Why Craters Are Round, Size of Life, The Real Length of Day – TYTYK

Shape of moon craters

0:00Thanks for showing up for these. You know how I come up with them? They're like things that people know about, but they don't know as much as they think they should know or might want to know after they learned all they could know. That was very Dr. Seuss-y of you.

0:18Welcome to StarTalk, your place in the universe where science and pop culture collide. StarTalk begins right now.

0:35Something very simple, the shape of craters. Shape of craters? Yeah, a picture of the moon, any photograph of the moon, what shape are the craters? Well, they're all circle, concave little circles. They're perfect circles. Every single one of them. The big ones, the little ones, the medium ones, they are perfect circles. Okay? So, this- Let me guess. Let me guess.

1:04Aliens, just like on Earth, the same way they make the crop circles. They make moon circles. That's how they make the moon circles.

1:15Moon circles. So, yeah, here's the thing. It's interesting that as humans, if there's something scientists don't understand, people rapidly just go to God or aliens. Well, that's why they got the explanation for why. All right. It turned out it was a mystery why all craters are perfect circles for the longest time. It was a mystery until about 100 years ago. Okay. And let me tell you why it was a mystery. All right? So, your first thought is, well,

1:48the moon doesn't have an atmosphere. Maybe it's getting slammed by meteors. Right. Okay? But is every meteor coming straight down at a 90-degree angle so that when it hits, it makes a perfect, surely some of them are at an angle. Right. You'd expect some craters to be elongated. Right. And you'd have all sort of shapes from circle to very flattened circles. You'd expect that.

2:20Some of them would be divots. Like when you swing a golf, you know, it'd just be like a- Oh, okay. Where it came in on an angle. Right. On the angle from, and with the, with the, with the, because you'd have a, the slope would be different on one side than the other. Then on the other. Right. Because it came, it slid in. It slid in. So, you'd expect that and it is nowhere to be found on the moon. So, this energized the geologists because we know that the dark areas of the moon is where lava has once flowed.

2:50Okay? Oh, cool. I think they call it basalt. Basalt? It's dark and it's lava fields. All right? And by the way, the Apollo astronauts, aimed for those places because they're flat. Right. If you're going to go a quarter million miles and land sideways and fall over, that's it. All right? So, they found the flattest areas. And those are called seas. These are, before people knew that the moon didn't have weather like we have or oceans, these large flat areas were called seas. The sea of tranquility. Okay? That's where it gets its name.

3:24Nice. And all the seas were named after psychological states or physiological states. So, there's a sea of fecundity. There's a sea. Oh, all right. Yeah, all right. So, there's a, anyway, but, so, they're flat because lava in liquid state flattens out, right? There you go. Okay. All right. So, wherever there was a crater, it was round. So, the geologist said, it's probably not these asteroids because they'd have to all come in perpendicular. They're probably all volcanic craters. Because volcanic craters, you punch up, it makes a hole, the

3:56hole is a circle. Right. Okay. You know, you can, you can pull that off. So, that's, that's how it stood for the longest while. Until people saw craters on top of places where it didn't look like lava had flowed. Okay. So, how are you going to have a crater and no lava signature anywhere near it? What's up with that? And so, this was a conundrum. Okay. I have a book from 1890. And it says,

4:32believed by many to be volcanic in origin, perhaps they're actually of asteroid impacts. We are not sure. Okay. It's, by the way, it was that way for most of the 20th century. Until computers came along. And you could simulate impacts. Nice.

High speed impact physics

4:56You want to simulate it. Okay. So, here you go. You ready? If you, if you have an impactor that comes in straight into a surface, it makes a circular crater. Right. If you send it at an angle, it will make an elongated crater. Okay. This will happen. So, they're saying, no, wait a minute, let's send it in faster.

5:18Turns out, there is a magic speed with which you can send in an asteroid. And on collision, it will make a perfect circle, no matter the angle it hits. No matter the angle it hits. Correct. And that speed. Okay. This is, this is what, okay. You ready? Let me guess. Let me guess. 88 miles per hour.

5:46Precisely. The same speed that it takes to propel you into the future.

5:54Because the flux capacitor. Because the flux capacitor.

