Astrophysicist Explains Black Holes in 5 Levels of Difficulty

Hi, I'm Janna Levin.

I'm a Professor of Physics and Astronomy

at Barnard College of Columbia University.

And today I've been asked to explain black holes

in five levels of increasing complexity.

A black hole might be different than you imagine.

To some extent it's a place and not a thing.

Black holes play an important role

in the history of the universe,

in sculpting galaxies that we live in,

and possibly in the ultimate fate of the universe.

[tense music]

Hi.

Hi, welcome.

Tell me your name.

Jude.

I wanted to ask you

if you have ever heard of a black hole?

Yeah, I think that they're scary and cool.

'Cause you can get sucked in and get lost forever

and get plopped out in a random place.

It's like a big, giant, black thingy.

So black holes, you describe them as huge.

The interesting thing about black holes

is they're hugely heavy,

but they're actually physically really small.

What really matters is the density.

Do you know what density is?

It's not weight, but it's how much of it is in it.

Here, let me show you something.

I can ask how heavy it is. Yeah.

I can also ask how big it is,

which is a question about its volume.

If I make it smaller,

then what's happening is it's becoming more dense.

So imagine I crushed this really, really small.

It would weigh the same, it would have the same mass,

but it would be much more dense.

How does it go that small?

If a star is heavy enough to explode into supernova

what's left begins to collapse under its own weight.

And if that's heavy enough,

the core will not be able to stop collapsing,

'cause it no longer has the thermonuclear fuel,

it's run outta fuel.

And if it's run outta fuel,

it's no longer shining and pushing outward.

And without that it itself begins to go dark

and there's nothing fighting the collapse anymore.

And that's when you form a black hole.

So if like the sun all collapsed on itself,

it would form a black hole?

Well, that's a really good question.

So interestingly, the sun itself isn't heavy enough.

So it has to be heavy enough

that when it begins to collapse,

it just overcomes all attempts to fight it.

If you made something really dense,

you would have to travel faster than the speed of light

to actually escape.

That's 300,000 kilometers per second.

So it goes so fast that there's it's all dark?

So it goes so fast that it goes completely dark.

Any light that veers too close will fall in,

will not be able to make it back out again.

If a light is shining from the sun near a black hole,

the black hole's not touching it.

Why does the light get pulled in?

Why does that happen?

Because the black hole is taking other stuff?

It is taking other stuff,

but the funny question was like,

if I wanted to move your chair,

you'd think it was really strange

if I didn't have to come near you

and actually grab the chair and move it.

One of the things Einstein thought about

is he imagined that

what the black hole is doing

is it's changing the shape of space around it.

What do you think of that idea?

It's crazy.

Isn't it crazy?

And then Einstein goes a step further

and thinks, well, what black holes must be doing

is curving the space so strongly

that even light gets caught.

Sometimes you can get light caught into a whole orbit,

literally the light going round and round in an orbit.

So black hole, it doesn't attract light,

it moves the space so that the curve is pointed towards it?

That's right.

We've been talking for a little while about black holes.

What are you gonna walk away with

in your impression of what a black hole is?

It's kind of curves in space

that are all coming to one point.

Everything that goes on those curves

changes directions to come in

and even light can't escape it, nothing can.

You said that very beautifully.

Does that feel like a different idea of a black hole

than the one you had before we spoke?

Yeah, a lot.

[midtempo music]

Have you heard about black holes?

Yes, I know it has a lot of mass, but it's very small.

I know that there are several theories about the universe

due to black holes,

like, around the universe and how it's made.

So a lot of times stars are born together

in two star systems

and when they die, if they're heavy enough,

they will collapse under their own weight

and form a black hole.

So here you have a black hole and a big fluffy star.

And what will happen is

it'll start to tear apart its neighboring star.

Literally parts of the star

will begin to spill onto the black hole

and splatter on the black hole.

But let's say both those stars formed black holes.

And what these black holes do

is they are like mallets on a drum.

They create literally waves

in the shape of spacetime as they're moving.

So imagine mallets on a drum,

how the drum ripples.

Depending on how the mallets are moving

you hear different sounds.

So effectively these black holes,

as they get very close together

in the final stages of their life together,

they're orbiting each other at hundreds of times a second.

