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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 a... | Intergalactic scale Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
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 a... | Intergalactic scale Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
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, sitt... | Intergalactic scale Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
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... | Intergalactic scale Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
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 gravitatio... | Intergalactic scale Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
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 i... | Intergalactic scale Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
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 ... | Intergalactic scale Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
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 ma... | Intergalactic scale Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
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 becom... | Intergalactic scale Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
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 f... | Intergalactic scale Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
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 ligh... | Intergalactic scale Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
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'... | Intergalactic scale Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
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... | Intergalactic scale Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
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... | Intergalactic scale Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
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. | Intergalactic scale Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
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 t... | Intergalactic scale Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
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 supercluste... | Intergalactic scale Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
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 th... | Intergalactic scale Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
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... | Intergalactic scale Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
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, f... | Intergalactic scale Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
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... | Intergalactic scale Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
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... | Intergalactic scale Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
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 ro... | Intergalactic scale Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
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... | Intergalactic scale Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
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 ... | Intergalactic scale Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
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, l... | Intergalactic scale Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
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, ... | Intergalactic scale Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
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 lig... | Intergalactic scale Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
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 ... | Intergalactic scale Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
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 actu... | Intergalactic scale Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
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 ... | Intergalactic scale Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
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 th... | Intergalactic scale Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
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 t... | Intergalactic scale Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
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... | Intergalactic scale Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
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 ... | Intergalactic scale Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
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 y... | Intergalactic scale Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
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 pri... | Intergalactic scale Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
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,... | Intergalactic scale Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
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 ... | Intergalactic scale Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
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... | Intergalactic scale Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
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 unknow... | Intergalactic scale Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
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 sustainin... | Detectable civilizations in our galaxy 2 Cosmology & Astronomy Khan Academy.mp3 |
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 do... | Detectable civilizations in our galaxy 2 Cosmology & Astronomy Khan Academy.mp3 |
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 c... | Detectable civilizations in our galaxy 2 Cosmology & Astronomy Khan Academy.mp3 |
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 wha... | Detectable civilizations in our galaxy 2 Cosmology & Astronomy Khan Academy.mp3 |
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 ... | Detectable civilizations in our galaxy 2 Cosmology & Astronomy Khan Academy.mp3 |
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 ... | Detectable civilizations in our galaxy 2 Cosmology & Astronomy Khan Academy.mp3 |
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 t... | Detectable civilizations in our galaxy 2 Cosmology & Astronomy Khan Academy.mp3 |
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 ... | Detectable civilizations in our galaxy 2 Cosmology & Astronomy Khan Academy.mp3 |
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 i... | Detectable civilizations in our galaxy 2 Cosmology & Astronomy Khan Academy.mp3 |
