Astrophysicist Explains One Concept in 5 Levels of Difficulty

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.

But she had just done the same experiment.

She was just floating in space thinking,

Am I moving?

There's no way to know which one of you is moving

and there's no meaning to the absolute motion.

The only thing that's true

is that you're in relative motion, that's true.

You both agree you're in relative motion,

and that's clear.

But neither of you can say it's actually you who's moving

and I'm stationary.

[laughs] I don't even know what to say to that.

So let me tell you where it gets really crazy.

[Maria laughs]

So, let's say you and Marina are floating in space

and you can't tell who's moving.

Let's say you both see a flash of light.

A flash of light comes from somewhere,

you don't know where.

So you measure the speed of light

to be 300,000 kilometers per second.

But here comes Marina and she's racing at the light pulse,

as far as you can tell.

Two cars driving towards each other

seem like they're going faster towards each other

than somebody who's standing still

relative to one of the cars,

right? Yeah.

So you would say, oh Marina is gonna measure

a different speed of light.

But she comes back and she says, No.

300,000 kilometers per second.

Because from her perspective, she's standing still,

and the laws of physics have better be the same for her.

The speed of light is a fact of nature

that's as true as the strength of gravity.

And the two of you are in this quandary

because if one of you is the preferred person

who correctly measures the speed of light,

that ruins everything about the idea

of the relativity of motion.

Which one of you should it be?

So Einstein decides they must both measure

the same speed of life.

How could that possibly, possibly be the case?

And he thinks, well, if speed is how far you travel,

your spatial distance, in a certain amount of time,

then there must be something wrong with space and time.

And he goes from the constancy of the speed of light

and a respect for this idea of relativity

to the idea that space and time must not be the same

for you and for Marina.

And that's how he gets the idea

of the relativity of space and time.

[laughs] You have the best expression on your face. [laughs]

It's pretty wild, but that is a starting point, actually,

of the whole theory of relativity.

That starting point leads to

this complete revolution in physics

where we suddenly have a Big Bang

and black holes and space-time.

Just from that one simple starting point.

So, is your impression of gravity different

than when we started the conversation?

Yeah, 'cause I knew that when I was on the train

it didn't feel like I was moving,

but I didn't know why or that it was a thing

and I wasn't crazy.

[Janna and Maria laugh]

And it's a really deep principle.

And what about the theory of gravity?

I don't know, usually when I just heard gravity

it's from my coaches,

but I didn't know it was all these things.

It's like a big paradigm.

[gentle music]

So, you're in college? Yeah.

[Janna] And what are you studying in college?

I'm a physics major.

So, from your perspective,

how would you describe gravity?

I'm taught that it's a force.

It's described by inverse law.

But I also know that it's a field.

And there's a recent discovery with gravitational waves,

although I don't know the specific details about that.

So, when you say it's an inverse-square law,

that means that the closer you are,

the more strongly you feel the gravitational pull.

And that makes sense.

There's very few things that are stronger

when you're further apart. Yeah.

So you can also think of a gravitational field,

something that permeates all of space.

Even though the earth is three stories below us,

it's not as though it's pulling at us from a distance.

We're actually interacting with the field at this point

and there's a real interaction right here at this point.

And that's nice, because people were worried

that if things acted at a distance,

that the way that old-fashioned

inverse-square force law describes it,

that it was as spooky as mind-bending a spoon,

that it was like telekinesis.

If you don't touch something, how do you affect it?

And so the first step was to start to think of gravity

as a field that permeates all of a space.

And it's weaker very far from the Earth

and it's closer very close to the Earth.

So one way to think of this field

as a field that's really describing

a curved space-time that is everywhere.

Forget the difficulty of the math,

just the intuition comes from

two kind of simple observations.

One was what Einstein described

as the happiest thought of his life.

So, right now, you might feel heavy in your chair,

and we might feel heavy on the floor and our feet,

or standing in an elevator cab.

And Einstein said, what does the chair have to do with it,

or the floor, or the elevator?

Those aren't gravitational objects.

So he wanted to eliminate them,

and one way to do the thought experiment

is to imagine standing in an elevator

that you can see out of, a black box.

