00:00
Hi, I’m Janna Levin, I’m an astrophysicist,
00:03
and I’ve been asked to explain gravity
00:05
in five levels of increasing complexity.
00:08
Gravity seems so familiar and so everyday,
00:12
and yet it’s this incredibly esoteric abstract subject
00:17
that has shaped the way we view the universe
00:19
on the larger scales,
00:20
has given us the strangest phenomena in the universe
00:25
that has changed the way we look at the entirety of physics.
00:28
It’s really been a revolution because of gravity.
00:36
Are you interested in science? Yes.
00:39
Do you know what gravity is?
00:40
It’s something that, so, right now,
00:43
we would be floating if there was no gravity,
00:45
but since there’s gravity
00:46
we’re sitting right down on these chairs.
00:49
That’s pretty good.
00:50
So gravity wants to attract us to the Earth,
00:55
and the Earth to us.
00:57
But the Earth is so much bigger
00:59
that even though we’re actually pulling the Earth
01:02
a little bit to us, you don’t notice it so much.
01:04
You know, the Moon pulls on the Earth a little bit.
01:07
Mm-hmm, just like the ocean tides.
01:10
[Janna] Exactly, the Moon is such a big body
01:13
compared to anything else very nearby
01:15
that it has the larger effect,
01:17
pulling the water of the Earth.
01:18
But more than the Moon, think about the Sun
01:20
pulling on the Earth.
01:21
We orbit the whole Sun,
01:22
just the way the Earth pulls on the Moon
01:25
and causes the Moon to orbit us.
01:27
All of those things are acting on you and me right now.
01:30
If gravity was too strong, would we be able to get up?
01:33
That’s such a good question.
01:35
No, we actually couldn’t.
01:37
In the Moon, gravity is weaker,
01:39
you can almost float between footsteps
01:41
if you look at the astronauts on the Moon.
01:44
On the Earth, it’s harder, ’cause it’s bigger.
01:46
If you go to a bigger, heavier planet,
01:49
it gets harder and harder.
01:50
But there are stars that have died
01:52
that are so dense that there’s no way
01:55
we could lift our arms,
01:57
no way we could step or walk.
01:59
The gravity is just way too strong.
02:02
Do you know how tall you are?
02:03
I’m in the fours. In the fours?
02:06
People think that while you’re sleeping,
02:09
your body has a chance to stretch out
02:11
and gravity isn’t crunching you together,
02:15
but when you’re standing or walking or sitting,
02:17
the gravity contracts your spine ever so slightly,
02:21
so that in the morning you might be a little bit taller
02:26
than in the evening.
02:27
See if it works for you.
02:32
So that was last night? Yes.
02:40
They say that astronauts in space,
02:42
definitely their spine elongates.
02:44
There were two twin astronauts,
02:46
one who stayed here on Earth
02:48
and the other who went to the International Space Station.
02:51
He was there for a long time, and when he came back,
02:55
he was actually taller than his twin brother.
02:58
Yeah, and that was because gravity
03:00
wasn’t compressing him all the time
03:01
and he was floating freely
03:04
in the International Space Station
03:05
and his spine just kind of elongated.
03:08
After a while here on Earth though he’ll readjust,
03:10
he’ll go back to the same size.
03:11
Have you ever heard of how gravity was discovered?
03:16
Isaac Newton would ponder,
03:18
how does the Earth cause things to fall?
03:21
There’s a famous story that Isaac Newton
03:23
was sitting under a tree
03:25
and the apple fell from the tree and hit him on the head
03:28
and he had an epiphany and understood this law,
03:32
this mathematical law for how that works.
03:35
I don’t actually think that’s a true story, though.
03:37
Yeah. But it’s a good story.
03:39
So Isaac Newton realized that even if you’re heavier,
03:42
you will fall at the same rate as something much lighter,
03:45
that that’s the same.
03:46
Once you hit the ground, if you’re heavier,
03:49
you’ll hit the ground with much greater force,
03:51
but you will hit the ground at the same time.
03:53
So, if we both dropped down from a plane,
03:56
we would both land at the same time,
03:58
but you would land heavier?
04:00
Yep, so like a penny from the Empire State Building
04:04
will fall at the same rate as a bowling ball.
04:06
Oh my God. Yeah, amazing.
04:08
Wanna try it? Yeah.
04:10
A light object, see how light that is.
04:13
That’s… Very light?
04:16
And a heavy object.
04:17
Oh my God. [Janna laughs]
04:19
They look the same, but this is much heavier, right?
04:22
Okay, so try it, just try holding your arms up front,
04:25
a little higher maybe, give them a chance to drop,
04:27
and then drop them.
04:28
[balls thud] [Janna laughs]
04:30
Did they fall at the same time?
04:31
Did they hit at the same time?
04:32
So, Isaac Newton, he was also the one who realized
04:36
that that’s the same force that keeps the Moon
04:39
in orbit around the Earth
04:40
and the Earth in orbit around the Sun,
04:42
and that’s a huge leap.
04:43
Here he is, looking at just things around him,
04:46
and then looks at the stars
04:48
and has this really big realization,
04:51
that that’s actually the same force.
04:53
So, what have you learned today talking about gravity?
04:56
I’ve learned that the person that learned about the apple.
05:01
He was learning about gravity
05:02
just about what he saw on this planet.
