involve a whole bunch of, so this might have been T. rex. There's a bunch of species that we could list over here. But after that mass extinction event, that was an endpoint for a ton of species, except for maybe this primitive rodent mole-like thing that was, maybe a lot of them died in this event, but just enough of them survived because they were underground or just in the right place, or they were in a mountain someplace."}, {"video_title": "Human evolution overview Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So before this mass extinction event, 65 million years ago, you had all these types of species here. Maybe this right up here, maybe this was, actually if I'm talking about species, maybe this was Tyrannosaurus rex, because the dinosaurs involve a whole bunch of, so this might have been T. rex. There's a bunch of species that we could list over here. But after that mass extinction event, that was an endpoint for a ton of species, except for maybe this primitive rodent mole-like thing that was, maybe a lot of them died in this event, but just enough of them survived because they were underground or just in the right place, or they were in a mountain someplace. Who knows where they were. And some of them were able to evolve into primitive primates. And some of those primitive primates, and this is, once again, these are pictures of primitive primates."}, {"video_title": "Human evolution overview Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But after that mass extinction event, that was an endpoint for a ton of species, except for maybe this primitive rodent mole-like thing that was, maybe a lot of them died in this event, but just enough of them survived because they were underground or just in the right place, or they were in a mountain someplace. Who knows where they were. And some of them were able to evolve into primitive primates. And some of those primitive primates, and this is, once again, these are pictures of primitive primates. Some of those primitive, and when I say primitive, these are modern versions of them. So primitive doesn't necessarily mean worse, because obviously these guys were able to find, even in today's world, they have a niche for themselves. They're able to find food and reproduce in ways that don't get in the way of other people, and other way people don't get in the way of them."}, {"video_title": "Human evolution overview Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And some of those primitive primates, and this is, once again, these are pictures of primitive primates. Some of those primitive, and when I say primitive, these are modern versions of them. So primitive doesn't necessarily mean worse, because obviously these guys were able to find, even in today's world, they have a niche for themselves. They're able to find food and reproduce in ways that don't get in the way of other people, and other way people don't get in the way of them. When I talk about primitive primate, I'm just talking about kind of an ancestral primate, maybe something that's not there today, although maybe some of its descendants look very much like it. But anyway, some of those primates evolve into primitive monkeys. Some of those primitive monkeys' descendants become modern monkeys."}, {"video_title": "Human evolution overview Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "They're able to find food and reproduce in ways that don't get in the way of other people, and other way people don't get in the way of them. When I talk about primitive primate, I'm just talking about kind of an ancestral primate, maybe something that's not there today, although maybe some of its descendants look very much like it. But anyway, some of those primates evolve into primitive monkeys. Some of those primitive monkeys' descendants become modern monkeys. So this is, I'll call it M monkeys for modern monkeys. And some of them evolve into primitive apes. And apes, their distinctive characteristic is that they're like monkeys, but they don't have tails, and they're larger than most monkeys."}, {"video_title": "Human evolution overview Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Some of those primitive monkeys' descendants become modern monkeys. So this is, I'll call it M monkeys for modern monkeys. And some of them evolve into primitive apes. And apes, their distinctive characteristic is that they're like monkeys, but they don't have tails, and they're larger than most monkeys. And so these primitive apes, some of their descendants are modern gorillas. At some point, they break off. Some of these descendants are an ancestor of both modern chimpanzees and of human beings."}, {"video_title": "Human evolution overview Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And apes, their distinctive characteristic is that they're like monkeys, but they don't have tails, and they're larger than most monkeys. And so these primitive apes, some of their descendants are modern gorillas. At some point, they break off. Some of these descendants are an ancestor of both modern chimpanzees and of human beings. And we think, just looking at the DNA evidence, we think that this departure right here, and the fossil evidence, was about 7 million years ago. That's our best guess for when we as human beings had a common ancestor with the chimpanzees. Now, you have that common ancestor."}, {"video_title": "Human evolution overview Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Some of these descendants are an ancestor of both modern chimpanzees and of human beings. And we think, just looking at the DNA evidence, we think that this departure right here, and the fossil evidence, was about 7 million years ago. That's our best guess for when we as human beings had a common ancestor with the chimpanzees. Now, you have that common ancestor. Some of that common ancestor's descendants became modern chimpanzees. And some of them, maybe they explored the right ecosystem where it was more advantageous to do so, started to walk on two legs. And the most famous fossil of this is the Australopithecine fossil of Lucy that was discovered 3.2 million years..."}, {"video_title": "Human evolution overview Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Now, you have that common ancestor. Some of that common ancestor's descendants became modern chimpanzees. And some of them, maybe they explored the right ecosystem where it was more advantageous to do so, started to walk on two legs. And the most famous fossil of this is the Australopithecine fossil of Lucy that was discovered 3.2 million years... It was discovered more recently. It's 3.2 million years old. So the whole genus, and genus is kind of one level of categorization above species, the whole genus of Australopithecine, these were 4 to 2 million years ago."}, {"video_title": "Human evolution overview Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And the most famous fossil of this is the Australopithecine fossil of Lucy that was discovered 3.2 million years... It was discovered more recently. It's 3.2 million years old. So the whole genus, and genus is kind of one level of categorization above species, the whole genus of Australopithecine, these were 4 to 2 million years ago. And we never know. You could always find a fossil that's older than this, maybe newer than this. I read one account that says maybe 1 million years ago."}, {"video_title": "Human evolution overview Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So the whole genus, and genus is kind of one level of categorization above species, the whole genus of Australopithecine, these were 4 to 2 million years ago. And we never know. You could always find a fossil that's older than this, maybe newer than this. I read one account that says maybe 1 million years ago. But give or take, the Lucy fossil, which is the most well-established Australopithecine fossil, is about 3 million years old. And this is a reconstruction I have over here of Lucy. So this is probably what Lucy looked like."}, {"video_title": "Human evolution overview Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "I read one account that says maybe 1 million years ago. But give or take, the Lucy fossil, which is the most well-established Australopithecine fossil, is about 3 million years old. And this is a reconstruction I have over here of Lucy. So this is probably what Lucy looked like. And once again, there were many Lucys. It wasn't just there was one Lucy and we're all descended from Lucys. And it's actually not even clear that we are even descended directly from Australopithecine."}, {"video_title": "Human evolution overview Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So this is probably what Lucy looked like. And once again, there were many Lucys. It wasn't just there was one Lucy and we're all descended from Lucys. And it's actually not even clear that we are even descended directly from Australopithecine. We might be a cousin species, or a cousin genus, I should say. Genus is the category right above species. So if you fast forward a little bit more, you go to about 2.3 to 1.4 million years ago."}, {"video_title": "Human evolution overview Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And it's actually not even clear that we are even descended directly from Australopithecine. We might be a cousin species, or a cousin genus, I should say. Genus is the category right above species. So if you fast forward a little bit more, you go to about 2.3 to 1.4 million years ago. We see fossils that they're standing upright. The brain size is bigger. Because if you look at the Australopithecine fossils, they are standing upright."}, {"video_title": "Human evolution overview Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So if you fast forward a little bit more, you go to about 2.3 to 1.4 million years ago. We see fossils that they're standing upright. The brain size is bigger. Because if you look at the Australopithecine fossils, they are standing upright. But their cranial capacity isn't that different than chimpanzees. You fast forward to 2.3 million to 1.4 million years ago. We start to see fossils where they're standing upright still."}, {"video_title": "Human evolution overview Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Because if you look at the Australopithecine fossils, they are standing upright. But their cranial capacity isn't that different than chimpanzees. You fast forward to 2.3 million to 1.4 million years ago. We start to see fossils where they're standing upright still. And the cranial capacity has grown. And you're starting to see primitive stone tools around the bone fossils. And so we believe that these are one of the first."}, {"video_title": "Human evolution overview Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "We start to see fossils where they're standing upright still. And the cranial capacity has grown. And you're starting to see primitive stone tools around the bone fossils. And so we believe that these are one of the first. And this is really just how we categorize it. But these are some of the first fossils that we categorize as belonging to the same genus as ours. And the genus is Homo."}, {"video_title": "Human evolution overview Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And so we believe that these are one of the first. And this is really just how we categorize it. But these are some of the first fossils that we categorize as belonging to the same genus as ours. And the genus is Homo. And Homo just means man. So it's the group right above species of man. And we call them similar to man because it looks like they're starting to make primitive stone tools."}, {"video_title": "Human evolution overview Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And the genus is Homo. And Homo just means man. So it's the group right above species of man. And we call them similar to man because it looks like they're starting to make primitive stone tools. They stand upright like us. And they have larger cranial capacities than the Australopithecine fossils or modern chimpanzees. And once again, we don't know if Homo habilis, which literally means, so the Homo part means man."}, {"video_title": "Human evolution overview Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And we call them similar to man because it looks like they're starting to make primitive stone tools. They stand upright like us. And they have larger cranial capacities than the Australopithecine fossils or modern chimpanzees. And once again, we don't know if Homo habilis, which literally means, so the Homo part means man. Habilis means handy because he liked to, I guess, make tools or whatever else. We don't know if Homo habilis is a descendant of Lucy's species of Australopithecus or maybe a cousin species. Maybe they're both descendants from some common ancestor."}, {"video_title": "Human evolution overview Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And once again, we don't know if Homo habilis, which literally means, so the Homo part means man. Habilis means handy because he liked to, I guess, make tools or whatever else. We don't know if Homo habilis is a descendant of Lucy's species of Australopithecus or maybe a cousin species. Maybe they're both descendants from some common ancestor. We're not quite sure. Then you fast forward a little bit more. We're talking now about 1.8."}, {"video_title": "Human evolution overview Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Maybe they're both descendants from some common ancestor. We're not quite sure. Then you fast forward a little bit more. We're talking now about 1.8. So now we're talking about 1.8 to 1.3 million years ago. And we start seeing fossils where the cranial capacity larger than Homo habilis getting closer in size to kind of what we, what our notion is of kind of a modern person's cranial capacity, at least relative to body size. And this is Homo erectus."}, {"video_title": "Human evolution overview Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "We're talking now about 1.8. So now we're talking about 1.8 to 1.3 million years ago. And we start seeing fossils where the cranial capacity larger than Homo habilis getting closer in size to kind of what we, what our notion is of kind of a modern person's cranial capacity, at least relative to body size. And this is Homo erectus. And once again, we don't know if Homo erectus is a descendant of Homo habilis. Maybe they have a common ancestor. Who knows?"}, {"video_title": "Human evolution overview Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And this is Homo erectus. And once again, we don't know if Homo erectus is a descendant of Homo habilis. Maybe they have a common ancestor. Who knows? And it looks from the fossil evidence that there was, especially when you look at this range here, that there was some overlap where you had both Homo erectus and Homo habilis living on the same planet at the same time. Now you fast forward even more and we think about 600,000 to 300,000. Once again, you know, all of these are constantly being modified as we get better at finding new fossils or interpreting the fossils we have or we look at DNA evidence or whatever."}, {"video_title": "Human evolution overview Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Who knows? And it looks from the fossil evidence that there was, especially when you look at this range here, that there was some overlap where you had both Homo erectus and Homo habilis living on the same planet at the same time. Now you fast forward even more and we think about 600,000 to 300,000. Once again, you know, all of these are constantly being modified as we get better at finding new fossils or interpreting the fossils we have or we look at DNA evidence or whatever. About 600,000 to 300,000 years ago, you have the Neanderthals appear. And Neanderthals are in the same genus as humans. So it's really Homo neanderthalensis."}, {"video_title": "Human evolution overview Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Once again, you know, all of these are constantly being modified as we get better at finding new fossils or interpreting the fossils we have or we look at DNA evidence or whatever. About 600,000 to 300,000 years ago, you have the Neanderthals appear. And Neanderthals are in the same genus as humans. So it's really Homo neanderthalensis. I always have trouble saying this. So this is still part of Homo. And a common misconception is that the Neanderthals are somehow a more primitive version of humans, that they're somehow cavemen and we're modern men."}, {"video_title": "Human evolution overview Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So it's really Homo neanderthalensis. I always have trouble saying this. So this is still part of Homo. And a common misconception is that the Neanderthals are somehow a more primitive version of humans, that they're somehow cavemen and we're modern men. That's not the case. The belief is that Neanderthals are either a cousin species, we have a common ancestor, or that they're actually a subspecies of human beings. And there's some belief that they might have interbred with Homo sapiens and maybe some or a good number of us have Neanderthal genes."}, {"video_title": "Human evolution overview Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And a common misconception is that the Neanderthals are somehow a more primitive version of humans, that they're somehow cavemen and we're modern men. That's not the case. The belief is that Neanderthals are either a cousin species, we have a common ancestor, or that they're actually a subspecies of human beings. And there's some belief that they might have interbred with Homo sapiens and maybe some or a good number of us have Neanderthal genes. It's nothing to be ashamed of. It's just something, you know, unfortunately, that Neanderthals just get a bad name because of, I guess, our popular culture, if anything. So this is a drawing of a Neanderthal brain."}, {"video_title": "Human evolution overview Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And there's some belief that they might have interbred with Homo sapiens and maybe some or a good number of us have Neanderthal genes. It's nothing to be ashamed of. It's just something, you know, unfortunately, that Neanderthals just get a bad name because of, I guess, our popular culture, if anything. So this is a drawing of a Neanderthal brain. They actually had a fairly large cranial capacity, although scientists say they kind of make one reason or another why we think that they might have been more primitive than Homo sapiens. But who knows? We don't know."}, {"video_title": "Human evolution overview Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So this is a drawing of a Neanderthal brain. They actually had a fairly large cranial capacity, although scientists say they kind of make one reason or another why we think that they might have been more primitive than Homo sapiens. But who knows? We don't know. We're constantly learning things every day. But of course, the whole point of this is to talk about how humans showed up on this planet and the first really human fossils we find about 200,000 years ago. And this, remember, we're in the genus Homo, and now we finally found something that looks just like us anatomically at least."}, {"video_title": "Human evolution overview Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "We don't know. We're constantly learning things every day. But of course, the whole point of this is to talk about how humans showed up on this planet and the first really human fossils we find about 200,000 years ago. And this, remember, we're in the genus Homo, and now we finally found something that looks just like us anatomically at least. We can't study its behavior and all the rest. And now we get to Homo sapiens. The Homo part, once again, means man."}, {"video_title": "Human evolution overview Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And this, remember, we're in the genus Homo, and now we finally found something that looks just like us anatomically at least. We can't study its behavior and all the rest. And now we get to Homo sapiens. The Homo part, once again, means man. And the sapiens means thinking. So we can debate whether it's an appropriate title for our species, but it's thinking man. So once again, the Neanderthals, they were either a cousin species for a lot of this time, especially once Homo sapiens showed up."}, {"video_title": "Human evolution overview Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "The Homo part, once again, means man. And the sapiens means thinking. So we can debate whether it's an appropriate title for our species, but it's thinking man. So once again, the Neanderthals, they were either a cousin species for a lot of this time, especially once Homo sapiens showed up. And maybe Homo sapiens showed up before this. We just haven't found the fossils yet. They were maybe both inhabiting the same planet."}, {"video_title": "Human evolution overview Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So once again, the Neanderthals, they were either a cousin species for a lot of this time, especially once Homo sapiens showed up. And maybe Homo sapiens showed up before this. We just haven't found the fossils yet. They were maybe both inhabiting the same planet. Maybe there was some interbreeding. But the Neanderthals disappeared about 30,000 years ago. 30,000 years ago, these guys disappeared."}, {"video_title": "Human evolution overview Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "They were maybe both inhabiting the same planet. Maybe there was some interbreeding. But the Neanderthals disappeared about 30,000 years ago. 30,000 years ago, these guys disappeared. Maybe some of them kind of got mixed in with the Homo sapiens, started to interbreed with them, or they might have just been killed off because they were fighting over the same ecosystems. And I've made a little sample here of Homo sapiens just in case. Well, I'm assuming most of you watching this video are one, but just in case, here's my little sample."}, {"video_title": "Supernova clarification Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "That's actually not precisely right. The supernova was first observed 1,000 years ago. Or the light from the first explosion was observed by astronomers, we believe, 1,000 years ago. But we have to be very clear here. Because the Crab Nebula, at its core, is roughly 6,500 light years away, even this light, even this image we see right here, is that nebula as it was 6,500 years ago. And so the supernova itself, if we think about when it actually occurred, it actually must have occurred about 7,500 years ago. So it must have occurred about 7,500 years ago."}, {"video_title": "Supernova clarification Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But we have to be very clear here. Because the Crab Nebula, at its core, is roughly 6,500 light years away, even this light, even this image we see right here, is that nebula as it was 6,500 years ago. And so the supernova itself, if we think about when it actually occurred, it actually must have occurred about 7,500 years ago. So it must have occurred about 7,500 years ago. And that first light from that first explosion, from that first energetic event, reached us about 1,000 years ago. So it took 6,500 years to get to us and reached us 1,000 years ago. So first light 1,000 years ago."}, {"video_title": "Supernova clarification Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So it must have occurred about 7,500 years ago. And that first light from that first explosion, from that first energetic event, reached us about 1,000 years ago. So it took 6,500 years to get to us and reached us 1,000 years ago. So first light 1,000 years ago. I just want to make that clear. It might have been obvious to some of you all. But always important to think about it."}, {"video_title": "Supernova clarification Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So first light 1,000 years ago. I just want to make that clear. It might have been obvious to some of you all. But always important to think about it. When I said 1,000 years ago, I really should have said it was first observed. The explosion was observed 1,000 years ago. But since it's so far, the actual event must have occurred 7,500 years ago."}, {"video_title": "Cosmological time scale 1 Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So just to start off, I have here, this is the best depiction I could find where it didn't have copyrights. This is from NASA, of the Big Bang. And I've talked about it several times. The Big Bang occurred 13.7 billion years ago. And then if we go a little bit forward, actually a lot forward, we get to the formation of our actual solar system and the Earth. This is kind of the protoplanetary disk, or a depiction of a protoplanetary disk forming around our young sun. And so this right here is 4.5 billion years ago."}, {"video_title": "Cosmological time scale 1 Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "The Big Bang occurred 13.7 billion years ago. And then if we go a little bit forward, actually a lot forward, we get to the formation of our actual solar system and the Earth. This is kind of the protoplanetary disk, or a depiction of a protoplanetary disk forming around our young sun. And so this right here is 4.5 billion years ago. Now this over here, once again, these aren't pictures of them, these are just depictions, because no one was there with a camera. This is what we think the asteroid that killed the dinosaurs looked like when it was impacting Earth. And it killed the dinosaurs 65 million years ago."}, {"video_title": "Cosmological time scale 1 Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And so this right here is 4.5 billion years ago. Now this over here, once again, these aren't pictures of them, these are just depictions, because no one was there with a camera. This is what we think the asteroid that killed the dinosaurs looked like when it was impacting Earth. And it killed the dinosaurs 65 million years ago. So until then, we had land dinosaurs, and then this, as far as the current theories go, got rid of them. Now we'll fast forward a little bit more. At about 3 million years ago, our ancestors look like this."}, {"video_title": "Cosmological time scale 1 Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And it killed the dinosaurs 65 million years ago. So until then, we had land dinosaurs, and then this, as far as the current theories go, got rid of them. Now we'll fast forward a little bit more. At about 3 million years ago, our ancestors look like this. This is Australopithecus afarensis. This is, I think, a depiction of Lucy. This is one of our, probably many of us share, I believe, the theory is that all of us have some DNA from her."}, {"video_title": "Cosmological time scale 1 Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "At about 3 million years ago, our ancestors look like this. This is Australopithecus afarensis. This is, I think, a depiction of Lucy. This is one of our, probably many of us share, I believe, the theory is that all of us have some DNA from her. But this was 3 million years ago. And you fast forward some more, and you actually have the first modern humans appearing on the planet, people that looked and thought like you and me. This is 200,000 years ago."}, {"video_title": "Cosmological time scale 1 Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "This is one of our, probably many of us share, I believe, the theory is that all of us have some DNA from her. But this was 3 million years ago. And you fast forward some more, and you actually have the first modern humans appearing on the planet, people that looked and thought like you and me. This is 200,000 years ago. That's right over here. Obviously, this drawing was done much later, but this is a depiction of a modern human. So 200,000 years ago."}, {"video_title": "Cosmological time scale 1 Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "This is 200,000 years ago. That's right over here. Obviously, this drawing was done much later, but this is a depiction of a modern human. So 200,000 years ago. And then you fast forward even more. And I don't want to keep picking on Jesus. I did that when him getting on the jetliner, and I genuinely don't mean any offense to anyone."}, {"video_title": "Cosmological time scale 1 Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So 200,000 years ago. And then you fast forward even more. And I don't want to keep picking on Jesus. I did that when him getting on the jetliner, and I genuinely don't mean any offense to anyone. I just keep picking Jesus because, frankly, our calendar is kind of, he's a good person that most people know about 2,000 years ago. And so when we associate a lot of modern history occurring after his birth. So I'll put this right here is obviously a painting of the birth of Jesus."}, {"video_title": "Cosmological time scale 1 Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "I did that when him getting on the jetliner, and I genuinely don't mean any offense to anyone. I just keep picking Jesus because, frankly, our calendar is kind of, he's a good person that most people know about 2,000 years ago. And so when we associate a lot of modern history occurring after his birth. So I'll put this right here is obviously a painting of the birth of Jesus. And this is 2,000 years ago. And then this might be a little bit American-centric, but the Declaration of Independence, it was a major event. Actually, even on a worldwide basis, it was the first secular democracy based on a kind of constitutional democracy that showed up on the planet."}, {"video_title": "Cosmological time scale 1 Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So I'll put this right here is obviously a painting of the birth of Jesus. And this is 2,000 years ago. And then this might be a little bit American-centric, but the Declaration of Independence, it was a major event. Actually, even on a worldwide basis, it was the first secular democracy based on a kind of constitutional democracy that showed up on the planet. They had gotten rid of, they said, we don't want the King of England anymore. And this was about 234 years ago. And I always remember it because I was born almost on the 200th anniversary."}, {"video_title": "Cosmological time scale 1 Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Actually, even on a worldwide basis, it was the first secular democracy based on a kind of constitutional democracy that showed up on the planet. They had gotten rid of, they said, we don't want the King of England anymore. And this was about 234 years ago. And I always remember it because I was born almost on the 200th anniversary. So you just have to add my age to 200. So this is 234 years ago. So these are all events or periods of time that we've heard about and we've talked about, and people throw around these type of years."}, {"video_title": "Cosmological time scale 1 Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And I always remember it because I was born almost on the 200th anniversary. So you just have to add my age to 200. So this is 234 years ago. So these are all events or periods of time that we've heard about and we've talked about, and people throw around these type of years. But what I want to do in this video is relate it to time scales that we can comprehend. So instead of the Big Bang occurring 13.7 billion years ago, let's pretend like it occurred 10 years ago. Because most of us, especially if you're over the age of 10, can kind of understand what 10 years is."}, {"video_title": "Cosmological time scale 1 Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So these are all events or periods of time that we've heard about and we've talked about, and people throw around these type of years. But what I want to do in this video is relate it to time scales that we can comprehend. So instead of the Big Bang occurring 13.7 billion years ago, let's pretend like it occurred 10 years ago. Because most of us, especially if you're over the age of 10, can kind of understand what 10 years is. It's a very, very long period of time, but something that's well within our lifetimes, well within our experience. So let's say the 13.7 billion, instead of saying the Big Bang occurred 13.7 billion years ago, let's pretend like it occurred 10 years ago. And if we pretend that it occurred 10 years ago, let's think about how many years or minutes or hours ago each of these events would have occurred."}, {"video_title": "Cosmological time scale 1 Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Because most of us, especially if you're over the age of 10, can kind of understand what 10 years is. It's a very, very long period of time, but something that's well within our lifetimes, well within our experience. So let's say the 13.7 billion, instead of saying the Big Bang occurred 13.7 billion years ago, let's pretend like it occurred 10 years ago. And if we pretend that it occurred 10 years ago, let's think about how many years or minutes or hours ago each of these events would have occurred. So if the Big Bang, which is really 13.7 billion years, if it really had occurred 10 years ago and we scaled everything down, then the Earth would have been created about 3.3 years ago. So this would have been 3.3 years ago. So still nothing kind of amazing about this."}, {"video_title": "Cosmological time scale 1 Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And if we pretend that it occurred 10 years ago, let's think about how many years or minutes or hours ago each of these events would have occurred. So if the Big Bang, which is really 13.7 billion years, if it really had occurred 10 years ago and we scaled everything down, then the Earth would have been created about 3.3 years ago. So this would have been 3.3 years ago. So still nothing kind of amazing about this. This is a significant fraction of the age of the universe. So not that mind-blowing just yet. But if we go all the way to when the dinosaurs were extinct, the last land dinosaurs, now the 65 million years, and this will give you an appreciation of the difference between million and billion, if the universe was only 10 years old, then the dinosaurs would have been extinct 17 days ago."}, {"video_title": "Cosmological time scale 1 Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So still nothing kind of amazing about this. This is a significant fraction of the age of the universe. So not that mind-blowing just yet. But if we go all the way to when the dinosaurs were extinct, the last land dinosaurs, now the 65 million years, and this will give you an appreciation of the difference between million and billion, if the universe was only 10 years old, then the dinosaurs would have been extinct 17 days ago. Not even a month ago, the dinosaurs would have been extinct. So if the universe was created when I just graduated, well, I'm in my 30s now, so when I was 24, just last month, the dinosaurs would have gone extinct. And it gets even crazier."}, {"video_title": "Cosmological time scale 1 Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But if we go all the way to when the dinosaurs were extinct, the last land dinosaurs, now the 65 million years, and this will give you an appreciation of the difference between million and billion, if the universe was only 10 years old, then the dinosaurs would have been extinct 17 days ago. Not even a month ago, the dinosaurs would have been extinct. So if the universe was created when I just graduated, well, I'm in my 30s now, so when I was 24, just last month, the dinosaurs would have gone extinct. And it gets even crazier. 17 days ago, the dinosaurs would have extinct. Australopithecus afarensis would have walked on the Earth 19 hours ago yesterday. Yesterday, 19 hours ago, she would have been walking around on the planet."}, {"video_title": "Cosmological time scale 1 Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And it gets even crazier. 17 days ago, the dinosaurs would have extinct. Australopithecus afarensis would have walked on the Earth 19 hours ago yesterday. Yesterday, 19 hours ago, she would have been walking around on the planet. And modern humans wouldn't have shown up until 80 minutes ago. A little over an hour. There wasn't even a modern human."}, {"video_title": "Cosmological time scale 1 Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Yesterday, 19 hours ago, she would have been walking around on the planet. And modern humans wouldn't have shown up until 80 minutes ago. A little over an hour. There wasn't even a modern human. Then the universe was 10 years. It didn't take until just very recently, the last hour, for us to see someone that looks something like us, looks and thinks something like us. Fast forward even more."}, {"video_title": "Cosmological time scale 1 Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "There wasn't even a modern human. Then the universe was 10 years. It didn't take until just very recently, the last hour, for us to see someone that looks something like us, looks and thinks something like us. Fast forward even more. The birth of Jesus. If the universe was 10 years old instead of 13.7 billion, and we scaled everything down, then the birth of Jesus would have been 46 seconds ago. And then if we fast forward all the way to the Declaration of Independence, this would have occurred 5 seconds ago."}, {"video_title": "Cosmological time scale 1 Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Fast forward even more. The birth of Jesus. If the universe was 10 years old instead of 13.7 billion, and we scaled everything down, then the birth of Jesus would have been 46 seconds ago. And then if we fast forward all the way to the Declaration of Independence, this would have occurred 5 seconds ago. So this isn't quite as mind-blowing as the scale of the universe. But in my mind, this is still pretty amazing. All that's happened since 1776 on a global basis could have been encapsulated in 5 seconds if the age of the universe was 10 years."}, {"video_title": "Cosmological time scale 1 Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And then if we fast forward all the way to the Declaration of Independence, this would have occurred 5 seconds ago. So this isn't quite as mind-blowing as the scale of the universe. But in my mind, this is still pretty amazing. All that's happened since 1776 on a global basis could have been encapsulated in 5 seconds if the age of the universe was 10 years. So hopefully that gives you a little bit of a perspective. In the next video, I'm going to compare, instead of condensing things in time, I'm going to compare this scale to kind of a distance scale. So we can kind of say, hey, if the universe was the number of pixels on my screen, how big would each of these things be?"}, {"video_title": "Scale of the large Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "What we'll see in this video is that we can't even grasp things that are actually super small compared to the size of the universe. And we actually don't even know what the entire size of the universe is. But with that said, let's actually just try to appreciate how small we are. So this is me right over here. I am 5'9\", depending on whether I'm wearing shoes, maybe 5'10 with shoes. But for the sake of this video, let's just roughly approximate around 6 feet, or around roughly, I'm not going to go into the details of the math, around 2 meters. Now if I were to lie down 10 times in a row, you'd get about the length of an 18-wheeler."}, {"video_title": "Scale of the large Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So this is me right over here. I am 5'9\", depending on whether I'm wearing shoes, maybe 5'10 with shoes. But for the sake of this video, let's just roughly approximate around 6 feet, or around roughly, I'm not going to go into the details of the math, around 2 meters. Now if I were to lie down 10 times in a row, you'd get about the length of an 18-wheeler. It's about 60 feet long, so this is times 10. Now if you were to put an 18-wheeler, if you were to make it tall as opposed to long, somehow stand it up, and you were to do that 10 times in a row, you'll get to the height of roughly a 60-story skyscraper. So once again, if you took me and you piled me up 100 times, you'll get about a 60-story skyscraper."}, {"video_title": "Scale of the large Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Now if I were to lie down 10 times in a row, you'd get about the length of an 18-wheeler. It's about 60 feet long, so this is times 10. Now if you were to put an 18-wheeler, if you were to make it tall as opposed to long, somehow stand it up, and you were to do that 10 times in a row, you'll get to the height of roughly a 60-story skyscraper. So once again, if you took me and you piled me up 100 times, you'll get about a 60-story skyscraper. Now if you took that skyscraper, and if you were to lie it down 10 times in a row, you'd get something of the length of the Golden Gate Bridge. And once again, I'm not giving you the exact numbers, it's not always going to be exactly 10, but we're now getting to about something that's a little on the order of a mile long. The Golden Gate Bridge is actually longer than a mile, but if you go within the twin spans, it's roughly about a mile."}, {"video_title": "Scale of the large Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So once again, if you took me and you piled me up 100 times, you'll get about a 60-story skyscraper. Now if you took that skyscraper, and if you were to lie it down 10 times in a row, you'd get something of the length of the Golden Gate Bridge. And once again, I'm not giving you the exact numbers, it's not always going to be exactly 10, but we're now getting to about something that's a little on the order of a mile long. The Golden Gate Bridge is actually longer than a mile, but if you go within the twin spans, it's roughly about a mile. It's actually a little longer than that, but that gives you a sense of a mile. Now, if you multiply that by 10, you get to the size of a large city. And this right here is a satellite photograph of San Francisco."}, {"video_title": "Scale of the large Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "The Golden Gate Bridge is actually longer than a mile, but if you go within the twin spans, it's roughly about a mile. It's actually a little longer than that, but that gives you a sense of a mile. Now, if you multiply that by 10, you get to the size of a large city. And this right here is a satellite photograph of San Francisco. This is the actual Golden Gate Bridge here, and when I copy and pasted this picture, I tried to make it roughly 10 miles by 10 miles, just so you appreciate the scale. And what's interesting here, and this picture's interesting, because this is the first time we can kind of relate to cities, but when you look at a city on this scale, it's starting to get larger than what we're used to processing on a daily basis. A bridge, we've been on a bridge, we know what a bridge looks like."}, {"video_title": "Scale of the large Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And this right here is a satellite photograph of San Francisco. This is the actual Golden Gate Bridge here, and when I copy and pasted this picture, I tried to make it roughly 10 miles by 10 miles, just so you appreciate the scale. And what's interesting here, and this picture's interesting, because this is the first time we can kind of relate to cities, but when you look at a city on this scale, it's starting to get larger than what we're used to processing on a daily basis. A bridge, we've been on a bridge, we know what a bridge looks like. We know that a bridge is huge, but it doesn't feel like something that we can't comprehend. Already a city is something that we can't comprehend all at once. We can drive across a city, we can look at satellite imagery, but if I were to show a human on this, it would be unbelievably, unbelievably small."}, {"video_title": "Scale of the large Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "A bridge, we've been on a bridge, we know what a bridge looks like. We know that a bridge is huge, but it doesn't feel like something that we can't comprehend. Already a city is something that we can't comprehend all at once. We can drive across a city, we can look at satellite imagery, but if I were to show a human on this, it would be unbelievably, unbelievably small. You wouldn't actually be able to see it. It would be less than a pixel on this image. A house is less than a pixel on this image."}, {"video_title": "Scale of the large Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "We can drive across a city, we can look at satellite imagery, but if I were to show a human on this, it would be unbelievably, unbelievably small. You wouldn't actually be able to see it. It would be less than a pixel on this image. A house is less than a pixel on this image. But let's keep multiplying by 10. If you multiply by 10 again, you get to something roughly the size of the San Francisco Bay Area. This whole square over here is roughly that square right over there."}, {"video_title": "Scale of the large Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "A house is less than a pixel on this image. But let's keep multiplying by 10. If you multiply by 10 again, you get to something roughly the size of the San Francisco Bay Area. This whole square over here is roughly that square right over there. Let's multiply by 10 again. So this square is about 100 miles by 100 miles. So this one would be about 1,000 miles by 1,000 miles."}, {"video_title": "Scale of the large Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "This whole square over here is roughly that square right over there. Let's multiply by 10 again. So this square is about 100 miles by 100 miles. So this one would be about 1,000 miles by 1,000 miles. Now you're including a big part of the western United States. You have California here, you have Nevada here, you have Arizona and New Mexico. So a big chunk of a big continent we're already including."}, {"video_title": "Scale of the large Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So this one would be about 1,000 miles by 1,000 miles. Now you're including a big part of the western United States. You have California here, you have Nevada here, you have Arizona and New Mexico. So a big chunk of a big continent we're already including. Frankly, this is beyond the scale that we're used to operating. We've seen maps, so maybe we're a little used to it, but if you ever had to walk across this type of distance, it would take you a while. To some degree, the fact that planes go so fast, almost unimaginably fast for us, that it's made it feel like things like continents aren't as big because you can fly across them in five or six hours."}, {"video_title": "Scale of the large Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So a big chunk of a big continent we're already including. Frankly, this is beyond the scale that we're used to operating. We've seen maps, so maybe we're a little used to it, but if you ever had to walk across this type of distance, it would take you a while. To some degree, the fact that planes go so fast, almost unimaginably fast for us, that it's made it feel like things like continents aren't as big because you can fly across them in five or six hours. But these are already huge, huge, huge distances. But once again, you take this square that's about 1,000 miles by 1,000 miles and you multiply that by 10 and you get pretty close, a little bit over the diameter of the Earth. But once again, we're on the Earth."}, {"video_title": "Scale of the large Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "To some degree, the fact that planes go so fast, almost unimaginably fast for us, that it's made it feel like things like continents aren't as big because you can fly across them in five or six hours. But these are already huge, huge, huge distances. But once again, you take this square that's about 1,000 miles by 1,000 miles and you multiply that by 10 and you get pretty close, a little bit over the diameter of the Earth. But once again, we're on the Earth. We kind of relate to the Earth. If you look carefully at the horizon, you might see a little bit of a curvature, especially if you were to get into the plane. Even though this is, frankly, larger than my brain can really grasp, we can kind of relate to the Earth."}, {"video_title": "Scale of the large Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But once again, we're on the Earth. We kind of relate to the Earth. If you look carefully at the horizon, you might see a little bit of a curvature, especially if you were to get into the plane. Even though this is, frankly, larger than my brain can really grasp, we can kind of relate to the Earth. Now, you multiply the diameter of Earth times 10 and you get to the diameter of Jupiter. So if you were to sit Earth right next to Jupiter, obviously they're nowhere near that close. That would destroy both of the planets."}, {"video_title": "Scale of the large Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Even though this is, frankly, larger than my brain can really grasp, we can kind of relate to the Earth. Now, you multiply the diameter of Earth times 10 and you get to the diameter of Jupiter. So if you were to sit Earth right next to Jupiter, obviously they're nowhere near that close. That would destroy both of the planets. Actually, it would definitely destroy Earth. It would probably just kind of be merged into Jupiter. So if you put Earth next to Jupiter, it would look something like that, right over there."}, {"video_title": "Scale of the large Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "That would destroy both of the planets. Actually, it would definitely destroy Earth. It would probably just kind of be merged into Jupiter. So if you put Earth next to Jupiter, it would look something like that, right over there. So I would say that Jupiter is definitely, oh, you went on this kind of diagram that I'm drawing here, is definitely the first thing that I can't comprehend. The Earth itself is so vastly huge. Jupiter is 10 times bigger in diameter."}, {"video_title": "Scale of the large Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So if you put Earth next to Jupiter, it would look something like that, right over there. So I would say that Jupiter is definitely, oh, you went on this kind of diagram that I'm drawing here, is definitely the first thing that I can't comprehend. The Earth itself is so vastly huge. Jupiter is 10 times bigger in diameter. It's much larger in terms of mass and volume and all the rest. But just in terms of diameter, it is 10 times bigger. But let's keep going."}, {"video_title": "Scale of the large Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Jupiter is 10 times bigger in diameter. It's much larger in terms of mass and volume and all the rest. But just in terms of diameter, it is 10 times bigger. But let's keep going. 10 times Jupiter gets us to the Sun. This is times 10. So if this is the Sun and if I were to draw Jupiter, it would look something like, I'll do Jupiter in pink, Jupiter would be around that big."}, {"video_title": "Scale of the large Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But let's keep going. 10 times Jupiter gets us to the Sun. This is times 10. So if this is the Sun and if I were to draw Jupiter, it would look something like, I'll do Jupiter in pink, Jupiter would be around that big. And then the Earth would be around that big, if you were to put them all next to each other. So the Sun, once again, is huge. Even though we see it almost every day, it is unimaginably huge."}, {"video_title": "Scale of the large Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So if this is the Sun and if I were to draw Jupiter, it would look something like, I'll do Jupiter in pink, Jupiter would be around that big. And then the Earth would be around that big, if you were to put them all next to each other. So the Sun, once again, is huge. Even though we see it almost every day, it is unimaginably huge. Even the Earth is kind of unimaginably huge, and the Sun is 100 times more unimaginably bigger. Now we're going to start getting really, really, really wacky. You multiply the diameter of the Sun, which is already 100 times the diameter of the Earth."}, {"video_title": "Scale of the large Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Even though we see it almost every day, it is unimaginably huge. Even the Earth is kind of unimaginably huge, and the Sun is 100 times more unimaginably bigger. Now we're going to start getting really, really, really wacky. You multiply the diameter of the Sun, which is already 100 times the diameter of the Earth. You multiply that times 100, and that is the distance from the Earth to the Sun. So I've drawn the Sun here as a little pixel, and I didn't even draw the Earth as a pixel, because a pixel would be way too large. It would have to be a hundredth of a pixel in order to draw the Earth properly."}, {"video_title": "Scale of the large Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "You multiply the diameter of the Sun, which is already 100 times the diameter of the Earth. You multiply that times 100, and that is the distance from the Earth to the Sun. So I've drawn the Sun here as a little pixel, and I didn't even draw the Earth as a pixel, because a pixel would be way too large. It would have to be a hundredth of a pixel in order to draw the Earth properly. So this is an unbelievable distance between the Earth and the Sun. It's 100 times the distance of the diameter of the Sun itself. So it's massive, massive."}, {"video_title": "Scale of the large Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It would have to be a hundredth of a pixel in order to draw the Earth properly. So this is an unbelievable distance between the Earth and the Sun. It's 100 times the distance of the diameter of the Sun itself. So it's massive, massive. But once again, these things are relatively close compared to where we're about to go. Because if we want to get to the nearest star... So remember, the Sun is 100 times the diameter of the Earth."}, {"video_title": "Scale of the large Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So it's massive, massive. But once again, these things are relatively close compared to where we're about to go. Because if we want to get to the nearest star... So remember, the Sun is 100 times the diameter of the Earth. The distance between the Sun and the Earth is 100 times that. Or you could say it's 10,000 times the diameter of the Earth. So these are unimaginable distances."}, {"video_title": "Scale of the large Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So remember, the Sun is 100 times the diameter of the Earth. The distance between the Sun and the Earth is 100 times that. Or you could say it's 10,000 times the diameter of the Earth. So these are unimaginable distances. But to get to the nearest star, which is 4.2 light years away, it's 200,000 times... And once again, unimaginable. It's 200,000 times the distance between the Earth and the Sun. And to give you a rough sense of how far apart these things are, if the Sun was roughly the size of a basketball, if the average star was about the size of a basketball, in our part of the galaxy, in a volume the size of the Earth..."}, {"video_title": "Scale of the large Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So these are unimaginable distances. But to get to the nearest star, which is 4.2 light years away, it's 200,000 times... And once again, unimaginable. It's 200,000 times the distance between the Earth and the Sun. And to give you a rough sense of how far apart these things are, if the Sun was roughly the size of a basketball, if the average star was about the size of a basketball, in our part of the galaxy, in a volume the size of the Earth... So if you had just like a big volume, the size of the Earth, if the stars were the sizes of basketballs, in our part of the galaxy, you would only have a handful of basketballs per that volume. So unbelievably sparse. Even though the galaxy looks like..."}, {"video_title": "Scale of the large Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And to give you a rough sense of how far apart these things are, if the Sun was roughly the size of a basketball, if the average star was about the size of a basketball, in our part of the galaxy, in a volume the size of the Earth... So if you had just like a big volume, the size of the Earth, if the stars were the sizes of basketballs, in our part of the galaxy, you would only have a handful of basketballs per that volume. So unbelievably sparse. Even though the galaxy looks like... Even though when you look at the galaxy, and this is just an artist's depiction of it, it looks like something that has kind of this spray of stars and it looks reasonably dense. There is actually a huge amount of space. The great, great, great, great, great majority of the volume in the galaxy is just empty, empty space."}, {"video_title": "Scale of the large Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Even though the galaxy looks like... Even though when you look at the galaxy, and this is just an artist's depiction of it, it looks like something that has kind of this spray of stars and it looks reasonably dense. There is actually a huge amount of space. The great, great, great, great, great majority of the volume in the galaxy is just empty, empty space. There's no stars, no planets, no nothing. I mean, this is a huge jump that I'm talking about. And then if you really want to realize how large a galaxy itself can be, you take this distance between the Sun or between our solar system and the nearest star."}, {"video_title": "Scale of the large Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "The great, great, great, great, great majority of the volume in the galaxy is just empty, empty space. There's no stars, no planets, no nothing. I mean, this is a huge jump that I'm talking about. And then if you really want to realize how large a galaxy itself can be, you take this distance between the Sun or between our solar system and the nearest star. So that's 200,000 times the distance between the Earth and the Sun. And you multiply that distance by 25,000. So if the Sun is right here, our nearest star will be in that same pixel."}, {"video_title": "Scale of the large Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And then if you really want to realize how large a galaxy itself can be, you take this distance between the Sun or between our solar system and the nearest star. So that's 200,000 times the distance between the Earth and the Sun. And you multiply that distance by 25,000. So if the Sun is right here, our nearest star will be in that same pixel. They'll actually be within... I mean, you'd have to get a ton of stars within that one pixel, even though they're so far apart. And then this whole thing is 100,000 light years."}, {"video_title": "Scale of the large Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So if the Sun is right here, our nearest star will be in that same pixel. They'll actually be within... I mean, you'd have to get a ton of stars within that one pixel, even though they're so far apart. And then this whole thing is 100,000 light years. It's 25,000 times the distance between the Sun and the nearest star. So we're talking about unimaginable, unfathomable distances just for a galaxy. And now we're going to get our..."}, {"video_title": "Scale of the large Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And then this whole thing is 100,000 light years. It's 25,000 times the distance between the Sun and the nearest star. So we're talking about unimaginable, unfathomable distances just for a galaxy. And now we're going to get our... Frankly, my brain is already well beyond anything that it can really process. At this point, it almost just becomes abstract thinking. It just becomes playing with numbers and mathematics."}, {"video_title": "Scale of the large Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And now we're going to get our... Frankly, my brain is already well beyond anything that it can really process. At this point, it almost just becomes abstract thinking. It just becomes playing with numbers and mathematics. But if we get a sense of the universe itself, the observable universe, and we have to be clear because we can only observe light that started leaving from its source 13.7 billion years ago because that's how old the universe is. The observable universe is about 93 billion light years across. And the reason why it's larger than 13.7 billion is that the points in space that emitted light 13.7 billion years ago, those have been going away from us."}, {"video_title": "Scale of the large Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It just becomes playing with numbers and mathematics. But if we get a sense of the universe itself, the observable universe, and we have to be clear because we can only observe light that started leaving from its source 13.7 billion years ago because that's how old the universe is. The observable universe is about 93 billion light years across. And the reason why it's larger than 13.7 billion is that the points in space that emitted light 13.7 billion years ago, those have been going away from us. So now they're on the order of 40 billion light years away. But this isn't about cosmology. This is just about scale and appreciating how huge the universe is."}, {"video_title": "Scale of the large Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And the reason why it's larger than 13.7 billion is that the points in space that emitted light 13.7 billion years ago, those have been going away from us. So now they're on the order of 40 billion light years away. But this isn't about cosmology. This is just about scale and appreciating how huge the universe is. Just in the part of the universe that we can theoretically observe, you have to get, and that we can observe just because we're getting electromagnetic radiation from those parts of the universe, you would have to multiply this number. So let me make this clear. 100,000 light years, that's the diameter of the Milky Way."}, {"video_title": "Scale of the large Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "This is just about scale and appreciating how huge the universe is. Just in the part of the universe that we can theoretically observe, you have to get, and that we can observe just because we're getting electromagnetic radiation from those parts of the universe, you would have to multiply this number. So let me make this clear. 100,000 light years, that's the diameter of the Milky Way. So 100,000 light years, that's the diameter of the Milky Way. You would have to multiply it not by 1,000. 1,000 would get you to 100 million light years."}, {"video_title": "Scale of the large Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "100,000 light years, that's the diameter of the Milky Way. So 100,000 light years, that's the diameter of the Milky Way. You would have to multiply it not by 1,000. 1,000 would get you to 100 million light years. This is 100,000 times 1,000 is 100 million. You have to multiply by 1,000 again to get to 100 billion light years. The universe, for all we know, might be much, much, much larger."}, {"video_title": "Scale of the large Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "1,000 would get you to 100 million light years. This is 100,000 times 1,000 is 100 million. You have to multiply by 1,000 again to get to 100 billion light years. The universe, for all we know, might be much, much, much larger. It might even be infinite. Who knows? But to get from just the diameter of the Milky Way to the observable universe, you have to multiply by a million."}, {"video_title": "Solar and lunar eclipses.mp3", "Sentence": "In a previous video, we asked ourselves a very important question. As the moon has its 28-day cycle around the Earth, we talked about how a new moon is when the moon is between the Earth and the sun. And so from the Earth's point of view, or from the point of view of someone standing on Earth, you're seeing the side of the moon that is not lit up. But an obvious question is, is why doesn't that block out the sun every time we are in a new moon position? After all, the sun would be out here, 93 million miles away, and so wouldn't the moon block out the sun in that scenario? And we also talked about when the moon is on the other side of the Earth. When we would typically see a full moon, the Earth is between the moon and the sun."}, {"video_title": "Solar and lunar eclipses.mp3", "Sentence": "But an obvious question is, is why doesn't that block out the sun every time we are in a new moon position? After all, the sun would be out here, 93 million miles away, and so wouldn't the moon block out the sun in that scenario? And we also talked about when the moon is on the other side of the Earth. When we would typically see a full moon, the Earth is between the moon and the sun. Why doesn't the Earth block the light from the sun that's being reflected on the moon? So the big question is, why don't we see a solar eclipse every new moon, and why don't we see a lunar eclipse every full moon? If we wanna ask the same question, looking at the scale of the Earth and the moon, we can see it right over here."}, {"video_title": "Solar and lunar eclipses.mp3", "Sentence": "When we would typically see a full moon, the Earth is between the moon and the sun. Why doesn't the Earth block the light from the sun that's being reflected on the moon? So the big question is, why don't we see a solar eclipse every new moon, and why don't we see a lunar eclipse every full moon? If we wanna ask the same question, looking at the scale of the Earth and the moon, we can see it right over here. And so this would be the new moon position, where the moon is between the Earth and the sun. The key explanation for why we do not have a solar eclipse every time we are in the new moon position is that the rotation of the moon around the Earth is not in the exact same plane as the rotation of the Earth around the sun. It actually turns out that the plane of the moon's rotation is at a five-degree angle with the plane of the Earth and the sun."}, {"video_title": "Solar and lunar eclipses.mp3", "Sentence": "If we wanna ask the same question, looking at the scale of the Earth and the moon, we can see it right over here. And so this would be the new moon position, where the moon is between the Earth and the sun. The key explanation for why we do not have a solar eclipse every time we are in the new moon position is that the rotation of the moon around the Earth is not in the exact same plane as the rotation of the Earth around the sun. It actually turns out that the plane of the moon's rotation is at a five-degree angle with the plane of the Earth and the sun. If this is the sun here, and this is not drawn to scale by any means, this is the Earth, and so the Earth is in its orbit, something like that. The moon's orbit does not sit exactly in this plane. It does not sit exactly in this plane."}, {"video_title": "Solar and lunar eclipses.mp3", "Sentence": "It actually turns out that the plane of the moon's rotation is at a five-degree angle with the plane of the Earth and the sun. If this is the sun here, and this is not drawn to scale by any means, this is the Earth, and so the Earth is in its orbit, something like that. The moon's orbit does not sit exactly in this plane. It does not sit exactly in this plane. The moon's orbit is at a five-degree angle, is at a five-degree angle to this plane. So depending what time of year you are at and where the moon is in its cycle, the moon will often sit above or below the plane of the Earth's orbit. So for example, from the vantage point of the Earth, the moon will vary up to five degrees above the plane of Earth's rotation around the sun, often known as the ecliptic, and five degrees below that."}, {"video_title": "Solar and lunar eclipses.mp3", "Sentence": "It does not sit exactly in this plane. The moon's orbit is at a five-degree angle, is at a five-degree angle to this plane. So depending what time of year you are at and where the moon is in its cycle, the moon will often sit above or below the plane of the Earth's orbit. So for example, from the vantage point of the Earth, the moon will vary up to five degrees above the plane of Earth's rotation around the sun, often known as the ecliptic, and five degrees below that. And so you can see here, the shadow of the moon will only fall on the Earth when the moon is crossing through the plane of the rotation of Earth around the sun. Many times, the moon might be here or here or here, and in those times, the shadow will not hit the Earth. Now what you do see here, depicted in this picture, would be a solar eclipse."}, {"video_title": "Solar and lunar eclipses.mp3", "Sentence": "So for example, from the vantage point of the Earth, the moon will vary up to five degrees above the plane of Earth's rotation around the sun, often known as the ecliptic, and five degrees below that. And so you can see here, the shadow of the moon will only fall on the Earth when the moon is crossing through the plane of the rotation of Earth around the sun. Many times, the moon might be here or here or here, and in those times, the shadow will not hit the Earth. Now what you do see here, depicted in this picture, would be a solar eclipse. Now there's two things that I've drawn here, and this is important for understanding a solar eclipse. What you see in those yellow lines, that's the umbra of the moon. So if you are at that point right over here in this picture, then the moon will completely block out the sun."}, {"video_title": "Solar and lunar eclipses.mp3", "Sentence": "Now what you do see here, depicted in this picture, would be a solar eclipse. Now there's two things that I've drawn here, and this is important for understanding a solar eclipse. What you see in those yellow lines, that's the umbra of the moon. So if you are at that point right over here in this picture, then the moon will completely block out the sun. And it turns out, depending on the solar eclipse, where that umbra is hitting the Earth, it might only be a few hundred miles where the moon completely blocks out the sun. What you see in this dark blue color, that is the penumbra. That is the outer shadow."}, {"video_title": "Solar and lunar eclipses.mp3", "Sentence": "So if you are at that point right over here in this picture, then the moon will completely block out the sun. And it turns out, depending on the solar eclipse, where that umbra is hitting the Earth, it might only be a few hundred miles where the moon completely blocks out the sun. What you see in this dark blue color, that is the penumbra. That is the outer shadow. And if you're in one of those areas, you will see the moon partially block out the sun. Now this is the exact same reason, the fact that the plane of its rotation is at a five degree angle with the plane of rotation of the Earth around the sun. That's also the reason why we don't see a lunar eclipse every 28 days."}, {"video_title": "Solar and lunar eclipses.mp3", "Sentence": "That is the outer shadow. And if you're in one of those areas, you will see the moon partially block out the sun. Now this is the exact same reason, the fact that the plane of its rotation is at a five degree angle with the plane of rotation of the Earth around the sun. That's also the reason why we don't see a lunar eclipse every 28 days. Here we see different scenarios where the moon is on the other side of Earth from the sun, when the moon is what we would call a full moon position. And as we can see, it can vary five degrees above the plane of the Earth and the sun, or five degrees below that. And you're only going to get a scenario of a lunar eclipse when the moon happens to fall in the shadow of the Earth."}, {"video_title": "Solar and lunar eclipses.mp3", "Sentence": "That's also the reason why we don't see a lunar eclipse every 28 days. Here we see different scenarios where the moon is on the other side of Earth from the sun, when the moon is what we would call a full moon position. And as we can see, it can vary five degrees above the plane of the Earth and the sun, or five degrees below that. And you're only going to get a scenario of a lunar eclipse when the moon happens to fall in the shadow of the Earth. And here, once again, between the yellow lines, you see the umbra, and between the blue lines, you see the penumbra, which would be kind of a partial shadow. So next time you look up at the moon, hopefully you are armed with a lot more information to think about how it's oriented with respect to the Earth. In fact, now when you look at the moon at night, you can usually look at the moon and tell which direction the sun is in."}, {"video_title": "Solar and lunar eclipses.mp3", "Sentence": "And you're only going to get a scenario of a lunar eclipse when the moon happens to fall in the shadow of the Earth. And here, once again, between the yellow lines, you see the umbra, and between the blue lines, you see the penumbra, which would be kind of a partial shadow. So next time you look up at the moon, hopefully you are armed with a lot more information to think about how it's oriented with respect to the Earth. In fact, now when you look at the moon at night, you can usually look at the moon and tell which direction the sun is in. And based on that, you can even think about what time of day it is, and you could figure out east, west, north, and south. And we've talked a lot about the sun. In fact, this entire video and the one before it is all about light from the sun."}, {"video_title": "Solar and lunar eclipses.mp3", "Sentence": "In fact, now when you look at the moon at night, you can usually look at the moon and tell which direction the sun is in. And based on that, you can even think about what time of day it is, and you could figure out east, west, north, and south. And we've talked a lot about the sun. In fact, this entire video and the one before it is all about light from the sun. And just to finish off with a little bit of awe, let's just appreciate how large and how far the sun is. As we mentioned before, this is a scaled representation of the Earth and the moon, the moon being roughly 240,000 miles away from Earth. If we were to try to model the sun here, it is 400 times as far."}, {"video_title": "Solar and lunar eclipses.mp3", "Sentence": "In fact, this entire video and the one before it is all about light from the sun. And just to finish off with a little bit of awe, let's just appreciate how large and how far the sun is. As we mentioned before, this is a scaled representation of the Earth and the moon, the moon being roughly 240,000 miles away from Earth. If we were to try to model the sun here, it is 400 times as far. Depending on your screen size, it would be a football field to the right of your screen, and you would have something roughly the size of a beach ball to represent the sun. If you wanted to just compare the sizes of the sun to the Earth and the moon, here you go. The sun is huge."}, {"video_title": "Lifecycle of massive stars Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Massive stars. And when I'm talking about massive stars, I'm talking about stars that have masses greater than 9 times the sun. So the general idea is exactly the same. You're going to start off with this huge cloud of mainly hydrogen, and now this cloud is going to have to be bigger than the clouds that condensed to form stars like our sun. But you're going to start with that, and eventually, gravity is going to pull it together, and the core of it is going to get hot and dense enough for hydrogen to ignite, for hydrogen to start fusing. So this is hydrogen, and it is now fusing. Let me write it."}, {"video_title": "Lifecycle of massive stars Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "You're going to start off with this huge cloud of mainly hydrogen, and now this cloud is going to have to be bigger than the clouds that condensed to form stars like our sun. But you're going to start with that, and eventually, gravity is going to pull it together, and the core of it is going to get hot and dense enough for hydrogen to ignite, for hydrogen to start fusing. So this is hydrogen, and it is now fusing. Let me write it. It is now fusing. Hydrogen fusion. Let me write it like this."}, {"video_title": "Lifecycle of massive stars Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Let me write it. It is now fusing. Hydrogen fusion. Let me write it like this. You now have hydrogen fusion in the middle, so it's ignited, and around it, you have just the other material of the cloud, so the rest of the hydrogen. And now, since it's so heated, it's really a plasma. It's really kind of a soup of electrons and nucleuses as opposed to well-formed atoms, especially close to the core."}, {"video_title": "Lifecycle of massive stars Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Let me write it like this. You now have hydrogen fusion in the middle, so it's ignited, and around it, you have just the other material of the cloud, so the rest of the hydrogen. And now, since it's so heated, it's really a plasma. It's really kind of a soup of electrons and nucleuses as opposed to well-formed atoms, especially close to the core. So now you have hydrogen fusion. We saw this happens at around 10 million Kelvin. And I want to make it very clear."}, {"video_title": "Lifecycle of massive stars Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It's really kind of a soup of electrons and nucleuses as opposed to well-formed atoms, especially close to the core. So now you have hydrogen fusion. We saw this happens at around 10 million Kelvin. And I want to make it very clear. Since we're talking about more massive stars, even at this stage, there's going to be more gravitational pressure, even at this stage, during the main sequence of the star, because it is more massive. And so this is going to burn faster and hotter. So this is going to be faster and hotter than something the mass of our sun."}, {"video_title": "Lifecycle of massive stars Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And I want to make it very clear. Since we're talking about more massive stars, even at this stage, there's going to be more gravitational pressure, even at this stage, during the main sequence of the star, because it is more massive. And so this is going to burn faster and hotter. So this is going to be faster and hotter than something the mass of our sun. Faster and hotter. And so even this stage is going to happen over a much shorter period of time than for a star the mass of our sun. Our sun's life is going to be 10 or 11 billion total years."}, {"video_title": "Lifecycle of massive stars Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So this is going to be faster and hotter than something the mass of our sun. Faster and hotter. And so even this stage is going to happen over a much shorter period of time than for a star the mass of our sun. Our sun's life is going to be 10 or 11 billion total years. Here, we're going to be talking about things in maybe the tens of millions of years, so a factor of 1,000 shorter lifespan. But anyway, let's think about what happens. And so far, just the pattern of what happens, it's going to happen faster because we have more pressure, more gravity, more temperature."