And imagine the cable is cut

and you and the elevator begin to fall.

So, in free fall?

You're in total free fall.

Now, because things fall at the same rate,

including the elevator and you,

you can actually float in the elevator.

If you just floated in the elevator,

the two of you would drop,

and you might not even know you're falling.

You could take an apple and drop it in front of you,

and it would float in front of you.

You would actually experience weightlessness.

It's called the equivalence principle.

It was Einstein's happiest thought

that what you're really doing

when you're experiencing gravity

isn't being heavy in your chair,

it's falling weightlessly in the gravitational field.

And that was the first step,

to think of gravity as weightlessness and falling.

I know zero-gravity experiences

that are done with planes, I believe?

Yeah, exactly. Yeah, yeah.

You can make somebody look like

they're in the International Space Station

by flying up in a plane and then just free-falling,

the plane just drops out of the air.

And while it's falling, they will float weightlessly,

and there's been a lot of experiments about it,

but you don't want it to end unhappily,

so the plane has to scoop back up,

and then you see them

become pinned to the floor of the plane,

because then the plane is interrupting their fall.

So that's the first thought,

and then the next is, what is the shape that's chased?

So if you were floating in empty space,

really empty space, and you had an apple,

and you threw the apple,

what shape do you think it would chase, the path?

Well, if I threw it straight,

I would think it would go straight.

Yeah, it would just go straight.

But if you did that on the Earth, what would happen?

It would just go down.

Yeah, it would chase a curve, it would chase an arc.

And the faster you throw it, the kind of longer the arc.

So the second step to think about curved space-time

is to say that when things fall freely

around a body like the Earth, they trace curved paths,

as though space-time itself, space itself was curved.

Oh.

You had that moment,

I saw that it your face! Yeah, yeah, yeah.

You went, Oh.

[Janna and Lisa laugh]

So, that's the intuition,

that's how Einstein gets from thinking

that space-time is curved from the idea that, well,

there's this field that permeates all of space,

and what is really describing is the curves

that things fall along.

And from there, it's a very long path

to finding the mathematics and the right description,

that's really hard.

But that intuition is so elegant and so beautiful

and just comes from these two simple thought experiments.

That's amazing.

Isn't it kind of amazing? Yeah. [laughs]

So you described learning in a class about light

the theory of special relativity

where Einstein is really adhering

to the constancy of the speed of light

and questioning the absolute nature of space and time.

And it seems like that has nothing to do with gravity,

but he later begins to think about

the incompatibility of gravity

with his theory of relativity.

So suppose the Sun were to disappear tomorrow.

Some evil genius comes and just figures out a way

to evaporate the Sun.

In Newton's understanding of gravity,

we would instantaneously know about it

all the way over here at the Earth.

And that's incompatible with the concept

that nothing can travel faster than the speed of light.

No information, not even information about the Sun,

could possibly travel faster than the speed of light.

So we shouldn't know about what happened to the Sun

for a full eight minutes,

which is the time it would take light to travel to us.

And so he begins to question

why gravity is so incompatible with relativity,

but he already knows he's thinking about

space and time in relativity.

So then he gets to his general theory of relativity

where he realizes if I eliminate everything

but just the gravitational field of let's say the Earth

and I look at how things fall

and I see that they follow curves,

well, then he realizes that space and time

don't just contract or dilate,

that they can really warp,

that they can bend and that they can curve.

And then he finds a way

to make gravity compatible with relativity

by saying if the Sun were to disappear tomorrow,

the curves that the Sun imprinted in space-time

would actually begin to ripple,

and those are the gravitational waves,

and they would change and they would flatten out,

'cause the Sun was no longer there.

And that would take the light-travel time to get to us

to tell us that the Sun was gone,

and then we would stop orbiting

and just travel along a straight line.

Wow. Wow.

[Janna and Lisa laughs]

Well, let's hope it doesn't happen.

Yeah. [Janna laughs]

So what do you think you walk away with?

What do you think you learned?

Well, I learned more about the intuitions

behind the concept.

'Cause we already just do the problems

but sometimes you get lost in the math,

but speaking like this it really helps build my intuition.