05:05
I also learned that if you drop one light thing
05:08
and one heavy thing at the same height at the same time,
05:11
they’re both gonna drop at the same time
05:13
but one’s gonna drop a little heavier than the other.
05:16
That’s beautiful, I’m impressed.
05:22
So, Maria, you’re in high school?
05:24
Yeah, I’m a junior.
05:25
[Janna] And are you studying any sciences in high school?
05:27
I’m taking physics right now.
05:28
Do you think of yourself as curious about science?
05:31
Well, there are some things that interest me
05:33
and others that bore me, so it depends.
05:37
What interests you?
05:38
Well, I’m a gymnast, so in physics they talk about
05:41
force and stuff and then I think of how I use physics
05:45
What’s your impression of what gravity is?
05:47
I think that if there’s no gravity,
05:49
everyone would float everywhere.
05:51
It pulls things down,
05:53
and without it, everything would be chaos.
05:56
So you’re saying gravity pulls things down,
05:59
yet we’ve launched things into space.
06:01
Do you ever wonder how we do that?
06:03
Isn’t it like a slingshot,
06:04
like if you pull something back enough
06:06
it’ll go in the opposite direction?
06:08
Well, that’s true, we do use slingshot technology
06:10
once things are out in the solar system.
06:12
So, for instance, we use Jupiter and other planets
06:15
so that when some of the spacecraft gets close,
06:19
it’ll slingshot around and it’ll cause it to speed up.
06:22
But mostly, around the Earth, gravity pulls things down,
06:25
so when we want to send a rocket into space,
06:29
when we wanna go to the Moon,
06:30
when we wanna send supplies
06:32
to the International Space Station,
06:34
the trick is to get something moving fast enough
06:37
that it escapes the gravitational pull of the Earth.
06:40
Have you heard the expression what goes up must come down?
06:43
It’s actually not true.
06:45
If you throw it fast enough,
06:47
you can actually get something
06:48
that doesn’t come back down again,
06:50
and that’s basically how rocket launches work.
06:53
You have to get the rocket for the Earth
06:55
to go more than 11 kilometers a second.
06:58
Think of how fast it is.
06:59
Just one breath and it’s gone 11 kilometers.
07:02
If you get it to go that fast,
07:03
it’s not gonna come back down again.
07:05
So you know the International Space Station
07:06
which is orbiting the Earth?
07:08
That’s going around the Earth at 17,000 miles an hour.
07:13
It has no engines anymore, the engines are turned off.
07:15
So it’s just there falling forever.
07:17
So once it’s out there, it’s not coming back down
07:20
as long as it’s cruising like that.
07:21
And does the gravity pull it or is it just floating?
07:24
In a weird way, that is gravity pulling it.
07:26
So have you ever had a yo-yo
07:28
where you swing it around like this?
07:30
The string is pulling it in at all times,
07:33
but you’ve also given it this angular momentum.
07:36
And as long as you give it the angular momentum,
07:39
pulling it in actually keeps it in orbit.
07:41
And so the Earth is pulling it in at all times,
07:44
so that’s why it doesn’t just travel off in a straight line.
07:47
It keeps coming back around.
07:49
So it’s funny, people think
07:50
that the International Space Station
07:52
is so far away that they’re not feeling gravity,
07:54
and that’s not the case at all.
07:55
They’re absolutely feeling gravity.
07:58
They’re just cruising so fast that,
08:00
even though they’re being pulled in,
08:02
they never get pulled to the surface.
08:04
It’s like that ride at the rollercoasters
08:06
where you go in and it’s spins super fast
08:09
and you can’t feel it spinning fast but–
08:11
Yeah, you feel pinned to that.
08:13
It’s exactly like that.
08:14
There’s something called the equivalence principle
08:17
where people realized, especially Einstein,
08:19
that if you were in outer space in a rocket ship
08:22
and it was dark and painted and it was accelerating
08:25
at exactly the right rate,
08:26
you actually wouldn’t know if you were sitting
08:28
on the floor of a building around the Earth
08:30
or if you were on a rocket ship that was accelerating.
08:32
That’s crazy. Yeah.
08:34
You ever had that experience where you’re sitting in a train
08:35
and the other one moves and for a second
08:38
you’re not sure if you’re the one moving?
08:39
Yeah, ’cause I go on the train every day
08:42
but I never feel like I’m moving when I’m in the train,
08:45
and then I’m like, wait, what?
08:46
That’s because in some sense, you’re really not.
08:48
Imagine you’re in this train
08:50
and it’s going near the speed of light
08:51
relative to the platform,
08:53
but it’s so smooth,
08:55
then you should be in a situation
08:57
in which there’s no meaning to your absolute motion,
08:59
there’s no absolute motion.
09:01
So that if you throw a ball up,
09:02
you might think from the outside of the platform,
09:05
be confused that when gravity pulls that back down,
09:08
it’s gonna hit you or something,
09:09
but it’ll land in your palm
09:11
as surely as if you were in your living room.
09:14
Isn’t that kinda crazy? Amazing.
09:15
So imagine you were an astronaut
09:16
and you were floating in empty space.
09:18
You can’t see anything.
09:19
There’s no stars, there’s no Earth.
09:21
You can ask yourself, am I moving?
09:24
There’s really no way for you to tell.
09:25
So you would probably conclude, well, I’m not moving.