}, {"video_title": "Lifecycle of massive stars Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Our sun's life is going to be 10 or 11 billion total years. Here, we're going to be talking about things in maybe the tens of millions of years, so a factor of 1,000 shorter lifespan. But anyway, let's think about what happens. And so far, just the pattern of what happens, it's going to happen faster because we have more pressure, more gravity, more temperature. But it's going to happen in pretty much the same way as what we saw with a star the mass of the sun. Eventually, that hydrogen is going to fuse into a helium core that's going to have a hydrogen shell around it. A hydrogen fusion shell around it, and then you have the rest of the star around that."}, {"video_title": "Lifecycle of massive stars Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And so far, just the pattern of what happens, it's going to happen faster because we have more pressure, more gravity, more temperature. But it's going to happen in pretty much the same way as what we saw with a star the mass of the sun. Eventually, that hydrogen is going to fuse into a helium core that's going to have a hydrogen shell around it. A hydrogen fusion shell around it, and then you have the rest of the star around that. So let me label it. This right here is our helium core. And more and more helium is going to be built up as this hydrogen and this shell fuses."}, {"video_title": "Lifecycle of massive stars Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "A hydrogen fusion shell around it, and then you have the rest of the star around that. So let me label it. This right here is our helium core. And more and more helium is going to be built up as this hydrogen and this shell fuses. And this is, in a star the size of our sun or the mass of our sun, this is when it starts to become a red giant. Because this core is getting denser and denser and denser as more and more helium is produced. And as it gets denser and denser and denser, there's more and more gravitational pressure being put on this hydrogen shell out here where we have fusion still happening."}, {"video_title": "Lifecycle of massive stars Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And more and more helium is going to be built up as this hydrogen and this shell fuses. And this is, in a star the size of our sun or the mass of our sun, this is when it starts to become a red giant. Because this core is getting denser and denser and denser as more and more helium is produced. And as it gets denser and denser and denser, there's more and more gravitational pressure being put on this hydrogen shell out here where we have fusion still happening. And so that's going to release more outward energy to push out the radius of the actual star. But then when you fast forward there, so the general process, and we're going to see this as the star gets more and more massive, is we're going to have heavier and heavier elements forming in the core. Those heavier and heavier elements, as the star gets denser and denser, will eventually ignite, kind of supporting the core."}, {"video_title": "Lifecycle of massive stars Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And as it gets denser and denser and denser, there's more and more gravitational pressure being put on this hydrogen shell out here where we have fusion still happening. And so that's going to release more outward energy to push out the radius of the actual star. But then when you fast forward there, so the general process, and we're going to see this as the star gets more and more massive, is we're going to have heavier and heavier elements forming in the core. Those heavier and heavier elements, as the star gets denser and denser, will eventually ignite, kind of supporting the core. But because the core itself is getting denser and denser and denser, material is getting pushed further and further out or with more and more energy. Although if the star is massive enough, it's not going to be able to be pushed out as far as you will have in a kind of a red giant with kind of a sun-like star. Let's just think about how this pattern is going to continue."}, {"video_title": "Lifecycle of massive stars Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Those heavier and heavier elements, as the star gets denser and denser, will eventually ignite, kind of supporting the core. But because the core itself is getting denser and denser and denser, material is getting pushed further and further out or with more and more energy. Although if the star is massive enough, it's not going to be able to be pushed out as far as you will have in a kind of a red giant with kind of a sun-like star. Let's just think about how this pattern is going to continue. So eventually, that helium is going to, once it gets dense enough, it's going to ignite. And it's going to fuse into carbon. And you're going to have a carbon core forming."}, {"video_title": "Lifecycle of massive stars Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Let's just think about how this pattern is going to continue. So eventually, that helium is going to, once it gets dense enough, it's going to ignite. And it's going to fuse into carbon. And you're going to have a carbon core forming. So that is carbon core. That's a carbon core. Around that, you have a helium core."}, {"video_title": "Lifecycle of massive stars Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And you're going to have a carbon core forming. So that is carbon core. That's a carbon core. Around that, you have a helium core. And near the center of the helium core, you have a shell of helium fusion. That's helium, not hydrogen. Turning into carbon, making that carbon core denser and hotter."}, {"video_title": "Lifecycle of massive stars Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Around that, you have a helium core. And near the center of the helium core, you have a shell of helium fusion. That's helium, not hydrogen. Turning into carbon, making that carbon core denser and hotter. And then around that, you have hydrogen fusion. Have to be very careful. You have hydrogen fusion."}, {"video_title": "Lifecycle of massive stars Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Turning into carbon, making that carbon core denser and hotter. And then around that, you have hydrogen fusion. Have to be very careful. You have hydrogen fusion. And then around that, you have the rest of the star. And so this process is going to keep continuing eventually. That carbon is going to start fusing."}, {"video_title": "Lifecycle of massive stars Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "You have hydrogen fusion. And then around that, you have the rest of the star. And so this process is going to keep continuing eventually. That carbon is going to start fusing. And you're going to have heavier and heavier elements forming the core. And so this is a depiction off of Wikipedia of a fairly mature, massive star. And you keep forming these shells of heavier and heavier elements and cores of heavier and heavier elements until eventually you get to iron."}, {"video_title": "Lifecycle of massive stars Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "That carbon is going to start fusing. And you're going to have heavier and heavier elements forming the core. And so this is a depiction off of Wikipedia of a fairly mature, massive star. And you keep forming these shells of heavier and heavier elements and cores of heavier and heavier elements until eventually you get to iron. And in particular, we're talking about iron 56. Iron with an atomic mass of 56. Here on this periodic table, that 26 is its atomic number."}, {"video_title": "Lifecycle of massive stars Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And you keep forming these shells of heavier and heavier elements and cores of heavier and heavier elements until eventually you get to iron. And in particular, we're talking about iron 56. Iron with an atomic mass of 56. Here on this periodic table, that 26 is its atomic number. It's how many protons it has. 56 is kind of viewed as a count of the protons and neutrons, although it's not exact. But at this point, the reason why you stop here is that you cannot get energy by fusing iron."}, {"video_title": "Lifecycle of massive stars Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Here on this periodic table, that 26 is its atomic number. It's how many protons it has. 56 is kind of viewed as a count of the protons and neutrons, although it's not exact. But at this point, the reason why you stop here is that you cannot get energy by fusing iron. Fusing iron into heavier elements beyond iron actually requires energy. So it would actually be an endothermic process. So to fuse iron actually won't help support the core."}, {"video_title": "Lifecycle of massive stars Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But at this point, the reason why you stop here is that you cannot get energy by fusing iron. Fusing iron into heavier elements beyond iron actually requires energy. So it would actually be an endothermic process. So to fuse iron actually won't help support the core. So what I want to do in this, well, just to be very clear, this is how the heavy elements actually formed. We started with hydrogen. Hydrogen fusing into helium."}, {"video_title": "Lifecycle of massive stars Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So to fuse iron actually won't help support the core. So what I want to do in this, well, just to be very clear, this is how the heavy elements actually formed. We started with hydrogen. Hydrogen fusing into helium. Helium fusing into carbon. And then all of these things in various combinations, and I won't go into all of the details, are fusing heavier and heavier elements. Neon, oxygen, and you see it right over here, silicon."}, {"video_title": "Lifecycle of massive stars Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Hydrogen fusing into helium. Helium fusing into carbon. And then all of these things in various combinations, and I won't go into all of the details, are fusing heavier and heavier elements. Neon, oxygen, and you see it right over here, silicon. And these aren't the only elements that are forming, but these are kind of the main core elements that are forming. But along the way, you have all this other stuff. Lithium, beryllium, boron, all of this other stuff is also forming."}, {"video_title": "Lifecycle of massive stars Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Neon, oxygen, and you see it right over here, silicon. And these aren't the only elements that are forming, but these are kind of the main core elements that are forming. But along the way, you have all this other stuff. Lithium, beryllium, boron, all of this other stuff is also forming. So this is how you form elements up to iron 56. And also, actually, this is actually how you can form up to nickel 56, just to be exact. There will also be some nickel 56, which has the same mass as iron 56, just has two fewer neutrons and two more protons."}, {"video_title": "Lifecycle of massive stars Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Lithium, beryllium, boron, all of this other stuff is also forming. So this is how you form elements up to iron 56. And also, actually, this is actually how you can form up to nickel 56, just to be exact. There will also be some nickel 56, which has the same mass as iron 56, just has two fewer neutrons and two more protons. So it's nickel 56. Will also form, can also kind of be a nickel iron core. But that's about how far a star can get, regardless of how massive it is, at least by going through traditional fusion, through the traditional ignition mechanism."}, {"video_title": "Development of agriculture and writing Cosmology & Astronomy Khan Academy.mp3", "Sentence": "In this video, I'm going to use words like eras, periods, and ages to refer to segments of time in the human or in the pre-human past. And what I want to clarify right from the get-go, because frankly this is something that's confused me in the past, is that archaeologists will refer to eras, periods, and ages in the human past, and they're usually referring to periods of tens of thousands of years or thousands of years. But these are different eras, periods, and ages than the ones that geologists would refer to when they're talking about geological time. In geological time, era means several hundred millions of years. Periods and ages mean millions of years. When archaeologists, when we're starting the human past, they're just generally talking about long segments of human time, but not in the millions of years, usually in the thousands or the 10,000s of years. So what I want to do with that out of the way is talk about what has happened in the distant human past or the distant pre-human past, and also touch on some of the classifications for these segments of time, because they actually tell us what were the interesting developments that happened to humanity over the 200,000 years that Homo sapiens have been on this planet, or that we believe that Homo sapiens have been on this planet."}, {"video_title": "Development of agriculture and writing Cosmology & Astronomy Khan Academy.mp3", "Sentence": "In geological time, era means several hundred millions of years. Periods and ages mean millions of years. When archaeologists, when we're starting the human past, they're just generally talking about long segments of human time, but not in the millions of years, usually in the thousands or the 10,000s of years. So what I want to do with that out of the way is talk about what has happened in the distant human past or the distant pre-human past, and also touch on some of the classifications for these segments of time, because they actually tell us what were the interesting developments that happened to humanity over the 200,000 years that Homo sapiens have been on this planet, or that we believe that Homo sapiens have been on this planet. So the longest period of time in human past, or the category of human time, and there's different ways you can categorize it, is the Paleolithic era right over here, and what really makes that period of time. So this begins even in pre-history or pre-human history, so before Homo sapiens even existed, you have the beginning of the Paleolithic era that really began with the development of stone tools, and as we learned in the video on human evolution, there were pre- Homo sapiens species that were using stone tools. And so the Paleolithic era, it's really kind of signified by one, the stone tools, but even more that either the pre-humans, or once you go about 200,000 years ago, the humans showed up."}, {"video_title": "Development of agriculture and writing Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So what I want to do with that out of the way is talk about what has happened in the distant human past or the distant pre-human past, and also touch on some of the classifications for these segments of time, because they actually tell us what were the interesting developments that happened to humanity over the 200,000 years that Homo sapiens have been on this planet, or that we believe that Homo sapiens have been on this planet. So the longest period of time in human past, or the category of human time, and there's different ways you can categorize it, is the Paleolithic era right over here, and what really makes that period of time. So this begins even in pre-history or pre-human history, so before Homo sapiens even existed, you have the beginning of the Paleolithic era that really began with the development of stone tools, and as we learned in the video on human evolution, there were pre- Homo sapiens species that were using stone tools. And so the Paleolithic era, it's really kind of signified by one, the stone tools, but even more that either the pre-humans, or once you go about 200,000 years ago, the humans showed up. It's kind of distinguished by humans being hunter-gatherers, which essentially means to survive, we used to walk around a lot, if we couldn't see something obvious to hunt, maybe a woolly mammoth or something, if we didn't see something obvious to hunt, we would look around for snails or mushrooms or whatever else, and that's how we would survive, that's how we would live. And because we were constantly adapting to our environment based on the seasons, we were maybe following animals as they migrated, hunter-gatherers were fundamentally nomadic, which means that they never settled in one place for a long time. They were always ready to kind of pick up, probably, their tents and follow the herd or follow whatever animals they were hunting or follow the seasons so they could go to warmer climates, maybe where there's more likely to find something to find on the ground to eat, maybe, during the winter, who knows."}, {"video_title": "Development of agriculture and writing Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And so the Paleolithic era, it's really kind of signified by one, the stone tools, but even more that either the pre-humans, or once you go about 200,000 years ago, the humans showed up. It's kind of distinguished by humans being hunter-gatherers, which essentially means to survive, we used to walk around a lot, if we couldn't see something obvious to hunt, maybe a woolly mammoth or something, if we didn't see something obvious to hunt, we would look around for snails or mushrooms or whatever else, and that's how we would survive, that's how we would live. And because we were constantly adapting to our environment based on the seasons, we were maybe following animals as they migrated, hunter-gatherers were fundamentally nomadic, which means that they never settled in one place for a long time. They were always ready to kind of pick up, probably, their tents and follow the herd or follow whatever animals they were hunting or follow the seasons so they could go to warmer climates, maybe where there's more likely to find something to find on the ground to eat, maybe, during the winter, who knows. So the Paleolithic era is really distinguished by that. It's a huge swath of time in human history, and it doesn't come to an end until you get to the advent of farming. So the Paleolithic era, I mean, we're literally talking about over 2 million years ago is when it starts, before Homo sapiens even existed as a species, and it goes all the way to the advent of farming that we believe first came about around 11,000 to 7,000 years ago."}, {"video_title": "Development of agriculture and writing Cosmology & Astronomy Khan Academy.mp3", "Sentence": "They were always ready to kind of pick up, probably, their tents and follow the herd or follow whatever animals they were hunting or follow the seasons so they could go to warmer climates, maybe where there's more likely to find something to find on the ground to eat, maybe, during the winter, who knows. So the Paleolithic era is really distinguished by that. It's a huge swath of time in human history, and it doesn't come to an end until you get to the advent of farming. So the Paleolithic era, I mean, we're literally talking about over 2 million years ago is when it starts, before Homo sapiens even existed as a species, and it goes all the way to the advent of farming that we believe first came about around 11,000 to 7,000 years ago. And this abbreviation right here, this BP, this does not stand for British Petroleum. It stands for Before Present or Before the Present Time. So one more acronym to kind of have in your toolkit when you see things."}, {"video_title": "Development of agriculture and writing Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So the Paleolithic era, I mean, we're literally talking about over 2 million years ago is when it starts, before Homo sapiens even existed as a species, and it goes all the way to the advent of farming that we believe first came about around 11,000 to 7,000 years ago. And this abbreviation right here, this BP, this does not stand for British Petroleum. It stands for Before Present or Before the Present Time. So one more acronym to kind of have in your toolkit when you see things. And obviously, if we're 11,000 years before the present, that's the same thing as 9,000 years before Christ or before the Common Era because Christ was, we believe, born 2,000 years ago. Now, it may or may not be obvious to you, but the advent of agriculture is a super big deal, arguably the biggest deal in the development of human civilization or in all of human history. And you might say, hey, what's the big deal about agriculture?"}, {"video_title": "Development of agriculture and writing Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So one more acronym to kind of have in your toolkit when you see things. And obviously, if we're 11,000 years before the present, that's the same thing as 9,000 years before Christ or before the Common Era because Christ was, we believe, born 2,000 years ago. Now, it may or may not be obvious to you, but the advent of agriculture is a super big deal, arguably the biggest deal in the development of human civilization or in all of human history. And you might say, hey, what's the big deal about agriculture? These characters over here look pretty happy. They're able to walk around a lot. They're able to hunt."}, {"video_title": "Development of agriculture and writing Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And you might say, hey, what's the big deal about agriculture? These characters over here look pretty happy. They're able to walk around a lot. They're able to hunt. What's the big deal of all of a sudden people plowing fields and domesticating cattle and having chickens to lay eggs and whatever else? And the big deal about that, besides the fact that it would change people's diet, is that for the first time, it allowed them to not be nomadic. It allowed them to, and you could have probably had some hunters who were somewhat settled, maybe living near the ocean, maybe they did some fishing and all the rest."}, {"video_title": "Development of agriculture and writing Cosmology & Astronomy Khan Academy.mp3", "Sentence": "They're able to hunt. What's the big deal of all of a sudden people plowing fields and domesticating cattle and having chickens to lay eggs and whatever else? And the big deal about that, besides the fact that it would change people's diet, is that for the first time, it allowed them to not be nomadic. It allowed them to, and you could have probably had some hunters who were somewhat settled, maybe living near the ocean, maybe they did some fishing and all the rest. But for the most part, with the development of agriculture, it forced people to stay in one place. So you have the Paleolithic era all the way to the advent of agriculture, which was about 11,000 to 7,000 years ago, and besides the fact that it changed people's diet, it allowed them to settle. So agriculture allowed human beings to settle down in one area."}, {"video_title": "Development of agriculture and writing Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It allowed them to, and you could have probably had some hunters who were somewhat settled, maybe living near the ocean, maybe they did some fishing and all the rest. But for the most part, with the development of agriculture, it forced people to stay in one place. So you have the Paleolithic era all the way to the advent of agriculture, which was about 11,000 to 7,000 years ago, and besides the fact that it changed people's diet, it allowed them to settle. So agriculture allowed human beings to settle down in one area. And it wasn't just that they were settling in one area, but because they were able to control their environment. They were able to increase the density of things, of crops that humans could consume, and animals that humans could consume, and lower the density of crops that humans can't consume, and animals that they can't consume, or that they don't want around, like pests of some type. What it allowed them to do is also settle in more dense environments."}, {"video_title": "Development of agriculture and writing Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So agriculture allowed human beings to settle down in one area. And it wasn't just that they were settling in one area, but because they were able to control their environment. They were able to increase the density of things, of crops that humans could consume, and animals that humans could consume, and lower the density of crops that humans can't consume, and animals that they can't consume, or that they don't want around, like pests of some type. What it allowed them to do is also settle in more dense environments. You can imagine, when you just have people walking around, you need a lot of land to support even the calorie requirements of one human being. But all of a sudden, if you are able to develop agriculture, you're able to domesticate animals, all of a sudden you can have, in the same amount of land, you can have more calories being generated. And because you have more calories being generated in a smaller amount of land, people can settle, and they can settle in a denser environment."}, {"video_title": "Development of agriculture and writing Cosmology & Astronomy Khan Academy.mp3", "Sentence": "What it allowed them to do is also settle in more dense environments. You can imagine, when you just have people walking around, you need a lot of land to support even the calorie requirements of one human being. But all of a sudden, if you are able to develop agriculture, you're able to domesticate animals, all of a sudden you can have, in the same amount of land, you can have more calories being generated. And because you have more calories being generated in a smaller amount of land, people can settle, and they can settle in a denser environment. And so agriculture was really this necessary requirement for people to kind of develop civilization, or to develop villages, and cities. And maybe also giving them the free time to start thinking about, hey, maybe we want to think about how we can record what we know, how we can develop even more technologies. And so just to give us a sense of the categorization that an archaeologist would use for these different periods of time, I told you we start with the Paleolithic era, with the advent of stone tools, pre-humans, most of human time on this planet."}, {"video_title": "Development of agriculture and writing Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And because you have more calories being generated in a smaller amount of land, people can settle, and they can settle in a denser environment. And so agriculture was really this necessary requirement for people to kind of develop civilization, or to develop villages, and cities. And maybe also giving them the free time to start thinking about, hey, maybe we want to think about how we can record what we know, how we can develop even more technologies. And so just to give us a sense of the categorization that an archaeologist would use for these different periods of time, I told you we start with the Paleolithic era, with the advent of stone tools, pre-humans, most of human time on this planet. And then about 11,000 years ago, the development of agriculture. And it developed independently at different places around the world, which is by itself an interesting phenomenon. And people think that it might just be that the climate might have warmed up a little bit, so that people, maybe naturally there were some human edible crops that existed in a little bit of a denser environment, and humans learned to optimize that slowly, and they did that independently."}, {"video_title": "Development of agriculture and writing Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And so just to give us a sense of the categorization that an archaeologist would use for these different periods of time, I told you we start with the Paleolithic era, with the advent of stone tools, pre-humans, most of human time on this planet. And then about 11,000 years ago, the development of agriculture. And it developed independently at different places around the world, which is by itself an interesting phenomenon. And people think that it might just be that the climate might have warmed up a little bit, so that people, maybe naturally there were some human edible crops that existed in a little bit of a denser environment, and humans learned to optimize that slowly, and they did that independently. But it's an interesting question of why did it develop just then, after 180,000, 190,000 years, why did agriculture all of a sudden happen? But just to get the terminology, the Paleolithic era is that period before agriculture. And then once agriculture starts developing, we are now in the Neolithic era."}, {"video_title": "Development of agriculture and writing Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And people think that it might just be that the climate might have warmed up a little bit, so that people, maybe naturally there were some human edible crops that existed in a little bit of a denser environment, and humans learned to optimize that slowly, and they did that independently. But it's an interesting question of why did it develop just then, after 180,000, 190,000 years, why did agriculture all of a sudden happen? But just to get the terminology, the Paleolithic era is that period before agriculture. And then once agriculture starts developing, we are now in the Neolithic era. And some archaeologists will describe a transition period between the Paleolithic and the Neolithic era, called the Mesolithic. And just so you know what these words mean, because they actually make sense when you know what they mean, paleo means old, and lithic means stone, or of stone. So they're really talking about the old stone age."}, {"video_title": "Development of agriculture and writing Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And then once agriculture starts developing, we are now in the Neolithic era. And some archaeologists will describe a transition period between the Paleolithic and the Neolithic era, called the Mesolithic. And just so you know what these words mean, because they actually make sense when you know what they mean, paleo means old, and lithic means stone, or of stone. So they're really talking about the old stone age. Neolithic, as you can imagine, means new, new stone. So it's kind of the new stone age. And meso means middle."}, {"video_title": "Development of agriculture and writing Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So they're really talking about the old stone age. Neolithic, as you can imagine, means new, new stone. So it's kind of the new stone age. And meso means middle. So it is the middle stone age. So another way of thinking about this whole period, from when people were hunter-gatherers all the way to about 11,000 to 7,000 years ago, when they developed agriculture, this whole period is called the stone age. And the stone age is kind of this biggest age."}, {"video_title": "Development of agriculture and writing Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And meso means middle. So it is the middle stone age. So another way of thinking about this whole period, from when people were hunter-gatherers all the way to about 11,000 to 7,000 years ago, when they developed agriculture, this whole period is called the stone age. And the stone age is kind of this biggest age. And there's just different ways of describing it. Because if you just call it the stone age, you're really making importance out of the actual tools that people could shape, they weren't able to use metal at this point. When you refer to Paleolithic and Neolithic, you're maybe referring a little bit more, and there's other ways to think about it, but you're referring a little bit more to the lifestyles of the human beings."}, {"video_title": "Development of agriculture and writing Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And the stone age is kind of this biggest age. And there's just different ways of describing it. Because if you just call it the stone age, you're really making importance out of the actual tools that people could shape, they weren't able to use metal at this point. When you refer to Paleolithic and Neolithic, you're maybe referring a little bit more, and there's other ways to think about it, but you're referring a little bit more to the lifestyles of the human beings. Paleolithic being hunter-gatherers, Neolithic having actually settled, having actually started to develop kind of primitive villages and even cities. And then, of course, Mesolithic is in between. And just for kind of a pop culture reference, you might have heard of the Paleolithic diet that some people are going on now."}, {"video_title": "Development of agriculture and writing Cosmology & Astronomy Khan Academy.mp3", "Sentence": "When you refer to Paleolithic and Neolithic, you're maybe referring a little bit more, and there's other ways to think about it, but you're referring a little bit more to the lifestyles of the human beings. Paleolithic being hunter-gatherers, Neolithic having actually settled, having actually started to develop kind of primitive villages and even cities. And then, of course, Mesolithic is in between. And just for kind of a pop culture reference, you might have heard of the Paleolithic diet that some people are going on now. And those are people who try to live like hunter-gatherers. Their belief is that most of human evolution occurred while we were hunter-gatherers. And so that's what our bodies are most accustomed to."}, {"video_title": "Development of agriculture and writing Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And just for kind of a pop culture reference, you might have heard of the Paleolithic diet that some people are going on now. And those are people who try to live like hunter-gatherers. Their belief is that most of human evolution occurred while we were hunter-gatherers. And so that's what our bodies are most accustomed to. So they like to eat meat, and they like to eat a lot of nuts. And I even met a co-worker once who used to only eat raw meat. And I don't know if that is even justified or that's even somehow validated by the archaeological record."}, {"video_title": "Development of agriculture and writing Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And so that's what our bodies are most accustomed to. So they like to eat meat, and they like to eat a lot of nuts. And I even met a co-worker once who used to only eat raw meat. And I don't know if that is even justified or that's even somehow validated by the archaeological record. These people probably did cook their meat. Now, at the end of the Stone Age, we would have, I would say, the number two most significant development in human history. And now we're talking about 3,000 BC, which is about 5,000 years ago."}, {"video_title": "Development of agriculture and writing Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And I don't know if that is even justified or that's even somehow validated by the archaeological record. These people probably did cook their meat. Now, at the end of the Stone Age, we would have, I would say, the number two most significant development in human history. And now we're talking about 3,000 BC, which is about 5,000 years ago. And this is the development of writing. So we were hunter-gatherers about 9,000 to 10,000, 11,000 years ago. People start developing agriculture."}, {"video_title": "Development of agriculture and writing Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And now we're talking about 3,000 BC, which is about 5,000 years ago. And this is the development of writing. So we were hunter-gatherers about 9,000 to 10,000, 11,000 years ago. People start developing agriculture. It allows them to settle in more dense environments. It also gives them a little bit more free time because they don't have to hunt and gather all the time. And then you go."}, {"video_title": "Development of agriculture and writing Cosmology & Astronomy Khan Academy.mp3", "Sentence": "People start developing agriculture. It allows them to settle in more dense environments. It also gives them a little bit more free time because they don't have to hunt and gather all the time. And then you go. And once again, we'll probably discover things as we go forward in time that maybe these dates need to be pushed back or whatever else. Or we discover new civilizations or who knows. But our best sense is you have these villages, you have these civilizations developing."}, {"video_title": "Development of agriculture and writing Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And then you go. And once again, we'll probably discover things as we go forward in time that maybe these dates need to be pushed back or whatever else. Or we discover new civilizations or who knows. But our best sense is you have these villages, you have these civilizations developing. And by about 5,000 years ago, so this would be 5,000 before the present or 3,000 BC, before Christ, you have people saying, hey, why don't we start trying to write down what we know so that when I tell someone orally, it doesn't actually lose information there. And so our descendants can slowly collect all of the knowledge we have and maybe accelerate. I don't know if they did it explicitly thinking of these, but let's just write down what we know."}, {"video_title": "Development of agriculture and writing Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But our best sense is you have these villages, you have these civilizations developing. And by about 5,000 years ago, so this would be 5,000 before the present or 3,000 BC, before Christ, you have people saying, hey, why don't we start trying to write down what we know so that when I tell someone orally, it doesn't actually lose information there. And so our descendants can slowly collect all of the knowledge we have and maybe accelerate. I don't know if they did it explicitly thinking of these, but let's just write down what we know. And so at about that period of time, you have, as far as we can tell, the first development of a pictogram based system of writing. And the earliest system of writing we know is cuneiform, which is from the Sumerian civilization, which is now in what present day Iraq. And what's the really big deal about this is that this is, on some level, the beginning of recorded history."}, {"video_title": "Development of agriculture and writing Cosmology & Astronomy Khan Academy.mp3", "Sentence": "I don't know if they did it explicitly thinking of these, but let's just write down what we know. And so at about that period of time, you have, as far as we can tell, the first development of a pictogram based system of writing. And the earliest system of writing we know is cuneiform, which is from the Sumerian civilization, which is now in what present day Iraq. And what's the really big deal about this is that this is, on some level, the beginning of recorded history. We could talk about the word history. You could say that history is all of the past. And we could use the archaeological record to figure out stuff before people started to write things down."}, {"video_title": "Development of agriculture and writing Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And what's the really big deal about this is that this is, on some level, the beginning of recorded history. We could talk about the word history. You could say that history is all of the past. And we could use the archaeological record to figure out stuff before people started to write things down. But when they started to write things down, now it was recorded. Now we're actually getting actual accounts of what people know, of actual people's knowledge. And the reason why this is a big deal, I mean, agriculture, hopefully you now appreciate that it was a pretty big deal."}, {"video_title": "Development of agriculture and writing Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And we could use the archaeological record to figure out stuff before people started to write things down. But when they started to write things down, now it was recorded. Now we're actually getting actual accounts of what people know, of actual people's knowledge. And the reason why this is a big deal, I mean, agriculture, hopefully you now appreciate that it was a pretty big deal. But the reason why writing was a big deal is that now civilization could collect its knowledge. And it could build upon it generation after generation without having to worry about people forgetting it or information getting distorted verbally from ancestor to descendant. And with that, you also have the beginning of the Bronze Age, and the Bronze Age is kind of known for this beginning of, even though it's referring to a material which comes from the first time that people started using bronze as a tool, or using bronze for their tools and for their weapons."}, {"video_title": "Development of agriculture and writing Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And the reason why this is a big deal, I mean, agriculture, hopefully you now appreciate that it was a pretty big deal. But the reason why writing was a big deal is that now civilization could collect its knowledge. And it could build upon it generation after generation without having to worry about people forgetting it or information getting distorted verbally from ancestor to descendant. And with that, you also have the beginning of the Bronze Age, and the Bronze Age is kind of known for this beginning of, even though it's referring to a material which comes from the first time that people started using bronze as a tool, or using bronze for their tools and for their weapons. And bronze, just so you know, it's a mixture of mostly copper and a little bit of tin. But the Bronze Age, at least in my mind, the biggest deal of what started at the beginning of the Bronze Age really, really was the writing. So once again, just as a review, because I actually find this kind of confusing, our current understanding, most of human prehistory, and even pre-human prehistory, were spent as hunter-gatherers using stone tools until about 11,000 years ago."}, {"video_title": "Development of agriculture and writing Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And with that, you also have the beginning of the Bronze Age, and the Bronze Age is kind of known for this beginning of, even though it's referring to a material which comes from the first time that people started using bronze as a tool, or using bronze for their tools and for their weapons. And bronze, just so you know, it's a mixture of mostly copper and a little bit of tin. But the Bronze Age, at least in my mind, the biggest deal of what started at the beginning of the Bronze Age really, really was the writing. So once again, just as a review, because I actually find this kind of confusing, our current understanding, most of human prehistory, and even pre-human prehistory, were spent as hunter-gatherers using stone tools until about 11,000 years ago. And then we became a little bit more settled. We became farmers, essentially, using stone tools. And then you fast forward another about 5,000, 6,000 years, and then we started to become farmers who started to write down the things that we knew, and we started to use bronze tools."}, {"video_title": "Detectable civilizations in our galaxy 1 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And maybe an even more interesting question is, can we detect any of those civilizations? Have they gotten to the level of technological progress like us, that they're emitting electromagnetic waves into space that other civilizations like ours can detect and say, hey, there's someone else out there watching television or using radio or whatever else they might be doing. And so what I want to do in this video is not answer that question. It's a big, open question. We don't know the answer. We don't have anywhere near enough information to definitively answer that question. But what I want to do is come up with a framework for at least thinking about that question, a way of actually estimating how many detectable civilizations there are in just our galaxy."}, {"video_title": "Detectable civilizations in our galaxy 1 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It's a big, open question. We don't know the answer. We don't have anywhere near enough information to definitively answer that question. But what I want to do is come up with a framework for at least thinking about that question, a way of actually estimating how many detectable civilizations there are in just our galaxy. And there's a formula that you may or may not have heard of called the Drake equation. And what we're going to do is independently derive our own version of the Drake equation. It's going to be slightly different, but it's the same thought process."}, {"video_title": "Detectable civilizations in our galaxy 1 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But what I want to do is come up with a framework for at least thinking about that question, a way of actually estimating how many detectable civilizations there are in just our galaxy. And there's a formula that you may or may not have heard of called the Drake equation. And what we're going to do is independently derive our own version of the Drake equation. It's going to be slightly different, but it's the same thought process. And in a future video, I'm going to maybe reconcile what we come up with with the Drake equation. And just so you know, the Drake equation is named for Frank Drake, who is a professor at University of California, Santa Cruz. He first kind of put some structure around this problem, and that's why the formula or the equation has his name."}, {"video_title": "Detectable civilizations in our galaxy 1 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It's going to be slightly different, but it's the same thought process. And in a future video, I'm going to maybe reconcile what we come up with with the Drake equation. And just so you know, the Drake equation is named for Frank Drake, who is a professor at University of California, Santa Cruz. He first kind of put some structure around this problem, and that's why the formula or the equation has his name. But the equation, it's not an equation that you can apply on a daily basis and get results that you can use to build things. But what it is, is it structures our thinking around this question of how many detectable civilizations are there in our galaxy. Now to answer this question, I'm going to start a little bit differently than Frank Drake did."}, {"video_title": "Detectable civilizations in our galaxy 1 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "He first kind of put some structure around this problem, and that's why the formula or the equation has his name. But the equation, it's not an equation that you can apply on a daily basis and get results that you can use to build things. But what it is, is it structures our thinking around this question of how many detectable civilizations are there in our galaxy. Now to answer this question, I'm going to start a little bit differently than Frank Drake did. He starts with the number of new stars that are born each year. And we'll see that our definitions are actually pretty close to each other. What I want to do is start with the total number of stars."}, {"video_title": "Detectable civilizations in our galaxy 1 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Now to answer this question, I'm going to start a little bit differently than Frank Drake did. He starts with the number of new stars that are born each year. And we'll see that our definitions are actually pretty close to each other. What I want to do is start with the total number of stars. So what we're trying to come up with is, I'll call it n, and this is the number of detectable civilizations in the Milky Way, in our galaxy. And once again, there could be civilizations, looking back at this star field right over here. This star right over here, maybe it has a planet that's in the right place."}, {"video_title": "Detectable civilizations in our galaxy 1 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "What I want to do is start with the total number of stars. So what we're trying to come up with is, I'll call it n, and this is the number of detectable civilizations in the Milky Way, in our galaxy. And once again, there could be civilizations, looking back at this star field right over here. This star right over here, maybe it has a planet that's in the right place. It has liquid water. And maybe there's intelligent life on that planet. But they might not be detectable because they aren't technologically advanced enough that they're using electromagnetic radiation."}, {"video_title": "Detectable civilizations in our galaxy 1 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "This star right over here, maybe it has a planet that's in the right place. It has liquid water. And maybe there's intelligent life on that planet. But they might not be detectable because they aren't technologically advanced enough that they're using electromagnetic radiation. Or maybe they just figured out some other way to communicate. Or maybe they're beyond using electromagnetic radiation, radio waves and all the rest, to communicate. And so we'll never be able to detect them."}, {"video_title": "Detectable civilizations in our galaxy 1 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But they might not be detectable because they aren't technologically advanced enough that they're using electromagnetic radiation. Or maybe they just figured out some other way to communicate. Or maybe they're beyond using electromagnetic radiation, radio waves and all the rest, to communicate. And so we'll never be able to detect them. We're talking about civilizations like ours that are, to some degree, using technology not too different than our own. So that's what we mean by detectable. So let's think about that a little bit."}, {"video_title": "Detectable civilizations in our galaxy 1 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And so we'll never be able to detect them. We're talking about civilizations like ours that are, to some degree, using technology not too different than our own. So that's what we mean by detectable. So let's think about that a little bit. So I like to start with just the total number of stars in our solar system. So let's just start with, I'll call it n star and asterisk. And this is the number of stars in our, not in our solar system, number of stars in our galaxy."}, {"video_title": "Detectable civilizations in our galaxy 1 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So let's think about that a little bit. So I like to start with just the total number of stars in our solar system. So let's just start with, I'll call it n star and asterisk. And this is the number of stars in our, not in our solar system, number of stars in our galaxy. Number of stars in the galaxy. And our best guess, I said, is this is going to be 100 billion to 400 billion stars. We don't even know how many there are."}, {"video_title": "Detectable civilizations in our galaxy 1 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And this is the number of stars in our, not in our solar system, number of stars in our galaxy. Number of stars in the galaxy. And our best guess, I said, is this is going to be 100 billion to 400 billion stars. We don't even know how many there are. Some of them are undetectable. In the center of our galaxy, it's just a big blur to us. And we don't even know what's on the other side of that."}, {"video_title": "Detectable civilizations in our galaxy 1 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "We don't even know how many there are. Some of them are undetectable. In the center of our galaxy, it's just a big blur to us. And we don't even know what's on the other side of that. And we can't even see all the stars that are packed into the center. So this is our best guess, 100 billion to 400 billion stars. Now obviously, it's going to be a subset of those stars that even have planets."}, {"video_title": "Detectable civilizations in our galaxy 1 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And we don't even know what's on the other side of that. And we can't even see all the stars that are packed into the center. So this is our best guess, 100 billion to 400 billion stars. Now obviously, it's going to be a subset of those stars that even have planets. So let's multiply it times that subset. So let's multiply it times the frequency of having a planet. If you're a star, this is the percent chance or the frequency or the fraction of these stars that have planets."}, {"video_title": "Detectable civilizations in our galaxy 1 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Now obviously, it's going to be a subset of those stars that even have planets. So let's multiply it times that subset. So let's multiply it times the frequency of having a planet. If you're a star, this is the percent chance or the frequency or the fraction of these stars that have planets. So I'll write it this way. Fraction that have planets. So if this is 100 billion, and let's say I'm making a guess here, and we're learning more about this every day, there are all these discoveries of exoplanets, planets outside of our solar system, maybe this is 1 4th."}, {"video_title": "Detectable civilizations in our galaxy 1 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "If you're a star, this is the percent chance or the frequency or the fraction of these stars that have planets. So I'll write it this way. Fraction that have planets. So if this is 100 billion, and let's say I'm making a guess here, and we're learning more about this every day, there are all these discoveries of exoplanets, planets outside of our solar system, maybe this is 1 4th. Then we could say, well, that means that 100 billion times 1 4th means that there are 25 billion stars that have planets around them. But that's still not enough to go to civilizations. We also need to think about planets."}, {"video_title": "Detectable civilizations in our galaxy 1 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So if this is 100 billion, and let's say I'm making a guess here, and we're learning more about this every day, there are all these discoveries of exoplanets, planets outside of our solar system, maybe this is 1 4th. Then we could say, well, that means that 100 billion times 1 4th means that there are 25 billion stars that have planets around them. But that's still not enough to go to civilizations. We also need to think about planets. Because a planet could be a planet like Jupiter, and we don't know how life as we know it can survive on a planet like Jupiter or Neptune or Mercury. It has to have planets that are good for sustaining life. Preferably have a rocky core, liquid water on the outside."}, {"video_title": "Detectable civilizations in our galaxy 1 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "We also need to think about planets. Because a planet could be a planet like Jupiter, and we don't know how life as we know it can survive on a planet like Jupiter or Neptune or Mercury. It has to have planets that are good for sustaining life. Preferably have a rocky core, liquid water on the outside. That's what we think are the ingredients that you need for life. Maybe we're just not being creative enough. That's what we know as life as being."}, {"video_title": "Detectable civilizations in our galaxy 1 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Preferably have a rocky core, liquid water on the outside. That's what we think are the ingredients that you need for life. Maybe we're just not being creative enough. That's what we know as life as being. So let's multiply this times the average number of life-sustaining or planets that could sustain life on them. So we don't necessarily that they're going to have life, but they seem like they're just the right distance from the star, not too hot, not too cold. They have the right amount of gravity, water, all the other stuff."}, {"video_title": "Detectable civilizations in our galaxy 1 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "That's what we know as life as being. So let's multiply this times the average number of life-sustaining or planets that could sustain life on them. So we don't necessarily that they're going to have life, but they seem like they're just the right distance from the star, not too hot, not too cold. They have the right amount of gravity, water, all the other stuff. And we still don't know exactly what this means. But this means average number. So given that there's a solar system with planets, what's the average number of planets that are capable of sustaining life?"}, {"video_title": "Detectable civilizations in our galaxy 1 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "They have the right amount of gravity, water, all the other stuff. And we still don't know exactly what this means. But this means average number. So given that there's a solar system with planets, what's the average number of planets that are capable of sustaining life? And once again, we don't know this answer. Maybe it's 0.1. It's probably less than 1."}, {"video_title": "Detectable civilizations in our galaxy 1 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So given that there's a solar system with planets, what's the average number of planets that are capable of sustaining life? And once again, we don't know this answer. Maybe it's 0.1. It's probably less than 1. For any given solar system that has planets, the average number capable of sustaining life, maybe it's 0.1. Maybe it's more than 1. I don't know."}, {"video_title": "Detectable civilizations in our galaxy 1 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It's probably less than 1. For any given solar system that has planets, the average number capable of sustaining life, maybe it's 0.1. Maybe it's more than 1. I don't know. We don't know the exact answer here. But I'll throw out a guess. Maybe it is 0.1."}, {"video_title": "Detectable civilizations in our galaxy 1 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "I don't know. We don't know the exact answer here. But I'll throw out a guess. Maybe it is 0.1. And here, the fraction that have planets, I don't know. I'll throw that out as. And once again, I'm just making up these numbers."}, {"video_title": "Detectable civilizations in our galaxy 1 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Maybe it is 0.1. And here, the fraction that have planets, I don't know. I'll throw that out as. And once again, I'm just making up these numbers. We really don't know the right answer. This is 1 4th. But if we were to multiply this out, we would have the average number of planets in our solar system that are capable of sustaining life, that are around stars that have planets."}, {"video_title": "Detectable civilizations in our galaxy 1 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And once again, I'm just making up these numbers. We really don't know the right answer. This is 1 4th. But if we were to multiply this out, we would have the average number of planets in our solar system that are capable of sustaining life, that are around stars that have planets. And these planets are capable of sustaining life. Now, and this would give us the total number, because this is average per solar system that has planets. This is the total number of solar systems with planets."}, {"video_title": "Detectable civilizations in our galaxy 1 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But if we were to multiply this out, we would have the average number of planets in our solar system that are capable of sustaining life, that are around stars that have planets. And these planets are capable of sustaining life. Now, and this would give us the total number, because this is average per solar system that has planets. This is the total number of solar systems with planets. You multiply it out. Total number of planets in our galaxy capable of sustaining life. Now, just because you have liquid water and the right temperature and all of the rest ingredients doesn't necessarily mean that you will actually have life happening on your planet."}, {"video_title": "Detectable civilizations in our galaxy 1 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "This is the total number of solar systems with planets. You multiply it out. Total number of planets in our galaxy capable of sustaining life. Now, just because you have liquid water and the right temperature and all of the rest ingredients doesn't necessarily mean that you will actually have life happening on your planet. So let's multiply that times the fraction that actually generate life. So this is the fraction that actually have life. And this is actually a very, we don't know this answer."}, {"video_title": "Detectable civilizations in our galaxy 1 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Now, just because you have liquid water and the right temperature and all of the rest ingredients doesn't necessarily mean that you will actually have life happening on your planet. So let's multiply that times the fraction that actually generate life. So this is the fraction that actually have life. And this is actually a very, we don't know this answer. So this is a fraction that have life on them. And this is a really big open question. Maybe if you have the ingredients, almost every planet has life."}, {"video_title": "Detectable civilizations in our galaxy 1 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And this is actually a very, we don't know this answer. So this is a fraction that have life on them. And this is a really big open question. Maybe if you have the ingredients, almost every planet has life. Maybe it's a frequent thing that's happening in our galaxy and frankly our universe. Or maybe it's a very infrequent thing. Maybe it's just the right kind of freak set of circumstances that just have to happen."}, {"video_title": "Detectable civilizations in our galaxy 1 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Maybe if you have the ingredients, almost every planet has life. Maybe it's a frequent thing that's happening in our galaxy and frankly our universe. Or maybe it's a very infrequent thing. Maybe it's just the right kind of freak set of circumstances that just have to happen. I'll throw out a number just for the sake of just to have a number there. Maybe it's 1 out of every 10 planets that have all of the right ingredients for life actually do generate life. My personal guess is probably higher than that, given that life seems such a robust and flexible thing that we've been seeing in all sorts of weird circumstances."}, {"video_title": "Detectable civilizations in our galaxy 1 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Maybe it's just the right kind of freak set of circumstances that just have to happen. I'll throw out a number just for the sake of just to have a number there. Maybe it's 1 out of every 10 planets that have all of the right ingredients for life actually do generate life. My personal guess is probably higher than that, given that life seems such a robust and flexible thing that we've been seeing in all sorts of weird circumstances. Actually, let me make it even a higher number than that. So let me make it 1 half, assuming that we have all of the ingredients. So this should tell us essentially how many planets, if we were to multiply all of these, how many planets in our galaxy have had life on them at some point in those planets' lives."}, {"video_title": "Detectable civilizations in our galaxy 1 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "My personal guess is probably higher than that, given that life seems such a robust and flexible thing that we've been seeing in all sorts of weird circumstances. Actually, let me make it even a higher number than that. So let me make it 1 half, assuming that we have all of the ingredients. So this should tell us essentially how many planets, if we were to multiply all of these, how many planets in our galaxy have had life on them at some point in those planets' lives. The life might have come and gone. It maybe destroyed itself through nuclear war or whatever. But this would tell us the number of life planets in our galaxy that have had life on them at at least one point in their history."}, {"video_title": "Detectable civilizations in our galaxy 1 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So this should tell us essentially how many planets, if we were to multiply all of these, how many planets in our galaxy have had life on them at some point in those planets' lives. The life might have come and gone. It maybe destroyed itself through nuclear war or whatever. But this would tell us the number of life planets in our galaxy that have had life on them at at least one point in their history. Now, we care about civilization. So let's multiply this times. If you make all of these, you even get to the point that you have life, we care about, well, do you get intelligent life?"}, {"video_title": "Detectable civilizations in our galaxy 1 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But this would tell us the number of life planets in our galaxy that have had life on them at at least one point in their history. Now, we care about civilization. So let's multiply this times. If you make all of these, you even get to the point that you have life, we care about, well, do you get intelligent life? So maybe if the asteroid never hit Earth, the dinosaurs would have stayed on Earth, and they would have never evolved to the point of generating radios and TVs and telephones and all the rest. And so it's kind of a freak circumstance that we were, because they were destroyed, these gaps in the ecosystem developed so that we could emerge and be intelligent and do all of these crazy things like make YouTube videos and all the rest. So let's multiply this times the fraction."}, {"video_title": "Detectable civilizations in our galaxy 1 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "If you make all of these, you even get to the point that you have life, we care about, well, do you get intelligent life? So maybe if the asteroid never hit Earth, the dinosaurs would have stayed on Earth, and they would have never evolved to the point of generating radios and TVs and telephones and all the rest. And so it's kind of a freak circumstance that we were, because they were destroyed, these gaps in the ecosystem developed so that we could emerge and be intelligent and do all of these crazy things like make YouTube videos and all the rest. So let's multiply this times the fraction. If you get all of this, the fraction that actually end up having intelligent life. And maybe this fraction is intelligent life over here. So intelligent life, this is maybe out to our number 1 10th."}, {"video_title": "Detectable civilizations in our galaxy 1 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So let's multiply this times the fraction. If you get all of this, the fraction that actually end up having intelligent life. And maybe this fraction is intelligent life over here. So intelligent life, this is maybe out to our number 1 10th. And probably in the next video, I'll calculate it all. And this is very important to realize, because once again, you could have life. These are all examples of life right over here."}, {"video_title": "Detectable civilizations in our galaxy 1 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So intelligent life, this is maybe out to our number 1 10th. And probably in the next video, I'll calculate it all. And this is very important to realize, because once again, you could have life. These are all examples of life right over here. This is actually life on our planet, even though this looks quite alien. This is a weevil that kind of looked very close up. But there's all sorts of forms of life, many of which we probably can't even begin to imagine."}, {"video_title": "Detectable civilizations in our galaxy 1 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "These are all examples of life right over here. This is actually life on our planet, even though this looks quite alien. This is a weevil that kind of looked very close up. But there's all sorts of forms of life, many of which we probably can't even begin to imagine. But what we care is that intelligent life starts to emerge on the planet. Because only intelligent life has a chance, we believe, of being able to eventually communicate in ways that are detectable by us. Now, I said intelligent life, but maybe not all intelligent life will eventually get to the technological sophistication where they will be using radio waves and electromagnetic radiation to communicate with each other."}, {"video_title": "Detectable civilizations in our galaxy 1 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But there's all sorts of forms of life, many of which we probably can't even begin to imagine. But what we care is that intelligent life starts to emerge on the planet. Because only intelligent life has a chance, we believe, of being able to eventually communicate in ways that are detectable by us. Now, I said intelligent life, but maybe not all intelligent life will eventually get to the technological sophistication where they will be using radio waves and electromagnetic radiation to communicate with each other. Maybe we might have stagnated at this stage if the right things didn't happen. So what we need to do now is multiply this. So right here, we would have the number of planets in our galaxy that have had intelligent life on them at some point in their history, maybe not at a time that coincides with ours."}, {"video_title": "Detectable civilizations in our galaxy 1 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Now, I said intelligent life, but maybe not all intelligent life will eventually get to the technological sophistication where they will be using radio waves and electromagnetic radiation to communicate with each other. Maybe we might have stagnated at this stage if the right things didn't happen. So what we need to do now is multiply this. So right here, we would have the number of planets in our galaxy that have had intelligent life on them at some point in their history, maybe not at a time that coincides with ours. But what we want to do is whittle it down even more to the percentage that get to the point that they can develop technology that allows us to detect them. So let me multiply it times the fraction that are, I'll put a c here for maybe they're using communications, c for communications, that allow us to detect them. So this is detectable, the fraction that are detectable."}, {"video_title": "Detectable civilizations in our galaxy 1 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So right here, we would have the number of planets in our galaxy that have had intelligent life on them at some point in their history, maybe not at a time that coincides with ours. But what we want to do is whittle it down even more to the percentage that get to the point that they can develop technology that allows us to detect them. So let me multiply it times the fraction that are, I'll put a c here for maybe they're using communications, c for communications, that allow us to detect them. So this is detectable, the fraction that are detectable. Detectable. Now you might think that we're done. This would give you the total number of civilizations or life forms in our galaxy, or the planets that have life forms that developed detectable technologies at some point in their history."}, {"video_title": "Detectable civilizations in our galaxy 1 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So this is detectable, the fraction that are detectable. Detectable. Now you might think that we're done. This would give you the total number of civilizations or life forms in our galaxy, or the planets that have life forms that developed detectable technologies at some point in their history. Now it would be nice if civilizations did not kind of be born and then die. But the reality is they do die. They might destroy themselves or whatever."}, {"video_title": "Detectable civilizations in our galaxy 1 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "This would give you the total number of civilizations or life forms in our galaxy, or the planets that have life forms that developed detectable technologies at some point in their history. Now it would be nice if civilizations did not kind of be born and then die. But the reality is they do die. They might destroy themselves or whatever. And they might exist for only a small period of time for the history of that planet or the history of that solar system. So in order to make it the number of civilizations that are in existence now, and I'll clarify what now means in the next video. Because it's really, if we're detecting something from a star that's 10,000 light years away, our now means we're just receiving their signals, which means that they released the signals 10,000 years ago."}, {"video_title": "Detectable civilizations in our galaxy 1 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "They might destroy themselves or whatever. And they might exist for only a small period of time for the history of that planet or the history of that solar system. So in order to make it the number of civilizations that are in existence now, and I'll clarify what now means in the next video. Because it's really, if we're detecting something from a star that's 10,000 light years away, our now means we're just receiving their signals, which means that they released the signals 10,000 years ago. So what I want to do is, what is the fraction of these whose signals are achieving, are reaching us right now? And here I'm going to say, well, what's the average lifespan of a civilization? I'll put that L. Who knows what that is?"}, {"video_title": "Detectable civilizations in our galaxy 1 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Because it's really, if we're detecting something from a star that's 10,000 light years away, our now means we're just receiving their signals, which means that they released the signals 10,000 years ago. So what I want to do is, what is the fraction of these whose signals are achieving, are reaching us right now? And here I'm going to say, well, what's the average lifespan of a civilization? I'll put that L. Who knows what that is? Maybe 10,000 years. So civilization lifespan. And it's going to be that over the life of the star."}, {"video_title": "Detectable civilizations in our galaxy 1 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "I'll put that L. Who knows what that is? Maybe 10,000 years. So civilization lifespan. And it's going to be that over the life of the star. So that is over, I'll put a T here. T for the star. So the average lifespan for the star."}, {"video_title": "Detectable civilizations in our galaxy 1 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And it's going to be that over the life of the star. So that is over, I'll put a T here. T for the star. So the average lifespan for the star. And I could say the average lifespan for the planet or whatever, but we're assuming that once our star supernovas, you're not going to have any chance for Earth to develop life on it anymore. So maybe this thing up here is 10,000 years, and this down here is maybe 10 billion years. And if you were to multiply all of this out, you should get the number of detectable civilizations in our galaxy right now."}, {"video_title": "Detectable civilizations in our galaxy 1 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So the average lifespan for the star. And I could say the average lifespan for the planet or whatever, but we're assuming that once our star supernovas, you're not going to have any chance for Earth to develop life on it anymore. So maybe this thing up here is 10,000 years, and this down here is maybe 10 billion years. And if you were to multiply all of this out, you should get the number of detectable civilizations in our galaxy right now. I'll leave you there for this video. In the next video, we'll discuss it a little bit more and reconcile it with the more famous version of Drake's equation. And I'll also try to talk about this piece a little bit, because I think this might be a little bit confusing."}, {"video_title": "Galactic collisions Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And I threw out in the context of maybe the supermassive black holes at the galactic cores of each of those galaxies will start getting a little bit more material when that collision happens. And maybe quasars will happen. I don't know. But given the interest in that, what I wanted to do here is kind of an unconventional thing for the Khan Academy and actually show a video. And before I play the video, I have to give credit where credit is due. This is a supercomputer simulation made at the National Center for Supercomputing Applications in NASA. And it's by B. Robertson of Caltech and L. Hernquist of Harvard University."}, {"video_title": "Galactic collisions Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But given the interest in that, what I wanted to do here is kind of an unconventional thing for the Khan Academy and actually show a video. And before I play the video, I have to give credit where credit is due. This is a supercomputer simulation made at the National Center for Supercomputing Applications in NASA. And it's by B. Robertson of Caltech and L. Hernquist of Harvard University. And what I want you to remember is this is super sped up in time. Just to give an idea, the amount of time it takes for a star about as far away as the sun to make one orbit around the galactic core is 250 million years. And you're going to see that this is happening multiple times over the course of this video."}, {"video_title": "Galactic collisions Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And it's by B. Robertson of Caltech and L. Hernquist of Harvard University. And what I want you to remember is this is super sped up in time. Just to give an idea, the amount of time it takes for a star about as far away as the sun to make one orbit around the galactic core is 250 million years. And you're going to see that this is happening multiple times over the course of this video. So this video is actually spanning billions of years. But when you actually speed up time like that, you'll see that it really gives you a sense of the actual dynamics of these interactions. The other thing I want to talk about before I actually start the video is to make you realize when we talk about galaxies colliding, it doesn't mean that the stars are colliding."}, {"video_title": "Galactic collisions Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And you're going to see that this is happening multiple times over the course of this video. So this video is actually spanning billions of years. But when you actually speed up time like that, you'll see that it really gives you a sense of the actual dynamics of these interactions. The other thing I want to talk about before I actually start the video is to make you realize when we talk about galaxies colliding, it doesn't mean that the stars are colliding. In fact, there are going to be very few stars that actually collide. The probability of a star-star collision is very low. And that's because we learned, when we learned about interstellar scale, that there's mostly free space in between stars."}, {"video_title": "Galactic collisions Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "The other thing I want to talk about before I actually start the video is to make you realize when we talk about galaxies colliding, it doesn't mean that the stars are colliding. In fact, there are going to be very few stars that actually collide. The probability of a star-star collision is very low. And that's because we learned, when we learned about interstellar scale, that there's mostly free space in between stars. The closest star to us is 4.2 light years away. And that's roughly 30 million times the diameter of the sun. So you have a lot more free space than star space or even solar system space."