Yeah, it does for me too, so thank you. [laughs]

[gentle music]

So you're getting your PhD in physics?

That's right.

Theoretical high energy physics.

Basically the physics of

really, really small fundamental things.

So what would that have to do

with gravity or astrophysics?

Well, what I'm looking at is states of matter

that might exist inside neutron stars.

So, when a star dies, if the star is massive enough,

there's a huge explosion, called a supernova,

and the stuff that's left behind

that doesn't get blown away

collapses into a tiny compact blob

called a neutron star.

So what I love about neutron stars personally

is that they're kind of city-sized,

right? That's right.

[Janna] They're about the size of a city.

So you're imagining something

more than the mass of the Sun.

[Will] Yeah, or about the mass of the Sun,

condensed to the size of a city.

It's dense enough that one teaspoon-full

would weigh about a billion tons here on Earth.

Now, that makes the gravitational field incredibly strong

around the neutron star.

So what would happen if we were on a neutron star,

because of the gravity?

We would immediately be crushed into the ground,

I think our bodies would be shred

into their subatomic particles.

So what's the connection

between neutron stars and black holes?

So, as I understand it,

a black hole is sort of like a neutron star's big brother.

It's more intense, though.

If you have so much matter when a star is collapsing

that it can't hold itself up, it collapses to a black hole,

and those are so dense that space-time breaks down

in some way or another.

Black holes are so amazing

that when the neutron star stops

and there's something actually there.

There's material there.

If it's so heavy it becomes a black hole,

so it keeps falling,

once the event horizon of the black hole forms,

which is the shadow,

the curve that's so strong that not even light can escape,

the material keeps falling.

And like you said, maybe space-time breaks down

right at the center there, but whatever happens,

the star's gone, that black hole is empty.

So in a weird way black holes are a place and not a thing.

So is there a sensible way to talk

about what's inside a black hole,

or is that, should you think of it

as there is no space-time inside?

There isn't a sensible way to talk about it yet,

and that probably means that's where Einstein's

theory of gravity as a curved space-time

is beginning to break down,

and we need to take the extra step

of going to some kind of quantum theory of gravity.

And we don't have that yet.

So even though the black hole isn't completely understood,

we do know that they form astronomically,

that in the universe things like neutron stars form

and things like black holes form.

The consequences are very much speaking

to this curved space-time.

So, for instance, if two black holes orbit each other,

they're like mallets on a drum,

and they actually cause space-time to ring,

and it's very much part of gravitation.

The ringing of space-time itself,

we call gravitational waves.

And this was something Einstein thought about

right away in 1950-1960, he was thinking about that.

Those waves are very exciting for me too

because neutron stars orbiting each other

also give off gravitational waves

and we might be able to get some data

about neutron star material from that kind of signal.

[Janna] Yes, they ring space-time also like a drum,

and you can record the sound of that ringing

after a billion years,

when it's traveled through the universe.

But then the next thing that happens is

those neutron stars collide,

and because of this incredibly high energy state of matter,

which you study,

it becomes this firework of different explosions.

It's really quite spectacular.

That's right, in fact,

when we recorded that for the first time

with gravitational waves,

we then pointed telescopes at it

and were able to see it optically as well,

and that gave scientists a lot of data.

Yeah, it was, to my knowledge,

the most widely studied astronomical event

in the history of humanity.

Wow, that's amazing.

So when the gravitational waves were recorded

and they realized, oh this sounds like,

you can reconstruct the shape and size

of the mallets of the drum from the sound,

these sounds like neutron stars colliding, not black holes.

And so, like you said, there was a trigger

for satellites and experiments all over the world

to point roughly in the direction

that the sound was coming from.

So, from your point of view,

they're like two super-conducting giant magnets colliding,

an experiment you could never do on Earth.

That's just the most tremendous scales

and peculiarities of matter.

Absolutely.

I've heard statistics like many Earth masses worth of gold

were created, forged in the neutron star collision

that caused that.

We used to think that most elements in the universe

were created in supernova, which is when stars explode,

because there's so much violent activity at the center

that you need that kind of energy to create new elements.

