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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.

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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.

I have spent hours for You, please spend 5 seconds for me
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