It's this really crazy event,

but it's happening in complete darkness.

Eventually they bobble together and they merge

and then they wring out,

the spacetime's going crazy around them,

it's this storm in spacetime,

and they settle down to a quiet black hole.

Then those waves that they created

travel through the universe, basically undisturbed.

For a long time people thought,

well, even if black holes are out there,

they're impossible to observe.

And then they got very clever.

You might wonder how we could possibly hear black holes,

that sounds crazy.

So I'm gonna show you, but I'm gonna need your help.

This demo involves an electric guitar.

Do you play at all? A little bit?

Okay, you wanna do the demo for me?

So the LIGO instrument electronically records

the ringing of the shape of space

with its very complicated instrument.

It stands for

Laser Interferometric Gravitational-Waves Observatory

and the design was incredibly difficult

and they didn't know if they would succeed.

I think of the instrument

as like the body of the electric guitar.

And then they take the readout

of the motions of the waves that they're recording,

just like that guitar

is recording the motions of the waves on the string.

Now just play it like a little bit.

And you can't hear anything, right?

You're not meant to hear an electric guitar

when it's not plugged in.

What's happening is the guitar strings are ringing,

but so quietly that we can't actually hear the sound.

And this is like the gravitational waves,

which are ringing the drum of spacetime,

but so quietly that they're not move the air

and we're not hearing them.

So now play and I'm gonna turn the volume up a little bit.

[electric guitar music]

While I can't actually hear

the ringing of the strings themselves,

I can hear the data of the shape of the string

recorded and played through this amplifier.

And that's kind of the idea behind the LIGO instrument.

How do you know that it's

like the black hole that's making this sound

and nothing else?

It's a great question.

If I didn't see you playing the guitar,

I would recognize the sound of a guitar.

And even if I had never heard of a guitar before

I could figure out the frequencies

that the string was playing,

I could tell how strongly it had been plucked,

and I could tell the length of it

and where it was pinned down

from the harmonics of the string.

And I can tell the different lengths of the strings

from the notes that they play.

So I can actually

reconstruct the instrument that's playing it.

And it's very similar to LIGO,

we can listen to the notes, the amplitude, the harmonics,

and we can deduce the size

and shape of the objects doing that.

And they're very massive and they're very small

and they have all the markings of a black hole.

Is there anything that like,

gets affected on Earth because of those waves?

It's a really good question.

Only this instrument,

and that's why it was so hard to build.

And by the time it gets here, it's so weak

that it's only squeezing and stretching space

at like the fraction of a nucleus over very large distances.

Has your understanding of black holes changed

over the course of our conversation?

I knew there were waves for like everything,

but I never thought specifically,

oh yeah, black holes have, like, waves.

I know more and less.

I know what you mean.

[gentle music]

I'm Jayda, It's nice to meet you.

Nice to meet you, and where are you studying?

I'm a senior at NYU.

I'm studying physics and environmental studies.

What is your impression of what a black hole is?

So it's a star that has collapsed.

It has so much concentrated mass and gravity

that there's a point outside of the black hole

called the event horizon.

So once you get past the event horizon,

nothing, not even light can escape from that.

So that's a great definition

and I wanna pick that apart a little bit.

So what you described is just right.

Stars, when they run out of thermonuclear fuel

are gonna collapse under their own weight.

It'll explode in a supernova, it'll leave a core,

and if the core itself is heavy enough,

it will keep collapsing.

It does, as you say, reach this point

where not even light can escape.

But the amazing thing is it leaves that point,

you called it rightly the event horizon,

it leaves it behind kind of like an archeological record

because the start itself

can no more sit at the event horizon

than it can race outward at the speed of light.

So the core of the star keeps collapsing

and where it goes nobody knows.

So in a weird way,

the black hole isn't anymore a crush of matter.

It left it behind in its wake,

but the stuff of the star is gone.

I've heard of Schwarzschild black holes,

which is a black hole that is static,

a Kerr black hole or a Kerr-Newman black hole,

which is a black hole that rotates,

but what makes a black hole static versus rotating?

And what's more common?

It turns out that there are only three quantities

that define a black hole,

its electric charge, its mass, and its spin.