}, {"video_title": "Galactic collisions Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And that's because we learned, when we learned about interstellar scale, that there's mostly free space in between stars. The closest star to us is 4.2 light years away. And that's roughly 30 million times the diameter of the sun. So you have a lot more free space than star space or even solar system space. So let's start up this animation. It's pretty amazing. And what you're going to see here, so these are just the, obviously, so one rotation is actually 250 million years, give or take."}, {"video_title": "Galactic collisions Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So you have a lot more free space than star space or even solar system space. So let's start up this animation. It's pretty amazing. And what you're going to see here, so these are just the, obviously, so one rotation is actually 250 million years, give or take. But now you see these stars right here are starting to get attracted to this core. And then they're actually attracted to that core. And then some of the stuff in that core was attracted to those stars."}, {"video_title": "Galactic collisions Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And what you're going to see here, so these are just the, obviously, so one rotation is actually 250 million years, give or take. But now you see these stars right here are starting to get attracted to this core. And then they're actually attracted to that core. And then some of the stuff in that core was attracted to those stars. And they get pulled away. That was the first pass of these two galaxies. Some stuff is just being thrown off into intergalactic space."}, {"video_title": "Galactic collisions Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And then some of the stuff in that core was attracted to those stars. And they get pulled away. That was the first pass of these two galaxies. Some stuff is just being thrown off into intergalactic space. And you might worry, maybe that'll happen to the Earth. And there's some probability that it would happen to the Earth, but it really wouldn't affect what happens within those stars' solar systems. This is happening so slow."}, {"video_title": "Galactic collisions Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Some stuff is just being thrown off into intergalactic space. And you might worry, maybe that'll happen to the Earth. And there's some probability that it would happen to the Earth, but it really wouldn't affect what happens within those stars' solar systems. This is happening so slow. You wouldn't feel like some type of acceleration or something. And then this is the second pass. So they passed one pass."}, {"video_title": "Galactic collisions Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "This is happening so slow. You wouldn't feel like some type of acceleration or something. And then this is the second pass. So they passed one pass. And once again, we're doing this. This is occurring over hundreds of millions or billions of years. And on the second pass, they finally are able to merge."}, {"video_title": "Galactic collisions Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So they passed one pass. And once again, we're doing this. This is occurring over hundreds of millions or billions of years. And on the second pass, they finally are able to merge. And all of these interactions are just through the gravity over interstellar, almost you could call it, intergalactic distances. You can see they merge into what could be called as a milk-o-meta, or maybe the Andromedae way. I don't know, whatever you want to call it."}, {"video_title": "Galactic collisions Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And on the second pass, they finally are able to merge. And all of these interactions are just through the gravity over interstellar, almost you could call it, intergalactic distances. You can see they merge into what could be called as a milk-o-meta, or maybe the Andromedae way. I don't know, whatever you want to call it. But even though they've merged, a lot of the stuff has still been thrown off into intergalactic space. But this is a pretty amazing animation to me. One, it's amazing to think about how this could happen over galactic space scales and time scales."}, {"video_title": "Galactic collisions Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "I don't know, whatever you want to call it. But even though they've merged, a lot of the stuff has still been thrown off into intergalactic space. But this is a pretty amazing animation to me. One, it's amazing to think about how this could happen over galactic space scales and time scales. But it's also pretty neat how a supercomputer can do all of the computations to figure out what every particle, which is really a star, a cluster of stars, or group of stars, is actually doing to actually give us a sense of the actual dynamics here. But this is pretty neat. Look at that."}, {"video_title": "Big bang introduction Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Big Bang Theory. And really, it's just this idea that the universe started as kind of this infinitely small point, this infinitely small singularity, and then it just had a big bang, or it just expanded from that state to the universe that we know right now. And when I first imagined this, it's also a byproduct of how it's named Big Bang. You kind of imagine this type of explosion. You kind of imagine this type of explosion, that everything was infinitely packed in together, and then it exploded, and then it exploded outward. And then as all of the matter exploded outward, it started to condense, and then you have these little galaxies and super clusters of galaxies, and they started to condense, and within them, planets condensed and stars condensed, and we have the type of universe that we have right now. But this model for visualizing the Big Bang has a couple of problems."}, {"video_title": "Big bang introduction Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "You kind of imagine this type of explosion. You kind of imagine this type of explosion, that everything was infinitely packed in together, and then it exploded, and then it exploded outward. And then as all of the matter exploded outward, it started to condense, and then you have these little galaxies and super clusters of galaxies, and they started to condense, and within them, planets condensed and stars condensed, and we have the type of universe that we have right now. But this model for visualizing the Big Bang has a couple of problems. One is, when we talk about the Big Bang, we're not talking about the matter, just the mass or just the matter in the universe being in one point. We're talking about actual space expanding. So we're not just talking about something inside of space, like the physical mass, the physical matter expanding."}, {"video_title": "Big bang introduction Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But this model for visualizing the Big Bang has a couple of problems. One is, when we talk about the Big Bang, we're not talking about the matter, just the mass or just the matter in the universe being in one point. We're talking about actual space expanding. So we're not just talking about something inside of space, like the physical mass, the physical matter expanding. We're talking about space itself. And so when you have this type of model, you have all of this stuff expanding, but you're like, well, look, isn't it expanding into something else? Maybe if the furthest out parts of this matter is right over here, what's this stuff over here?"}, {"video_title": "Big bang introduction Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So we're not just talking about something inside of space, like the physical mass, the physical matter expanding. We're talking about space itself. And so when you have this type of model, you have all of this stuff expanding, but you're like, well, look, isn't it expanding into something else? Maybe if the furthest out parts of this matter is right over here, what's this stuff over here? And so you say, well, wouldn't that be space? So how can you say space itself is expanding? And another idea that a Big Bang also implies is if this is the furthest stuff out there, would this be the edge of the universe?"}, {"video_title": "Big bang introduction Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Maybe if the furthest out parts of this matter is right over here, what's this stuff over here? And so you say, well, wouldn't that be space? So how can you say space itself is expanding? And another idea that a Big Bang also implies is if this is the furthest stuff out there, would this be the edge of the universe? Does the universe have an edge? And the answer to either of those questions, and that's what we're going to try to tackle in this, is that one, the universe does not have an edge, and two, there is no outside space. We are not expanding into another space, and I'm going to explain that."}, {"video_title": "Big bang introduction Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And another idea that a Big Bang also implies is if this is the furthest stuff out there, would this be the edge of the universe? Does the universe have an edge? And the answer to either of those questions, and that's what we're going to try to tackle in this, is that one, the universe does not have an edge, and two, there is no outside space. We are not expanding into another space, and I'm going to explain that. Hopefully we'll see why that is the case right now. So the best way to view it, and we're going to view it by analogy, if I were to tell you that I have a two-dimensional space that has a finite area, so it's not infinite, and it also has no edge, this once again, when you first look at it, it seems difficult. How do I construct something that has a finite area but still has no edge?"}, {"video_title": "Big bang introduction Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "We are not expanding into another space, and I'm going to explain that. Hopefully we'll see why that is the case right now. So the best way to view it, and we're going to view it by analogy, if I were to tell you that I have a two-dimensional space that has a finite area, so it's not infinite, and it also has no edge, this once again, when you first look at it, it seems difficult. How do I construct something that has a finite area but still has no edge? Every time I try to draw an area, it looks like I have to have some edges. And then you might remember, what if that two-dimensional space is curved? What happens?"}, {"video_title": "Big bang introduction Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "How do I construct something that has a finite area but still has no edge? Every time I try to draw an area, it looks like I have to have some edges. And then you might remember, what if that two-dimensional space is curved? What happens? And I think the easiest example of that is the surface of a sphere. The surface of a sphere, let me draw a sphere over here. So this right here is a sphere."}, {"video_title": "Big bang introduction Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "What happens? And I think the easiest example of that is the surface of a sphere. The surface of a sphere, let me draw a sphere over here. So this right here is a sphere. Let me draw some longitude and latitude and lines on this sphere. On this sphere, all of a sudden, and I'll shade it in a little bit, make it look nice. This type of a sphere, you have a finite area."}, {"video_title": "Big bang introduction Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So this right here is a sphere. Let me draw some longitude and latitude and lines on this sphere. On this sphere, all of a sudden, and I'll shade it in a little bit, make it look nice. This type of a sphere, you have a finite area. You could imagine the surface of a balloon or the surface of a bubble or the surface of the Earth. You have a finite area, but you have no edge. If you keep going forever in one direction, you're just going to go all the way around and come back to the other side."}, {"video_title": "Big bang introduction Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "This type of a sphere, you have a finite area. You could imagine the surface of a balloon or the surface of a bubble or the surface of the Earth. You have a finite area, but you have no edge. If you keep going forever in one direction, you're just going to go all the way around and come back to the other side. Now, to imagine a three-dimensional space that has these same properties, a finite area and, and I don't want to say finite area anymore because we're not talking about a three-dimensional space. Let me draw it over here. So let's think about a three-dimensional space."}, {"video_title": "Big bang introduction Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "If you keep going forever in one direction, you're just going to go all the way around and come back to the other side. Now, to imagine a three-dimensional space that has these same properties, a finite area and, and I don't want to say finite area anymore because we're not talking about a three-dimensional space. Let me draw it over here. So let's think about a three-dimensional space. So three-dimensional space. Instead of area, since we're in three dimensions now, I want to talk about a finite volume, finite volume, and no edge. How do I do that?"}, {"video_title": "Big bang introduction Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So let's think about a three-dimensional space. So three-dimensional space. Instead of area, since we're in three dimensions now, I want to talk about a finite volume, finite volume, and no edge. How do I do that? When you think about it superficially, well, look, if I have a finite volume, it's going to be in, maybe it'll be contained in some type of a cube, and then we clearly have edges in those situations, or you could even think about a finite volume as being the inside of a sphere, and that clearly has an edge, this entire surface over there. So how do you construct a three-dimensional space that has finite volume and no edge? And that, I'm going to tell you right now, it's very hard for us to visualize it, but in order to visualize it, I'm essentially going to draw the same thing as I drew right here."}, {"video_title": "Big bang introduction Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "How do I do that? When you think about it superficially, well, look, if I have a finite volume, it's going to be in, maybe it'll be contained in some type of a cube, and then we clearly have edges in those situations, or you could even think about a finite volume as being the inside of a sphere, and that clearly has an edge, this entire surface over there. So how do you construct a three-dimensional space that has finite volume and no edge? And that, I'm going to tell you right now, it's very hard for us to visualize it, but in order to visualize it, I'm essentially going to draw the same thing as I drew right here. What you have to imagine, and you almost have to imagine it by analogy unless you have some type of a profound brain wired for more than three spatial dimensions, is a sphere, but, so let me make it clear. This is a two-dimensional surface, right, on the surface of the sphere, you can only move in two directions, two perpendicular directions. You can move like that, or you can move like that."}, {"video_title": "Big bang introduction Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And that, I'm going to tell you right now, it's very hard for us to visualize it, but in order to visualize it, I'm essentially going to draw the same thing as I drew right here. What you have to imagine, and you almost have to imagine it by analogy unless you have some type of a profound brain wired for more than three spatial dimensions, is a sphere, but, so let me make it clear. This is a two-dimensional surface, right, on the surface of the sphere, you can only move in two directions, two perpendicular directions. You can move like that, or you can move like that. You can move left and right, or you can move up and down. So it's a two-dimensional surface of a three-dimensional sphere. So if we take it by analogy, let's imagine, and it's hard to imagine, a three-dimensional surface, and you can do it mathematically."}, {"video_title": "Big bang introduction Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "You can move like that, or you can move like that. You can move left and right, or you can move up and down. So it's a two-dimensional surface of a three-dimensional sphere. So if we take it by analogy, let's imagine, and it's hard to imagine, a three-dimensional surface, and you can do it mathematically. The math here is actually not that difficult. It's a three-dimensional surface of a four-dimensional sphere. And I'm going to draw it the same way, so if we kind of view those three dimensions as just these two dimensions of the surface, the same thing."}, {"video_title": "Big bang introduction Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So if we take it by analogy, let's imagine, and it's hard to imagine, a three-dimensional surface, and you can do it mathematically. The math here is actually not that difficult. It's a three-dimensional surface of a four-dimensional sphere. And I'm going to draw it the same way, so if we kind of view those three dimensions as just these two dimensions of the surface, the same thing. It's the same thing. And if you imagine that, and I'm not saying that this is actually the shape of the universe. We don't know the actual shape, but we do know that it does have a slight curvature."}, {"video_title": "Big bang introduction Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And I'm going to draw it the same way, so if we kind of view those three dimensions as just these two dimensions of the surface, the same thing. It's the same thing. And if you imagine that, and I'm not saying that this is actually the shape of the universe. We don't know the actual shape, but we do know that it does have a slight curvature. We don't know the actual shape, but a sphere is the simplest. There's other ones we could do. A toroid would also fit the bill of having a finite volume with no edge."}, {"video_title": "Big bang introduction Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "We don't know the actual shape, but we do know that it does have a slight curvature. We don't know the actual shape, but a sphere is the simplest. There's other ones we could do. A toroid would also fit the bill of having a finite volume with no edge. And another thing, I want to make it clear. We actually don't even know whether it has just a finite volume. That's still an open question, but what I want to do is show you that it can have a finite volume and also have no edge."}, {"video_title": "Big bang introduction Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "A toroid would also fit the bill of having a finite volume with no edge. And another thing, I want to make it clear. We actually don't even know whether it has just a finite volume. That's still an open question, but what I want to do is show you that it can have a finite volume and also have no edge. And most people believe, and I want to say believe here because we can just go based on evidence and all of that, that we are talking about something with a finite volume, especially when you talk about the Big Bang Theory. That kind of, on some dimension, implies a finite volume, although it could be a super large, unfathomably large volume, it is finite. Now, if you have this, let's imagine this sphere."}, {"video_title": "Big bang introduction Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "That's still an open question, but what I want to do is show you that it can have a finite volume and also have no edge. And most people believe, and I want to say believe here because we can just go based on evidence and all of that, that we are talking about something with a finite volume, especially when you talk about the Big Bang Theory. That kind of, on some dimension, implies a finite volume, although it could be a super large, unfathomably large volume, it is finite. Now, if you have this, let's imagine this sphere. Once again, if you're on this surface of this four-dimensional sphere, I obviously cannot draw a four-dimensional sphere, but if you're on the surface of this four-dimensional sphere, if you go in any direction, you will come back out and come back to where you started. If you go that way, you'll come back around here. Now, the universe is super huge, so even light, maybe light itself would take an unbelievable amount of time to traverse it."}, {"video_title": "Big bang introduction Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Now, if you have this, let's imagine this sphere. Once again, if you're on this surface of this four-dimensional sphere, I obviously cannot draw a four-dimensional sphere, but if you're on the surface of this four-dimensional sphere, if you go in any direction, you will come back out and come back to where you started. If you go that way, you'll come back around here. Now, the universe is super huge, so even light, maybe light itself would take an unbelievable amount of time to traverse it. And if this sphere itself is expanding, it might be expanding so fast that light might not ever be able to come back around it, but in theory, if something were fast enough, if something were to keep going around, it could eventually go back to this point. Now, when we talk about a three-dimensional surface, it's a three-dimensional surface of a four-dimensional sphere, that means that any of the three dimensions, over here on the surface, I can only draw two, but that means if this is true, if the universe is a three-dimensional surface of a four-dimensional sphere, that means that if you go up and you just keep going up, you'll eventually come back from the bottom. So if you keep going all the way up, you'll eventually come back to the point that you were."}, {"video_title": "Big bang introduction Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Now, the universe is super huge, so even light, maybe light itself would take an unbelievable amount of time to traverse it. And if this sphere itself is expanding, it might be expanding so fast that light might not ever be able to come back around it, but in theory, if something were fast enough, if something were to keep going around, it could eventually go back to this point. Now, when we talk about a three-dimensional surface, it's a three-dimensional surface of a four-dimensional sphere, that means that any of the three dimensions, over here on the surface, I can only draw two, but that means if this is true, if the universe is a three-dimensional surface of a four-dimensional sphere, that means that if you go up and you just keep going up, you'll eventually come back from the bottom. So if you keep going all the way up, you'll eventually come back to the point that you were. It might be an unbelievably large distance, but you'll eventually get back where you were. If you go to the right, you'll eventually come back all the way around to the point where you were, and if you were to go into the page, so if you were to go into the page, let me draw it that way, if you were to go into the page, you would eventually come back from above the page and come back to the point that you are. So that's what this implication would be, that you would eventually get back to where you are."}, {"video_title": "Big bang introduction Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So if you keep going all the way up, you'll eventually come back to the point that you were. It might be an unbelievably large distance, but you'll eventually get back where you were. If you go to the right, you'll eventually come back all the way around to the point where you were, and if you were to go into the page, so if you were to go into the page, let me draw it that way, if you were to go into the page, you would eventually come back from above the page and come back to the point that you are. So that's what this implication would be, that you would eventually get back to where you are. So let's go back to the question of an expanding universe. An expanding universe that's not expanding into any other space, that is all of the space, but it's still expanding. Well, this is the model."}, {"video_title": "Big bang introduction Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So that's what this implication would be, that you would eventually get back to where you are. So let's go back to the question of an expanding universe. An expanding universe that's not expanding into any other space, that is all of the space, but it's still expanding. Well, this is the model. So you could imagine, shortly after the Big Bang, our four-dimensional sphere looked like this. Maybe it was a little small four-dimensional sphere. At some, you know, maybe, you know, right at the Big Bang, it was like this little unbelievably small sphere, then a little bit later, it's this larger sphere."}, {"video_title": "Big bang introduction Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Well, this is the model. So you could imagine, shortly after the Big Bang, our four-dimensional sphere looked like this. Maybe it was a little small four-dimensional sphere. At some, you know, maybe, you know, right at the Big Bang, it was like this little unbelievably small sphere, then a little bit later, it's this larger sphere. Let me just shade it in to show you that it has three, that it has, that it's kind of popping out of the page, that it's a sphere. And then at a later time, the sphere might look like this. The sphere might look like this."}, {"video_title": "Big bang introduction Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "At some, you know, maybe, you know, right at the Big Bang, it was like this little unbelievably small sphere, then a little bit later, it's this larger sphere. Let me just shade it in to show you that it has three, that it has, that it's kind of popping out of the page, that it's a sphere. And then at a later time, the sphere might look like this. The sphere might look like this. Now, your temptation might be to say, wait, Sal, isn't this stuff outside of the sphere? Isn't that some type of a space that it's expanding into? Isn't that somehow part of the universe?"}, {"video_title": "Big bang introduction Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "The sphere might look like this. Now, your temptation might be to say, wait, Sal, isn't this stuff outside of the sphere? Isn't that some type of a space that it's expanding into? Isn't that somehow part of the universe? And I would say, if you're talking in three dimensions, no, it's not. The entire universe is this surface. It is this surface of this four-dimensional sphere."}, {"video_title": "Big bang introduction Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Isn't that somehow part of the universe? And I would say, if you're talking in three dimensions, no, it's not. The entire universe is this surface. It is this surface of this four-dimensional sphere. If you start talking about more dimensions, then yes, you could talk about maybe things outside of our three-dimensional universe. So as this expands in space-time, so you can, you know, one way to view the fourth dimension is it is time itself, things are just getting further and further apart. And I'll talk about more evidence in future videos for why we, why the Big Bang is the best theory we have out there right now."}, {"video_title": "Big bang introduction Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It is this surface of this four-dimensional sphere. If you start talking about more dimensions, then yes, you could talk about maybe things outside of our three-dimensional universe. So as this expands in space-time, so you can, you know, one way to view the fourth dimension is it is time itself, things are just getting further and further apart. And I'll talk about more evidence in future videos for why we, why the Big Bang is the best theory we have out there right now. But as you can imagine, if we have two points on this sphere that are that far apart, as this sphere expands, this four-dimensional sphere, as this bubble blows up or this balloon blows up, those two points are just, let me draw three points. Let's say those are three points. Those three points are just going to get further and further apart."}, {"video_title": "Big bang introduction Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And I'll talk about more evidence in future videos for why we, why the Big Bang is the best theory we have out there right now. But as you can imagine, if we have two points on this sphere that are that far apart, as this sphere expands, this four-dimensional sphere, as this bubble blows up or this balloon blows up, those two points are just, let me draw three points. Let's say those are three points. Those three points are just going to get further and further apart. And that's actually one of the main points that, or one of the first reasons why it made sense to believe in the Big Bang is that everything is expanding not from some central point, but everything is expanding from everything. That if you go in any direction from any point in the universe, everything else is expanding away. And the further away you go, it looks like the faster it's expanding away from you."}, {"video_title": "Big bang introduction Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Those three points are just going to get further and further apart. And that's actually one of the main points that, or one of the first reasons why it made sense to believe in the Big Bang is that everything is expanding not from some central point, but everything is expanding from everything. That if you go in any direction from any point in the universe, everything else is expanding away. And the further away you go, it looks like the faster it's expanding away from you. So I'll leave you there. Something for you to kind of think about a little bit. And then we'll build on some of this to think about what it means to kind of observe the observable universe."}, {"video_title": "Random predictions for 2060 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "That's obviously what I'm working on now, so I have some opinions about that. But what I want to do in this video is do maybe slightly more broad and wild predictions. And my one prediction I'll make is I'm probably completely not going to predict the real reality of 2060. And probably the really big things to predict I will completely miss. But with that out of the way, it is fun to predict things, so let's give a shot. So the first area that I will predict is what's going to happen in the field of medicine. In particular, I think that the human lifespan is going to increase dramatically."}, {"video_title": "Random predictions for 2060 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And probably the really big things to predict I will completely miss. But with that out of the way, it is fun to predict things, so let's give a shot. So the first area that I will predict is what's going to happen in the field of medicine. In particular, I think that the human lifespan is going to increase dramatically. So I'll be conservative and say that the lifespan, the average human lifespan, is going to be, especially in the developed world, and once again, hopefully by 2060, most of the world is developed, the average human lifespan is going to be over 100 years old. And I won't say whether that's going to be a good or a bad thing. There's arguments either way about the proper way to live and what happens to global populations."}, {"video_title": "Random predictions for 2060 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "In particular, I think that the human lifespan is going to increase dramatically. So I'll be conservative and say that the lifespan, the average human lifespan, is going to be, especially in the developed world, and once again, hopefully by 2060, most of the world is developed, the average human lifespan is going to be over 100 years old. And I won't say whether that's going to be a good or a bad thing. There's arguments either way about the proper way to live and what happens to global populations. But if the world is, for the most part, developed and educated, you actually will probably have a lower rate of reproduction, and so you might actually have some space in the world for older people. But this, I think, is, there's a very strong chance of this happening because we are starting to understand the molecular basis of aging. It's not a given thing that because of some form of wear and tear, things have to die after 70 years or 80 years or 90 years, and we're starting to understand the mechanisms and how to maybe improve the repair mechanisms or how to augment it in some way."}, {"video_title": "Random predictions for 2060 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "There's arguments either way about the proper way to live and what happens to global populations. But if the world is, for the most part, developed and educated, you actually will probably have a lower rate of reproduction, and so you might actually have some space in the world for older people. But this, I think, is, there's a very strong chance of this happening because we are starting to understand the molecular basis of aging. It's not a given thing that because of some form of wear and tear, things have to die after 70 years or 80 years or 90 years, and we're starting to understand the mechanisms and how to maybe improve the repair mechanisms or how to augment it in some way. So I definitely think this is going to happen. I don't know whether it's going to be a positive or negative, but it's likely to happen. The next thing, and this is kind of closely related, this is still biological, is you're going to have a close integration between the digital and the biological."}, {"video_title": "Random predictions for 2060 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It's not a given thing that because of some form of wear and tear, things have to die after 70 years or 80 years or 90 years, and we're starting to understand the mechanisms and how to maybe improve the repair mechanisms or how to augment it in some way. So I definitely think this is going to happen. I don't know whether it's going to be a positive or negative, but it's likely to happen. The next thing, and this is kind of closely related, this is still biological, is you're going to have a close integration between the digital and the biological. So digital and biological. And once again, I won't make any statement of whether this is a good or a bad thing, but it seems like it's just an extrapolation of the direction we're already going in, digital and biological integration. So already, you're getting more and more in your handheld devices."}, {"video_title": "Random predictions for 2060 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "The next thing, and this is kind of closely related, this is still biological, is you're going to have a close integration between the digital and the biological. So digital and biological. And once again, I won't make any statement of whether this is a good or a bad thing, but it seems like it's just an extrapolation of the direction we're already going in, digital and biological integration. So already, you're getting more and more in your handheld devices. Imagine when your screen is now no longer in your palm, but it's being projected onto your retina from some little thing. And then eventually you have a direct connection with your retina, and your brain can directly access areas of memory through some biological and digital interface. So I definitely think this is going to happen."}, {"video_title": "Random predictions for 2060 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So already, you're getting more and more in your handheld devices. Imagine when your screen is now no longer in your palm, but it's being projected onto your retina from some little thing. And then eventually you have a direct connection with your retina, and your brain can directly access areas of memory through some biological and digital interface. So I definitely think this is going to happen. This is a big deal, because this is starting to, and I think it's already happening with a lot of what you see around technology, it's really going to reshape what the individual human experience is going to be. We already see people kind of living in virtual realities and playing these immersive games and spending all of their time on social networking platforms. What happens when they're literally plugged in all the time, when almost their cell phone is in the brain?"}, {"video_title": "Random predictions for 2060 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So I definitely think this is going to happen. This is a big deal, because this is starting to, and I think it's already happening with a lot of what you see around technology, it's really going to reshape what the individual human experience is going to be. We already see people kind of living in virtual realities and playing these immersive games and spending all of their time on social networking platforms. What happens when they're literally plugged in all the time, when almost their cell phone is in the brain? I don't know. I'm not going to comment whether it's a good or a bad thing, but it does look like a trend that's going to keep on going through the next 50 years. Now, if we take that even further, we're talking about a digital and biological integration, but if you go at the extreme, and actually it's probably both of these top two things combined in some way, is that we are learning, and once again, not making a comment whether it's good or bad, it's just an extrapolation of what we're already seeing, we are seeing more and more ability to understand our genome, to molecularly target things, to manipulate actual biology."}, {"video_title": "Random predictions for 2060 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "What happens when they're literally plugged in all the time, when almost their cell phone is in the brain? I don't know. I'm not going to comment whether it's a good or a bad thing, but it does look like a trend that's going to keep on going through the next 50 years. Now, if we take that even further, we're talking about a digital and biological integration, but if you go at the extreme, and actually it's probably both of these top two things combined in some way, is that we are learning, and once again, not making a comment whether it's good or bad, it's just an extrapolation of what we're already seeing, we are seeing more and more ability to understand our genome, to molecularly target things, to manipulate actual biology. And so what you have is that you can actually have, let me write this, manipulation of biology. And why this is on some level creepy, it could be creepy or it could be exciting, depending on how it all plays out, but it could do things like augment intelligence, augment human intelligence, which would, if you think about all the progress of society and all the things that are already accelerating, imagine how society will change if intelligence itself is augmented. As someone with a limited intelligence, I can't even imagine what will happen as soon as you do this."}, {"video_title": "Random predictions for 2060 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Now, if we take that even further, we're talking about a digital and biological integration, but if you go at the extreme, and actually it's probably both of these top two things combined in some way, is that we are learning, and once again, not making a comment whether it's good or bad, it's just an extrapolation of what we're already seeing, we are seeing more and more ability to understand our genome, to molecularly target things, to manipulate actual biology. And so what you have is that you can actually have, let me write this, manipulation of biology. And why this is on some level creepy, it could be creepy or it could be exciting, depending on how it all plays out, but it could do things like augment intelligence, augment human intelligence, which would, if you think about all the progress of society and all the things that are already accelerating, imagine how society will change if intelligence itself is augmented. As someone with a limited intelligence, I can't even imagine what will happen as soon as you do this. And obviously, the more you augment intelligence, the more that you can learn how to augment intelligence and increase lifespan and do digital and biological integration even more. So these things I see as some form of a trend. We'll see how it all plays out, and hopefully it plays out in the feel-good care bear version versus some type of crazy society and we all turn into the Borg in some way."}, {"video_title": "Random predictions for 2060 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "As someone with a limited intelligence, I can't even imagine what will happen as soon as you do this. And obviously, the more you augment intelligence, the more that you can learn how to augment intelligence and increase lifespan and do digital and biological integration even more. So these things I see as some form of a trend. We'll see how it all plays out, and hopefully it plays out in the feel-good care bear version versus some type of crazy society and we all turn into the Borg in some way. Now the other trend, and these are the things that just popped into my brain today when I pressed record, but the other trend that I think is interesting is some of the things that we've taken for granted in terms of how nations interact with each other, and nation states in particular interact with each other. And just as a kind of an overview, so there's a notion called a nation state. And in everyday language, the word nation and the word state almost means the same thing, but they kind of mean different things if you want to be a little bit more formal about it."}, {"video_title": "Random predictions for 2060 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "We'll see how it all plays out, and hopefully it plays out in the feel-good care bear version versus some type of crazy society and we all turn into the Borg in some way. Now the other trend, and these are the things that just popped into my brain today when I pressed record, but the other trend that I think is interesting is some of the things that we've taken for granted in terms of how nations interact with each other, and nation states in particular interact with each other. And just as a kind of an overview, so there's a notion called a nation state. And in everyday language, the word nation and the word state almost means the same thing, but they kind of mean different things if you want to be a little bit more formal about it. That's why people use the word nation state. That's like, to a lot of people, it doesn't mean saying like state, state. The difference between a nation and a state is these are a group of people that feel some type of common identity."}, {"video_title": "Random predictions for 2060 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And in everyday language, the word nation and the word state almost means the same thing, but they kind of mean different things if you want to be a little bit more formal about it. That's why people use the word nation state. That's like, to a lot of people, it doesn't mean saying like state, state. The difference between a nation and a state is these are a group of people that feel some type of common identity. It could be a language, it could be a culture, it could be a value system. So this is some kind of common identity. And it often is somehow associated with geography, but it does not have to be associated with geography."}, {"video_title": "Random predictions for 2060 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "The difference between a nation and a state is these are a group of people that feel some type of common identity. It could be a language, it could be a culture, it could be a value system. So this is some kind of common identity. And it often is somehow associated with geography, but it does not have to be associated with geography. It could even be a religion or it could be whatever else. That's what a nation is. A state is a formal governance structure that makes the laws and has the institutions to make society function."}, {"video_title": "Random predictions for 2060 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And it often is somehow associated with geography, but it does not have to be associated with geography. It could even be a religion or it could be whatever else. That's what a nation is. A state is a formal governance structure that makes the laws and has the institutions to make society function. Now, a nation state is what most of us live in today because it both has an identity and some type of formal institutions. So a very pure nation state would be someplace like Japan where there's relatively uniform in terms of ethnicity and religion and in terms of culture. And you have that same group of people are governing themselves."}, {"video_title": "Random predictions for 2060 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "A state is a formal governance structure that makes the laws and has the institutions to make society function. Now, a nation state is what most of us live in today because it both has an identity and some type of formal institutions. So a very pure nation state would be someplace like Japan where there's relatively uniform in terms of ethnicity and religion and in terms of culture. And you have that same group of people are governing themselves. In a place like the United States, ethnicity, religion, that's diverse, but what gives identity is a notion of shared values and a notion of maybe a common history or whatever else or a certain kind of world view. And obviously there's a formal state structure. Now, what I think is going to be interesting here, and I actually have no idea how all of this is going to play out, but when you see things like some of the revolutions in the Middle East due to things like people being able to communicate irrespective of the traditional media, I think there's going to be some interesting questions on what happens to the nation state, especially nation states that are able to secure their power by kind of having a bottleneck on access to information."}, {"video_title": "Random predictions for 2060 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And you have that same group of people are governing themselves. In a place like the United States, ethnicity, religion, that's diverse, but what gives identity is a notion of shared values and a notion of maybe a common history or whatever else or a certain kind of world view. And obviously there's a formal state structure. Now, what I think is going to be interesting here, and I actually have no idea how all of this is going to play out, but when you see things like some of the revolutions in the Middle East due to things like people being able to communicate irrespective of the traditional media, I think there's going to be some interesting questions on what happens to the nation state, especially nation states that are able to secure their power by kind of having a bottleneck on access to information. And all of that is, I think, going to change in a very dramatic way as you have more and more integration between people, cross-border communications, when people realize that the people on the other side of the border really aren't that different than themselves. And another interesting thing, even the notion of democracy, and once again, I don't know how it's going to play out, but all notions of representational democracy that we have today are somewhat based on geography. They're somewhat based on geography, and that's because when the major representational democracies came about, that was the best way to represent each other."}, {"video_title": "Random predictions for 2060 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Now, what I think is going to be interesting here, and I actually have no idea how all of this is going to play out, but when you see things like some of the revolutions in the Middle East due to things like people being able to communicate irrespective of the traditional media, I think there's going to be some interesting questions on what happens to the nation state, especially nation states that are able to secure their power by kind of having a bottleneck on access to information. And all of that is, I think, going to change in a very dramatic way as you have more and more integration between people, cross-border communications, when people realize that the people on the other side of the border really aren't that different than themselves. And another interesting thing, even the notion of democracy, and once again, I don't know how it's going to play out, but all notions of representational democracy that we have today are somewhat based on geography. They're somewhat based on geography, and that's because when the major representational democracies came about, that was the best way to represent each other. Hey, let me pick some representatives from our county or from our region, and they'll go elect other representatives, and they'll go to the national government. But now that we have this instantaneous communication with people, you might be able to have different types of a representational democracy, or maybe you could even have more direct democracies. Who knows?"}, {"video_title": "Random predictions for 2060 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "They're somewhat based on geography, and that's because when the major representational democracies came about, that was the best way to represent each other. Hey, let me pick some representatives from our county or from our region, and they'll go elect other representatives, and they'll go to the national government. But now that we have this instantaneous communication with people, you might be able to have different types of a representational democracy, or maybe you could even have more direct democracies. Who knows? Because of things like communication and technology and whatever else. And then the other way that I think nation-states, or the way they fundamentally interact is going to change is actually in things like warfare. And once again, already seeing this trend."}, {"video_title": "Random predictions for 2060 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Who knows? Because of things like communication and technology and whatever else. And then the other way that I think nation-states, or the way they fundamentally interact is going to change is actually in things like warfare. And once again, already seeing this trend. In particular, I think developed countries are not going to have humans on the front line. No humans on the front line. And depending on your point of view, this could be a very good thing, or it could be a kind of a scary thing."}, {"video_title": "Random predictions for 2060 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And once again, already seeing this trend. In particular, I think developed countries are not going to have humans on the front line. No humans on the front line. And depending on your point of view, this could be a very good thing, or it could be a kind of a scary thing. Because if you have no humans on the front line, and you're already seeing things like this with drones, predator drones, and you see these kind of robot bomb detectors and things like that. And it doesn't even have to be these big things. There's already a DARPA-funded project to work on miniature insects that could be used as some form of reconnaissance."}, {"video_title": "Random predictions for 2060 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And depending on your point of view, this could be a very good thing, or it could be a kind of a scary thing. Because if you have no humans on the front line, and you're already seeing things like this with drones, predator drones, and you see these kind of robot bomb detectors and things like that. And it doesn't even have to be these big things. There's already a DARPA-funded project to work on miniature insects that could be used as some form of reconnaissance. Or you could imagine eventually they could have these little things on them that could knock someone out, or who knows what they do. The exciting thing is that all of a sudden a human won't be there to get shot, and so hopefully military casualties would go down. The scary thing here is if you don't have humans on the front line, nations might be willing to enter into war, especially developed nations."}, {"video_title": "Random predictions for 2060 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "There's already a DARPA-funded project to work on miniature insects that could be used as some form of reconnaissance. Or you could imagine eventually they could have these little things on them that could knock someone out, or who knows what they do. The exciting thing is that all of a sudden a human won't be there to get shot, and so hopefully military casualties would go down. The scary thing here is if you don't have humans on the front line, nations might be willing to enter into war, especially developed nations. They might not take it as seriously, and so it might be something that they do a little bit more when they're in the mood. And it would actually create a huge disparity between developed and developing nations when this happens. And you already see that to a certain degree."}, {"video_title": "Random predictions for 2060 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "The scary thing here is if you don't have humans on the front line, nations might be willing to enter into war, especially developed nations. They might not take it as seriously, and so it might be something that they do a little bit more when they're in the mood. And it would actually create a huge disparity between developed and developing nations when this happens. And you already see that to a certain degree. A developed nation, they don't have to put as many humans in harm's way, and it's purely driven by their wealth to have capital that can go, these robots and these drones, and their technological innovation, while in the developing countries they actually would have to use human beings. So their costs would be much, much, much, much higher. So it's an interesting question, and once again, who knows how this plays out, whether it's a good or bad thing in the long term."}, {"video_title": "Random predictions for 2060 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And you already see that to a certain degree. A developed nation, they don't have to put as many humans in harm's way, and it's purely driven by their wealth to have capital that can go, these robots and these drones, and their technological innovation, while in the developing countries they actually would have to use human beings. So their costs would be much, much, much, much higher. So it's an interesting question, and once again, who knows how this plays out, whether it's a good or bad thing in the long term. I think a similar thing with this is, I think you're going to see more and more non-lethal weapons, which once again, it's very similar to this. It sounds like it's a good thing. If there's a gun that instead of having to kill someone, it incapacitates them in some way, or it stuns them in some way, kind of the classic Star Trek, put your phasers on, stun things."}, {"video_title": "Random predictions for 2060 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So it's an interesting question, and once again, who knows how this plays out, whether it's a good or bad thing in the long term. I think a similar thing with this is, I think you're going to see more and more non-lethal weapons, which once again, it's very similar to this. It sounds like it's a good thing. If there's a gun that instead of having to kill someone, it incapacitates them in some way, or it stuns them in some way, kind of the classic Star Trek, put your phasers on, stun things. I guess the scarier version of non-lethal weapons is the threshold for using it becomes much lower. So if a government wants to subject its citizens or subject another group of citizens, it can literally just stun them, or it can make them pass out, or it can control them in some way. So this could be a little bit scarier."}, {"video_title": "Random predictions for 2060 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "If there's a gun that instead of having to kill someone, it incapacitates them in some way, or it stuns them in some way, kind of the classic Star Trek, put your phasers on, stun things. I guess the scarier version of non-lethal weapons is the threshold for using it becomes much lower. So if a government wants to subject its citizens or subject another group of citizens, it can literally just stun them, or it can make them pass out, or it can control them in some way. So this could be a little bit scarier. So who knows how all of this plays out. So those are my predictions or things to think about over the next 50 years. These are just the things that happened to jump into my brain today, probably based on some of the science fiction books I've been reading or whatever else."}, {"video_title": "Collective learning Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "There are many things that differentiate human beings from other species, but the one thing that probably differentiates humans even from our closest relatives in the animal kingdom in a really big way is the notion that humans are collective learners. Collective learners. And to understand that, let's think about how even our closest relative in the animal kingdom, the chimpanzee, might communicate. So you might have one chimpanzee, and over the course of his or her lifetime, they're able to learn a bunch of cool experiences, and they're even able to learn to use tools, manipulate tools, and who knows, maybe even make tools, maybe even get a twig someplace and take off the leaves and then use that to go get ants out of a hole or whatever else. So they're able to learn all of this stuff over a lifetime. Now, unfortunate for chimpanzees, well, what is fortunate for chimpanzees is they do teach some of these things that they've learned to other members of their group, often their offspring, but what's unfortunate for chimpanzees is they don't have a great way to communicate with each other. So for most chimpanzees, the way that they're able to teach is essentially by kind of showing, not showing and telling, just showing."}, {"video_title": "Collective learning Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So you might have one chimpanzee, and over the course of his or her lifetime, they're able to learn a bunch of cool experiences, and they're even able to learn to use tools, manipulate tools, and who knows, maybe even make tools, maybe even get a twig someplace and take off the leaves and then use that to go get ants out of a hole or whatever else. So they're able to learn all of this stuff over a lifetime. Now, unfortunate for chimpanzees, well, what is fortunate for chimpanzees is they do teach some of these things that they've learned to other members of their group, often their offspring, but what's unfortunate for chimpanzees is they don't have a great way to communicate with each other. So for most chimpanzees, the way that they're able to teach is essentially by kind of showing, not showing and telling, just showing. And so because this is such a unprecise or not exact and such an inefficient way of communication, they're really, all of the nuances of what this chimpanzee might be able to accumulate over his or her lifetime aren't able to be conveyed to the next generation or to the other chimpanzees around. So you have tremendous energy loss. And in particular, all that can be conveyed are maybe the specific movements or what you might be able to kind of observe in the present, all of the other things that maybe the chimpanzee is learning about the times of year where this is appropriate."}, {"video_title": "Collective learning Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So for most chimpanzees, the way that they're able to teach is essentially by kind of showing, not showing and telling, just showing. And so because this is such a unprecise or not exact and such an inefficient way of communication, they're really, all of the nuances of what this chimpanzee might be able to accumulate over his or her lifetime aren't able to be conveyed to the next generation or to the other chimpanzees around. So you have tremendous energy loss. And in particular, all that can be conveyed are maybe the specific movements or what you might be able to kind of observe in the present, all of the other things that maybe the chimpanzee is learning about the times of year where this is appropriate. And maybe they can convey some of that by showing them at the right times of year. But other nuanced aspects of it or particular ways to hold something or twist something can only be shown, it can't be described in a very precise way. So you have all of this loss of experience, this loss of information."}, {"video_title": "Collective learning Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And in particular, all that can be conveyed are maybe the specific movements or what you might be able to kind of observe in the present, all of the other things that maybe the chimpanzee is learning about the times of year where this is appropriate. And maybe they can convey some of that by showing them at the right times of year. But other nuanced aspects of it or particular ways to hold something or twist something can only be shown, it can't be described in a very precise way. So you have all of this loss of experience, this loss of information. And then over the course of these animals' life, they may be able to learn the same amount again. They're able to learn maybe the same amount again. But then when they need to communicate it, they have the exact same problem."}, {"video_title": "Collective learning Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So you have all of this loss of experience, this loss of information. And then over the course of these animals' life, they may be able to learn the same amount again. They're able to learn maybe the same amount again. But then when they need to communicate it, they have the exact same problem. It's hard to communicate it with what they have at their disposal, which is really just showing the other chimpanzees what they've done. And so once again, you have a loss of information. And what you have in this type of circumstances, generation after generation, even though there is learning over the course of an individual chimpanzee's life, and even though they can communicate to some of that to each other, that form of communication is so, it loses so much information and so much nuance that you never have an overall accumulation of knowledge and wisdom in this chimpanzee population."}, {"video_title": "Collective learning Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But then when they need to communicate it, they have the exact same problem. It's hard to communicate it with what they have at their disposal, which is really just showing the other chimpanzees what they've done. And so once again, you have a loss of information. And what you have in this type of circumstances, generation after generation, even though there is learning over the course of an individual chimpanzee's life, and even though they can communicate to some of that to each other, that form of communication is so, it loses so much information and so much nuance that you never have an overall accumulation of knowledge and wisdom in this chimpanzee population. Now, humans, on the other hand, have something called symbolic language. And I'll talk about this in a second. But for now, it's safe to say that human language is far more precise and far more efficient than just being able to show someone something."}, {"video_title": "Collective learning Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And what you have in this type of circumstances, generation after generation, even though there is learning over the course of an individual chimpanzee's life, and even though they can communicate to some of that to each other, that form of communication is so, it loses so much information and so much nuance that you never have an overall accumulation of knowledge and wisdom in this chimpanzee population. Now, humans, on the other hand, have something called symbolic language. And I'll talk about this in a second. But for now, it's safe to say that human language is far more precise and far more efficient than just being able to show someone something. Imagine if you had to learn how to do something without being able to communicate verbally, if you just had to look at someone else's actions. And then you'd have a good idea of how difficult it is for chimpanzees to teach each other. But in the case of human beings, we have this thing called symbolic language that's a very precise, a very efficient way of communicating."}, {"video_title": "Collective learning Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But for now, it's safe to say that human language is far more precise and far more efficient than just being able to show someone something. Imagine if you had to learn how to do something without being able to communicate verbally, if you just had to look at someone else's actions. And then you'd have a good idea of how difficult it is for chimpanzees to teach each other. But in the case of human beings, we have this thing called symbolic language that's a very precise, a very efficient way of communicating. So from one human being to another, you can actually communicate a good deal, maybe not every single nuance and every single experience, but a good chunk of it. So right here, I'm drawing about that much of it to the next, to some other human being. Maybe it is the offspring, maybe it is another member of the tribe or the group, whatever it is."}, {"video_title": "Collective learning Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But in the case of human beings, we have this thing called symbolic language that's a very precise, a very efficient way of communicating. So from one human being to another, you can actually communicate a good deal, maybe not every single nuance and every single experience, but a good chunk of it. So right here, I'm drawing about that much of it to the next, to some other human being. Maybe it is the offspring, maybe it is another member of the tribe or the group, whatever it is. And then this human being might come up with some other innovations. They're able to build off of all of this learning from that previous generation or from that other human being that's around, and they're able to come up with their own nuances and their own innovations. And this one right over here might come up with his or her own nuances and innovations."}, {"video_title": "Collective learning Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Maybe it is the offspring, maybe it is another member of the tribe or the group, whatever it is. And then this human being might come up with some other innovations. They're able to build off of all of this learning from that previous generation or from that other human being that's around, and they're able to come up with their own nuances and their own innovations. And this one right over here might come up with his or her own nuances and innovations. And because they have a good communication mechanism, this one could even communicate to that one what he's learned or what she's learned and communicate a good chunk of that. Maybe not all of it, but maybe a reasonable bit. They can describe exactly how they do something, the times of years, when it's good to do it, when it's not good to do it, how to plan for the future, what's the history of this new learning."}, {"video_title": "Collective learning Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And this one right over here might come up with his or her own nuances and innovations. And because they have a good communication mechanism, this one could even communicate to that one what he's learned or what she's learned and communicate a good chunk of that. Maybe not all of it, but maybe a reasonable bit. They can describe exactly how they do something, the times of years, when it's good to do it, when it's not good to do it, how to plan for the future, what's the history of this new learning. And so what you have going on here is because of this strong communication mechanism, so strong, precise, efficient communication, what you have is a human group, or eventually a human civilization, is able to have a collective memory. In the case of the chimpanzees, they're every generation, every chimpanzee is having to relearn the things that the other chimpanzees might have already done in previous generations. They're not able to really move forward or build on those in significant ways."}, {"video_title": "Collective learning Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "They can describe exactly how they do something, the times of years, when it's good to do it, when it's not good to do it, how to plan for the future, what's the history of this new learning. And so what you have going on here is because of this strong communication mechanism, so strong, precise, efficient communication, what you have is a human group, or eventually a human civilization, is able to have a collective memory. In the case of the chimpanzees, they're every generation, every chimpanzee is having to relearn the things that the other chimpanzees might have already done in previous generations. They're not able to really move forward or build on those in significant ways. In humans, as information is learned and experience gained, a good bit of that is able to be passed on to other humans. And so this might be passed on. So all of this might be passed on, or a good chunk of this could be passed on to the next generation."}, {"video_title": "Collective learning Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "They're not able to really move forward or build on those in significant ways. In humans, as information is learned and experience gained, a good bit of that is able to be passed on to other humans. And so this might be passed on. So all of this might be passed on, or a good chunk of this could be passed on to the next generation. And I'm not even talking about written language yet. This could still just be oral communication, which is still a very strong, precise, efficient means of communication. Written communication takes it to another level."}, {"video_title": "Collective learning Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So all of this might be passed on, or a good chunk of this could be passed on to the next generation. And I'm not even talking about written language yet. This could still just be oral communication, which is still a very strong, precise, efficient means of communication. Written communication takes it to another level. But then this person over here, maybe she comes up with other innovations. And at some point you might say, well look, if everyone keeps having innovations and they keep learning what everyone learned in previous generations, maybe this will tap out the total amount of memory that a human being even has. And there's actually a case that maybe this is one of the reasons why humans even have larger memories, because there is all of this collective knowledge to gain from one generation to another, from one human being to another."}, {"video_title": "Collective learning Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Written communication takes it to another level. But then this person over here, maybe she comes up with other innovations. And at some point you might say, well look, if everyone keeps having innovations and they keep learning what everyone learned in previous generations, maybe this will tap out the total amount of memory that a human being even has. And there's actually a case that maybe this is one of the reasons why humans even have larger memories, because there is all of this collective knowledge to gain from one generation to another, from one human being to another. But there are some limits to this. And this is the other element where this collective aspect of collective memory and collective learning becomes really powerful. A human being, because of the strong communication mechanism is not just limited to the knowledge and the experience in their memory."}, {"video_title": "Collective learning Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And there's actually a case that maybe this is one of the reasons why humans even have larger memories, because there is all of this collective knowledge to gain from one generation to another, from one human being to another. But there are some limits to this. And this is the other element where this collective aspect of collective memory and collective learning becomes really powerful. A human being, because of the strong communication mechanism is not just limited to the knowledge and the experience in their memory. They are able to tap into, so this human being right over here does not have this skill set. And that skill set maybe gets passed on to another human being. So let me copy and paste that."}, {"video_title": "Collective learning Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "A human being, because of the strong communication mechanism is not just limited to the knowledge and the experience in their memory. They are able to tap into, so this human being right over here does not have this skill set. And that skill set maybe gets passed on to another human being. So let me copy and paste that. So let's say you copy, let's say you paste that. This other human being that's maybe living at the same time, and when that becomes relevant, when that becomes relevant, they could actually tap into it. And maybe they could learn it from that human being, or maybe it's in a different part of society and this human being can build certain tools or build certain things using this information, using that knowledge right over there."}, {"video_title": "Collective learning Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So let me copy and paste that. So let's say you copy, let's say you paste that. This other human being that's maybe living at the same time, and when that becomes relevant, when that becomes relevant, they could actually tap into it. And maybe they could learn it from that human being, or maybe it's in a different part of society and this human being can build certain tools or build certain things using this information, using that knowledge right over there. And then this human being doesn't need to know that information. They can just leverage the output of that information to then build on top of it. So what it allows human beings to do is not only convey information and build on information from generation to generation, human to human, it allows all of the human brains collectively at any given point of time to be one collective memory bank that can be used to develop or innovate in specific domains and adapt to specific parts of the ecosystem or to teach each other."}, {"video_title": "Collective learning Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And maybe they could learn it from that human being, or maybe it's in a different part of society and this human being can build certain tools or build certain things using this information, using that knowledge right over there. And then this human being doesn't need to know that information. They can just leverage the output of that information to then build on top of it. So what it allows human beings to do is not only convey information and build on information from generation to generation, human to human, it allows all of the human brains collectively at any given point of time to be one collective memory bank that can be used to develop or innovate in specific domains and adapt to specific parts of the ecosystem or to teach each other. So all of a sudden, this is really unique as far as we can tell in the animal kingdom. All of a sudden, it's not all about the brain or the memory of one individual member of a species. It now becomes about the brain or the memory of the entire civilization or the entire group of the species."}, {"video_title": "Collective learning Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So what it allows human beings to do is not only convey information and build on information from generation to generation, human to human, it allows all of the human brains collectively at any given point of time to be one collective memory bank that can be used to develop or innovate in specific domains and adapt to specific parts of the ecosystem or to teach each other. So all of a sudden, this is really unique as far as we can tell in the animal kingdom. All of a sudden, it's not all about the brain or the memory of one individual member of a species. It now becomes about the brain or the memory of the entire civilization or the entire group of the species. And just as an example of that, there's, as far as I know, there's no human being who knows how to do everything that all human beings know how to do. I could imagine that there is a chimpanzee that knows how to do everything that any other chimpanzee knows how to do. There are no humans that can be a fighter pilot, a doctor, a gymnast, a lawyer, understands philosophy, speaks 20 different languages."}, {"video_title": "Collective learning Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It now becomes about the brain or the memory of the entire civilization or the entire group of the species. And just as an example of that, there's, as far as I know, there's no human being who knows how to do everything that all human beings know how to do. I could imagine that there is a chimpanzee that knows how to do everything that any other chimpanzee knows how to do. There are no humans that can be a fighter pilot, a doctor, a gymnast, a lawyer, understands philosophy, speaks 20 different languages. As far as I know, that human being does not exist. And that's okay because they can tap into the experiences, the abilities of other human beings to build up their civilization. None of us, as far as we know, knows how to do everything that we need to actually build our civilization."}, {"video_title": "Collective learning Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "There are no humans that can be a fighter pilot, a doctor, a gymnast, a lawyer, understands philosophy, speaks 20 different languages. As far as I know, that human being does not exist. And that's okay because they can tap into the experiences, the abilities of other human beings to build up their civilization. None of us, as far as we know, knows how to do everything that we need to actually build our civilization. But the information is in our collective memory to actually do it. Now, the next thing you might say is, okay, I started with this premise that we have a strong, precise, efficient means of communication and that other animals don't. But don't other animals actually have some form of language?"