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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 s... | Cosmic background radiation 2 Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
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 ... | Cosmic background radiation 2 Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
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 billio... | Cosmic background radiation 2 Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
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 ... | Cosmic background radiation 2 Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
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... | Cosmic background radiation 2 Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
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 afte... | Cosmic background radiation 2 Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
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. | Cosmic background radiation 2 Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
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 g... | Cosmic background radiation 2 Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
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 t... | Cosmic background radiation 2 Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
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 th... | Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3 |
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 ... | Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3 |
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. An... | Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3 |
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 did... | Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3 |
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... | Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3 |
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... | Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3 |
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... | Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3 |
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 ... | Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3 |
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... | Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3 |
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 i... | Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3 |
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... | Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3 |
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 Cep... | Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3 |
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... | Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3 |
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 wha... | Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3 |
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... | Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3 |
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 appr... | Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3 |
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 t... | Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3 |
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 furt... | Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3 |
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 ... | Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3 |
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 loga... | Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3 |
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... | Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3 |
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 no... | Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3 |
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, th... | Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3 |
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 loo... | Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3 |
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... | Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3 |
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 ga... | Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3 |
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 rel... | Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3 |
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... | Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3 |
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 ... | Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3 |
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 r... | Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3 |
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 m... | Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3 |
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 ... | Cepheid variables 1 Stars, black holes and galaxies Cosmology & Astronomy Khan Academy (2).mp3 |
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 t... | Red shift Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
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... | Red shift Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
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 mo... | Red shift Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
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. No... | Red shift Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
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 ... | Red shift Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
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 ... | Red shift Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
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. | Red shift Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
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. | Red shift Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
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. | Red shift Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
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... | Red shift Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
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 p... | Red shift Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
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 th... | Red shift Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
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 sh... | Red shift Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
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 statio... | Red shift Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
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 li... | Red shift Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
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 m... | Red shift Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
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 ver... | Red shift Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
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 no... | Red shift Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
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. | Red shift Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
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 wav... | Red shift Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
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 havi... | Red shift Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
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 ... | Red shift Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
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... | Red shift Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
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 gree... | Red shift Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
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... | Red shift Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
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 aw... | Red shift Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
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... | Red shift Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
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 soun... | Red shift Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
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 sa... | Red shift Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
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... | Red shift Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
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 ... | Red shift Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
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 trave... | Red shift Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
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... | Red shift Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
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 eve... | Red shift Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
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 th... | Red shift Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
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 b... | Red shift Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
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 m... | Red shift Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
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 po... | Seasons aren't dictated by closeness to sun Cosmology & Astronomy Khan Academy.mp3 |
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... | Seasons aren't dictated by closeness to sun Cosmology & Astronomy Khan Academy.mp3 |
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 poin... | Seasons aren't dictated by closeness to sun Cosmology & Astronomy Khan Academy.mp3 |
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... | Seasons aren't dictated by closeness to sun Cosmology & Astronomy Khan Academy.mp3 |
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 ... | Seasons aren't dictated by closeness to sun Cosmology & Astronomy Khan Academy.mp3 |
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 p... | Seasons aren't dictated by closeness to sun Cosmology & Astronomy Khan Academy.mp3 |
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,... | Seasons aren't dictated by closeness to sun Cosmology & Astronomy Khan Academy.mp3 |
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 ... | Seasons aren't dictated by closeness to sun Cosmology & Astronomy Khan Academy.mp3 |
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... | Seasons aren't dictated by closeness to sun Cosmology & Astronomy Khan Academy.mp3 |
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 p... | Seasons aren't dictated by closeness to sun Cosmology & Astronomy Khan Academy.mp3 |
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 d... | Seasons aren't dictated by closeness to sun Cosmology & Astronomy Khan Academy.mp3 |
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 ... | Seasons aren't dictated by closeness to sun Cosmology & Astronomy Khan Academy.mp3 |
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 ... | Seasons aren't dictated by closeness to sun Cosmology & Astronomy Khan Academy.mp3 |
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 sa... | Seasons aren't dictated by closeness to sun Cosmology & Astronomy Khan Academy.mp3 |
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... | Energy inputs for tilling a hectare of land Cosmology & Astronomy Khan Academy.mp3 |
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 o... | Energy inputs for tilling a hectare of land Cosmology & Astronomy Khan Academy.mp3 |
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 th... | Energy inputs for tilling a hectare of land Cosmology & Astronomy Khan Academy.mp3 |
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 e... | Energy inputs for tilling a hectare of land Cosmology & Astronomy Khan Academy.mp3 |
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 chang... | Energy inputs for tilling a hectare of land Cosmology & Astronomy Khan Academy.mp3 |
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 agricultu... | Energy inputs for tilling a hectare of land Cosmology & Astronomy Khan Academy.mp3 |
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 mu... | Energy inputs for tilling a hectare of land Cosmology & Astronomy Khan Academy.mp3 |
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 ... | Energy inputs for tilling a hectare of land Cosmology & Astronomy Khan Academy.mp3 |
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