飛航物理家解釋重力從5個等級的難度（Astrophysicist Explains Gravity in 5 Levels of Difficulty

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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."
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.

飛航物理家解釋重力從5個等級的難度（Astrophysicist Explains Gravity in 5 Levels of Difficulty | WIRED）