Original subtitles

Cosmologists are battling over the universe's greatest enigma.

Black holes.

We've never seen them.

It's near impossible to study them.

And their existence challenges everything

we think we know about space.

The black hole represents the absolute limits

of what we understand about nature.

The truth is, we have almost no idea what these things are

and how they work.

Black holes are at the very heart of cosmology.

Yet, some scientists question if they're even real.

Black holes present lots of paradoxes,

and the simplest way to resolve the paradoxes would be

if black holes didn't exist at all.

Solving the mysteries of black holes

pushes our understanding of physics

to the edge of reason.

But it's the only way to discover

if black holes really exist.

captions paid for by discovery communications

Black holes are the monsters of the universe...

Terrifying cosmic beasts that devour all they encounter.

But black holes scare scientists for very different reasons.

They challenge our theories to the breaking point.

This is at the forefront of theoretical physics.

When it comes to the detailed nature of black holes,

it would not surprise me if we got it all wrong.

The science of black holes is so challenging

that some scientists question whether they exist at all.

Despite their fearsome reputation,

we've never actually seen one.

Black holes are everywhere. They're all over the universe.

They're all throughout our galaxy.

But that doesn't mean that they're easy to find.

They're black, and space is black,

and black-on-black is kind of hard

to see in a picture of space.

This is paradoxical

because scientists believe black holes are born

in some of the brightest explosions in the universe...

rising from the corpses of detonated stars

many times larger than our sun.

A star that burns for 10 million years

collapses to form a black hole in a period of seconds.

As it collapses,

the outer region of the star hits the core,

triggering a huge explosion... A supernova.

We see the bang, but not what's left behind...

a dead core with the enormous mass of the star

crushed down into an infinitesimal, tiny area.

From this minuscule high-mass core,

a black hole is born.

The flow of gravity is so strong that nothing can escape...

Not even light.

But how can scientists claim that black holes exist

if we can't even see them?

You could say that about the existence of the atom.

We knew they had existed for decades, centuries

before we had actually seen one in some sort of imaging device.

And so it's the same sort of thing with black holes.

Just because you can't see it doesn't mean it's not there.

Not seeing black holes but knowing they're there

is a possibility,

just like we know that wind is there,

even though we can't see air.

Air is invisible, but when the wind blows,

its effects can be measured.

It's the same with black holes.

You just need to know what to look for.

While they emit no light themselves,

black holes are tremendous sources of x-rays,

and that's because as things get close to a black hole,

they're accelerated by the gravity,

and they can heat up to millions of degrees.

Million-degree gas gives you lots of x-rays.

To find and measure these telltale x-rays,

scientists turn to the NuSTAR space telescope.

In 2017, it spots a burst of x-rays

in a cluster called 47 Tucanae,

at the edge of the milky way.

When scientists analyze the data,

they realize they're looking at two objects

orbiting each other very closely.

All we see is that there's a star being ripped apart,

and gas is spiraling down to a very dense, very dark object,

so something weird is going on.

As one of the objects accretes matter off the other,

it causes it to emit x-rays,

and those x-rays can be used, then, to trace out the orbits

and, therefore, extract the mass.

When scientists work out

the size and mass of the two objects,

they find the first is the fading corpse

of a sunlike star.

And while the second object is tiny,

it has the mass of a giant.

Is this an elusive black hole?

What we're talking about here is an object

that is very massive, very small, very dense,

with intense gravity.

But it turns out there are lots of different ways

to create an object like that.

There is another type of ultra-dense object out there

in the universe, called a neutron star.

Neutron stars form in the same way we think black holes form...

When stars die, explode,

but then collapse down into a tiny ball of matter.

The gravitational attraction of a neutron star is enormous,

pulling in gas, dust, and asteroids.

But light can still escape.

Black holes and neutron stars are kind of cousins,

but in the case of a neutron star,

it didn't have quite enough mass to collapse out of control.

So you can sort of think of it as just barely hanging back

from collapsing into a black hole.

The tiny object discovered by NuSTAR

does have enormous mass.

But size and mass alone

are not enough to prove it's a black hole.

Cosmologists need more evidence.

They can't see black holes, but is there another way?

What if they could hear them?

Think of two massive cars colliding... boom!

When they do, they radiate sound,

and then we can tell whether or not that collision occurred

and maybe even how far away it was.

It's like that when black holes collide.

So, by listening for a black hole crash...

could scientists conclusively prove they exist?

Black holes are gravitational giants of the universe.