09:27
So then your friend Marina comes cruising past you,
09:30
and maybe she’s going thousands of kilometers a second,
09:33
and you say, Marina, you’re cruising
09:35
at thousands of kilometers a second,
09:36
you’re going so fast.
09:37
But she had just done the same experiment.
09:39
She was just floating in space thinking,
09:43
There’s no way to know which one of you is moving
09:46
and there’s no meaning to the absolute motion.
09:49
The only thing that’s true
09:50
is that you’re in relative motion, that’s true.
09:53
You both agree you’re in relative motion,
09:57
But neither of you can say it’s actually you who’s moving
10:00
and I’m stationary.
10:02
[laughs] I don’t even know what to say to that.
10:06
So let me tell you where it gets really crazy.
10:10
So, let’s say you and Marina are floating in space
10:14
and you can’t tell who’s moving.
10:16
Let’s say you both see a flash of light.
10:19
A flash of light comes from somewhere,
10:21
you don’t know where.
10:21
So you measure the speed of light
10:23
to be 300,000 kilometers per second.
10:25
But here comes Marina and she’s racing at the light pulse,
10:28
as far as you can tell.
10:29
Two cars driving towards each other
10:32
seem like they’re going faster towards each other
10:34
than somebody who’s standing still
10:35
relative to one of the cars,
10:37
So you would say, oh Marina is gonna measure
10:39
a different speed of light.
10:40
But she comes back and she says, No.
10:41
300,000 kilometers per second.
10:44
Because from her perspective, she’s standing still,
10:46
and the laws of physics have better be the same for her.
10:49
The speed of light is a fact of nature
10:51
that’s as true as the strength of gravity.
10:54
And the two of you are in this quandary
10:56
because if one of you is the preferred person
10:59
who correctly measures the speed of light,
11:02
that ruins everything about the idea
11:04
of the relativity of motion.
11:05
Which one of you should it be?
11:07
So Einstein decides they must both measure
11:10
the same speed of life.
11:11
How could that possibly, possibly be the case?
11:14
And he thinks, well, if speed is how far you travel,
11:19
your spatial distance, in a certain amount of time,
11:23
then there must be something wrong with space and time.
11:26
And he goes from the constancy of the speed of light
11:29
and a respect for this idea of relativity
11:31
to the idea that space and time must not be the same
11:36
for you and for Marina.
11:38
And that’s how he gets the idea
11:39
of the relativity of space and time.
11:41
[laughs] You have the best expression on your face. [laughs]
11:46
It’s pretty wild, but that is a starting point, actually,
11:49
of the whole theory of relativity.
11:51
That starting point leads to
11:52
this complete revolution in physics
11:54
where we suddenly have a Big Bang
11:56
and black holes and space-time.
11:57
Just from that one simple starting point.
11:59
So, is your impression of gravity different
12:01
than when we started the conversation?
12:03
Yeah, ’cause I knew that when I was on the train
12:06
it didn’t feel like I was moving,
12:07
but I didn’t know why or that it was a thing
12:10
and I wasn’t crazy.
12:11
[Janna and Maria laugh]
12:12
And it’s a really deep principle.
12:15
And what about the theory of gravity?
12:17
I don’t know, usually when I just heard gravity
12:19
it’s from my coaches,
12:20
but I didn’t know it was all these things.
12:24
It’s like a big paradigm.
12:29
So, you’re in college? Yeah.
12:31
[Janna] And what are you studying in college?
12:32
I’m a physics major.
12:33
So, from your perspective,
12:35
how would you describe gravity?
12:37
I’m taught that it’s a force.
12:39
It’s described by inverse law.
12:40
But I also know that it’s a field.
12:42
And there’s a recent discovery with gravitational waves,
12:45
although I don’t know the specific details about that.
12:49
So, when you say it’s an inverse-square law,
12:51
that means that the closer you are,
12:54
the more strongly you feel the gravitational pull.
12:56
And that makes sense.
12:57
There’s very few things that are stronger
12:59
when you’re further apart. Yeah.
13:00
So you can also think of a gravitational field,
13:04
something that permeates all of space.
13:06
Even though the earth is three stories below us,
13:10
it’s not as though it’s pulling at us from a distance.
13:13
We’re actually interacting with the field at this point
13:16
and there’s a real interaction right here at this point.
13:19
And that’s nice, because people were worried
13:21
that if things acted at a distance,
13:24
that the way that old-fashioned
13:25
inverse-square force law describes it,
13:27
that it was as spooky as mind-bending a spoon,
13:30
that it was like telekinesis.
13:32
If you don’t touch something, how do you affect it?
13:35
And so the first step was to start to think of gravity
13:37
as a field that permeates all of a space.
13:39
And it’s weaker very far from the Earth
13:41
and it’s closer very close to the Earth.
13:43
So one way to think of this field
13:45
as a field that’s really describing
13:47
a curved space-time that is everywhere.
13:49
Forget the difficulty of the math,
13:51
just the intuition comes from
13:53
two kind of simple observations.
13:56
One was what Einstein described
13:58
as the happiest thought of his life.
14:00
So, right now, you might feel heavy in your chair,
14:04
and we might feel heavy on the floor and our feet,
14:07
or standing in an elevator cab.
14:08
And Einstein said, what does the chair have to do with it,
14:12
or the floor, or the elevator?
14:14
Those aren’t gravitational objects.