6:01Okay. 89 miles per hour. All right. So, here's the, so here's what's going on. Let's, let's take a rock. And you can ask, why does the rock sort of hang together, right? Why isn't it just decomposed into sand? Well, because there are molecules that are attached. All right. It's molecularly attached within itself. Right. Right. These are electromagnetic forces. You can, we know what they are. Okay. All right. You can add up how much energy is contained in all of these molecular bonds.

6:37Okay. And you can come, write down that number. And you say, this rock is held together by this much energy. You can do that. Okay. Okay. All right. Some rocks are held together with not much energy. Like if you take a sort of a semi, a kind of solid thing. Like a snowball. Or I was going to get to that in a minute. Right. Okay. Like a sand ball. Right. Right. That if you just sort of punch it, the whole thing just crumbles back into sand, but it holds its shape. It held together just for a little while. Not much energy is holding that together. Snowball. Another example, not much energy

7:12is holding, unless you like really pack it in. And what you're doing is you're melting the snow and have refreeze and that holds it up better. We had a whole, we had a whole explainer on the freezing and melting of ice. So what you're really doing is saying to the other person you're having a snowball fight with, I hate you. I hate you even more. Even more. Cause this is going to be a nice little ice ball. So that helps. And if it's held together only loosely, you can't even throw it without the thing flying apart. Okay. So you can write down how much energy that is.

7:47Calculate it and write it down. Now move the object through space or through the air. There's a kinetic energy it has. And this is the energy of motion. It's called kinetic energy. There's a formula. It's one half times the mass of the object times the velocity squared. Okay. All right. Okay. Now the moment, the kinetic energy that the object has exceeds the binding energy of

8:19the molecules, right? You have to ask it. Well, if the thing was going at that speed and then after it hits, it's going at zero speed, then what happened to all that kinetic energy? It got pumped back into the object. Right. But that's more energy than what is holding the object together in the first place. Right. So if you have more kinetic energy than the binding energy of the thing that has the kinetic energy, it's going to explode. On impact, it explodes precisely. Wow.

8:55So we call these high speed collisions and it's not just, oh, cause it's going fast. No, it is higher speed than the energy that's holding it together. So on impact, it's an explosion and explosions happen in all directions. Now, is that because the moon has no atmosphere? Because that doesn't happen on earth. So, so, so let's, I'll get to that in a minute. Okay. Let me get to that. Just imagine the air is not a thing. Just what you're talking about. Okay. Imagine air is not a thing. Energy. Just talking about the energy. Okay. Okay. All right. So, so there you have it. That's the entire

9:31reason. So in fact, all the craters on the moon, damn near all the craters on them, there might be one or two that were volcano. All the rest are asteroids, meteors coming in at whatever angle they choose. Doesn't make a difference. That doesn't make a difference. They all explode on impact. They explode on impact because they are coming in at what we call a high speed collision. Now, now let's look at snowballs. You should try this. Okay. When you throw a snowball at a wall,

10:04winter's coming up. Okay. We're recording this in November. So you throw it at a wall. When it hits the wall, it doesn't stay as a snowball. No. It explodes. It basically pops. It goes everywhere. It goes everywhere. And try this. Except for one little white dot that's stuck to the wall. One little white dot that's stuck. So you've done this before. Yeah, I can. This to all our Hawaii friends and other friends who don't have snowball fights. So try throwing it at different angles when winter comes. You will see that every time it explodes on impact and you see the scatter all around it in

10:41equal directions. That's because even at your speed, okay, you're not going hypersonic speeds. It's just the speed that you throw it. That's more kinetic energy than is the binding energy of the snowball. Right. So that's why a high speed collision, you have to ask what's binding the object and how much energy does it have of motion. And it's the relationship of those two numbers that'll tell you if it's a high speed collision. So a snowball on someone's chest exploding is a high speed collision. Nice. That's super cool. Yeah. Yeah. And so let's just hope though that you never

11:18throw a snowball hard enough to leave a crater in somebody. That would be more energy than is necessary to explode the snowball at the other side, but correct. Yeah. You don't want to be leaving craters. All right. So let's get back to the atmosphere. Very important and interesting question. Okay. So now an asteroid comes and it sees the atmosphere. Right. Well, the atmosphere

11:49is going to slow it down a bit, isn't it? Okay. It's going to slow it down. If it slows it down enough so that it's kinetic energy is less than the energy that binds it together, it will survive on impact. Right. We do find meteorites. We had the ITE after it's on the ground, meteor coming through the air, meteorite. We find there are meteorites that make it to the ground. Okay. Wow. Yeah. Right.