[Janna] The way you do in a bomb.

It's basically nuclear fusion.

Sure, but we now think that that kind of fusion happens

when two neutron stars collide.

If you think about it,

you have two massive blobs of neutrons.

When you smush them together, you've got neutrons colliding.

It creates the conditions where new elements can be created.

Yeah, it's amazing.

It's literally populating the periodic table.

Yes, we now think that most of the heavy elements

after some number are created in neutron star collisions.

So you are already a PhD student,

you know a lot about gravity,

but what do you think you've taken away

from this conversation?

Well, I've definitely taken away

that the way that we think about gravity today

is very different from how Newton thought about it,

and that even though we have a very good understanding,

there's lots of things that we don't fully understand.

There's still a lot of questions to be answered,

which I think is really exciting.

See, you're a scientist. [laughs]

Isn't the best part being able to ask the questions?

Oh yeah.

[gentle music]

So we've been talking about gravity

from Newton and celestial bodies, the Earth, the Moon,

pulling on each other in the conventional sense

of gravity being an attractive force,

to the Earth creating curves in space-time,

then we moved on to just diffused seas of energy

and space-time as the real universe

and gravitation is really just talking about

space-time in general, and here we are,

and you're really hardcore in theoretical physics.

Where would you take the exposition of gravity

from that point?

Well, one thing is quantum mechanics.

Quantum mechanics is the most successful theory

in the history of science,

it explains the most different phenomena the most precisely.

Yet many people would still say we don't understand

even the basics of it.

So when we think about quantum mechanics,

we think about particles and their quantum charges

in the Feynman way, the way that Feynman taught us.

They come in and they exchange a force carrier

and then they come out again,

so that's how we think of an electron and light scattering,

for instance, or something like that.

And the language that Einstein gave us is so different.

It's completely geometric, it's all this space-time.

And it's also unnecessary.

Yeah, for me, the beauty of the theory of gravity is

the way Einstein formulated it,

as a theory of geometry, of curved space and time.

I think, like you, that's one of the things

that really pulled me into it.

Is there really space-time

or are we just using unnecessary language

because it's elegant and we like it and it's beautiful?

Well, I think there's really space-time

in the sense that it's a description that works really well,

so there has to be something right about it.

I mean, if we're gonna talk about

what's really, really underlying that

and we're gonna put quantum mechanics into the mix,

then there should be some

quantum mechanical wave function for space-time.

You should be able to take two different space-times

and add them together,

'cause one of the crazy things about quantum mechanics,

as you know, it's--

To have the waves together.

Yeah, and in two states

and in two possible states of the world,

you can just literally put a plus sign between them

and that's a sensible state, that's a good state,

it makes sense.

So do you think there's some sense

in which we shouldn't be thinking

about individual universes, individual space-time,

so we should be thinking about

superpositions of space-times?

Yeah, I think so.

I think if you were to go far enough back

in the history of the universe,

back to when it was very, very dense, very small,

and when quantum mechanics was certainly important,

then it must have been like that.

I mean, if we believe that

the dominant standard model of cosmology,

something had to produce the density perturbations,

the things that seeded all the galaxies and stars

and everything else in the world.

So there's a galaxy over there, let's say,

and not over there, so how did that happen?

Why is there a galaxy there and not there?

In the standard theory, as you know,

that was a quantum event, a random event.

And it doesn't mean that if happened there and not there

'cause you flipped a coin,

it actually happened in both places.

There's gotta be a wave function

where in one branch of the wave function

there's a galaxy there and not there,

and on the other branch it's the opposite.

So when we're talking about

the multiverse or the Big Bang,

we are really talking about gravity ultimately,

and we're talking about how a theory of gravitation

which we know think of as a theory of space-time

has a quantum explanation,

has a quantum paradigm imposed on it

that will help us understand these things,

and we don't have that yet.

One of the things that I think is so amazing

is that the terrains in which we're going to understand

quantum gravity are very few.

It's the Big Bang, because that's where we know

that quantum and gravity both were called into action.

And there's black holes.

One of the most interesting discoveries

is of course Hawking's discovery,

kick-started a kind of crisis, right?