14:16
So he wanted to eliminate them,
14:18
and one way to do the thought experiment
14:20
is to imagine standing in an elevator
14:22
that you can see out of, a black box.
14:24
And imagine the cable is cut
14:26
and you and the elevator begin to fall.
14:30
You’re in total free fall.
14:31
Now, because things fall at the same rate,
14:33
including the elevator and you,
14:36
you can actually float in the elevator.
14:38
If you just floated in the elevator,
14:39
the two of you would drop,
14:41
and you might not even know you’re falling.
14:43
You could take an apple and drop it in front of you,
14:46
and it would float in front of you.
14:47
You would actually experience weightlessness.
14:50
It’s called the equivalence principle.
14:52
It was Einstein’s happiest thought
14:54
that what you’re really doing
14:56
when you’re experiencing gravity
14:58
isn’t being heavy in your chair,
14:59
it’s falling weightlessly in the gravitational field.
15:04
And that was the first step,
15:05
to think of gravity as weightlessness and falling.
15:07
I know zero-gravity experiences
15:09
that are done with planes, I believe?
15:13
Yeah, exactly. Yeah, yeah.
15:14
You can make somebody look like
15:15
they’re in the International Space Station
15:17
by flying up in a plane and then just free-falling,
15:21
the plane just drops out of the air.
15:23
And while it’s falling, they will float weightlessly,
15:26
and there’s been a lot of experiments about it,
15:28
but you don’t want it to end unhappily,
15:30
so the plane has to scoop back up,
15:32
and then you see them
15:33
become pinned to the floor of the plane,
15:35
because then the plane is interrupting their fall.
15:38
So that’s the first thought,
15:39
and then the next is, what is the shape that’s chased?
15:43
So if you were floating in empty space,
15:45
really empty space, and you had an apple,
15:48
and you threw the apple,
15:49
what shape do you think it would chase, the path?
15:52
Well, if I threw it straight,
15:54
I would think it would go straight.
15:55
Yeah, it would just go straight.
15:57
But if you did that on the Earth, what would happen?
15:59
It would just go down.
16:00
Yeah, it would chase a curve, it would chase an arc.
16:04
And the faster you throw it, the kind of longer the arc.
16:08
So the second step to think about curved space-time
16:11
is to say that when things fall freely
16:14
around a body like the Earth, they trace curved paths,
16:19
as though space-time itself, space itself was curved.
16:25
You had that moment,
16:26
I saw that it your face! Yeah, yeah, yeah.
16:28
[Janna and Lisa laugh]
16:29
So, that’s the intuition,
16:31
that’s how Einstein gets from thinking
16:34
that space-time is curved from the idea that, well,
16:37
there’s this field that permeates all of space,
16:39
and what is really describing is the curves
16:41
that things fall along.
16:42
And from there, it’s a very long path
16:45
to finding the mathematics and the right description,
16:48
that’s really hard.
16:49
But that intuition is so elegant and so beautiful
16:52
and just comes from these two simple thought experiments.
16:55
Isn’t it kind of amazing? Yeah. [laughs]
16:57
So you described learning in a class about light
17:01
the theory of special relativity
17:03
where Einstein is really adhering
17:05
to the constancy of the speed of light
17:07
and questioning the absolute nature of space and time.
17:10
And it seems like that has nothing to do with gravity,
17:13
but he later begins to think about
17:15
the incompatibility of gravity
17:17
with his theory of relativity.
17:19
So suppose the Sun were to disappear tomorrow.
17:22
Some evil genius comes and just figures out a way
17:25
to evaporate the Sun.
17:27
In Newton’s understanding of gravity,
17:29
we would instantaneously know about it
17:31
all the way over here at the Earth.
17:33
And that’s incompatible with the concept
17:35
that nothing can travel faster than the speed of light.
17:39
No information, not even information about the Sun,
17:41
could possibly travel faster than the speed of light.
17:44
So we shouldn’t know about what happened to the Sun
17:46
for a full eight minutes,
17:48
which is the time it would take light to travel to us.
17:50
And so he begins to question
17:52
why gravity is so incompatible with relativity,
17:56
but he already knows he’s thinking about
17:57
space and time in relativity.
17:59
So then he gets to his general theory of relativity
18:02
where he realizes if I eliminate everything
18:06
but just the gravitational field of let’s say the Earth
18:10
and I look at how things fall
18:12
and I see that they follow curves,
18:14
well, then he realizes that space and time
18:16
don’t just contract or dilate,
18:18
that they can really warp,
18:20
that they can bend and that they can curve.
18:22
And then he finds a way
18:23
to make gravity compatible with relativity
18:27
by saying if the Sun were to disappear tomorrow,
18:29
the curves that the Sun imprinted in space-time
18:33
would actually begin to ripple,
18:35
and those are the gravitational waves,
18:37
and they would change and they would flatten out,
18:40
’cause the Sun was no longer there.
18:41
And that would take the light-travel time to get to us
18:45
to tell us that the Sun was gone,
18:47
and then we would stop orbiting
18:48
and just travel along a straight line.
18:51
[Janna and Lisa laughs]
18:52
Well, let’s hope it doesn’t happen.
18:54
Yeah. [Janna laughs]
18:55
So what do you think you walk away with?
18:57
What do you think you learned?
18:58
Well, I learned more about the intuitions
19:00
behind the concept.