But we've found only circumstantial evidence

that they exist.

To make it a slam dunk,

cosmologists are listening for proof

in the hidden world of gravitational waves.

There are gravitational waves going through this room

all the time.

Every time I move my hands like I just did,

I create gravitational waves.

The problem is, gravity is so weak

that you don't detect those gravitational waves.

In order to detect those disturbances of space and time,

you have to have cataclysmic events

involving massive objects.

Black holes are some of the densest objects in the universe.

So we should be able to hear and measure the waves

created when they collide.

LIGO, the laser interferometer gravitational-wave observatory,

listens for waves that can come

from over a billion light-years away.

In 2017, LIGO heard an enormous crash.

Two very massive objects

collided at near the speed of light

in one of the most energetic events

that we've ever witnessed in the history of humankind.

Two ultra-heavy, ultra-dense objects whirl around each other,

hurling powerful gravitational waves through space.

The closer they fall toward each other,

the more gravitational energy they throw out.

Finally, they collide,

in one of the most violent events in the universe.

The smash sends out immense gravitational waves

that ripple across intergalactic space,

until, eventually, LIGO detects them.

Listening to a gravitational wave

is like listening to a musical instrument.

If it's making certain tones or certain vibrations,

you can figure out the size of the musical instrument,

the type of the musical instrument,

who's playing the musical instrument.

The thing that's really amazing about the LIGO detection

is it allowed us to measure the mass of these objects

and how quickly they coalesce together.

So we actually have an idea how dense they must have been,

and with modern physics, we say,

"well, it has to be a black hole."

But the question is, "have we missed something?"

the information gathered by LIGO is groundbreaking.

But some scientists think that the gravitational waves

could have come not from black holes,

but from something even more mysterious.

It's possible that what we identify as black holes

in our universe are really another object like gravastar.

Possible... there's a capital "P" on that "possible."

A gravastar is what scientists call an exotic compact object.

This bizarre theoretical body has exactly the same mass

and gravitational pull as a black hole,

but it's made of exotic matter.

A gravastar would be impossible to see with the naked eye,

but because it forms differently than a black hole,

it has a strange, incredibly dense surface.

In the formation of what we think of as a black hole,

the catastrophic gravitational collapse of a dense object,

maybe it doesn't go all the way down

to become an infinitely dense point.

Instead, maybe there's some interaction

that prevents the formation of the black hole,

and instead, you have a tight, little, dense ball,

which is what we call a gravastar.

So the LIGO data could be the signature

of two black holes colliding, but it also could be

the signature of two gravastars colliding.

Right now, we can't tell them apart.

So, for now, gravitational waves have led to a dead end

in the hunt for black holes.

We can't be completely sure that we're hearing them,

and we already know we can't see them.

But what about the mayhem they leave behind?

Even though you can't see the black hole itself,

it's going to leave behind a trail of destruction,

and that is something you can see.

By picking apart a cosmic crime scene,

can scientists finally solve the mystery of black holes?

This is the hydra "a" galaxy cluster.

840 million light-years from Earth,

it's a region of space filled with galaxies

and dense intergalactic gas.

But a dark and massively destructive force

is at work here.

And it's blasting holes in the gas

that are bigger than the milky way.

Could a black hole be responsible?

It's almost as if an intergalactic bomb has exploded

to blow these cavities out.

Some of these cavities are tens

or even hundreds of thousands of light-years across,

and to create a cavity that large

requires an immense source of energy,

a powerful engine that's driving it.

To discover what's creating the cavities,

scientists combine images taken at different wavelengths.

They reveal something remarkable.

The cavities are being carved out by enormous jets

emanating from a galaxy in the center of the cluster.

As the jets are ejected from the galaxy,

they can actually slam into the surrounding material

and form a shock wave, which we can see.

And eventually, that jet inflates a bubble, a cavity,

and the cavity grows as it's inflated

with hot, dense plasma ejected from this jet.

These jets are incredibly large and incredibly powerful.

This is like a death star, but for real.

It's much, much more energy

than it would take to blow up a planet.

Carnage like this could have been created

by the energetics of a supermassive black hole

in the form of a quasar.

Supermassive black holes

are millions or even billions of times the mass of the sun.

They can be voracious overeaters,

cramming in huge amounts of matter.

As gas and dust swirl toward the supermassive black hole,

it rubs together, causing friction

that heats the material up to millions of degrees Fahrenheit.

The magnetic field around the black hole

forms the material into twin jets

that spin out at enormous speeds

for hundreds of millions of light-years.

