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Of all the objects in the cosmos...

Planets...

Stars...

Galaxies...

None are as strange,

mysterious,

or powerful...

as black holes.

Black holes are the most
mind-blowing things in the universe.

They could swallow a star
completely intact.

Black holes have these powerful jets
that just spew matter out.

First discovered on paper...

On the back of an envelope,

some squiggles of the pen.

...the bizarre solution
to a seemingly unsolvable equation...

A mathematical enigma...

...Einstein himself could not accept
black holes as real.

People didn't even believe
for many years that they existed.

Nature doesn't work that way.

Yet slowly, as scientists
investigate black holes

by observing the effect they have

on their surroundings,

evidence begins to mount...

That is the proof of a black hole.

Millions of times the mass of the sun.

Cutting-edge discoveries show...

We did it!

...black holes are very real.

I thought it was crazy.

I said, "Holy."

But what exactly are they?

If we could visit one, what might we see?

With their immense power,
do black holes somehow shape

the very structure of the universe?

Is it possible we might
not exist without them?

It's quite a journey.

"Black Hole Apocalypse."

Right now, on NOVA.

There are apocalyptic objects
in the universe:

engines of destruction,

menacing and mysterious.

Black holes.

Even scientists who study them

find them astonishing.

Black holes can sort of blow your mind.

I'm amazed that
these objects actually exist.

Black holes defy
our understanding of nature.

Black holes are
the greatest mystery in the universe.

They're completely invisible,

yet powerful beyond imagining.

They can tear a star to shreds.

Black holes actually will eat anything
that comes in their path.

You really want to avoid them at all cost.

Black holes even slow time.

Once thought too strange to be real...

black holes shatter
our very understanding of physics.

But we're learning they may
somehow be necessary

for the universe we know to exist.

They might well be the key
players in the universe.

What are these strange,
powerful objects...

outrageous and surprising?

Where are they, and how do
they control the universe?

The search for black holes is on.

And it will be a wild ride
across the cosmos

to places where everything
you think you know is challenged...

where space and time, even reality,
are stranger than fiction.

And we're starting that journey
at a very unlikely place:

here, at a remote location
in Washington state,

where, for the first time,

a radical new experiment
has detected black holes.

It originated over 50 years ago,

when a few visionary scientists

imagine a technology
that hasn't yet been invented...

Searching for something
no one is certain can be found.

The experiment is daring and risky.

Failure could mark their lives forever.

But they don't fail.

Right here, in these facilities,

they make a remarkable discovery.

In the early hours of September 14, 2015,

they record a message.

It looks and sounds like this.

Just a little chirp.

But that chirp is epic, monumental.

The signal traveled over
a billion light years to reach us.

It started far, far away.

And what it tells us is this:

somewhere in the cosmos,
over a billion years ago,

two massive black holes circle
each other in a fatal encounter.

Closer and closer they come,

swirling faster and faster,

until finally, they slam together.

The black holes create waves
that spread outward.

Just like vibrations on a drum...

a ringing in the fabric of space itself.

The collision creates a massive blast,

putting out 50 times as much power
as the entire visible universe.

It sends out a wave not of heat,
or light, or sound,

but of gravity.

This gravity wave is moving
its way through the universe

at the speed of light.

The wave races by stars.

On the young Earth,
supercontinents are forming.

Microscopic organisms have just appeared.

Washing over one galaxy
after another, after another.

Dinosaurs roam the Earth.

The wave is still moving.

It zooms through clouds of dust.

And then it nears the Milky Way Galaxy.

The Ice Age is just beginning.

We're troglodytes, drawing in caves.

The wave reaches nearby stars.

Albert Einstein is in the sixth grade.

The wave approaches as close
as Alpha Centauri.

At midnight on September 13, 2015,

it is as close as Saturn.

Finally, over a billion years
after the black holes collide,

the wave reaches us.

It strikes a pair
of revolutionary new observatories,

the sites of the daring experiment.

This is LIGO,

the Laser Interferometer
Gravitational-Wave Observatory.

The experiment 50 years in the making
has finally hit the jackpot

and opened an entirely new way
of exploring the universe.

For 400 years, almost everything
we've observed in space

has come to us in some form
of electromagnetic energy.

That little chirp is different.

What hits the Earth in September 2015

is a gravitational wave...

a squeezing and stretching
of the very fabric of space.

It produced no light,

no telescope could ever see the collision.

We needed an entirely
new kind of observatory to detect it.

That wave is new and direct evidence

of one of the strangest mysteries
in our universe:

black holes.

Most of us have heard of black holes.

They're invisible, powerful.

We are talking about things that are
a billion times the mass of the sun.

Bizarre.

A physical entity

with infinite density.

No beginning, no end.

They pull things in.

And warp light.

Approach one,
and time itself begins to change.

The gravity
is so intense that a moving clock

will tick slower.

Time will become so slow for you...

that you will watch the entire
future of the universe

unfold before your very eyes.

Fall in, and you'd be
squeezed as thin as a noodle.

You'll be extruded through
the fabric of space and time

like toothpaste through a tube.

Today, we know more about
black holes than ever before.

But the more we learn,
the more mysterious they become.

They're the most exotic
objects in the universe.

We don't have the physics
to describe them.

No matter how well you understand them,

they remain unreachable in some sense.

Now man is about to enter
the black hole!

So black holes have
a pretty fierce reputation.

And if you want a villain for
a sci-fi movie, cast a black hole.

But in reality,
what exactly is a black hole?

And where do they come from?

You might think
a black hole is like this...

an object.

But it's not.

It's a hole in the fabric of space.

A place where there is nothing,
nothing except gravity,

gravity at its most intense
and overwhelming.

So if black holes are all about gravity,

gravity at its most extreme,

what exactly is gravity?

We're all familiar with gravity.

Yep, it's Friday.

It rules our lives.

But even so, for a very long time,

how gravity actually works

was one of the greatest mysteries.

Over 300 years ago,

Isaac Newton was fascinated
with the behavior of moving objects.

Eventually he figured out
his laws of motion.

They work so well,
we still use them today.

Lift-off, we have lift-off at 9:34 a.m.

But Newton's laws
can only describe gravity's effects,

not explain what it is.

And here's where Albert Einstein comes in.

Like Newton,
he thinks about objects in motion.

And he wonders what gravity actually is.

Is it a force?

Or could it be something else?

Here's what concerns Einstein.

Take this apple.

I can't move it without touching it.

But if I drop the apple...

it moves toward the Earth.

But what if I take my hand away,

and the floor, and the basement,
and the floor below that?

Then what happens?

The apple just keeps falling.

Einstein realized that gravity

had something to do with falling.

Now, if I throw the apple...

it falls along a curved path.

But imagine I could get
the apple moving much faster.

Eventually, if I get the apple
moving really, really fast,

say, 17,000 miles an hour...

its curved path matches
the curve of the Earth.

The apple is in orbit, falling freely,

just like the International Space Station

and the astronauts inside it.

According to Einstein,

the apple and the space station,
and the astronauts...

are all falling freely
along a curved path in space.

And what makes that path curved?

The mass of the Earth.

Einstein came up with
a supremely simple concept,

and that is that space and time
is bent by the Earth,

and by the sun, and by
all the objects in the world.

So according to Einstein,

the mass of every object...

causes the space around it to curve.

And that was Einstein's conception.

There are no forces anymore.

There's just objects bending space-time

and other objects following
the straightest line through it.

All objects in motion
follow the curves in space.

So how does the Earth
move the apple without touching it?

The Earth curves space,

and the apple falls freely
along those curves.

That, according to Einstein's
general theory of relativity,

is gravity: curved space.

And that understanding of gravity,

that an object causes
the space around it to curve

leads directly to black holes.

But it's not Albert Einstein
who first makes the connection

between gravity and black holes.

It's another scientist.

Karl Schwarzschild

was a German astronomer,

head of the Potsdam Observatory
in Germany.

Ever since he was a teenager,
he had been calculating

complicated features of planetary orbits.

As Einstein unveils
his theory of gravity in 1915,

Karl Schwarzschild is in the German army,

calculating artillery
trajectories in World War I.

And just weeks after
Einstein presented his papers,

Schwarzschild,
then on the Russian front...

quickly got a copy

and was mapping
the gravitational field around a star.