19:02
‘Cause we already just do the problems
19:05
but sometimes you get lost in the math,
19:07
but speaking like this it really helps build my intuition.
19:11
Yeah, it does for me too, so thank you. [laughs]
19:20
So you’re getting your PhD in physics?
19:23
Theoretical high energy physics.
19:24
Basically the physics of
19:25
really, really small fundamental things.
19:28
So what would that have to do
19:29
with gravity or astrophysics?
19:31
Well, what I’m looking at is states of matter
19:33
that might exist inside neutron stars.
19:36
So, when a star dies, if the star is massive enough,
19:39
there’s a huge explosion, called a supernova,
19:41
and the stuff that’s left behind
19:43
that doesn’t get blown away
19:45
collapses into a tiny compact blob
19:47
called a neutron star.
19:48
So what I love about neutron stars personally
19:50
is that they’re kind of city-sized,
19:53
right? That’s right.
19:53
[Janna] They’re about the size of a city.
19:54
So you’re imagining something
19:56
more than the mass of the Sun.
19:57
[Will] Yeah, or about the mass of the Sun,
19:59
condensed to the size of a city.
20:01
It’s dense enough that one teaspoon-full
20:03
would weigh about a billion tons here on Earth.
20:05
Now, that makes the gravitational field incredibly strong
20:11
around the neutron star.
20:12
So what would happen if we were on a neutron star,
20:15
because of the gravity?
20:16
We would immediately be crushed into the ground,
20:18
I think our bodies would be shred
20:20
into their subatomic particles.
20:22
So what’s the connection
20:22
between neutron stars and black holes?
20:24
So, as I understand it,
20:25
a black hole is sort of like a neutron star’s big brother.
20:29
It’s more intense, though.
20:30
If you have so much matter when a star is collapsing
20:33
that it can’t hold itself up, it collapses to a black hole,
20:37
and those are so dense that space-time breaks down
20:41
in some way or another.
20:41
Black holes are so amazing
20:43
that when the neutron star stops
20:45
and there’s something actually there.
20:47
There’s material there.
20:48
If it’s so heavy it becomes a black hole,
20:50
so it keeps falling,
20:51
once the event horizon of the black hole forms,
20:54
which is the shadow,
20:56
the curve that’s so strong that not even light can escape,
20:59
the material keeps falling.
21:00
And like you said, maybe space-time breaks down
21:03
right at the center there, but whatever happens,
21:05
the star’s gone, that black hole is empty.
21:08
So in a weird way black holes are a place and not a thing.
21:11
So is there a sensible way to talk
21:12
about what’s inside a black hole,
21:14
or is that, should you think of it
21:15
as there is no space-time inside?
21:19
There isn’t a sensible way to talk about it yet,
21:21
and that probably means that’s where Einstein’s
21:24
theory of gravity as a curved space-time
21:26
is beginning to break down,
21:27
and we need to take the extra step
21:29
of going to some kind of quantum theory of gravity.
21:32
And we don’t have that yet.
21:34
So even though the black hole isn’t completely understood,
21:37
we do know that they form astronomically,
21:39
that in the universe things like neutron stars form
21:43
and things like black holes form.
21:45
The consequences are very much speaking
21:48
to this curved space-time.
21:50
So, for instance, if two black holes orbit each other,
21:53
they’re like mallets on a drum,
21:55
and they actually cause space-time to ring,
21:58
and it’s very much part of gravitation.
21:59
The ringing of space-time itself,
22:01
we call gravitational waves.
22:03
And this was something Einstein thought about
22:05
right away in 1950-1960, he was thinking about that.
22:09
Those waves are very exciting for me too
22:11
because neutron stars orbiting each other
22:13
also give off gravitational waves
22:15
and we might be able to get some data
22:17
about neutron star material from that kind of signal.
22:20
[Janna] Yes, they ring space-time also like a drum,
22:23
and you can record the sound of that ringing
22:25
after a billion years,
22:27
when it’s traveled through the universe.
22:29
But then the next thing that happens is
22:30
those neutron stars collide,
22:32
and because of this incredibly high energy state of matter,
22:37
it becomes this firework of different explosions.
22:43
It’s really quite spectacular.
22:45
That’s right, in fact,
22:45
when we recorded that for the first time
22:47
with gravitational waves,
22:48
we then pointed telescopes at it
22:50
and were able to see it optically as well,
22:52
and that gave scientists a lot of data.
22:55
Yeah, it was, to my knowledge,
22:57
the most widely studied astronomical event
23:00
in the history of humanity.
23:01
Wow, that’s amazing.
23:02
So when the gravitational waves were recorded
23:04
and they realized, oh this sounds like,
23:06
you can reconstruct the shape and size
23:08
of the mallets of the drum from the sound,
23:11
these sounds like neutron stars colliding, not black holes.
23:14
And so, like you said, there was a trigger
23:16
for satellites and experiments all over the world
23:19
to point roughly in the direction
23:21
that the sound was coming from.
23:22
So, from your point of view,
23:24
they’re like two super-conducting giant magnets colliding,
23:27
an experiment you could never do on Earth.
23:29
That’s just the most tremendous scales
23:32
and peculiarities of matter.
23:35
I’ve heard statistics like many Earth masses worth of gold
23:38
were created, forged in the neutron star collision
23:43
We used to think that most elements in the universe
23:45
were created in supernova, which is when stars explode,
23:49
because there’s so much violent activity at the center
23:52
that you need that kind of energy to create new elements.