A quasar is born.

The jets emitted by black holes are not only

incredibly high-energy, but incredibly intense.

More energy than is emitted in almost any other object

is observed from quasars.

Those quasars are intense enough

to vaporize objects that they hit.

They're deadly rays from space.

They almost sound like science-fiction objects.

In order to generate that much energy,

what kind of physical process do you need?

And pretty much the only answer we have is a giant black hole.

Jets are absolutely

really, really convincing smoking-gun evidence

for the existence of black holes.

The awesome power of a black hole

can explain the vast mysterious cavities

in the galaxy cluster hydra "a."

So, is the case closed?

Can we say conclusively that black holes are real?

Although there's great evidence for black holes,

we have to keep questioning whether they are real.

As a scientist, I'd much rather have questions I can't answer

than answers I can't question.

One question scientists are struggling to answer

is how black holes work.

Are cosmology's greatest minds in danger of flunking out?

There's always going to be details

that we still have yet to figure out.

That's true for black holes right now

and a lot of the problems are big ones,

like how do they even exist, how do they behave?

I mean, we're seeing that the math doesn't seem to work out,

so there is an issue here.

To show that black holes exist,

scientists need to solve the math.

If they can't,

they might fail to prove that black holes exist at all.

Cosmologists say the universe is filled with black holes...

Big and small.

But the evidence for them is not conclusive.

Even if there's 9 out of 10 pieces of evidence

for black holes, there's still one piece of evidence left.

That can open it up to the possibility

that maybe black holes don't exist.

Experts instead look to the theoretical science

of how black holes work.

They're thought to have the superdense collapsed core

of a star at their center.

Around this is a sphere known as the event horizon,

a place where the rules of physics go out the window.

The event horizon, in many ways,

cuts a black hole off from the rest of the universe.

Whatever comes in can never come back out.

It's almost like an invisible line in space.

It's not until you try to turn around and leave

that you realize you're never going to escape.

It's Einstein's theory of general relativity

that tells us nothing can leave a black hole.

This set of rules

governs the giant structures of the universe...

Galaxies, star systems, and planets.

General relativity is Einstein's theory of gravity.

It is the all-encompassing theory

that describes everything we know about gravity

and how the universe on large scales functions.

But the universe functions on small scales, too.

Everything in the universe... Including us...

Is made up of tiny bits of matter.

These are governed by another set of rules,

known as quantum mechanics.

Quantum mechanics allows you to understand

at the smallest levels

the smallest scales of what builds our universe.

Weird stuff happens in the quantum world.

It defies all intuition,

and one of the weird things that can happen is you can have

the empty space of a vacuum creating particles.

In the quantum world,

tiny particles can pop spontaneously into existence.

They're drawn to each other like magnets.

But when they collide, they annihilate each other.

So, you've got two particles... They can pop out of the vacuum,

and they annihilate very quickly.

That doesn't break any laws.

Now in the vicinity of a black hole, though,

things get kind of complicated.

Gravity is usually too weak

to affect the particles in the quantum world.

But British physicist Stephen Hawking theorized

that at the event horizon of a black hole,

the normal rules don't apply.

What Stephen Hawking realized is that

if you have a pair of particles

that pop up at the edge of a black hole

and one gets sucked into the black hole,

then the other is forced to become

a real particle in our universe.

In order to do so, it takes energy from the black hole,

and in that mechanism,

black holes very slowly over time lose mass.

With Hawking's work, we see

that the black hole will eventually evaporate,

and that was a complete shift in how to think about black holes.

According to Hawking,

when black holes aren't eating material,

they actually shrink by emitting thermal radiation or heat.

In this process,

a black hole will eventually evaporate completely,

and this creates a problem for scientists.

Everything in the universe,

from atoms to planets to spacecrafts,

carry information about what they're made of,

how fast they're going, and where they've been.

The laws of physics state

that this information can't be lost from the universe,

and that should apply to black holes.

So if an object passes through

the event horizon of a black hole,

all the information about that object

becomes part of the black hole itself.

Once they've fallen in, all you can see is a heavier black hole,

and you don't know what fell in if you weren't watching it.

Well, that doesn't violate the laws of physics,

but if the black hole continues to evaporate,

evaporate, evaporate with thermal radiation

and then disappear,

then all you have afterwards is thermal radiation.

You have no information, even in principle,

about what fell in.

And that information is precisely

what quantum mechanics says can't be lost.