Einstein had gotten at it
through a series of approximations.

But Schwarzschild...

...sitting on the front
with bullets and bombs flying,

calculated an exact solution
to Einstein's theory

and sent it to Einstein.

Einstein was astonished.

He hadn't even imagined that
you could solve these equations exactly.

But Schwarzschild isn't done.

In his solution to Einstein's equations,

he discovers something
Einstein himself had not anticipated.

Schwarzschild said,
"I can calculate this strange distance

from a gravitating object

that represents a kind of boundary."

Schwarzschild mathematically,
concentrates a mass,

for example, a star,

into a single point.

Then he calculates how that mass
would bend space

and curve rays of light passing nearby.

As he, through his mathematics,

aimed particles of light
or matter towards this point,

there was this boundary
surrounding the point

at which the particles would just stop.

The particles disappeared.

Time stopped.

Schwarzschild has discovered

that a concentration of mass
will warp space

to such an extreme
that it creates a region of no return.

Anything that enters that region
will be trapped,

unable to escape... even light.

It's like those roach motels.

You can check in, but you can't check out.

Once you go across that boundary,

even if you can sail through,

there's nothing you can do to get out,

there's nothing you can do to signal out.

It becomes this strange,
cut-off portion of space-time.

What Karl Schwarzschild has discovered

is that any mass,
compressed into a small enough space,

creates what we today call a black hole.

But Albert Einstein

whose own theory of gravity
predicts such a thing

cannot believe it can happen
in the real world.

Einstein didn't think that

nature would act like this.

He didn't like this idea.

Karl Schwarzschild becomes ill

and dies before he has a chance
to further investigate his own discovery.

Two-and-a-half years later,
in November 1918,

World War I ends.

The strange theoretical sphere
discovered by Karl Schwarzschild

seems destined to be forgotten,

nothing but a curious historical footnote.

But in the coming decades,

physicists learn more about the atom...

and about how fusing atoms powers stars,

a process called nuclear fusion.

Some begin to wonder
if something like a black hole

could actually come from a star.

But not just any star,
it would have to be big.

Stars are born in litters,

and you get a distribution
of sizes and masses...

thousands of little stars

and a few big stars, very big stars,

incredibly massive.

Stars are in
many ways similar to living creatures.

Like humans, they have life cycles.

Investigating stars' life cycles
in the 1930s, two visionaries

Subrahmanyan Chandrasekhar
and Robert Oppenheimer

discover that the most massive stars

end their lives very differently
from smaller ones.

The life cycle of a star
really depends on its mass.

The mass of a star determines
what's going to happen

after it finishes burning
its hydrogen fuel.

All stars start out
burning hydrogen, the lightest atom,

fusing hydrogen atoms into helium,

working their way up to heavier elements.

Gravity wants to crush
the entire mass of the star,

but the enormous energy
released by fusion pushes outward,

preventing the star from collapsing.

Stars are stable because
you have an outward-moving pressure

due to nuclear fusion,

and that's balancing with
the inward force of gravity.

Smaller stars can't fuse elements
heavier than helium.

But in the most massive stars,

fusion crushes heavier and heavier atoms

all the way up to iron.

Iron is such a massive element,
it has so many protons in it,

that by the time you fuse iron,

you don't get any energy back out.

Iron is a dead end for stars.

Fusing atoms larger than iron

doesn't release enough energy
to support the star.

And without enough energy from fusion

keeping the star inflated,

there's nothing to fight gravity.

And gravity wins.

And so the entire star collapses.

Very rapidly, trillions of tons
of material come crashing down,

hit the dense core, and bounce back out,

blowing off the outer layers of the star
in a massive explosion...

...a supernova.

The more mass, the more gravity.

So if the remaining core
is massive enough,

gravity becomes unstoppable.

There's no known force

to prevent the collapse
to an infinitesimally small dot.

Gravity crushes the stellar core down,

smaller and smaller and smaller,

until all its mass is compressed
in an infinitely small point...

a black hole.

The theory makes sense,

but most physicists
remain skeptical about black holes.

Einstein and Eddington,
all the sort of, you know,

pre-eminent astrophysicists
in the 1930s through 1950s,

did not believe
that they were actually real.

It remained a solution,
a mathematical enigma,

for a very long time.

So it took a long time for people
to even start looking for them.

It's not until the 1960s

that the idea of a supernova
creating a black hole is taken seriously.

Princeton physicist John Wheeler,
who had originally been a skeptic,

begins to use a name from history
for these invisible objects...

black hole.

The term "black hole"
actually originates in India.

The Black Hole was the name
of an infamous prison in Calcutta.

Still, no one has ever detected
any sign of a black hole.

Then, in 1967, graduate student
Jocelyn Bell

discovers a strange,
extremely tiny dead star

that gives off very little light...

a neutron star.

The cold remains of a stellar collapse,

the neutron star gives
astronomers more confidence

that black holes, much heavier dead stars,

might also exist.

A half-century after Karl Schwarzschild

mathematically showed that black holes
were theoretically possible...

scientists have identified
a natural process

that might create them:
the death of large stars.

So these giant supernova explosions
of extremely massive stars

make black holes.

Any star that is born with a mass

that's about ten times
the mass of the sun or higher,

will end in a black hole.

So our galaxy is replete
with little black holes,

which are the stellar corpses
of generations of stars

that have come and gone.

So what are these invisible,
stellar corpses like?

Imagine I'm exploring space

with some advanced technology
for interstellar travel,

so that we could visit a black hole...

maybe one in
our own galactic neighborhood.

This particular black hole isn't very big,

only about ten solar masses,

meaning ten times the mass of the sun.

And like all black holes,
it has an event horizon...

a distinct edge to the darkness.

That's the boundary
Karl Schwarzschild first discovered,

where gravity is so strong
that nothing can escape,

not even light.

And that's where we're going.

As we get closer, some very
strange things begin to happen.

Look at the edge of the black hole,

see how the image of distant stars
is distorted and smeared into a circle?

That's gravitational lensing.

The black hole's extreme gravity
bends the path of light passing by,

so that a single point of light,
like a star,

briefly appears as a ring
around the event horizon.

I'm now deep in
the black hole's gravity well,

and we're going to start
experiencing the effects.

The extreme gravity
actually slows down time

relative to the Earth.

From their point of view,

I appear to be slowing down.

But from my point of view,
time on Earth is speeding up.

Now, let's say I want to get even closer,

by taking a spacewalk.

The way the black hole slows down time

is about to get even more pronounced.

To keep track of the changes
I'm about to experience,

I'm turning on this strobe light.

It'll blink once a second.

From here, I can see the shadow
of the event horizon approaching

and my light blinking normally.

But watching from the ship,

the closer I move toward the black hole,

the more slowly I appear to move.

The pulses are nearly infinitely spaced...

so it looks as though I'm frozen in time.

For me, everything is completely normal.

Even when I reach the event horizon.

If you waited long enough...

maybe millions or billions of years,

the ship would finally see me disappear.

And that's the last you'd see of me.

What's inside a black hole?

That's still a mystery.

And even if I find out,
I can never go back and tell you.

But I can say this:
black holes may be dark from the outside,

but inside, they can be bright.

I can watch the light from the galaxy
that's fallen in behind me.

And that's the last thing I'll ever see.

Unfortunately, the fun is about to end.

Now that I've crossed the event horizon,

I'm falling toward the center,

where all of the mass
of the black hole is concentrated.

And I'm beginning to get stretched.

As I fall in,
the gravitational pull at my feet

is stronger than at my head,

and my body is starting
to get pulled apart.

I'll be stretched as long and thin
as a noodle... spaghettified.

And ultimately, I'll end up

completely disintegrating
into my fundamental particles,

which are then crushed
to an infinitely small point.

A singularity,

where everything we understand
about space and time breaks down.

Or maybe the black hole,

less than 40 miles across
on the outside,

is as big as a universe on the inside.

And as I pass through,

my particles will join the primordial soup
of a new beginning.

So that's what theory tells us
we might experience

if we could travel to a black hole.

But how can we know for sure?

How do you investigate something
you can't even see?

There are ways to investigate

if something is happening somewhere,

even if I can't see that thing directly.

Take Yankee Stadium...

What's happening inside there?