23:55
[Janna] The way you do in a bomb.
23:57
It’s basically nuclear fusion.
23:58
Sure, but we now think that that kind of fusion happens
24:02
when two neutron stars collide.
24:04
If you think about it,
24:05
you have two massive blobs of neutrons.
24:07
When you smush them together, you’ve got neutrons colliding.
24:11
It creates the conditions where new elements can be created.
24:14
Yeah, it’s amazing.
24:15
It’s literally populating the periodic table.
24:17
Yes, we now think that most of the heavy elements
24:20
after some number are created in neutron star collisions.
24:24
So you are already a PhD student,
24:26
you know a lot about gravity,
24:27
but what do you think you’ve taken away
24:28
from this conversation?
24:29
Well, I’ve definitely taken away
24:31
that the way that we think about gravity today
24:33
is very different from how Newton thought about it,
24:36
and that even though we have a very good understanding,
24:39
there’s lots of things that we don’t fully understand.
24:41
There’s still a lot of questions to be answered,
24:43
which I think is really exciting.
24:45
See, you’re a scientist. [laughs]
24:46
Isn’t the best part being able to ask the questions?
24:54
So we’ve been talking about gravity
24:56
from Newton and celestial bodies, the Earth, the Moon,
25:00
pulling on each other in the conventional sense
25:02
of gravity being an attractive force,
25:03
to the Earth creating curves in space-time,
25:07
then we moved on to just diffused seas of energy
25:11
and space-time as the real universe
25:15
and gravitation is really just talking about
25:18
space-time in general, and here we are,
25:20
and you’re really hardcore in theoretical physics.
25:23
Where would you take the exposition of gravity
25:27
Well, one thing is quantum mechanics.
25:28
Quantum mechanics is the most successful theory
25:30
in the history of science,
25:32
it explains the most different phenomena the most precisely.
25:35
Yet many people would still say we don’t understand
25:39
even the basics of it.
25:40
So when we think about quantum mechanics,
25:41
we think about particles and their quantum charges
25:44
in the Feynman way, the way that Feynman taught us.
25:47
They come in and they exchange a force carrier
25:50
and then they come out again,
25:51
so that’s how we think of an electron and light scattering,
25:53
for instance, or something like that.
25:54
And the language that Einstein gave us is so different.
25:57
It’s completely geometric, it’s all this space-time.
26:00
And it’s also unnecessary.
26:02
Yeah, for me, the beauty of the theory of gravity is
26:05
the way Einstein formulated it,
26:06
as a theory of geometry, of curved space and time.
26:09
I think, like you, that’s one of the things
26:11
that really pulled me into it.
26:12
Is there really space-time
26:13
or are we just using unnecessary language
26:17
because it’s elegant and we like it and it’s beautiful?
26:20
Well, I think there’s really space-time
26:21
in the sense that it’s a description that works really well,
26:24
so there has to be something right about it.
26:26
I mean, if we’re gonna talk about
26:27
what’s really, really underlying that
26:29
and we’re gonna put quantum mechanics into the mix,
26:31
then there should be some
26:32
quantum mechanical wave function for space-time.
26:35
You should be able to take two different space-times
26:38
and add them together,
26:39
’cause one of the crazy things about quantum mechanics,
26:41
as you know, it’s–
26:42
To have the waves together.
26:43
Yeah, and in two states
26:44
and in two possible states of the world,
26:47
you can just literally put a plus sign between them
26:49
and that’s a sensible state, that’s a good state,
26:52
So do you think there’s some sense
26:54
in which we shouldn’t be thinking
26:55
about individual universes, individual space-time,
26:57
so we should be thinking about
26:58
superpositions of space-times?
27:01
I think if you were to go far enough back
27:02
in the history of the universe,
27:03
back to when it was very, very dense, very small,
27:05
and when quantum mechanics was certainly important,
27:08
then it must have been like that.
27:09
I mean, if we believe that
27:10
the dominant standard model of cosmology,
27:13
something had to produce the density perturbations,
27:15
the things that seeded all the galaxies and stars
27:17
and everything else in the world.
27:19
So there’s a galaxy over there, let’s say,
27:20
and not over there, so how did that happen?
27:22
Why is there a galaxy there and not there?
27:24
In the standard theory, as you know,
27:25
that was a quantum event, a random event.
27:27
And it doesn’t mean that if happened there and not there
27:29
’cause you flipped a coin,
27:30
it actually happened in both places.
27:32
There’s gotta be a wave function
27:33
where in one branch of the wave function
27:34
there’s a galaxy there and not there,
27:35
and on the other branch it’s the opposite.
27:37
So when we’re talking about
27:39
the multiverse or the Big Bang,
27:41
we are really talking about gravity ultimately,
27:43
and we’re talking about how a theory of gravitation
27:45
which we know think of as a theory of space-time
27:47
has a quantum explanation,
27:50
has a quantum paradigm imposed on it
27:52
that will help us understand these things,
27:53
and we don’t have that yet.
27:54
One of the things that I think is so amazing
27:56
is that the terrains in which we’re going to understand
27:59
quantum gravity are very few.
28:02
It’s the Big Bang, because that’s where we know
28:04
that quantum and gravity both were called into action.
28:09
And there’s black holes.