}, {"video_title": "Collective learning Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "None of us, as far as we know, knows how to do everything that we need to actually build our civilization. But the information is in our collective memory to actually do it. Now, the next thing you might say is, okay, I started with this premise that we have a strong, precise, efficient means of communication and that other animals don't. But don't other animals actually have some form of language? So for example, don't some, you know, for example, even monkeys, when they screech, when they are in danger, that's a form of communication, maybe a form of language. Maybe certain animals, birds, monkeys, maybe they have a song that they sing that can convey certain things. Maybe it's when they are looking for a mate."}, {"video_title": "Collective learning Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But don't other animals actually have some form of language? So for example, don't some, you know, for example, even monkeys, when they screech, when they are in danger, that's a form of communication, maybe a form of language. Maybe certain animals, birds, monkeys, maybe they have a song that they sing that can convey certain things. Maybe it's when they are looking for a mate. Isn't that a form of communication? And these are, these are a form of communication and a form of language. But these don't really come in play in terms of the teaching learning."}, {"video_title": "Collective learning Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Maybe it's when they are looking for a mate. Isn't that a form of communication? And these are, these are a form of communication and a form of language. But these don't really come in play in terms of the teaching learning. You don't see one chimpanzee making screeching sounds or learning sounds. They might do a little bit just to warn, maybe as a warning, but there's no deep nuance or deep precision that's being able to convey by these one-off sounds or even one-off gestures. And what's particularly powerful about human language is that it is a symbolic language."}, {"video_title": "Collective learning Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But these don't really come in play in terms of the teaching learning. You don't see one chimpanzee making screeching sounds or learning sounds. They might do a little bit just to warn, maybe as a warning, but there's no deep nuance or deep precision that's being able to convey by these one-off sounds or even one-off gestures. And what's particularly powerful about human language is that it is a symbolic language. It is a symbolic language. And when I say it's a symbolic language, I'm even saying it in a broader sense than even just written symbols. I'm talking about even the sounds themselves."}, {"video_title": "Collective learning Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And what's particularly powerful about human language is that it is a symbolic language. It is a symbolic language. And when I say it's a symbolic language, I'm even saying it in a broader sense than even just written symbols. I'm talking about even the sounds themselves. So let's go to a time where we did not even have writings. And when we talk about symbolic languages, let's think about a non-symbolic language. So in a non-symbolic language, you might have some sound, let's call it sound one, and it has some meaning."}, {"video_title": "Collective learning Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "I'm talking about even the sounds themselves. So let's go to a time where we did not even have writings. And when we talk about symbolic languages, let's think about a non-symbolic language. So in a non-symbolic language, you might have some sound, let's call it sound one, and it has some meaning. Let's call it meaning one. Meaning one. So this might be a certain type of scream."}, {"video_title": "Collective learning Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So in a non-symbolic language, you might have some sound, let's call it sound one, and it has some meaning. Let's call it meaning one. Meaning one. So this might be a certain type of scream. It means that a predator is approaching. Then you might have something like a sound two or gesture two, and then it has some other meaning. It has meaning two."}, {"video_title": "Collective learning Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So this might be a certain type of scream. It means that a predator is approaching. Then you might have something like a sound two or gesture two, and then it has some other meaning. It has meaning two. It might be a certain type of song, which means that I am in the mood to reproduce or whatever else. You might have gesture three. Gesture three."}, {"video_title": "Collective learning Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It has meaning two. It might be a certain type of song, which means that I am in the mood to reproduce or whatever else. You might have gesture three. Gesture three. That has some direct meaning. It might mean that I have found food or something like that. So meaning three."}, {"video_title": "Collective learning Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Gesture three. That has some direct meaning. It might mean that I have found food or something like that. So meaning three. What humans have, they can do this, where particular sounds have particular meanings. So for example, in humans, you could have sound one. It refers to meaning one."}, {"video_title": "Collective learning Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So meaning three. What humans have, they can do this, where particular sounds have particular meanings. So for example, in humans, you could have sound one. It refers to meaning one. I'll just refer it to meaning one. You could have sound two that refers to meaning two. You could have sound three that refers to meaning three."}, {"video_title": "Collective learning Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It refers to meaning one. I'll just refer it to meaning one. You could have sound two that refers to meaning two. You could have sound three that refers to meaning three. So these are just direct representations, but what is really powerful about symbolic languages is that these oral symbols can be combined according to set rules or grammars to have an infinite number of meanings. So this is what really makes human language transcend other languages and really makes it this robust, precise communication mechanism is you could have combinations. Sound one, sound two, sound three will now have another meaning, meaning four."}, {"video_title": "Collective learning Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "You could have sound three that refers to meaning three. So these are just direct representations, but what is really powerful about symbolic languages is that these oral symbols can be combined according to set rules or grammars to have an infinite number of meanings. So this is what really makes human language transcend other languages and really makes it this robust, precise communication mechanism is you could have combinations. Sound one, sound two, sound three will now have another meaning, meaning four. Then you could maybe have a combination where you have sound three, sound one, and sound two might have meaning five. And if you have tens of thousands of sounds, when really our oral words are those sounds in a given language, then all of a sudden you can have infinite meanings by putting them in different combinations. And if you think this is a little bit abstract, imagine that sound one is the sound me saying the word dog."}, {"video_title": "Collective learning Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Sound one, sound two, sound three will now have another meaning, meaning four. Then you could maybe have a combination where you have sound three, sound one, and sound two might have meaning five. And if you have tens of thousands of sounds, when really our oral words are those sounds in a given language, then all of a sudden you can have infinite meanings by putting them in different combinations. And if you think this is a little bit abstract, imagine that sound one is the sound me saying the word dog. And I'm not even gonna write it down because I wanna imagine a world even before written communication. So sound one is the sound dog. Sound two is the sound eats."}, {"video_title": "Collective learning Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And if you think this is a little bit abstract, imagine that sound one is the sound me saying the word dog. And I'm not even gonna write it down because I wanna imagine a world even before written communication. So sound one is the sound dog. Sound two is the sound eats. And sound three is the sound man. So literally, sound one, if you heard dog, you'd think, okay, I'd visualize a dog of some type. And even there, you'd have some visualization of a dog, and we all have one maybe."}, {"video_title": "Collective learning Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Sound two is the sound eats. And sound three is the sound man. So literally, sound one, if you heard dog, you'd think, okay, I'd visualize a dog of some type. And even there, you'd have some visualization of a dog, and we all have one maybe. Sound two, if you heard eats, you'd say, okay, I imagine eating in some way. And sound three, man, you have some visualization of it. And if it was a non-symbolic language, that's all you could get out of those three sounds."}, {"video_title": "Collective learning Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And even there, you'd have some visualization of a dog, and we all have one maybe. Sound two, if you heard eats, you'd say, okay, I imagine eating in some way. And sound three, man, you have some visualization of it. And if it was a non-symbolic language, that's all you could get out of those three sounds. But now in a symbolic language, we can combine those. We could say dog eats man. So once again, we just reused the three sounds, the three symbols, but now they're referring to a whole new, a much more complex meaning than just referring to certain objects or certain actions."}, {"video_title": "Collective learning Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And if it was a non-symbolic language, that's all you could get out of those three sounds. But now in a symbolic language, we can combine those. We could say dog eats man. So once again, we just reused the three sounds, the three symbols, but now they're referring to a whole new, a much more complex meaning than just referring to certain objects or certain actions. Or you could have man eats dog. And it's not pleasant, but I guess in a desperate situation. But once again, it is another meaning that we can get out of the same sounds."}, {"video_title": "What causes precession and other orbital changes Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And I'm not going to go into the physics of it, it's a little bit beyond this discussion right over here. But they're really a byproduct of the Earth and Sun's interactions with Earth and with the fact that Earth is not a perfect sphere. So if I draw Earth, so this is my little drawing of Earth, and let me put the poles over here, North Pole and South Pole, it actually turns out that Earth is fatter than it is taller. So if you were to measure Earth's diameter along the equator, it is 43 kilometers, or about 27, 43 kilometers, which is approximately 27 miles longer than if you were to measure its diameter from pole to pole. So longer than the pole to pole diameter. And the fact that Earth has this equatorial bulge, that it's not a perfect sphere, and once again I'm not going to go into the math here, is the interactions between that, I guess you could call it that one asymmetry of the Earth, it's that interaction between that and the pull of gravity between the Earth and the Sun and the Moon that causes these long-term cycles, this axial precession. And other, I guess, less noticeable changes in Earth's orbit."}, {"video_title": "What causes precession and other orbital changes Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So if you were to measure Earth's diameter along the equator, it is 43 kilometers, or about 27, 43 kilometers, which is approximately 27 miles longer than if you were to measure its diameter from pole to pole. So longer than the pole to pole diameter. And the fact that Earth has this equatorial bulge, that it's not a perfect sphere, and once again I'm not going to go into the math here, is the interactions between that, I guess you could call it that one asymmetry of the Earth, it's that interaction between that and the pull of gravity between the Earth and the Sun and the Moon that causes these long-term cycles, this axial precession. And other, I guess, less noticeable changes in Earth's orbit. And as we'll see in the next video, these aren't the only types of changes in orbits we have. We also have changes in the actual ellipse that Earth's orbit has actually rotates over time. But that's due more to interactions with Earth's orbit and the orbit of other planets in our solar system."}, {"video_title": "How earth's tilt causes seasons Cosmology & Astronomy Khan Academy.mp3", "Sentence": "In the last video, we talk about how seasons on Earth are not caused by how close Earth is to the sun in its orbit. And we also hint at the fact that it's actually caused by the tilt of the Earth. And so in this video, I want to show you how the tilt of the Earth causes the seasons to happen. So let's draw as many diagrams as possible here, because at least for my brain, they help me visualize what's actually going on. So we could imagine a top view first. So let's have a top view. That is the sun right over there."}, {"video_title": "How earth's tilt causes seasons Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So let's draw as many diagrams as possible here, because at least for my brain, they help me visualize what's actually going on. So we could imagine a top view first. So let's have a top view. That is the sun right over there. And let me draw Earth's orbit. So Earth's orbit maybe looks something like that. It is almost circular, so I'll draw it as something that's pretty close to a circle right over here."}, {"video_title": "How earth's tilt causes seasons Cosmology & Astronomy Khan Academy.mp3", "Sentence": "That is the sun right over there. And let me draw Earth's orbit. So Earth's orbit maybe looks something like that. It is almost circular, so I'll draw it as something that's pretty close to a circle right over here. And I'm going to draw Earth at different points in its orbit. And I'm going to try to depict the tilt of its rotational axis. And obviously, this is not drawn anywhere near close to scale."}, {"video_title": "How earth's tilt causes seasons Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It is almost circular, so I'll draw it as something that's pretty close to a circle right over here. And I'm going to draw Earth at different points in its orbit. And I'm going to try to depict the tilt of its rotational axis. And obviously, this is not drawn anywhere near close to scale. Earth is much further away from the sun and much, much smaller than the sun as well. So I'll draw the Earth at that point. And at this point, the Earth will be tilted away from the sun."}, {"video_title": "How earth's tilt causes seasons Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And obviously, this is not drawn anywhere near close to scale. Earth is much further away from the sun and much, much smaller than the sun as well. So I'll draw the Earth at that point. And at this point, the Earth will be tilted away from the sun. So Earth's tilt does not change, if you think about the direction, or at least over the course of a year, if we think about relatively small periods of time, it does not change relative to the direction that it's pointing at in the universe. And we'll talk about that in a second. But let's say right over here, we are pointed away from the sun."}, {"video_title": "How earth's tilt causes seasons Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And at this point, the Earth will be tilted away from the sun. So Earth's tilt does not change, if you think about the direction, or at least over the course of a year, if we think about relatively small periods of time, it does not change relative to the direction that it's pointing at in the universe. And we'll talk about that in a second. But let's say right over here, we are pointed away from the sun. So we're up and out of this page. So we are up and out. So if I wanted to put some perspective on an arrow, it would be up and out."}, {"video_title": "How earth's tilt causes seasons Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But let's say right over here, we are pointed away from the sun. So we're up and out of this page. So we are up and out. So if I wanted to put some perspective on an arrow, it would be up and out. It would be more like up and out of this page. So that's the direction if you were to come straight out of the North Pole. And if you were to go straight out of the South Pole, you would go below that circle right over there."}, {"video_title": "How earth's tilt causes seasons Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So if I wanted to put some perspective on an arrow, it would be up and out. It would be more like up and out of this page. So that's the direction if you were to come straight out of the North Pole. And if you were to go straight out of the South Pole, you would go below that circle right over there. And if I wanted to draw the same position, but if we're looking sideways along the plane, the orbital plane, or the plane of Earth's orbit. So if we're looking at it from that direction, so let me do it this way. If we're looking at it directly sideways, this is the sun right over here."}, {"video_title": "How earth's tilt causes seasons Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And if you were to go straight out of the South Pole, you would go below that circle right over there. And if I wanted to draw the same position, but if we're looking sideways along the plane, the orbital plane, or the plane of Earth's orbit. So if we're looking at it from that direction, so let me do it this way. If we're looking at it directly sideways, this is the sun right over here. This is the sun. That is the sun. And this is Earth at that position."}, {"video_title": "How earth's tilt causes seasons Cosmology & Astronomy Khan Academy.mp3", "Sentence": "If we're looking at it directly sideways, this is the sun right over here. This is the sun. That is the sun. And this is Earth at that position. This is Earth right over there. If I were to draw an arrow pointing straight out of the North Pole, it would look something like this. So this arrow and this arrow, they are both popping straight out of the North Pole."}, {"video_title": "How earth's tilt causes seasons Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And this is Earth at that position. This is Earth right over there. If I were to draw an arrow pointing straight out of the North Pole, it would look something like this. So this arrow and this arrow, they are both popping straight out of the North Pole. And so when we talk about the tilt of the Earth, we're talking about the tilt of its orbital axis, kind of this pole that could go straight between the South Pole and the North Pole. The angle between that and a pole that would actually be at a 90 degree angle, or perpendicular to the plane of its orbit. And so compared to if it was just straight up and down, relative to the plane of the orbit."}, {"video_title": "How earth's tilt causes seasons Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So this arrow and this arrow, they are both popping straight out of the North Pole. And so when we talk about the tilt of the Earth, we're talking about the tilt of its orbital axis, kind of this pole that could go straight between the South Pole and the North Pole. The angle between that and a pole that would actually be at a 90 degree angle, or perpendicular to the plane of its orbit. And so compared to if it was just straight up and down, relative to the plane of the orbit. So this right here is the angle of Earth's tilt. Let me draw that a little bit bigger, just so it becomes a little bit clearer. So if this is the plane of the orbit, we're looking sideways along the plane of the orbit."}, {"video_title": "How earth's tilt causes seasons Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And so compared to if it was just straight up and down, relative to the plane of the orbit. So this right here is the angle of Earth's tilt. Let me draw that a little bit bigger, just so it becomes a little bit clearer. So if this is the plane of the orbit, we're looking sideways along the plane of the orbit. And this is Earth right over here. So this is Earth. My best attempt to draw a circle."}, {"video_title": "How earth's tilt causes seasons Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So if this is the plane of the orbit, we're looking sideways along the plane of the orbit. And this is Earth right over here. So this is Earth. My best attempt to draw a circle. That is Earth. Earth does not rotate. Its axis of rotation is not perpendicular to the plane of the orbit."}, {"video_title": "How earth's tilt causes seasons Cosmology & Astronomy Khan Academy.mp3", "Sentence": "My best attempt to draw a circle. That is Earth. Earth does not rotate. Its axis of rotation is not perpendicular to the plane of the orbit. So this is how Earth would orbit. This is how Earth would rotate if it was. Earth rotates."}, {"video_title": "How earth's tilt causes seasons Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Its axis of rotation is not perpendicular to the plane of the orbit. So this is how Earth would orbit. This is how Earth would rotate if it was. Earth rotates. Earth's rotational axis is at an angle to that vertical relative to the plane of its orbit, I guess you could say it. It rotates at an angle like this. So this would be the North Pole."}, {"video_title": "How earth's tilt causes seasons Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Earth rotates. Earth's rotational axis is at an angle to that vertical relative to the plane of its orbit, I guess you could say it. It rotates at an angle like this. So this would be the North Pole. That is the South Pole. This is the South Pole. And so it rotates like this."}, {"video_title": "How earth's tilt causes seasons Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So this would be the North Pole. That is the South Pole. This is the South Pole. And so it rotates like this. And that angle relative to being vertical with respect to the orbital plane, this angle right here for Earth right now is 23.4 degrees. And if we're talking about relatively short periods of time like our lifespans, that is constant. But it is actually changing over long periods of time."}, {"video_title": "How earth's tilt causes seasons Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And so it rotates like this. And that angle relative to being vertical with respect to the orbital plane, this angle right here for Earth right now is 23.4 degrees. And if we're talking about relatively short periods of time like our lifespans, that is constant. But it is actually changing over long periods of time. That is changing between, and these are rough numbers, it is changing between 22.1 degrees and 24.5 degrees, if my sources are correct. But that gives a rough estimate of what is changing between. But I want to make it clear, this is not happening overnight."}, {"video_title": "How earth's tilt causes seasons Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But it is actually changing over long periods of time. That is changing between, and these are rough numbers, it is changing between 22.1 degrees and 24.5 degrees, if my sources are correct. But that gives a rough estimate of what is changing between. But I want to make it clear, this is not happening overnight. The period for it to go from roughly a 22 degree angle to a 24.5 degree angle and back to a 22 degree angle is 41,000 years. And this long-term change in the tilt, this might play into some of the long-term climactic change. Maybe it might contribute on some level to some of the ice ages that have formed over Earth's past."}, {"video_title": "How earth's tilt causes seasons Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But I want to make it clear, this is not happening overnight. The period for it to go from roughly a 22 degree angle to a 24.5 degree angle and back to a 22 degree angle is 41,000 years. And this long-term change in the tilt, this might play into some of the long-term climactic change. Maybe it might contribute on some level to some of the ice ages that have formed over Earth's past. But for the sake of thinking about our annual seasons, you don't have to worry too much, or you don't have to worry at all really about this variation. You really just have to know that it is tilted. And right now it is tilted at an angle of 23.4 degrees."}, {"video_title": "How earth's tilt causes seasons Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Maybe it might contribute on some level to some of the ice ages that have formed over Earth's past. But for the sake of thinking about our annual seasons, you don't have to worry too much, or you don't have to worry at all really about this variation. You really just have to know that it is tilted. And right now it is tilted at an angle of 23.4 degrees. Now, you might say, OK, I understand what the tilt is, but how does that change the seasons in either the northern or the southern hemisphere? And to do that, I'm going to imagine the Earth when the northern hemisphere is most tilted away from the sun and when it is most tilted towards the sun. So remember, this tilt, the direction this arrow points into relative to the rest of the universe, if we assume that this tilt is at 23.4%, it's not changing throughout the year."}, {"video_title": "How earth's tilt causes seasons Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And right now it is tilted at an angle of 23.4 degrees. Now, you might say, OK, I understand what the tilt is, but how does that change the seasons in either the northern or the southern hemisphere? And to do that, I'm going to imagine the Earth when the northern hemisphere is most tilted away from the sun and when it is most tilted towards the sun. So remember, this tilt, the direction this arrow points into relative to the rest of the universe, if we assume that this tilt is at 23.4%, it's not changing throughout the year. But depending on where it is in the orbit, it's either going to be tilting away from the sun, as it is in this example right over here, or it will be tilting towards the sun. I'll do the towards the sun in this magenta color. Or it would be tilting towards the sun."}, {"video_title": "How earth's tilt causes seasons Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So remember, this tilt, the direction this arrow points into relative to the rest of the universe, if we assume that this tilt is at 23.4%, it's not changing throughout the year. But depending on where it is in the orbit, it's either going to be tilting away from the sun, as it is in this example right over here, or it will be tilting towards the sun. I'll do the towards the sun in this magenta color. Or it would be tilting towards the sun. So six months later, when the Earth is over here, it's going to, relative to the rest of the universe, it will be tilted in that same direction. It will be tilted in that same direction up and up, out of this page and to the right. So out of this page and to the right again, just like it was over here."}, {"video_title": "How earth's tilt causes seasons Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Or it would be tilting towards the sun. So six months later, when the Earth is over here, it's going to, relative to the rest of the universe, it will be tilted in that same direction. It will be tilted in that same direction up and up, out of this page and to the right. So out of this page and to the right again, just like it was over here. But now that it's on the other side of the sun, that makes it tilt a little bit more towards the sun. If I were to draw it right over here, it is now tilted towards the sun. It is now tilted towards the sun."}, {"video_title": "How earth's tilt causes seasons Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So out of this page and to the right again, just like it was over here. But now that it's on the other side of the sun, that makes it tilt a little bit more towards the sun. If I were to draw it right over here, it is now tilted towards the sun. It is now tilted towards the sun. And what I want to think about is, how much sunlight will different parts of the planet receive? And I'll focus on the northern hemisphere, but you can make a similar argument for the southern hemisphere. I want to think about how much sunlight they receive when it's tilted away or tilted towards the sun."}, {"video_title": "How earth's tilt causes seasons Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It is now tilted towards the sun. And what I want to think about is, how much sunlight will different parts of the planet receive? And I'll focus on the northern hemisphere, but you can make a similar argument for the southern hemisphere. I want to think about how much sunlight they receive when it's tilted away or tilted towards the sun. And so let's think about those two situations. So first of all, let's think about this situation here where we are tilted away from the sun. So let me zoom in a little bit."}, {"video_title": "How earth's tilt causes seasons Cosmology & Astronomy Khan Academy.mp3", "Sentence": "I want to think about how much sunlight they receive when it's tilted away or tilted towards the sun. And so let's think about those two situations. So first of all, let's think about this situation here where we are tilted away from the sun. So let me zoom in a little bit. So this is the situation where we're tilted away from the sun. So if this is the vertical, so let me draw it. I could actually just use this diagram, but let me make it."}, {"video_title": "How earth's tilt causes seasons Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So let me zoom in a little bit. So this is the situation where we're tilted away from the sun. So if this is the vertical, so let me draw it. I could actually just use this diagram, but let me make it. So we're tilted away from the sun like this. Let me do this in a different color. So if we have an arrow coming straight out of the North Pole, it would look like this."}, {"video_title": "How earth's tilt causes seasons Cosmology & Astronomy Khan Academy.mp3", "Sentence": "I could actually just use this diagram, but let me make it. So we're tilted away from the sun like this. Let me do this in a different color. So if we have an arrow coming straight out of the North Pole, it would look like this. If we have an arrow coming straight out of the North Pole and we are rotating around like that. So we're out of the page on the left-hand side and then into the page on the right-hand side. So we're rotating towards the east constantly."}, {"video_title": "How earth's tilt causes seasons Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So if we have an arrow coming straight out of the North Pole, it would look like this. If we have an arrow coming straight out of the North Pole and we are rotating around like that. So we're out of the page on the left-hand side and then into the page on the right-hand side. So we're rotating towards the east constantly. So this arrow is in the direction of that arrow is in the direction of the east. So when we are at this point in Earth's orbit, and actually let me copy and paste this, and I'm going to use the same exact diagram for the different seasons. So let me copy and then let me paste this exact diagram."}, {"video_title": "How earth's tilt causes seasons Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So we're rotating towards the east constantly. So this arrow is in the direction of that arrow is in the direction of the east. So when we are at this point in Earth's orbit, and actually let me copy and paste this, and I'm going to use the same exact diagram for the different seasons. So let me copy and then let me paste this exact diagram. I'll do it over here for two different points. So when we are here in Earth's orbit, where is the sunlight coming from? Well, it's going to be coming from the left, at least the way I've drawn the diagram right over here."}, {"video_title": "How earth's tilt causes seasons Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So let me copy and then let me paste this exact diagram. I'll do it over here for two different points. So when we are here in Earth's orbit, where is the sunlight coming from? Well, it's going to be coming from the left, at least the way I've drawn the diagram right over here. So the sunlight is coming from the left. Sunlight is coming from the left in this situation. And so if you think about it, what half of the Earth is being, or what part of the Earth is being lit by sunlight?"}, {"video_title": "How earth's tilt causes seasons Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Well, it's going to be coming from the left, at least the way I've drawn the diagram right over here. So the sunlight is coming from the left. Sunlight is coming from the left in this situation. And so if you think about it, what half of the Earth is being, or what part of the Earth is being lit by sunlight? Or what part of the Earth is in daylight, the way I've drawn it right over here? Well, the part that is facing the sun. So all of this right over here is going to be in daylight."}, {"video_title": "How earth's tilt causes seasons Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And so if you think about it, what half of the Earth is being, or what part of the Earth is being lit by sunlight? Or what part of the Earth is in daylight, the way I've drawn it right over here? Well, the part that is facing the sun. So all of this right over here is going to be in daylight. As we rotate, whatever part of the surface of the Earth enters into this yellow part right over here will be in daylight. But let's think about what's happening at different parts of the Earth. So let me draw the equator, which separates our northern and southern hemispheres."}, {"video_title": "How earth's tilt causes seasons Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So all of this right over here is going to be in daylight. As we rotate, whatever part of the surface of the Earth enters into this yellow part right over here will be in daylight. But let's think about what's happening at different parts of the Earth. So let me draw the equator, which separates our northern and southern hemispheres. So this is the equator. And then let me go into the northern hemisphere. And I want to show you why, when the North Pole is pointed away from the sun, why this is our winter."}, {"video_title": "How earth's tilt causes seasons Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So let me draw the equator, which separates our northern and southern hemispheres. So this is the equator. And then let me go into the northern hemisphere. And I want to show you why, when the North Pole is pointed away from the sun, why this is our winter. So when we're pointed away from the sun, well, if we go to the Arctic Circle. So let me go right over here. Let me go to some point in the Arctic Circle."}, {"video_title": "How earth's tilt causes seasons Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And I want to show you why, when the North Pole is pointed away from the sun, why this is our winter. So when we're pointed away from the sun, well, if we go to the Arctic Circle. So let me go right over here. Let me go to some point in the Arctic Circle. As it goes, as the Earth rotates every 24 hours, this point on the globe will just rotate around just like that. It will just keep rotating around just like that. And so my question is, that point in the Arctic Circle as it rotates, will it ever see sunlight?"}, {"video_title": "How earth's tilt causes seasons Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Let me go to some point in the Arctic Circle. As it goes, as the Earth rotates every 24 hours, this point on the globe will just rotate around just like that. It will just keep rotating around just like that. And so my question is, that point in the Arctic Circle as it rotates, will it ever see sunlight? Well, no. It will never see sunlight, because the North Pole is tilted away from the sun. So what I'm shading here in purple, that part of the Earth, when it's completely tilted away, will never see sunlight."}, {"video_title": "How earth's tilt causes seasons Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And so my question is, that point in the Arctic Circle as it rotates, will it ever see sunlight? Well, no. It will never see sunlight, because the North Pole is tilted away from the sun. So what I'm shading here in purple, that part of the Earth, when it's completely tilted away, will never see sunlight. Or at least it won't see sunlight while it's tilted away, while it's in this position, or in this position in the orbit. Never. I won't say never, because once it becomes summer, they will be able to see it."}, {"video_title": "How earth's tilt causes seasons Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So what I'm shading here in purple, that part of the Earth, when it's completely tilted away, will never see sunlight. Or at least it won't see sunlight while it's tilted away, while it's in this position, or in this position in the orbit. Never. I won't say never, because once it becomes summer, they will be able to see it. So no sunlight, no day, I guess you could say. No daylight. If you go to slightly more southern latitudes, so let's say you go over here."}, {"video_title": "How earth's tilt causes seasons Cosmology & Astronomy Khan Academy.mp3", "Sentence": "I won't say never, because once it becomes summer, they will be able to see it. So no sunlight, no day, I guess you could say. No daylight. If you go to slightly more southern latitudes, so let's say you go over here. So maybe that's the latitude of something like, I don't know, New York or San Francisco or something like that. Let's think about what it would see as the Earth rotates every 24 hours. So this would be daylight, daylight, daylight, daylight."}, {"video_title": "How earth's tilt causes seasons Cosmology & Astronomy Khan Academy.mp3", "Sentence": "If you go to slightly more southern latitudes, so let's say you go over here. So maybe that's the latitude of something like, I don't know, New York or San Francisco or something like that. Let's think about what it would see as the Earth rotates every 24 hours. So this would be daylight, daylight, daylight, daylight. Then nighttime, nighttime, nighttime, nighttime. This is now going behind the globe. Nighttime, nighttime, nighttime, nighttime, nighttime."}, {"video_title": "How earth's tilt causes seasons Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So this would be daylight, daylight, daylight, daylight. Then nighttime, nighttime, nighttime, nighttime. This is now going behind the globe. Nighttime, nighttime, nighttime, nighttime, nighttime. Daylight, daylight, daylight, daylight. So if you just compare this, so let me do the daylight in orange. So daylight is in orange."}, {"video_title": "How earth's tilt causes seasons Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Nighttime, nighttime, nighttime, nighttime, nighttime. Daylight, daylight, daylight, daylight. So if you just compare this, so let me do the daylight in orange. So daylight is in orange. And then nighttime I will do in this bluish, purplish color. So nighttime over here. So if you go to really northern latitudes, like the Arctic Circle, they don't get any daylight when we're tilted away from the Earth."}, {"video_title": "How earth's tilt causes seasons Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So daylight is in orange. And then nighttime I will do in this bluish, purplish color. So nighttime over here. So if you go to really northern latitudes, like the Arctic Circle, they don't get any daylight when we're tilted away from the Earth. And if we go to slightly still northern latitudes, but not as north as the Arctic Circle, it does get daylight, but it gets a lot less daylight. It spends a lot less time in the daylight than in the nighttime. So notice, if you say that this circumference represents the positions over 24 hours, it spends much less time in the daylight than it does in the nighttime."}, {"video_title": "How earth's tilt causes seasons Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So if you go to really northern latitudes, like the Arctic Circle, they don't get any daylight when we're tilted away from the Earth. And if we go to slightly still northern latitudes, but not as north as the Arctic Circle, it does get daylight, but it gets a lot less daylight. It spends a lot less time in the daylight than in the nighttime. So notice, if you say that this circumference represents the positions over 24 hours, it spends much less time in the daylight than it does in the nighttime. So because while the northern hemisphere is tilted away from the Earth, the latitudes in the northern hemisphere are getting less daylight. So they're getting less daylight. They're also getting less energy from the sun."}, {"video_title": "How earth's tilt causes seasons Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So notice, if you say that this circumference represents the positions over 24 hours, it spends much less time in the daylight than it does in the nighttime. So because while the northern hemisphere is tilted away from the Earth, the latitudes in the northern hemisphere are getting less daylight. So they're getting less daylight. They're also getting less energy from the sun. And so that's what leads to winter, or just being generally colder. And to see what happens in the summer, let's just go the other side. So now we're going to the other side of our orbit around the sun."}, {"video_title": "How earth's tilt causes seasons Cosmology & Astronomy Khan Academy.mp3", "Sentence": "They're also getting less energy from the sun. And so that's what leads to winter, or just being generally colder. And to see what happens in the summer, let's just go the other side. So now we're going to the other side of our orbit around the sun. This is going to be six months later. And notice, the actual direction relative to the rest of the universe has not changed. We're still pointed in that same direction."}, {"video_title": "How earth's tilt causes seasons Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So now we're going to the other side of our orbit around the sun. This is going to be six months later. And notice, the actual direction relative to the rest of the universe has not changed. We're still pointed in that same direction. We still have a 23.4 degree tilt relative to, I guess, being straight up and down. But now, once we're over here, the light from the sun is going to be coming from the right. So the light from the sun is going to be coming from the right, just like that."}, {"video_title": "How earth's tilt causes seasons Cosmology & Astronomy Khan Academy.mp3", "Sentence": "We're still pointed in that same direction. We still have a 23.4 degree tilt relative to, I guess, being straight up and down. But now, once we're over here, the light from the sun is going to be coming from the right. So the light from the sun is going to be coming from the right, just like that. And now, if on this diagram at least, this is the side of the Earth that is going to be getting the sunlight. And let me draw the equator again, or my best attempt to draw the equator. I'll draw the equator in that same color, actually, in that green color."}, {"video_title": "How earth's tilt causes seasons Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So the light from the sun is going to be coming from the right, just like that. And now, if on this diagram at least, this is the side of the Earth that is going to be getting the sunlight. And let me draw the equator again, or my best attempt to draw the equator. I'll draw the equator in that same color, actually, in that green color. So this separates the northern and the southern hemisphere. And now, let's think about the Arctic Circle. So let's say I'm sitting here in the Arctic Circle."}, {"video_title": "How earth's tilt causes seasons Cosmology & Astronomy Khan Academy.mp3", "Sentence": "I'll draw the equator in that same color, actually, in that green color. So this separates the northern and the southern hemisphere. And now, let's think about the Arctic Circle. So let's say I'm sitting here in the Arctic Circle. As the day goes on, as 24 hours go around, I'll keep rotating around here. But notice, the whole time I am inside of the sun. I'm getting no night time."}, {"video_title": "How earth's tilt causes seasons Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So let's say I'm sitting here in the Arctic Circle. As the day goes on, as 24 hours go around, I'll keep rotating around here. But notice, the whole time I am inside of the sun. I'm getting no night time. There is no night in the Arctic Circle while we are tilted towards the sun. And if we still do that fairly northern latitude, but not as far as the Arctic Circle, maybe in San Francisco or New York or something like that, if we go to that latitude, notice how much time we spend in the sun. So maybe we just enter."}, {"video_title": "How earth's tilt causes seasons Cosmology & Astronomy Khan Academy.mp3", "Sentence": "I'm getting no night time. There is no night in the Arctic Circle while we are tilted towards the sun. And if we still do that fairly northern latitude, but not as far as the Arctic Circle, maybe in San Francisco or New York or something like that, if we go to that latitude, notice how much time we spend in the sun. So maybe we just enter. So this is right at sunrise. And then as the day goes on, we are in sunlight, sunlight, sunlight, sunlight, sunlight, sunlight, sunlight, sunlight. Then we hit sunset."}, {"video_title": "How earth's tilt causes seasons Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So maybe we just enter. So this is right at sunrise. And then as the day goes on, we are in sunlight, sunlight, sunlight, sunlight, sunlight, sunlight, sunlight, sunlight. Then we hit sunset. Then we hit night time, night time. Then we hit night time. And then we get sunrise again."}, {"video_title": "How earth's tilt causes seasons Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Then we hit sunset. Then we hit night time, night time. Then we hit night time. And then we get sunrise again. And so when you look at the amount of time that something in the northern hemisphere spends in the daylight versus sunlight, you'll see it spends a lot more time in the daylight when the northern hemisphere is tilted towards the sun. So this is more day, less night. So it is getting more energy from the sun."}, {"video_title": "How earth's tilt causes seasons Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And then we get sunrise again. And so when you look at the amount of time that something in the northern hemisphere spends in the daylight versus sunlight, you'll see it spends a lot more time in the daylight when the northern hemisphere is tilted towards the sun. So this is more day, less night. So it is getting more energy from the sun. So when it's tilted towards the sun, it is getting more energy from the sun. So things will generally be warmer. And so you are now talking about summer in the northern hemisphere."}, {"video_title": "How earth's tilt causes seasons Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So it is getting more energy from the sun. So when it's tilted towards the sun, it is getting more energy from the sun. So things will generally be warmer. And so you are now talking about summer in the northern hemisphere. And the arguments for the southern hemisphere are identical. You could even play it right over here. When the northern hemisphere is tilted away from the sun, then the southern hemisphere is tilted towards the sun."}, {"video_title": "How earth's tilt causes seasons Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And so you are now talking about summer in the northern hemisphere. And the arguments for the southern hemisphere are identical. You could even play it right over here. When the northern hemisphere is tilted away from the sun, then the southern hemisphere is tilted towards the sun. And so for example, the South Pole will have all daylight and no night time. And southern latitudes will have more daylight than night time. And so the south will have summer."}, {"video_title": "How earth's tilt causes seasons Cosmology & Astronomy Khan Academy.mp3", "Sentence": "When the northern hemisphere is tilted away from the sun, then the southern hemisphere is tilted towards the sun. And so for example, the South Pole will have all daylight and no night time. And southern latitudes will have more daylight than night time. And so the south will have summer. So this is summer in the south, in the southern hemisphere. And it's winter in the north. And then down here, the southern hemisphere is pointed away from the sun."}, {"video_title": "How earth's tilt causes seasons Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And so the south will have summer. So this is summer in the south, in the southern hemisphere. And it's winter in the north. And then down here, the southern hemisphere is pointed away from the sun. So this is winter in the southern hemisphere. And you might be saying, hey, Sal, what about, you haven't talked a lot about spring and fall. Well, let's think about it."}, {"video_title": "How earth's tilt causes seasons Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And then down here, the southern hemisphere is pointed away from the sun. So this is winter in the southern hemisphere. And you might be saying, hey, Sal, what about, you haven't talked a lot about spring and fall. Well, let's think about it. Well, if we're talking about the northern hemisphere, this over here we just tided was winter in the northern hemisphere. And we are going to rotate around the sun. And at some point, we're going to get over here."}, {"video_title": "How earth's tilt causes seasons Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Well, let's think about it. Well, if we're talking about the northern hemisphere, this over here we just tided was winter in the northern hemisphere. And we are going to rotate around the sun. And at some point, we're going to get over here. And then because of this tilt, we aren't pointed away or towards the sun. We're kind of pointed sideways relative to the direction of the sun. But this doesn't favor one hemisphere over the other."}, {"video_title": "How earth's tilt causes seasons Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And at some point, we're going to get over here. And then because of this tilt, we aren't pointed away or towards the sun. We're kind of pointed sideways relative to the direction of the sun. But this doesn't favor one hemisphere over the other. So when we're over here in, and this will actually be the spring now, both hemispheres are getting the equal amount of daylight and sunlight. Or for a given latitude above or below the equator, they're getting the same amount. And the same thing is true over here when we get to, so this is the spring, this is the summer in the northern hemisphere."}, {"video_title": "How earth's tilt causes seasons Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But this doesn't favor one hemisphere over the other. So when we're over here in, and this will actually be the spring now, both hemispheres are getting the equal amount of daylight and sunlight. Or for a given latitude above or below the equator, they're getting the same amount. And the same thing is true over here when we get to, so this is the spring, this is the summer in the northern hemisphere. Now this will be the fall in the northern hemisphere. And once again, we're tilted in this direction. And so the northern hemisphere isn't tilted away or towards the sun."}, {"video_title": "How earth's tilt causes seasons Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And the same thing is true over here when we get to, so this is the spring, this is the summer in the northern hemisphere. Now this will be the fall in the northern hemisphere. And once again, we're tilted in this direction. And so the northern hemisphere isn't tilted away or towards the sun. And so both hemispheres are going to get the same amount of radiation from the sun. So you really see the extremes in the winters and the summers. Now one thing I do want to make clear, and I started off with just the length of day and nighttime, because frankly that's maybe a little bit, or at least in my brain, a little bit easier to visualize."}, {"video_title": "How earth's tilt causes seasons Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And so the northern hemisphere isn't tilted away or towards the sun. And so both hemispheres are going to get the same amount of radiation from the sun. So you really see the extremes in the winters and the summers. Now one thing I do want to make clear, and I started off with just the length of day and nighttime, because frankly that's maybe a little bit, or at least in my brain, a little bit easier to visualize. But that by itself does not account for all of the difference between summer and winter. Another cause, and actually this is probably the biggest cause, is if you think about the total amount of sun. So let's talk about the northern hemisphere winter."}, {"video_title": "How earth's tilt causes seasons Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Now one thing I do want to make clear, and I started off with just the length of day and nighttime, because frankly that's maybe a little bit, or at least in my brain, a little bit easier to visualize. But that by itself does not account for all of the difference between summer and winter. Another cause, and actually this is probably the biggest cause, is if you think about the total amount of sun. So let's talk about the northern hemisphere winter. And let's say there's a certain amount of sunlight that is reaching the Earth. So there is a certain amount of sunlight that is reaching the Earth. So this is the total amount of sunlight that's reaching the Earth at any point in time."}, {"video_title": "How earth's tilt causes seasons Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So let's talk about the northern hemisphere winter. And let's say there's a certain amount of sunlight that is reaching the Earth. So there is a certain amount of sunlight that is reaching the Earth. So this is the total amount of sunlight that's reaching the Earth at any point in time. You see that much more of that is hitting the southern hemisphere than the northern hemisphere here. If you imagine it, all of these rays right over here are hitting the southern hemisphere. So a majority of the rays are hitting the southern hemisphere and much fewer are hitting the northern hemisphere."}, {"video_title": "How earth's tilt causes seasons Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So this is the total amount of sunlight that's reaching the Earth at any point in time. You see that much more of that is hitting the southern hemisphere than the northern hemisphere here. If you imagine it, all of these rays right over here are hitting the southern hemisphere. So a majority of the rays are hitting the southern hemisphere and much fewer are hitting the northern hemisphere. So actually a smaller amount of the radiation period, at even a given period in time, not even talking about the amount of time you're facing the sun, but at any given moment in time, more energy is hitting the southern hemisphere than the northern. And the opposite is true when the tilt is then towards the sun. Now a disproportionate amount of the sun's energy is hitting the northern hemisphere."}, {"video_title": "How earth's tilt causes seasons Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So a majority of the rays are hitting the southern hemisphere and much fewer are hitting the northern hemisphere. So actually a smaller amount of the radiation period, at even a given period in time, not even talking about the amount of time you're facing the sun, but at any given moment in time, more energy is hitting the southern hemisphere than the northern. And the opposite is true when the tilt is then towards the sun. Now a disproportionate amount of the sun's energy is hitting the northern hemisphere. So if you just think that this is all of the energy from the sun, most of it, all of these rays up here, are hitting the northern hemisphere. And only these down here are hitting the southern hemisphere. And on top of that, what makes it even more extreme is that the actual angle that the, and of course this is to some degree is due to the fact of where the angle of the sun relative to the horizon or where you are on Earth."}, {"video_title": "How earth's tilt causes seasons Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Now a disproportionate amount of the sun's energy is hitting the northern hemisphere. So if you just think that this is all of the energy from the sun, most of it, all of these rays up here, are hitting the northern hemisphere. And only these down here are hitting the southern hemisphere. And on top of that, what makes it even more extreme is that the actual angle that the, and of course this is to some degree is due to the fact of where the angle of the sun relative to the horizon or where you are on Earth. But even more than that, if you are on, let's say that this is the land. And we're talking about the winter in the northern hemisphere. So let's say you're talking about, let's say we're up over here at this northern latitude."}, {"video_title": "How earth's tilt causes seasons Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And on top of that, what makes it even more extreme is that the actual angle that the, and of course this is to some degree is due to the fact of where the angle of the sun relative to the horizon or where you are on Earth. But even more than that, if you are on, let's say that this is the land. And we're talking about the winter in the northern hemisphere. So let's say you're talking about, let's say we're up over here at this northern latitude. And this is just what, and we're looking, we're just looking at the sun here. And over here you could see even when we are closest to the sun, the sun is not directly overhead. When we're closest to the sun, the sun still is pretty low on the horizon."}, {"video_title": "How earth's tilt causes seasons Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So let's say you're talking about, let's say we're up over here at this northern latitude. And this is just what, and we're looking, we're just looking at the sun here. And over here you could see even when we are closest to the sun, the sun is not directly overhead. When we're closest to the sun, the sun still is pretty low on the horizon. So maybe right over here when we're closest to the sun in the winter, the sun might be right over here. But if you look at that same latitude in the summer, when it is closest to the sun, the sun is more close to being directly overhead. It still won't be directly overhead because we're still at a relatively northern latitude."}, {"video_title": "How earth's tilt causes seasons Cosmology & Astronomy Khan Academy.mp3", "Sentence": "When we're closest to the sun, the sun still is pretty low on the horizon. So maybe right over here when we're closest to the sun in the winter, the sun might be right over here. But if you look at that same latitude in the summer, when it is closest to the sun, the sun is more close to being directly overhead. It still won't be directly overhead because we're still at a relatively northern latitude. But the sun is going to be much higher in the sky. And these are all related to each other. It's kind of connected with this idea that more energy is hitting one hemisphere or the other."}, {"video_title": "How earth's tilt causes seasons Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It still won't be directly overhead because we're still at a relatively northern latitude. But the sun is going to be much higher in the sky. And these are all related to each other. It's kind of connected with this idea that more energy is hitting one hemisphere or the other. But also when you have a, I guess you could say, a steeper angle from the rays of the sun with the earth, it's actually going to be dissipated less by the atmosphere. And let me just make it clear how this is. So in the summer, so let's say that that's the land."}, {"video_title": "How earth's tilt causes seasons Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It's kind of connected with this idea that more energy is hitting one hemisphere or the other. But also when you have a, I guess you could say, a steeper angle from the rays of the sun with the earth, it's actually going to be dissipated less by the atmosphere. And let me just make it clear how this is. So in the summer, so let's say that that's the land. And let's say that, let me draw the atmosphere. I'll draw the atmosphere in white. So all of this area right over here, this is the atmosphere."}, {"video_title": "How earth's tilt causes seasons Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So in the summer, so let's say that that's the land. And let's say that, let me draw the atmosphere. I'll draw the atmosphere in white. So all of this area right over here, this is the atmosphere. And obviously, there's not a hard boundary for the atmosphere. But let's just say this is the densest part of the atmosphere. In the summer, when the sun is higher in the sky, the rays from the sun are dissipated by less atmosphere."}, {"video_title": "How earth's tilt causes seasons Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So all of this area right over here, this is the atmosphere. And obviously, there's not a hard boundary for the atmosphere. But let's just say this is the densest part of the atmosphere. In the summer, when the sun is higher in the sky, the rays from the sun are dissipated by less atmosphere. So they have to get through this much atmosphere. And they're bounced off. And they heat some of that atmosphere."}, {"video_title": "How earth's tilt causes seasons Cosmology & Astronomy Khan Academy.mp3", "Sentence": "In the summer, when the sun is higher in the sky, the rays from the sun are dissipated by less atmosphere. So they have to get through this much atmosphere. And they're bounced off. And they heat some of that atmosphere. And they're absorbed before they get to the ground. In the winter, when the sun is lower in the sky, so maybe the sun is out here. Let me draw it a little bit."}, {"video_title": "How earth's tilt causes seasons Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And they heat some of that atmosphere. And they're absorbed before they get to the ground. In the winter, when the sun is lower in the sky, so maybe the sun is out here. Let me draw it a little bit. So when the sun is lower in the sky relative to this point, you see that the rays of sunlight have to travel through a lot more atmosphere. So they get dissipated much more before they get to this point on the planet. So all in all, it is the tilt that is causing the changes in the season."}, {"video_title": "How earth's tilt causes seasons Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Let me draw it a little bit. So when the sun is lower in the sky relative to this point, you see that the rays of sunlight have to travel through a lot more atmosphere. So they get dissipated much more before they get to this point on the planet. So all in all, it is the tilt that is causing the changes in the season. But it's causing it for multiple reasons. One is when you're tilted towards the sun, you're getting more absolute hours of daylight. Not only are you getting more absolute hours of daylight, but at any given moment, most or more of the sun's total rays that are hitting the Earth are hitting the northern hemisphere as opposed to the southern hemisphere."}, {"video_title": "Star field and nebula images Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Since we've been talking about how stars form and the evolution of stars, I thought it was about time that we looked at some cool pictures of stars forming or stars themselves or evolution of stars. So this right here is from the Eagle Nebula. And just so you know, the word nebula is kind of a general word for any interstellar cloud of gas or dust. So when we're talking about the Eagle Nebula, we're talking about a huge nebula. And actually, it's a nebula that spans. And just so you have a sense, this is just one of the pillars in the, this is called the Pillars of Creation. You've probably seen this image before."}, {"video_title": "Star field and nebula images Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So when we're talking about the Eagle Nebula, we're talking about a huge nebula. And actually, it's a nebula that spans. And just so you have a sense, this is just one of the pillars in the, this is called the Pillars of Creation. You've probably seen this image before. There's these three pillars here. And this is just a small part of the actual Eagle Nebula. And just this pillar right over here, just so that you have a sense of how large it is."}, {"video_title": "Star field and nebula images Cosmology & Astronomy Khan Academy.mp3", "Sentence": "You've probably seen this image before. There's these three pillars here. And this is just a small part of the actual Eagle Nebula. And just this pillar right over here, just so that you have a sense of how large it is. Just this pillar itself is 7 light years. It is 7 light years tall. So this is an enormous amount of distance."}, {"video_title": "Star field and nebula images Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And just this pillar right over here, just so that you have a sense of how large it is. Just this pillar itself is 7 light years. It is 7 light years tall. So this is an enormous amount of distance. Remember, the distance from Earth to the nearest star was about 4 light years. It would take Voyager, if it was pointed in the right direction, at moving at 60,000 kilometers per hour, it would take Voyager 80,000 years to go 4 light years. Just this pillar is 7 light years."}, {"video_title": "Star field and nebula images Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So this is an enormous amount of distance. Remember, the distance from Earth to the nearest star was about 4 light years. It would take Voyager, if it was pointed in the right direction, at moving at 60,000 kilometers per hour, it would take Voyager 80,000 years to go 4 light years. Just this pillar is 7 light years. But I wanted to show you this because these type of nebulae, as the plural of nebula, are where stars can form. So this right here, you actually see, this is actually kind of the breeding ground for the birth of new stars. This gas is condensing, just like we talked about a couple of videos ago, until it gets to that critical temperature, that critical density, where you can actually get fusion of hydrogen."}, {"video_title": "Star field and nebula images Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Just this pillar is 7 light years. But I wanted to show you this because these type of nebulae, as the plural of nebula, are where stars can form. So this right here, you actually see, this is actually kind of the breeding ground for the birth of new stars. This gas is condensing, just like we talked about a couple of videos ago, until it gets to that critical temperature, that critical density, where you can actually get fusion of hydrogen. So this is just a huge interstellar cloud of hydrogen gas. And over here, you can just see it's just this breeding ground for stars. And we actually think that this structure doesn't even exist anymore."}, {"video_title": "Star field and nebula images Cosmology & Astronomy Khan Academy.mp3", "Sentence": "This gas is condensing, just like we talked about a couple of videos ago, until it gets to that critical temperature, that critical density, where you can actually get fusion of hydrogen. So this is just a huge interstellar cloud of hydrogen gas. And over here, you can just see it's just this breeding ground for stars. And we actually think that this structure doesn't even exist anymore. Because remember, this thing is very, very far away from us. In fact, it is, just so you have the number, this is 7,000 light years away. Which means that what we are seeing now, the photons that are reaching us right now, reaching our eyes, are reaching our telescopes right now, left this region of space 7,000 years ago."}, {"video_title": "Star field and nebula images Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And we actually think that this structure doesn't even exist anymore. Because remember, this thing is very, very far away from us. In fact, it is, just so you have the number, this is 7,000 light years away. Which means that what we are seeing now, the photons that are reaching us right now, reaching our eyes, are reaching our telescopes right now, left this region of space 7,000 years ago. So we're seeing it as it was 7,000 years ago. So a lot of this gas, a lot of this hydrogen, may have already condensed into many, many, many more stars. So the structure might not be the way it looks right now."}, {"video_title": "Star field and nebula images Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Which means that what we are seeing now, the photons that are reaching us right now, reaching our eyes, are reaching our telescopes right now, left this region of space 7,000 years ago. So we're seeing it as it was 7,000 years ago. So a lot of this gas, a lot of this hydrogen, may have already condensed into many, many, many more stars. So the structure might not be the way it looks right now. And actually, there was another supernova that happened that we think might have kind of blown away a lot of this dust. And we won't even be able to see what the effects of that supernova were for another 1,000 years. But anyway, this is just a pretty amazing photograph, in my opinion."}, {"video_title": "Star field and nebula images Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So the structure might not be the way it looks right now. And actually, there was another supernova that happened that we think might have kind of blown away a lot of this dust. And we won't even be able to see what the effects of that supernova were for another 1,000 years. But anyway, this is just a pretty amazing photograph, in my opinion. Especially, and it's beautiful at any scale, but it's even more mind-blowing when you think that this is 7, this is a structure that is 7 light years tall. And this is really just part of the Eagle Nebula, one of the pillars of creation. This right here is a star field."}, {"video_title": "Star field and nebula images Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But anyway, this is just a pretty amazing photograph, in my opinion. Especially, and it's beautiful at any scale, but it's even more mind-blowing when you think that this is 7, this is a structure that is 7 light years tall. And this is really just part of the Eagle Nebula, one of the pillars of creation. This right here is a star field. And this is as we're looking towards the center of our galaxy, the Milky Way. So this is the Sagittarius star field. And the neat thing here is you just see such a diversity in stars."}, {"video_title": "Star field and nebula images Cosmology & Astronomy Khan Academy.mp3", "Sentence": "This right here is a star field. And this is as we're looking towards the center of our galaxy, the Milky Way. So this is the Sagittarius star field. And the neat thing here is you just see such a diversity in stars. And this is also kind of mind-numbing, because every one of these stars are inside of our galaxy. This is looking towards the center of our galaxy. We're not looking at, this isn't one of those where we're looking beyond our galaxy or looking at clusters of galaxies."}, {"video_title": "Star field and nebula images Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And the neat thing here is you just see such a diversity in stars. And this is also kind of mind-numbing, because every one of these stars are inside of our galaxy. This is looking towards the center of our galaxy. We're not looking at, this isn't one of those where we're looking beyond our galaxy or looking at clusters of galaxies. This is just stars here. But what's neat here is you see a huge variety. You see some stars that are shining red right over here."}, {"video_title": "Star field and nebula images Cosmology & Astronomy Khan Academy.mp3", "Sentence": "We're not looking at, this isn't one of those where we're looking beyond our galaxy or looking at clusters of galaxies. This is just stars here. But what's neat here is you see a huge variety. You see some stars that are shining red right over here. And obviously, the apparent size, you cannot completely tell, because the stars are at different distances and at different intensities. But the redder stars, these are stars at their red giant phase, or they're probably at their red giant phase. I haven't done specific research on these stars, but that's what we suspect, that they're at the red giant phase."}, {"video_title": "Star field and nebula images Cosmology & Astronomy Khan Academy.mp3", "Sentence": "You see some stars that are shining red right over here. And obviously, the apparent size, you cannot completely tell, because the stars are at different distances and at different intensities. But the redder stars, these are stars at their red giant phase, or they're probably at their red giant phase. I haven't done specific research on these stars, but that's what we suspect, that they're at the red giant phase. The ones that are kind of in the yellowish-white part of the spectrum, these are stars probably in their main sequence, probably not too different than our own sun. The ones that are in the yellowish-white, or closer to orange, yellowish-white part of the spectrum. And the ones that look a little bit more bluish, or a little bit more greenish, these are burning super fast."}, {"video_title": "Star field and nebula images Cosmology & Astronomy Khan Academy.mp3", "Sentence": "I haven't done specific research on these stars, but that's what we suspect, that they're at the red giant phase. The ones that are kind of in the yellowish-white part of the spectrum, these are stars probably in their main sequence, probably not too different than our own sun. The ones that are in the yellowish-white, or closer to orange, yellowish-white part of the spectrum. And the ones that look a little bit more bluish, or a little bit more greenish, these are burning super fast. Let me see if I can find. This one looks a little bit bluish to me. Sorry, I had to pause the video for a cough."}, {"video_title": "Star field and nebula images Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And the ones that look a little bit more bluish, or a little bit more greenish, these are burning super fast. Let me see if I can find. This one looks a little bit bluish to me. Sorry, I had to pause the video for a cough. These are burning super, super fast. And so the supermassive stars, they burn kind of fast and furious and then just die out. While the smaller stars, the ones with less mass, they burn slower over a much, much, much longer period of time."}, {"video_title": "Star field and nebula images Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Sorry, I had to pause the video for a cough. These are burning super, super fast. And so the supermassive stars, they burn kind of fast and furious and then just die out. While the smaller stars, the ones with less mass, they burn slower over a much, much, much longer period of time. So the ones that are burning really fast are emitting a lot of energy at the smaller wavelength part of the light spectrum, and that's why they look bluer or greener. And these are going to be more massive stars, the ones that look whiter or bluer or greener. While the redder ones are less massive stars that are in kind of their supergiant phase, and so they are at this point cooler than kind of the main sequence stars."}, {"video_title": "Star field and nebula images Cosmology & Astronomy Khan Academy.mp3", "Sentence": "While the smaller stars, the ones with less mass, they burn slower over a much, much, much longer period of time. So the ones that are burning really fast are emitting a lot of energy at the smaller wavelength part of the light spectrum, and that's why they look bluer or greener. And these are going to be more massive stars, the ones that look whiter or bluer or greener. While the redder ones are less massive stars that are in kind of their supergiant phase, and so they are at this point cooler than kind of the main sequence stars. This right here is the Cat's Eye Nebula. And the word nebula, this is actually a planetary nebula, and I want to differentiate it. This right here is a planetary nebula."}, {"video_title": "Star field and nebula images Cosmology & Astronomy Khan Academy.mp3", "Sentence": "While the redder ones are less massive stars that are in kind of their supergiant phase, and so they are at this point cooler than kind of the main sequence stars. This right here is the Cat's Eye Nebula. And the word nebula, this is actually a planetary nebula, and I want to differentiate it. This right here is a planetary nebula. And it's called a nebula because it is kind of this gas that's kind of floating out in space, but it's at a completely different scale than the Eagle Nebula that we drew over here. So normally when people just talk about nebula, they're talking about something like the Eagle Nebula, this huge masses of interstellar gas. When people talk about planetary nebula, this is still actually a huge, huge radius, but nowhere near 7 light years, but it's still a huge."}, {"video_title": "Star field and nebula images Cosmology & Astronomy Khan Academy.mp3", "Sentence": "This right here is a planetary nebula. And it's called a nebula because it is kind of this gas that's kind of floating out in space, but it's at a completely different scale than the Eagle Nebula that we drew over here. So normally when people just talk about nebula, they're talking about something like the Eagle Nebula, this huge masses of interstellar gas. When people talk about planetary nebula, this is still actually a huge, huge radius, but nowhere near 7 light years, but it's still a huge. But this is the byproduct of a star shedding off all of its outer material. So at the center of this, we see a fairly mature star here, and it's shed off kind of its outer layers, and it did that while it was in its red giant phase. So the core would keep flaring up, keep having these hot explosions, and every time you had one of those hot explosions, you had more and more of its outer layers getting pushed off into space, forming this planetary nebula."