Is there a game going on?

I can't see the field.

I can't see any players,
or baseballs, or bats.

But I can definitely tell
if there's activity around the park.

It's pretty clear something is going on.

It might seem obvious, but whatever it is,

I can learn a lot just by observing
the happenings around the stadium.

And these do look
a lot like baseball fans.

And that's the way
we investigate black holes:

by observing the effect
they have on their surroundings.

But what sort of effects?

How might a black hole reveal itself?

Starting just before World War II,

two monumental discoveries
are about to radically change astronomy.

In 1931, Bell Labs engineer Karl Jansky

picks up mysterious radio waves
emanating from deep space.

Then the sky gets even stranger...

when scientists mount Geiger counters
on captured German rockets

and discover the cosmos
is also full of X-rays.

These discoveries give
astronomers important new tools

that will revolutionize
the hunt for black holes

and dramatically expand our vision.

What our eyes can perceive

is a very narrow part
of the electromagnetic spectrum.

If the electromagnetic spectrum
were laid out along the Brooklyn Bridge,

the portion we can see with our eyes
would be just a few feet wide.

Electromagnetic radiation

includes waves of
many different frequencies:

radio waves, microwaves,
infrared and ultraviolet light,

X-rays, and gamma rays.

Radio and X-ray astronomy open up the sky,

revealing dim or even invisible objects

blasting out powerful energy
no one knew was there.

They began to realize that this
very placid thing that we see out there,

all this very quiet thing that
looks like nothing is happening

and the only thing
that's moving is the planets,

found out that there
was madness going out there.

It was chaos out there!

X-rays come from
the high-energy end of the spectrum.

What is creating all this energy?

This much is certain:
whatever the source,

it is invisible to ordinary telescopes.

And it is hot.

X-rays come from things

which are at temperatures
of millions of degrees.

Even tens of millions.

One of the first
of these X-ray sources

to catch the attention of astronomers

is named Cygnus X-1.

Cygnus,
it was in the constellation Cygnus,

X, it was an x-ray source,
one, it was the first one you found.

In 1970, Paul Murdin
is a young English astronomer

trying to secure his next job.

I was a research fellow,

I was coming to the end
of my three-year contract,

and I thought, "What can I contribute
to finding out what these things are?"

Murdin works in a 15th-century castle

surrounded by telescopes...

the Royal Observatory.

Using the largest telescope in England,

he begins searching the area
of the constellation Cygnus, the swan.

He decides to hunt for pairs of stars.

Pairs of stars are called binaries.

They may sound exotic,
but they're not at all uncommon.

Many of the stars we see, perhaps half,

are actually binaries,

pairs of orbiting stars
locked together by gravity.

But Murdin wonders:
Is it possible there are binaries

where only one of the stars is visible?

I thought that maybe there was
a kind of a star system

in which there was a star,
one ordinary star that made light,

and then there was another star
nearby that made X-rays.

The telltale sign of a binary

is that the stars are
moving around each other.

So Murdin begins searching
for a visible star

that shows signs of motion.

Sometimes it's coming towards you,
sometimes it's coming away.

Sometimes it's coming towards you,
sometimes it's coming away.

When the star is moving toward us,
it appears more blue,

as the wavelength of its light
gets shorter.

Moving away, it appears more red,

as the wavelength of its light
gets longer.

This is known as Doppler shift.

After looking for color changes

in hundreds of stars
in the area of Cygnus,

Murdin spots a possible suspect,

a visible star whose light is shifting,
as though moving around.

It very clearly was
a binary star, a double star.

The star was moving
around and around with a period,

going around once, every 5.6 days.

But whatever it's going around
can't be seen.

There was no trace
in the spectrum of the second star.

There was one star there.
There wasn't the second star there.

Murdin has a binary pair
in which only one star is visible.

The second object emits X-rays,

has enough mass and gravity
to dramatically move a star,

but gives off no light.

Could it be the corpse of a star
massive enough to become a black hole?

The crucial issue in deciding
whether Cygnus X-1 was a black hole

was to measure the mass
of the X-ray-emitting object.

It would have to be very massive,

at least three times the mass of our sun.

If not, it's probably
just a neutron star...

a collapsed star that's dense,
but not heavy enough to be a black hole.

So the observers needed
to come up with a conclusion

that the dark object,
the X-ray-emitting object in Cygnus X-1,

was heavier,
hopefully substantially heavier,

than three solar masses.

From his observations,
Murdin is able to make an estimate

of the mass of the invisible partner.

And the answer came out to be...

six times the mass of the sun.

So there was a story, then,
that Cygnus X-1 was a black hole.

And the key to the argument was

that the mass of the star you couldn't see

was more than three solar masses.

When I'd finished writing it all out,
I sat back and thought,

"It's a black hole."

This would be the first
actual detection of a black hole.

It's a huge claim,
and Murdin will have to convince skeptics,

starting with his boss.

The Astronomer Royal, Sir Richard Woolley.

He didn't really go for black holes.

"It's all fanciful..."

It's kind of... a lot of people
in California were talking about this.

There are a lot of
funny people in California.

You know,
a lot of hippie-type people.

People like theorist Kip Thorne.

So I was nervous about it.

I was nervous about
the scale of the discovery.

And actually so were
other people all around me.

I was working with
a fellow scientist, Louise Webster.

And we were modest about
the claim that we were making

because we knew
what people would think of it.

And if you look at the paper we published,

it just mentions the word
"black hole" once, right at the end.

"We think this might be a black hole."

The Paul Murdin-Louise Webster
paper appears in September 1971.

Other astronomers agree:
It could be a black hole.

But no one knows for sure.

Three years later,

Kip Thorne and the noted
British physicist Stephen Hawking

make a now-famous wager about Cygnus X-1.

We made a bet as to whether Cygnus X-1
really was a black hole or not.

The bet is partly in jest.

Both men hope it is a black hole.

But Hawking, not wanting to jinx it,
bets against his own wishes.

Stephen claims that Cygnus X-1
is not a black hole.

And I claim it is a black hole.

And so we signed that bet
in December 1974.

And gradually, the case
that it really was a black hole

became stronger and stronger and stronger.

So in June of 1990,
Stephen broke into my office

and he thumb-printed off on this bet,

conceded the bet in my absence.

I came back from Russia
and discovered that he had conceded.

Now, by 1990, the evidence of
Cygnus X-1's mass

may be strong enough
to settle a bet between two friends.

But the original estimate wasn't
precise enough to be definitive.

In order to calculate mass,

Paul Murdin had to rely on rough estimates

of the distance to Cygnus X-1,

which varied by a factor of ten.

And the question wouldn't be
answered for another 20 years,

until astronomer Mark Reid
became intrigued by the puzzle.

Reid is an astronomer

at the Harvard-Smithsonian
Center for Astrophysics

when he sets out to conclusively prove

that Cygnus X-1 is a black hole

by measuring its precise mass.

But how can you measure the mass
of an invisible object?

Using laws developed by German astronomer
Johannes Kepler in the 1600s,

it's possible to calculate
the mass of a celestial object,

but only if you know its distance.

Distance in astronomy
is absolutely fundamental.

If you don't know distance,
you don't know what the object is.

It could be a very nearby
firefly-like thing.

It could be a very distant, huge star,
much, much bigger than the sun.

So to get the true,
precise mass of Cygnus X-1

and confirm that it is a black hole,

Reid needs to know how far away it is.

But how can he measure
the distance to a star?

The secret lies in a familiar phenomenon,
parallax.

It's what our eyes and brains
use to see in three dimensions.

You can put your finger up
at arm's length,

look at it, and close one eye.

I'm closing my left eye.

And I'm looking at my finger

relative to the wall
in the background there.

And now if I open my eye,
close my right eye,

I see my finger has appeared to move

with respect to the original position.

And that's because our eyes are separated,

and we view from different vantage points.

To use parallax to measure
distance to an object in the sky,

astronomers let the motion of the Earth

provide the two different vantage points.

Imagine Cygnus X-1 is right here.

And the Earth and the sun are over there.

Now, the Earth goes
around the sun once a year.

And in the springtime,
the Earth ends up on one side of the sun,

and we observe Cygnus X-1
along a ray path like this.

Then six months later,
the Earth goes around the sun

to the other side.