28:10
One of the most interesting discoveries
28:12
is of course Hawking’s discovery,
28:14
kick-started a kind of crisis, right?
28:18
In thinking about why quantum mechanics and gravity
28:20
were so knocking heads.
28:22
It was one of the most beautiful examples.
28:24
Sure, yeah, it is a beautiful, beautiful idea.
28:27
So, first of all, to be totally clear, though,
28:28
we’ve never observed Hawking radiation,
28:30
which is what he predicted, directly.
28:32
I don’t think very many people doubt that it’s there,
28:34
but yeah, Hawking discovered mathematically
28:36
that when you have a black hole, it’s got an event horizon,
28:40
it’s got a surface which is a point of no return.
28:43
If you fall through that surface, no matter what you have,
28:46
no matter how powerful the rocket you’ve got,
28:48
even if you beam a flashlight back behind you
28:50
in the direction you fall from,
28:51
nothing escapes, not even light.
28:52
It all gets sucked in and spaghettified
28:54
and destroyed at the singularity,
28:55
or something, something happens, but it doesn’t get out.
28:58
But in quantum mechanics,
28:59
you can’t really pin down
29:00
the location of something precisely.
29:02
If you try to pin down an electron
29:04
in a tiny circuit in a microchip,
29:05
sometimes you discover it’s not actually there
29:07
and then your computer crashes.
29:09
This is the Heisenberg’s uncertainty principle in reality.
29:12
You can’t precisely say where the electron is,
29:15
and you can’t precisely say how quickly it’s moving.
29:18
Exactly, yeah, so when you get the blue screen of death,
29:20
that might be because of quantum mechanics.
29:23
You know, you try to pin something down
29:24
near a black hole, well, it’s a surface,
29:25
it’s got a particular radius for a round black hole,
29:27
and wanna say something is inside or outside,
29:29
well, you can’t absolutely say that in quantum mechanics.
29:31
And this kind of uncertainty produces a radiation,
29:34
which you can think of as pulling some of the energy
29:36
out of the black hole.
29:38
The black hole is formed out of some mass
29:40
and there’s an energy in that.
29:41
If you think of pulling some energy out of that
29:43
and sending it off to infinity
29:44
in the form of particles being admitted.
29:46
And what Hawking found is that it’s a thermal spectrum,
29:48
it looks like a hot, or not so hot for a large black hole,
29:51
but like an oven, the kind of radiation
29:52
that comes out of a cast iron.
29:54
This idea that the darkest phenomenon in the universe
29:57
actually is forced to radiate quantum particles
30:01
I think everyone understood
30:02
that it was a correct calculation,
30:05
but I don’t think a lot of people
30:06
understood the implications,
30:08
that it meant something really terrible was happening.
30:11
Because this black hole,
30:12
which could have been made of who knows what,
30:14
is disappearing into these quantum particles
30:17
which, in some sense, have nothing to do
30:19
with the material that went in.
30:20
So do you think that’s a big crisis?
30:22
The black hole evaporates, the information is lost?
30:25
It’s a crisis because of some of the details of it,
30:27
but I would say the way you just described,
30:29
I mean, if I build a big bonfire or an incinerator
30:33
and I throw an encyclopedia into it,
30:35
good luck reconstructing what was in that encyclopedia.
30:37
The information is lost for all practical purpose.
30:39
Practical purposes. Yes.
30:40
So this is a huge crisis
30:41
’cause either quantum mechanics is wrong,
30:43
and as you described it,
30:44
it’s the most accurately-tested paradigm
30:46
in the history of physics,
30:47
how could it be wrong, right?
30:50
Or the event horizon is letting information out
30:52
and violating one of the most
30:54
sacred principles of relativity.
30:56
One thing about quantum mechanics is that
30:58
any time you have a state of the world
31:00
and another state of the world,
31:01
you can literally add them together
31:02
and get a third possible state,
31:04
as crazy as that sounds.
31:05
And so if you’re gonna have a quantum theory of gravity,
31:08
then we can’t really talk about there being a black hole
31:11
or not a black hole,
31:12
or an event horizon or not an event horizon,
31:14
because we could always a state
31:17
that had an event horizon and a state that doesn’t,
31:19
or has the event horizon
31:19
in a slightly different position, maybe,
31:21
and add them together.
31:22
So the existence or position of an event horizon
31:25
can’t possibly be determined as a fact
31:27
any more than the position of an electron is determined.
31:29
So I think that’s the loophole.
31:30
That’s a nice way of looking at it.
31:32
So that you’re not actually violating classical relativity
31:36
once you’re in a regime where the wave function
31:39
has really peaked around a very well-defined stage.
31:43
That’s right, and one of the most exciting developments
31:45
in the last 10 or 20 years is called holography,
31:48
and it’s called holography because
31:50
a hologram is a two-dimensional surface
31:51
that creates a three-dimensional image.
31:53
It’s got sort of 3D information built into it.
31:55
And this, in a fundamental way,
31:57
really has that 3D or higher dimensional information
32:00
It’s exactly the same as this theory of gravity
32:02
and more dimensions.
32:04
Yes, so one of the things I like to think of
32:05
with holography is that I can pack
32:07
a certain amount of information in a black hole.
32:09
I mean, you can literally think of it
32:09
as throwing things into it.
32:11
So let’s say I have information in some volume
32:13
and I’m under the illusion
32:14
that I can just keep packing information in that volume,
32:17
as much as the volume will contain.