12:19We have them. They're there. Okay. They slowed down enough. Put the brakes on them. Put the brakes on them and it did not completely explode. Now in the explosion, there could be some pieces that remain intact just in the explosion. So in Meteor Crater, Arizona, which used to be called Barringer Crater and Pete, and Georgia said, this has got to be volcanic because it's a perfect circle, but there's no volcanic activity in Arizona. Right. There's just, there's no volcano for miles. All right. So that was a conundrum when we figured it out, it got renamed Meteor Crater. Wow. And those,

12:52those computer simulations were abundant in the early seventies when we were able to program computers in the science laboratories. So that, that asteroid, they were saying, well, where is the asteroid? If an asteroid did it, let's dig and find the big asteroid. All right. And so they dug and they didn't find anything because they didn't know that the sucker blew up. Okay. And so a person bought that land, a mining company bought that land on the expectation that they could mine the metals of the asteroid that must be buried deep in it. And it was nowhere to, 90% of it vaporized on impact.

13:27Yes. They're pieces that are scattered around. And, and those pieces are now in collections. We have some at the museum, but most of it is gone for that reason. And that's why Crater's around. There you go. Okay. You know, we keep doing this. We'll run out of stuff to explain.

13:45That's like reaching the end of the internet, which I don't think it's going to happen. Well, the end of Netflix movies, right? Right. It's just, so the next one.

Earth rotation and time

14:11Here's yet another fact from Lost in Space, the latest collaboration between StarTalk and National Geographic. Saturn is the only planet in the solar system whose average density is less than that of water, which means if you found a bathtub big enough, Saturn would float. If you like that fact, you can find 4,999 more in Lost in Space. 5,000 facts to help navigate the

14:51universe. Lost in Space is now available for pre-order wherever books are sold. How long does Earth take to rotate on its axis? From what I've been told, 24 hours. 24 hours. Okay. In fact, we kind of defined 24 hours to mean that.

15:21Right. So, that's how we divide up time. Okay. So, as you might have guessed, it doesn't take 24 hours. It actually takes 23 hours, 56 minutes, and 4 seconds. Okay. Really? Yes. Now, wait a minute. Wait, wait, wait. Hold on. So, let me just be clear about what I mean. 23 hours. Get rid of the entire, just get rid of the entire solar system. Okay. Watch Earth rotate. Okay? And stand there and time it. It will, that spot that's in front of you will

15:51come back around in 23 hours, 56 minutes, and 4 seconds. One full revolution. Okay. Yes. Okay. That's called a sidereal day. I like this. Because sidereal means stars, star day. A sidereal day. So, you're imagining with respect to the universe.

16:07Okay? Nice. However, we don't base our lives on when stars return to the same spot in the sky, in the night sky. We base our days on when the sun, which also happens to be a star, returns to its spot on the sky. Okay? It turns out that takes longer than 23 hours and 56. Why? I'll tell you why. Because in the time it took Earth to rotate, 23 hours and 56 minutes, it actually moved almost one degree in its orbit around the sun.

16:43Gosh. So, it rotates back to where its previous line, but it has to turn a little bit extra. A little bit extra to get back to the same thing. A little bit extra to put the sun back in the same spot on the sky. Right. And that's a little few minutes extra. That's fantastic. Oh my God, that's, that's just, I love that. I've only just begun. So, so, that little extra four minutes. So, I said we move a degree in our orbit each day. Right. You know, I didn't just pull that out. How many, how many days are there in a year?

17:14Uh, 365.

17:17Okay. And how many degrees in a circle? 360. Yeah. So, it's about a degree a day. It's about a degree a day. Yeah. We, we, so technically it's one degree and five, 365ths. Oh, wow. Of a degree. But it's basically a degree a day. And in that degree, you have to turn that little extra. So, just imagine you keep having to turn your head. Just a little bit more. Extra amount. Okay. Because you move just a little bit. So, now you got to look back just a little bit. Correct. That's cool. Well, let's keep going. Okay. All right. Earth's orbit around the sun is not a perfect circle. Oh, by the way.