We get a different vantage point
from Cygnus X-1.

Now he has a triangle that goes
between the Earth at its two positions

and Cygnus X-1.

We know the base of the triangle,

the diameter of Earth's orbit.

And the principles of geometry tell us

that all we need to calculate the distance

is the size of the angle at the top.

And we measure this very small angle here,

at the point at Cygnus X-1.

And then from direct geometry,

we can calculate the distance
to Cygnus X-1,

and from that infer a very accurate mass.

The concept is simple.

But Cygnus X-1 is so far away

that the angle to be measured
is minuscule,

a tiny fraction of one degree.

It's smaller than the angle spanned
by Abraham Lincoln's nose

on a penny in San Francisco
viewed from New York.

Because the angle is so very tiny,

it can't be measured by any one telescope.

But Reid's team has a solution.

We take ten radio telescopes

that are spread
across the continental U.S.

and to Hawaii and to St. Croix
in the Virgin Islands.

We use these telescopes simultaneously,

and we synthesize in a computer

a telescope that has a diameter
of the size of the Earth.

That gives you incredible
angular resolution.

Using this technique,
Reid's team determines

that Cygnus X-1 is 6,000 light years away.

With the new distance we got,
the 6,000-light-year distance,

we're able to determine that the mass
is about 15 solar masses,

easily a black hole.

Forty years after it was
identified as a possibility,

Cygnus X-1 is now widely accepted

as the first confirmed black hole.

It's an understated paper,

and the fact that my name was on it
and Louise Webster's was on it,

did us a lot of good in our careers.

I think as a result of this discovery,
I got offered a permanent job.

And it was a great celebration
for the family.

So it worked out very well for me...

as well as getting
the intellectual satisfaction

of solving a problem.

So finally,
after years of speculation,

we have a real black hole.

Not only that, but a black hole
that's blasting out X-rays

and has a companion star.

If we could visit
in my imaginary spaceship,

what would we see?

The distance to Cygnus X-1

has been established
at 6,000 light years from Earth.

And its mass is 15 solar masses,
or 15 times the mass of the sun.

And Cygnus X-1 is surrounded
by an accretion disk...

a disk-shaped cloud of gas
and dust outside its event horizon,

the point of no return.

As gravity pulls matter
toward the black hole,

the cloud starts rotating,

just like water being pulled down a drain.

Within that accretion disk,

particles closest
to the black hole whip around

at half the speed of light.

It's like a giant
particle accelerator in space.

But why does it emit X-rays?

As those particles race around,
they collide,

which heats them up
to millions of degrees.

When they get that hot,
particles blast out X-rays.

And it's those X-rays that
first led astronomer Paul Murdin

to investigate this black hole
nearly five decades ago.

And there's something else
about Cygnus that's different:

It has a companion star.

This blue super-giant star

orbits the black hole once every 5.6 days.

It orbits so close to Cygnus X-1

that the black hole
strips material off the star

and pulls it into the accretion disk.

Some of that material
will cross the event horizon

and get swallowed up, but not all of it.

Some of the stuff actually comes back out

before ever entering the black hole.

Kind of like a toddler eating:
Half the pasta ends up on the floor,

half of it may be on the ceiling,
and some of it in the mouth.

One of the most striking
and enigmatic features of Cygnus X-1

is its enormous jets.

These beams of particles
and radiation stream outward

from Cygnus's north and south poles,

perpendicular to the accretion disk.

There's still a lot
we don't know about these jets,

but they are tightly focused
and extremely powerful,

blasting out at nearly the speed of light

and extending well beyond Cygnus.

When gas gets to these
high temperatures and produces the light,

there's also a little bit
of a magnetic field

that forms around them.

And we don't understand exactly how,

but these magnetic fields

help collimate these massive outflows
from black holes,

powerful hoses if you will,
that just spew matter out.

So that's Cygnus X-1,
if we could see it up close,

a growing, feeding black hole
with huge jets...

blasting particles way out
into the universe.

They're almost these breathing,

fire-eating demons, if you will.

They flicker, they have bursts,

it's a very violent fireball, very active.

What was once
a bizarre mathematical curiosity

has now become quite real.

After decades of skepticism,

scientists now accept
that burned-out corpses

of large stars can trap light inside them,

warp space and time around them,

attract matter, and accelerate it
to mind-boggling speeds.

Black holes seemed like
such a radical idea

that we shouldn't accept it.

But bit by bit,
the evidence for black holes

has gotten stronger and stronger.

And we've seen these amazing things.

At least 20 black holes
have been found in our galaxy,

X-ray binaries, like Cygnus X-1.

And there are probably millions more
of these massive stellar corpses

in our galaxy alone.

Still, a stunning surprise awaits.

Everything astronomers
think they know about black holes,

and much of what they believe
about the universe itself

will be upended
by a shocking discovery.

The revelations begin when radio
telescope surveys of the sky

detect mysterious hot spots
emitting radio energy.

They were coming from
what looked like stars.

Because these objects resemble stars,

but were discovered through radio signals,

astronomers name them
quasi-stellar radio sources...

quasars.

But are they stars or not?

The first step in investigating them

is to figure out what they're made of.

To do that, astronomers analyze
the electromagnetic energy they emit.

Every element has
a unique spectral fingerprint.

For example, carbon.

Helium.

Hydrogen.

These lines reveal
the chemical make-up of a star.

But the spectrum of a quasar
turns out to be incomprehensible.

They looked at it and it was gibberish.

It didn't look like
there were any emissions

from elements that they knew.

What are they missing?

There has to be a clue somewhere.

Finally, in 1963,
Caltech astronomer Maarten Schmidt

finds it hiding in plain sight.

Buried in the quasar's spectrum
is the fingerprint of hydrogen.

He noticed something familiar,
but it was in the wrong place.

The fingerprints of hydrogen
had been shifted way off to the red.

It was hard to spot
because the spectral lines of hydrogen

were radically shifted toward
the lower-frequency end

of the spectrum.

And that could only mean one thing.

The quasar is moving away
from us at fantastic speed.

But astronomers have never
before seen light shifted

to such an extreme.

Like a familiar sound shifting
too low to understand...

...the light from quasars
has shifted to such a degree...

that hydrogen is unrecognizable.

This extreme amount of shift

means quasars are racing away
from us at blinding speeds.

The reason?

It's the legacy of an event

that occurred almost
14 billion years ago: the Big Bang.

The beginning of our universe.

And ever since, the universe
has been expanding,

carrying with it all the objects
it contains, including quasars.

No one had ever seen anything
moving away at that high speed.

This made this object
the furthest-away thing

that had ever been seen,

which meant the thing itself
had to be so luminous,

and you had to account for that.

Two billion light years away,
putting out the energy

of a trillion suns each second.

What could possibly create that?

No one had any idea
what could be powering these things.

Where could all of this energy come from?

If you work out through calculations,

it can't be chemical energy.

They knew it couldn't be nuclear energy.

There's no way a quasar could be a star.

No amount of nuclear fusion
could produce that much star power.

The only engine that could possibly
put out that much energy is gravity.

Gravity.

In everyday life,
we can overcome gravity easily.

But when concentrated
to an extreme by a black hole,

gravity is overwhelmingly powerful.

A handful of scientists start wondering:

Could quasars perhaps
be powered by gravity engines?

What if the energy
blasting out from quasars

is coming from bright
accretion disks around black holes?

To produce that kind of energy,

that kind of brightness,

it has to involve a black hole.

But not just any black hole.

Whatever was the source
of the emission from a quasar...

had to be massive.

How massive?

Well, millions or billions
of times heavier than the sun.

Millions or billions of times
heavier than the sun.

Cygnus X-1 is only 15 times
the mass of the sun.

The black holes powering quasars

are an entirely different
category of black hole:

supermassives.

And they seem to be located
in the centers of galaxies.

But what about our own galaxy?

Could there be any supermassive
black holes closer to home?

The center, where any
supermassive would be found,

lies in the direction
of the constellation Sagittarius,

the Archer.

Now, Sagittarius
isn't just any constellation.

It's in the direction of the center
of our own Milky Way Galaxy.

But since we live inside the Milky Way,

we can't see the galaxy
the way a space traveler would.

But I can use my trusted
imaginary star machine

to show us the galaxy from the outside.