32:19
Eventually I’ll make a black hole
32:21
and I’ll find out that the maximum amount of information
32:23
I can pack into anything in the entire universe
32:26
is what I can pack on the area.
32:27
And since area is projecting the illusion, maybe, of volume,
32:31
maybe the whole world is just a hologram.
32:32
It’s not a principle that only applies to black holes.
32:37
if this theory of quantum gravity is correct,
32:39
then this while three-dimensionality is an utter illusion
32:42
and really the universe is two-dimensional.
32:44
That’s crazy. That’s true.
32:46
And as practically speaking,
32:47
you mentioned before in our conversation
32:49
that it’s really interesting
32:50
that the Heisenberg uncertainty principle
32:51
is a practical limit now in microchips.
32:54
If we make microchips much smaller than they already are,
32:56
even as they already are, it causes errors,
32:59
’cause you don’t know that the electron’s in.
33:00
If holography, if this limit on how much information
33:03
you can ever pack, if that ever become a limit,
33:05
as far as we know that’s an absolute limit.
33:07
We started off with clay tablets,
33:09
not so much information per cubit centimeter or whatever.
33:12
Then we had written stuff that’s getting better,
33:14
encyclopedias with thin paper that’s even better, CDs.
33:17
A smaller and smaller space,
33:18
trying to pack it denser and denser,
33:20
until eventually we make a black hole.
33:21
Yeah, at some point you try to fill up
33:24
your encyclopedia with knowledge
33:25
and you get swallowed up by a black hole.
33:27
And the most knowledge you could ever have
33:29
would only be on a two-dimensional surface.
33:31
Right, and as big as the universe, and then you’re done.
33:34
So, you know, not likely
33:36
that we’re ever gonna hit that limit any time soon.
33:37
Do you think it’s possible
33:39
that gravity is really ultimately just quantum mechanics
33:44
and doesn’t exist at all in the fundamental ways
33:47
that we’ve been talking about so far,
33:49
like the Newtonian way and the space-time way,
33:51
that those are just these kind of macroscopic illusions?
33:55
Sometimes I talk about it in terms of temperature.
33:56
Temperature is not a thing.
33:59
There is no single thing called temperature.
34:01
It’s a macroscopic illusion
34:03
that comes from the collective behavior,
34:05
really quantum behavior of random motions of atoms.
34:08
And is it possible that the whole of gravity
34:10
is some kind of emergent illusion
34:13
from what’s really quantum phenomenon underlying it?
34:16
If we buy the idea of holography, then absolutely,
34:18
that’s for sure, that’s what it’s telling us.
34:21
Although which side is the illusion
34:22
and which side is the reality?
34:24
I mean, temperature is still great to talk about.
34:25
It doesn’t mean we shouldn’t talk about temperature.
34:27
I mean, we should absolutely adjust our thermostats
34:29
and talk about temperature.
34:30
But if we look at it closer and closer and closer,
34:33
we realize there’s not a thing in the world
34:34
that has as a quantum value temperature, isolated.
34:38
And so maybe there is no such thing as gravity
34:41
isolated from quantum mechanics.
34:43
Right, so I guess with the holographic description
34:45
we’ve got two sides, which are actually secretly the same.
34:48
On one side there’s definitely no gravity.
34:50
On the other side, well,
34:51
it’s a quantum theory of gravity, whatever that means.
34:54
But the point is you can get it out,
34:56
it’s equivalent to this theory.
34:57
So that’s just like saying
34:59
there’s the idea of a dual description.
35:01
It’s just saying there’s a perfect dictionary
35:02
between these two descriptions,
35:03
and so to belabor which one’s real is silly.
35:06
It’s like saying, is French or is English real?
35:09
Yeah, an example I like to give is
35:11
if you take some extra dimensions
35:13
and you compactify them, let’s say just one,
35:15
all that is, it’s exactly prevalent
35:17
to whatever particles you had,
35:18
whatever fields you had in your original theory
35:19
before you added it,
35:20
you just added an infinite tower of new particles
35:22
with certain properties that are all easy to calculate.
35:25
For me, it’s a question of which description
35:27
I mean, if you wanna say gravity is an illusion
35:29
and it’s all quantum, that’s great,
35:30
but then you fall down the stairs and bang your head.
35:33
It’s sort of like there’s a description
35:35
that works pretty well.
35:37
Yeah, you don’t go to the doctor and say,
35:38
Heisenberg’s uncertainty principle caused
35:41
a series of fluctuations.
35:43
Right, would you help me?
35:44
So there’s so many open questions.
35:46
The fact that they are all these fundamental issues
35:48
that we really don’t understand.
35:50
But, on the other hand, there’s all these moving parts
35:52
that fit together so neatly.
35:53
There’s definitely something that’s working here.
35:55
But ultimately what is gonna emerge from that,
35:57
what structure is lying under it, we just don’t know.
36:00
But I think the fact that there are
36:02
so many fundamental questions
36:04
that we just don’t know the answer to,
36:05
that is an opportunity, that’s exciting, it’s great.
36:08
Thanks so much for coming.
36:09
It’s really good to have you here.
36:10
Thank you very much, Janna, it was my pleasure.
36:19
I hoped you learned something about gravity
36:21
you hadn’t thought of before,
36:22
and I hope even more that it provoked some questions.
36:25
So thank you for watching.