So the most general black hole can also spin

and it can also be electrically charged.

Whether or not they are has to do with how they formed.

If a star collapses,

it will likely be spinning when it collapses

and the remnant black hole that forms

will likely be spinning.

A black hole of a certain mass, charge, and spin

is indistinguishable from any other black hole

with those same properties.

So in some sense, they're like fundamental particles,

which makes them absolutely exceptional

for any other astrophysical object.

Have you heard the stories about what happens

inside a black hole?

I remember that once you pass the event horizon

space becomes time and time becomes space,

in like, a coordinate sense.

So from the outside, if you're an astronaut,

you're watching your friend,

another astronaut going into the black hole,

it's as though your times become rotated

relative to each other.

So the profound thing is as an astronaut on the outside,

looking at this round event horizon,

you think of the center of black hole as a point in space,

but to the person who's fallen in,

it's not a point in space at all, it's a point in time.

The singularity, or the end of it all,

the crush in the center of a black hole

is in their future.

So they can no more avoid the singularity

than you can avoid the next instant of time coming.

So the death in the singularity is inevitable.

Although we don't really think

the singularity necessarily exists.

I sort of know what a singularity is.

I think of it as something

where everything is compacted into one single point,

it's a place where the laws of physics

don't exactly work out.

What did you mean when you said

that you don't think the singularity really exists?

So the singularity is definitely predicted

in Einstein's general theory of relativity

and that's purely a theory of spacetime.

And in the theory of spacetime,

there is no question that a singularity would form

when the star collapses catastrophically

inside the black hole.

Now, even when people talked about singularities

back in the '60s, they thought, you know, quantum mechanics

is part of the story of the whole of physics.

It's not just gravity.

And if we understand quantum gravity

we'll realize that singularity

probably doesn't ever actually form.

So since we obviously have never been to a black hole,

how do we know for sure,

like, what happens after you cross the event horizon

or what happens inside a black hole?

Is it just like, inferred from the math?

I would say to some extent we don't know for sure.

What we have found is that

the mathematics is so unbelievably powerful

that we're able to disprove wrong ideas

just in pen and paper.

Just very recently, within the past couple of years,

the first ever human-procured image of a black hole

showed us what we expected to see of the event horizon.

So Jayda, after our conversation today,

what would you say a black hole is?

Something that I had never thought of before is

a black hole as kind of

a type of quantum fundamental particle.

I've also learned how event horizon of a black hole

kind of hides a singularity.

The beauty of being a student

of something like black holes

is you never stop

having new impressions of what this enigmatic phenomena is.

So in a year, I'll tell you what I learned that's new.

Awesome!

[classical music]

I'm Clare.

And you're in graduate school

and you're getting your PhD.

What year are you?

I'm a second year.

So I'm measuring star formation histories

in the Small and Large Magellanic Clouds.

Does the Large Magellanic Cloud have a big black hole?

So, I think the prevailing wisdom for a while was no,

but my answer honestly is I'm not sure.

Yeah, and probably nobody is. [women laughing]

Have you heard a lot in your studies

about these super massive black holes

that we think lurk in the centers

of very nearly every galaxy?

So I don't study AGN a lot,

but I do have a long term interest in black holes,

it's one of the reasons I entered the field.

I always was curious about

how a black hole of that size was able to form.

Was it the result of mergers between smaller black holes,

ultimately creating gravitational well deep enough

to contract a protogenic disc for a whole galaxy?

Or, man, what happened?

Yeah, I think it's a really good question.

The only mechanism that we know for sure

can form black holes is to collapse of very massive stars.

So it's sensible to think,

well maybe some very massive stars in a young universe

collapsed under their own weight and then they merged

and after some time they got big enough,

but the black holes from stars

can be tens of times the mass of the sun,

maybe hundreds of times to the mass of the sun

if they merge.

To get to millions and billions,

and if you just do the simple arithmetic

of how many years that would take,

there aren't enough years

in the 14 billion years of the universe's lifetime.

So they must have come from somewhere else.

I am at a loss

to think of what could have possibly happened

in between the start of the universe

and the formation of our galaxy

that could create such a massive object.