Our home is a spiral galaxy,
hundreds of billions of stars,

drawn together into a gigantic disk.

It's wide,
about 100,000 light years across.

But it's relatively thin,

only about 1,000 light years thick.

And the whole spiral slowly rotates.

Our solar system is here.

And here, 26,000 light years
from the Earth, is the center,

which we see
in the direction of Sagittarius.

In this dense center,
there are millions of stars,

and lots and lots of dust and gas.

So that's the view of our galaxy
from the outside,

thanks to my imaginary technology.

But since we live inside the Milky Way,

when we look towards the center,

we're looking through much
of our own galaxy,

which means it appears to us

as a band of stars and dust
across the sky... a milky way.

Deep inside this band of stars and dust,

could a supermassive
black hole be lurking?

The data that we're getting now...

In the 1990s, astronomers
grow determined to solve the mystery,

to peer through the murky Milky Way,

and learn what, if anything,
is at its center.

One of them is Andrea Ghez.

One in 20...

Ghez takes on
a daunting challenge.

She will try to track individual stars

orbiting the center of the galaxy.

The essence of this experiment
comes from watching

stars orbit the center of the galaxy.

So you want to find the stars

that are as close to
the center of the galaxy as possible.

Which means that I want to get access

to the largest telescope
I can possibly get my hands on.

And that means coming here.

The summit of Mauna Kea, a dormant volcano

almost 14,000 feet
above the beaches of Hawaii.

High altitude and low humidity
make this the ideal place for astronomy.

The instrument Ghez uses
is Mauna Kea's Keck Observatory,

one of the largest in the world.

But despite its size,
Keck has the same problem

as all telescopes on Earth:

atmospheric distortion.

Think about looking at a pebble

at the bottom of a river.

The river is moving very quickly

and your view of that pebble is distorted.

Like a river,

the Earth's atmosphere
is constantly changing,

bending light like a funhouse mirror.

To compensate for this,
Keck pioneers the scientific use

of a declassified military technology

called adaptive optics.

First, they shine a laser into the sky,

creating an artificial guide star.

The turbulent atmosphere
distorts the guide star,

but the computer knows
what it should look like,

and adjusts the telescope mirror
accordingly.

So if you look at yourself
in a circus funhouse mirror,

you look completely distorted.

And the goal of the adaptive optics system

is to introduce a second mirror
that's the exact opposite shape

and make you look flat again.

Buried deep inside the telescope,

the deformable mirror changes shape
up to 2,000 times a second

to reverse the atmosphere's distortion.

And it has allowed us
to take the sharpest images

ever obtained of the center of the galaxy.

The sharpness
of those images allows Ghez

to make out individual stars
near the center,

a huge advance in astronomy.

She begins recording
their positions in 1995.

And every year since then,

we've taken an image...
just take a picture.

Putting those annual snapshots together

creates a time-lapse movie
of stellar orbits.

And what those movies reveal
is astounding.

The stars are whipping around
the center of the Milky Way

at phenomenal speeds.

These things are moving
at several thousand,

up to 10,000 kilometers per second,

or ten million miles per hour.

They're, they're really hauling.

To go that fast,
the stars must be orbiting

something extremely massive.

The mass that we infer

is four million times the mass of the Sun.

What could be four million times
the mass of the Sun

yet be completely invisible?

That is the proof of a black hole.

And not just any black hole,

a supermassive, silent and sleeping,

right in the center of our own galaxy.

In fact, this is the best evidence to date

that we have for the existence
of supermassive black holes,

not only in the center of our own galaxy,

but anywhere in the universe.

A supermassive black hole

four million times the mass of the Sun,

in the very center
of our own Milky Way galaxy.

From a cosmic perspective,
it's right next door.

And it raises a profound question.

There are billions of galaxies out there.

If ours has a supermassive
black hole at its center,

and if quasars are found
at the centers of their galaxies,

what about the others?

Are there black holes
at the centers of galaxies?

If they are, how common are they?

We simply didn't know.

Could astronomers ever hope
to find what lurks

at the centers of other galaxies,

millions of light years away,
as Ghez did in our Milky Way?

Five, four...

It would take another innovation
in astronomy to make that possible.

And lift-off
of the space shuttle Discovery,

with the Hubble Space Telescope,
our window on the universe.

When the Hubble Space Telescope
starts delivering clear images

of distant galaxies,

a team of astronomers gets to work.

They become known as "the Nukers"

because their focus is galactic nuclei,

the centers of galaxies.

One of them is Tod Lauer.

Step one, we take a picture of the galaxy

with the Hubble Space Telescope.

It shows us where the stars
in the galaxy are.

It tells us its structure
in exquisite resolution.

The key to finding
supermassive black holes

is to learn how fast the stars
in the galaxy are moving.

Galaxies outside our own
are much too far away

to measure the speed of individual stars.

But by analyzing the way light
is shifted from blue to red

at different points in the galaxy,

astronomers can put together
an average speed of stars

orbiting the center.

It's accurate enough
to create a replica in a computer.

The second step,
where the real work begins,

is to try to model the observations.

And we actually do that
by building models of galaxies

in the computer.

It's known as Schwarzschild's method,

developed by Princeton
astronomer Martin Schwarzschild,

son of Karl Schwarzschild,

whose mathematics first described

the possibility of black holes.

Martin Schwarzschild's trick was,

he would actually build up
a model of the galaxy

that not only had where the mass was,

but it also had how the stars were moving.

For each galaxy they investigate,

the Nukers painstakingly build
a computer model,

and then using trial and error,

adjust the parameters
of mass and velocity,

trying to make the model
match the original observations

they got from the Hubble.

And we say, "Let's try a star here,

let's try one over here.

Let's have it go around this way.

Let's have this one go around that way."

And we do this thousands
and thousands of times

until we build up a library

of how stars can orbit in this galaxy.

Success is when observations of the model

match the observations taken
with the Hubble Space Telescope.

But that doesn't happen.

The models are missing something.

We try it again and again and again,

all with no black hole yet, and we say,

"Gee, we really can't get
the observations explained

by the model."

Only when they add
an enormous invisible mass

at the galaxy's center

does the model match
the Hubble observations.

Almost always we have to put in

a black hole at the center.

We can't match the observations

without a black hole in the model.

Of roughly three dozen galaxies
that the Nukers investigate,

virtually all of them require
a supermassive black hole.

And since then,
other observations have made us

even more certain

that supermassives and galaxies
go together.

Every galaxy we've looked for one,

we have found a supermassive
black hole in its center.

It's a stunning revelation.

Supermassives,

once an entirely unexpected
category of black holes,

may be common,

not only at the center of our galaxy,

but of all galaxies.

Take galaxy M31,

also known as the Great Andromeda Galaxy.

It's two-and-a-half million
light years away.

On a clear night,
you can see it from Earth.

But even with the Hubble Space Telescope,

we can't make out
precise details of its center.

Still, we're pretty sure

there's something extremely
massive hiding there.

What if we could take a closer look?

What if we could visit
a galaxy far, far away?

As we enter the outer part of Andromeda,

we're still too far away to see
what's lurking at the center.

But we can make out
a dense cluster of stars in the core,

and that could be a sign

that there's a giant black hole nearby.

Billions of years ago,
it would have been surrounded

by gas and stars
and other small black holes.

The black hole may have powered a quasar,

feeding mad, and blasting out
blinding radiation.

Over hundreds of millions of years,

it would have consumed

all the available gas
and the closest stars.

These days it's relatively quiet.

But it has some distinctive features

we've never seen before.

First, it's colossal.

If it were dropped in our solar system,

Mercury, Venus, Earth, and Mars

would all be trapped inside
the event horizon.

That's big, but it's nothing
compared to the sheer mass:

100 million times the mass of the Sun.

And the destruction won't end there.

Jupiter won't last long.

The gravitational field
of the supermassive

will grab hold and swallow it whole.

Eventually, Saturn will suffer
the same fate.

The outer planets might survive,
but in cold and dark orbits.

This black hole rotates rapidly,

distorting and dragging
the fabric of space-time.

Like all black holes,

the event horizon
is completely featureless.

Remember, there's nothing there.

It's just a boundary
that conceals the interior.

But the accretion disk can tell us a lot
about what's going on.