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 ... | Detectable civilizations in our galaxy 2 Cosmology & Astronomy Khan Academy.mp3 |
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 ra... | Detectable civilizations in our galaxy 2 Cosmology & Astronomy Khan Academy.mp3 |
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 y... | Detectable civilizations in our galaxy 2 Cosmology & Astronomy Khan Academy.mp3 |
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. Signa... | Detectable civilizations in our galaxy 2 Cosmology & Astronomy Khan Academy.mp3 |
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 ... | Detectable civilizations in our galaxy 2 Cosmology & Astronomy Khan Academy.mp3 |
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 un... | Detectable civilizations in our galaxy 2 Cosmology & Astronomy Khan Academy.mp3 |
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 ... | Detectable civilizations in our galaxy 2 Cosmology & Astronomy Khan Academy.mp3 |
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. M... | Detectable civilizations in our galaxy 2 Cosmology & Astronomy Khan Academy.mp3 |
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 poin... | Detectable civilizations in our galaxy 2 Cosmology & Astronomy Khan Academy.mp3 |
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 detectab... | Detectable civilizations in our galaxy 2 Cosmology & Astronomy Khan Academy.mp3 |
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 civilizations in our galaxy 2 Cosmology & Astronomy Khan Academy.mp3 |
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... | Detectable civilizations in our galaxy 2 Cosmology & Astronomy Khan Academy.mp3 |
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 mee... | Detectable civilizations in our galaxy 2 Cosmology & Astronomy Khan Academy.mp3 |
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 li... | Detectable civilizations in our galaxy 2 Cosmology & Astronomy Khan Academy.mp3 |
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 jus... | Detectable civilizations in our galaxy 2 Cosmology & Astronomy Khan Academy.mp3 |
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... | Detectable civilizations in our galaxy 2 Cosmology & Astronomy Khan Academy.mp3 |
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 ... | Detectable civilizations in our galaxy 2 Cosmology & Astronomy Khan Academy.mp3 |
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... | Potassium-argon (K-Ar) dating Cosmology & Astronomy Khan Academy.mp3 |
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, an... | Potassium-argon (K-Ar) dating Cosmology & Astronomy Khan Academy.mp3 |
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 pota... | Potassium-argon (K-Ar) dating Cosmology & Astronomy Khan Academy.mp3 |
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 potas... | Potassium-argon (K-Ar) dating Cosmology & Astronomy Khan Academy.mp3 |
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 pota... | Potassium-argon (K-Ar) dating Cosmology & Astronomy Khan Academy.mp3 |
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 ... | Potassium-argon (K-Ar) dating Cosmology & Astronomy Khan Academy.mp3 |
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 op... | Potassium-argon (K-Ar) dating Cosmology & Astronomy Khan Academy.mp3 |
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 s... | Potassium-argon (K-Ar) dating Cosmology & Astronomy Khan Academy.mp3 |
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 physi... | Potassium-argon (K-Ar) dating Cosmology & Astronomy Khan Academy.mp3 |
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 proto... | Potassium-argon (K-Ar) dating Cosmology & Astronomy Khan Academy.mp3 |
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 liqui... | Potassium-argon (K-Ar) dating Cosmology & Astronomy Khan Academy.mp3 |
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 he... | Potassium-argon (K-Ar) dating Cosmology & Astronomy Khan Academy.mp3 |
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 a... | Potassium-argon (K-Ar) dating Cosmology & Astronomy Khan Academy.mp3 |
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,... | Potassium-argon (K-Ar) dating Cosmology & Astronomy Khan Academy.mp3 |
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 ma... | Potassium-argon (K-Ar) dating Cosmology & Astronomy Khan Academy.mp3 |
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... | Potassium-argon (K-Ar) dating Cosmology & Astronomy Khan Academy.mp3 |
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 ... | Potassium-argon (K-Ar) dating Cosmology & Astronomy Khan Academy.mp3 |
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... | Potassium-argon (K-Ar) dating Cosmology & Astronomy Khan Academy.mp3 |
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 nec... | Potassium-argon (K-Ar) dating Cosmology & Astronomy Khan Academy.mp3 |
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... | Potassium-argon (K-Ar) dating Cosmology & Astronomy Khan Academy.mp3 |
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 potassi... | Potassium-argon (K-Ar) dating Cosmology & Astronomy Khan Academy.mp3 |
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 on... | Potassium-argon (K-Ar) dating Cosmology & Astronomy Khan Academy.mp3 |
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 ... | Potassium-argon (K-Ar) dating Cosmology & Astronomy Khan Academy.mp3 |
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... | Potassium-argon (K-Ar) dating Cosmology & Astronomy Khan Academy.mp3 |
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 the... | Potassium-argon (K-Ar) dating Cosmology & Astronomy Khan Academy.mp3 |
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... | Potassium-argon (K-Ar) dating Cosmology & Astronomy Khan Academy.mp3 |
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 t... | Potassium-argon (K-Ar) dating Cosmology & Astronomy Khan Academy.mp3 |
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 he... | Potassium-argon (K-Ar) dating Cosmology & Astronomy Khan Academy.mp3 |
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 at... | Birth of stars Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3 |
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 ma... | Birth of stars Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3 |
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 eac... | Birth of stars Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3 |
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. S... | Birth of stars Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3 |
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. An... | Birth of stars Stars, black holes and galaxies Cosmology & Astronomy Khan Academy.mp3 |
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