Yeah, I think that's right.

I think people are really perplexed

about how you make something so big

in such a short period of time.

It's kind of funny, the bigger you make a black hole,

it seems maybe counterintuitive,

but the less dense the material has to be

out of which you make it.

So you can, out of something the density almost of air,

you can make a supermassive black hole.

You can't make a star out of that,

but weirdly, if you skip the star phase altogether,

it's conceivable that they directly collapse.

And so there's suddenly a new way to make black holes

that nature has figured out.

We spend all of our time,

when we learn about black holes in school,

predominantly through star collapse.

[Janna] Yeah.

I didn't even realize that there

was an alternate route to creating a black hole.

There might be many alternate routes.

It might be in the very early universe

that bubbles in unusual phase transitions

from very high energy universe to a low energy universe

can make black holes.

Like, we haven't really thought of

the range of possibilities.

And so there could also be primordial black holes

that are still around

that also skipped the star stage altogether

that were formed really in the very earliest phases.

And I think the interesting thing is,

with your looking at like the Large Magellanic Cloud,

is to wonder if we're gonna merge.

Absolutely.

We thought the canonical picture of the Clouds

was essentially that they had formed with the Milky Way,

maybe in its halo,

and had been in a stable orbit for about a Hubble time,

or about 14 billion years.

Young guns in the field have thrown a wrench in that theory

that they've always been orbiting

and that perhaps they're on their first orbit,

they're on an unstable orbit.

Will they join us?

Can you tell us about Andromeda?

Andromeda is part of the big three in the local group.

The local group being a group of galaxies

that are not expanding

with the expansion of the universe away from each other,

they're trapped.

Gravitationally, all friends.

Yes, they're all friends.

And Andromeda is one of the few galaxies

that is traveling towards us

and do for a merger event at some point.

So given a sufficiently low velocity,

we would just have two big galaxies that,

for the most part,

pass through each other, pass by each other.

But given a sufficiently high velocity,

we will have some crazy black hole interactions

and some crazy star interactions.

But when we do merge with Andromeda,

presumably our black holes will merge

and Andromeda indeed has

a very big black hole as well at its center.

And then we'll have this just gigantic-

Supermassive black hole.

Yeah, and it's very possible that as you said,

the collision won't be so severe

that it'll be very disruptive.

So our entire solar system could stay intact

and here we would go with the sun and all the other planets

in orbit around a new black hole.

They're kind of misunderstood giants in a way.

So I was curious,

have you heard anything new or interesting

in the field of black holes

that will shape future discussions?

We work a lot right now

on thinking of black holes as batteries.

So a black hole that can take, like a giant magnet,

astronomical magnet in the form of another collapsed star,

like a neutron star,

and flip it around so fast, near the speed of light,

that it actually creates an electronic circuit

out of this moving magnet.

And so that the power

that can come out of these electronic circuits

created by these batteries can be tremendous.

You know, I know that at a certain point

for our civilization to become sufficiently advanced,

to travel the cosmos beyond, you know, the moon or Mars,

we may have to be able to harness the power of our sun.

Would it be similarly possible to harness

the power of a black hole like you were mentioning,

to travel?

It's a great question.

I once did a calculation of

using a black hole made out of the moon

and the strongest magnet we could find on Earth

to see if I could make an electronic battery.

And honestly,

you only get about enough energy to power New York City.

But we have to find one in our neighborhood first.

Yeah, wouldn't be my favorite thing.

So Claire,

we've had this pretty fascinating conversation

about supermassive black holes in particular.

And after our discussion,

what is it that has changed for you in your perspective

or what is it that excites you?

Oh, I think our discussion kind of

exposed a piece of black holes that I don't think of often,

which is that they're not just life takers,

they're life givers.

And they inform a lot about,

not just how a galaxy is destroyed or made,

but how it's shaped and how it eventually, you know,

builds life like ours.

So maybe I have to give black holes a little bit more props.

[gentle music]

Hi, Dan, I'm so glad you could make it.

What have you been working on with black holes

in the time since I've last seen you?

There are a lot of aspects to black holes.

The one that's kind of interested me most lately

is trying to understand them

from the point of view of information,

how information is stored and processed

and recovered from black holes.