That's the fiery ring of gas
and dust around the black hole.

Imagine if we could release
a swarm of autonomous robots

to explore the accretion disk.

The disk is spinning
at an incredible speed,

as much as half the speed of light.

If Jupiter moved that fast,

it would complete
its entire orbit in a few hours.

The region around the black hole
is a cosmic tornado.

Now our swarm is caught
in the whirlwind, too.

They're like tracers
dropped into the storm

to map the movement.

The middle robot can send us images.

It's following the leader like a race car

speeding around the track.

From here, the extreme warping
of space-time around the black hole

plays crazy tricks on our eyes.

It looks like there's one accretion disk

whipping around the equator,

and another arcing
over and under the poles.

But that's an illusion.

The black hole's extreme gravity
bends the path of light

emitted behind the black hole,

and makes it look like the accretion disk

is both above and below.

There's actually nothing around the poles.

It's just the passing light rays.

That's gravitational lensing again.

Drawing much closer to the event horizon,

the gravitational lensing
would become so extreme

that one of my robots could look
straight ahead

and eventually see its own back,

the light forever trapped
in an eternal circle.

So that's our tour
of the supermassive black hole

at the center of the Andromeda Galaxy.

Pretty amazing.

Also amazing: nothing in the mathematics

led scientists to imagine that
black holes could get that big.

As strange as they are,

ordinary stellar-mass black holes

were at least predicted by theory.

Supermassives are a complete surprise.

For the stellar-mass black holes,

people thought about them
from a theoretical perspective.

And then we found them observationally.

The supermassive black holes,
the story has been inverted.

We actually found evidence
of them observationally first.

And now we're working on the theory

of, how did these things come into being?

We already know that stars can collapse

to create ordinary black holes.

But supermassives are bigger
by many orders of magnitude.

Cygnus X-1 is 15 times as big as our Sun.

The supermassive
at the center of our Milky Way

is four million times as big as our Sun.

The one in the Andromeda galaxy

is 100 million times as big as our Sun.

And it's not the biggest, not even close.

There are supermassives
ten, even 20 billion times

the mass of our Sun.

How is it possible to make
such gigantic black holes?

Could supermassives
have come from collapsed stars?

That seems very unlikely.

We don't know any stars
billions of times bigger than the Sun.

We know about black holes
you might get from a dying star.

They have several times
the mass of the Sun

contained within them.

But millions of times the mass of the Sun.

If that's the case, a dying star
cannot have possibly made it.

So do these supermassives,

millions or even billions
of times heavier than the Sun,

somehow just grow,
packing it on like voracious giants?

The wild thing about black holes
is that they feed.

They're constantly devouring
anything that comes

within their sphere of influence,

so they grow.

But how exactly do they grow?

What do they eat,
and where do they find it?

We believe that black holes grow

by accretion of gas.

And the way this works is that
you have a lot of gas around

in the center of a galaxy,

and this gas would then assemble
and form an accretion disk.

The accretion disk
is made up of hydrogen, helium,

and other elements in a gaseous form.

The immense gravity of the black hole

pulls the gas in toward it.

As it swirls around,

it orbits closer and closer
to the black hole,

and the feeding begins.

The stuff in the inner regions
would get slowly pulled in,

sped up, will reach the event horizon,

and then that's it.

Whatever gas crosses the event
horizon disappears forever.

The black hole has absorbed that material.

So it actually adds
to the mass of the black hole.

So this is one way
a black hole can grow:

gradually nibbling gas and dust.

But it's not the only way.

Cygnus X-1 has been
slowly stripping material

off a nearby star,

a process that will likely go on

for thousands or millions of years.

But what if a black hole
could rip an entire star apart

in just a matter of years, or even weeks?

That would be a very violent event.

And a team of space explorers
is on the lookout.

This is the Operations Control
Center for a space telescope...

I have you five-by-five, how may...

We show beginning of track at 0330.

...the Chandra X-Ray Observatory.

Orbiting up to 86,000 miles
above the Earth,

Chandra takes high-resolution images

of objects that emit X-rays.

This is one: a short-lived,
extremely violent event

called a transient,

which fascinates James Guillochon.

Supernovae,

the destruction of planets
by their host stars.

Yeah, I'm just fascinated with
destroying things for science.

James is investigating a mystery
discovered by a colleague,

Dacheng Lin.

This blur on James's screen

is actually a massive sudden
burst of X-ray energy,

caught by accident.

This little smudge popped up
in the background of this image.

And given its great distance,

it's actually tremendously bright.

Could it be a black hole
caught in the act of being born

in the violent collapse
of a huge star, a supernova?

Perhaps.

But the intense radiation
released by supernova

would only linger for a few months.

So how long has this mystery
object been blasting out X-rays?

To find out, they look at images
of that same part of the sky

taken at earlier dates.

2015.

2011.

2008.

2005, July.

2005, April.

No X-rays detected.

But the X-rays are there just
three months later, in July.

And the powerful, bright signal
has continued

for more than ten years,

from July 2005 to the present,

far too long to be a supernova.

So what could it be?

A black hole that's not feeding
is quiet and completely dark.

It won't show up on any telescope.

But a black hole that is feeding
is different.

When it feeds, it blasts out X-rays.

So could this be a black hole

that's suddenly begun devouring
something big?

What effect will this have
on anything that comes near?

What would it do to a star
that wanders too close?

Well, it will flay a star layer by layer,

ultimately devouring

the entire star.

Unlike Cygnus X-1,
this is no mere nibbling.

This is a ten-year feeding frenzy,

a massive black hole devouring
an entire star

in a cosmic blink of an eye.

It's the result of a chance collision,

when an unlucky star wanders too close,

and the black hole's extreme
gravity actually rips it apart.

The gravity from the black hole

will progressively get stronger
and stronger

as the star gets near.

And at that point, the star
will begin to deform.

It's called tidal disruption.

It's similar to the way our moon's gravity

easily moves all the world's oceans.

The tides caused by a black hole

would be billions of times stronger

and much more violent.

You have these events

where a star could be
ripped apart by the black hole.

So you would see sort of a plume of light

from the last gasp
of the material in the star.

But there is a chance
for some part of the star to escape,

as James illustrates.

As the star is elongated by
the black hole's tidal forces,

it will essentially be feeding
the black hole

at the same time as half of it
is trying to escape.

So everything above this point,
approximately,

will have the chance
of leaving the galaxy.

It's moving that rapidly.

And everything below this point

will fall back onto the black hole

and eventually be consumed by it.

So this is another way
for a black hole to gain weight.

Unlike the slow steady nibbling
of Cygnus X-1,

this black hole is devouring
most of an entire star

in one gulp.

But whether a black hole feeds suddenly,

by swallowing half a star,

or steadily, through accretion,

astronomers still face a problem
when they try to understand

how supermassives got so big,

the timing problem.

The trouble begins
with the very oldest supermassives:

quasars, those very bright, very distant,

and ancient objects

first discovered in the early 1960s.

The conundrum was
when we started finding these quasars,

very bright quasars,
very early on in the universe.

They're giving off so much energy

that they have to have very massive
supermassive black holes at their center.

But quasars are extremely far away,

which means that they're part
of the very early universe,

which began nearly 14 billion years ago.

Bright quasars,

600 million years after the Big Bang.

A fraction of today's age.

And, they're enormous.

So billion-solar-mass
black holes, these behemoths,

had to be in place when the universe

was about 550 million years old.

Now you have a problem.

Because you have to grow
something really big, really fast.

And you are bumping up against
sort of physical limits.

Whether a black hole is nibbling

or gulping down its meal,

it turns out that accretion,
how black holes feed,

has a speed limit.

Named after English astronomer
Arthur Eddington,

the Eddington Limit
will not allow a black hole

to feed too fast

because of the light blasting out

from its own accretion disk.

Light has a pressure.

So photons can impart
a force on something.

We see this in winds from stars:
Light is pushing out gas.

So there's a limit to how fast
you can feed a black hole

before its own luminosity
quenches its own growth.

So given this speed limit,
how did early supermassives,

quasars, get so big, so fast?

Could there be a way to bypass
the speed limit entirely?

The problem is still time itself.

How do you grow them

to a billion times the mass of the Sun?

What are the conditions that you
need for that kind of growth?