The thermal radiation coming out of the black hole

contains no information.

It's a blank slate.

If this continues until the black hole evaporates,

then all the information is completely lost

when the black hole disappears.

If that information disappears,

then the laws of quantum mechanics are violated.

And we don't know how to solve that paradox yet.

We're trying to combine quantum mechanics and relativity,

and the first time we're able to do it

leads to this giant mess that calls into question

fundamental assumptions about the way the universe works,

so... that's kind of a problem.

The problem is known as the information paradox,

and if scientists are going to find an answer to it,

they'll have to unify their two theories...

General relativity, the rules of the large,

and quantum mechanics, the rules of the tiny.

And so far, that's proved impossible.

I think the single biggest embarrassment in physics

is that we have these two theories...

General relativity describing the big

and quantum mechanics describing the small...

That simply don't get along.

Nature obviously knows what it's doing,

but we just don't have a single, unified explanation.

The past few decades,

as we've tried to describe black holes fully,

the more work we've done,

the more of a tangled mess we get.

It will take at least one brilliant mind

to figure this out.

Maybe someone already has.

The classical black hole has a shell-like event horizon

beyond which nothing can escape.

And according to the information paradox,

information is lost when a black hole evaporates.

But here's a radical, mind-bending idea...

What if black holes have hair?

What Hawking and his collaborators pointed out

is that black holes can have soft hair,

and if that's true, that would be a way

of encapsulating all the information

that went into the black hole.

Scientists propose that these so-called hairs

somehow store the information

of whatever has fallen into the black hole.

The information is then imprinted

on the thermal radiation emitted as the black hole evaporates.

These hairy black holes could solve the information paradox,

but they're entirely theoretical.

These are really new ideas,

and people are still trying to figure them out.

So it's hard to give a really good explanation

of something that is...

This is theory in the making.

People are working this out now.

Solving the information paradox

is pushing science to its limits.

But an even bigger problem

is sitting at the heart of a black hole.

The big problem about the idea of black holes

is that at its center, there's what we call a singularity.

At this singularity, matter has infinite density

and space is infinitely curved.

That's not something that really sounds like physics.

Singularities do not exist in nature,

and when they appear in the mathematics,

that's a signal that you're doing something wrong,

that you're incomplete.

It's like the ultimate curse word,

and mother nature doesn't like it when we curse.

To complete the math and understand black holes,

cosmologists grapple

with one of science's most mind-bending concepts...

Infinity.

Black holes... They're thought to be

a fundamental part of the universe.

But scientists are not only struggling

to explain how they work,

they're struggling to prove they even exist.

So, black holes have some problems.

Every time we try to think of something,

some creative mathematical solution

to describe black holes fully,

it breaks down, and we just can't make any progress.

We can't make reliable predictions,

we can't compare to observations.

What is the solution?

We honestly don't know.

Scientists are finding

that general relativity and quantum mechanics

go haywire at the edge of a black hole.

But inside a black hole, things could be even weirder.

When a giant dying star collapses,

the mass of the star falls in and keeps falling in,

crunching down into an infinitely small point.

This is called the singularity.

A singularity is troubling

because although it sounds cool and scientific,

it's really just a fancy word of saying,

"oh, we have no clue what happens here."

the way our physics describes black holes when they form,

you're taking a finite amount of mass

and you're collapsing it down.

And its volume should shrink all the way to zero,

but that means it has infinite density and infinite gravity,

and that doesn't make sense.

If you make a prediction and the answer is infinite,

it tells you there's something wrong with your prediction,

'cause we've never seen an infinity in the universe today.

Once again, quantum mechanics is at the heart of the problem.

Have you ever thought about the term "quantum mechanics"

and thought about what those words mean?

Well, everything in the universe

is broken up into tiny, little units.

There really is a basic unit of energy, a unit of time,

even a unit of space that cannot be divided any further.

There's a limit to how small things can be.

The smallest unit of space in the universe

is what's known as a Planck length.

If you took a human hair

and blew it up to the width of the observable universe,

one Planck length would be about 1/4000th of an inch.

If there is a universal limit on the smallest size,

then something infinitely small can't exist.

Well, if infinity doesn't exist, then singularities don't exist,

and if singularities don't exist,

then Einstein's theory of general relativity

is not correct.

The simplest thing to do is to say,

"well, let's choose some new equations.

Let's change Einstein's theory of gravity somehow.

Let's invent what we would call exotic speculative physics."

This speculative physics has led scientists to invent

the idea of the Planck star.

