字幕表 動画を再生する 英語字幕をプリント - Hi, I'm Janna Levin, I'm an astrophysicist, and I've been asked to explain gravity in five levels of increasing complexity. Gravity seems so familiar and so everyday, and yet it's this incredibly esoteric abstract subject that has shaped the way we view the universe on the larger scales, has given us the strangest phenomena in the universe like black holes that has changed the way we look at the entirety of physics. It's really been a revolution because of gravity. [gentle music] Are you interested in science? - Yes. - Yes, you are? - Yes. - Do you know what gravity is? - It's something that, so, right now, we would be floating if there was no gravity, but since there's gravity we're sitting right down on these chairs. - That's pretty good. So gravity wants to attract us to the Earth, and the Earth to us. But the Earth is so much bigger that even though we're actually pulling the Earth a little bit to us, you don't notice it so much. You know, the Moon pulls on the Earth a little bit. - Mm-hmm, just like the ocean tides. - [Janna] Exactly, the Moon is such a big body compared to anything else very nearby that it has the larger effect, pulling the water of the Earth. But more than the Moon, think about the Sun pulling on the Earth. We orbit the whole Sun, just the way the Earth pulls on the Moon and causes the Moon to orbit us. All of those things are acting on you and me right now. - If gravity was too strong, would we be able to get up? - That's such a good question. No, we actually couldn't. In the Moon, gravity is weaker, you can almost float between footsteps if you look at the astronauts on the Moon. On the Earth, it's harder, 'cause it's bigger. If you go to a bigger, heavier planet, it gets harder and harder. But there are stars that have died that are so dense that there's no way we could lift our arms, no way we could step or walk. The gravity is just way too strong. Do you know how tall you are? - I'm in the fours. - In the fours? - Maybe four three. - People think that while you're sleeping, your body has a chance to stretch out and gravity isn't crunching you together, but when you're standing or walking or sitting, the gravity contracts your spine ever so slightly, so that in the morning you might be a little bit taller than in the evening. See if it works for you. - [Woman] Wow. - So that was last night? - Yes. [Bonet screams] - Ooh. - They say that astronauts in space, definitely their spine elongates. There were two twin astronauts, one who stayed here on Earth and the other who went to the International Space Station. He was there for a long time, and when he came back, he was actually taller than his twin brother. - Wow. - Yeah, and that was because gravity wasn't compressing him all the time and he was floating freely in the International Space Station and his spine just kind of elongated. After a while here on Earth though he'll readjust, he'll go back to the same size. Have you ever heard of how gravity was discovered? - Mm-hmm. - Isaac Newton would ponder, how does the Earth cause things to fall? There's a famous story that Isaac Newton was sitting under a tree and the apple fell from the tree and hit him on the head and he had an epiphany and understood this law, this mathematical law for how that works. I don't actually think that's a true story, though. - Yeah. - But it's a good story. So Isaac Newton realized that even if you're heavier, you will fall at the same rate as something much lighter, that that's the same. Once you hit the ground, if you're heavier, you'll hit the ground with much greater force, but you will hit the ground at the same time. - So, if we both dropped down from a plane, we would both land at the same time, but you would land heavier? - Yep, so like a penny from the Empire State Building will fall at the same rate as a bowling ball. - Oh my God. - Yeah, amazing. Wanna try it? - Yeah. - A light object, see how light that is. - That's... - Very light? - Yeah. And a heavy object. - Oh my God. [Janna laughs] - They look the same, but this is much heavier, right? Okay, so try it, just try holding your arms up front, a little higher maybe, give them a chance to drop, and then drop them. [balls thud] [Janna laughs] Did they fall at the same time? Did they hit at the same time? - So, Isaac Newton, he was also the one who realized that that's the same force that keeps the Moon in orbit around the Earth and the Earth in orbit around the Sun, and that's a huge leap. Here he is, looking at just things around him, and then looks at the stars and has this really big realization, that that's actually the same force. So, what have you learned today talking about gravity? - I've learned that the person that learned about the apple. - Newton. - He was learning about gravity just about what he saw on this planet. I also learned that if you drop one light thing and one heavy thing at the same height at the same