In thinking about why quantum mechanics and gravity

were so knocking heads.

It was one of the most beautiful examples.

Sure, yeah, it is a beautiful, beautiful idea.

So, first of all, to be totally clear, though,

we've never observed Hawking radiation,

which is what he predicted, directly.

I don't think very many people doubt that it's there,

but yeah, Hawking discovered mathematically

that when you have a black hole, it's got an event horizon,

it's got a surface which is a point of no return.

If you fall through that surface, no matter what you have,

no matter how powerful the rocket you've got,

even if you beam a flashlight back behind you

in the direction you fall from,

nothing escapes, not even light.

It all gets sucked in and spaghettified

and destroyed at the singularity,

or something, something happens, but it doesn't get out.

But in quantum mechanics,

you can't really pin down

the location of something precisely.

If you try to pin down an electron

in a tiny circuit in a microchip,

sometimes you discover it's not actually there

and then your computer crashes.

This is the Heisenberg's uncertainty principle in reality.

You can't precisely say where the electron is,

and you can't precisely say how quickly it's moving.

Exactly, yeah, so when you get the blue screen of death,

that might be because of quantum mechanics.

Right.

You know, you try to pin something down

near a black hole, well, it's a surface,

it's got a particular radius for a round black hole,

and wanna say something is inside or outside,

well, you can't absolutely say that in quantum mechanics.

And this kind of uncertainty produces a radiation,

which you can think of as pulling some of the energy

out of the black hole.

The black hole is formed out of some mass

and there's an energy in that.

If you think of pulling some energy out of that

and sending it off to infinity

in the form of particles being admitted.

And what Hawking found is that it's a thermal spectrum,

it looks like a hot, or not so hot for a large black hole,

but like an oven, the kind of radiation

that comes out of a cast iron.

This idea that the darkest phenomenon in the universe

actually is forced to radiate quantum particles

is pretty wild.

I think everyone understood

that it was a correct calculation,

but I don't think a lot of people

understood the implications,

that it meant something really terrible was happening.

Because this black hole,

which could have been made of who knows what,

is disappearing into these quantum particles

which, in some sense, have nothing to do

with the material that went in.

So do you think that's a big crisis?

The black hole evaporates, the information is lost?

It's a crisis because of some of the details of it,

but I would say the way you just described,

I mean, if I build a big bonfire or an incinerator

and I throw an encyclopedia into it,

good luck reconstructing what was in that encyclopedia.

The information is lost for all practical purpose.

Practical purposes. Yes.

So this is a huge crisis

'cause either quantum mechanics is wrong,

and as you described it,

it's the most accurately-tested paradigm

in the history of physics,

how could it be wrong, right?

Or the event horizon is letting information out

and violating one of the most

sacred principles of relativity.

One thing about quantum mechanics is that

any time you have a state of the world

and another state of the world,

you can literally add them together

and get a third possible state,

as crazy as that sounds.

And so if you're gonna have a quantum theory of gravity,

then we can't really talk about there being a black hole

or not a black hole,

or an event horizon or not an event horizon,

because we could always a state

that had an event horizon and a state that doesn't,

or has the event horizon

in a slightly different position, maybe,

and add them together.

So the existence or position of an event horizon

can't possibly be determined as a fact

any more than the position of an electron is determined.

So I think that's the loophole.

That's a nice way of looking at it.

So that you're not actually violating classical relativity

once you're in a regime where the wave function

has really peaked around a very well-defined stage.

That's right, and one of the most exciting developments

in the last 10 or 20 years is called holography,

and it's called holography because

a hologram is a two-dimensional surface

that creates a three-dimensional image.

It's got sort of 3D information built into it.

And this, in a fundamental way,

really has that 3D or higher dimensional information

built into it.

It's exactly the same as this theory of gravity

and more dimensions.

Yes, so one of the things I like to think of

with holography is that I can pack

a certain amount of information in a black hole.

I mean, you can literally think of it

as throwing things into it.

So let's say I have information in some volume

and I'm under the illusion

that I can just keep packing information in that volume,

as much as the volume will contain.

