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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 physic... | Introduction to light Electronic structure of atoms Chemistry Khan Academy.mp3 |
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,... | Introduction to light Electronic structure of atoms Chemistry Khan Academy.mp3 |
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 imag... | Introduction to light Electronic structure of atoms Chemistry Khan Academy.mp3 |
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 wa... | Introduction to light Electronic structure of atoms Chemistry Khan Academy.mp3 |
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 nan... | Introduction to light Electronic structure of atoms Chemistry Khan Academy.mp3 |
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 l... | Introduction to light Electronic structure of atoms Chemistry Khan Academy.mp3 |
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 littl... | Introduction to light Electronic structure of atoms Chemistry Khan Academy.mp3 |
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 electromagneti... | Introduction to light Electronic structure of atoms Chemistry Khan Academy.mp3 |
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 all... | Introduction to light Electronic structure of atoms Chemistry Khan Academy.mp3 |
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. Infrare... | Introduction to light Electronic structure of atoms Chemistry Khan Academy.mp3 |
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 exampl... | Introduction to light Electronic structure of atoms Chemistry Khan Academy.mp3 |
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 ... | Introduction to light Electronic structure of atoms Chemistry Khan Academy.mp3 |
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 thin... | Introduction to light Electronic structure of atoms Chemistry Khan Academy.mp3 |
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 tha... | Introduction to light Electronic structure of atoms Chemistry Khan Academy.mp3 |
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 cr... | Structure of the Earth.mp3 |
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... | Structure of the Earth.mp3 |
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... | Structure of the Earth.mp3 |
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 ... | Structure of the Earth.mp3 |
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 so... | Structure of the Earth.mp3 |
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 kilom... | Structure of the Earth.mp3 |
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 chemic... | Structure of the Earth.mp3 |
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 t... | Structure of the Earth.mp3 |
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 thi... | Structure of the Earth.mp3 |
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 mant... | Structure of the Earth.mp3 |
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. An... | Structure of the Earth.mp3 |
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 ... | Structure of the Earth.mp3 |
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. | Structure of the Earth.mp3 |
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. | Structure of the Earth.mp3 |
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. S... | Structure of the Earth.mp3 |
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 th... | Structure of the Earth.mp3 |
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 talk... | Structure of the Earth.mp3 |
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 exact... | Structure of the Earth.mp3 |
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... | Structure of the Earth.mp3 |
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 asthen... | Structure of the Earth.mp3 |
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... | Structure of the Earth.mp3 |
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 h... | Structure of the Earth.mp3 |
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 t... | Structure of the Earth.mp3 |
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.... | Structure of the Earth.mp3 |
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 ge... | Structure of the Earth.mp3 |
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, ... | Structure of the Earth.mp3 |
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 kilomete... | Structure of the Earth.mp3 |
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 cor... | Structure of the Earth.mp3 |
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 ki... | Structure of the Earth.mp3 |
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... | Structure of the Earth.mp3 |
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 ni... | Structure of the Earth.mp3 |
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 ... | Structure of the Earth.mp3 |
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 ... | Structure of the Earth.mp3 |
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. Chemical... | Structure of the Earth.mp3 |
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... | Structure of the Earth.mp3 |
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 t... | Structure of the Earth.mp3 |
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 ... | Structure of the Earth.mp3 |
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 loo... | Land productivity limiting human population Cosmology & Astronomy Khan Academy.mp3 |
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 human... | Land productivity limiting human population Cosmology & Astronomy Khan Academy.mp3 |
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, ... | Land productivity limiting human population Cosmology & Astronomy Khan Academy.mp3 |
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... | Land productivity limiting human population Cosmology & Astronomy Khan Academy.mp3 |
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 t... | Land productivity limiting human population Cosmology & Astronomy Khan Academy.mp3 |
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 whe... | Land productivity limiting human population Cosmology & Astronomy Khan Academy.mp3 |
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... | Land productivity limiting human population Cosmology & Astronomy Khan Academy.mp3 |
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 ... | Land productivity limiting human population Cosmology & Astronomy Khan Academy.mp3 |
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... | Land productivity limiting human population Cosmology & Astronomy Khan Academy.mp3 |
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 lower... | Land productivity limiting human population Cosmology & Astronomy Khan Academy.mp3 |
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... | Land productivity limiting human population Cosmology & Astronomy Khan Academy.mp3 |
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 i... | Land productivity limiting human population Cosmology & Astronomy Khan Academy.mp3 |
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 ki... | Land productivity limiting human population Cosmology & Astronomy Khan Academy.mp3 |
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 pastora... | Land productivity limiting human population Cosmology & Astronomy Khan Academy.mp3 |
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 ove... | Land productivity limiting human population Cosmology & Astronomy Khan Academy.mp3 |
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,... | Land productivity limiting human population Cosmology & Astronomy Khan Academy.mp3 |
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... | Land productivity limiting human population Cosmology & Astronomy Khan Academy.mp3 |
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,... | Land productivity limiting human population Cosmology & Astronomy Khan Academy.mp3 |
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 defin... | Land productivity limiting human population Cosmology & Astronomy Khan Academy.mp3 |
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 ... | Land productivity limiting human population Cosmology & Astronomy Khan Academy.mp3 |
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... | Land productivity limiting human population Cosmology & Astronomy Khan Academy.mp3 |
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 yo... | Land productivity limiting human population Cosmology & Astronomy Khan Academy.mp3 |
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 thi... | Land productivity limiting human population Cosmology & Astronomy Khan Academy.mp3 |
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 pe... | Land productivity limiting human population Cosmology & Astronomy Khan Academy.mp3 |
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. ... | Land productivity limiting human population Cosmology & Astronomy Khan Academy.mp3 |
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 m... | Land productivity limiting human population Cosmology & Astronomy Khan Academy.mp3 |
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... | Land productivity limiting human population Cosmology & Astronomy Khan Academy.mp3 |
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 th... | Land productivity limiting human population Cosmology & Astronomy Khan Academy.mp3 |
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 th... | Land productivity limiting human population Cosmology & Astronomy Khan Academy.mp3 |
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 t... | Land productivity limiting human population Cosmology & Astronomy Khan Academy.mp3 |
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 countr... | Land productivity limiting human population Cosmology & Astronomy Khan Academy.mp3 |
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 d... | Land productivity limiting human population Cosmology & Astronomy Khan Academy.mp3 |
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 o... | Land productivity limiting human population Cosmology & Astronomy Khan Academy.mp3 |
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 l... | Land productivity limiting human population Cosmology & Astronomy Khan Academy.mp3 |
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, ... | Land productivity limiting human population Cosmology & Astronomy Khan Academy.mp3 |
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 prod... | Land productivity limiting human population Cosmology & Astronomy Khan Academy.mp3 |
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 productivity limiting human population Cosmology & Astronomy Khan Academy.mp3 |
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, ever... | Scale of distance to closest stars Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
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... | Scale of distance to closest stars Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
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 ... | Scale of distance to closest stars Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
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 i... | Scale of distance to closest stars Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
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 ... | Scale of distance to closest stars Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
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 w... | Scale of distance to closest stars Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
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... | Scale of distance to closest stars Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
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 i... | Scale of distance to closest stars Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
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 count... | Scale of distance to closest stars Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
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... | Scale of distance to closest stars Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
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 al... | Scale of distance to closest stars Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
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 ... | Scale of distance to closest stars Scale of the universe Cosmology & Astronomy Khan Academy.mp3 |
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