Some scientists are now asking:

What if there's a way
to create a black hole

that's already much more massive
from birth,

giving it a head start?

If there was a physical mechanism

that would allow you to make
a black hole seed

which was much more massive
from the get-go,

then the timing crunch
is not as much of an issue,

and the growing problem is not as acute.

The answer, some believe,

is to create a black hole
directly from a cloud of gas:

a scenario called direct collapse.

It starts with gas clouds
made of hydrogen, helium,

and other elements.

The same raw materials
from which stars are born.

The denser clouds will start to collapse
under their own gravity.

As they collapse,
parts that are more dense

will collapse more quickly.

And so what happens is,
the cloud fragments.

Those fragments continue collapsing

until the hydrogen atoms
within them begin to merge.

Nuclear fusion begins,
and stars are created.

But what if a giant gas cloud
collapsed without making stars?

We realized that there are
a set of physical conditions

that would allow you
to form a very large gas disk

prior to the formation of any stars.

So this gas disk starts getting unstable.

That would allow the mass
to sort of flow into the center

very, very rapidly
and make a very massive black hole.

It's something we've all seen in nature,

from tornadoes to bathtubs,

a vortex.

But on a supermassive scale.

If you're in a bathtub
and you pull the plug out

and you see the water flowing in a vortex,

very fast down to the center,

that's exactly what happens.

Direct collapse might be a way

to create very large black holes
early in the universe

from enormous gas clouds,

completely skipping the star stage.

Because they would be
so large already at birth,

these direct-collapse black holes

would have a head start,
helping them to quickly grow

into the enormous young
supermassives we see

in the distant universe.

You could potentially
have these direct-collapse black holes.

So black holes whose original masses,

seed masses, the initial masses,

are about 10,000 to maybe 100,000 times

the mass of the Sun,

and that they form
from the get-go with that mass.

Direct collapse may explain
how enormous early supermassives

got their start.

But there's another fundamental
question about supermassives.

What is their role in the universe?

Is their existence
just a matter of chance?

Or are they connected in some larger way

to the very structure of the cosmos?

Supermassive black holes
don't exist in isolation.

They seem to live in partnership
with galaxies.

Collections of millions, billions,

or even trillions of stars
bound together by gravity,

galaxies are the fundamental
building blocks of our universe.

So are the supermassive
black holes at their centers

somehow fundamental
to their very existence?

We now just assume every galaxy,

even ones we have yet to confirm,

will have a supermassive
black hole in their center.

It could be that instead
of simply being oddities,

that they are a key component to galaxies,

a key component to the universe.

We've come in a very short time to realize

that they likely inhabit
the centers of all the galaxies.

And that can really only happen

if there's some symbiotic relationship

between the evolution of a galaxy

and the supermassive black hole
in its core.

What could that relationship be?

One intriguing clue relates to size.

The bigger the galaxy is,

the more massive the black hole
appears to be.

So these black holes at the center

seem to know about
their larger-scale environment.

So which comes first,

the galaxy or the supermassive black hole?

It's not that simple.

It appears they somehow grow in tandem.

It's hard for one to form first
and affect the other.

So today we think that whatever formed one

had to form the other
as a by-product of that process.

And that there has to be
some feedback mechanism

between the black hole and the galaxy

that keeps the growth of the two
in lock sync.

The way galaxies grow
is by forming new stars

from clouds of hydrogen gas.

Gas is essentially
the fuel for star formation,

just like gas is the fuel for our cars.

And so if you run out of gas,
you run out of new stars.

So are supermassive black holes

somehow interfering with star formation?

When a black hole is growing,

a tremendous amount of energy
is being liberated

and sent out into the galaxy.

And so we think that some of
that energy goes to warm up gas.

And gas that's too warm will not
form stars anymore.

The heat produced
by a growing black hole

makes it impossible
for stars to form nearby.

And so,
one way that a growing black hole

can influence its host galaxy
is by quenching the star formation.

In effect,
the growth of the supermassive

determines whether or not
its host galaxy grows or stagnates.

They have a kind of eating phase,

and then a quiescent phase.

So they seem to be involved

with the formation of the galaxy
in that way,

and then stabilizing
of the galaxy at the same time.

So these mysterious supermassives

may actually control
the building of the universe,

not so much by their size,

but by the way the energy
they generate shapes galaxies.

By mass, if you count up all
the black holes in the universe,

the tiny ones as well
as the supermassive ones,

the ultra-massive ones,

black holes are nothing.

However, energetically,
how much power the galaxy gets

and at what time as it assembles,

seems to be dictated
by the central black hole.

So they might well be
the key players in the universe.

In the next two years,
NASA plans to launch

the James Webb Space Telescope.

Humanity's most powerful telescope ever,

the James Webb is designed
to look in the infrared,

allowing it to see farther
back in time than Hubble,

getting a look
at the first stars and galaxies

that formed after the Big Bang.

Hopes are high
that the James Webb Space Telescope

will help solve
many of the remaining mysteries

about the earliest
supermassive black holes.

The James Webb Space Telescope
is tuned specifically

to observe the early universe
when galaxies were being born.

That could give us deeper understanding

of how you end up
with a supermassive black hole

in your galaxy to begin with.

Technology is moving really fast,

and as a result, we have
really fundamental new views

of the universe.

I think we are really living
in a golden era of astronomy.

And the James Webb Space Telescope

isn't the only new development
that promises to solve

some of the mysteries around black holes.

I believe have
infrared components...

A group of scientists
led by Shep Doeleman

is now attempting the impossible:

to take a picture of a black hole.

It's interesting that we can say something

about the accretion flow
near the black hole at all.

And if some of this
linear behavior survives,

maybe we'll have a way of interpreting it.

The project is called
the Event Horizon Telescope.

The basic goal
of the Event Horizon Telescope

is really to see the unseeable.

It's to bring into focus

something that science
has told us for many, many years

is precisely something we can't observe,

the black hole.

Their primary target is Sagittarius A*,

the supermassive in the center
of our Milky Way Galaxy.

They're using a global network
of radio telescopes.

We need good weather
at eight different telescopes

all around the world,

and that is a tall order.

But if black holes are invisible,

what exactly do they hope to photograph?

What we are trying to photograph
really is the shadow.

So, as this gas around the black hole

swirls inwards and actually hits
the event horizon,

it leaves a silhouette,

a very well-defined shadow
on the surrounding light.

So really it should look like a donut,

with its very well-defined hole.

And that's the picture that we're after.

If I convert that into frequencies,

I get two-pi-square there.

The team has conducted
their first observing run

and is processing the data now.

Okay, you're saying the velocity...

It's hoped that
these new technologies

will give us an unprecedented
view of black holes in our universe.

But there is one new technology
that is already delivering results.

And that brings us back here, to LIGO,

a key player in the black hole drama,

to an idea that took root
way ahead of its time:

gravitational waves.

With general relativity,
his theory of gravity,

Einstein predicts
that when an object moves,

it can create ripples in space and time,

an actual squeezing
and stretching of space itself.

One of the holy grails
of 20th-century physics

was to detect these gravitational waves.

That was not easy to do

with general relativity,

because all the effects
that you could think of

were infinitesimally small.

Very, very difficult to measure.

The thinking was,

if gravitational waves could be measured,

it would confirm Einstein's prediction.

And there could be an added benefit,

it might also prove
the existence of black holes

and help solve the mystery
of how supermassives grow.

But how to detect gravitational waves?

In 1970, the problem caught the attention

of a young experimental
physicist, Rai Weiss.

Rai had the perfect background
to hunt for gravitational waves.

For decades, he'd been working
with more familiar waves:

sound waves.

We were immigrants,

we were German Jews.

And a lot of our friends were
very, very interested in music.

Rai devoted himself

to coaxing every subtle nuance he could

out of recorded music.

Those records had a terrible problem.

When the music was loud,
it sounded wonderful.

When the music was real quiet and slow,

what you heard was this...

...like that.

A hissing noise.

And that was so annoying.

The lessons he learns
trying to eliminate noise in recordings

will pay off later,

when Rai turns his attention

to detecting gravitational waves.

You have to understand

how a gravitational wave
does its dirty work.

As a physics problem,

gravitational waves
are not unlike sound waves.

Let's suppose the wave
comes from something

that is in some way
moving and oscillating.