17:47So, the first day is a sidereal day. The second day is a solar day. Gotcha. Okay? So, for obvious reasons. And they don't, they're not the same. All right. Now, Earth's orbit around the sun is not a perfect circle. Right. Sometimes, which means sometimes we're farther away, sometimes we're closer. When we are closer, we are moving faster in our orbit than when we are farther away. Okay. Okay? Okay. That's how gravity works. So, it turns out that extra four minutes is not the same if we're farther from the sun than if we're closer to the sun.

18:26Because we move more than our allocated fraction of a circle when we're close to the sun and less when we're farther away from the sun. Gotcha. Okay. So, the length of the solar day is changing continually throughout the year. Wow. And sometimes it's less than 24 hours. Sometimes it's more than 24 hours. So, sometimes the sun gets to its highest point in the sky before clock noon.

18:59Right. And sometimes it gets… So, what we do is we just average that over the whole year and say, sun, you are average 24 hours and there you go. Wow. By the way, if you ever look at sundials, there's a map on the sundial that corrects for the sun being early and the sun being late. You either add or subtract up to 14 minutes of the day from what the sun time reads in order to get your clock time. Wow. All right. Every sundial has it.

19:30So, it's a figure eight. It's called an analemma. It's got a name, an analemma. But I don't want to talk about analemmas right now. So, what you have on your clock is the average length of a day over the 365 days of the year. All right. So, now we define that as 24 hours, each hour 60 minutes, each minute 60 seconds. Got it. Okay. Well, what is defining this? Well, it's the rotation of the earth. Right. Okay. Makes sense. Well, all right. Well, how stable is that?

20:00Even in the perfect average that we're taking? Well, you'll never know, will you? Because you're using earth to define the time frame. Right. Exactly. If earth is your measure and you are slowing down, you will never know this. Just that there's no way to know it. Right. Okay. Now, back when I was growing up, watches, they would boast. This is how old I am. They would say, get this watch. It's so accurate. It's accurate to two minutes a month.

20:30Wow. What? That was like accurate. That is not a good advertising. Okay. Well, then it was because, you know, cheap watches were 10 minutes a month. You'd have to reset your watch every day. Why do you think heist movies would always say, let's synchronize our clocks. Right. Before they perform the heist. That's cool. They knew that the clocks were not keeping good time. Earth was keeping the best time of them all. Okay. So now, you pose the question, maybe earth is not the best time king of the heist.

21:01Let's offload the responsibility of keeping time to something else. Like a vibrating atom. So we did that. That sounds good. So the cesium-137. There's a electron transition between two energy levels that has a very precise frequency. Okay. Very precise. You can measure. What's good about that is any lab can get some cesium and measure this and then define the length of a second in their lab. Okay.

21:31Okay. When you do this, you multiply it by 60. You multiply that by 60, you get the hour. You multiply that by 24. You do this and you find out that earth is slowing down. Well, you know, it's kind of old.

21:50I'm just saying. It's been added. It's tired. It's a long time. It's tired. If you're 4.6 billion years old, you might slow down a little bit too. You might be tired too. I'm just saying. You might lose a step on your game. That's very sweet of you, Chuck. That's very geriatrically sensitive of you. So you only would know that earth was slowing down once you offload it to something else that tracks time. Right. And what we found out is that the sloshing of tides on the ocean floor, on the beachfronts, actually works to slow down the rotation of the earth.

22:30Oh, wow. Okay. Because the moon is causing these tides. Right. Okay. And we're rotating faster than the moon is orbiting us. So the moon is tugging on tides backwards on our attempt to rotate. Right. Okay. So it's almost like there's a counterweight. A counterbalance. Exactly. A counterbalance that's pulling against us. Against us. Wow. Now, tides are slowing down the rotation of the earth.

23:01And in response, the moon is spiraling away from us by a couple inches a year. Okay. That's in response to earth slowing down. It all relates to what's called the conservation of angular momentum, but it's a big ballet. Okay. And so we've been slowing down. And Chuck, that's, so what we could do is say, let's redefine the second, the length of the second, so that we always have 24 hours and 60 minutes and 60 seconds. But that's messy. Right. Every year, here's a new definition of the second, folks.