}, {"video_title": "Star field and nebula images Cosmology & Astronomy Khan Academy.mp3", "Sentence": "When people talk about planetary nebula, this is still actually a huge, huge radius, but nowhere near 7 light years, but it's still a huge. But this is the byproduct of a star shedding off all of its outer material. So at the center of this, we see a fairly mature star here, and it's shed off kind of its outer layers, and it did that while it was in its red giant phase. So the core would keep flaring up, keep having these hot explosions, and every time you had one of those hot explosions, you had more and more of its outer layers getting pushed off into space, forming this planetary nebula. So as we see it right now, it's still not yet a white dwarf, it is still an active star, fusion is still occurring in this star, but it's well on its way to becoming a white dwarf. It wants all the fuel runs out. So it's past the red giant phase, it's thrown all of this material into space, and it's on its way to becoming a white dwarf."}, {"video_title": "Star field and nebula images Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So the core would keep flaring up, keep having these hot explosions, and every time you had one of those hot explosions, you had more and more of its outer layers getting pushed off into space, forming this planetary nebula. So as we see it right now, it's still not yet a white dwarf, it is still an active star, fusion is still occurring in this star, but it's well on its way to becoming a white dwarf. It wants all the fuel runs out. So it's past the red giant phase, it's thrown all of this material into space, and it's on its way to becoming a white dwarf. Anyway, hopefully you enjoyed that. I actually find all of these images to be pretty captivating, especially the star field one, because this is just inside of our galaxy. So hopefully it gives even more appreciation for how many stars there are."}, {"video_title": "Seismic waves Earth geological and climatic history Cosmology & Astronomy Khan Academy.mp3", "Sentence": "One, because they're interesting by themselves, but they're also really useful for figuring out what the actual composition of the Earth is. You've seen my video on the actual layers of the Earth. And seismic waves are crucial to actually realizing how people figure it out what the different layers of the Earth are. And just to be clear, seismic waves, they're normally associated with earthquakes. But there are any waves that travel through the Earth that could be due to an earthquake, or just really any kind of large explosion, an explosion or anything that really essentially starts sending energy through the rock on Earth, or really through Earth itself. Now, there's two fundamentally different types of seismic waves. And we're going to focus on one more than the other."}, {"video_title": "Seismic waves Earth geological and climatic history Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And just to be clear, seismic waves, they're normally associated with earthquakes. But there are any waves that travel through the Earth that could be due to an earthquake, or just really any kind of large explosion, an explosion or anything that really essentially starts sending energy through the rock on Earth, or really through Earth itself. Now, there's two fundamentally different types of seismic waves. And we're going to focus on one more than the other. One is surface waves, and the other is body waves. Now, surface waves are ones that literally travel across the surface of something. In this case, we're talking about the surface of the ground."}, {"video_title": "Seismic waves Earth geological and climatic history Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And we're going to focus on one more than the other. One is surface waves, and the other is body waves. Now, surface waves are ones that literally travel across the surface of something. In this case, we're talking about the surface of the ground. And this right here is a depiction of surface waves. And these really are more analogous to the type of waves we normally associate with the surface of water. And there's two types of surface waves, rally waves and love waves."}, {"video_title": "Seismic waves Earth geological and climatic history Cosmology & Astronomy Khan Academy.mp3", "Sentence": "In this case, we're talking about the surface of the ground. And this right here is a depiction of surface waves. And these really are more analogous to the type of waves we normally associate with the surface of water. And there's two types of surface waves, rally waves and love waves. We won't go into a lot of details. But you can see that rally waves are kind of the ground moving up and down. Here the ground is moving up."}, {"video_title": "Seismic waves Earth geological and climatic history Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And there's two types of surface waves, rally waves and love waves. We won't go into a lot of details. But you can see that rally waves are kind of the ground moving up and down. Here the ground is moving up. Here it's moving down. Here it's moving up. Here it's moving down."}, {"video_title": "Seismic waves Earth geological and climatic history Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Here the ground is moving up. Here it's moving down. Here it's moving up. Here it's moving down. So you can kind of view it as kind of a ground roll. The love waves are essentially the ground shifting left and right. So here it's not moving up and down, but here it's moving."}, {"video_title": "Seismic waves Earth geological and climatic history Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Here it's moving down. So you can kind of view it as kind of a ground roll. The love waves are essentially the ground shifting left and right. So here it's not moving up and down, but here it's moving. If you're facing the direction of the wave movement, it's moving to the left here. Here it's moving to the right. Here it's moving to the left."}, {"video_title": "Seismic waves Earth geological and climatic history Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So here it's not moving up and down, but here it's moving. If you're facing the direction of the wave movement, it's moving to the left here. Here it's moving to the right. Here it's moving to the left. Here it's moving to the right. In both cases, the movement of the surface wave is perpendicular to the direction of motion. So we sometimes call these transverse waves."}, {"video_title": "Seismic waves Earth geological and climatic history Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Here it's moving to the left. Here it's moving to the right. In both cases, the movement of the surface wave is perpendicular to the direction of motion. So we sometimes call these transverse waves. And these are essentially analogous to, as I said, kind of what we see in water waves. Now the more interesting thing are the body waves. Because the body waves, first of all, they're the fastest moving waves."}, {"video_title": "Seismic waves Earth geological and climatic history Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So we sometimes call these transverse waves. And these are essentially analogous to, as I said, kind of what we see in water waves. Now the more interesting thing are the body waves. Because the body waves, first of all, they're the fastest moving waves. And these are also the waves that are used to figure out the structure of the Earth. So the body waves come in two varieties. You have your P waves or primary waves."}, {"video_title": "Seismic waves Earth geological and climatic history Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Because the body waves, first of all, they're the fastest moving waves. And these are also the waves that are used to figure out the structure of the Earth. So the body waves come in two varieties. You have your P waves or primary waves. You have your P waves, and you have your S waves, or secondary waves. And they're depicted right over here. And this is actually energy that's being transferred through a body."}, {"video_title": "Seismic waves Earth geological and climatic history Cosmology & Astronomy Khan Academy.mp3", "Sentence": "You have your P waves or primary waves. You have your P waves, and you have your S waves, or secondary waves. And they're depicted right over here. And this is actually energy that's being transferred through a body. So it's not just moving along the surface of one. And so here in this diagram that I got from Wikipedia, which I think Wikipedia got from the US Geological Survey, we have a hammer being hit on some rock or whatever. And what you see is right when the hammer gets hit at this end of the rock, and I can zoom in a little bit."}, {"video_title": "Seismic waves Earth geological and climatic history Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And this is actually energy that's being transferred through a body. So it's not just moving along the surface of one. And so here in this diagram that I got from Wikipedia, which I think Wikipedia got from the US Geological Survey, we have a hammer being hit on some rock or whatever. And what you see is right when the hammer gets hit at this end of the rock, and I can zoom in a little bit. So let's say I have this rock over here. And I hit it right over here with a hammer or something. What that's immediately going to do is it's going to compress the rock that the hammer comes in touch with."}, {"video_title": "Seismic waves Earth geological and climatic history Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And what you see is right when the hammer gets hit at this end of the rock, and I can zoom in a little bit. So let's say I have this rock over here. And I hit it right over here with a hammer or something. What that's immediately going to do is it's going to compress the rock that the hammer comes in touch with. It's going to compress that rock. But then that energy, essentially the molecules, are going to bump into the adjacent molecules. And then those adjacent molecules are then going to bump into the molecules right next to it."}, {"video_title": "Seismic waves Earth geological and climatic history Cosmology & Astronomy Khan Academy.mp3", "Sentence": "What that's immediately going to do is it's going to compress the rock that the hammer comes in touch with. It's going to compress that rock. But then that energy, essentially the molecules, are going to bump into the adjacent molecules. And then those adjacent molecules are then going to bump into the molecules right next to it. And then they're going to bump into the molecules right next to it. So you're going to have this kind of compressed part of rock moving through the wave. So these are compressed."}, {"video_title": "Seismic waves Earth geological and climatic history Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And then those adjacent molecules are then going to bump into the molecules right next to it. And then they're going to bump into the molecules right next to it. So you're going to have this kind of compressed part of rock moving through the wave. So these are compressed. And those molecules are going to bump into the adjacent molecules. So immediately after that, the rock will be denser right over here. The first things that were bumped, those will essentially bump into the ones right above them."}, {"video_title": "Seismic waves Earth geological and climatic history Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So these are compressed. And those molecules are going to bump into the adjacent molecules. So immediately after that, the rock will be denser right over here. The first things that were bumped, those will essentially bump into the ones right above them. And then they will move back to where they were. And so now the compression will have moved. And if you fast forward, it will have moved a little bit forward."}, {"video_title": "Seismic waves Earth geological and climatic history Cosmology & Astronomy Khan Academy.mp3", "Sentence": "The first things that were bumped, those will essentially bump into the ones right above them. And then they will move back to where they were. And so now the compression will have moved. And if you fast forward, it will have moved a little bit forward. So you essentially have this compression wave. You hit the hammer here. And you essentially have a changing density that is moving in the same direction of the wave."}, {"video_title": "Seismic waves Earth geological and climatic history Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And if you fast forward, it will have moved a little bit forward. So you essentially have this compression wave. You hit the hammer here. And you essentially have a changing density that is moving in the same direction of the wave. In this situation, that is the direction of the wave. And you see that the molecules are kind of going back and forth along that same axis. They're going along the same direction as the wave."}, {"video_title": "Seismic waves Earth geological and climatic history Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And you essentially have a changing density that is moving in the same direction of the wave. In this situation, that is the direction of the wave. And you see that the molecules are kind of going back and forth along that same axis. They're going along the same direction as the wave. So those are P waves. And P waves can travel through air. That's what essentially sound waves are, compression waves."}, {"video_title": "Seismic waves Earth geological and climatic history Cosmology & Astronomy Khan Academy.mp3", "Sentence": "They're going along the same direction as the wave. So those are P waves. And P waves can travel through air. That's what essentially sound waves are, compression waves. They can travel through liquid. And they can obviously travel through solids. And depending on the air, they'll travel the slowest."}, {"video_title": "Seismic waves Earth geological and climatic history Cosmology & Astronomy Khan Academy.mp3", "Sentence": "That's what essentially sound waves are, compression waves. They can travel through liquid. And they can obviously travel through solids. And depending on the air, they'll travel the slowest. They'll essentially move at the speed of sound, 330 meters per second, which isn't really slow by everyday human standards. In a liquid, they'll move about 1,500 meters per second. And then in granite, which is most of the crustal material of the Earth, they'll move at around 5,000 meters per second."}, {"video_title": "Seismic waves Earth geological and climatic history Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And depending on the air, they'll travel the slowest. They'll essentially move at the speed of sound, 330 meters per second, which isn't really slow by everyday human standards. In a liquid, they'll move about 1,500 meters per second. And then in granite, which is most of the crustal material of the Earth, they'll move at around 5,000 meters per second. Let me write that down. So 5,000 meters per second, or essentially 5 kilometers per second if they're moving through granite. Now, S waves are essentially if you were to hit a hammer on the side of this rock."}, {"video_title": "Seismic waves Earth geological and climatic history Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And then in granite, which is most of the crustal material of the Earth, they'll move at around 5,000 meters per second. Let me write that down. So 5,000 meters per second, or essentially 5 kilometers per second if they're moving through granite. Now, S waves are essentially if you were to hit a hammer on the side of this rock. So let me draw another diagram since this is pretty small. If you were to hit a hammer right over here, what it would do is it would temporarily kind of push all the rock over here. It would deform it a little bit."}, {"video_title": "Seismic waves Earth geological and climatic history Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Now, S waves are essentially if you were to hit a hammer on the side of this rock. So let me draw another diagram since this is pretty small. If you were to hit a hammer right over here, what it would do is it would temporarily kind of push all the rock over here. It would deform it a little bit. And that would pull a little bit of the rock back with it. And then this rock that's right above it would slowly be pulled down while this rock that was initially hit will be moved back up. So you fast forward maybe a millisecond."}, {"video_title": "Seismic waves Earth geological and climatic history Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It would deform it a little bit. And that would pull a little bit of the rock back with it. And then this rock that's right above it would slowly be pulled down while this rock that was initially hit will be moved back up. So you fast forward maybe a millisecond. And now the next layer of rock right above that will be kind of deformed to the right. And if you keep fast forwarding it, the deformation will move upwards. And notice over here, once again, the movement of the wave is upwards."}, {"video_title": "Seismic waves Earth geological and climatic history Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So you fast forward maybe a millisecond. And now the next layer of rock right above that will be kind of deformed to the right. And if you keep fast forwarding it, the deformation will move upwards. And notice over here, once again, the movement of the wave is upwards. But now the movement of the material is not going along the same axis that we saw with the P waves or the compression waves. It's now moving perpendicular. It's now moving along a perpendicular axis."}, {"video_title": "Seismic waves Earth geological and climatic history Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And notice over here, once again, the movement of the wave is upwards. But now the movement of the material is not going along the same axis that we saw with the P waves or the compression waves. It's now moving perpendicular. It's now moving along a perpendicular axis. Or you could call this a transverse wave. The movement of the particles is now on a perpendicular axis to the actual movement of the waves. And so that's what an S wave is."}, {"video_title": "Seismic waves Earth geological and climatic history Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It's now moving along a perpendicular axis. Or you could call this a transverse wave. The movement of the particles is now on a perpendicular axis to the actual movement of the waves. And so that's what an S wave is. And they move a little bit slower than the P waves. So if an earthquake were to happen, you would see the P waves first. And then at about 60% of the speed of the P waves, you would see the S waves."}, {"video_title": "Seismic waves Earth geological and climatic history Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And so that's what an S wave is. And they move a little bit slower than the P waves. So if an earthquake were to happen, you would see the P waves first. And then at about 60% of the speed of the P waves, you would see the S waves. Now the most important thing to think about, especially from the point of view of figuring out the composition of the Earth, is that the S waves can only travel through solid. And you might say, wait, I've seen transverse waves on water that look like this. Remember, that is a surface wave."}, {"video_title": "Seismic waves Earth geological and climatic history Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And then at about 60% of the speed of the P waves, you would see the S waves. Now the most important thing to think about, especially from the point of view of figuring out the composition of the Earth, is that the S waves can only travel through solid. And you might say, wait, I've seen transverse waves on water that look like this. Remember, that is a surface wave. We're talking about body waves. We're talking about things that are actually going through the body of water. And one way to think about this is if I had some water over here."}, {"video_title": "Seismic waves Earth geological and climatic history Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Remember, that is a surface wave. We're talking about body waves. We're talking about things that are actually going through the body of water. And one way to think about this is if I had some water over here. So let's say that this is a pool. I'll draw a cross section of water. If I have a cross section of water right over here, let's think about it."}, {"video_title": "Seismic waves Earth geological and climatic history Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And one way to think about this is if I had some water over here. So let's say that this is a pool. I'll draw a cross section of water. If I have a cross section of water right over here, let's think about it. And hopefully it'll make intuitive sense to you. If I were to compress some of the water, if I were to kind of slam some part of the water here with like a big, I don't know, some type of, I would just compress it really fast, it would do, a P wave could transmit. Because those water molecules would bump into the water molecules next to it, which would bump into the water molecules next to that."}, {"video_title": "Seismic waves Earth geological and climatic history Cosmology & Astronomy Khan Academy.mp3", "Sentence": "If I have a cross section of water right over here, let's think about it. And hopefully it'll make intuitive sense to you. If I were to compress some of the water, if I were to kind of slam some part of the water here with like a big, I don't know, some type of, I would just compress it really fast, it would do, a P wave could transmit. Because those water molecules would bump into the water molecules next to it, which would bump into the water molecules next to that. And so you would have a compression wave or a P wave moving in the direction of my bump. So P waves, it makes sense. And the same thing is true with air or sound waves."}, {"video_title": "Seismic waves Earth geological and climatic history Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Because those water molecules would bump into the water molecules next to it, which would bump into the water molecules next to that. And so you would have a compression wave or a P wave moving in the direction of my bump. So P waves, it makes sense. And the same thing is true with air or sound waves. That it makes sense that it could travel through a liquid. But let's say that you, let's say that if, and remember, we're under the water. We're not thinking about the surface."}, {"video_title": "Seismic waves Earth geological and climatic history Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And the same thing is true with air or sound waves. That it makes sense that it could travel through a liquid. But let's say that you, let's say that if, and remember, we're under the water. We're not thinking about the surface. We're thinking about moving through the body of the water. Let's say that you were to kind of take that hammer, if you were to take a hammer and kind of slap the side of this little volume of water here. Well, essentially all that would do, it would send a compression wave in that direction."}, {"video_title": "Seismic waves Earth geological and climatic history Cosmology & Astronomy Khan Academy.mp3", "Sentence": "We're not thinking about the surface. We're thinking about moving through the body of the water. Let's say that you were to kind of take that hammer, if you were to take a hammer and kind of slap the side of this little volume of water here. Well, essentially all that would do, it would send a compression wave in that direction. It really wouldn't do anything. It wouldn't allow a transverse wave to go that way because it's not going to, the water doesn't allow it to kind of, it doesn't have this elastic property where if something bounces that way, it's going to immediately bounce back that way. It's not being pulled back like a solid would."}, {"video_title": "Cosmic background radiation Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Let's think a little bit about what the Big Bang theory suggests, and then based on the theory, what we should be observing today. So the Big Bang starts with all of the mass and space in the universe, an infinitely dense singularity. The singularity is just something that the math doesn't even apply to. We don't even know how to understand that. But immediately after the Big Bang, so this occurred 13.7 billion years ago, immediately after this little tiny, infinitely small singularity begins to expand. And so for the first 100,000 years, it's still pretty dense. So let me just show this right now."}, {"video_title": "Cosmic background radiation Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "We don't even know how to understand that. But immediately after the Big Bang, so this occurred 13.7 billion years ago, immediately after this little tiny, infinitely small singularity begins to expand. And so for the first 100,000 years, it's still pretty dense. So let me just show this right now. So then it starts to expand. So maybe it gets to this level right over here. And I do not know if the entire universe is infinite or finite, whether it's a four-dimensional sphere, or whether it goes infinitely in every direction, or whether it's just slightly curved here and there and maybe flat everywhere else."}, {"video_title": "Cosmic background radiation Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So let me just show this right now. So then it starts to expand. So maybe it gets to this level right over here. And I do not know if the entire universe is infinite or finite, whether it's a four-dimensional sphere, or whether it goes infinitely in every direction, or whether it's just slightly curved here and there and maybe flat everywhere else. I won't go into all of that. But then it starts to expand a little bit from the singularity, but it's still extremely dense. So dense that atoms can't even form."}, {"video_title": "Cosmic background radiation Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And I do not know if the entire universe is infinite or finite, whether it's a four-dimensional sphere, or whether it goes infinitely in every direction, or whether it's just slightly curved here and there and maybe flat everywhere else. I won't go into all of that. But then it starts to expand a little bit from the singularity, but it's still extremely dense. So dense that atoms can't even form. So you just have the basic fundamental building blocks of atoms. They're just all flying around. Electrons and protons, they're just flying around in just this ultra-hot, white-hot, I could say, or maybe even white-hot plasma."}, {"video_title": "Cosmic background radiation Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So dense that atoms can't even form. So you just have the basic fundamental building blocks of atoms. They're just all flying around. Electrons and protons, they're just flying around in just this ultra-hot, white-hot, I could say, or maybe even white-hot plasma. So this is, I'll call it white-hot plasma. And then if we fast forward a little bit more, and now this is a point that we think we understand well, but this number, I actually looked at some old physics books, and this number has changed in really the last 15, 20 years. So maybe it'll change more."}, {"video_title": "Cosmic background radiation Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Electrons and protons, they're just flying around in just this ultra-hot, white-hot, I could say, or maybe even white-hot plasma. So this is, I'll call it white-hot plasma. And then if we fast forward a little bit more, and now this is a point that we think we understand well, but this number, I actually looked at some old physics books, and this number has changed in really the last 15, 20 years. So maybe it'll change more. But after 380,000 years from the beginning of the Big Bang, and obviously this is give or take a couple of years, the universe expands enough. The universe is now large enough, and obviously I'm not drawing things to scale, and sparse enough that it can cool down a little bit. You don't have as much bumping around."}, {"video_title": "Cosmic background radiation Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So maybe it'll change more. But after 380,000 years from the beginning of the Big Bang, and obviously this is give or take a couple of years, the universe expands enough. The universe is now large enough, and obviously I'm not drawing things to scale, and sparse enough that it can cool down a little bit. You don't have as much bumping around. It's still a hot place, but now it cools down enough that electrons can be captured by protons, and you could actually have the first hydrogen atoms can begin to form. The first hydrogen atoms begin to form. They actually condense."}, {"video_title": "Cosmic background radiation Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "You don't have as much bumping around. It's still a hot place, but now it cools down enough that electrons can be captured by protons, and you could actually have the first hydrogen atoms can begin to form. The first hydrogen atoms begin to form. They actually condense. And we estimate this temperature to be around 3,000 Kelvin. So we've cooled the 3,000 Kelvin, but this is still a temperature that you would not want to hang out in. It's still extremely, extremely hot."}, {"video_title": "Cosmic background radiation Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "They actually condense. And we estimate this temperature to be around 3,000 Kelvin. So we've cooled the 3,000 Kelvin, but this is still a temperature that you would not want to hang out in. It's still extremely, extremely hot. Now why is this moment important, the first atoms forming? So let's think about what's happening here. You have all of this bumping and interactions."}, {"video_title": "Cosmic background radiation Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It's still extremely, extremely hot. Now why is this moment important, the first atoms forming? So let's think about what's happening here. You have all of this bumping and interactions. And if because of a bump or some energy release, or because of the heat temperature, if a photon is released, it'll be immediately absorbed by something else. If something gets, if some energy gets released, it'll immediately be absorbed by something else, because the universe is so dense, especially with charged particles. Here, all of a sudden, it's not that dense."}, {"video_title": "Cosmic background radiation Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "You have all of this bumping and interactions. And if because of a bump or some energy release, or because of the heat temperature, if a photon is released, it'll be immediately absorbed by something else. If something gets, if some energy gets released, it'll immediately be absorbed by something else, because the universe is so dense, especially with charged particles. Here, all of a sudden, it's not that dense. So over here, things that were being emitted could not travel long distances. They would immediately bump into something else. While you go over here, and the universe is starting to look like the universe we recognize."}, {"video_title": "Cosmic background radiation Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Here, all of a sudden, it's not that dense. So over here, things that were being emitted could not travel long distances. They would immediately bump into something else. While you go over here, and the universe is starting to look like the universe we recognize. All of a sudden, if one of these really hot, and it's still nowhere near as hot as this universe over here, but if one of these hot atoms emits a photon, and they would because they are at 3,000 Kelvin, if they emit a photon, all of a sudden, there's actually space for that photon to travel. So for the first time in the history of the universe, 380,000 years after the Big Bang, you now have photons. You now have electromagnetic radiation."}, {"video_title": "Cosmic background radiation Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "While you go over here, and the universe is starting to look like the universe we recognize. All of a sudden, if one of these really hot, and it's still nowhere near as hot as this universe over here, but if one of these hot atoms emits a photon, and they would because they are at 3,000 Kelvin, if they emit a photon, all of a sudden, there's actually space for that photon to travel. So for the first time in the history of the universe, 380,000 years after the Big Bang, you now have photons. You now have electromagnetic radiation. You now have information that can travel over long, long distances. So given that this happened, it's still roughly 13.7 billion years ago. 380,000 years is not a lot when you talk about 13.7."}, {"video_title": "Cosmic background radiation Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "You now have electromagnetic radiation. You now have information that can travel over long, long distances. So given that this happened, it's still roughly 13.7 billion years ago. 380,000 years is not a lot when you talk about 13.7. It still wouldn't even really change the dial, because we're talking in the hundreds of thousands. 0.7 is 700 million years. So this is actually a very small number."}, {"video_title": "Cosmic background radiation Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "380,000 years is not a lot when you talk about 13.7. It still wouldn't even really change the dial, because we're talking in the hundreds of thousands. 0.7 is 700 million years. So this is actually a very small number. So it's still approximately 13.7 billion. It's really 13.7 minus 380,000 years. But given that this was the first time that information could travel, that photons could travel through space without most of them having to bump into something, especially something that's probably charged."}, {"video_title": "Cosmic background radiation Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So this is actually a very small number. So it's still approximately 13.7 billion. It's really 13.7 minus 380,000 years. But given that this was the first time that information could travel, that photons could travel through space without most of them having to bump into something, especially something that's probably charged. The other interesting thing is that these atoms that formed are now neutral. What could we expect to see today? Well, let's think about it."}, {"video_title": "Cosmic background radiation Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But given that this was the first time that information could travel, that photons could travel through space without most of them having to bump into something, especially something that's probably charged. The other interesting thing is that these atoms that formed are now neutral. What could we expect to see today? Well, let's think about it. These photons were emitted 13.7 billion years ago. And they were emitted from every point in the universe. So this is every point in the universe."}, {"video_title": "Cosmic background radiation Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Well, let's think about it. These photons were emitted 13.7 billion years ago. And they were emitted from every point in the universe. So this is every point in the universe. The universe was a pretty uniform place at that time. Very minor irregularities, but you could see, because it was this white hot thing, just began to condense. It hadn't formed a lot of the structures that we now associate with the universe."}, {"video_title": "Cosmic background radiation Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So this is every point in the universe. The universe was a pretty uniform place at that time. Very minor irregularities, but you could see, because it was this white hot thing, just began to condense. It hadn't formed a lot of the structures that we now associate with the universe. It was just kind of a fairly uniform spread of, at that time, reasonably hot hydrogen atoms. So this is every point in the universe. So let's think about what's going on here."}, {"video_title": "Cosmic background radiation Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It hadn't formed a lot of the structures that we now associate with the universe. It was just kind of a fairly uniform spread of, at that time, reasonably hot hydrogen atoms. So this is every point in the universe. So let's think about what's going on here. Let me draw another diagram. So we're talking about this point in the universe right over here. The universe is, even 380,000 years after the Big Bang, still much, much, much, much smaller than the universe today."}, {"video_title": "Cosmic background radiation Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So let's think about what's going on here. Let me draw another diagram. So we're talking about this point in the universe right over here. The universe is, even 380,000 years after the Big Bang, still much, much, much, much smaller than the universe today. But let's say that this is the point in the universe where we happen to be now. At this point in time, there was no Earth, there was no solar system, there was no Milky Way. It was just a bunch of hot hydrogen atoms."}, {"video_title": "Cosmic background radiation Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "The universe is, even 380,000 years after the Big Bang, still much, much, much, much smaller than the universe today. But let's say that this is the point in the universe where we happen to be now. At this point in time, there was no Earth, there was no solar system, there was no Milky Way. It was just a bunch of hot hydrogen atoms. Now, if we were at this point in the universe, there must have been points in the universe at that exact same time that were emitting this radiation. And there were actually every point in the universe was emitting this radiation. The point in the universe where we are now is emitting this radiation."}, {"video_title": "Cosmic background radiation Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It was just a bunch of hot hydrogen atoms. Now, if we were at this point in the universe, there must have been points in the universe at that exact same time that were emitting this radiation. And there were actually every point in the universe was emitting this radiation. The point in the universe where we are now is emitting this radiation. And some of that radiation, so the points that were closer to us, it was emitting that radiation, but it got to us much sooner. It got to us billions of years ago. But there were some points that were far enough that that radiation must be getting to us right now."}, {"video_title": "Cosmic background radiation Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "The point in the universe where we are now is emitting this radiation. And some of that radiation, so the points that were closer to us, it was emitting that radiation, but it got to us much sooner. It got to us billions of years ago. But there were some points that were far enough that that radiation must be getting to us right now. Or another way to think about it is that radiation has taken 13.7 billion years to reach us. So if I were to draw the visible universe today, and you know from the video about the size, so it's not going to be a scale, it would have to be far, far larger than the circle I drew here. But let's say that this is the visible universe today."}, {"video_title": "Cosmic background radiation Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But there were some points that were far enough that that radiation must be getting to us right now. Or another way to think about it is that radiation has taken 13.7 billion years to reach us. So if I were to draw the visible universe today, and you know from the video about the size, so it's not going to be a scale, it would have to be far, far larger than the circle I drew here. But let's say that this is the visible universe today. We should be receiving, and we're in the center of it because we can only look roughly the same distance in every direction. We're not the center of the universe. I want to be clear."}, {"video_title": "Cosmic background radiation Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But let's say that this is the visible universe today. We should be receiving, and we're in the center of it because we can only look roughly the same distance in every direction. We're not the center of the universe. I want to be clear. We're the center of the observable universe because we can only observe the same distance in all directions. Now, we're receiving some light from 100,000 light years away, and then we're looking 100,000 years in the past. We should be receiving some light, maybe a million light years that was first emitted a million light years before."}, {"video_title": "Cosmic background radiation Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "I want to be clear. We're the center of the observable universe because we can only observe the same distance in all directions. Now, we're receiving some light from 100,000 light years away, and then we're looking 100,000 years in the past. We should be receiving some light, maybe a million light years that was first emitted a million light years before. And that's like looking a million years in the past because the light we see was emitted a million years ago. I think that's a bit redundant. We could see light that was emitted that's just getting to us after traveling for a billion years."}, {"video_title": "Cosmic background radiation Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "We should be receiving some light, maybe a million light years that was first emitted a million light years before. And that's like looking a million years in the past because the light we see was emitted a million years ago. I think that's a bit redundant. We could see light that was emitted that's just getting to us after traveling for a billion years. And so we're actually looking at those objects a billion years ago because that's when they emitted the light. So the same way, we can look at objects that emitted their light 13.7 billion years ago, right at the beginning, right at this stage over here, right after 380,000 years after the Big Bang. And so since that light is only just reaching us, we will see it as it was 13.7 billion years ago."}, {"video_title": "Cosmic background radiation Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "We could see light that was emitted that's just getting to us after traveling for a billion years. And so we're actually looking at those objects a billion years ago because that's when they emitted the light. So the same way, we can look at objects that emitted their light 13.7 billion years ago, right at the beginning, right at this stage over here, right after 380,000 years after the Big Bang. And so since that light is only just reaching us, we will see it as it was 13.7 billion years ago. So we should see this type of radiation. Now, the other thing to remember, the universe was expanding. When this was emitted, the universe was expanding."}, {"video_title": "Cosmic background radiation Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And so since that light is only just reaching us, we will see it as it was 13.7 billion years ago. So we should see this type of radiation. Now, the other thing to remember, the universe was expanding. When this was emitted, the universe was expanding. The universe was expanding at a very, well, it's all relative, what's a fast rate and all of that, but it was expanding. And we learned on the video in Redshift that when the source of the light is moving away from you, or the source of the electromagnetic radiation is moving away from you, the radiation itself gets redshifted. So even though this is at a relatively high frequency, you could almost imagine it was kind of red hot gas, it was at 3,000 Kelvin, because it was moving away from us, these things, and we learned in the video on the actual size of the observable universe."}, {"video_title": "Cosmic background radiation Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "When this was emitted, the universe was expanding. The universe was expanding at a very, well, it's all relative, what's a fast rate and all of that, but it was expanding. And we learned on the video in Redshift that when the source of the light is moving away from you, or the source of the electromagnetic radiation is moving away from you, the radiation itself gets redshifted. So even though this is at a relatively high frequency, you could almost imagine it was kind of red hot gas, it was at 3,000 Kelvin, because it was moving away from us, these things, and we learned in the video on the actual size of the observable universe. Even though these electromagnetic waves are taking 13.7 billion years to reach us, in that time, this point in space, the point in space that emitted those electromagnetic waves, are about 46 billion light years away, so that's our best estimate. So this is still stretching away. So theory, if you believe all of this, that this was about 3,000 Kelvin, and it gets redshifted, theory would have it that we should see not something analogous to electromagnetic waves being released from a 3,000 degree temperature atom, we should see something redshifted into the radio spectrum."}, {"video_title": "Cosmic background radiation Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So even though this is at a relatively high frequency, you could almost imagine it was kind of red hot gas, it was at 3,000 Kelvin, because it was moving away from us, these things, and we learned in the video on the actual size of the observable universe. Even though these electromagnetic waves are taking 13.7 billion years to reach us, in that time, this point in space, the point in space that emitted those electromagnetic waves, are about 46 billion light years away, so that's our best estimate. So this is still stretching away. So theory, if you believe all of this, that this was about 3,000 Kelvin, and it gets redshifted, theory would have it that we should see not something analogous to electromagnetic waves being released from a 3,000 degree temperature atom, we should see something redshifted into the radio spectrum. So we should be observing radio waves. And the reason why we're observing radio waves, and not something of a higher frequency, is because it got redshifted. It got redshifted down into a lower frequency."}, {"video_title": "Cosmic background radiation Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So theory, if you believe all of this, that this was about 3,000 Kelvin, and it gets redshifted, theory would have it that we should see not something analogous to electromagnetic waves being released from a 3,000 degree temperature atom, we should see something redshifted into the radio spectrum. So we should be observing radio waves. And the reason why we're observing radio waves, and not something of a higher frequency, is because it got redshifted. It got redshifted down into a lower frequency. And remember, we should be seeing it from every point in the universe where the photons have been traveling for 13.7 billion years. We should see it all around us. This is almost a necessity for us to really believe in the current Big Bang theory."}, {"video_title": "Cosmic background radiation Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It got redshifted down into a lower frequency. And remember, we should be seeing it from every point in the universe where the photons have been traveling for 13.7 billion years. We should see it all around us. This is almost a necessity for us to really believe in the current Big Bang theory. And it turns out that we did observe this. And I want to make this very unintuitive, because you look at any other point in the universe, it's non-uniform. Every other point in the universe, you have stars and galaxies."}, {"video_title": "Cosmic background radiation Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "This is almost a necessity for us to really believe in the current Big Bang theory. And it turns out that we did observe this. And I want to make this very unintuitive, because you look at any other point in the universe, it's non-uniform. Every other point in the universe, you have stars and galaxies. These aren't atoms anymore. These are stars and galaxies and whatnot. And so there's some points in the universe where you see a lot of radiation."}, {"video_title": "Cosmic background radiation Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Every other point in the universe, you have stars and galaxies. These aren't atoms anymore. These are stars and galaxies and whatnot. And so there's some points in the universe where you see a lot of radiation. And there's other points in the universe where you see nothing, it's just black. But if this is correct, if this really did happen, we should be able to observe uniform radio waves from every direction around us. And you go more than 360 degrees, we're going in three dimensions, any direction you point an antenna, a radio antenna, you should be receiving these radio waves that were at much higher frequency when they were emitted, they'd been redshifted then, but they were emitted 13.7 billion years ago."}, {"video_title": "Cosmic background radiation Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And so there's some points in the universe where you see a lot of radiation. And there's other points in the universe where you see nothing, it's just black. But if this is correct, if this really did happen, we should be able to observe uniform radio waves from every direction around us. And you go more than 360 degrees, we're going in three dimensions, any direction you point an antenna, a radio antenna, you should be receiving these radio waves that were at much higher frequency when they were emitted, they'd been redshifted then, but they were emitted 13.7 billion years ago. And it turns out in the late 1960s, they did find these radio waves from every direction. And these are called the cosmic microwave background radiation. And it's this in combination, so it's this data that we're getting, this observation, in combination with the fact that the further we look out to galaxies and clusters of galaxies, they all seem to be moving away from us."}, {"video_title": "Cosmic background radiation Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And you go more than 360 degrees, we're going in three dimensions, any direction you point an antenna, a radio antenna, you should be receiving these radio waves that were at much higher frequency when they were emitted, they'd been redshifted then, but they were emitted 13.7 billion years ago. And it turns out in the late 1960s, they did find these radio waves from every direction. And these are called the cosmic microwave background radiation. And it's this in combination, so it's this data that we're getting, this observation, in combination with the fact that the further we look out to galaxies and clusters of galaxies, they all seem to be moving away from us. They're all redshifted. And they get redshifted more and more the further we look out. So this and everything being redshifted away from us are the best two points of evidence for the actual Big Bang, so hopefully you found that reasonably interesting."}, {"video_title": "Apsidal precession (perihelion precession) and Milankovitch cycles Khan Academy.mp3", "Sentence": "And so if our rotational axis is pointed in a different direction after a long enough time, then the absolute point in our orbit, if we use the sun as our frame of reference, the point in our orbit when we are most pointed away from the sun, or when the Northern Hemisphere is most pointed away from the sun, will be earlier in the orbit. Now I emphasize that that won't necessarily mean earlier in our calendar, because our calendar, by definition, takes into consideration, I guess, or it's more based on when we are furthest away from, furthest tilted away from the sun, or furthest tilted towards the sun. So, even though, if we wait 1,800 years, like the example I gave, we will be most tilted away from the sun, we will have our, or the Northern Hemisphere will have its winter, will have its winter equinox at an earlier point in the orbit, according to our calendar, it will still be December 22nd. If our calendar instead was based, and it's not based on this, but if our calendar was based on the exact point in orbit, if our calendar was based on the exact point in orbit, then our year would be about 20-25 minutes longer every year, and then the dates actually would, then the date for the start of winter actually would go back, it would, you know, 1,800 years later, the date of the start of winter would be November 22nd. But that's not how we measure our calendar. Our calendar is actually measured from equinox to equinox, from December 22nd or 21st, there's slight fluctuations depending on the calendar, but that will always be the date that we are most pointed away from the sun. That will not be necessarily the date that we are at this exact position, at this exact position relative to the sun itself."}, {"video_title": "Apsidal precession (perihelion precession) and Milankovitch cycles Khan Academy.mp3", "Sentence": "If our calendar instead was based, and it's not based on this, but if our calendar was based on the exact point in orbit, if our calendar was based on the exact point in orbit, then our year would be about 20-25 minutes longer every year, and then the dates actually would, then the date for the start of winter actually would go back, it would, you know, 1,800 years later, the date of the start of winter would be November 22nd. But that's not how we measure our calendar. Our calendar is actually measured from equinox to equinox, from December 22nd or 21st, there's slight fluctuations depending on the calendar, but that will always be the date that we are most pointed away from the sun. That will not be necessarily the date that we are at this exact position, at this exact position relative to the sun itself. And that's why the actual perihelion does change, because if this is always December 22nd, and if we at first assume that the perihelion is always at the same fixed point in space relative to the sun, although that's not exactly the case, but if we make that assumption, then it will be further and further after that December 22nd, further and further after that time that we are most pointed away from the sun. And that's why you have this kind of pushing back of the perihelion. Now, what I want to add to this video is that the perihelion itself is also changing."}, {"video_title": "Apsidal precession (perihelion precession) and Milankovitch cycles Khan Academy.mp3", "Sentence": "That will not be necessarily the date that we are at this exact position, at this exact position relative to the sun itself. And that's why the actual perihelion does change, because if this is always December 22nd, and if we at first assume that the perihelion is always at the same fixed point in space relative to the sun, although that's not exactly the case, but if we make that assumption, then it will be further and further after that December 22nd, further and further after that time that we are most pointed away from the sun. And that's why you have this kind of pushing back of the perihelion. Now, what I want to add to this video is that the perihelion itself is also changing. So if I draw the sun again, and right now our orbit looks something like this, and I'm going to exaggerate the eccentricity of it, I'm going to exaggerate the eccentricity of it, just so that the perihelion and the aphelion are a little bit clearer. So right now this is the perihelion, this is the aphelion, based on the way I drew it right over there. We drew it in different colors."}, {"video_title": "Apsidal precession (perihelion precession) and Milankovitch cycles Khan Academy.mp3", "Sentence": "Now, what I want to add to this video is that the perihelion itself is also changing. So if I draw the sun again, and right now our orbit looks something like this, and I'm going to exaggerate the eccentricity of it, I'm going to exaggerate the eccentricity of it, just so that the perihelion and the aphelion are a little bit clearer. So right now this is the perihelion, this is the aphelion, based on the way I drew it right over there. We drew it in different colors. I'm not going to show that that's necessarily where Earth is. Perihelion and aphelion. There is also a rotation of this, of the perihelion, and sometimes this is called the precession of the perihelion, or perihelion precession, or apsidal precession."}, {"video_title": "Apsidal precession (perihelion precession) and Milankovitch cycles Khan Academy.mp3", "Sentence": "We drew it in different colors. I'm not going to show that that's necessarily where Earth is. Perihelion and aphelion. There is also a rotation of this, of the perihelion, and sometimes this is called the precession of the perihelion, or perihelion precession, or apsidal precession. These are all very hard to say. And so if we wait several thousands of years, our orbit might look a little bit like this. Our orbit will look like this."}, {"video_title": "Apsidal precession (perihelion precession) and Milankovitch cycles Khan Academy.mp3", "Sentence": "There is also a rotation of this, of the perihelion, and sometimes this is called the precession of the perihelion, or perihelion precession, or apsidal precession. These are all very hard to say. And so if we wait several thousands of years, our orbit might look a little bit like this. Our orbit will look like this. The actual perihelion will have rotated. So our orbit will look like this. The actual ellipse would have rotated a little bit."}, {"video_title": "Apsidal precession (perihelion precession) and Milankovitch cycles Khan Academy.mp3", "Sentence": "Our orbit will look like this. The actual perihelion will have rotated. So our orbit will look like this. The actual ellipse would have rotated a little bit. You wait a little bit longer, it will go, it will look like, it might look like this. And obviously I'm once again talking about over thousands and thousands of years. From a year-to-year basis, you really wouldn't notice the difference."}, {"video_title": "Apsidal precession (perihelion precession) and Milankovitch cycles Khan Academy.mp3", "Sentence": "The actual ellipse would have rotated a little bit. You wait a little bit longer, it will go, it will look like, it might look like this. And obviously I'm once again talking about over thousands and thousands of years. From a year-to-year basis, you really wouldn't notice the difference. But what that does is, is we talked about the axial precession, that this change in direction of our rotational axis, it takes 26,000 years to complete one period. 26,000 years. So 26,000 years from today, our polar axis, if we don't think about our rotational axis, if we aren't too concerned about the actual change in tilt, which there will be some small change in tilt, but 26,000 years from now, our pole will roughly point in the same direction again."}, {"video_title": "Apsidal precession (perihelion precession) and Milankovitch cycles Khan Academy.mp3", "Sentence": "From a year-to-year basis, you really wouldn't notice the difference. But what that does is, is we talked about the axial precession, that this change in direction of our rotational axis, it takes 26,000 years to complete one period. 26,000 years. So 26,000 years from today, our polar axis, if we don't think about our rotational axis, if we aren't too concerned about the actual change in tilt, which there will be some small change in tilt, but 26,000 years from now, our pole will roughly point in the same direction again. We would have completed one whole period of axial precession. However, it does not take 26,000 years for whatever our date of perihelion is today, so it's in January, I actually don't know the exact date, you could look that up, but whatever that date is in January, it will not take 26,000 years for it to be that date again. And it would have taken 26,000 years if the perihelion itself were not changing, if it always stayed fixed over here."}, {"video_title": "Apsidal precession (perihelion precession) and Milankovitch cycles Khan Academy.mp3", "Sentence": "So 26,000 years from today, our polar axis, if we don't think about our rotational axis, if we aren't too concerned about the actual change in tilt, which there will be some small change in tilt, but 26,000 years from now, our pole will roughly point in the same direction again. We would have completed one whole period of axial precession. However, it does not take 26,000 years for whatever our date of perihelion is today, so it's in January, I actually don't know the exact date, you could look that up, but whatever that date is in January, it will not take 26,000 years for it to be that date again. And it would have taken 26,000 years if the perihelion itself were not changing, if it always stayed fixed over here. If we did not have this abscidal precession. But since it is also changing, you can kind of say it is over thousands of years moving in that direction, while our date, our January date, is moving in that direction, they will actually meet sooner, so that the precession will be back on whatever date it is on January, less than 26,000 years from now, and actually the exact time, and I haven't done the calculation, but this is what I've read, is that it will be 21,000 years from now. And then on top of that, if that's not enough for you, that not only is the direction of Earth's rotational axis changing, and the tilt is changing, and that the perihelion and the aphelion are also rotating around, it's also the case that the eccentricity of the orbit itself is changing."}, {"video_title": "Apsidal precession (perihelion precession) and Milankovitch cycles Khan Academy.mp3", "Sentence": "And it would have taken 26,000 years if the perihelion itself were not changing, if it always stayed fixed over here. If we did not have this abscidal precession. But since it is also changing, you can kind of say it is over thousands of years moving in that direction, while our date, our January date, is moving in that direction, they will actually meet sooner, so that the precession will be back on whatever date it is on January, less than 26,000 years from now, and actually the exact time, and I haven't done the calculation, but this is what I've read, is that it will be 21,000 years from now. And then on top of that, if that's not enough for you, that not only is the direction of Earth's rotational axis changing, and the tilt is changing, and that the perihelion and the aphelion are also rotating around, it's also the case that the eccentricity of the orbit itself is changing. So over long periods of time, Earth's orbit becomes more or less eccentric, and we've learned that almost circular has, well if you're circular you have no eccentricity, and then you can become more and more eccentric, which means you're more and more of kind of this flattened out ellipse. And these cycles occur, so these eccentricity cycles occur over approximately 100,000 years. And so to revisit the Milankovitch cycles, and once again this is a theory, we're not sure whether this is necessarily causing our ice ages, or whether this is necessarily a major influence over long-term climate change, but the Milankovitch cycle, or the theory of Milankovitch cycles, is that over long periods of time, if the eccentricity changes enough, and if it coincides with when the perihelion and the seasons also coincide, maybe that's enough to start an ice age, or maybe that's enough to take us out of an ice age."}, {"video_title": "Apsidal precession (perihelion precession) and Milankovitch cycles Khan Academy.mp3", "Sentence": "And then on top of that, if that's not enough for you, that not only is the direction of Earth's rotational axis changing, and the tilt is changing, and that the perihelion and the aphelion are also rotating around, it's also the case that the eccentricity of the orbit itself is changing. So over long periods of time, Earth's orbit becomes more or less eccentric, and we've learned that almost circular has, well if you're circular you have no eccentricity, and then you can become more and more eccentric, which means you're more and more of kind of this flattened out ellipse. And these cycles occur, so these eccentricity cycles occur over approximately 100,000 years. And so to revisit the Milankovitch cycles, and once again this is a theory, we're not sure whether this is necessarily causing our ice ages, or whether this is necessarily a major influence over long-term climate change, but the Milankovitch cycle, or the theory of Milankovitch cycles, is that over long periods of time, if the eccentricity changes enough, and if it coincides with when the perihelion and the seasons also coincide, maybe that's enough to start an ice age, or maybe that's enough to take us out of an ice age. And actually if you want to throw something even more on that, the actual plane of our orbit also changes over time, mainly because of interactions with the outer planets. Anyway, I'll leave you there. I mentioned this is a very complex topic, but hopefully you now have an appreciation of all the different ways our orbit can change, and maybe start to think about how that might affect our weather, although we don't necessarily know how it does it, or whether it even really does affect going into or out of ice ages."}, {"video_title": "Plate tectonics Evidence of plate movement Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And the most obvious one of these is when you look at this kind of little pointy part of South America. And if you have a more detailed map, it really is amazing how well it seems to fit into the Nigerian basin right here in Africa. It looks like at one time this little pointy part was nudged into this part of Africa, that they were actually connected. And if you're a little bit more creative, there are other parts of the world that you can kind of start to see how they might have fit in with each other in the past. And that by itself, that's just a very small clue, but it kind of hints at, well maybe if at one time they were next to each other, or if this was kind of connected, then they've had to move the part at some time. Although it doesn't tell us that it's still moving, or what might have caused the movement. And it definitely doesn't definitively tell us that they even moved."}, {"video_title": "Plate tectonics Evidence of plate movement Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And if you're a little bit more creative, there are other parts of the world that you can kind of start to see how they might have fit in with each other in the past. And that by itself, that's just a very small clue, but it kind of hints at, well maybe if at one time they were next to each other, or if this was kind of connected, then they've had to move the part at some time. Although it doesn't tell us that it's still moving, or what might have caused the movement. And it definitely doesn't definitively tell us that they even moved. Maybe this is just a coincidence that this coast of South America looks very similar to this coast right here of Africa. Now the next clues, which really came over, I would say about the last 60 or 70 years. The first clue is that, okay, if you go to the mid-Atlantic ridge right here, so if you look at the Atlantic Ocean, let me look at this photograph right over here."}, {"video_title": "Plate tectonics Evidence of plate movement Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And it definitely doesn't definitively tell us that they even moved. Maybe this is just a coincidence that this coast of South America looks very similar to this coast right here of Africa. Now the next clues, which really came over, I would say about the last 60 or 70 years. The first clue is that, okay, if you go to the mid-Atlantic ridge right here, so if you look at the Atlantic Ocean, let me look at this photograph right over here. So this is, it's a little, you don't normally see the oceans highlighted like this, so let me make it very clear to you. This right here is South America. This is South America."}, {"video_title": "Plate tectonics Evidence of plate movement Cosmology & Astronomy Khan Academy.mp3", "Sentence": "The first clue is that, okay, if you go to the mid-Atlantic ridge right here, so if you look at the Atlantic Ocean, let me look at this photograph right over here. So this is, it's a little, you don't normally see the oceans highlighted like this, so let me make it very clear to you. This right here is South America. This is South America. This right here is Africa. This right here is North America. So if you actually look at the elevations in the middle of the ocean, people noticed in the middle of the 20th century that, gee, there's a ridge in the middle of the Atlantic Ocean."}, {"video_title": "Plate tectonics Evidence of plate movement Cosmology & Astronomy Khan Academy.mp3", "Sentence": "This is South America. This right here is Africa. This right here is North America. So if you actually look at the elevations in the middle of the ocean, people noticed in the middle of the 20th century that, gee, there's a ridge in the middle of the Atlantic Ocean. There's kind of a mountain ridge that goes straight up the middle of the Atlantic Ocean. So that by itself doesn't tell you that you have these plates that are moving apart, but it is kind of a curious thing to look at. And not only is there a ridge, there's a lot of underwater volcanic activity."}, {"video_title": "Plate tectonics Evidence of plate movement Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So if you actually look at the elevations in the middle of the ocean, people noticed in the middle of the 20th century that, gee, there's a ridge in the middle of the Atlantic Ocean. There's kind of a mountain ridge that goes straight up the middle of the Atlantic Ocean. So that by itself doesn't tell you that you have these plates that are moving apart, but it is kind of a curious thing to look at. And not only is there a ridge, there's a lot of underwater volcanic activity. You have magma flowing out and lava flowing into the water, and it's kind of forming this ridge that really goes across the whole Atlantic Ocean. There are other ridges in the world like that, underwater ridges. You have one over here in the Pacific Ocean."}, {"video_title": "Plate tectonics Evidence of plate movement Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And not only is there a ridge, there's a lot of underwater volcanic activity. You have magma flowing out and lava flowing into the water, and it's kind of forming this ridge that really goes across the whole Atlantic Ocean. There are other ridges in the world like that, underwater ridges. You have one over here in the Pacific Ocean. You have these here in the Indian Ocean. So that by itself, that's just a little clue, but that by itself doesn't explain, doesn't tell you that these plates are actually moving apart at the ridge. The more conclusive, this is just the beginning of the clue, but what made this conclusive is, one, the separate discovery."}, {"video_title": "Plate tectonics Evidence of plate movement Cosmology & Astronomy Khan Academy.mp3", "Sentence": "You have one over here in the Pacific Ocean. You have these here in the Indian Ocean. So that by itself, that's just a little clue, but that by itself doesn't explain, doesn't tell you that these plates are actually moving apart at the ridge. The more conclusive, this is just the beginning of the clue, but what made this conclusive is, one, the separate discovery. This is what's interesting, is that you have these separate discoveries in different domains that eventually let you kind of come to a pretty neat conclusion. So you've had a separate discovery that if you look at different eras of magnetic rock, or maybe I should say different magnetic rock from different periods in geologic time, and you can tell where they are in geologic time by how they're layered. So this would be newer rock, this would be newer, and then this would be a little bit older, and then this would be even older."}, {"video_title": "Plate tectonics Evidence of plate movement Cosmology & Astronomy Khan Academy.mp3", "Sentence": "The more conclusive, this is just the beginning of the clue, but what made this conclusive is, one, the separate discovery. This is what's interesting, is that you have these separate discoveries in different domains that eventually let you kind of come to a pretty neat conclusion. So you've had a separate discovery that if you look at different eras of magnetic rock, or maybe I should say different magnetic rock from different periods in geologic time, and you can tell where they are in geologic time by how they're layered. So this would be newer rock, this would be newer, and then this would be a little bit older, and then this would be even older. Geologists noticed something interesting. If I were to take magnetic rock, and if it was molten lava, and if it were to harden, remember, it's magnetic rock, so it would want to align with the poles the same way a compass would. So if I had a bunch of magnetic, so let's say this is some lava right here, and so the molecules can align themselves."}, {"video_title": "Plate tectonics Evidence of plate movement Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So this would be newer rock, this would be newer, and then this would be a little bit older, and then this would be even older. Geologists noticed something interesting. If I were to take magnetic rock, and if it was molten lava, and if it were to harden, remember, it's magnetic rock, so it would want to align with the poles the same way a compass would. So if I had a bunch of magnetic, so let's say this is some lava right here, and so the molecules can align themselves. Since when it's liquid and they can align themselves, they're going to naturally want to align with the poles. So they'll naturally all want to align in one direction because of Earth's magnetic field. And so when that lava hardens into actual rock, that alignment will kind of be frozen."}, {"video_title": "Plate tectonics Evidence of plate movement Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So if I had a bunch of magnetic, so let's say this is some lava right here, and so the molecules can align themselves. Since when it's liquid and they can align themselves, they're going to naturally want to align with the poles. So they'll naturally all want to align in one direction because of Earth's magnetic field. And so when that lava hardens into actual rock, that alignment will kind of be frozen. Now, if Earth's magnetic field was constant over time, then when you look at magnetic rocks from any period, you would expect them all to be aligned in the same direction. So since we're taking a cross-section of rock here, let's say an alignment towards the North Pole looks like this. I draw it like that, that's kind of an arrow pointing into our screen."}, {"video_title": "Plate tectonics Evidence of plate movement Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And so when that lava hardens into actual rock, that alignment will kind of be frozen. Now, if Earth's magnetic field was constant over time, then when you look at magnetic rocks from any period, you would expect them all to be aligned in the same direction. So since we're taking a cross-section of rock here, let's say an alignment towards the North Pole looks like this. I draw it like that, that's kind of an arrow pointing into our screen. And let's say an alignment pointing to the South Pole would look like this. This would be an arrow pointing out of our screen. So what you would expect is the newer rock, that kind of the alignment of the rock would go into the screen."}, {"video_title": "Plate tectonics Evidence of plate movement Cosmology & Astronomy Khan Academy.mp3", "Sentence": "I draw it like that, that's kind of an arrow pointing into our screen. And let's say an alignment pointing to the South Pole would look like this. This would be an arrow pointing out of our screen. So what you would expect is the newer rock, that kind of the alignment of the rock would go into the screen. And then the older rock, it would still go older into the screen. So if I were to draw a top view, let me draw it like this, just so I make sure that everyone is on the same page. Let me just draw a cross-section like this."}, {"video_title": "Plate tectonics Evidence of plate movement Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So what you would expect is the newer rock, that kind of the alignment of the rock would go into the screen. And then the older rock, it would still go older into the screen. So if I were to draw a top view, let me draw it like this, just so I make sure that everyone is on the same page. Let me just draw a cross-section like this. So we know what we're talking about. Let me draw a cross-section like this. So this is the surface up here."}, {"video_title": "Plate tectonics Evidence of plate movement Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Let me just draw a cross-section like this. So we know what we're talking about. Let me draw a cross-section like this. So this is the surface up here. This up here is the surface. When I talk about going into the page, that means that the magnetic rock would be aligned in that direction. And when I talk about going out of the page, that means that the magnetic rock would be aligned in that direction."}, {"video_title": "Plate tectonics Evidence of plate movement Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So this is the surface up here. This up here is the surface. When I talk about going into the page, that means that the magnetic rock would be aligned in that direction. And when I talk about going out of the page, that means that the magnetic rock would be aligned in that direction. Now, like I said, if the magnetic field of Earth never changed, then lava that essentially turns into hard, cools down into non-lava rock, or you can say freezes into rock, it would all point in the same direction regardless of when it hardened. This would be the situation in a constant magnetic field. But what we've seen is that that's not the case."}, {"video_title": "Plate tectonics Evidence of plate movement Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And when I talk about going out of the page, that means that the magnetic rock would be aligned in that direction. Now, like I said, if the magnetic field of Earth never changed, then lava that essentially turns into hard, cools down into non-lava rock, or you can say freezes into rock, it would all point in the same direction regardless of when it hardened. This would be the situation in a constant magnetic field. But what we've seen is that that's not the case. When you look at older magnetic rock, and depending on how old you go, you have the newer rock that's aligned with our current magnetic field. You go a little bit older, and right now we think it's about 780,000 years ago. Roughly, you have to find rock of that age, magnetic rock that hardened at that time."}, {"video_title": "Plate tectonics Evidence of plate movement Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But what we've seen is that that's not the case. When you look at older magnetic rock, and depending on how old you go, you have the newer rock that's aligned with our current magnetic field. You go a little bit older, and right now we think it's about 780,000 years ago. Roughly, you have to find rock of that age, magnetic rock that hardened at that time. It's actually in the opposite direction. So it's actually the magnetic rock has hardened in a way so that it's pointing. It's as if the North Pole was at the South Pole now, the magnetic North Pole."