A sound wave
compresses and expands air.

A gravitational wave
compresses and expands space

and everything in it.

If a wave came through the Earth,

it would cause space

to expand momentarily
and then contract again.

It keeps doing it, so it's this thing

that goes blip, blip, blip,
right along like that.

So how to measure

the extremely tiny expansion
and contraction of space?

Rai's idea was to use light.

Send a beam of light

from one place to another,

and measure the time it takes
to get there.

That's how the exact distance
to the moon was calculated:

bouncing a laser beam from the Earth

off a mirror left behind
by Apollo 11 astronauts.

From the duration of the round trip,

scientists could determine the distance.

Rai came up with an ingenious design

for an instrument
that uses lasers and mirrors

to detect the faint expansions
and contractions of space

that would be caused
by a gravitational wave.

It's called a laser interferometer.

It works by firing a laser
into a splitter.

Half of the light continues straight ahead

towards one mirror,

while the other half
is sent towards another mirror.

The distant mirrors bounce
the light beams back,

where they rejoin at a photo detector.

If the distances the two beams
travel are exactly the same,

the system is designed
so the two beams cancel each other out,

the detector sees nothing.

You've set the trap to measure
the gravitational wave.

Now comes the gravitational wave
that's coming,

let's say, at this structure.

If a gravitational wave passes through,

it briefly changes the length of the arms.

The light beams no longer
arrive back at the same time

to cancel each other out.

A gravitational wave hits.

Light appears at the detector.

The trap has sprung.

That's the basic idea.

It's a very straightforward measurement.

A clever idea,
and simple in principle.

But the devil, and the Nobel Prize,

lie in the details.

The difference in length
between the two arms

would be tiny beyond imagining.

How tiny? Well, take the size of an atom.

It's less than that.

Go down by a factor of 100,000.

That's the nucleus of an atom.

It's less than that.

It was 100 times below that.

So we're talking about
really itsy-bitsy, teeny-weeny.

I thought it was crazy.

I think everybody's
initial reaction to the idea

was, this is going to be impossible.

In 1973, Kip Thorne puts
his skepticism on the record

in a classic textbook,

doubting it will ever work.

But Kip has never heard Rai Weiss

explain his plan in detail.

And when he does...

We spent the whole night talking.

And, so I said, "No, no, no,
it's very possible."

And within no time at all,
20 minutes, maybe half an hour,

Kip was solidly understanding
this thing, and he says, "Yup!"

And I ate crow the rest of my career,

because once I had talked
with Rai about it in detail,

I decided I would spend a large fraction

of the rest of my career
helping the experimenters.

But it will take 40 years,

and enormous sums of money,

to bring Rai and Kip's vision to reality.

Getting LIGO funded

was extremely controversial.

Hundreds of millions of dollars

to detect a signal
that had never been seen before.

There were many people who feared

that LIGO would suck the money
out of the room.

And so there was a lot of controversy.

What everybody could agree on was,

this was extremely difficult.

With such a sensitive instrument,

one of the biggest challenges

is Rai Weiss's old hi-fi nemesis: noise.

Ground motion.

The seismic motion of the Earth.

Acoustics' noise,

sounds...

Everything would tend to move that mirror.

Turns out, even the emptiness
of a total vacuum

creates a potentially crippling problem.

At subatomic distances,

the weird randomness of the quantum world

causes a ruckus in the mirrors.

This quantum noise is due
to quantum fluctuations.

These mirrors are doing what
an electron does inside an atom,

they're jiggling around.

Exquisite sensitivity,

extreme vacuum,

hundreds of thousands
of electronic circuits...

LIGO is one of
the most complex instruments

in the history of science.

And as a final means
of eliminating false signals,

they build not one,

but two complete installations:

one in Washington state
and another in Louisiana.

And so the LIGO designers did it right.

They designed more than one detector,

separated from one another
by great distances,

so that if you detect something
in one and not in the other,

then, you know, go back
and check your electronics.

Check to see if it was April Fools' Day

and somebody didn't just tweak the knobs.

Early fall 2015.

Both locations are operating,

but the first official science run
has not yet begun.

They're still testing.

In the early hours
of Sunday, September 14, 2015,

a scientist in Louisiana
makes a fateful decision.

Robert Schofield
has been working all weekend

doing final calibrations.

All righty, let's take a spectrum.

He has one last test.

So let's see where this
computer's getting its power.

But it's late,
and the equipment is not cooperating.

It was about 4:00 or so in the morning,

and we still had about
another hour of work to do.

And we were, like, "Yeah,
things aren't working so well,

and I'm really tired.

Let's not do this last hour
or so of work."

They call it a night.

And 40 minutes later,

in the silence of their inactivity,

they open the door to history.

A powerful gravitational wave
rumbles through both detectors,

Louisiana and Washington.

Had Robert Schofield worked
40 more minutes that night,

with the instruments in test mode,

a signal that had been
on its way for 1.3 billion years

would never have been recorded.

I like to say,

you know, one of my biggest
contributions to LIGO

has been my laziness that day.

I got an email from somebody here saying,

"Hey, look, look at this place
on the web."

I looked at that and I said,
"Holy."

It was so strong

that you could see it by eye in the data.

It was too good to be true.

But it was true.

In fact, it was loud,
and surprisingly clear.

And it just sang at you.

There it was, standing out.

The signal lasted less than a second,

but in that briefest of moments

it delivered a cosmically profound message

more than a billion years in the making,

proving the existence of black holes.

So what we saw in the signal

involved oscillations of the
mirrors that were slow at first,

became faster and faster and faster.

And this was precisely
the kind of behavior

that you would expect
from gravitational waves

caused by two black holes
going around each other,

spiraling together.

Two massive black holes,

one 29 times the mass of the Sun,

the other 36 times the mass of the Sun,

whipping around each other
hundreds of times a second,

finally completing their act of
mutual destruction by merging...

Creating a single, larger
black hole of 62 solar masses.

The violent merger
converts some of the mass

into an apocalyptic release of energy

beyond anything ever before witnessed.

The collision, in effect,

creates a very... a veritable
storm in the fabric or the shape

of space and time,

as though you had taken three suns,

you had annihilated them completely,

converted it into gravitational waves.

The power was 50 times higher
than the output power

of all the stars
in the universe put together

in a fraction of a second.

But the most powerful explosion

that humans have ever had any evidence for

with the exception of the Big Bang.

Since that very first signal
in September 2015,

LIGO has detected several more
collisions of black holes.

In October 2017, Rai Weiss, Kip Thorne,

and LIGO's former director Barry Barisch

received the Nobel Prize.

The LIGO discoveries prove
that black holes can merge,

one way they can grow bigger quickly.

More and more evidence of these
merging black holes tells us

there are a lot of these
stellar black holes around,

that they can find each other
and, and merge.

And the discovery opened
an entirely new way

of observing the universe.

We always thought of astronomy
as an observational field

where we are looking at radiation.

We are seeing things.

But this is not radiation.

This is something much more fundamental.

These are sort of fundamental
tremors in space-time itself.

We can now hear the universe.

For the first time,

astronomers have simultaneously
seen and heard a cosmic event.

In August 2017, LIGO detected
gravitational waves

from a collision of two neutron stars.

Black holes are empty space,

but neutron stars are dense dead stars

that can crash together
and light up the skies.

When telescopes and satellites
around the globe

pointed in the direction of the sound,

the world saw fireworks

in an explosive collision and afterglow.

Possibly, the collision resulted
in the creation

of a new black hole.

But unless we observe
the formation of a black hole,

there is much we will never know.

Because so much about black holes

is irretrievably out of our reach,

we can never know where they came from,

what's inside, or their history.

But we can imagine their future.

The number of black holes
in the universe is increasing.

And they're getting bigger.

Stars collapse,

black holes feed and merge,

new ones form.

Could it be that one day,

everything will end up inside them

and they will rule the universe?

Untold trillions upon trillions
of years after this happens,

and the last bits of matter
cross their event horizons,

black holes themselves may radiate away

and vanish from this reality.

Their mysteries are many,
and we're just starting

to unlock the secrets of these
strange, powerful places.

But one thing is certain.

Black holes will continue to intrigue us,

tantalize us,

and challenge both our science
and our imaginations.