23:32Go to your lab, redefine the cesium atom. It's much easier to accept the fact that, you know, we're breaking up with the moon, but, you know, it's just taking some time. Breaking up is hard to do. Everybody has to be on the same page so the breakup can be as amicable as possible. So that's, that's, that's, that's a nice way to think about it. So, so this, so we, the way we compensate for this is how we add a leap second. When, when do we, oh, well, it has to be every four years.

24:03Then we add a. No, no, no, no. No. Did I mention years at all? I'm talking about days, dude. Days. Every four days? No, no, no, no, no, no. What I'm saying is we monitor when we have fell behind by a second. Okay. So when the second, when we know that we've lost a second based on our atomic measurement. Correct. Then internationally we say time to throw in a leap second. Now we throw it in. By convention, we throw it in on June 30th or on December 31st. And we can throw in one at each time if we needed it.

24:35Okay. Since 1973 or four, something like early seventies, there's been 23, 24 leap seconds added to the calendar. Sweet. So when they're going to add a leap second, they choose which of those days it will be. When they do so, that final minute has 61 seconds in it. Nice. Yes. It's very cool. It's very cool. And so, so this is what we do. The longest minute of. Yes. It's the longest minute because it's actually 61 seconds. 61 seconds. Okay.

25:05So newspapers like having fun with this. They say this year will be slightly longer than other years because you have to throw in the leap second. That leap second happens on Greenwich time. So for us, it would take place at like, I guess, 7 p.m. Right. Because we're five hours behind them. On the Greenwich Mean Time. Right. Greenwich Mean Time. Right. So I was at a dinner party once in the June 30th leap second and 7 p.m. That's when you're having dinner. And I said, pause. I got out the atomic clock and we just watched that sucker tick.

25:3761 seconds went by. We toasted it with a sip of champagne and then went on our way. Okay. Why are you walking around with an atomic clock? No, no. I have, wait, just don't have access. Don't just, let's not just, I got people. Okay, right on.

Factors shifting earth rotation

25:53All right. So a couple more things. Other things that can change the rotation of the earth. Not just the tidal sloshing. Right. Okay. But here's one for you. Are you ready? Go ahead. All right. You've seen skaters who want to speed up their speed. So what do they do? Their hands start out extended. Uh-huh. And they start with a slight rotation. They bring in their hands and they spin faster. Right. Right. What they've done is they've changed where the mass is relative to their rotation axis. Right. So the farther away the mass is, the slower they're going to rotate. The closer it is to the rotation, the faster they'll turn.

26:26Right. Okay. If earth has an earthquake and one of the continental slabs shifts north, okay, then there's mass on earth's surface that used to be closer to the equator the day before and has now moved closer to the pole, that will have the effect of speeding up the rotation of the earth. Right. Large animals that migrate, okay, will change where the mass of the earth is from the northern

27:03climes down to the south and they're farther away from the rotation of the axis. So not only does, do tides affect the rotation of the earth, my seasonal migration of animals affects the rotation of the earth. And so does earthquakes, so do volcanoes and anything else that's remapping the surface of the earth. And you know what else will change it? The melting of land glaciers. Uh-oh, we're in trouble. Okay. You got Antarctica has land glaciers as it melts.

27:34What used to be right there near the pole now melts and goes closer to the equator. That will slow down the rotation of the earth because that's like the skater bringing their hands farther away. Right. So all of these factors combined, basically, the earth today has been increasing rather than, in principle, it could decrease depending on what's going on. Right. Then I had this diabolical idea. We'd go set up jet engines, anchor them to the earth, face them either due east or due west and ignite them.

28:04And then the exhaust would help speed up or slow down the earth. I thought we might do that. Then I did the math on that. And? And no, it's not going to work. It's not enough. It's just not enough. You would have made a good Bond villain, though.

28:20Yeah. It's just, you know, it's like Lex Luthor saying, let me create an earthquake to make these front properties in California. So, yeah, it's geoengineering on a huge scale. But in principle, you could do that. But if you take the greatest engines and fire them, the mass of the earth is so huge. You just have essentially no effect. No effect at all. Yeah. So, I'm just saying the rotation rate of the earth is susceptible to all these small effects that can add up.