}, {"video_title": "Plate tectonics Evidence of plate movement Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Roughly, you have to find rock of that age, magnetic rock that hardened at that time. It's actually in the opposite direction. So it's actually the magnetic rock has hardened in a way so that it's pointing. It's as if the North Pole was at the South Pole now, the magnetic North Pole. So it's aligned in the opposite direction. So it's kind of pointing out of the page here. And if you get even older rock, it's more aligned with our traditional direction."}, {"video_title": "Plate tectonics Evidence of plate movement Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It's as if the North Pole was at the South Pole now, the magnetic North Pole. So it's aligned in the opposite direction. So it's kind of pointing out of the page here. And if you get even older rock, it's more aligned with our traditional direction. So it's more aligned than that. And so the only conclusion, the only reasonable conclusion that we can draw from this is that Earth's magnetic field has actually fluctuated over time. Earth's magnetic field fluctuates."}, {"video_title": "Plate tectonics Evidence of plate movement Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And if you get even older rock, it's more aligned with our traditional direction. So it's more aligned than that. And so the only conclusion, the only reasonable conclusion that we can draw from this is that Earth's magnetic field has actually fluctuated over time. Earth's magnetic field fluctuates. Now you're probably thinking, Sal, how is this relevant to plate tectonics? Well, once you accept that magnetic fields fluctuate over the history of the Earth, there's another interesting observation you can make about the rock that's kind of at the basin of the ocean floor. So not only do you have this mid-Atlantic ridge, do you have these volcanoes spewing kind of new rock into the ocean, creating this kind of underwater mountain ridge, but it also turns out that the rock that forms the sea floor also contains a lot of magnetite, which is magnetic."}, {"video_title": "Plate tectonics Evidence of plate movement Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Earth's magnetic field fluctuates. Now you're probably thinking, Sal, how is this relevant to plate tectonics? Well, once you accept that magnetic fields fluctuate over the history of the Earth, there's another interesting observation you can make about the rock that's kind of at the basin of the ocean floor. So not only do you have this mid-Atlantic ridge, do you have these volcanoes spewing kind of new rock into the ocean, creating this kind of underwater mountain ridge, but it also turns out that the rock that forms the sea floor also contains a lot of magnetite, which is magnetic. And what's really interesting about that, so let me draw, so let's say that this is, so we have a top view, just like we have over here. So let's say that this is the mid-Atlantic ridge right here. So mid-Atlantic ridge."}, {"video_title": "Plate tectonics Evidence of plate movement Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So not only do you have this mid-Atlantic ridge, do you have these volcanoes spewing kind of new rock into the ocean, creating this kind of underwater mountain ridge, but it also turns out that the rock that forms the sea floor also contains a lot of magnetite, which is magnetic. And what's really interesting about that, so let me draw, so let's say that this is, so we have a top view, just like we have over here. So let's say that this is the mid-Atlantic ridge right here. So mid-Atlantic ridge. Now this is really cool. So when they look at rocks that are very close to the mid-Atlantic ridge, they're aligned, and once again we're looking at rocks at the floor of the ocean, they're aligned in a way that you would expect with the current magnetic field. They're aligned just like that, the way that you would expect when you're looking at the magnetic rock that's close to the ridge."}, {"video_title": "Plate tectonics Evidence of plate movement Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So mid-Atlantic ridge. Now this is really cool. So when they look at rocks that are very close to the mid-Atlantic ridge, they're aligned, and once again we're looking at rocks at the floor of the ocean, they're aligned in a way that you would expect with the current magnetic field. They're aligned just like that, the way that you would expect when you're looking at the magnetic rock that's close to the ridge. But if you go a little bit further, and when I say a little bit, I'm talking about thousands of miles, but when you go further out from that, you have stripes of other, you have other magnetic rock that is going in the opposite direction. It's going in the opposite direction. It's going like this."}, {"video_title": "Plate tectonics Evidence of plate movement Cosmology & Astronomy Khan Academy.mp3", "Sentence": "They're aligned just like that, the way that you would expect when you're looking at the magnetic rock that's close to the ridge. But if you go a little bit further, and when I say a little bit, I'm talking about thousands of miles, but when you go further out from that, you have stripes of other, you have other magnetic rock that is going in the opposite direction. It's going in the opposite direction. It's going like this. And what's even cooler than the idea that it's switched directions depending on how far you've gone from the rift, is that there's a symmetric stripe of magnetic rock on exactly the same distance, or roughly the same distance away from the rift, that's also pointing in that same direction. And you go a little bit further out, and you'll find some rock that's pointing in the original direction. And even better, you go on the symmetric other side of the actual ridge, and you find another set of rocks that's doing the exact same thing."}, {"video_title": "Plate tectonics Evidence of plate movement Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It's going like this. And what's even cooler than the idea that it's switched directions depending on how far you've gone from the rift, is that there's a symmetric stripe of magnetic rock on exactly the same distance, or roughly the same distance away from the rift, that's also pointing in that same direction. And you go a little bit further out, and you'll find some rock that's pointing in the original direction. And even better, you go on the symmetric other side of the actual ridge, and you find another set of rocks that's doing the exact same thing. So if you accept that Earth's magnetic field has kind of been flip-flopping over time, the only reasonable conclusion, at least that I could think of, or that the geologists can think of, is that, sure, all of this was formed at a similar period in time. This came out as lava, magnetic lava, and then it all aligned with Earth's magnetic field, and that's why it looks similar. You fast-forward in time some, and the only way that these could have formed, and they could have been so similar, so if we rewind in time, the only way that these purple magnetic rocks could have aligned this way, in exactly the same way, at exactly the same distance, is that at some point, they were much closer to each other, if they were actually connected."}, {"video_title": "Plate tectonics Evidence of plate movement Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And even better, you go on the symmetric other side of the actual ridge, and you find another set of rocks that's doing the exact same thing. So if you accept that Earth's magnetic field has kind of been flip-flopping over time, the only reasonable conclusion, at least that I could think of, or that the geologists can think of, is that, sure, all of this was formed at a similar period in time. This came out as lava, magnetic lava, and then it all aligned with Earth's magnetic field, and that's why it looks similar. You fast-forward in time some, and the only way that these could have formed, and they could have been so similar, so if we rewind in time, the only way that these purple magnetic rocks could have aligned this way, in exactly the same way, at exactly the same distance, is that at some point, they were much closer to each other, if they were actually connected. So if we rewind in time, maybe at the mid-Atlantic rift, you had all of the purple rock coming out from those underwater volcanoes, and at that time, Earth's magnetic field was the opposite as it is right now. And then, of course, you had this blue rock that is looking like that. And so this seems like a reasonable explanation."}, {"video_title": "Plate tectonics Evidence of plate movement Cosmology & Astronomy Khan Academy.mp3", "Sentence": "You fast-forward in time some, and the only way that these could have formed, and they could have been so similar, so if we rewind in time, the only way that these purple magnetic rocks could have aligned this way, in exactly the same way, at exactly the same distance, is that at some point, they were much closer to each other, if they were actually connected. So if we rewind in time, maybe at the mid-Atlantic rift, you had all of the purple rock coming out from those underwater volcanoes, and at that time, Earth's magnetic field was the opposite as it is right now. And then, of course, you had this blue rock that is looking like that. And so this seems like a reasonable explanation. This rock and this rock were at some point touching. They were actually formed at the exact place and at the exact same time. And so if this is the case, if at one point, this purple rock was all together, and they formed at the same time at the mid-Atlantic rift, we're assuming all the rock was... Well, we don't have to make that assumption, but if you assume that they formed at the same time, and that based on the pattern, it really does look like they do, and it's a symmetric distance away from that rift, then the only reasonable conclusion I can think of is that the rift has had to move apart."}, {"video_title": "Plate tectonics Evidence of plate movement Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And so this seems like a reasonable explanation. This rock and this rock were at some point touching. They were actually formed at the exact place and at the exact same time. And so if this is the case, if at one point, this purple rock was all together, and they formed at the same time at the mid-Atlantic rift, we're assuming all the rock was... Well, we don't have to make that assumption, but if you assume that they formed at the same time, and that based on the pattern, it really does look like they do, and it's a symmetric distance away from that rift, then the only reasonable conclusion I can think of is that the rift has had to move apart. The rift has had to move apart from this period to that period. And there was a time when all this blue rock was together. So that by itself, that frankly, is the most definitive evidence in the 1960s where it kind of became conclusive that you did have these plates that were moving away from each other."}, {"video_title": "Plate tectonics Evidence of plate movement Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And so if this is the case, if at one point, this purple rock was all together, and they formed at the same time at the mid-Atlantic rift, we're assuming all the rock was... Well, we don't have to make that assumption, but if you assume that they formed at the same time, and that based on the pattern, it really does look like they do, and it's a symmetric distance away from that rift, then the only reasonable conclusion I can think of is that the rift has had to move apart. The rift has had to move apart from this period to that period. And there was a time when all this blue rock was together. So that by itself, that frankly, is the most definitive evidence in the 1960s where it kind of became conclusive that you did have these plates that were moving away from each other. You did have these plates that were moving away from each other. And obviously, if the plates are moving away from each other at some point, and that means that, you know, just based on the way the map looks, at some point, they're also going to be moving into each other. We could talk more about that in future videos."}, {"video_title": "Plate tectonics Evidence of plate movement Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So that by itself, that frankly, is the most definitive evidence in the 1960s where it kind of became conclusive that you did have these plates that were moving away from each other. You did have these plates that were moving away from each other. And obviously, if the plates are moving away from each other at some point, and that means that, you know, just based on the way the map looks, at some point, they're also going to be moving into each other. We could talk more about that in future videos. But, you know, right, at certain points, they're actually moving on one plate, is moving under another. And we'll talk about how that might partially explain, and we'll talk about all of the explanations for why we think the plates might actually be moving. But now if we fast forward to more present times, now that we have GPS satellites and all the rest, we can actually measure the movement of the plates."}, {"video_title": "Plate tectonics Evidence of plate movement Cosmology & Astronomy Khan Academy.mp3", "Sentence": "We could talk more about that in future videos. But, you know, right, at certain points, they're actually moving on one plate, is moving under another. And we'll talk about how that might partially explain, and we'll talk about all of the explanations for why we think the plates might actually be moving. But now if we fast forward to more present times, now that we have GPS satellites and all the rest, we can actually measure the movement of the plates. This is actually an image from NASA showing the vector of the movements at different points on the surface of the planet. And you can see we've gotten a lot of vectors from the United States, so it's almost hard to read since it's so chock full of vectors. But you can see right over here in Hawaii, the Pacific plate at that point is moving in this northwest direction, as measured by GPS satellites."}, {"video_title": "Plate tectonics Evidence of plate movement Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But now if we fast forward to more present times, now that we have GPS satellites and all the rest, we can actually measure the movement of the plates. This is actually an image from NASA showing the vector of the movements at different points on the surface of the planet. And you can see we've gotten a lot of vectors from the United States, so it's almost hard to read since it's so chock full of vectors. But you can see right over here in Hawaii, the Pacific plate at that point is moving in this northwest direction, as measured by GPS satellites. And I want to make clear, this movement is relatively slow. It's roughly the speed at which fingernails grow. But if you do it over millions of years, that actually amounts to thousands of miles."}, {"video_title": "Plate tectonics Evidence of plate movement Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But you can see right over here in Hawaii, the Pacific plate at that point is moving in this northwest direction, as measured by GPS satellites. And I want to make clear, this movement is relatively slow. It's roughly the speed at which fingernails grow. But if you do it over millions of years, that actually amounts to thousands of miles. So we're talking on the order of about a centimeter a year for most of the plates. Some of the plates might be moving a little bit faster, maybe close to 10 or 15 centimeters. But most are moving about a centimeter a year, at the same rate your fingernails are going."}, {"video_title": "Plate tectonics Evidence of plate movement Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But if you do it over millions of years, that actually amounts to thousands of miles. So we're talking on the order of about a centimeter a year for most of the plates. Some of the plates might be moving a little bit faster, maybe close to 10 or 15 centimeters. But most are moving about a centimeter a year, at the same rate your fingernails are going. But this is fascinating because we can actually measure it because GPS is so accurate. Over here it looks like the North American plate is kind of rotating generally in that direction. We have the Nazca plate right here is moving roughly in that direction, moving into the South American plate."}, {"video_title": "Plate tectonics Evidence of plate movement Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But most are moving about a centimeter a year, at the same rate your fingernails are going. But this is fascinating because we can actually measure it because GPS is so accurate. Over here it looks like the North American plate is kind of rotating generally in that direction. We have the Nazca plate right here is moving roughly in that direction, moving into the South American plate. I'll leave you there right now. Actually, before I leave you there, this is another thing that I got off of Wikipedia that shows that same magnetic striping. It's maybe a slightly neater drawing."}, {"video_title": "Plate tectonics Evidence of plate movement Cosmology & Astronomy Khan Academy.mp3", "Sentence": "We have the Nazca plate right here is moving roughly in that direction, moving into the South American plate. I'll leave you there right now. Actually, before I leave you there, this is another thing that I got off of Wikipedia that shows that same magnetic striping. It's maybe a slightly neater drawing. I don't know which one might be more helpful for you. But I'll leave you there in this video. In the next video, we'll think about some of the theories."}, {"video_title": "The Moon.mp3", "Sentence": "When you look at the moon in the night sky, it might seem reasonably close, but it's actually incredibly far away. Right over here is a scaled picture of the distance between the Earth and the moon. Earth has a diameter of approximately 8,000 miles, while the moon has a diameter of approximately 2,200 miles, so a little bit more than 1 4th the diameter of Earth. Now the distance between the two is 239,000 miles, which you can imagine is incredibly far. Now one thing that is very interesting, and this is why the moon looks like it's the same size as the sun in the sky, even though it is 1 4 100th of the diameter, is the ratio between the distance to the moon and the diameter of the moon is roughly the same as the ratio of the distance of the sun to the diameter of the sun. So for the moon, that ratio, the distance to the moon, 239,000, we'll do everything in miles, over the radius of the moon, 2,200, it's approximately the same as the ratio between the distance from Earth to the sun, which would be 93 million miles over 865,000 miles in diameter. So this number is roughly 400 times larger than that number, and this number is roughly 400 times larger than that number, but what's neat is, and this depends on where Earth is in its orbit, but this is approximately equal to 108."}, {"video_title": "The Moon.mp3", "Sentence": "Now the distance between the two is 239,000 miles, which you can imagine is incredibly far. Now one thing that is very interesting, and this is why the moon looks like it's the same size as the sun in the sky, even though it is 1 4 100th of the diameter, is the ratio between the distance to the moon and the diameter of the moon is roughly the same as the ratio of the distance of the sun to the diameter of the sun. So for the moon, that ratio, the distance to the moon, 239,000, we'll do everything in miles, over the radius of the moon, 2,200, it's approximately the same as the ratio between the distance from Earth to the sun, which would be 93 million miles over 865,000 miles in diameter. So this number is roughly 400 times larger than that number, and this number is roughly 400 times larger than that number, but what's neat is, and this depends on where Earth is in its orbit, but this is approximately equal to 108. This gives me goosebumps whenever I think about it, because it didn't have to be this way, that the moon and the sun, even though they have these vastly different diameters, that they look roughly the same size in the sky because the ratio of the distances is comparable to the ratio of their diameters. And this is why the number 108 is actually an auspicious number in some cultures, especially in Hinduism. Now when you're discussing the moon, and especially the moon as viewed from Earth, one of the obvious questions is, why does the moon go through phases?"}, {"video_title": "The Moon.mp3", "Sentence": "So this number is roughly 400 times larger than that number, and this number is roughly 400 times larger than that number, but what's neat is, and this depends on where Earth is in its orbit, but this is approximately equal to 108. This gives me goosebumps whenever I think about it, because it didn't have to be this way, that the moon and the sun, even though they have these vastly different diameters, that they look roughly the same size in the sky because the ratio of the distances is comparable to the ratio of their diameters. And this is why the number 108 is actually an auspicious number in some cultures, especially in Hinduism. Now when you're discussing the moon, and especially the moon as viewed from Earth, one of the obvious questions is, why does the moon go through phases? Why do we see different portions of the moon lit up from day to day? This right over here is a diagram from NASA's website, and what you see here, clearly this is not at scale. This picture over here, this inner picture, the size of the moon and the Earth is roughly at scale, but clearly the distances between them are not."}, {"video_title": "The Moon.mp3", "Sentence": "Now when you're discussing the moon, and especially the moon as viewed from Earth, one of the obvious questions is, why does the moon go through phases? Why do we see different portions of the moon lit up from day to day? This right over here is a diagram from NASA's website, and what you see here, clearly this is not at scale. This picture over here, this inner picture, the size of the moon and the Earth is roughly at scale, but clearly the distances between them are not. You don't see that 239,000 miles between them. In this inner circle, what you see is that the moon and the Earth is always lit up from the right. So it's assuming that the sun is 93 million miles in that direction, and it is lighting up both the Earth and the moon from the right."}, {"video_title": "The Moon.mp3", "Sentence": "This picture over here, this inner picture, the size of the moon and the Earth is roughly at scale, but clearly the distances between them are not. You don't see that 239,000 miles between them. In this inner circle, what you see is that the moon and the Earth is always lit up from the right. So it's assuming that the sun is 93 million miles in that direction, and it is lighting up both the Earth and the moon from the right. Now as the moon rotates around the Earth in its 28-day cycle, and if you're wondering, gee, a 28-day cycle seems awfully close to a month, that is not a coincidence. The notion of a month comes from the cycles of the moon. In fact, even the word month and the word moon have the exact same root in Old English and in Proto-Germanic."}, {"video_title": "The Moon.mp3", "Sentence": "So it's assuming that the sun is 93 million miles in that direction, and it is lighting up both the Earth and the moon from the right. Now as the moon rotates around the Earth in its 28-day cycle, and if you're wondering, gee, a 28-day cycle seems awfully close to a month, that is not a coincidence. The notion of a month comes from the cycles of the moon. In fact, even the word month and the word moon have the exact same root in Old English and in Proto-Germanic. They are essentially the same word. But what you see is the moon goes in this 28-day cycle. When the moon is roughly between the Earth and the sun, the lit up half of the moon is away from what we can see here on Earth, and so we see the non-lit up side, which would be a new moon."}, {"video_title": "The Moon.mp3", "Sentence": "In fact, even the word month and the word moon have the exact same root in Old English and in Proto-Germanic. They are essentially the same word. But what you see is the moon goes in this 28-day cycle. When the moon is roughly between the Earth and the sun, the lit up half of the moon is away from what we can see here on Earth, and so we see the non-lit up side, which would be a new moon. Now as the moon goes, this is viewing from above the Earth, this would be the North Pole right over here, as it goes counterclockwise, as viewed from above the Earth, we start to be able to see a little bit of that half of the moon that gets lit up. So when the moon is in this position, we see us from Earth, from this vantage point, we're able to see a little bit of the lit up side. When the moon is over here, we're able to see half the lit up side and half the non-lit up side, and that's called a first quarter moon."}, {"video_title": "The Moon.mp3", "Sentence": "When the moon is roughly between the Earth and the sun, the lit up half of the moon is away from what we can see here on Earth, and so we see the non-lit up side, which would be a new moon. Now as the moon goes, this is viewing from above the Earth, this would be the North Pole right over here, as it goes counterclockwise, as viewed from above the Earth, we start to be able to see a little bit of that half of the moon that gets lit up. So when the moon is in this position, we see us from Earth, from this vantage point, we're able to see a little bit of the lit up side. When the moon is over here, we're able to see half the lit up side and half the non-lit up side, and that's called a first quarter moon. And that keeps on going. When we're halfway through our cycle here, at a full moon, the Earth is between the moon and the sun. And so from our vantage point, we are able to see the entire lit up side, and that's why it is a full moon."}, {"video_title": "The Moon.mp3", "Sentence": "When the moon is over here, we're able to see half the lit up side and half the non-lit up side, and that's called a first quarter moon. And that keeps on going. When we're halfway through our cycle here, at a full moon, the Earth is between the moon and the sun. And so from our vantage point, we are able to see the entire lit up side, and that's why it is a full moon. And then we keep going all the way until we get back to a new moon. Now one question that might be bothering you, it bothered me for many years, is as soon as I understood this, the cycle of the moon, how the moon has this 28-day cycle as Earth rotates around the sun, I always wondered, well in a new moon, it looks like the moon is between the Earth and the sun. Why doesn't it block out the sun every time we have a new moon?"}, {"video_title": "The Moon.mp3", "Sentence": "And so from our vantage point, we are able to see the entire lit up side, and that's why it is a full moon. And then we keep going all the way until we get back to a new moon. Now one question that might be bothering you, it bothered me for many years, is as soon as I understood this, the cycle of the moon, how the moon has this 28-day cycle as Earth rotates around the sun, I always wondered, well in a new moon, it looks like the moon is between the Earth and the sun. Why doesn't it block out the sun every time we have a new moon? Why don't we have a solar eclipse every 28 days? Similarly, for the full moons, when we're in this position, it looks like the Earth is between the moon and the sun. Why doesn't Earth's shadow block out the sun so that we have a lunar eclipse instead of a full moon, and we would have one of those every 28 days?"}, {"video_title": "The Moon.mp3", "Sentence": "Why doesn't it block out the sun every time we have a new moon? Why don't we have a solar eclipse every 28 days? Similarly, for the full moons, when we're in this position, it looks like the Earth is between the moon and the sun. Why doesn't Earth's shadow block out the sun so that we have a lunar eclipse instead of a full moon, and we would have one of those every 28 days? Why don't we see that? Think about it yourself. What's a plausible explanation?"}, {"video_title": "Quasar correction Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "In that video, I said, and I mistakenly said, that the Cretan disk that's really forming or releasing the energy of the quasar, that's releasing energy predominantly in the X-ray part of the electromagnetic spectrum. And that was incorrect. Most quasars are actually emitting electromagnetic radiation across the spectrum, all the way from X-rays. As high frequency as X-rays, all the way down to infrared. And some quasars even release super high frequency gamma rays, and they'll release low frequency electromagnetic waves all the way down to radio waves. So I just wanted to make that correction. It's not predominantly in the X-ray part of the spectrum."}, {"video_title": "Quasar correction Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "As high frequency as X-rays, all the way down to infrared. And some quasars even release super high frequency gamma rays, and they'll release low frequency electromagnetic waves all the way down to radio waves. So I just wanted to make that correction. It's not predominantly in the X-ray part of the spectrum. It's across the spectrum right over here. It's this entire range of the spectrum, and sometimes even a wider range. Now, the other thing I want to clarify is this is the range of the spectrum that's being emitted."}, {"video_title": "Quasar correction Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It's not predominantly in the X-ray part of the spectrum. It's across the spectrum right over here. It's this entire range of the spectrum, and sometimes even a wider range. Now, the other thing I want to clarify is this is the range of the spectrum that's being emitted. But we have to remember that most, or actually all of these quasars are quite far away. The closest is 780 million light years away. Many of them are many, many billions of light years away."}, {"video_title": "Quasar correction Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Now, the other thing I want to clarify is this is the range of the spectrum that's being emitted. But we have to remember that most, or actually all of these quasars are quite far away. The closest is 780 million light years away. Many of them are many, many billions of light years away. And so they're moving away from us at a very fast speed. Or they're getting redshifted because the universe is expanding so fast relative to us at that point. Or that coordinate is moving so fast away from our coordinate."}, {"video_title": "Quasar correction Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Many of them are many, many billions of light years away. And so they're moving away from us at a very fast speed. Or they're getting redshifted because the universe is expanding so fast relative to us at that point. Or that coordinate is moving so fast away from our coordinate. And so even though this is the spectrum that's being emitted, it's all going to be redshifted. It's all going to be redshifted down. And so we are going to observe things at a much lower frequency, maybe around the radio part of the frequency."}, {"video_title": "Quasar correction Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Or that coordinate is moving so fast away from our coordinate. And so even though this is the spectrum that's being emitted, it's all going to be redshifted. It's all going to be redshifted down. And so we are going to observe things at a much lower frequency, maybe around the radio part of the frequency. So everything will be redshifted down. And that's why these were originally called quasi-stellar radio sources. Anyway, hopefully that clears things up a little bit."}, {"video_title": "First living things on land clarification Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And someone brought up what I thought was a very good question. If the animals were the first, what did they eat? So I thought that that was, one, a good question. So it justified a whole video on clarifying exactly who was first on the land. So right here, this is a picture of algae on the coast. This is kind of algal scum right over here. So this right here is algae."}, {"video_title": "First living things on land clarification Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So it justified a whole video on clarifying exactly who was first on the land. So right here, this is a picture of algae on the coast. This is kind of algal scum right over here. So this right here is algae. And just to be clear, sometimes cyanobacteria, which we talked about as the first photosynthetic organism, sometimes that's called blue-green algae. But that's really bacteria. Algae is considered to be eukaryotic."}, {"video_title": "First living things on land clarification Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So this right here is algae. And just to be clear, sometimes cyanobacteria, which we talked about as the first photosynthetic organism, sometimes that's called blue-green algae. But that's really bacteria. Algae is considered to be eukaryotic. And it just doesn't have the structures of modern plants. So this is algae right here. And our best estimate is that algae actually colonized kind of coastal rocks about 1.2 billion years ago."}, {"video_title": "First living things on land clarification Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Algae is considered to be eukaryotic. And it just doesn't have the structures of modern plants. So this is algae right here. And our best estimate is that algae actually colonized kind of coastal rocks about 1.2 billion years ago. So this is 1.2 billion years ago. G for giga, billion years ago. So if you wanted the first thing that even resembled or was close to plants or animals, and if you consider algae close to a plant, then this would be the winner of who got on land first."}, {"video_title": "First living things on land clarification Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And our best estimate is that algae actually colonized kind of coastal rocks about 1.2 billion years ago. So this is 1.2 billion years ago. G for giga, billion years ago. So if you wanted the first thing that even resembled or was close to plants or animals, and if you consider algae close to a plant, then this would be the winner of who got on land first. This is 1.2 billion years ago. Now, in the last video where I talk about animals colonizing the land first, they weren't animals that only existed on land. They would have been animals that probably spent most of their time in the ocean collecting food or whatever."}, {"video_title": "First living things on land clarification Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So if you wanted the first thing that even resembled or was close to plants or animals, and if you consider algae close to a plant, then this would be the winner of who got on land first. This is 1.2 billion years ago. Now, in the last video where I talk about animals colonizing the land first, they weren't animals that only existed on land. They would have been animals that probably spent most of their time in the ocean collecting food or whatever. And then they would show up on the land maybe to lay eggs. And if you think about it, back then, the land would have been a really good place to lay eggs because there wouldn't have been much else on the land. So you would have been protected from predators."}, {"video_title": "First living things on land clarification Cosmology & Astronomy Khan Academy.mp3", "Sentence": "They would have been animals that probably spent most of their time in the ocean collecting food or whatever. And then they would show up on the land maybe to lay eggs. And if you think about it, back then, the land would have been a really good place to lay eggs because there wouldn't have been much else on the land. So you would have been protected from predators. So it might have been slug-like creatures like this. Some people talk about kind of spider-like creatures. But it still would have been at the coast."}, {"video_title": "First living things on land clarification Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So you would have been protected from predators. So it might have been slug-like creatures like this. Some people talk about kind of spider-like creatures. But it still would have been at the coast. And these would have been creatures that would have spent a lot of time in the ocean and some time in their land. So this is what I was referring to as kind of animals colonizing the land before plants. And this would have happened about 530 million years ago."}, {"video_title": "First living things on land clarification Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But it still would have been at the coast. And these would have been creatures that would have spent a lot of time in the ocean and some time in their land. So this is what I was referring to as kind of animals colonizing the land before plants. And this would have happened about 530 million years ago. 530 million years ago. Now, the first living organisms to fully live on the land, their whole life is on the land, those would be plants. So it depends."}, {"video_title": "First living things on land clarification Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And this would have happened about 530 million years ago. 530 million years ago. Now, the first living organisms to fully live on the land, their whole life is on the land, those would be plants. So it depends. If you think about things that live part of their life, you'd get the animals, things that live their whole life on the land. Then you go back to the plants. So this right here, this is what we think the first primitive plants would have looked like."}, {"video_title": "First living things on land clarification Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So it depends. If you think about things that live part of their life, you'd get the animals, things that live their whole life on the land. Then you go back to the plants. So this right here, this is what we think the first primitive plants would have looked like. And the evidence, we actually don't have fossils from these plants themselves. We have fossils of their spores. But we think that the earliest fossils of their spores, which show that these existed, were about 475 million years ago."}, {"video_title": "First living things on land clarification Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So this right here, this is what we think the first primitive plants would have looked like. And the evidence, we actually don't have fossils from these plants themselves. We have fossils of their spores. But we think that the earliest fossils of their spores, which show that these existed, were about 475 million years ago. So this is 400. Let me do this in another color. This right over here is 475 million years ago."}, {"video_title": "First living things on land clarification Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But we think that the earliest fossils of their spores, which show that these existed, were about 475 million years ago. So this is 400. Let me do this in another color. This right over here is 475 million years ago. So 1.2 billion years ago, you have the algae. 530 million years ago, we have evidence of things kind of oozing out of the ocean and maybe laying their eggs or something. 475 million years ago, we have evidence of what we would kind of call really plants."}, {"video_title": "First living things on land clarification Cosmology & Astronomy Khan Academy.mp3", "Sentence": "This right over here is 475 million years ago. So 1.2 billion years ago, you have the algae. 530 million years ago, we have evidence of things kind of oozing out of the ocean and maybe laying their eggs or something. 475 million years ago, we have evidence of what we would kind of call really plants. But the evidence is really the fossils of their spores. And then the first evidence of real, I guess you could call them animals, that spend their entire life on the land. The oldest fossil we have, it was discovered in Scotland fairly recently in 2004."}, {"video_title": "First living things on land clarification Cosmology & Astronomy Khan Academy.mp3", "Sentence": "475 million years ago, we have evidence of what we would kind of call really plants. But the evidence is really the fossils of their spores. And then the first evidence of real, I guess you could call them animals, that spend their entire life on the land. The oldest fossil we have, it was discovered in Scotland fairly recently in 2004. And this is the fossil right over here. It was actually discovered by a bus driver, by Mike Newman. Mike Newman, who was a bus driver in Scotland."}, {"video_title": "First living things on land clarification Cosmology & Astronomy Khan Academy.mp3", "Sentence": "The oldest fossil we have, it was discovered in Scotland fairly recently in 2004. And this is the fossil right over here. It was actually discovered by a bus driver, by Mike Newman. Mike Newman, who was a bus driver in Scotland. And they actually named the thing after him. It's called Pneumodesmus pneumoni. So they got the pneumoni from Mike Newman."}, {"video_title": "First living things on land clarification Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Mike Newman, who was a bus driver in Scotland. And they actually named the thing after him. It's called Pneumodesmus pneumoni. So they got the pneumoni from Mike Newman. And this fossil is 428 million years old. And right now, it's the oldest fossil we have of a true land animal. So if you think about true plants versus true land animals, things that spent their entire life on the land, the plants do win out."}, {"video_title": "First living things on land clarification Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So they got the pneumoni from Mike Newman. And this fossil is 428 million years old. And right now, it's the oldest fossil we have of a true land animal. So if you think about true plants versus true land animals, things that spent their entire life on the land, the plants do win out. If you think about things that spend part of their time on the land, then the animals probably won out. If you view algae as plants, then the plants won out. So it depends where you want to draw the line."}, {"video_title": "First living things on land clarification Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So if you think about true plants versus true land animals, things that spent their entire life on the land, the plants do win out. If you think about things that spend part of their time on the land, then the animals probably won out. If you view algae as plants, then the plants won out. So it depends where you want to draw the line. And this first fossil, this is of a myriapod, which just means a lot of legs. Let me write over here. Myriapod."}, {"video_title": "First living things on land clarification Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So it depends where you want to draw the line. And this first fossil, this is of a myriapod, which just means a lot of legs. Let me write over here. Myriapod. You probably know the word myriad. Myriad means a bunch of things, or a huge amount. So myriapod, a huge amount of legs."}, {"video_title": "First living things on land clarification Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Myriapod. You probably know the word myriad. Myriad means a bunch of things, or a huge amount. So myriapod, a huge amount of legs. And you might be familiar with the millipedes and centipedes. Those are myriapods. And so those first primitive myriapods, 428 million years ago, and they would have lived off of plants, and maybe other myriapods, and other slugs, and whatever other animals they might have found, might have looked something like that."}, {"video_title": "First living things on land clarification Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So myriapod, a huge amount of legs. And you might be familiar with the millipedes and centipedes. Those are myriapods. And so those first primitive myriapods, 428 million years ago, and they would have lived off of plants, and maybe other myriapods, and other slugs, and whatever other animals they might have found, might have looked something like that. So hopefully that gives a little bit of clarification over. It wasn't like you had dogs sitting on the land and they had nothing to eat. It was kind of a grayer in what you define a plant or an animal and who gets the bragging rights for being the first on the land."}, {"video_title": "Quasars Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And it's a short form for quasi-stellar radio sources. And this name is just a byproduct of the first observations of quasars. Because all they look like were these kind of point-like sources of electromagnetic radiation, mainly in the radio part of the spectrum. So that's why we call them quasi-stellar radio sources. Now it turns out that they are neither stars or even quasi-stellar. And their main energy isn't even being released in the radio band of the electromagnetic spectrum. They're far more energetic than that."}, {"video_title": "Quasars Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So that's why we call them quasi-stellar radio sources. Now it turns out that they are neither stars or even quasi-stellar. And their main energy isn't even being released in the radio band of the electromagnetic spectrum. They're far more energetic than that. What they really are are the active nucleuses of galaxies. So let's think about that a little bit. So if we have a supermassive black hole at the center of a galaxy, so let me draw that right over here."}, {"video_title": "Quasars Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "They're far more energetic than that. What they really are are the active nucleuses of galaxies. So let's think about that a little bit. So if we have a supermassive black hole at the center of a galaxy, so let me draw that right over here. So that's our supermassive black hole. Maybe that's the surface of the event horizon of the supermassive black hole. The actual mass of the black hole is in the center of that event horizon."}, {"video_title": "Quasars Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So if we have a supermassive black hole at the center of a galaxy, so let me draw that right over here. So that's our supermassive black hole. Maybe that's the surface of the event horizon of the supermassive black hole. The actual mass of the black hole is in the center of that event horizon. If there's material that's passing by this black hole, it's going to get attracted to it and it's going to form an accretion disk around it. This material is going to start rotating around this black hole and some of it, if it doesn't have enough velocity, is going to actually fall into the black hole. So you have all of this material going around the black hole and some of it, if it doesn't have enough angular velocity, not enough to orbit around the black hole, it's actually going to fall in."}, {"video_title": "Quasars Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "The actual mass of the black hole is in the center of that event horizon. If there's material that's passing by this black hole, it's going to get attracted to it and it's going to form an accretion disk around it. This material is going to start rotating around this black hole and some of it, if it doesn't have enough velocity, is going to actually fall into the black hole. So you have all of this material going around the black hole and some of it, if it doesn't have enough angular velocity, not enough to orbit around the black hole, it's actually going to fall in. Now while things, let me label this. This is the accretion disk. So as things are getting faster and faster, as they fall closer and closer to this black hole and bumping into each other more and more, that gravitational potential energy from things falling into it is being turned into actual energy, actual temperature."}, {"video_title": "Quasars Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So you have all of this material going around the black hole and some of it, if it doesn't have enough angular velocity, not enough to orbit around the black hole, it's actually going to fall in. Now while things, let me label this. This is the accretion disk. So as things are getting faster and faster, as they fall closer and closer to this black hole and bumping into each other more and more, that gravitational potential energy from things falling into it is being turned into actual energy, actual temperature. And so what you have is things start to get really unbelievably hot near the surface. They get hotter and hotter as they fall closer and closer to that event horizon. And so near the event horizon itself, things are so intense that they're actually releasing electromagnetic radiation, high frequency electromagnetic radiation, mainly in the x-ray part of the spectrum."}, {"video_title": "Quasars Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So as things are getting faster and faster, as they fall closer and closer to this black hole and bumping into each other more and more, that gravitational potential energy from things falling into it is being turned into actual energy, actual temperature. And so what you have is things start to get really unbelievably hot near the surface. They get hotter and hotter as they fall closer and closer to that event horizon. And so near the event horizon itself, things are so intense that they're actually releasing electromagnetic radiation, high frequency electromagnetic radiation, mainly in the x-ray part of the spectrum. Now I want to be very clear. So there's two things here. One is when you learn about quasars, or when I first was exposed to quasars in like a NOVA special, they make you think that the quasar, that the radiation, is somehow being released by the black hole itself."}, {"video_title": "Quasars Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And so near the event horizon itself, things are so intense that they're actually releasing electromagnetic radiation, high frequency electromagnetic radiation, mainly in the x-ray part of the spectrum. Now I want to be very clear. So there's two things here. One is when you learn about quasars, or when I first was exposed to quasars in like a NOVA special, they make you think that the quasar, that the radiation, is somehow being released by the black hole itself. And I would scratch my head because I was just told that nothing can escape the event horizon of a black hole, including electromagnetic radiation. So how could that be being emitted by the black hole? And the answer is it's not being emitted by the black hole, it's being emitted by the matter in the accretion disk that hasn't quite gotten to the event horizon yet."}, {"video_title": "Quasars Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "One is when you learn about quasars, or when I first was exposed to quasars in like a NOVA special, they make you think that the quasar, that the radiation, is somehow being released by the black hole itself. And I would scratch my head because I was just told that nothing can escape the event horizon of a black hole, including electromagnetic radiation. So how could that be being emitted by the black hole? And the answer is it's not being emitted by the black hole, it's being emitted by the matter in the accretion disk that hasn't quite gotten to the event horizon yet. Once it's inside of the event horizon, any electromagnetic radiation that it might emit will not be able to escape the black hole anymore, will not be able to escape the actual event horizon. So all of this is from the accretion disk around the supermassive black hole. And the other question that used to pop in my mind is why does it kind of come out at these kind of perpendicular, orthogonal, to the plane of the actual accretion disk?"}, {"video_title": "Quasars Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And the answer is it's not being emitted by the black hole, it's being emitted by the matter in the accretion disk that hasn't quite gotten to the event horizon yet. Once it's inside of the event horizon, any electromagnetic radiation that it might emit will not be able to escape the black hole anymore, will not be able to escape the actual event horizon. So all of this is from the accretion disk around the supermassive black hole. And the other question that used to pop in my mind is why does it kind of come out at these kind of perpendicular, orthogonal, to the plane of the actual accretion disk? And they're just, at least my logic tells me, well things aren't going to pop out along the direction of the accretion disk because then they're going to be absorbed by other things. In fact, that's what's going to cause other things to get heated up. Closer to the actual event horizon."}, {"video_title": "Quasars Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And the other question that used to pop in my mind is why does it kind of come out at these kind of perpendicular, orthogonal, to the plane of the actual accretion disk? And they're just, at least my logic tells me, well things aren't going to pop out along the direction of the accretion disk because then they're going to be absorbed by other things. In fact, that's what's going to cause other things to get heated up. Closer to the actual event horizon. So any energy that's going out in that direction is just going to be absorbed and make other things hotter. And only when you go roughly perpendicular to the plane of the accretion disk is that energy allowed to kind of go and transmit freely into space. Now I want to be very clear."}, {"video_title": "Quasars Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Closer to the actual event horizon. So any energy that's going out in that direction is just going to be absorbed and make other things hotter. And only when you go roughly perpendicular to the plane of the accretion disk is that energy allowed to kind of go and transmit freely into space. Now I want to be very clear. Quasars are the most luminous things that we know of in the universe. So most luminous things that we know of in the universe. The brightest, or actually many quasars, are on the order of a trillion suns in luminosity."}, {"video_title": "Quasars Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Now I want to be very clear. Quasars are the most luminous things that we know of in the universe. So most luminous things that we know of in the universe. The brightest, or actually many quasars, are on the order of a trillion suns in luminosity. So they can be brighter than an entire galaxy. And that's just coming from material around a fairly small region of space. Much smaller than an actual galaxy."}, {"video_title": "Quasars Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "The brightest, or actually many quasars, are on the order of a trillion suns in luminosity. So they can be brighter than an entire galaxy. And that's just coming from material around a fairly small region of space. Much smaller than an actual galaxy. It's the very center. It's kind of just the galactic core. Now another interesting thing about quasars, and this kind of gives credence to this notion of a constantly changing universe, and even to some degree the Big Bang itself, is you have these supermassive black holes that may be formed shortly after the Big Bang."}, {"video_title": "Quasars Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Much smaller than an actual galaxy. It's the very center. It's kind of just the galactic core. Now another interesting thing about quasars, and this kind of gives credence to this notion of a constantly changing universe, and even to some degree the Big Bang itself, is you have these supermassive black holes that may be formed shortly after the Big Bang. Now you can imagine, at an early stage in the universe's development, there would have been a lot of mass that would have been near these black holes, that didn't have quite the velocities to be able to escape them or be able to orbit around them. And so these would actually start falling into the black hole, and then over time, all of the mass that had to fall into the black hole, into the supermassive black hole, will have fallen into the supermassive black hole. And if you imagine some future period of time, you should still have the supermassive black hole, but all you should see is mostly things orbiting around it."}, {"video_title": "Quasars Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Now another interesting thing about quasars, and this kind of gives credence to this notion of a constantly changing universe, and even to some degree the Big Bang itself, is you have these supermassive black holes that may be formed shortly after the Big Bang. Now you can imagine, at an early stage in the universe's development, there would have been a lot of mass that would have been near these black holes, that didn't have quite the velocities to be able to escape them or be able to orbit around them. And so these would actually start falling into the black hole, and then over time, all of the mass that had to fall into the black hole, into the supermassive black hole, will have fallen into the supermassive black hole. And if you imagine some future period of time, you should still have the supermassive black hole, but all you should see is mostly things orbiting around it. Anything that had to fall into it would have already fallen into it. So you're just going to see things orbiting around it. And this is actually what we see."}, {"video_title": "Quasars Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And if you imagine some future period of time, you should still have the supermassive black hole, but all you should see is mostly things orbiting around it. Anything that had to fall into it would have already fallen into it. So you're just going to see things orbiting around it. And this is actually what we see. If we look around us, we look at our Milky Way galaxy, we don't observe a lot of things falling in. For example, the Milky Way galaxy does not have an active nucleus, an active core. It is not currently a quasar, the center of the Milky Way galaxy."}, {"video_title": "Quasars Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And this is actually what we see. If we look around us, we look at our Milky Way galaxy, we don't observe a lot of things falling in. For example, the Milky Way galaxy does not have an active nucleus, an active core. It is not currently a quasar, the center of the Milky Way galaxy. The supermassive black hole there is not, I guess we could say, digesting or consuming material. But you could imagine, at some point in the Milky Way's past, there might have been a lot of material that didn't have quite the velocity to be able to orbit. And so that was consumed."}, {"video_title": "Quasars Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It is not currently a quasar, the center of the Milky Way galaxy. The supermassive black hole there is not, I guess we could say, digesting or consuming material. But you could imagine, at some point in the Milky Way's past, there might have been a lot of material that didn't have quite the velocity to be able to orbit. And so that was consumed. And as it was consumed, it would emit all of this X-ray radiation and could be observed as a quasar. And that's actually what we observe. The closest quasars, and we've observed more than 200,000 quasars."}, {"video_title": "Quasars Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And so that was consumed. And as it was consumed, it would emit all of this X-ray radiation and could be observed as a quasar. And that's actually what we observe. The closest quasars, and we've observed more than 200,000 quasars. The closest quasars are on the order of 780 million light years away. So what does that mean? We don't observe quasars closer than 700 million light years."}, {"video_title": "Quasars Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "The closest quasars, and we've observed more than 200,000 quasars. The closest quasars are on the order of 780 million light years away. So what does that mean? We don't observe quasars closer than 700 million light years. So what that tells us is, at least in our region of the universe, the most recent quasars were 780 million years in the past. When we look at closer parts of the universe, let me draw, let's say this is the observable universe, this is us, so we only start to observe quasars at a certain distance away from us. And that distance is actually also a certain time in the past, because it took the light 780 million years to get to us."}, {"video_title": "Quasars Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "We don't observe quasars closer than 700 million light years. So what that tells us is, at least in our region of the universe, the most recent quasars were 780 million years in the past. When we look at closer parts of the universe, let me draw, let's say this is the observable universe, this is us, so we only start to observe quasars at a certain distance away from us. And that distance is actually also a certain time in the past, because it took the light 780 million years to get to us. And actually, most of the quasars are more than 3 billion light years away, which tells us that they only existed more than 3 billion years in the past, at a younger stage of the actual universe, when there was actual material for these supermassive black holes to consume at the center of galaxies. You move closer in time to us, and most of that material has actually been consumed. And we just have material orbiting around these supermassive black holes, which we call galaxies."}, {"video_title": "Quasars Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And that distance is actually also a certain time in the past, because it took the light 780 million years to get to us. And actually, most of the quasars are more than 3 billion light years away, which tells us that they only existed more than 3 billion years in the past, at a younger stage of the actual universe, when there was actual material for these supermassive black holes to consume at the center of galaxies. You move closer in time to us, and most of that material has actually been consumed. And we just have material orbiting around these supermassive black holes, which we call galaxies. And so we don't observe quasars anymore. And just to give an idea, I mean, everything we learn in cosmology, there's kind of these mind-bending concepts, unbelievable distances, unbelievable masses, unbelievable brightnesses, I guess you could think about it, but just to give a sense, the brightest known quasars devour on the order of 1,000 solar masses per year. So that's on the order of 10 Earths per second, if I did my math right."}, {"video_title": "Quasars Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And we just have material orbiting around these supermassive black holes, which we call galaxies. And so we don't observe quasars anymore. And just to give an idea, I mean, everything we learn in cosmology, there's kind of these mind-bending concepts, unbelievable distances, unbelievable masses, unbelievable brightnesses, I guess you could think about it, but just to give a sense, the brightest known quasars devour on the order of 1,000 solar masses per year. So that's on the order of 10 Earths per second, if I did my math right. 10 Earths per second are being devoured by the brightest quasars. And it's that energy of that mass that's accreting around it that's generating all of that energy. And actually, I should say, I shouldn't even talk about it in the present tense."}, {"video_title": "Quasars Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So that's on the order of 10 Earths per second, if I did my math right. 10 Earths per second are being devoured by the brightest quasars. And it's that energy of that mass that's accreting around it that's generating all of that energy. And actually, I should say, I shouldn't even talk about it in the present tense. These brightest quasars, this happened in the past. We're just observing it now. As far as all we know, the rest of the universe looks fairly similar to the way our universe does."}, {"video_title": "Quasars Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And actually, I should say, I shouldn't even talk about it in the present tense. These brightest quasars, this happened in the past. We're just observing it now. As far as all we know, the rest of the universe looks fairly similar to the way our universe does. And so there really aren't that many quasars around. Although, the other side of the coin might be, even though most of the material has already been consumed, maybe even by our own supermassive black hole in the center of the Milky Way, at some point in the future, maybe it will be able to consume on some more stellar material, some more, well, any type of material in the future. And that might happen about 4 or 5 billion years in the future when we actually collide with the Andromeda galaxy."}, {"video_title": "Beginnings of life Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It was named for Hades, or the ancient Greek underworld. Hades is also the name of the god that ran the Greek underworld, Zeus's oldest brother. And it was an appropriate name, although the idea of the ancient Greek notion of the underworld isn't exactly the more modern notion of hell. But it was a hellish environment. You had all this lava flowing around. You had things impacting the Earth from space. And as far as we can tell right now, it was completely inhospitable to life."}, {"video_title": "Beginnings of life Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But it was a hellish environment. You had all this lava flowing around. You had things impacting the Earth from space. And as far as we can tell right now, it was completely inhospitable to life. And to make matters worse, even though the Earth started to cool down a little bit, maybe the crust became a little bit more solid. Maybe the collisions started to happen less and less as we started to go a few hundred million years fast forward after Thea rammed into the early Earth and formed the moon. There was something called the late heavy bombardment."}, {"video_title": "Beginnings of life Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And as far as we can tell right now, it was completely inhospitable to life. And to make matters worse, even though the Earth started to cool down a little bit, maybe the crust became a little bit more solid. Maybe the collisions started to happen less and less as we started to go a few hundred million years fast forward after Thea rammed into the early Earth and formed the moon. There was something called the late heavy bombardment. And right now the consensus is that whatever we are descended from would have had to come about after the late heavy bombardment. Because this was a time where so many things from outer space were hitting Earth that it was so violent that it might have killed off any kind of primitive, self-replicating organisms or molecules that might have existed before it. And I won't go into the physics of the late heavy bombardment, but we believe that it happened because Uranus and Neptune, so this is the sun right here."}, {"video_title": "Beginnings of life Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "There was something called the late heavy bombardment. And right now the consensus is that whatever we are descended from would have had to come about after the late heavy bombardment. Because this was a time where so many things from outer space were hitting Earth that it was so violent that it might have killed off any kind of primitive, self-replicating organisms or molecules that might have existed before it. And I won't go into the physics of the late heavy bombardment, but we believe that it happened because Uranus and Neptune, so this is the sun right here. That is the sun. This is the asteroid belt that's outside the orbits of the inner rocky planets. That Uranus and Neptune, their orbits moved outward."}, {"video_title": "Beginnings of life Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And I won't go into the physics of the late heavy bombardment, but we believe that it happened because Uranus and Neptune, so this is the sun right here. That is the sun. This is the asteroid belt that's outside the orbits of the inner rocky planets. That Uranus and Neptune, their orbits moved outward. And I'm not going to go into the physics, but what that caused is gravitationally it caused a lot of the asteroids in the asteroid belt to move inward and start impacting the inner planets. And of course, Earth was one of the inner planets. And I should make the sun like orange or something, not blue."}, {"video_title": "Beginnings of life Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "That Uranus and Neptune, their orbits moved outward. And I'm not going to go into the physics, but what that caused is gravitationally it caused a lot of the asteroids in the asteroid belt to move inward and start impacting the inner planets. And of course, Earth was one of the inner planets. And I should make the sun like orange or something, not blue. I don't want you to think that's Earth. And it also impacted the moon. And it's more obvious on the moon because the moon does not have an atmosphere to kind of smooth over the impact."}, {"video_title": "Beginnings of life Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And I should make the sun like orange or something, not blue. I don't want you to think that's Earth. And it also impacted the moon. And it's more obvious on the moon because the moon does not have an atmosphere to kind of smooth over the impact. So the consensus is that only after the late heavy bombardment was Earth kind of ready for life. And we believe that the first life formed 3.8 to 4 billion years ago. Remember, G for giga, 4 billion years ago."}, {"video_title": "Beginnings of life Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And it's more obvious on the moon because the moon does not have an atmosphere to kind of smooth over the impact. So the consensus is that only after the late heavy bombardment was Earth kind of ready for life. And we believe that the first life formed 3.8 to 4 billion years ago. Remember, G for giga, 4 billion years ago. And when we talk about life at this period, we're not talking about squirrels or panda bears. We're talking about extremely simple life forms. We're talking about prokaryotes."}, {"video_title": "Beginnings of life Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Remember, G for giga, 4 billion years ago. And when we talk about life at this period, we're not talking about squirrels or panda bears. We're talking about extremely simple life forms. We're talking about prokaryotes. And let me give you a little primer on that right now, although we go into much more detail in the biology playlist, we're talking about prokaryotes. And I'll compare them to eukaryotes. Prokaryotes are, for the most part, unicellular organisms that have no nucleuses."}, {"video_title": "Beginnings of life Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "We're talking about prokaryotes. And let me give you a little primer on that right now, although we go into much more detail in the biology playlist, we're talking about prokaryotes. And I'll compare them to eukaryotes. Prokaryotes are, for the most part, unicellular organisms that have no nucleuses. They also don't have any other membrane-bound, what we'd call organelles, or these little parts of the cells that perform specific functions like mitochondria. So their DNA is just kind of floating around. So let me draw this character's DNA."}, {"video_title": "Beginnings of life Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Prokaryotes are, for the most part, unicellular organisms that have no nucleuses. They also don't have any other membrane-bound, what we'd call organelles, or these little parts of the cells that perform specific functions like mitochondria. So their DNA is just kind of floating around. So let me draw this character's DNA. So it's just floating around, just like that. And prokaryote literally means before kernel or before a nucleus. Eukaryotes do have a nucleus where all of their DNA is."}, {"video_title": "Beginnings of life Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So let me draw this character's DNA. So it's just floating around, just like that. And prokaryote literally means before kernel or before a nucleus. Eukaryotes do have a nucleus where all of their DNA is. So this is the nuclear membrane, and then all of its DNA is floating inside of the nucleus. And then it also has other membrane-bound organelles. Mitochondria is kind of the most famous of them."}, {"video_title": "Beginnings of life Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Eukaryotes do have a nucleus where all of their DNA is. So this is the nuclear membrane, and then all of its DNA is floating inside of the nucleus. And then it also has other membrane-bound organelles. Mitochondria is kind of the most famous of them. So it also has things like mitochondria. We'll learn more about that in future videos. Mitochondria, we believe, is essentially one prokaryote crawling inside of another prokaryote and kind of starting to become a symbiotic organism with each other."}, {"video_title": "Beginnings of life Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Mitochondria is kind of the most famous of them. So it also has things like mitochondria. We'll learn more about that in future videos. Mitochondria, we believe, is essentially one prokaryote crawling inside of another prokaryote and kind of starting to become a symbiotic organism with each other. But I won't go into that right now. But when we talk about life at this period, we're talking about prokaryotes. And we still have prokaryotes on the planet."}, {"video_title": "Beginnings of life Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Mitochondria, we believe, is essentially one prokaryote crawling inside of another prokaryote and kind of starting to become a symbiotic organism with each other. But I won't go into that right now. But when we talk about life at this period, we're talking about prokaryotes. And we still have prokaryotes on the planet. Bacteria and archaea are examples of prokaryotes. And just to give you a little bit of a tidbit right here, this kind of shows our current understanding of where we think things branched off from. So at this point of the tree is some common ancestor to prokaryotes and eukaryotes."}, {"video_title": "Beginnings of life Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And we still have prokaryotes on the planet. Bacteria and archaea are examples of prokaryotes. And just to give you a little bit of a tidbit right here, this kind of shows our current understanding of where we think things branched off from. So at this point of the tree is some common ancestor to prokaryotes and eukaryotes. So these are the prokaryotes right over here, the bacteria and the archaea. And here is the eukaryotes. And this first living thing, or this first set of living things, we think might have just been some type of self-replicating molecules."}, {"video_title": "Beginnings of life Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So at this point of the tree is some common ancestor to prokaryotes and eukaryotes. So these are the prokaryotes right over here, the bacteria and the archaea. And here is the eukaryotes. And this first living thing, or this first set of living things, we think might have just been some type of self-replicating molecules. And slowly some membrane might have come around and became a little bit more organized. DNA, RNA, maybe RNA was that original self-replicating molecule, became the method of kind of transmitting information from one generation to the next. So it's really still an open question of exactly what that first life is, or even how do you define that first life."}, {"video_title": "Beginnings of life Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And this first living thing, or this first set of living things, we think might have just been some type of self-replicating molecules. And slowly some membrane might have come around and became a little bit more organized. DNA, RNA, maybe RNA was that original self-replicating molecule, became the method of kind of transmitting information from one generation to the next. So it's really still an open question of exactly what that first life is, or even how do you define that first life. But based on studying the genetic makeup of current organisms, this is how we think the tree of life came about. So we have one common ancestor, then they broke apart, and then the archaea and eukaryotes have a common ancestor that's different from the bacteria. And we'll talk more about that in the future."}, {"video_title": "Beginnings of life Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So it's really still an open question of exactly what that first life is, or even how do you define that first life. But based on studying the genetic makeup of current organisms, this is how we think the tree of life came about. So we have one common ancestor, then they broke apart, and then the archaea and eukaryotes have a common ancestor that's different from the bacteria. And we'll talk more about that in the future. And this right here, just so you can visualize it, this is an example of bacteria. This is E. coli or Escherichia coli. It's just an example of bacteria."}, {"video_title": "Beginnings of life Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And we'll talk more about that in the future. And this right here, just so you can visualize it, this is an example of bacteria. This is E. coli or Escherichia coli. It's just an example of bacteria. It comes in a bunch of shapes and forms. But it's a prokaryotic life form. And the earliest life forms we also think were anaerobes."}, {"video_title": "Beginnings of life Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It's just an example of bacteria. It comes in a bunch of shapes and forms. But it's a prokaryotic life form. And the earliest life forms we also think were anaerobes. These are things that did not need, one, that they did not need oxygen, and they, for the most part, found oxygen poisonous. And the earliest life forms also probably did not perform photosynthesis. They might have gotten their energy from other sources, chemically, from this kind of extremely volatile environment that they were in at that time."