If you passed one in space,

it would look just like a black hole.

But a Planck star doesn't have a singularity at its core.

Maybe things can't collapse down

to less than the Planck's length,

that maybe you get stuck with this little Planck-size nugget

that stabilizes things, keeps everything finite.

So, where a singularity is at the center of a black hole,

a Planck-sized nugget is at the center of a Planck star.

A Planck star is just like a black hole,

except it plays by the rules of quantum mechanics.

The problem is, Planck stars are just another exotic theory.

The reason there are so many exotic alternatives

to black holes is because you can write down a gazillion

different postulated, mysterious new kinds of matter

and say, "suppose this kind of weird stuff exists.

Then maybe that could explain the data."

Problem is, there's no evidence

that any of that kind of stuff exists.

Scientists are getting creative

as they try to prove that black holes exist.

They've imagined strange objects and used exotic workarounds.

But one black hole idea is the strangest of all.

What if we are living inside of one?

It would be the most mind-blowing thing ever

if we were actually living inside a black hole.

And the crazy thing is, it could happen.

It could be true.

Black holes are full of theoretical holes.

Scientists say they're out there, but we can't see them.

Math says there's a singularity at a black hole's core,

but in nature, these don't exist.

The rules we use to understand the universe

simply don't seem to apply to black holes.

So where does that leave us?

Maybe a classical black hole

with an event horizon described by general relativity

just isn't the proper description of the physics.

This lack of understanding

opens the door to some outlandish theories

that aim to show black holes can exist.

But one idea stands out as the strangest of them all.

In our understanding of the universe,

there are two places where everything seems to break down.

One is inside the heart of a black hole.

The other is what happened right before the big bang.

And some people have wondered

if these two things could be linked.

It sounds crazy, but there's actually a model

that you could put together

in which all of our observations of the universe

are entirely consistent

with us actually being inside of a black hole.

How would that work?

Well, a black hole is supposed to have

a tiny, dense region at its core

containing trillions and trillions of tons of matter.

There is a theory that as matter is crushed

into the center of a black hole, it actually reaches a point

where it can be crushed no further.

An event like this could, in fact, lead to a big bang.

When the collapsing matter in the black hole

reaches a maximum density...

it bounces back,

expanding outwards in a cataclysmic explosion.

The matter gradually cools over time to form atoms,

building galaxies, stars, and planets.

If that sounds familiar,

it's because it's just like the universe we see today.

This is one idea for how a universe like ours is formed...

That, in fact, we all live inside of a black hole

that was created this way.

Within our galaxy alone,

there are tens of millions of black holes.

And just think... Inside each one of them

could be a baby universe waiting to be born.

That's incredible.

Are we inside of a black hole that exists in a universe

that has other black holes that has other universes

which exists inside of a black hole?

It goes on and on and on ad infinitum.

Once again, this is all theoretical,

but it's a compelling idea.

I mean, are we actually existing inside of a black hole?

I don't know, but, you know,

sometimes when I'm sitting in traffic waiting to get home,

it feels like time is stretched out infinitely.

I don't think that you should lose sleep at night

wondering if we're actually inside of a black hole.

The answer is "probably not."

But because black holes are just pushing at the edge

of what we understand about nature,

they are the perfect illustration

of everything that we don't know about the universe,

and that is a lot.

So, what do we know?

We can't see black holes.

We can only find circumstantial evidence of them.

They violate the laws of physics that predict them.

They may even be hairy.

So, do they actually exist?

So, I will tell you that right now in modern physics,

we have no idea what is going on

inside the heart of a black hole,

whether black holes in the true sense really exist at all,

but the wonderful thing is that physics is now taking us

down paths that we would never have imagined before.

Lack of evidence of how black holes work

is not evidence against the existence of black holes.

It's just evidence of lack of understanding.

If you ask me what I believe,

I'd have to tell you that I don't believe in black holes.

I believe in something which behaves like black holes.

It could be that all the objects in our universe

that we currently identify as black holes

aren't really black holes,

but if I were to bet, I would bet on black holes.

I think they're the simplest explanation.

Yep, they have a lot of problems that we have to resolve,

but I do... I believe in black holes.

Whatever it is that we're seeing,

it smells like a black hole,

walks like a black hole, it quacks like a black hole.

It's a black hole.

Right now, black holes are the best explanation

for what we see out there.

But if we can find a way to unify our theories,

we might finally prove they exist

and discover a whole lot more about how our universe works.

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