28:51And we deal with it. We, you know, deal with it. We throw in leap seconds. 23 hours, 56 minutes, and four seconds. That's our rotation rate to the stars. To the stars. It's exactly 24 hours on average. And that average is changing. Right. Oh, man. That is, that's so cool, man. Yeah. Just to show you that smart people think about this stuff. Yeah. And solve it so you don't have to. Because everyone, you just think everything, oh, just it works. Oh, the computer tells me what the right time is.

29:22And everything. And the bank knows what time it is. And my GPS. We've got top people thinking about this stuff. So, before the atomic clock, nobody knew what time it was. That's why they synchronized clocks at the beginning of heist movies. Everyone had different time, dude. Oh, my God. That's crazy. Oh, man. I wish I lived back then. I'd never be late for anything. It'd be like, Chuck, you're 10 minutes late. Not according to my watch. And as far as I could tell, don't nobody know what time it is.

29:55No, no.

29:59I'm not 10 minutes late. You're 10 minutes early. That's what the problem is.

30:04So, that's all I got to say. I mean, it's a fascinating thing. That is fascinating. And I love thinking about it just because of how many different branches of the science and technological world had to come together to figure this out. Sweet. Well, I got to go. I'm preparing for the next leap second. You're going to be like.

30:26So, all right, dude. By the way, if we have a leap second announced, we'll do a special show and maybe celebrate that. And I'll bring out my atomic clock and we can watch it. That'd be so cool. I'm all about it. Okay. Yes. And make sure. Don't forget the champagne because you said you had champagne the last time. So, don't even try to skip out on the champagne. You didn't forget that. That's right. Okay. I am just saying.

31:00Hi, I'm Ernie Carducci from Columbus, Ohio. I'm here with my son, Ernie, because we listen to StarTalk every night and support StarTalk on Patreon. This is StarTalk with Neil deGrasse Tyson.

Physics and body size

31:24Consider a few things. I want to talk about life. Well, okay. Yeah, it's relevant. And on size and life. Well, listen, the way I see it is this, okay?

31:41You know, size is a preference thing, okay? And not all of us can actually, you know, be what some people might have as a preference. And, you know, why are you looking at me like that? Because why? Perhaps you've been told something about, you know, my ethnicity and maybe I'm supposed to be a certain way. Well, it doesn't necessarily happen that way for every person in my position and someone

32:11who looks like me. We are by witness to Chuck digging himself out of a box. That's what that is, okay? So, here we go. So, you're in a room there, right? All right, it looks like a hotel room. All right, you can walk around on the floor. I can. Okay? If there were an ant walking around on the floor, it would be doing just what you're doing. Absolutely. However, an ant can go to the wall and then just walk up the wall. Right. How come you're not walking up the wall? Because I'm not Spider-Man. Okay. That's all.

32:42If I were Spider-Man, I'd be walking up the wall. Okay. Because Spider-Man is imitating a spider. Yes, he is. Okay. But there are no spiders the size of humans. This is true. That can just walk up the wall. Correct. All right? There's a reason for that. There's a very good reason for it. And it has to do with the physics of life. Okay. Nobody ever talks about it that way. People say, oh, it's the biology. This. You can talk any biology you want. Wow. At the end of the day, physics sets the rules. Take that, biologist.

33:15Here's a little smack down for you, you biologist. Physics sets the rules. Even in your world. Physics. All right. So, it all comes down to this. Go ahead. Jack. Okay. So, how strong are your limbs? Well, the strength of your limbs is measured by the cross-sectional area. Oh, God. All right. So, that tells you how big your muscles are. Well, you're just embarrassing me on every level today.

33:47First, we're talking about size. Now, we're talking about skinny wrists. Thanks. All right. So, just take any part of your body, your legs and your arms. Take the cross-section. And that tells you how big your muscles are. And when you say big muscles, you're measuring sort of circumference. Okay? Correct. Right. And that's related to the- That's always the way you measure muscles. The circumference of the area. Okay. Right. So, as you get bigger, your strength will grow as the area of your muscles.

34:20Right. The cross-sectional area of your muscles. Okay. However, your weight goes up as the cube of those dimensions. Oh, my goodness. Now, of course, I knew your weight goes up, but I didn't realize it. Okay? Wow. That's a lot. That's okay. So, that's why, okay, a 300-pound person does not look all that much bigger than a 200-pound person.