}, {"video_title": "Beginnings of life Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And the earliest life forms we also think were anaerobes. These are things that did not need, one, that they did not need oxygen, and they, for the most part, found oxygen poisonous. And the earliest life forms also probably did not perform photosynthesis. They might have gotten their energy from other sources, chemically, from this kind of extremely volatile environment that they were in at that time. So if we fast forward a little bit, and this is actually a major event in the history of Earth. And these are huge timescales we're talking about. I mean, remember, I'm kind of just nonchalantly saying, oh, 4.6 billion years ago to 3.8 billion years ago, that's just 800 million years."}, {"video_title": "Beginnings of life Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "They might have gotten their energy from other sources, chemically, from this kind of extremely volatile environment that they were in at that time. So if we fast forward a little bit, and this is actually a major event in the history of Earth. And these are huge timescales we're talking about. I mean, remember, I'm kind of just nonchalantly saying, oh, 4.6 billion years ago to 3.8 billion years ago, that's just 800 million years. Remember, and I'll talk about this, grass has only existed for 50 million years. This is 800 million years. Humans and chimpanzees only diverged 5 million years ago."}, {"video_title": "Beginnings of life Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "I mean, remember, I'm kind of just nonchalantly saying, oh, 4.6 billion years ago to 3.8 billion years ago, that's just 800 million years. Remember, and I'll talk about this, grass has only existed for 50 million years. This is 800 million years. Humans and chimpanzees only diverged 5 million years ago. This is 800 million years we're talking about. From ancient Greece to now, we're only talking about 2,500 years. You multiply that times 1,000."}, {"video_title": "Beginnings of life Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Humans and chimpanzees only diverged 5 million years ago. This is 800 million years we're talking about. From ancient Greece to now, we're only talking about 2,500 years. You multiply that times 1,000. You multiply that times 1,000, you get 2.5 million years. And this is 800 million years we're talking about. So these are extremely huge periods of time."}, {"video_title": "Beginnings of life Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "You multiply that times 1,000. You multiply that times 1,000, you get 2.5 million years. And this is 800 million years we're talking about. So these are extremely huge periods of time. And that's why we call them eons. Eons are 500 million to a billion years. Now, the dividing line between the Hadean Eon and the Archean Eon, and it's kind of a fuzzy dividing line, but most people place it about 3.8 billion years ago, is kind of the earliest rocks that we can observe."}, {"video_title": "Beginnings of life Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So these are extremely huge periods of time. And that's why we call them eons. Eons are 500 million to a billion years. Now, the dividing line between the Hadean Eon and the Archean Eon, and it's kind of a fuzzy dividing line, but most people place it about 3.8 billion years ago, is kind of the earliest rocks that we can observe. And so we have rocks that are roughly 3.8 billion years ago, so we kind of put that as the beginning of the Archean Eon. And so there's two things there. One, rocks have survived from the beginning of the Archean Eon, and also that's roughly when we think that the first life existed."}, {"video_title": "Beginnings of life Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Now, the dividing line between the Hadean Eon and the Archean Eon, and it's kind of a fuzzy dividing line, but most people place it about 3.8 billion years ago, is kind of the earliest rocks that we can observe. And so we have rocks that are roughly 3.8 billion years ago, so we kind of put that as the beginning of the Archean Eon. And so there's two things there. One, rocks have survived from the beginning of the Archean Eon, and also that's roughly when we think that the first life existed. And so we're now in the Archean Eon. And you might say, oh, maybe Earth is a more pleasant place now. But it would not be."}, {"video_title": "Beginnings of life Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "One, rocks have survived from the beginning of the Archean Eon, and also that's roughly when we think that the first life existed. And so we're now in the Archean Eon. And you might say, oh, maybe Earth is a more pleasant place now. But it would not be. It still has no to little oxygen in the environment. If you were to go to Earth at that time, it might have looked something like this. It would have been a reddish sky."}, {"video_title": "Beginnings of life Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But it would not be. It still has no to little oxygen in the environment. If you were to go to Earth at that time, it might have looked something like this. It would have been a reddish sky. You would have had nitrogen and methane and carbon dioxide in the atmosphere. There would have been nothing for you to breathe. There still would have been a lot of volcanic activity."}, {"video_title": "Beginnings of life Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It would have been a reddish sky. You would have had nitrogen and methane and carbon dioxide in the atmosphere. There would have been nothing for you to breathe. There still would have been a lot of volcanic activity. This right here, these are pictures of stromatolites. And these are formed from bacteria that are bringing in sediment particles, and over time, these things get built up. But the most significant event in the Archean period, at least in my humble opinion, was what we believe started to happen about 3.5 billion years ago."}, {"video_title": "Beginnings of life Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "There still would have been a lot of volcanic activity. This right here, these are pictures of stromatolites. And these are formed from bacteria that are bringing in sediment particles, and over time, these things get built up. But the most significant event in the Archean period, at least in my humble opinion, was what we believe started to happen about 3.5 billion years ago. And this is prokaryotes, or especially bacteria, evolving to actually utilize energy from the sun to actually do photosynthesis. And the real fascinating byproduct of that, other than the fact that they can now use energy directly from the sun, is that it started to produce oxygen. So it starts to produce oxygen."}, {"video_title": "Beginnings of life Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But the most significant event in the Archean period, at least in my humble opinion, was what we believe started to happen about 3.5 billion years ago. And this is prokaryotes, or especially bacteria, evolving to actually utilize energy from the sun to actually do photosynthesis. And the real fascinating byproduct of that, other than the fact that they can now use energy directly from the sun, is that it started to produce oxygen. So it starts to produce oxygen. And at first, this oxygen, even though it was being produced by the cyanobacteria, by this blue-green bacteria, it really didn't accumulate in the atmosphere, because you had all of this iron that was dissolved in the oceans. And let me be clear, all of the life that we're going to be talking about for really the next several billion years, it all occurred in the ocean. We had no ozone layer now."}, {"video_title": "Beginnings of life Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So it starts to produce oxygen. And at first, this oxygen, even though it was being produced by the cyanobacteria, by this blue-green bacteria, it really didn't accumulate in the atmosphere, because you had all of this iron that was dissolved in the oceans. And let me be clear, all of the life that we're going to be talking about for really the next several billion years, it all occurred in the ocean. We had no ozone layer now. The land was being irradiated. The land was just a completely inhospitable environment for life. So all of this was occurring in the ocean."}, {"video_title": "Beginnings of life Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "We had no ozone layer now. The land was being irradiated. The land was just a completely inhospitable environment for life. So all of this was occurring in the ocean. And so the first oxygen that actually got produced, it actually, instead of just being released into the atmosphere, it ended up bonding with the iron that was dissolved in the ocean at that time. So it actually didn't have a chance to accumulate in the atmosphere. And when we fast forward past the Archean period, we're going to see that once a lot of that iron was oxidized, and the oxygen really did start to get released in the atmosphere, it actually had, it's funny to say, a cataclysmic effect or a catastrophic effect on the other anaerobic life on the planet at the time."}, {"video_title": "Beginnings of life Life on earth and in the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So all of this was occurring in the ocean. And so the first oxygen that actually got produced, it actually, instead of just being released into the atmosphere, it ended up bonding with the iron that was dissolved in the ocean at that time. So it actually didn't have a chance to accumulate in the atmosphere. And when we fast forward past the Archean period, we're going to see that once a lot of that iron was oxidized, and the oxygen really did start to get released in the atmosphere, it actually had, it's funny to say, a cataclysmic effect or a catastrophic effect on the other anaerobic life on the planet at the time. And it's funny to say that, because it was a catastrophe for them, but it was kind of a necessary thing that had to happen for us to happen. So for us, it was a blessing that this cyanobacteria started to pump out a lot of oxygen and eventually oxidized all of the iron and eventually released a lot of oxygen in the atmosphere and killed off all of this anaerobic bacteria so that eventually we could, us oxygen-breathing organisms could come about. But that's not going to happen for a while."}, {"video_title": "Thomas Malthus and population growth Cosmology & Astronomy Khan Academy.mp3", "Sentence": "The 1700s in Europe are often referred to as the age of enlightenment. It was a time we had come out of the Renaissance. We had rediscovered science and reason. In the 1700s, we saw that come about with even more progress of society. As we exit the 1700s and enter into the 1800s, we start having the Industrial Revolution. People saw the steady march of human reason, of human progress. Because of this, a lot of people were saying, humanity will continue to improve and improve forever to a point that poverty will go away."}, {"video_title": "Thomas Malthus and population growth Cosmology & Astronomy Khan Academy.mp3", "Sentence": "In the 1700s, we saw that come about with even more progress of society. As we exit the 1700s and enter into the 1800s, we start having the Industrial Revolution. People saw the steady march of human reason, of human progress. Because of this, a lot of people were saying, humanity will continue to improve and improve forever to a point that poverty will go away. We will turn into this perfect utopian civilization without wars, without strife of any kind. There was something to be said about that. You had significant improvements."}, {"video_title": "Thomas Malthus and population growth Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Because of this, a lot of people were saying, humanity will continue to improve and improve forever to a point that poverty will go away. We will turn into this perfect utopian civilization without wars, without strife of any kind. There was something to be said about that. You had significant improvements. In fact, you had even more dramatic improvements once the Industrial Revolution started. But not everyone in the late 1700s was as optimistic. One of the more famous not-so-optimistic people was Thomas Malthus."}, {"video_title": "Thomas Malthus and population growth Cosmology & Astronomy Khan Academy.mp3", "Sentence": "You had significant improvements. In fact, you had even more dramatic improvements once the Industrial Revolution started. But not everyone in the late 1700s was as optimistic. One of the more famous not-so-optimistic people was Thomas Malthus. Thomas Malthus, right over here. And I will just quote him directly. This is from his essay on the principle of population."}, {"video_title": "Thomas Malthus and population growth Cosmology & Astronomy Khan Academy.mp3", "Sentence": "One of the more famous not-so-optimistic people was Thomas Malthus. Thomas Malthus, right over here. And I will just quote him directly. This is from his essay on the principle of population. The power of population is so superior to the power of the earth to produce subsistence for man that premature death must in some shape or other visit the human race. This is uplifting. The vices of mankind are active and able ministers of deep population."}, {"video_title": "Thomas Malthus and population growth Cosmology & Astronomy Khan Academy.mp3", "Sentence": "This is from his essay on the principle of population. The power of population is so superior to the power of the earth to produce subsistence for man that premature death must in some shape or other visit the human race. This is uplifting. The vices of mankind are active and able ministers of deep population. They are the precursors in the great army of destruction and often finish the dreadful work themselves. But should they fail in this war of extermination, sickly seasons, epidemics, pestilence and plague advance in terrific array and sweep off their thousands and tens of thousands, should success still be incomplete, gigantic inevitable famine stalks in the rear and with one mighty blow levels the population with the food of the world. So not so uplifting of a little quote right over here."}, {"video_title": "Thomas Malthus and population growth Cosmology & Astronomy Khan Academy.mp3", "Sentence": "The vices of mankind are active and able ministers of deep population. They are the precursors in the great army of destruction and often finish the dreadful work themselves. But should they fail in this war of extermination, sickly seasons, epidemics, pestilence and plague advance in terrific array and sweep off their thousands and tens of thousands, should success still be incomplete, gigantic inevitable famine stalks in the rear and with one mighty blow levels the population with the food of the world. So not so uplifting of a little quote right over here. But this was his general sense. He lived in a time where people were being very optimistic that the march of progress would go on forever until we got to some utopian civilization. But from Thomas Malthus' point of view, he felt that if people could reproduce and increase the population, they will, that there's no way of stopping them."}, {"video_title": "Thomas Malthus and population growth Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So not so uplifting of a little quote right over here. But this was his general sense. He lived in a time where people were being very optimistic that the march of progress would go on forever until we got to some utopian civilization. But from Thomas Malthus' point of view, he felt that if people could reproduce and increase the population, they will, that there's no way of stopping them. So from his point of view, the way he saw it, so let me on that axis, let's say that that is the population, and this axis right over here, let's say that that is time. So by his thinking and everything that he'd seen in reality up to that point would back this up, that if people had enough food and time, they would reproduce and they would reproduce in numbers that would grow the population. So in his mind, the population would just keep on increasing until it can't support itself anymore, until the actual productivity of the land can't produce enough calories to feed all of those people."}, {"video_title": "Thomas Malthus and population growth Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But from Thomas Malthus' point of view, he felt that if people could reproduce and increase the population, they will, that there's no way of stopping them. So from his point of view, the way he saw it, so let me on that axis, let's say that that is the population, and this axis right over here, let's say that that is time. So by his thinking and everything that he'd seen in reality up to that point would back this up, that if people had enough food and time, they would reproduce and they would reproduce in numbers that would grow the population. So in his mind, the population would just keep on increasing until it can't support itself anymore, until the actual productivity of the land can't produce enough calories to feed all of those people. So in his mind, there would be some natural upper bound based on the actual amount of food that the earth could support. Let me do that in a different color. So in his mind, there was some upper bound."}, {"video_title": "Thomas Malthus and population growth Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So in his mind, the population would just keep on increasing until it can't support itself anymore, until the actual productivity of the land can't produce enough calories to feed all of those people. So in his mind, there would be some natural upper bound based on the actual amount of food that the earth could support. Let me do that in a different color. So in his mind, there was some upper bound. And once you get to that upper bound, then all of a sudden the vices of mankind will show up, and if those don't start killing people, then all of these other things will. Epidemics, pestilence, plague, and then famine. People are actually starving to death."}, {"video_title": "Thomas Malthus and population growth Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So in his mind, there was some upper bound. And once you get to that upper bound, then all of a sudden the vices of mankind will show up, and if those don't start killing people, then all of these other things will. Epidemics, pestilence, plague, and then famine. People are actually starving to death. So in his mind, once you got to this level, maybe you had a couple of good crops, but then all of a sudden you have a bad crop. Or because you have a bad crop, people start fighting over resources and wars happen. Or maybe the population is so dense that a plague develops, and then you have a massive wave of depopulation."}, {"video_title": "Thomas Malthus and population growth Cosmology & Astronomy Khan Academy.mp3", "Sentence": "People are actually starving to death. So in his mind, once you got to this level, maybe you had a couple of good crops, but then all of a sudden you have a bad crop. Or because you have a bad crop, people start fighting over resources and wars happen. Or maybe the population is so dense that a plague develops, and then you have a massive wave of depopulation. And so you would just oscillate around this limit. And this limit, some people refer to as a Malthusian limit, but it's just really the limit at which the population can sustain itself. And from Thomas Malthus' point of view, he did recognize that there were technological improvements, especially in things like agriculture, and that this line was moving up."}, {"video_title": "Thomas Malthus and population growth Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Or maybe the population is so dense that a plague develops, and then you have a massive wave of depopulation. And so you would just oscillate around this limit. And this limit, some people refer to as a Malthusian limit, but it's just really the limit at which the population can sustain itself. And from Thomas Malthus' point of view, he did recognize that there were technological improvements, especially in things like agriculture, and that this line was moving up. He had seen it in his own lifetime that this line had moved up. But from his point of view, however far you move this line up, the population will always compensate for it and catch up to it and eventually get to this limit, and then the same kind of not-so-positive things that he talks about would actually happen. And some people now say, Oh, Thomas Malthus, he was so pessimistic."}, {"video_title": "Thomas Malthus and population growth Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And from Thomas Malthus' point of view, he did recognize that there were technological improvements, especially in things like agriculture, and that this line was moving up. He had seen it in his own lifetime that this line had moved up. But from his point of view, however far you move this line up, the population will always compensate for it and catch up to it and eventually get to this limit, and then the same kind of not-so-positive things that he talks about would actually happen. And some people now say, Oh, Thomas Malthus, he was so pessimistic. He was obviously wrong. Look at what's happened. We have so much food on this planet right now."}, {"video_title": "Thomas Malthus and population growth Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And some people now say, Oh, Thomas Malthus, he was so pessimistic. He was obviously wrong. Look at what's happened. We have so much food on this planet right now. We've gone through multiple agricultural revolutions. And they are right. In the last 200 years since Malthus, so since the early 1800s, we really have been able to outstrip population."}, {"video_title": "Thomas Malthus and population growth Cosmology & Astronomy Khan Academy.mp3", "Sentence": "We have so much food on this planet right now. We've gone through multiple agricultural revolutions. And they are right. In the last 200 years since Malthus, so since the early 1800s, we really have been able to outstrip population. So this line up here has been moving up much faster than even population. So right now we actually do have more calories per person on the planet than we've had at any time in history. But it's not saying that Thomas Malthus was wrong."}, {"video_title": "Thomas Malthus and population growth Cosmology & Astronomy Khan Academy.mp3", "Sentence": "In the last 200 years since Malthus, so since the early 1800s, we really have been able to outstrip population. So this line up here has been moving up much faster than even population. So right now we actually do have more calories per person on the planet than we've had at any time in history. But it's not saying that Thomas Malthus was wrong. It's just saying that maybe he was just a little bit pessimistic in when that limit will be reached. Now the other dimension where you might say that he was maybe wrong was in this principle that a population will increase if it can increase. If there is food and if there is time, people will reproduce."}, {"video_title": "Thomas Malthus and population growth Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But it's not saying that Thomas Malthus was wrong. It's just saying that maybe he was just a little bit pessimistic in when that limit will be reached. Now the other dimension where you might say that he was maybe wrong was in this principle that a population will increase if it can increase. If there is food and if there is time, people will reproduce. And a good counterpoint to that is what we've now observed in modern developed nations. And so this right over here shows the population growth. I got this from the World Bank."}, {"video_title": "Thomas Malthus and population growth Cosmology & Astronomy Khan Academy.mp3", "Sentence": "If there is food and if there is time, people will reproduce. And a good counterpoint to that is what we've now observed in modern developed nations. And so this right over here shows the population growth. I got this from the World Bank. But the population growth of some modern developed nations. And you can see the United States is pretty low, but it's still positive. It's still over half a percent."}, {"video_title": "Thomas Malthus and population growth Cosmology & Astronomy Khan Academy.mp3", "Sentence": "I got this from the World Bank. But the population growth of some modern developed nations. And you can see the United States is pretty low, but it's still positive. It's still over half a percent. But even that adds up when you compound it. But if you look over here, Japan and Germany have less immigration than the United States, especially Japan, they are actually negative. So just this population left to its own devices, especially if you account for people not going across borders, just the population itself growing, they actually have negative growth."}, {"video_title": "Thomas Malthus and population growth Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It's still over half a percent. But even that adds up when you compound it. But if you look over here, Japan and Germany have less immigration than the United States, especially Japan, they are actually negative. So just this population left to its own devices, especially if you account for people not going across borders, just the population itself growing, they actually have negative growth. So there's some reason to believe that this is evidence that Thomas Malthus was wrong, or not completely right. He didn't put into account that maybe once a society becomes rich enough and educated enough that they might not just populate the world or have as many kids as they want. They might try to do other things with their time, whatever that might be."}, {"video_title": "Thomas Malthus and population growth Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So just this population left to its own devices, especially if you account for people not going across borders, just the population itself growing, they actually have negative growth. So there's some reason to believe that this is evidence that Thomas Malthus was wrong, or not completely right. He didn't put into account that maybe once a society becomes rich enough and educated enough that they might not just populate the world or have as many kids as they want. They might try to do other things with their time, whatever that might be. So I just wanted to expose you to this idea. Time will tell if Thomas Malthus, if we can always keep this line of food productivity growing faster than the population, and time will tell whether our populations can become, I guess you could say, developed enough so that they don't inexorably, I can never say that word, they don't always just keep growing. Maybe they do become a Japan or a Germany situation in the world population, especially if we have a high rate of literacy eventually does level off, so it never even has a chance of hitting up against that Malthusian limit."}, {"video_title": "Thomas Malthus and population growth Cosmology & Astronomy Khan Academy.mp3", "Sentence": "They might try to do other things with their time, whatever that might be. So I just wanted to expose you to this idea. Time will tell if Thomas Malthus, if we can always keep this line of food productivity growing faster than the population, and time will tell whether our populations can become, I guess you could say, developed enough so that they don't inexorably, I can never say that word, they don't always just keep growing. Maybe they do become a Japan or a Germany situation in the world population, especially if we have a high rate of literacy eventually does level off, so it never even has a chance of hitting up against that Malthusian limit. But I thought I would introduce you to the idea, and now you can go to parties and you can talk about things like Malthusian limits. And if you want to know what country is maybe closest to the Malthusian limit right now, and we've talked about this before, but a good case example is something like Bangladesh. They are right now the most population-dense country in the world."}, {"video_title": "Thomas Malthus and population growth Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Maybe they do become a Japan or a Germany situation in the world population, especially if we have a high rate of literacy eventually does level off, so it never even has a chance of hitting up against that Malthusian limit. But I thought I would introduce you to the idea, and now you can go to parties and you can talk about things like Malthusian limits. And if you want to know what country is maybe closest to the Malthusian limit right now, and we've talked about this before, but a good case example is something like Bangladesh. They are right now the most population-dense country in the world. They have 900 people per square kilometer, and just to give you a sense of perspective, that's 30 times more dense than the United States is. So if you took every person in the United States and turned them into 30 people in the United States, that would give you a sense of how dense Bangladesh is. And it's probably due to a certain degree that it's very fertile land."}, {"video_title": "Thomas Malthus and population growth Cosmology & Astronomy Khan Academy.mp3", "Sentence": "They are right now the most population-dense country in the world. They have 900 people per square kilometer, and just to give you a sense of perspective, that's 30 times more dense than the United States is. So if you took every person in the United States and turned them into 30 people in the United States, that would give you a sense of how dense Bangladesh is. And it's probably due to a certain degree that it's very fertile land. It's the river delta of the Ganges that essentially makes up the entire country. But they have in the past had famines. They've gotten a little bit beyond that, but still you do have major problems with flooding and resources."}, {"video_title": "Precession causing perihelion to happen later Cosmology & Astronomy Khan Academy.mp3", "Sentence": "I get this comment here on the YouTube channel from Vic Soma. And he or she says, if I understood this correctly, precession changes the time of the seasons over long periods of time, and obliquity changes the strengths of the season over long periods of time. And so this is a good comment. So the first comment isn't exactly true, and that's what I'm going to focus on in this video. He says, or she says, precession changes the time of the seasons over long periods of time. This is kind of true, but I don't think in the sense that Vic Soma is referring to it. The second part is roughly true."}, {"video_title": "Precession causing perihelion to happen later Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So the first comment isn't exactly true, and that's what I'm going to focus on in this video. He says, or she says, precession changes the time of the seasons over long periods of time. This is kind of true, but I don't think in the sense that Vic Soma is referring to it. The second part is roughly true. Obliquity changes the strength of the season over long periods of time. If you are more tilted towards the sun in the extreme, then yes, you will have a bigger disparity than when you are less tilted, when you are tilted away from the sun. Or I guess you'd say, if you are more tilted to or away from the sun, the disparity between summer and winter will be greater than if you are less tilted."}, {"video_title": "Precession causing perihelion to happen later Cosmology & Astronomy Khan Academy.mp3", "Sentence": "The second part is roughly true. Obliquity changes the strength of the season over long periods of time. If you are more tilted towards the sun in the extreme, then yes, you will have a bigger disparity than when you are less tilted, when you are tilted away from the sun. Or I guess you'd say, if you are more tilted to or away from the sun, the disparity between summer and winter will be greater than if you are less tilted. So the second statement is true, although you always have to be careful with things as complicated as the climate, because it can really depend from parts of the globe to parts on the globe, depending on what are all of the other factors that play into it. So you would want to run some type of simulation or something like that. But the second part is roughly true."}, {"video_title": "Precession causing perihelion to happen later Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Or I guess you'd say, if you are more tilted to or away from the sun, the disparity between summer and winter will be greater than if you are less tilted. So the second statement is true, although you always have to be careful with things as complicated as the climate, because it can really depend from parts of the globe to parts on the globe, depending on what are all of the other factors that play into it. So you would want to run some type of simulation or something like that. But the second part is roughly true. But I want to focus on the first part, because I think it'll really give us a better understanding of what precession is. And so this last statement Vic Soma makes is actually not true. He says, I'm assuming that he means the she, so in several thousand years, if we still use the same calendar system, summer and winter will happen in different months, and they will be more mild or more harsh."}, {"video_title": "Precession causing perihelion to happen later Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But the second part is roughly true. But I want to focus on the first part, because I think it'll really give us a better understanding of what precession is. And so this last statement Vic Soma makes is actually not true. He says, I'm assuming that he means the she, so in several thousand years, if we still use the same calendar system, summer and winter will happen in different months, and they will be more mild or more harsh. And what we're going to see is that the second statement is not true, because our calendar is actually based on when we are most tilted away from or towards the sun. So our calendar is actually, to some degree, you could say, takes precession into account. And what actually does change according to our calendar is when we are closest or furthest away from the sun, the perihelion or the aphelion."}, {"video_title": "Precession causing perihelion to happen later Cosmology & Astronomy Khan Academy.mp3", "Sentence": "He says, I'm assuming that he means the she, so in several thousand years, if we still use the same calendar system, summer and winter will happen in different months, and they will be more mild or more harsh. And what we're going to see is that the second statement is not true, because our calendar is actually based on when we are most tilted away from or towards the sun. So our calendar is actually, to some degree, you could say, takes precession into account. And what actually does change according to our calendar is when we are closest or furthest away from the sun, the perihelion or the aphelion. And we're going to think about that in this video. So let me draw the sun here. Let me draw the sun right over here."}, {"video_title": "Precession causing perihelion to happen later Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And what actually does change according to our calendar is when we are closest or furthest away from the sun, the perihelion or the aphelion. And we're going to think about that in this video. So let me draw the sun here. Let me draw the sun right over here. And let me draw Earth's orbit around the sun. And I'm going to draw it with some eccentricity. And just so you know what I'm talking about when I say eccentricity, a circle has no eccentricity."}, {"video_title": "Precession causing perihelion to happen later Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Let me draw the sun right over here. And let me draw Earth's orbit around the sun. And I'm going to draw it with some eccentricity. And just so you know what I'm talking about when I say eccentricity, a circle has no eccentricity. An ellipse, this ellipse right here, has more eccentricity than the circle, which has no eccentricity. And an even more eccentric ellipse would look like this. So you can really think about eccentricity as a measure of how far you are away from being perfectly circular."}, {"video_title": "Precession causing perihelion to happen later Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And just so you know what I'm talking about when I say eccentricity, a circle has no eccentricity. An ellipse, this ellipse right here, has more eccentricity than the circle, which has no eccentricity. And an even more eccentric ellipse would look like this. So you can really think about eccentricity as a measure of how far you are away from being perfectly circular. So Earth's orbit around the sun is pretty close to circular, but it has some eccentricity. It is slightly elliptical. And I'm going to exaggerate it a bit in this drawing right over here."}, {"video_title": "Precession causing perihelion to happen later Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So you can really think about eccentricity as a measure of how far you are away from being perfectly circular. So Earth's orbit around the sun is pretty close to circular, but it has some eccentricity. It is slightly elliptical. And I'm going to exaggerate it a bit in this drawing right over here. So let's say that this is the closest point that Earth gets to the sun. So that is the perihelion. And let's say that this is the furthest point."}, {"video_title": "Precession causing perihelion to happen later Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And I'm going to exaggerate it a bit in this drawing right over here. So let's say that this is the closest point that Earth gets to the sun. So that is the perihelion. And let's say that this is the furthest point. And obviously, it's not this big of a difference. It's actually only a 3% difference right now. But we'll also learn that that's changing."}, {"video_title": "Precession causing perihelion to happen later Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And let's say that this is the furthest point. And obviously, it's not this big of a difference. It's actually only a 3% difference right now. But we'll also learn that that's changing. But it never becomes this dramatic. But this will help us visualize it. So Earth's orbit might look something like this."}, {"video_title": "Precession causing perihelion to happen later Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But we'll also learn that that's changing. But it never becomes this dramatic. But this will help us visualize it. So Earth's orbit might look something like this. Let me make it do a better job than that. Earth's orbit might look something like this. Obviously, I'm exaggerating the eccentricity here."}, {"video_title": "Precession causing perihelion to happen later Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So Earth's orbit might look something like this. Let me make it do a better job than that. Earth's orbit might look something like this. Obviously, I'm exaggerating the eccentricity here. So let's say that's Earth's orbit. This is the point in orbit where we're closest. So that's perihelion."}, {"video_title": "Precession causing perihelion to happen later Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Obviously, I'm exaggerating the eccentricity here. So let's say that's Earth's orbit. This is the point in orbit where we're closest. So that's perihelion. And this is when we are furthest. This is aphelion. And we saw in the first video when we discussed this that right now, this perihelion is occurring in January."}, {"video_title": "Precession causing perihelion to happen later Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So that's perihelion. And this is when we are furthest. This is aphelion. And we saw in the first video when we discussed this that right now, this perihelion is occurring in January. And this will change over time, as we'll see in this video. So January right now, and aphelion right now occurs in July. Now, the time where we are most tilted towards the sun is not at the perihelion right now."}, {"video_title": "Precession causing perihelion to happen later Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And we saw in the first video when we discussed this that right now, this perihelion is occurring in January. And this will change over time, as we'll see in this video. So January right now, and aphelion right now occurs in July. Now, the time where we are most tilted towards the sun is not at the perihelion right now. It actually occurs a few weeks before the perihelion. So when we are most tilted to the sun, this is our winter solstice. And this is when we are, actually, in the case of the northern hemisphere, this is when we are most tilted away from the sun, I should say."}, {"video_title": "Precession causing perihelion to happen later Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Now, the time where we are most tilted towards the sun is not at the perihelion right now. It actually occurs a few weeks before the perihelion. So when we are most tilted to the sun, this is our winter solstice. And this is when we are, actually, in the case of the northern hemisphere, this is when we are most tilted away from the sun, I should say. So if we were to draw our tilt here, if I were to come out of the North Pole, it would look like that. And this right over here, depending on the year and where you are and your time zone and everything, this is usually December 21 or 22. I'll just go with December 22 for now."}, {"video_title": "Precession causing perihelion to happen later Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And this is when we are, actually, in the case of the northern hemisphere, this is when we are most tilted away from the sun, I should say. So if we were to draw our tilt here, if I were to come out of the North Pole, it would look like that. And this right over here, depending on the year and where you are and your time zone and everything, this is usually December 21 or 22. I'll just go with December 22 for now. And when the northern hemisphere is most tilted towards the sun occurs on June 20 or 21. So roughly six months later. Or really, exactly six months later."}, {"video_title": "Precession causing perihelion to happen later Cosmology & Astronomy Khan Academy.mp3", "Sentence": "I'll just go with December 22 for now. And when the northern hemisphere is most tilted towards the sun occurs on June 20 or 21. So roughly six months later. Or really, exactly six months later. It's just that all the months have different numbers of days and you have leap years sometimes, with February sometimes having 28 or 29 days. But if you go half a year away, then you are at, in the case of the northern hemisphere, the summer solstice. And this is when we are most tilted towards the sun."}, {"video_title": "Precession causing perihelion to happen later Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Or really, exactly six months later. It's just that all the months have different numbers of days and you have leap years sometimes, with February sometimes having 28 or 29 days. But if you go half a year away, then you are at, in the case of the northern hemisphere, the summer solstice. And this is when we are most tilted towards the sun. And once again, it occurs right now, a few weeks before the aphelion, before we are furthest away from the sun. Now, I want to zoom in right over here on December 22. So right over here, let me zoom in."}, {"video_title": "Precession causing perihelion to happen later Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And this is when we are most tilted towards the sun. And once again, it occurs right now, a few weeks before the aphelion, before we are furthest away from the sun. Now, I want to zoom in right over here on December 22. So right over here, let me zoom in. So let's say that this is the Earth. I'm zooming in on this circle right over here. So let me box it to show this is the one I'm focused on right over here."}, {"video_title": "Precession causing perihelion to happen later Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So right over here, let me zoom in. So let's say that this is the Earth. I'm zooming in on this circle right over here. So let me box it to show this is the one I'm focused on right over here. And let me draw the axis of rotation. And we know that that angle versus the vertical, that you could call the tilt or the obliquity. And we know this is 23.4 degrees relative to the vertical, relative to perpendicular, compared to the plane of our orbit, I guess."}, {"video_title": "Precession causing perihelion to happen later Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So let me box it to show this is the one I'm focused on right over here. And let me draw the axis of rotation. And we know that that angle versus the vertical, that you could call the tilt or the obliquity. And we know this is 23.4 degrees relative to the vertical, relative to perpendicular, compared to the plane of our orbit, I guess. So if our orbital axis was straight up and down, it would look something like this. It's not. It tilts."}, {"video_title": "Precession causing perihelion to happen later Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And we know this is 23.4 degrees relative to the vertical, relative to perpendicular, compared to the plane of our orbit, I guess. So if our orbital axis was straight up and down, it would look something like this. It's not. It tilts. And this angle right over here is 23.4 degrees. And when I say straight up and down, I'm saying relative to the plane of Earth's orbit around the sun. So this right here is the obliquity."}, {"video_title": "Precession causing perihelion to happen later Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It tilts. And this angle right over here is 23.4 degrees. And when I say straight up and down, I'm saying relative to the plane of Earth's orbit around the sun. So this right here is the obliquity. And as Viksoma mentioned, this does change. It kind of goes between 22 degrees and 24 and 1 1\u20442 degrees over very long periods of time. But it does, I think it's 41,000 years if I remember that correctly."}, {"video_title": "Precession causing perihelion to happen later Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So this right here is the obliquity. And as Viksoma mentioned, this does change. It kind of goes between 22 degrees and 24 and 1 1\u20442 degrees over very long periods of time. But it does, I think it's 41,000 years if I remember that correctly. So it will affect, on some level, the severity. So this tilt is kind of going between that, where actually it's reducing right now. And it'll get to a minimum in a few thousand years."}, {"video_title": "Precession causing perihelion to happen later Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But it does, I think it's 41,000 years if I remember that correctly. So it will affect, on some level, the severity. So this tilt is kind of going between that, where actually it's reducing right now. And it'll get to a minimum in a few thousand years. So it'll get to some minimum, and then eventually it'll get back to some maximum tilt. So it kind of goes back and forth between those two over the course of several tens of thousands of years. But anyway, this is as it is zoomed in right now."}, {"video_title": "Precession causing perihelion to happen later Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And it'll get to a minimum in a few thousand years. So it'll get to some minimum, and then eventually it'll get back to some maximum tilt. So it kind of goes back and forth between those two over the course of several tens of thousands of years. But anyway, this is as it is zoomed in right now. And as we mentioned, precession, you can kind of view it as if this arrow right here actually existed, it would trace out a circle. And it's tracing out the circle over a huge period of time, over 26,000 years. Let me make everything clear here."}, {"video_title": "Precession causing perihelion to happen later Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But anyway, this is as it is zoomed in right now. And as we mentioned, precession, you can kind of view it as if this arrow right here actually existed, it would trace out a circle. And it's tracing out the circle over a huge period of time, over 26,000 years. Let me make everything clear here. Right now I'm just going to assume Earth is rotating. The orbital direction is in that direction. I'm going to assume, let me make that a little bit more curved, Earth, this is the rotation of the Earth."}, {"video_title": "Precession causing perihelion to happen later Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Let me make everything clear here. Right now I'm just going to assume Earth is rotating. The orbital direction is in that direction. I'm going to assume, let me make that a little bit more curved, Earth, this is the rotation of the Earth. It is in this direction right over here. And what we learned about precession, and actually to be particular, it's axial precession. There's multiple types of precession we'll talk about."}, {"video_title": "Precession causing perihelion to happen later Cosmology & Astronomy Khan Academy.mp3", "Sentence": "I'm going to assume, let me make that a little bit more curved, Earth, this is the rotation of the Earth. It is in this direction right over here. And what we learned about precession, and actually to be particular, it's axial precession. There's multiple types of precession we'll talk about. If someone says just precession, they're usually referring to axial precession. It's this idea that over 26,000 years, the tip of this arrow, or you can even imagine the poles themselves, will kind of trace out a circle. If you look at the same point in our orbit at any period in time, the circle it's tracing out is going to be going in this direction right over here."}, {"video_title": "Precession causing perihelion to happen later Cosmology & Astronomy Khan Academy.mp3", "Sentence": "There's multiple types of precession we'll talk about. If someone says just precession, they're usually referring to axial precession. It's this idea that over 26,000 years, the tip of this arrow, or you can even imagine the poles themselves, will kind of trace out a circle. If you look at the same point in our orbit at any period in time, the circle it's tracing out is going to be going in this direction right over here. So if we wait 1,800 years, and I want to make sure I get this right because it's important to see what happens to our calendar. If we wait 1,800 years, we might, this arrow, instead of, it'll still have a tilt of 23.4 degrees, but instead of pointing in this direction, it might be pointing in this direction. Or in fact, it is likely, it will be pointing in this direction."}, {"video_title": "Precession causing perihelion to happen later Cosmology & Astronomy Khan Academy.mp3", "Sentence": "If you look at the same point in our orbit at any period in time, the circle it's tracing out is going to be going in this direction right over here. So if we wait 1,800 years, and I want to make sure I get this right because it's important to see what happens to our calendar. If we wait 1,800 years, we might, this arrow, instead of, it'll still have a tilt of 23.4 degrees, but instead of pointing in this direction, it might be pointing in this direction. Or in fact, it is likely, it will be pointing in this direction. I'm obviously not drawing it that exact. And then the bottom of the arrow will come out over here. So if you think about that, if you wait 1,800 years, and once again, the tilt hasn't changed, or it's changed a little bit, but what the precession has done, tracing out the circle, has changed the direction of this arrow, changed the direction of our axis of rotation."}, {"video_title": "Precession causing perihelion to happen later Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Or in fact, it is likely, it will be pointing in this direction. I'm obviously not drawing it that exact. And then the bottom of the arrow will come out over here. So if you think about that, if you wait 1,800 years, and once again, the tilt hasn't changed, or it's changed a little bit, but what the precession has done, tracing out the circle, has changed the direction of this arrow, changed the direction of our axis of rotation. And if you wait 1,800 years, when will the northern hemisphere be pointed most away from the sun? Well, now it won't be pointed most away from the sun at this point in space relative to the sun anymore, because now its axis of rotation looks something like this. So now, if we wait, or I should say, in 1,800 years, it'll be most pointed away, or the northern hemisphere will be most pointed away from the sun about a month earlier."}, {"video_title": "Precession causing perihelion to happen later Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So if you think about that, if you wait 1,800 years, and once again, the tilt hasn't changed, or it's changed a little bit, but what the precession has done, tracing out the circle, has changed the direction of this arrow, changed the direction of our axis of rotation. And if you wait 1,800 years, when will the northern hemisphere be pointed most away from the sun? Well, now it won't be pointed most away from the sun at this point in space relative to the sun anymore, because now its axis of rotation looks something like this. So now, if we wait, or I should say, in 1,800 years, it'll be most pointed away, or the northern hemisphere will be most pointed away from the sun about a month earlier. So about a month earlier. It'll be most pointed away from the sun about a month earlier. So this is when it will be most pointed away from the sun."}, {"video_title": "Precession causing perihelion to happen later Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So now, if we wait, or I should say, in 1,800 years, it'll be most pointed away, or the northern hemisphere will be most pointed away from the sun about a month earlier. So about a month earlier. It'll be most pointed away from the sun about a month earlier. So this is when it will be most pointed away from the sun. But if we, to today's time, we would say, no, that's still not the most pointed away. But since we have this precession, since the direction of the tilt, I guess we could say, or the direction of our axis, our rotational axis is changing, we are now at a different point in orbit where we're most pointed away from the sun. So this is 1,800 years later, approximately."}, {"video_title": "Precession causing perihelion to happen later Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So this is when it will be most pointed away from the sun. But if we, to today's time, we would say, no, that's still not the most pointed away. But since we have this precession, since the direction of the tilt, I guess we could say, or the direction of our axis, our rotational axis is changing, we are now at a different point in orbit where we're most pointed away from the sun. So this is 1,800 years later, approximately. So now, based on this, and I think this is what Vick's almost might have been hinting at, you say, look, OK, it's earlier in our orbit. Wouldn't this now be like November? And the answer is no."}, {"video_title": "Precession causing perihelion to happen later Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So this is 1,800 years later, approximately. So now, based on this, and I think this is what Vick's almost might have been hinting at, you say, look, OK, it's earlier in our orbit. Wouldn't this now be like November? And the answer is no. It will still be December 22nd. This will still be December 21st or 22nd, depending on the year, still be December 22nd, still be the same date. And that's because our calendar is based on when we are most tilted away or when we are most tilted towards the sun."}, {"video_title": "Precession causing perihelion to happen later Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And the answer is no. It will still be December 22nd. This will still be December 21st or 22nd, depending on the year, still be December 22nd, still be the same date. And that's because our calendar is based on when we are most tilted away or when we are most tilted towards the sun. So by definition, this is when we're most tilted away. So this will be the winter solstice. So what happens is, every year, so the way I drew it right over here, and actually, this perihelion actually changes over time as well."}, {"video_title": "Precession causing perihelion to happen later Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And that's because our calendar is based on when we are most tilted away or when we are most tilted towards the sun. So by definition, this is when we're most tilted away. So this will be the winter solstice. So what happens is, every year, so the way I drew it right over here, and actually, this perihelion actually changes over time as well. There's a precession of the perihelion as well, but I'm not going to go into that right now. So if you fast forward 1,800 years, all that's going to happen is that what we consider by our calendar to be December 22nd, in an absolute point in our orbit, will be earlier in our orbit. But we're still going to call it December 22nd."}, {"video_title": "Precession causing perihelion to happen later Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So what happens is, every year, so the way I drew it right over here, and actually, this perihelion actually changes over time as well. There's a precession of the perihelion as well, but I'm not going to go into that right now. So if you fast forward 1,800 years, all that's going to happen is that what we consider by our calendar to be December 22nd, in an absolute point in our orbit, will be earlier in our orbit. But we're still going to call it December 22nd. And so the perihelion is going to be further away from that December 22nd. It's actually going to be a month further away. So the perihelion 1,800 years from now won't be in January."}, {"video_title": "Precession causing perihelion to happen later Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But we're still going to call it December 22nd. And so the perihelion is going to be further away from that December 22nd. It's actually going to be a month further away. So the perihelion 1,800 years from now won't be in January. It will be in February. So the real takeaway here is that our calendar is based not on the exact point in space relative to the sun. Our calendar is based on the maximum tilt towards or away from the sun."}, {"video_title": "Precession causing perihelion to happen later Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So the perihelion 1,800 years from now won't be in January. It will be in February. So the real takeaway here is that our calendar is based not on the exact point in space relative to the sun. Our calendar is based on the maximum tilt towards or away from the sun. And that, as we see, that is slightly changing in terms of where it occurs in the absolute point in space. I think it's changing by roughly 20 minutes a year. So every year, the perihelion is getting 20 minutes later."}, {"video_title": "Precession causing perihelion to happen later Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Our calendar is based on the maximum tilt towards or away from the sun. And that, as we see, that is slightly changing in terms of where it occurs in the absolute point in space. I think it's changing by roughly 20 minutes a year. So every year, the perihelion is getting 20 minutes later. If we wanted to use the perihelion, if we wanted to use the exact point in space as our calendar, our year would actually be about that, I don't know what it is, roughly 20 or 25 minutes longer. But it actually makes much more sense to think about it from the tilt, because that's what dictates the seasons. And that's actually what's most observable from Earth, where the sun is in the sky."}, {"video_title": "Scale of solar system Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "In fact, when I showed this sun over here that was about 5 or 6 inches across, I said the earth would be just this little speck about 40 feet. It wouldn't be this distance, it would be about 40 feet to the left or the right. Its orbit would have a radius of about 40 feet. You wouldn't even notice it if you were looking at this thing over here. It would be this little speck orbiting at this huge, huge distance. If you look at this sun over here, if I were to draw the whole sun, it looks like it would have a diameter of about 20 inches. In this situation, this earth right here, this is drawn to scale, this earth would not be anywhere near this close."}, {"video_title": "Scale of solar system Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "You wouldn't even notice it if you were looking at this thing over here. It would be this little speck orbiting at this huge, huge distance. If you look at this sun over here, if I were to draw the whole sun, it looks like it would have a diameter of about 20 inches. In this situation, this earth right here, this is drawn to scale, this earth would not be anywhere near this close. It would be about 200 feet that way, or about 60 or 70 meters. You can imagine if the sun was this size, sitting on something like a football field, this little speck of an earth, this little thing right here would be sitting on the other 40 yard line, 60 meters away. You wouldn't even notice it."}, {"video_title": "Scale of solar system Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "In this situation, this earth right here, this is drawn to scale, this earth would not be anywhere near this close. It would be about 200 feet that way, or about 60 or 70 meters. You can imagine if the sun was this size, sitting on something like a football field, this little speck of an earth, this little thing right here would be sitting on the other 40 yard line, 60 meters away. You wouldn't even notice it. You might notice this from a distance, but you wouldn't even see this thing over here. The other planets are even further. Well, not all of the other planets."}, {"video_title": "Scale of solar system Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "You wouldn't even notice it. You might notice this from a distance, but you wouldn't even see this thing over here. The other planets are even further. Well, not all of the other planets. Obviously, you have Mercury here. I think most of us are familiar with these, but I'll just list them here just in case. That's Mercury."}, {"video_title": "Scale of solar system Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Well, not all of the other planets. Obviously, you have Mercury here. I think most of us are familiar with these, but I'll just list them here just in case. That's Mercury. This is Venus. Mercury is the smallest of the planets where it's not debated whether it's a planet. Pluto is the smallest, but some people debate whether it's really a planet or just a large solar body or a dwarf planet or any of those type of things."}, {"video_title": "Scale of solar system Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "That's Mercury. This is Venus. Mercury is the smallest of the planets where it's not debated whether it's a planet. Pluto is the smallest, but some people debate whether it's really a planet or just a large solar body or a dwarf planet or any of those type of things. Then you have Venus, probably the closest in size to the earth, or it is the closest in size to the earth. Then you have Mars. Then you have Jupiter."}, {"video_title": "Scale of solar system Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Pluto is the smallest, but some people debate whether it's really a planet or just a large solar body or a dwarf planet or any of those type of things. Then you have Venus, probably the closest in size to the earth, or it is the closest in size to the earth. Then you have Mars. Then you have Jupiter. Just to give a sense of, once again, how far these things are, if I were to go back to the analogy of this being the size of the sun, then Jupiter is five times further than earth. If I were to actually do the scale distance, this would be 300 meters away. 300 meters."}, {"video_title": "Scale of solar system Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Then you have Jupiter. Just to give a sense of, once again, how far these things are, if I were to go back to the analogy of this being the size of the sun, then Jupiter is five times further than earth. If I were to actually do the scale distance, this would be 300 meters away. 300 meters. If I had a nice, big, maybe medicine-ball-sized sun right over here, maybe basketball-sized sun, a little bit bigger than a basketball, this looks on my screen, then I would put this little thing that's smaller than a ping-pong ball, I would put this three football fields away. That's how far Jupiter is. Then Saturn's about twice as far as that."}, {"video_title": "Scale of solar system Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "300 meters. If I had a nice, big, maybe medicine-ball-sized sun right over here, maybe basketball-sized sun, a little bit bigger than a basketball, this looks on my screen, then I would put this little thing that's smaller than a ping-pong ball, I would put this three football fields away. That's how far Jupiter is. Then Saturn's about twice as far as that. Saturn is about nine times the distance. Let me make it clear. The earth is one astronomical unit away from the sun, roughly."}, {"video_title": "Scale of solar system Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Then Saturn's about twice as far as that. Saturn is about nine times the distance. Let me make it clear. The earth is one astronomical unit away from the sun, roughly. Its distance changes. It's not a perfectly circular orbit. Jupiter is approximately a little bit five plus astronomical units, so a little bit more than five times the distance of the sun to the earth."}, {"video_title": "Scale of solar system Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "The earth is one astronomical unit away from the sun, roughly. Its distance changes. It's not a perfectly circular orbit. Jupiter is approximately a little bit five plus astronomical units, so a little bit more than five times the distance of the sun to the earth. Saturn is approximately nine astronomical units, or nine times the distance from the sun to the earth. Once again, this would be nine football fields away. Another way to think about it, it would be essentially a kilometer away."}, {"video_title": "Scale of solar system Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Jupiter is approximately a little bit five plus astronomical units, so a little bit more than five times the distance of the sun to the earth. Saturn is approximately nine astronomical units, or nine times the distance from the sun to the earth. Once again, this would be nine football fields away. Another way to think about it, it would be essentially a kilometer away. If we had a medicine-ball-sized sun, this little, smaller-than-a-ping-pong-balled Saturn would be a kilometer away. I just want to really reiterate that because you never visualize it that way. Just for the sake of being able to draw it on a page, you see diagrams that look like this."}, {"video_title": "Scale of solar system Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Another way to think about it, it would be essentially a kilometer away. If we had a medicine-ball-sized sun, this little, smaller-than-a-ping-pong-balled Saturn would be a kilometer away. I just want to really reiterate that because you never visualize it that way. Just for the sake of being able to draw it on a page, you see diagrams that look like this. They really don't give you a sense of how small these planets are relative to the sun, and especially relative to their distance from the sun. After Saturn, you have Uranus and then Neptune. Obviously, these guys are even further."}, {"video_title": "Scale of solar system Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Just for the sake of being able to draw it on a page, you see diagrams that look like this. They really don't give you a sense of how small these planets are relative to the sun, and especially relative to their distance from the sun. After Saturn, you have Uranus and then Neptune. Obviously, these guys are even further. Just to give you a sense, it's very easy to start talking about galaxies and universes or the universe. Already, what we've talked about, we're talking about huge distances, huge scale. We already talked about that it would take a jet plane 17 years to travel from the earth to the sun."}, {"video_title": "Scale of solar system Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Obviously, these guys are even further. Just to give you a sense, it's very easy to start talking about galaxies and universes or the universe. Already, what we've talked about, we're talking about huge distances, huge scale. We already talked about that it would take a jet plane 17 years to travel from the earth to the sun. Multiply that by 5, about 100 years to go from Jupiter to the sun, 200 years to go from Saturn to the sun. You could have had Abraham Lincoln get into a jet plane, and he still wouldn't have gotten, if he left from Saturn, he still would not have gotten to the sun. These are huge, huge distances, but we're not done with the solar system there."}, {"video_title": "Scale of solar system Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "We already talked about that it would take a jet plane 17 years to travel from the earth to the sun. Multiply that by 5, about 100 years to go from Jupiter to the sun, 200 years to go from Saturn to the sun. You could have had Abraham Lincoln get into a jet plane, and he still wouldn't have gotten, if he left from Saturn, he still would not have gotten to the sun. These are huge, huge distances, but we're not done with the solar system there. Just to give a sense of scale, this right here, that's the sun. Each of these planets are actually narrower than these orbits, so they just draw these orbits here, but you wouldn't actually even see the actual planets here at this type of a scale. This is one astronomical unit right over here, the distance from the sun to the earth."}, {"video_title": "Scale of solar system Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "These are huge, huge distances, but we're not done with the solar system there. Just to give a sense of scale, this right here, that's the sun. Each of these planets are actually narrower than these orbits, so they just draw these orbits here, but you wouldn't actually even see the actual planets here at this type of a scale. This is one astronomical unit right over here, the distance from the sun to the earth. Then you have Mars. Then you have the asteroid belt. Mars also has some pretty big things in it itself, and it has these things that are kind of considered almost dwarf planets, things like Ceres."}, {"video_title": "Scale of solar system Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "This is one astronomical unit right over here, the distance from the sun to the earth. Then you have Mars. Then you have the asteroid belt. Mars also has some pretty big things in it itself, and it has these things that are kind of considered almost dwarf planets, things like Ceres. You could look those type things up. Then you have Jupiter out here. Once again, we said it would take 100 years, or roughly 100 years for a jet plane to get from Jupiter to the sun."}, {"video_title": "Scale of solar system Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Mars also has some pretty big things in it itself, and it has these things that are kind of considered almost dwarf planets, things like Ceres. You could look those type things up. Then you have Jupiter out here. Once again, we said it would take 100 years, or roughly 100 years for a jet plane to get from Jupiter to the sun. Even if you take this whole box here, which is a huge amount of distance, roughly about 5 astronomical units, it would take about 40 minutes for light to get from the sun to Jupiter. This is a huge, huge distance, but even this huge distance, we can put it into this little box right over here. This whole box right over there can be fit into this box."}, {"video_title": "Scale of solar system Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Once again, we said it would take 100 years, or roughly 100 years for a jet plane to get from Jupiter to the sun. Even if you take this whole box here, which is a huge amount of distance, roughly about 5 astronomical units, it would take about 40 minutes for light to get from the sun to Jupiter. This is a huge, huge distance, but even this huge distance, we can put it into this little box right over here. This whole box right over there can be fit into this box. You need to do that in order to appreciate the orbits of the outer planets. On this scale, the earth and Venus and Mercury and Mars, their orbits look pretty much, you can't even differentiate them from the sun. They look so close."}, {"video_title": "Scale of solar system Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "This whole box right over there can be fit into this box. You need to do that in order to appreciate the orbits of the outer planets. On this scale, the earth and Venus and Mercury and Mars, their orbits look pretty much, you can't even differentiate them from the sun. They look so close. They almost look like they're part of the sun when you look at it on this scale. Then you have the outer planets, Saturn, Uranus, Neptune. Then we have a Kuiper belt."}, {"video_title": "Scale of solar system Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "They look so close. They almost look like they're part of the sun when you look at it on this scale. Then you have the outer planets, Saturn, Uranus, Neptune. Then we have a Kuiper belt. This is more asteroids, but these are kind of more frozen. When we think of ice, you always think of water ice. But out here, it's so cold and it's relatively getting dark now because we're pretty far from the sun that things that we normally associate as gases are going to be in their solid form out here."}, {"video_title": "Scale of solar system Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Then we have a Kuiper belt. This is more asteroids, but these are kind of more frozen. When we think of ice, you always think of water ice. But out here, it's so cold and it's relatively getting dark now because we're pretty far from the sun that things that we normally associate as gases are going to be in their solid form out here. This isn't just rocky elements. This will also be things that we normally associate as gases like methane, frozen methane. But even here, we're not done."}, {"video_title": "Scale of solar system Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But out here, it's so cold and it's relatively getting dark now because we're pretty far from the sun that things that we normally associate as gases are going to be in their solid form out here. This isn't just rocky elements. This will also be things that we normally associate as gases like methane, frozen methane. But even here, we're not done. We're not even out of the solar system yet. Actually, just to give you a sense of the scale we're operating right here, I have this chart right here from the Voyager mission. The Voyager missions, Voyager 1 and 2, actually Voyager 2 left a little bit earlier, a month earlier."}, {"video_title": "Scale of solar system Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But even here, we're not done. We're not even out of the solar system yet. Actually, just to give you a sense of the scale we're operating right here, I have this chart right here from the Voyager mission. The Voyager missions, Voyager 1 and 2, actually Voyager 2 left a little bit earlier, a month earlier. Voyager 1 is just traveling faster. They left about a year after I was born. Their current velocity, just to give you a sense of how fast, Voyager 1 right here, is right now traveling at 61,000 kilometers per hour."}, {"video_title": "Scale of solar system Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "The Voyager missions, Voyager 1 and 2, actually Voyager 2 left a little bit earlier, a month earlier. Voyager 1 is just traveling faster. They left about a year after I was born. Their current velocity, just to give you a sense of how fast, Voyager 1 right here, is right now traveling at 61,000 kilometers per hour. That's about 17 kilometers per second, the size of a city every second. It's going that fast. That's, at least in my mind, an unfathomably fast velocity."}, {"video_title": "Scale of solar system Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Their current velocity, just to give you a sense of how fast, Voyager 1 right here, is right now traveling at 61,000 kilometers per hour. That's about 17 kilometers per second, the size of a city every second. It's going that fast. That's, at least in my mind, an unfathomably fast velocity. This thing has been traveling roughly that fast. It's been going around planets and gaining acceleration as it went around orbits. But for most of the time, it's been going at a pretty fast speed."}, {"video_title": "Scale of solar system Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "That's, at least in my mind, an unfathomably fast velocity. This thing has been traveling roughly that fast. It's been going around planets and gaining acceleration as it went around orbits. But for most of the time, it's been going at a pretty fast speed. Just to translate it to people who don't relate to kilometers, that's about 38,000 miles per hour. This huge, huge, unfathomably fast speed. It's been doing it since 1977."}, {"video_title": "Scale of solar system Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But for most of the time, it's been going at a pretty fast speed. Just to translate it to people who don't relate to kilometers, that's about 38,000 miles per hour. This huge, huge, unfathomably fast speed. It's been doing it since 1977. I was learning to walk, and when I was learning to walk, it was traveling at this super fast speed. Then when I was learning to talk, our whole lives, when we're sleeping, everything, we're eating, I'm in elementary school, it's still rocketing out of the solar system at roughly this speed. Its velocity has changed, but especially once it got outside of the planets, it's been roughly at this velocity."}, {"video_title": "Scale of solar system Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It's been doing it since 1977. I was learning to walk, and when I was learning to walk, it was traveling at this super fast speed. Then when I was learning to talk, our whole lives, when we're sleeping, everything, we're eating, I'm in elementary school, it's still rocketing out of the solar system at roughly this speed. Its velocity has changed, but especially once it got outside of the planets, it's been roughly at this velocity. It's just been rocketing out, and I don't want to say only, but it's gotten this far. It's gotten this far. If we look at it on this scale, it's gotten about that far right there."}, {"video_title": "Scale of solar system Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Its velocity has changed, but especially once it got outside of the planets, it's been roughly at this velocity. It's just been rocketing out, and I don't want to say only, but it's gotten this far. It's gotten this far. If we look at it on this scale, it's gotten about that far right there. It's about 115, 116 astronomical units. To give a sense, there's two ways to think about it. One is, wow, that's really far, because we know that even on this scale, you can't even see Earth's orbit."