All language subtitles for 6CA2F88C48C4295B16DC2136E51F8902_eng

The Milky Way seems a tranquil place,
hundreds

of billions of stars serenely spinning
through the cosmos.

But journey inwards, through the gas and
dust that shrouds the galactic core,

and

you see a curious sight.

Stars orbiting seemingly empty space.

Something dark and ancient lives here.

A hole in the fabric of the universe.

Every one of those
points of light in the

night sky is a strange
and fascinating place.

Magnificent suns with countless planets
orbiting around them.

Alien worlds beyond imagination.

But the strangest
and most fascinating

places out there by far
are dark and unseen.

An invisible monster is lurking in the
centre of the Milky Way.

The Monster that drains
the colour from the universe,

with the power to destroy
worlds and stop time.

We've named the monster Sagittarius A-Star
and we believe it to be a black hole.

Sagittarius A-Star has played a major role
in the evolution of our galaxy.

The galaxy's ...and may
even have influenced

the formation of stars
and planets like ours.

But there is so much more,
because black holes like Sagittarius

A star present the most
profound intellectual challenge.

They are part of
nature, just like you and

me, so we should be
able to understand them.

But they are holes in the fabric of the
universe.

They are gravitational prisons from which
even light itself can't escape.

And in trying to understand them,
physicists have been led to completely

reassess our most basic understanding of
reality.

But there are answers hiding in the void
for those brave enough to seek them.

It is said that fact is sometimes stranger
than fiction.

But nowhere is that more true than in the
case of black holes.

Black holes are stranger than anything
dreamed up by science fiction writers.

But they are firmly matters of science
fact.

And we copy at go for deploy.

You look out and this
thing is so big you certainly

know that it's looming
over the head of the shuttle.

In the summer of 1999,
NASA's flagship mission for

X-ray astronomy was released
from the shuttle cargo bay.

Nothing as beautiful as Chandra is tailing
off on its way to work.

High above the Earth, Chandra
scanned the sky, hunting

for some of the hottest
regions in the universe.

Exploding stars and clusters of galaxies.

But

on September the 14th,
2013, after 14 years,

Chandra chanced on
something else entirely.

The telescope gazed
into the constellation of

Sagittarius, hoping to observe
a large cloud of hot gas.

Instead, it recorded a flash
of X-rays, just a few pixels

across, coming from the
apparently empty space in the galaxy.

Something had got very hot for a very
short period of time.

It's thought that the flash seen by
Chandra may have been an asteroid,

tens of kilometers across.

Ripped apart and burning up in a fireball
300 times brighter than the sun.

The culprit?

Sagittarius A-star.

The asteroid's destruction
was our galaxy's

black hole, signalling
its presence to the world.

Twenty years ago, we
didn't know for sure that

there is a black hole at
the centre of our galaxy.

But by measuring bursts of radiation and
observing in detail the orbits of stars

close to the galactic centre, we now know
that Sagittarius A-star definitely exists.

And we've been able to measure its length.

The mass?

It is around four million
times the mass of our

sun, which makes it a
supermassive black hole.

One of the strangest and most powerful
objects in the universe.

And we've begun to suspect that
Sagittarius A-star isn't just some strange

thing that sits tens
of thousands of light

years from Earth at
the centre of the galaxy.

It has played a crucial role in the
evolution of the Milky Way.

And the whole story began with the death.

It was the death of one massive star.

Soon after the dawn of time, the cosmos
was home to colossal stars.

Hundreds of times more massive than the
sun.

Stars that burnt blue with intense heat.

But the brightest stars are the shortest
lived.

One lived a particularly fast and furious
life.

Burning through its nuclear fuel in just a
few million years.

And with nothing left in the tank,
gravity took over.

The star collapsed.

Ever smaller.

Ever denser.

Until it seemingly disappeared.

The remnants of the star.

Now smaller than an atom.

Lost from the universe.

All that's left is a ghost.

A black hole.

The genesis of Sagittarius A-star wasn't
some bizarre one-off event.

It's possible that almost all early stars
became black holes when they died.

Black holes are simply what happens when
gravity goes unchecked.

Compacting matter so densely, it tears a
hole in the universe.

In the vicinity of a black hole, space and
time behave in very counterintuitive ways.

In fact, this river provides a beautiful
and surprisingly accurate analogy.

See, close to a black hole, you can think
of space itself.

It's a black hole.

It's flowing towards the black hole.

Now here, the flow is not too fast.

And so you can imagine, I could jump into
that river.

I can swim faster than the river of space.

And so I can escape out into the galaxy.