34:53Right. Okay? They're chubbier and they're big, you know, if they could be tall, whatever. But if you start increasing depth, height, and width, as your volume increases, so does your mass, but your strength necessary to sustain that volume and that weight does not go up in proportion. Right. So, your limbs, if you're going to hold up a heavy animal, your limbs have to be much fatter than they would otherwise be if you were smaller.

35:27Okay. And what are your heaviest animals on Earth? Your elephant, your whale. Yeah, on Earth's surface. Okay. The elephant. The elephant. Its legs are huge. Yes. Okay? They don't have spindly legs. No, they don't. Their legs are huge to compensate for the fact that strength only goes up as the area of their muscles, while the weight is going up as the volume. And volume rapidly outstrips area.

35:58Okay? Now, you can go the opposite direction. As you get smaller and smaller and smaller, the weight with your volume gets lower and lower and lower, and the strength of your limbs becomes better relative to your body weight. That's why an ant can lift. What is it? Not only is it why they can lift things relative to their body weight. Relative to their body weight. Okay. All right. Not only can they do that, it's why they can get around with really skinny legs.

36:29Right. Okay, yes, they have six of them, but they're really skinny compared to the thickness of their body, because the legs aren't holding up much weight relative to their own strength. Right. And they intermittent fast. They do intermittent fasting, too. Is that how that works? Thank you. I'll take so. Go to an ant colony. They have these diet books.

36:52Wow. So, you know what? You just gave me a really stark thought, which is I watched this documentary about this guy who climbed the only cliff face that these people die all the time climbing without any belay. And this guy was super skinny. Yep. Yep. Not strong like with big, giant muscles, but, I mean, his muscles, when you look at his

37:23musculature, this dude was ripped. He's ripped, but he doesn't weigh very much. Ripped, but he doesn't weigh much at all. Correct. Correct. And that's why in the movie Cliffhanger, which involved rock climbers and mountain climbers, it starred Sylvester Stallone, who's got all these muscles. It's like, no! Every mountain climber on Earth knew that that is not the body type, because he's got muscles where he does not need them. He's got body mass that is counter to his ability to support himself.

37:54Right. So the point is, generally, and there's certain exceptions depending on environments, the smaller you are, then the less you weigh, the thinner your legs can be and still hold up your weight. And the bigger you are in mass, the thicker your legs are. So rhinoceroses and hippopotamuses, hippopotami, and elephants all have really thick legs. Because as they got bigger, if they kept their thin legs, they'd break their legs.

38:27And that's the end of you. You get eaten, and you leave the gene pool very quickly. Right. When that happens. Okay. All the lions are missing you. Okay. Remember what we used to, remember those elephants with the skinny legs, man? And we used to just find them lying around. Oh, and they made such a delicious meal. Remember those days, we would just walk out on the plane, and we would find the elephants.

38:58Well, they'd be the elephants on crutches because they kept breaking their legs. Right. So, anyhow. Oh, snap. So, that's why you don't find very tiny creatures with really thick legs. That's that. So, because they don't need it. That's the point. Okay. So, now, so that's an important fact, number one. Whales are different because they don't weigh anything. Right. You got to go back to our weight, density, and whatever. I was just about to say. Did we talk about that? I think we talked about that. No, we didn't.

39:30But they're buoyant because they're at a certain depth. Right. So, they don't have to hold up their own weight. That's why they can get so big. Right. Okay. So, it doesn't, it's not fair to say how much does that weigh weigh weigh? It weighs 400 tons. No, it doesn't. It weighs zero because it is neutrally buoyant in the water. And if you bring it onto the land, it will die because it cannot hold up its own weight. It cannot do anything in its own weight. So, you cannot say it weighs that much because that's not where it lives. It makes so much sense because you've never seen a beach whale say, all right, I'm out of here.

40:06No, no.

40:08I'm good with the land thing. I tried the beach. I tried the beach. I'm going to roll myself back on it. I'm good. You never saw that. You never saw that. So, they don't weigh anything. They don't have to hold up their own weight. So, they're really not. So, everything I'm describing is sort of in the air on Earth's surface. Okay? All right. So, but I'm not done. Oh, cool.

