Janna Levin: Black Holes, Wormholes, Aliens, Paradoxes & Extra Dimensions | Lex Fridman Podcast #468

- Black holes curve space and time around them,

and the way that we've been describing,

things fall along the curves in space.

If the black holes move around,

the curves have to follow them, right?

But they can't travel faster than the speed of light either.

So, what happens is as these black holes,

let's say move around,

maybe I've got two black holes in orbit around each other,

that can happen,

it takes a while.

Wave is created in the actual shape of space,

and that wave follows the black holes

as black holes are undulating.

Eventually those two black holes will merge.

And as we were talking about,

it doesn't take an infinite time,

even though there's time dilation.

because they're both so big,

they're really deforming space-time a lot.

I don't have a little tiny marble

falling across an event horizon.

I have two event horizons.

And in the simulations you can see a bobble

and they merge together and they make one bigger black hole.

And then it radiates in the gravitational waves.

It radiates away all those imperfections,

and it settles down to one quiescent,

perfectly silent black hole that's spinning.

Beautiful stuff.

And it emits E=mc² energy.

So, the mass of the final black hole

will be less than the sum of the two starter black holes.

And that energy is radiated away

in this ringing of space-time.

It's really important to emphasize that it's not light.

None of this has to do literally with light

that we can detect with normal things that detect light.

X-rays are a form of light,

gamma rays are a form of light,

infrared, optical, this whole electromagnetic spectrum,

none of it is emitted as light.

It's completely dark.

It's only emitted in the rippling of the shape of space.

A lot of times it's likened closer to sound.

Technically, we've kind of argued, I mean,

I haven't done an anatomical calculation,

but if you're near enough to two colliding black holes,

they actually ring space-time in the human auditory range.

The frequency is actually in the human auditory range,

that the shape of space could squeeze

and stretch your eardrum, even in vacuum,

and you could literally hear these waves ringing.

- The following is a conversation with Janna Levin,

a theoretical physicist and cosmologist

specializing in black holes, cosmology of extra dimensions,

topology of the universe,

and gravitational waves in space-time.

She has also written some incredible books,

including "How the Universe Got Its Spots"

on the topic of the shape and the size of the universe,

"A Madman Dreams of Turing Machines"

on the topic of genius, madness,

and the limits of knowledge,

"Black Hole Blues and Other Songs from Outer Space"

on the topic of LIGO

and the detection of gravitational waves,

and "Black Hole Survival Guide"

all about black holes.

This was a fun and fascinating conversation.

This is The Lex Fridman Podcast.

To support it, please check out our sponsors

in the description.

And now, dear friends, here's Janna Levin.

I should say that you sent me a message

about not starting early in the morning,

and that made me feel like we're kindred spirits.

- Yeah.

- You wrote to me, "When the great physicist Sidney Coleman

was asked to attend a 9:00 AM meeting,

his reply was, 'I can't stay up that late.'"

- Yeah. Classic.

Sidney was beloved.

- I think all the best thoughts, honestly,

maybe the worst thoughts, too, all come at night.

There's something about the night.

Maybe it's the silence.

Maybe it's the peace all around.

Maybe it's the darkness.

And you just,

you could be with yourself

and you could think deeply.

- I feel like they're stolen hours

in the middle of the night because it's not busy.

Your gadgets aren't pinging.

There's really no pressure to do anything.

But I'm often awake in the middle of the night.

And so it's sort of like these extra hours of the day.

I think we were exchanging messages at 4:00 in the morning.

- Okay, so in that way,

and in many other ways, we're kindred spirits.

So, let's go into

one of the coolest objects in the universe,

black holes, what are they?

And maybe even a good way to start

is to talk about how are they formed?

- Mm. Yeah.

In a way, people often confuse how they're formed

with the concept of the black hole in the first place.

So, when black holes were first proposed,

Einstein was very surprised

that such a solution could be found so quickly.

But really thought nature would protect us

from their formation.

And then nature thinks of a way.

Nature thinks of a way to make these crazy objects,

which is to kill off a few stars.

But then I think that there's a confusion that dead stars,

these very, very massive stars that die,

are synonymous with the phenomenon of black hole.

And it's really not the case.

Black holes are more general

and more fundamental than just the death state of a star.

But even the history of how people realized

that stars could form black holes is quite fascinating.

Because the entire idea really just started

as a thought experiment.

And if you think of, it's 1915, 1916,

when Einstein fully describes relativity

in a way that's the canonical formulation.

It was a lot of changing back and forth before then.

And it's World War I.

And he gets a message from the Eastern Front,

from a friend of his, Karl Schwarzschild,

who's solved Einstein's equations, you know,

between sitting in the trenches and, like, cannon fire.

It was joked that he was calculating ballistic trajectories.

He's also perusing the proceedings

of the Prussian Academy of Sciences, as you do.

And he was an astronomer who had enlisted in his forties.

And he finds this really remarkable solution

to Einstein's equations.

And it's the first exact solution.

He doesn't call it a black hole,

it's not called a black hole for decades.

But what I love about what Schwarzschild did

is it's a thought experiment.

It's not about observations,

it's not about making these things in nature.

It's really just about the idea.

He sets up this completely untenable situation.

He says, "Imagine I crush all the mass of a star

to a point."

Don't ask how that's done.

Because that's really absurd, but let's just pretend.

And let's just imagine that that's a scenario.

And then he wants to decide what happens to space-time

if I set up this confounding,

but somehow very simple scenario.

And really what Einstein's equations were telling everybody

at the time was that matter and energy

curved space and time,

and then curved space-time tells matter and energy

how to fall once the space-time's shaped.

So, he finds this beautiful solution.

And the most amazing thing about his solution

is he finds this demarcation, which is the event horizon,

which is the region beyond which not even light can escape.

And if you were to ask me today, all these decades,

over a hundred years later,

I would say that is the black hole.

The black hole is not the mass crushed to a point.

The black hole is the event horizon.

And the event horizon is really just a point in space-time

or a region at space-time,

it's actually, in this case, a surface in space-time.

And it marks a separation in events,

which is why it's called an event horizon.

Everything outside is causally separated from the inside,

insofar as what's inside the event horizon

can't affect events outside.

What's outside can affect events inside.

I can throw a probe into a black hole

and cause something to happen on the inside.

But the opposite isn't true.

Somebody who fell in can't send a probe out.

And this one-way aspect really is

what's profound about the black hole.

Sometimes we talk about the black holes being nothing

because at the event horizon, there's really nothing there.

Sometimes, when we think about black holes,

we wanna imagine a really dense, dead star.

But if you go up to the event horizon,

it's an empty region of space-time.

It's more of a place than it is a thing.

And Einstein found this fascinating.

He helped get the work published,

but he really didn't think these would form in nature.

I doubt Carl Schwarzschild did either.

I think they thought they were