Eventually I'll make a black hole

and I'll find out that the maximum amount of information

I can pack into anything in the entire universe

is what I can pack on the area.

And since area is projecting the illusion, maybe, of volume,

maybe the whole world is just a hologram.

It's not a principle that only applies to black holes.

It's saying that,

if this theory of quantum gravity is correct,

then this while three-dimensionality is an utter illusion

and really the universe is two-dimensional.

That's crazy. That's true.

[Janna laughs]

And as practically speaking,

you mentioned before in our conversation

that it's really interesting

that the Heisenberg uncertainty principle

is a practical limit now in microchips.

If we make microchips much smaller than they already are,

even as they already are, it causes errors,

'cause you don't know that the electron's in.

If holography, if this limit on how much information

you can ever pack, if that ever become a limit,

as far as we know that's an absolute limit.

We started off with clay tablets,

not so much information per cubit centimeter or whatever.

Then we had written stuff that's getting better,

encyclopedias with thin paper that's even better, CDs.

A smaller and smaller space,

trying to pack it denser and denser,

until eventually we make a black hole.

Yeah, at some point you try to fill up

your encyclopedia with knowledge

and you get swallowed up by a black hole.

Right, exactly.

And the most knowledge you could ever have

would only be on a two-dimensional surface.

Right, and as big as the universe, and then you're done.

So, you know, not likely

that we're ever gonna hit that limit any time soon.

Do you think it's possible

that gravity is really ultimately just quantum mechanics

and doesn't exist at all in the fundamental ways

that we've been talking about so far,

like the Newtonian way and the space-time way,

that those are just these kind of macroscopic illusions?

Sometimes I talk about it in terms of temperature.

Temperature is not a thing.

There is no single thing called temperature.

It's a macroscopic illusion

that comes from the collective behavior,

really quantum behavior of random motions of atoms.

And is it possible that the whole of gravity

is some kind of emergent illusion

from what's really quantum phenomenon underlying it?

If we buy the idea of holography, then absolutely,

that's for sure, that's what it's telling us.

Although which side is the illusion

and which side is the reality?

They're the same.

I mean, temperature is still great to talk about.

It doesn't mean we shouldn't talk about temperature.

I mean, we should absolutely adjust our thermostats

and talk about temperature.

But if we look at it closer and closer and closer,

we realize there's not a thing in the world

that has as a quantum value temperature, isolated.

And so maybe there is no such thing as gravity

isolated from quantum mechanics.

Right, so I guess with the holographic description

we've got two sides, which are actually secretly the same.

On one side there's definitely no gravity.

On the other side, well,

it's a quantum theory of gravity, whatever that means.

But the point is you can get it out,

it's equivalent to this theory.

So that's just like saying

there's the idea of a dual description.

It's just saying there's a perfect dictionary

between these two descriptions,

and so to belabor which one's real is silly.

It's like saying, is French or is English real?

Yeah, an example I like to give is

if you take some extra dimensions

and you compactify them, let's say just one,

all that is, it's exactly prevalent

to whatever particles you had,

whatever fields you had in your original theory

before you added it,

you just added an infinite tower of new particles

with certain properties that are all easy to calculate.

For me, it's a question of which description

is most useful.

I mean, if you wanna say gravity is an illusion

and it's all quantum, that's great,

but then you fall down the stairs and bang your head.

[Janna laughs]

It's sort of like there's a description

that works pretty well.

Yeah, you don't go to the doctor and say,

Heisenberg's uncertainty principle caused

a series of fluctuations.

Right, would you help me?

So there's so many open questions.

The fact that they are all these fundamental issues

that we really don't understand.

But, on the other hand, there's all these moving parts

that fit together so neatly.

There's definitely something that's working here.

But ultimately what is gonna emerge from that,

what structure is lying under it, we just don't know.

But I think the fact that there are

so many fundamental questions

that we just don't know the answer to,

that is an opportunity, that's exciting, it's great.

Thanks so much for coming.

It's really good to have you here.

Thank you very much, Janna, it was my pleasure.

[gentle music]

I hoped you learned something about gravity

you hadn't thought of before,

and I hope even more that it provoked some questions.

So thank you for watching.