time, they're both gonna drop at the same time but one's gonna drop a little heavier than the other. - That's beautiful, I'm impressed. [gentle music] So, Maria, you're in high school? - Yeah, I'm a junior. - [Janna] And are you studying any sciences in high school? - I'm taking physics right now. - Do you think of yourself as curious about science? - Well, there are some things that interest me and others that bore me, so it depends. - What interests you? - Well, I'm a gymnast, so in physics they talk about force and stuff and then I think of how I use physics in my own life. - What's your impression of what gravity is? - I think that if there's no gravity, everyone would float everywhere. It pulls things down, and without it, everything would be chaos. - So you're saying gravity pulls things down, yet we've launched things into space. Do you ever wonder how we do that? - Isn't it like a slingshot, like if you pull something back enough it'll go in the opposite direction? - Well, that's true, we do use slingshot technology once things are out in the solar system. So, for instance, we use Jupiter and other planets so that when some of the spacecraft gets close, it'll slingshot around and it'll cause it to speed up. But mostly, around the Earth, gravity pulls things down, so when we want to send a rocket into space, when we wanna go to the Moon, when we wanna send supplies to the International Space Station, the trick is to get something moving fast enough that it escapes the gravitational pull of the Earth. Have you heard the expression what goes up must come down? It's actually not true. If you throw it fast enough, you can actually get something that doesn't come back down again, and that's basically how rocket launches work. You have to get the rocket for the Earth to go more than 11 kilometers a second. Think of how fast it is. Just one breath and it's gone 11 kilometers. If you get it to go that fast, it's not gonna come back down again. So you know the International Space Station which is orbiting the Earth? That's going around the Earth at 17,000 miles an hour. It has no engines anymore, the engines are turned off. So it's just there falling forever. So once it's out there, it's not coming back down as long as it's cruising like that. - And does the gravity pull it or is it just floating? - In a weird way, that is gravity pulling it. So have you ever had a yo-yo where you swing it around like this? The string is pulling it in at all times, but you've also given it this angular momentum. And as long as you give it the angular momentum, pulling it in actually keeps it in orbit. And so the Earth is pulling it in at all times, so that's why it doesn't just travel off in a straight line. It keeps coming back around. So it's funny, people think that the International Space Station is so far away that they're not feeling gravity, and that's not the case at all. They're absolutely feeling gravity. They're just cruising so fast that, even though they're being pulled in, they never get pulled to the surface. - It's like that ride at the rollercoasters where you go in and it's spins super fast and you can't feel it spinning fast but-- - Yeah, you feel pinned to that. It's exactly like that. There's something called the equivalence principle where people realized, especially Einstein, that if you were in outer space in a rocket ship and it was dark and painted and it was accelerating at exactly the right rate, you actually wouldn't know if you were sitting on the floor of a building around the Earth or if you were on a rocket ship that was accelerating. - That's crazy. - Yeah. You ever had that experience where you're sitting in a train and the other one moves and for a second you're not sure if you're the one moving? - Yeah, 'cause I go on the train every day to go to school, but I never feel like I'm moving when I'm in the train, and then I'm like, wait, what? - That's because in some sense, you're really not. Imagine you're in this train and it's going near the speed of light relative to the platform, but it's so smooth, then you should be in a situation in which there's no meaning to your absolute motion, there's no absolute motion. So that if you throw a ball up, you might think from the outside of the platform, be confused that when gravity pulls that back down, it's gonna hit you or something, but it'll land in your palm as surely as if you were in your living room. Isn't that kinda crazy? - Amazing. - So imagine you were an astronaut and you were floating in empty space. You can't see anything. There's no stars, there's no Earth. You can ask yourself, am I moving? There's really no way for you to tell. So you would probably conclude, well, I'm not moving. So then your friend Marina comes cruising past you, and maybe she's going thousands of kilometers a second, and you say, "Marina, you're cruising "at thousands of kilometers a second, "you're going so fast."