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  • - 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."