But as you get closer
to the black hole, the

river of space flows
faster and faster and faster.

A collapsed star is so small.

It's so massive that close to it, the
gravitational forces become overwhelming.

There is no limit to the speed at which
the river of space can flow.

There's a place where the
river becomes a waterfall, where

no matter how fast I swim, I
could never escape upstream.

And that's what happens in the vicinity of
a black hole.

The river of space flows at and then
faster than the speed of light.

Light itself can't move fast enough to
escape.

And that's what Sagittarius A star is.

It's a waterfall in the fabric of the
universe.

From the moment the black
hole formed, the seed of

Sagittarius A star had a
heart of pure darkness.

The interior forever hidden from view.

Shielded from the rest of the universe by
a boundary in space.

The event horizon.

The ultimate point of no return.

As we approach the event
horizon, we get our first

glimpse of the true
weirdness of black holes.

The Black Hole The Black Hole The Black
Hole Space and time are not what they seem.

They are merged together into a kind of a
fabric, the fabric of the universe.

It's called space-time.

And Einstein also taught us that the
presence of massive objects, stars and

planets and galaxies, curve and distort
the fabric of the universe.

And that's what we feel as the force of
gravity.

But that distortion is not only in space.

It's also in time.

As you go closer and
closer to a massive

object, the rate that
time passes slows down.

So if I look towards a black hole,
I see time passing slower and slower and

slower until, on the event horizon,
time stops.

Sagittarius A star was born a waterfall in
the fabric of the universe.

Where space flows faster than light.

And time grinds to a halt.

But our black hole was still just a baby.

Dwarfed by the stars around it.

It was nothing like the monster it would
become.

Sagittarius A star today is four million
times the mass of the sun.

There has never been a star that massive.

It must have formed by the collapse of
something much smaller.

And then it must have grown over the
lifetime of the Milky Way by eating stuff.

Unfortunately, there has been a lot of
stuff around for it to eat.

The young black hole's inexorable
gravitational pull meant there was no

escape for anything that strayed too
close.

Sagittarius A star began to grow.

Pulling on nearby stars.

Before ripping them to shreds and feasting
on the hot plasma.

The black hole gaining more mass.

And more gravitational power.

But we don't think there were
enough stars nearby for the

black hole to grow supermassive
on a diet of stars alone.

Instead, it developed a taste for more
massive prey.

When another black hole
passed close to Sagittarius A

star, they became locked
in a gravitational embrace.

Spiralling towards each other.

Approaching half the speed of light.

Sagittarius A star cannibalised its
cousin.

Creating ripples in the fabric of the
universe itself.

Meals were to follow.

Black holes, stars, gas clouds.

Whatever ventured too deep into its lair.

And as our black hole's power and influence
grew, its surroundings were changing too.

Around the galactic core.

Hundreds of billions of stars.

Were in orbit.

Slowly spinning.

Around their common centre of mass.

Evolving.

Into the familiar spiralling disc.

The majestic Milky Way.

With Sagittarius A star.

At its core.

Sagittarius A star became what we now call
a supermassive black hole.

Many tens or even hundreds
of thousands of times

more massive than
any star in the universe.

And Sagittarius A star is not unique.

We now think that
virtually every large galaxy

has a supermassive
black hole at its heart.

Chandra has looked beyond the Milky Way.

And observed countless supermassive black
holes.

Lurking at the hearts of the myriad
galaxies that litter the cosmos.

These monsters aren't obscure quirks of
nature.

They are fundamental features of it.

And far from being bit players.

We're starting to realise that black holes
have the power and reach.

To sculpt the galaxies around them.

The centre of our young galaxy was rich in
gas and dust.

The centre of our young galaxy was rich in
gas and dust.

This

was a gluttonous period.

That marked a new era for the Milky Way's
supermassive black hole.

When the invisible monster became sculptor
of the galaxy.

Creation and destruction often go hand in
hand in the universe.

And black holes are no exception.

Sagittarius A star is certainly not only
an agent of destruction.

Because the material that falls inwards
towards the black hole.

Doesn't all vanish across the event
horizon.

A lot of it goes into orbit around the
black hole.

And that region is tremendously violent.

There are magnetic fields that swirl
around and become twisted and distorted.

And they can throw material out along the
magnetic poles of the black hole.

Making jets that sweep through the galaxy.

It's only recently that
we've grasped the

true scale of Sagittarius
A star's eruptions.

Engine start.

One zero.

Liftoff.

Carrying

a gamma ray telescope.

Searching for unseen physics in the stars
of the galaxies.

Just over a decade ago.