Which turns out to be a really interesting perspective.

Talk us through Hawking's initial revolution

that led to a lot of these conversations

about the information around black holes.

Hawking's big insight was that

he had to apply both the rules of quantum mechanics

and the rules of gravity

to really understand how black holes behaved.

But Hawking took a point of view

where he brought quantum mechanics into the game.

He really that if you took that into account,

that it's actually not quite true

that black holes are black,

that actually things can escape from black holes.

So what you're describing is the famous Hawking radiation

where a black hole cleverly kind of steals energy

from the quantum vacuum

and radiates and in the process of evaporates.

And of course this caused a big kerfuffle

because when the black hole evaporates,

eventually that event horizons is yanked up.

And the question is, where did everything go

that had once fallen in?

A way to think about Hawking radiation

is to imagine that pairs of particles and antiparticles

appear out of the quantum vacuum

and the particle can escape the black hole,

but the antiparticle falls in.

But the particle and antiparticle are a pair

and if the antiparticle really falls into the black hole

and is destroyed at the singularity,

that poor particle outside the black hole

has lost his partner.

It also violates the rules of quantum mechanics.

If you have two particles that are entangled,

that has to be preserved.

Now, to be clear,

nobody disputes that black holes will quantum radiate,

that Hawking radiation is a solid prediction.

The black holes should in fact evaporate,

that's not disputed, right?

That's right.

It would be wonderful if we could have

some experimental of evidence for this,

if we could really build a black hole in the lab

and test to see whether it behaves this way.

But I think there is hope

that we'll be able to detect some of these effects

either indirectly,

by looking at black holes out in the universe,

or also maybe indirectly in the laboratory

by looking at systems which aren't black holes,

but which radiate in kind of similar ways.

There's this domain of black holes in astrophysics

where we see stars collapse,

and we know that they exist

and there's whole observational astronomy around them.

And then there's this domain that we're talking about,

where, as you said, black holes are so special

because they're kind of guiding us in the right direction

to understand the very nature of reality.

And that makes them really unusually special.

And one of the things I wanted to draw out is that

we talk about the fundamental forces of nature.

So there's the matter forces,

and then the outlier is gravity.

We've quantized all the matter forces

in a way that we're rather comfortable with.

Gravity keeps resisting quantization of gravity itself.

And now we're thinking in a way that you're describing

that, well, maybe it's only the quantum forces altogether.

The pursuit of quantum gravity

has taken us to places we never expected to be.

I think what's exciting about physics,

about theoretical physics,

that you start following a thread,

you start developing a chain of logic,

and you never know where it's gonna end up.

Do you think there's ever a hope that

the kind of information that you think about,

the quantum gravity aspects of the universe

that you think about,

whether it will pan out or not,

will ever be viably observed

in these astronomical pursuits of the event horizon?

It's a real challenge, but astronomical observations

have gotten so fantastically precise.

And there is some hope that if you looked at things

like two black holes merging,

each black hole comes in with its own event horizon,

but then when the black holes merge,

there's a very complicated process

where these two event horizons merge

and oscillate and vibrate,

and then settle down into a single event horizon

for the final black hole.

There is some hope that if we can make

sufficiently detailed observations of this process,

if we could really see

the way the event horizon is behaving

as it settles down to this final state,

that maybe that could reveal

some of these quantum effects that we've been talking about.

It is amazing in the numerical simulations

of two black holes merging,

you really see the event horizons bobble around.

And we were talking earlier about how

really black holes are flawless,

they don't tolerate those kinds of imperfections.

And so you can so quickly watch the system wring away

that misshapen merger.

And it comes out in the gravitational waves,

which is literally the ripples in the shape of spacetime

until it settles down,

and then it's utterly flawless again.

It really happens fast.

It's quite amazing.

Yeah, it's a spectacular process.

In some sense, black holes aren't anything anymore.

They're just empty, curved spacetimes and nothing is there.

How would you possibly make one?

And then it becomes,

why are there so many

and where are they all?

Being a black hole scientist means

each question leads to more questions.

We know more and more,

but we also see how much more there is to understand.

[mellow music]

I hope you've learned something about black holes.

Thank you so much for watching.