Surface tension and scale

40:31Not only that, the laws of physics don't all manifest equally at all size scales. Interesting. Okay? So, if you're going to drink water, you'll put it in a glass and then you'll drink it like this. Right. Why do you put it in a glass? Why don't you just sort of pour it in front of you? Well, because it'll flatten out on the table and you'll make a mess. Right. Okay? But, suppose I am an insect. Again, let's suppose I'm an ant.

41:02Does an ant need a vessel in which to put the water in order for it to drink? No. Because water beads up. There's a bead of water that does not continue to spread. It could just walk up and suck the juice right out of the bead of water until it's gone. Okay. Nice. Now, the producers and writers of the movie Bugs Life knew this. So, when the mosquito went to the bar and ordered a drink, and what drink did the mosquito order?

41:33A Bloody Mary, of course. I just turned it to my mother. I can't believe it. You turned it to your parents. That's right. I turned it to my parents then. Oh, my God. Okay. Bloody Mary. Bloody Mary, O positive. Okay? And so, the bartender, which is another insect, I don't remember what kind, has a spigot, pours out a blob of Bloody Mary, and plunks it down in front of the mosquito, and it's just a ball.

42:05It's just a ball. Held up by surface tension. Surface tension is this sort of imaginary film that exists on the surfaces of liquids that you have to sort of puncture that in order to fall through. But that sort of film actually contains it in small enough amounts. Surface tension is not strong enough to contain a big blob of water like this because gravity wins. But when you're small enough, gravity loses against surface tension. Surface tension wins. They plop this drop of Bloody Mary in front of the mosquito.

42:39Mosquito put it in its nose, sucked it out, and then that was the end of that scene. This was brilliant. And it's in that movie. It's a Pixar movie. They had people who knew physics and chemistry writing the script for that movie. So, different forces operate when you're small than when you are large. And so, that's why they have – there's something called – is it a Jesus spider? A water strider? It's sometimes called – there's something that walks on the water swiftly, and in some regions they call it a Jesus spider, I think.

43:13Anyway, because Jesus in the Bible walked on water. Well, except Jesus weighs more than the surface tension would have held him up. So, Jesus would have just sunk without some kind of magic or spirit or miracle, okay? Right, right. Whereas, if you're small enough – Well, maybe he had just giant foam sandals. Foam sandals. We didn't check. It was – right. He was the first person to wear the giant foam sandals. The foam sandals. And they're just like, oh, my God, Jesus is a miracle.

43:45People misinterpreted it, right? Right. They were like, hey. But go ahead. So, with the – so, all I'm saying is that if you're small enough, that surface tension, you don't weigh enough to break the surface tension. Right. So, then you can just sort of walk on the water as some insects do. So, my point is size is everything in this world. And you might say, well, could you have something really big, like a life form the size of a galaxy? Probably not.

44:16Here's why. Because suppose you're that big – by the way, the galaxy is 100,000 light years across. Suppose you have your head itches, and then you send the signal to the brain, and then it goes to your finger, and you've got to bring your finger up to scratch your head. Well, that took – that took about 30 million years. It took – it's going to take at least 100,000 years to get your arm to your head moving at the speed of light. And you're probably not moving at the speed of light.

44:48So, you just – you're not going to be able to respond to stimulus the way life needs to in order to thrive in an environment. So, there's certain sizes above which are not realistic. So, physics contains the largest and the small. And by the way, you can't be so small that quantum physics prevents your molecules from binding. So, binding, you can't be smaller than the sizes of molecules. Right. Because what are you at that point?

45:20Okay? If molecules comprise life, you can't be life that's smaller than a molecule. It's just not – there's no – so, the chemistry and physics contain all the realms in which you will find biology. And that's just the beginning of it. I'm giving you the basics of it, where that's the entry level sort of size and life 101. So, there you have it. So, Spider-Man, he's got the powers of a spider? No, not if he's that size. He doesn't. No, he doesn't.

45:50No, he doesn't. Right. No, he doesn't. He can't. No. He basically just has the powers of a really good rock climber. Rock climber.

46:04Yes. That's about it. That's about it. All right. We got to go there. Chuck, good to have you. Always a pleasure. Neil deGrasse Tyson, signing off, bidding you. Keep looking now.