A completely unexpected discovery was
made.

Likened to finding a brand new continent
here on Earth.

The Fermi Space Telescope was built to
detect gamma rays.

The most energetic radiation in the
universe.

As Fermi orbited the Earth.

It constructed a map of the sky.

And saw.

Emerging from the plane of the Milky Way.

Two colossal bubbles of material.

Each 25,000 light years across.

These bubbles are superheated gas.

If our eyes were sensitive
to the wavelengths

of light emitted
by those bubbles.

They would span half the sky.

As seen from here on Earth.

And they point back to the centre of the
galaxy.

It looks like their origin.

Is Sagittarius A star.

Think about that.

Sagittarius A star is big.

But not big on a galactic scale.

It would fit comfortably inside the orbit
of Mercury.

In our solar system.

Though our black hole is only a fraction
of the size of the galaxy around it.

It had become sculptor.

Of the Milky Way.

Every few million years.

The dense ring of material circling our
black hole.

Was accelerated.

By twisting magnetic fields.

Into fiery jets.

Of superheated matter.

Jets so powerful.

They strip the atmospheres from any
planets in their path.

And radiation.

Rendered every Earth-like world.

Within a thousand light years.

Uninhabitable.

But such was the scale.

Of Sagittarius A star's outbursts.

That far, far out in the galaxy.

Destruction.

Turned to creation.

If you are looking for reasons why.

Life not only began here on Earth.

But was able to prosper.

For thousands of years.

After the almost four billion years it
took.

For it to evolve.

Into the complex living world that we see
today.

Then it might seem a bit of a stretch.

To point to a supermassive black hole.

At the centre of our galaxy.

And say that's one of the reasons why.

But we are now beginning to suspect.

That those great outpourings of energy.

From Sagittarius A star.

Played a crucial role.

In making this region of the galaxy.

One in which life.

Can flourish.

Because the hot gas.

Produced by Sagittarius A star.

Had a calming effect.

On the galaxy.

Now you might think that a hot gas cloud.

Would produce more stars.

But in fact the opposite is true.

Because hot means that everything is
moving around very fast.

And that makes it more difficult.

For gravity to grab hold of everything.

And collapse it.

To form stars.

So Sagittarius A star reduced the number
of stars.

That formed.

In this region of the galaxy.

And that's a good thing.

Because there was a giant star.

That had formed close by.

That exploded in a supernova explosion.

And that would not be a good thing.

If you're an amoeba.

And you have designs one day.

On evolving into Einstein.

So Sagittarius A star.

Turned what is potentially.

A violent region of our galaxy.

Into a peaceful one.

The warm gases.

Pushed out by Sagittarius A star.

Slowed the rate of star formation.

Around

one small yellow star.

In a quiet region.

At the unfashionable end.

Of the outer spiral arms.

Of the galaxy.

Four billion years of stability.

Made all the difference.

Now of course.

There are many things.

That are necessary for life.

To exist on a planet.

The list is unimaginably vast.

But I think it is interesting.

That on that list.

There is the presence.

Of this strange object.

A black hole.

Sagittarius A star.

Tens of thousands of light years away.

The centre of our galaxy.

Having cleared out.

Much of the gas.

Dust and stars.

That once lay close by.

There was little left.

To feast on.

Our black hole.

Fell silent.

The enormous.

Bubbles.

Spotted by Fermi.

Echoes.

Of a glorious past.

Today.

Sagittarius A star.

Is a sleeping giant.

A brooding beast.

Operating.

On a slow simmer.

Sagittarius A star's journey.

From violent destroyer.

To sculptor of the galaxy.

To the sleeping giant.

That we see today.

Together.

Over the last 20 years.

By observational data.

From telescopes.

Such as Chandra.

And Fermi.

But there's a very big difference indeed.

Between knowing how a black hole.

Interacts with its environment.

How it sculpts a galaxy.

And what it actually is.

At a deep level.

What is it really like.

Inside.

Black holes challenge.

The most basic principle.

About the predictability.

Of the universe.

And the certainty of history.

Nothing could get out.

Of a black hole.

Or so it was thought.

It's by looking.

Into the future.

That we're beginning to explore.

The deep mystery.

Of black holes.

Dozens of stars.

Passing just a few billion miles.

From the event horizon.

A hair's breadth.

On galactic scales.

These flybys.

Could have fatal consequences.

Some of these stars.

Will likely have planets in orbit.

Planets that may stray.

Too close.

To the beast.

A moth to a flame.

Or to the sun.

Pulled from its parent star.

Towards the abyss.

If the planet survives.

Its journey inwards.