}, {"video_title": "Scale of solar system Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "If we look at it on this scale, it's gotten about that far right there. It's about 115, 116 astronomical units. To give a sense, there's two ways to think about it. One is, wow, that's really far, because we know that even on this scale, you can't even see Earth's orbit. This looks like it's a pretty, pretty far distance. Just to give you a sense of how far 116 astronomical units are, if 2,000 years ago, Jesus got on a plane, I actually pasted a copy of Jesus just for visualization purposes, but if he got on a jetliner at 1,000 kilometers per hour and went straight in that direction, in the direction of Voyager, Voyager would only just now be catching up to Jesus. This is a huge, huge, huge, huge distance."}, {"video_title": "Scale of solar system Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "One is, wow, that's really far, because we know that even on this scale, you can't even see Earth's orbit. This looks like it's a pretty, pretty far distance. Just to give you a sense of how far 116 astronomical units are, if 2,000 years ago, Jesus got on a plane, I actually pasted a copy of Jesus just for visualization purposes, but if he got on a jetliner at 1,000 kilometers per hour and went straight in that direction, in the direction of Voyager, Voyager would only just now be catching up to Jesus. This is a huge, huge, huge, huge distance. At the same time, even though it's a huge distance, especially relative to everything else we've talked about, relative to just even the outer reaches of the solar system, we're still talking in terms of a small scale. That's how far Voyager is. Just to give a sense, on this scale, this whole box over here can be contained in this box."}, {"video_title": "Scale of solar system Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "This is a huge, huge, huge, huge distance. At the same time, even though it's a huge distance, especially relative to everything else we've talked about, relative to just even the outer reaches of the solar system, we're still talking in terms of a small scale. That's how far Voyager is. Just to give a sense, on this scale, this whole box over here can be contained in this box. When you look at this box, Voyager's only gotten about that far. After traveling at this unbelievable velocity for over 30 years, for about 33 years. Just to give you an idea of these other things, Sedna right here is one of the..."}, {"video_title": "Scale of solar system Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Just to give a sense, on this scale, this whole box over here can be contained in this box. When you look at this box, Voyager's only gotten about that far. After traveling at this unbelievable velocity for over 30 years, for about 33 years. Just to give you an idea of these other things, Sedna right here is one of the... It's a reasonably large-sized outer solar system object. It's one of the furthest objects that we know of in the solar system. It has this very eccentric orbit."}, {"video_title": "Scale of solar system Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Just to give you an idea of these other things, Sedna right here is one of the... It's a reasonably large-sized outer solar system object. It's one of the furthest objects that we know of in the solar system. It has this very eccentric orbit. It gets, I don't want to say relatively close, but not unreasonably far away. Then it gets really far away from the sun. Even Sedna's orbit, if I were to look at this, this whole box over here can be contained right over here."}, {"video_title": "Scale of solar system Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It has this very eccentric orbit. It gets, I don't want to say relatively close, but not unreasonably far away. Then it gets really far away from the sun. Even Sedna's orbit, if I were to look at this, this whole box over here can be contained right over here. In this diagram right here, you wouldn't even be able to see, it would be like a speck how far Voyager has traveled in 33 years at 38,000 miles per hour. You would not even be able to notice that distance. Even though you can't even notice that distance, we still have the sun's influence."}, {"video_title": "Scale of solar system Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Even Sedna's orbit, if I were to look at this, this whole box over here can be contained right over here. In this diagram right here, you wouldn't even be able to see, it would be like a speck how far Voyager has traveled in 33 years at 38,000 miles per hour. You would not even be able to notice that distance. Even though you can't even notice that distance, we still have the sun's influence. The gravitational pull is still attracting things to it. This right here, we speculate that there's the Oort cloud. This is where the comets originate from."}, {"video_title": "Scale of solar system Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Even though you can't even notice that distance, we still have the sun's influence. The gravitational pull is still attracting things to it. This right here, we speculate that there's the Oort cloud. This is where the comets originate from. This is just a bunch of frozen gases and ice particles and things like that. We're starting to get to the outer reaches of the solar system. This distance right here is about 50,000 astronomical units."}, {"video_title": "Scale of solar system Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "This is where the comets originate from. This is just a bunch of frozen gases and ice particles and things like that. We're starting to get to the outer reaches of the solar system. This distance right here is about 50,000 astronomical units. Just to give a scale, because you hear a lot about light years and all of that, light years are about 63,000 astronomical units. If you go a light year out from the sun, you'll end up in the Oort cloud, the hypothesized Oort cloud. Just to give a sense, another scale, the Oort cloud is actually, most of the planets' orbits are roughly in the same plane."}, {"video_title": "Scale of solar system Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "This distance right here is about 50,000 astronomical units. Just to give a scale, because you hear a lot about light years and all of that, light years are about 63,000 astronomical units. If you go a light year out from the sun, you'll end up in the Oort cloud, the hypothesized Oort cloud. Just to give a sense, another scale, the Oort cloud is actually, most of the planets' orbits are roughly in the same plane. This right here is the orbit of the planets. Once again, these lines are drawn too thick. They're just drawn the thinnest possible so that you can see them, but they're still drawn too thick."}, {"video_title": "Scale of solar system Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Just to give a sense, another scale, the Oort cloud is actually, most of the planets' orbits are roughly in the same plane. This right here is the orbit of the planets. Once again, these lines are drawn too thick. They're just drawn the thinnest possible so that you can see them, but they're still drawn too thick. This gets us all the way to the Kuiper belt, but all of this over here, all the way out to the Kuiper belt, all the way out to all of the major planets, this is Pluto's orbit right over here. This whole diagram is only sitting in right over there. You can barely see it."}, {"video_title": "Scale of solar system Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "They're just drawn the thinnest possible so that you can see them, but they're still drawn too thick. This gets us all the way to the Kuiper belt, but all of this over here, all the way out to the Kuiper belt, all the way out to all of the major planets, this is Pluto's orbit right over here. This whole diagram is only sitting in right over there. You can barely see it. This whole diagram is just that dot in this. Then you can see the Oort cloud all around it. It's more of a spherical cloud."}, {"video_title": "Scale of solar system Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "You can barely see it. This whole diagram is just that dot in this. Then you can see the Oort cloud all around it. It's more of a spherical cloud. We think it exists. Obviously, it's hard to observe things at that distance. Hopefully, that gives you a beginning sense of the scale of the solar system."}, {"video_title": "Scale of solar system Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It's more of a spherical cloud. We think it exists. Obviously, it's hard to observe things at that distance. Hopefully, that gives you a beginning sense of the scale of the solar system. What's really going to blow your mind, if this hasn't blown your mind already, is that this whole thing is going to start looking like a speck. When you even just look at the local area around our galaxy, much less the galaxy as a whole, much less the universe as a whole. Anyway, this is starting to get crazy."}, {"video_title": "Detectable civilizations in our galaxy 3 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "The number of detectable civilizations in the galaxy is equal to, and they'll have this. And this is not the number of stars in the galaxy. This is the average rate of star formation per year in the galaxy. So star, so let me write this down. Average rate of star formation, which seems kind of unintuitive, and frankly it is, but hopefully we'll reconcile to show you that this and what we're going to show with the traditional Drake equation are actually the same thing. So that's the average rate of star formation. So I don't know what it is."}, {"video_title": "Detectable civilizations in our galaxy 3 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So star, so let me write this down. Average rate of star formation, which seems kind of unintuitive, and frankly it is, but hopefully we'll reconcile to show you that this and what we're going to show with the traditional Drake equation are actually the same thing. So that's the average rate of star formation. So I don't know what it is. Maybe it's 10 stars a year or something on that order. And then the rest of it looks pretty similar. So times the fraction of stars that have planets."}, {"video_title": "Detectable civilizations in our galaxy 3 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So I don't know what it is. Maybe it's 10 stars a year or something on that order. And then the rest of it looks pretty similar. So times the fraction of stars that have planets. So this product would give you the average stars with planet formations per year. You multiply that times the average number of planets capable of sustaining life for a star that has planets, for a solar system that has planets. So essentially if you multiply this, this is the average new planets per year in our galaxy capable of sustaining life."}, {"video_title": "Detectable civilizations in our galaxy 3 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So times the fraction of stars that have planets. So this product would give you the average stars with planet formations per year. You multiply that times the average number of planets capable of sustaining life for a star that has planets, for a solar system that has planets. So essentially if you multiply this, this is the average new planets per year in our galaxy capable of sustaining life. You multiply that times this, which is the same exact fraction, the fraction of those planets that are capable of sustaining life. Or actually, this is capable of sustaining life. Now we're saying that the fraction that actually do develop life."}, {"video_title": "Detectable civilizations in our galaxy 3 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So essentially if you multiply this, this is the average new planets per year in our galaxy capable of sustaining life. You multiply that times this, which is the same exact fraction, the fraction of those planets that are capable of sustaining life. Or actually, this is capable of sustaining life. Now we're saying that the fraction that actually do develop life. And then of the life, we care about the fraction that actually does become intelligent. And of the fraction that actually does become intelligent, we care about the fraction that eventually becomes detectable, that can actually communicate. And then in the traditional Drake equation, we multiply that times this L over here, times the detectable life of the civilization."}, {"video_title": "Detectable civilizations in our galaxy 3 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Now we're saying that the fraction that actually do develop life. And then of the life, we care about the fraction that actually does become intelligent. And of the fraction that actually does become intelligent, we care about the fraction that eventually becomes detectable, that can actually communicate. And then in the traditional Drake equation, we multiply that times this L over here, times the detectable life of the civilization. So how long is that civilization detectable? Are they releasing radio waves or something like it that a civilization like ours can detect? Maybe there are other ways to communicate, and we're just not advanced enough."}, {"video_title": "Detectable civilizations in our galaxy 3 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And then in the traditional Drake equation, we multiply that times this L over here, times the detectable life of the civilization. So how long is that civilization detectable? Are they releasing radio waves or something like it that a civilization like ours can detect? Maybe there are other ways to communicate, and we're just not advanced enough. Maybe in a few years we'll discover, in a few decades or a few hundreds of years, we'll discover that all of the other advanced civilizations are using a much more sophisticated way of communicating that doesn't involve electromagnetic waves. Who knows? But this is what we're thinking right now."}, {"video_title": "Detectable civilizations in our galaxy 3 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Maybe there are other ways to communicate, and we're just not advanced enough. Maybe in a few years we'll discover, in a few decades or a few hundreds of years, we'll discover that all of the other advanced civilizations are using a much more sophisticated way of communicating that doesn't involve electromagnetic waves. Who knows? But this is what we're thinking right now. But anyway, the whole point here is to reconcile this thing, which is less intuitive, for me at least, than with this thing. Because I started up here with the total number of stars in the galaxy. The traditional Drake equation starts with the average rate of star formation."}, {"video_title": "Detectable civilizations in our galaxy 3 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But this is what we're thinking right now. But anyway, the whole point here is to reconcile this thing, which is less intuitive, for me at least, than with this thing. Because I started up here with the total number of stars in the galaxy. The traditional Drake equation starts with the average rate of star formation. So I was like, well, how does the average rate of star formation gel with the total number of stars or civilizations that are now detectable? What I want to do is diagram that out a little bit. And I'm going to make a few assumptions."}, {"video_title": "Detectable civilizations in our galaxy 3 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "The traditional Drake equation starts with the average rate of star formation. So I was like, well, how does the average rate of star formation gel with the total number of stars or civilizations that are now detectable? What I want to do is diagram that out a little bit. And I'm going to make a few assumptions. I'm going to assume that this is kind of constant, that we're in a steady state. So this is constant, and we are in a steady state. The reality is that what would matter is the rate of star formation maybe 4, 5, 6 billion years ago."}, {"video_title": "Detectable civilizations in our galaxy 3 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And I'm going to make a few assumptions. I'm going to assume that this is kind of constant, that we're in a steady state. So this is constant, and we are in a steady state. The reality is that what would matter is the rate of star formation maybe 4, 5, 6 billion years ago. I don't know how long it has to be ago. So that now it starts to become realistic for real intelligence and real detectable intelligence to exist. But let's just assume that this number is constant for most of the life of the galaxy."}, {"video_title": "Detectable civilizations in our galaxy 3 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "The reality is that what would matter is the rate of star formation maybe 4, 5, 6 billion years ago. I don't know how long it has to be ago. So that now it starts to become realistic for real intelligence and real detectable intelligence to exist. But let's just assume that this number is constant for most of the life of the galaxy. Obviously, we're making all sorts of crazy assumptions here, so why not make another one? But what I want to show is that this is equivalent to the number of stars in the galaxy divided by the average life of a star or the average life of a solar system. And if n divided by this t sub s, if that's the same thing as r star, then essentially we have the same formulas."}, {"video_title": "Detectable civilizations in our galaxy 3 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But let's just assume that this number is constant for most of the life of the galaxy. Obviously, we're making all sorts of crazy assumptions here, so why not make another one? But what I want to show is that this is equivalent to the number of stars in the galaxy divided by the average life of a star or the average life of a solar system. And if n divided by this t sub s, if that's the same thing as r star, then essentially we have the same formulas. And to see that they're the same, imagine this, that this year, so this is this year. So this is 2000 and, well, let me say this year. This year, let's say that we have r star, let's say that this number is 10."}, {"video_title": "Detectable civilizations in our galaxy 3 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And if n divided by this t sub s, if that's the same thing as r star, then essentially we have the same formulas. And to see that they're the same, imagine this, that this year, so this is this year. So this is 2000 and, well, let me say this year. This year, let's say that we have r star, let's say that this number is 10. We have 10 new stars in the galaxy. So this is, I'll just say it's 10. So r star is equal to 10."}, {"video_title": "Detectable civilizations in our galaxy 3 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "This year, let's say that we have r star, let's say that this number is 10. We have 10 new stars in the galaxy. So this is, I'll just say it's 10. So r star is equal to 10. So this height over here is 10. That's what I'm depicting. So if I were to slide, I could show this is 10 units high or whatever."}, {"video_title": "Detectable civilizations in our galaxy 3 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So r star is equal to 10. So this height over here is 10. That's what I'm depicting. So if I were to slide, I could show this is 10 units high or whatever. And then last year, there was also 10, so on and so forth. Now let's go to whatever, let's say that this number right over here is 10 billion years. The average star life is 10 billion years."}, {"video_title": "Detectable civilizations in our galaxy 3 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So if I were to slide, I could show this is 10 units high or whatever. And then last year, there was also 10, so on and so forth. Now let's go to whatever, let's say that this number right over here is 10 billion years. The average star life is 10 billion years. So let's go back 10 billion years into the past. So this, so the average life of a star is equal to 10 billion years. And we're assuming that this is constant."}, {"video_title": "Detectable civilizations in our galaxy 3 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "The average star life is 10 billion years. So let's go back 10 billion years into the past. So this, so the average life of a star is equal to 10 billion years. And we're assuming that this is constant. So 10 billion years ago this year, there were also 10 new stars came about. And every year in between, every year in between, you had 10 stars come about. Now, how many total stars would there be in our galaxy?"}, {"video_title": "Detectable civilizations in our galaxy 3 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And we're assuming that this is constant. So 10 billion years ago this year, there were also 10 new stars came about. And every year in between, every year in between, you had 10 stars come about. Now, how many total stars would there be in our galaxy? Well, any star that came about, so we could go beyond that. We could go to stars that were born more than 10 billion years ago, more than this T sub s years ago. So you could have a star that was born 10 billion and 1 years ago, on average."}, {"video_title": "Detectable civilizations in our galaxy 3 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Now, how many total stars would there be in our galaxy? Well, any star that came about, so we could go beyond that. We could go to stars that were born more than 10 billion years ago, more than this T sub s years ago. So you could have a star that was born 10 billion and 1 years ago, on average. We're talking about on averages here. On average, that star will not exist anymore. So that star is not in existence."}, {"video_title": "Detectable civilizations in our galaxy 3 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So you could have a star that was born 10 billion and 1 years ago, on average. We're talking about on averages here. On average, that star will not exist anymore. So that star is not in existence. The stars that are in existence, once again, on average, are the ones that were born 10 billion years ago, all the way to the ones that were born this year. So you have 10 billion years of star birth, the ones that are still around. Each year, there's 10 of those years."}, {"video_title": "Detectable civilizations in our galaxy 3 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So that star is not in existence. The stars that are in existence, once again, on average, are the ones that were born 10 billion years ago, all the way to the ones that were born this year. So you have 10 billion years of star birth, the ones that are still around. Each year, there's 10 of those years. So the total number of stars should be equal to the number of stars that are born each year, assuming that that is constant, times the average lifespan of the stars. And if you look at it, and once again, this works because the stars that were born before this lifespan don't exist anymore. They've died out, on average."}, {"video_title": "Detectable civilizations in our galaxy 3 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Each year, there's 10 of those years. So the total number of stars should be equal to the number of stars that are born each year, assuming that that is constant, times the average lifespan of the stars. And if you look at it, and once again, this works because the stars that were born before this lifespan don't exist anymore. They've died out, on average. We care about this area right over here. 10 stars per year times 10 billion years. And now, if you manipulate this a little bit, you'll see that we'll get the result we want."}, {"video_title": "Detectable civilizations in our galaxy 3 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "They've died out, on average. We care about this area right over here. 10 stars per year times 10 billion years. And now, if you manipulate this a little bit, you'll see that we'll get the result we want. Let's solve for r, so we can just divide both sides by this t. So you get n star. So the number of stars in our galaxy now, making a bunch of assumptions, divided by the average life of the stars, is equal to the average number of new stars per year. And we get our result."}, {"video_title": "Detectable civilizations in our galaxy 3 Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And now, if you manipulate this a little bit, you'll see that we'll get the result we want. Let's solve for r, so we can just divide both sides by this t. So you get n star. So the number of stars in our galaxy now, making a bunch of assumptions, divided by the average life of the stars, is equal to the average number of new stars per year. And we get our result. If you replace this with the total number of stars divided by t, you get the exact same result that we had before. You just change the order a bit. We can take this divided by t, put it under this n, take it out of here, and then you get the exact same thing."}, {"video_title": "Cosmological time scale 2 Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And we saw that these huge periods of time, everything that's happened since the United States Declaration of Independence, all gets compressed into just five seconds. And that's not much when you think about 10 years. I mean, 10 years, you could wait around a lot. That's a huge amount of time. So hopefully that put things in perspective a little bit. What I want to do in this video is kind of do the same thing, but relate the 13.7 billion years, if that were a distance or the length of a timeline, how big each of these periods would be. So just to start off, the timeline, I'm recording this in high definition right now."}, {"video_title": "Cosmological time scale 2 Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "That's a huge amount of time. So hopefully that put things in perspective a little bit. What I want to do in this video is kind of do the same thing, but relate the 13.7 billion years, if that were a distance or the length of a timeline, how big each of these periods would be. So just to start off, the timeline, I'm recording this in high definition right now. There should be 1,280 pixels wide, depending on where you're watching it. So maybe you're watching it on a high def TV. If the timeline was the width of my screen right over here, so if that was 13.7 billion years from there, let's say this is the beginning of the universe and this is our present time."}, {"video_title": "Cosmological time scale 2 Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So just to start off, the timeline, I'm recording this in high definition right now. There should be 1,280 pixels wide, depending on where you're watching it. So maybe you're watching it on a high def TV. If the timeline was the width of my screen right over here, so if that was 13.7 billion years from there, let's say this is the beginning of the universe and this is our present time. If that was 13.7 billion years, the amount of time that humans have been on this planet, modern humans, the people who look and think like us, the amount of time that we have been on this planet will not even be a pixel. This little dot I drew is multiple pixels. The amount of time we've been here would not even be a pixel."}, {"video_title": "Cosmological time scale 2 Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "If the timeline was the width of my screen right over here, so if that was 13.7 billion years from there, let's say this is the beginning of the universe and this is our present time. If that was 13.7 billion years, the amount of time that humans have been on this planet, modern humans, the people who look and think like us, the amount of time that we have been on this planet will not even be a pixel. This little dot I drew is multiple pixels. The amount of time we've been here would not even be a pixel. In fact, a pixel, and that dot I drew here is about 4 pixels, but a pixel, just one little pixel on my screen, would represent 11 million years. 11, let me scroll over, it would represent 11 million years. So if this was the timeline, the dinosaurs would have been extinct about 6 pixels from the end."}, {"video_title": "Cosmological time scale 2 Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "The amount of time we've been here would not even be a pixel. In fact, a pixel, and that dot I drew here is about 4 pixels, but a pixel, just one little pixel on my screen, would represent 11 million years. 11, let me scroll over, it would represent 11 million years. So if this was the timeline, the dinosaurs would have been extinct about 6 pixels from the end. When the dinosaurs would have gotten extinct, and the amount of time that modern humans are on the planet wouldn't even register here. It would be 1 20th of that pixel over there. In fact, if we just expanded that very last pixel, I'll do it in yellow, that very last tiny pixel that you can't see, if we were to expand that to the entire length of this screen again, so if we were to just expand just that very last pixel, so I'm saying everything on this yellow line could be contained in that very last dot right over there, then every pixel in this would still be 8,000 years."}, {"video_title": "Cosmological time scale 2 Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So if this was the timeline, the dinosaurs would have been extinct about 6 pixels from the end. When the dinosaurs would have gotten extinct, and the amount of time that modern humans are on the planet wouldn't even register here. It would be 1 20th of that pixel over there. In fact, if we just expanded that very last pixel, I'll do it in yellow, that very last tiny pixel that you can't see, if we were to expand that to the entire length of this screen again, so if we were to just expand just that very last pixel, so I'm saying everything on this yellow line could be contained in that very last dot right over there, then every pixel in this would still be 8,000 years. So the entire period of time that humanity has been on this planet on this scale, so we've broken out this little tiny pixel here all the way out here, 200,000 years, that's on the order of about 25 of these pixels. So 25 of these pixels would be something like that. Not even that."}, {"video_title": "Cosmological time scale 2 Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "In fact, if we just expanded that very last pixel, I'll do it in yellow, that very last tiny pixel that you can't see, if we were to expand that to the entire length of this screen again, so if we were to just expand just that very last pixel, so I'm saying everything on this yellow line could be contained in that very last dot right over there, then every pixel in this would still be 8,000 years. So the entire period of time that humanity has been on this planet on this scale, so we've broken out this little tiny pixel here all the way out here, 200,000 years, that's on the order of about 25 of these pixels. So 25 of these pixels would be something like that. Not even that. So 25 pixels on this screen, at least at my resolution, is about that distance. So this is the fraction of just that pixel. That is the amount of time humans have been on the planet."}, {"video_title": "Cosmological time scale 2 Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Not even that. So 25 pixels on this screen, at least at my resolution, is about that distance. So this is the fraction of just that pixel. That is the amount of time humans have been on the planet. If you want the time since Jesus, it would be 1 4th of a pixel on this yellow scale. On this yellow scale, it would be 1 4th of a pixel. And the amount of time since the Declaration of Independence would be even a more minuscule amount of time."}, {"video_title": "Cosmological time scale 2 Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "That is the amount of time humans have been on the planet. If you want the time since Jesus, it would be 1 4th of a pixel on this yellow scale. On this yellow scale, it would be 1 4th of a pixel. And the amount of time since the Declaration of Independence would be even a more minuscule amount of time. So that's one way to think about it. But these timelines, to some degree, also don't give justice. So another way to think about it, what if we had a timeline that stretched from here, where I am, in the San Francisco Bay Area, if it stretched 1,300 kilometers to Seattle."}, {"video_title": "Cosmological time scale 2 Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And the amount of time since the Declaration of Independence would be even a more minuscule amount of time. So that's one way to think about it. But these timelines, to some degree, also don't give justice. So another way to think about it, what if we had a timeline that stretched from here, where I am, in the San Francisco Bay Area, if it stretched 1,300 kilometers to Seattle. So if this thing stretched 1,300 kilometers all the way to Seattle, then how? So this thing, the Big Bang, would be sitting in Seattle. Well, the formation of the Earth and the solar system, this would be about 200 kilometers away, a little over 200 kilometers."}, {"video_title": "Cosmological time scale 2 Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So another way to think about it, what if we had a timeline that stretched from here, where I am, in the San Francisco Bay Area, if it stretched 1,300 kilometers to Seattle. So if this thing stretched 1,300 kilometers all the way to Seattle, then how? So this thing, the Big Bang, would be sitting in Seattle. Well, the formation of the Earth and the solar system, this would be about 200 kilometers away, a little over 200 kilometers. And in the direction of Seattle, I don't know, that would get us in probably some part of northern California with redwoods and whatnot. But just to give an idea of 200 kilometers, that would get me from where I am near the coast of California to about the Nevada border. So still a pretty good distance relative to the entire timeline."}, {"video_title": "Cosmological time scale 2 Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Well, the formation of the Earth and the solar system, this would be about 200 kilometers away, a little over 200 kilometers. And in the direction of Seattle, I don't know, that would get us in probably some part of northern California with redwoods and whatnot. But just to give an idea of 200 kilometers, that would get me from where I am near the coast of California to about the Nevada border. So still a pretty good distance relative to the entire timeline. The last land dinosaurs, when the Earth got hit by an asteroid, this, so remember, our whole timeline now stretches all the way to Seattle. This event now would have only occurred about 3,000 meters away, or maybe I should just say 3 kilometers away, or roughly about 2 miles. So that still seems like a pretty good distance."}, {"video_title": "Cosmological time scale 2 Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So still a pretty good distance relative to the entire timeline. The last land dinosaurs, when the Earth got hit by an asteroid, this, so remember, our whole timeline now stretches all the way to Seattle. This event now would have only occurred about 3,000 meters away, or maybe I should just say 3 kilometers away, or roughly about 2 miles. So that still seems like a pretty good distance. But remember, our timeline stretches all the way to Seattle from the San Francisco area. So it's a pretty big timeline. So this is already a pretty small distance."}, {"video_title": "Cosmological time scale 2 Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So that still seems like a pretty good distance. But remember, our timeline stretches all the way to Seattle from the San Francisco area. So it's a pretty big timeline. So this is already a pretty small distance. But it gets even smaller. Australopithecus, this right over here, when they were roaming the Earth 3 million years ago, this gets reduced to a little bit more than a football field. About 150 meters, not kilometers."}, {"video_title": "Cosmological time scale 2 Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So this is already a pretty small distance. But it gets even smaller. Australopithecus, this right over here, when they were roaming the Earth 3 million years ago, this gets reduced to a little bit more than a football field. About 150 meters, not kilometers. Let me write that in a color where there's some contrast. 150 meters is where, so 150 meters in the direction of Seattle, if that's the timeline that we're talking about. And then the first humans, even shorter."}, {"video_title": "Cosmological time scale 2 Scale of the universe Cosmology & Astronomy Khan Academy.mp3", "Sentence": "About 150 meters, not kilometers. Let me write that in a color where there's some contrast. 150 meters is where, so 150 meters in the direction of Seattle, if that's the timeline that we're talking about. And then the first humans, even shorter. It's only going to be 10 meters of this timeline that stretches all the way to Seattle. The birth of Jesus 2,000 years ago would be 10 centimeters. 10 centimeters on a timeline that stretches from San Francisco to Seattle."}, {"video_title": "Plate Tectonics-- Geological features of Convergent Plate Boundaries.mp3", "Sentence": "We've already talked a lot about plate boundaries, where essentially new crust material is being created, and the plates are actually moving apart. We call these divergent boundaries. And the example we showed of this was the Mid-Atlantic Ridge, where essentially new crustal material is being created. Now on the other side of the equation, you have areas where plates are ramming into each other. We see that over here, where the Nazca plate is running into the South American plate. We see it over here, where the Pacific plate is running into the Filipino plate. They're running into each other."}, {"video_title": "Plate Tectonics-- Geological features of Convergent Plate Boundaries.mp3", "Sentence": "Now on the other side of the equation, you have areas where plates are ramming into each other. We see that over here, where the Nazca plate is running into the South American plate. We see it over here, where the Pacific plate is running into the Filipino plate. They're running into each other. So what happens over there? So what we're going to do is just go through the different scenarios. The general idea is that one plate is going to get subducted under another."}, {"video_title": "Plate Tectonics-- Geological features of Convergent Plate Boundaries.mp3", "Sentence": "They're running into each other. So what happens over there? So what we're going to do is just go through the different scenarios. The general idea is that one plate is going to get subducted under another. They're ramming into each other. One is going to get essentially pushed under the other one. This diagram shows some subduction over here."}, {"video_title": "Plate Tectonics-- Geological features of Convergent Plate Boundaries.mp3", "Sentence": "The general idea is that one plate is going to get subducted under another. They're ramming into each other. One is going to get essentially pushed under the other one. This diagram shows some subduction over here. This is essentially an oceanic plate being subducted under another oceanic plate. So not too different than what might happen where the Pacific plate runs into the Filipino plate right over here. And then on this side of the diagram, we see an oceanic plate and the oceanic crust getting subducted under a continental plate right over here."}, {"video_title": "Plate Tectonics-- Geological features of Convergent Plate Boundaries.mp3", "Sentence": "This diagram shows some subduction over here. This is essentially an oceanic plate being subducted under another oceanic plate. So not too different than what might happen where the Pacific plate runs into the Filipino plate right over here. And then on this side of the diagram, we see an oceanic plate and the oceanic crust getting subducted under a continental plate right over here. And this is what's happening when the Nazca plate is getting subducted under the South American plate. And when that happens, a couple of things. So you have the oceanic plate being pushed under."}, {"video_title": "Plate Tectonics-- Geological features of Convergent Plate Boundaries.mp3", "Sentence": "And then on this side of the diagram, we see an oceanic plate and the oceanic crust getting subducted under a continental plate right over here. And this is what's happening when the Nazca plate is getting subducted under the South American plate. And when that happens, a couple of things. So you have the oceanic plate being pushed under. And what happens at the same time, the continental plate gets pushed upwards, causing mountain ranges like the Andes, and that's exactly what has created the Andes. It's that upward force from the Nazca plate being pushed under the South American plate at that coastline. And what you're also going to see is, and you can imagine, you have these huge plates grinding past each other."}, {"video_title": "Plate Tectonics-- Geological features of Convergent Plate Boundaries.mp3", "Sentence": "So you have the oceanic plate being pushed under. And what happens at the same time, the continental plate gets pushed upwards, causing mountain ranges like the Andes, and that's exactly what has created the Andes. It's that upward force from the Nazca plate being pushed under the South American plate at that coastline. And what you're also going to see is, and you can imagine, you have these huge plates grinding past each other. And it's not a very smooth process. Every now and then, you kind of reach a breaking point and huge amounts of energy get released. So you're also going to see a lot of earthquakes in those areas."}, {"video_title": "Plate Tectonics-- Geological features of Convergent Plate Boundaries.mp3", "Sentence": "And what you're also going to see is, and you can imagine, you have these huge plates grinding past each other. And it's not a very smooth process. Every now and then, you kind of reach a breaking point and huge amounts of energy get released. So you're also going to see a lot of earthquakes in those areas. And we know that Chile has a lot of earthquakes. And then on top of that, this is going to result in a lot of heat and a lot of the friction of the plates grinding past each other, essentially allowing magma to form at that part of the rock. Because it's getting so heated, and so you'll also have volcanoes in these areas, where essentially something is being subducted underneath a continental plate."}, {"video_title": "Plate Tectonics-- Geological features of Convergent Plate Boundaries.mp3", "Sentence": "So you're also going to see a lot of earthquakes in those areas. And we know that Chile has a lot of earthquakes. And then on top of that, this is going to result in a lot of heat and a lot of the friction of the plates grinding past each other, essentially allowing magma to form at that part of the rock. Because it's getting so heated, and so you'll also have volcanoes in these areas, where essentially something is being subducted underneath a continental plate. Now, we also talked about what's happening in the Pacific, where we have the Pacific plate being subducted under the Filipino plate. That's what we kept referring to over here. And that's doing a couple of interesting things."}, {"video_title": "Plate Tectonics-- Geological features of Convergent Plate Boundaries.mp3", "Sentence": "Because it's getting so heated, and so you'll also have volcanoes in these areas, where essentially something is being subducted underneath a continental plate. Now, we also talked about what's happening in the Pacific, where we have the Pacific plate being subducted under the Filipino plate. That's what we kept referring to over here. And that's doing a couple of interesting things. Whenever you have subduction, you have trenches. But it's most interesting, or at least in my mind, the deepest trenches have been created where you have an oceanic plate being subducted under another oceanic plate. So a couple of things are going to happen."}, {"video_title": "Plate Tectonics-- Geological features of Convergent Plate Boundaries.mp3", "Sentence": "And that's doing a couple of interesting things. Whenever you have subduction, you have trenches. But it's most interesting, or at least in my mind, the deepest trenches have been created where you have an oceanic plate being subducted under another oceanic plate. So a couple of things are going to happen. You're going to have a very deep, you're going to have trenches form. Over here, we see in this diagram, we also have a trench in the first example. But you have trenches form where one oceanic plate is being subducted under another."}, {"video_title": "Plate Tectonics-- Geological features of Convergent Plate Boundaries.mp3", "Sentence": "So a couple of things are going to happen. You're going to have a very deep, you're going to have trenches form. Over here, we see in this diagram, we also have a trench in the first example. But you have trenches form where one oceanic plate is being subducted under another. And then you have that same type of friction that you saw over here create volcanoes. And those volcanoes will initially be underwater volcanoes, since these are both oceanic plates. Or we're dealing with oceanic crust at that point of the plate."}, {"video_title": "Plate Tectonics-- Geological features of Convergent Plate Boundaries.mp3", "Sentence": "But you have trenches form where one oceanic plate is being subducted under another. And then you have that same type of friction that you saw over here create volcanoes. And those volcanoes will initially be underwater volcanoes, since these are both oceanic plates. Or we're dealing with oceanic crust at that point of the plate. It doesn't have to be entirely an oceanic plate. And they'll first be underwater volcanoes. But as the lava piles up and hardens, it'll eventually turn into a group of islands."}, {"video_title": "Plate Tectonics-- Geological features of Convergent Plate Boundaries.mp3", "Sentence": "Or we're dealing with oceanic crust at that point of the plate. It doesn't have to be entirely an oceanic plate. And they'll first be underwater volcanoes. But as the lava piles up and hardens, it'll eventually turn into a group of islands. And we have that happening where the Pacific plate runs into the Filipino plate. And first, we have the trench. So let me just draw everything right here."}, {"video_title": "Plate Tectonics-- Geological features of Convergent Plate Boundaries.mp3", "Sentence": "But as the lava piles up and hardens, it'll eventually turn into a group of islands. And we have that happening where the Pacific plate runs into the Filipino plate. And first, we have the trench. So let me just draw everything right here. So this is the boundary, roughly speaking, between the two plates. This is the Pacific plate. And this is the Filipino plate, right over here."}, {"video_title": "Plate Tectonics-- Geological features of Convergent Plate Boundaries.mp3", "Sentence": "So let me just draw everything right here. So this is the boundary, roughly speaking, between the two plates. This is the Pacific plate. And this is the Filipino plate, right over here. And so where it's being subducted, you have the Mariana Trench, which is the deepest trench in the world. It goes down 11 kilometers, 11,000 meters. That's deeper than Mount Everest is high."}, {"video_title": "Plate Tectonics-- Geological features of Convergent Plate Boundaries.mp3", "Sentence": "And this is the Filipino plate, right over here. And so where it's being subducted, you have the Mariana Trench, which is the deepest trench in the world. It goes down 11 kilometers, 11,000 meters. That's deeper than Mount Everest is high. Mount Everest is about 9,000 meters high. And we'll see that's also due to another convergent plate boundary, another place where plates are running into each other. So not only do you see the Mariana Trench here, because one plate is being subducted under the other, you see the formation of the Mariana Islands, which are essentially created from underwater volcanoes because of all of the energy being released."}, {"video_title": "Plate Tectonics-- Geological features of Convergent Plate Boundaries.mp3", "Sentence": "That's deeper than Mount Everest is high. Mount Everest is about 9,000 meters high. And we'll see that's also due to another convergent plate boundary, another place where plates are running into each other. So not only do you see the Mariana Trench here, because one plate is being subducted under the other, you see the formation of the Mariana Islands, which are essentially created from underwater volcanoes because of all of the energy being released. And this is actually a depiction of what's the subduction that's happening at the Mariana Trench. You have this subduction over here, and then you have the Mariana Islands being created by essentially the energy causing a magma and lava, essentially magma before it surfaces, to flow to the top. And as lava just goes and starts building these islands."}, {"video_title": "Plate Tectonics-- Geological features of Convergent Plate Boundaries.mp3", "Sentence": "So not only do you see the Mariana Trench here, because one plate is being subducted under the other, you see the formation of the Mariana Islands, which are essentially created from underwater volcanoes because of all of the energy being released. And this is actually a depiction of what's the subduction that's happening at the Mariana Trench. You have this subduction over here, and then you have the Mariana Islands being created by essentially the energy causing a magma and lava, essentially magma before it surfaces, to flow to the top. And as lava just goes and starts building these islands. Now the last type of convergent boundary is when you have two parts of continental crust running into each other. So that's the situation that we have where the Indian plate is running into the Eurasian plate. And I think you might already guess what's going to happen there."}, {"video_title": "Plate Tectonics-- Geological features of Convergent Plate Boundaries.mp3", "Sentence": "And as lava just goes and starts building these islands. Now the last type of convergent boundary is when you have two parts of continental crust running into each other. So that's the situation that we have where the Indian plate is running into the Eurasian plate. And I think you might already guess what's going to happen there. When you have two pieces of continental crust running into each other, one isn't more or less dense than the other. And so at least the crustal portions of them are just going to keep jamming into each other. And so they're just going to push things upward."}, {"video_title": "Plate Tectonics-- Geological features of Convergent Plate Boundaries.mp3", "Sentence": "And I think you might already guess what's going to happen there. When you have two pieces of continental crust running into each other, one isn't more or less dense than the other. And so at least the crustal portions of them are just going to keep jamming into each other. And so they're just going to push things upward. This is a depiction right here that I got from the USGS. And what's kind of depicting is this is the Indian plate, this is the Eurasian plate. This is if you rewind a good bit before they've really had a chance to jam into each other."}, {"video_title": "Plate Tectonics-- Geological features of Convergent Plate Boundaries.mp3", "Sentence": "And so they're just going to push things upward. This is a depiction right here that I got from the USGS. And what's kind of depicting is this is the Indian plate, this is the Eurasian plate. This is if you rewind a good bit before they've really had a chance to jam into each other. But as they're jamming into each other, the Indian plate is kind of digging in a little bit, not being fully subducted, and it's causing the land to rise. And what that essentially ends up with is you end up with something like the Himalayas. And this right here is a picture of Mount Everest, which is almost 9,000 meters high, 9,000 meters above sea level."}, {"video_title": "Supernova (supernovae) Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "A star that started forming a core of iron. It has enormous pressure, enormous inward pressure on this core. Because as we form heavier and heavier elements in the core, the core gets denser and denser and denser. And so we keep fusing more and more elements into iron. This iron core becomes more and more massive, more and more dense. It's squeezing in on itself. And it's not fusing."}, {"video_title": "Supernova (supernovae) Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And so we keep fusing more and more elements into iron. This iron core becomes more and more massive, more and more dense. It's squeezing in on itself. And it's not fusing. That is not exothermic anymore. If iron were to fuse, it would not even be an exothermic process. It would require energy."}, {"video_title": "Supernova (supernovae) Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And it's not fusing. That is not exothermic anymore. If iron were to fuse, it would not even be an exothermic process. It would require energy. So it wouldn't be even something that could be helped to fend off this squeezing, to fend off this increasing density of the core. So we have this iron here, and it just gets more and more massive, more and more dense. And so at some mass, already a reasonably high mass, the only thing that's keeping this from just completely collapsing is what we could call electron degeneracy pressure."}, {"video_title": "Supernova (supernovae) Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "It would require energy. So it wouldn't be even something that could be helped to fend off this squeezing, to fend off this increasing density of the core. So we have this iron here, and it just gets more and more massive, more and more dense. And so at some mass, already a reasonably high mass, the only thing that's keeping this from just completely collapsing is what we could call electron degeneracy pressure. So let me write this here. Electron degeneracy pressure. And all this means is we have all of these iron atoms."}, {"video_title": "Supernova (supernovae) Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And so at some mass, already a reasonably high mass, the only thing that's keeping this from just completely collapsing is what we could call electron degeneracy pressure. So let me write this here. Electron degeneracy pressure. And all this means is we have all of these iron atoms. We have all of these iron atoms getting really, really, really close to each other. The only thing that keeps it from collapsing at this earlier stage, the only thing that keeps it from collapsing altogether is that they have these electrons. You have these electrons, and these are being squeezed together now."}, {"video_title": "Supernova (supernovae) Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And all this means is we have all of these iron atoms. We have all of these iron atoms getting really, really, really close to each other. The only thing that keeps it from collapsing at this earlier stage, the only thing that keeps it from collapsing altogether is that they have these electrons. You have these electrons, and these are being squeezed together now. We're talking about unbelievably dense states of matter. And electron degeneracy pressure is essentially what it's saying. These electrons don't want to be in the same place at the same time."}, {"video_title": "Supernova (supernovae) Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "You have these electrons, and these are being squeezed together now. We're talking about unbelievably dense states of matter. And electron degeneracy pressure is essentially what it's saying. These electrons don't want to be in the same place at the same time. I won't go into the quantum mechanics of it, but they cannot be squeezed into each other anymore. So that, at least temporarily, holds this thing from collapsing even further. And in the case of a less massive star, in the case of a white dwarf, that's how a white dwarf actually maintains its shape, because of the electron degeneracy pressure."}, {"video_title": "Supernova (supernovae) Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "These electrons don't want to be in the same place at the same time. I won't go into the quantum mechanics of it, but they cannot be squeezed into each other anymore. So that, at least temporarily, holds this thing from collapsing even further. And in the case of a less massive star, in the case of a white dwarf, that's how a white dwarf actually maintains its shape, because of the electron degeneracy pressure. But as this iron core gets even more massive, more dense, and we get more and more gravitational pressure, so this is our core now, even more gravitational pressure, eventually even this electron degeneracy, I guess we could call it force or pressure, this outward pressure, this thing that keeps it from collapsing, even that gives in. And then we have something called electron capture. Which is essentially the electrons get captured by protons in the nucleus."}, {"video_title": "Supernova (supernovae) Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And in the case of a less massive star, in the case of a white dwarf, that's how a white dwarf actually maintains its shape, because of the electron degeneracy pressure. But as this iron core gets even more massive, more dense, and we get more and more gravitational pressure, so this is our core now, even more gravitational pressure, eventually even this electron degeneracy, I guess we could call it force or pressure, this outward pressure, this thing that keeps it from collapsing, even that gives in. And then we have something called electron capture. Which is essentially the electrons get captured by protons in the nucleus. They start collapsing into the nucleus. It's kind of the opposite of beta negative decay, where you have the electrons get captured, protons get turned into neutrons, you have neutrinos being released, but you can imagine, an enormous amount of energy is also being released. So this is kind of a temporary, and then all of a sudden this collapses."}, {"video_title": "Supernova (supernovae) Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Which is essentially the electrons get captured by protons in the nucleus. They start collapsing into the nucleus. It's kind of the opposite of beta negative decay, where you have the electrons get captured, protons get turned into neutrons, you have neutrinos being released, but you can imagine, an enormous amount of energy is also being released. So this is kind of a temporary, and then all of a sudden this collapses. This collapses even more, until all you have, and all the protons are turning into neutrons, because they're capturing electrons. So then what you eventually have is this entire core is collapsing into a dense ball of neutrons. So dense neutrons."}, {"video_title": "Supernova (supernovae) Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So this is kind of a temporary, and then all of a sudden this collapses. This collapses even more, until all you have, and all the protons are turning into neutrons, because they're capturing electrons. So then what you eventually have is this entire core is collapsing into a dense ball of neutrons. So dense neutrons. You can kind of view them as just one really, really, really, really, really massive atom, because it's just a dense ball of neutrons. At the same time, when this collapse happens, you have an enormous amount of energy being released in the form of neutrinos. Did I say that neutrons are being released?"}, {"video_title": "Supernova (supernovae) Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So dense neutrons. You can kind of view them as just one really, really, really, really, really massive atom, because it's just a dense ball of neutrons. At the same time, when this collapse happens, you have an enormous amount of energy being released in the form of neutrinos. Did I say that neutrons are being released? No, no, no. The electrons are being captured by the protons, protons turning into neutrons, this dense ball of neutrons right here, and in the process neutrinos get released, these fundamental particles. We won't go into the details here."}, {"video_title": "Supernova (supernovae) Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Did I say that neutrons are being released? No, no, no. The electrons are being captured by the protons, protons turning into neutrons, this dense ball of neutrons right here, and in the process neutrinos get released, these fundamental particles. We won't go into the details here. But it's an enormous amount of energy. And this actually is not really, really well understood of all of the dynamics here, because at the same time, that this iron core is undergoing through this, it first kind of pauses due to the electron degeneracy pressure, and then it finally gives in because it's so massive, and then it collapses into this dense ball of neutrons. But when it does it, all of this energy is released."}, {"video_title": "Supernova (supernovae) Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "We won't go into the details here. But it's an enormous amount of energy. And this actually is not really, really well understood of all of the dynamics here, because at the same time, that this iron core is undergoing through this, it first kind of pauses due to the electron degeneracy pressure, and then it finally gives in because it's so massive, and then it collapses into this dense ball of neutrons. But when it does it, all of this energy is released. It's not clear how, because it has to be a lot of energy, because remember, this is a massive star, so you have a lot of mass in this area over here. But it's so much energy that it causes the rest of the star to explode outward in an unbelievable, I guess, unbelievably bright or energetic explosion, and that's called a supernova. And the reason why it's called nova, it comes from, I believe, I'm not an expert here, Latin for new."}, {"video_title": "Supernova (supernovae) Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But when it does it, all of this energy is released. It's not clear how, because it has to be a lot of energy, because remember, this is a massive star, so you have a lot of mass in this area over here. But it's so much energy that it causes the rest of the star to explode outward in an unbelievable, I guess, unbelievably bright or energetic explosion, and that's called a supernova. And the reason why it's called nova, it comes from, I believe, I'm not an expert here, Latin for new. And the first time people observed a nova, they thought it was a new star, because all of a sudden, something they didn't see before, all of a sudden, it looks like a star appeared, because maybe it wasn't bright enough for us to observe it before, but then when the nova occurred, it did become bright enough, so it comes from the idea of new. But a supernova is when you have a pretty massive star's core collapsing, and that energy is being released to explode the rest of the star out at unbelievable velocities. So just to kind of fathom the amount of energy that's being released in a supernova, it can temporarily outshine an entire galaxy, and in a galaxy, we're talking about hundreds of billions of stars."}, {"video_title": "Supernova (supernovae) Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And the reason why it's called nova, it comes from, I believe, I'm not an expert here, Latin for new. And the first time people observed a nova, they thought it was a new star, because all of a sudden, something they didn't see before, all of a sudden, it looks like a star appeared, because maybe it wasn't bright enough for us to observe it before, but then when the nova occurred, it did become bright enough, so it comes from the idea of new. But a supernova is when you have a pretty massive star's core collapsing, and that energy is being released to explode the rest of the star out at unbelievable velocities. So just to kind of fathom the amount of energy that's being released in a supernova, it can temporarily outshine an entire galaxy, and in a galaxy, we're talking about hundreds of billions of stars. Or another way to think about it, in that very short period of time, it can release as much energy as the sun will in its entire lifetime. So these are unbelievably energetic events. And so you actually have the material that's not in the core being shot out of the star at appreciable percentages of the actual speed of light."}, {"video_title": "Supernova (supernovae) Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So just to kind of fathom the amount of energy that's being released in a supernova, it can temporarily outshine an entire galaxy, and in a galaxy, we're talking about hundreds of billions of stars. Or another way to think about it, in that very short period of time, it can release as much energy as the sun will in its entire lifetime. So these are unbelievably energetic events. And so you actually have the material that's not in the core being shot out of the star at appreciable percentages of the actual speed of light. So we're talking about things being shot out at up to 10%, 10% the speed of light. That's 30,000 kilometers per second. That's almost circumnavigating the Earth every second."}, {"video_title": "Supernova (supernovae) Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And so you actually have the material that's not in the core being shot out of the star at appreciable percentages of the actual speed of light. So we're talking about things being shot out at up to 10%, 10% the speed of light. That's 30,000 kilometers per second. That's almost circumnavigating the Earth every second. So this is unbelievably energetic events that we're talking about here. And so if the star, if the original star was, and these are rough estimates, people don't have kind of a hard limit here. If the original star is 9 to 20, I'll say approximately 9 to 20 times the mass of the sun, then it will supernova, and the core will turn into what's called a neutron star."}, {"video_title": "Supernova (supernovae) Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "That's almost circumnavigating the Earth every second. So this is unbelievably energetic events that we're talking about here. And so if the star, if the original star was, and these are rough estimates, people don't have kind of a hard limit here. If the original star is 9 to 20, I'll say approximately 9 to 20 times the mass of the sun, then it will supernova, and the core will turn into what's called a neutron star. This is a neutron star. Which you can imagine is just this dense ball, it's this dense ball of neutrons. And just to give you a sense of it, it'll be something about maybe 2 times the mass of the sun, give or take, 1.5 to 3 times the mass of the sun."}, {"video_title": "Supernova (supernovae) Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "If the original star is 9 to 20, I'll say approximately 9 to 20 times the mass of the sun, then it will supernova, and the core will turn into what's called a neutron star. This is a neutron star. Which you can imagine is just this dense ball, it's this dense ball of neutrons. And just to give you a sense of it, it'll be something about maybe 2 times the mass of the sun, give or take, 1.5 to 3 times the mass of the sun. So this is 1.5 to 3 times the mass of the sun in a volume that has a diameter of about, on the order of tens of kilometers. So roughly the size of a city, in a diameter of a city. So this is unbelievably dense, diameter of a city."}, {"video_title": "Supernova (supernovae) Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And just to give you a sense of it, it'll be something about maybe 2 times the mass of the sun, give or take, 1.5 to 3 times the mass of the sun. So this is 1.5 to 3 times the mass of the sun in a volume that has a diameter of about, on the order of tens of kilometers. So roughly the size of a city, in a diameter of a city. So this is unbelievably dense, diameter of a city. I mean, we know how much larger the sun is relative to the earth, and we know how much larger the earth is relative to a city, but this is something more mass than the sun being squeezed into the density or into the size of the city. So unbelievably dense. Now, if the original star is even more massive, if it's more than 20 times the sun, so let me write it over here, let me scroll up."}, {"video_title": "Supernova (supernovae) Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So this is unbelievably dense, diameter of a city. I mean, we know how much larger the sun is relative to the earth, and we know how much larger the earth is relative to a city, but this is something more mass than the sun being squeezed into the density or into the size of the city. So unbelievably dense. Now, if the original star is even more massive, if it's more than 20 times the sun, so let me write it over here, let me scroll up. If it's greater than 20 times the sun, then even the neutron degeneracy pressure, even the pressure, even the neutrons' inability to squeeze further will give up and it'll turn into a black hole. I can do many videos on that, and that's actually an open area of research still on exactly what's going on inside of a black hole. But then you turn into a black hole where essentially all of the mass gets condensed into an infinitely small and dense point, so something unbelievably hard to imagine."}, {"video_title": "Supernova (supernovae) Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Now, if the original star is even more massive, if it's more than 20 times the sun, so let me write it over here, let me scroll up. If it's greater than 20 times the sun, then even the neutron degeneracy pressure, even the pressure, even the neutrons' inability to squeeze further will give up and it'll turn into a black hole. I can do many videos on that, and that's actually an open area of research still on exactly what's going on inside of a black hole. But then you turn into a black hole where essentially all of the mass gets condensed into an infinitely small and dense point, so something unbelievably hard to imagine. And just to give you a sense of it, this will be more mass than even 3 times the mass of the sun, so we're talking about an incredibly high amount of mass. So just to kind of visualize things, here's actually a remnant of a supernova. This is the Crab Nebula."}, {"video_title": "Supernova (supernovae) Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "But then you turn into a black hole where essentially all of the mass gets condensed into an infinitely small and dense point, so something unbelievably hard to imagine. And just to give you a sense of it, this will be more mass than even 3 times the mass of the sun, so we're talking about an incredibly high amount of mass. So just to kind of visualize things, here's actually a remnant of a supernova. This is the Crab Nebula. This right here is the Crab Nebula. And it's about 6,500 light years away. So it's still, you know, from galactic scale, if you think of our galaxy as being 100,000 light years in diameter, it's still not too far from us on those scales, but it's an enormous distance."}, {"video_title": "Supernova (supernovae) Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "This is the Crab Nebula. This right here is the Crab Nebula. And it's about 6,500 light years away. So it's still, you know, from galactic scale, if you think of our galaxy as being 100,000 light years in diameter, it's still not too far from us on those scales, but it's an enormous distance. The closest star to us is 4 light years away, and it would take Voyager traveling at 60,000 kilometers an hour 80,000 years to get there. So this is a very, very... That's only 4 light years. That is 6,500 light years."}, {"video_title": "Supernova (supernovae) Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So it's still, you know, from galactic scale, if you think of our galaxy as being 100,000 light years in diameter, it's still not too far from us on those scales, but it's an enormous distance. The closest star to us is 4 light years away, and it would take Voyager traveling at 60,000 kilometers an hour 80,000 years to get there. So this is a very, very... That's only 4 light years. That is 6,500 light years. So this supernova, it's believed happened 1,000 years ago, right at the center. And so at the center here, we should have a neutron star. And this cloud, the shock wave that you see here, this is still the material traveling outward from that supernova over 1,000 years."}, {"video_title": "Supernova (supernovae) Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "That is 6,500 light years. So this supernova, it's believed happened 1,000 years ago, right at the center. And so at the center here, we should have a neutron star. And this cloud, the shock wave that you see here, this is still the material traveling outward from that supernova over 1,000 years. This shock wave, or the diameter of the sphere of material, is 6 light years. So we could say this distance right here is 6 light years. So this is an enormously big shock wave cloud."}, {"video_title": "Supernova (supernovae) Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And this cloud, the shock wave that you see here, this is still the material traveling outward from that supernova over 1,000 years. This shock wave, or the diameter of the sphere of material, is 6 light years. So we could say this distance right here is 6 light years. So this is an enormously big shock wave cloud. And actually, we believe that our solar system started to form, it started to condense because of a shock wave created by a supernova relatively near to us. And just to answer another question that was kind of jumping up probably in the last video, and this is still not really, really well understood, we talk about how elements up to iron or maybe nickel can be formed inside of the cores of massive stars. So you can imagine when the star explodes, a lot of that material is released into the universe."}, {"video_title": "Supernova (supernovae) Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So this is an enormously big shock wave cloud. And actually, we believe that our solar system started to form, it started to condense because of a shock wave created by a supernova relatively near to us. And just to answer another question that was kind of jumping up probably in the last video, and this is still not really, really well understood, we talk about how elements up to iron or maybe nickel can be formed inside of the cores of massive stars. So you can imagine when the star explodes, a lot of that material is released into the universe. And so that's why we have a lot of these materials in our own bodies. In fact, we could not exist if these heavier elements were not formed inside of the cores of primitive stars, stars that have supernovaed a long time ago. Now, the question is, how do these heavier elements form?"}, {"video_title": "Supernova (supernovae) Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So you can imagine when the star explodes, a lot of that material is released into the universe. And so that's why we have a lot of these materials in our own bodies. In fact, we could not exist if these heavier elements were not formed inside of the cores of primitive stars, stars that have supernovaed a long time ago. Now, the question is, how do these heavier elements form? How do we get all of this other stuff on the periodic table? How do we get all these other heavier elements? And they're formed during the supernova itself."}, {"video_title": "Supernova (supernovae) Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "Now, the question is, how do these heavier elements form? How do we get all of this other stuff on the periodic table? How do we get all these other heavier elements? And they're formed during the supernova itself. It's so energetic. You have all sorts of particles streaming out and streaming in, streaming out because of the force of the shock wave, streaming in because of the gravity. You have all sorts of kind of a mishmash of elements forming, and that's actually where you have your heavier elements forming."}, {"video_title": "Supernova (supernovae) Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "And they're formed during the supernova itself. It's so energetic. You have all sorts of particles streaming out and streaming in, streaming out because of the force of the shock wave, streaming in because of the gravity. You have all sorts of kind of a mishmash of elements forming, and that's actually where you have your heavier elements forming. And because, and I'll talk more about this in future videos, most of the uranium, or actually all of the uranium on Earth right now must have been formed in some type of a supernova explosion, at least based on our current understanding. And it looks to be about 4.6 billion years old. So given that it looks to be about 4.6 billion years old, based on how fast it's decayed, and I'll do a whole video on that, that's why we think that Earth was probably, that our solar system was first formed from some type of supernova explosion because that uranium would have been formed right at about the birth of our solar system."}, {"video_title": "Supernova (supernovae) Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "You have all sorts of kind of a mishmash of elements forming, and that's actually where you have your heavier elements forming. And because, and I'll talk more about this in future videos, most of the uranium, or actually all of the uranium on Earth right now must have been formed in some type of a supernova explosion, at least based on our current understanding. And it looks to be about 4.6 billion years old. So given that it looks to be about 4.6 billion years old, based on how fast it's decayed, and I'll do a whole video on that, that's why we think that Earth was probably, that our solar system was first formed from some type of supernova explosion because that uranium would have been formed right at about the birth of our solar system. Anyway, hopefully you found that interesting. This is a fascinating picture, and if you go to Wikipedia and look up the Crab Nebula, keep clicking on the image and eventually you'll get a zoomed in picture, and that's just kind of even more mind-blowing because you can see all the intricacy in this actual photo."}, {"video_title": "Supernova (supernovae) Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3", "Sentence": "So given that it looks to be about 4.6 billion years old, based on how fast it's decayed, and I'll do a whole video on that, that's why we think that Earth was probably, that our solar system was first formed from some type of supernova explosion because that uranium would have been formed right at about the birth of our solar system. Anyway, hopefully you found that interesting. This is a fascinating picture, and if you go to Wikipedia and look up the Crab Nebula, keep clicking on the image and eventually you'll get a zoomed in picture, and that's just kind of even more mind-blowing because you can see all the intricacy in this actual photo."}] \ No newline at end of file