And we could stand.

On its surface.

And look out.

Into the universe beyond.

We would see space.

And time.

Becoming increasingly distorted.

But eventually.

Tidal gravitational forces.

Become too strong.

Inextricably.

The singularity.

Awaits.

The end of time.

Where all paths.

Terminate.

Over trillions of years.

All the stars.

Around Sagittarius A star.

Will gradually fade.

And die.

On more and more alien worlds.

The dawn.

Will fail to come.

But our supermassive monster.

Will go on.

Its secrets.

Sealed away inside.

Seemingly.

Forever.

We predict.

That one day.

Black holes.

Will be all that remains.

In the universe.

The final.

Dark age.

If nothing can ever escape.

From black holes.

Sagittarius A star.

Really is.

An eternal prison.

Then this is the end.

Of the story of the universe.

Darkness.

Littered with holes in space time.

But we don't think.

This is the end of the story.

We now believe.

That even black holes.

Die.

And their deaths come.

At the hands of what might seem.

An inconsequential detail.

Discovered.

Almost five decades ago.

In 1975.

Stephen Hawking.

Published a remarkable paper.

In which he showed that black holes.

Are not completely black.

They glow faintly.

They have a temperature.

And here's his beautiful equation.

For the temperature of a black hole.

And you can see.

That there's something deep.

Going on.

Because this has got.

All the physics in it.

With this thing here.

H bar is Planck's constant.

That's to do with quantum mechanics.

A subatomic world.

C is the speed of light.

G is the strength of gravity.

This Kb here is Boltzmann's constant.

That's to do with temperature.

And thermodynamics.

And this M here is the mass of the black
hole.

It's even got circles.

Hawking's conclusion.

Proved to be irrefutable.

And the implications.

Are huge.

If something has a temperature.

Then it radiates.

That's why if you put your hand.

Near something that's hot.

You can feel it.

And so over time scales.

That are billions and billions and
billions.

And billions of times longer.

Than the current age of the universe.

Sagittarius A star will eventually.

Evaporate away.

Very gradually.

Hawking radiation.

Will erode.

Sagittarius A star.

Smaller.

And smaller.

Until many trillions.

And trillions of years.

Into the future.

In a final burst of light.

Our black hole.

Will darken.

Then there will be darkness.

For all.

Eternity.

Now you may say.

Quite legitimately.

Well why do we care.

Why does it matter.

If black holes evaporate away.

Sometime in the far far future of the
universe.

There will be nobody around to see it.

But the discovery.

That black holes evaporate.

Raises what I think is the most profound
question.

In the history of physics.

In the first hundred years.

And that's no exaggeration.

See what happens.

If I set fire.

To this piece of paper.

With Stephen Hawking's equation written on
it.

I cause it.

To evaporate away.

Do I destroy.

Everything.

Do I remove every piece of information.

Including the equation from the universe.

When it burns away.

Well the answer is no.

If I could collect.

Every ash.

Every molecule of gas.

That burns off into the atmosphere.

Then in principle.

I could reconstruct the piece of paper.

And everything it contains.

Every piece of information.

On this piece of paper.

Including.

Stephen Hawking's equation.

But can that be true.

For black holes.

The ultimate gravitational prisons.

These objects in the sky.

From which even light itself.

Can't escape.

When they evaporate away.

Do they return the information.

About everything that ever fell in.

Back to the universe.

Black holes.

As black as they are painted.

They are not the eternal prisons.

They were once thought.

Things can get out of control.

From a black hole.

Both to the outside.

And possibly.

To another universe.

So.

If you feel you are in a black hole.

Don't give up.

There's a way out.

If information somehow escapes.

From Sagittarius A star.

As it evaporates away.

The implication.

Is profound.

Black holes aren't tombs.

They're gateways.

We now believe.

That anything.

That falls into Sagittarius A star.

Will live on.

Not as a physical object.

But as information.

Escaping from the heart of darkness.

Encoded in the hawking radiation.

In the far future.

And in the future.

The memory of all those worlds.

That fell into Sagittarius A star.

Over the entire history.

Of the Milky Way galaxy.

Is somehow written.

In the ashes of the universe.

In the far future.

But the real treasure.

Lies in the explanation of how.

The information gets out.

From those eternal prisons.

Now what I'm going to tell you.

Is going to sound absolutely bizarre.

It's going to sound like science fiction.

But here goes.

When the black hole.

Has evaporated away.

About half of it has gone.

The interior.

Becomes in some sense the same place.

As the distant.

Hawking radiation.

That was emitted eons ago.

That's out there in the far reaches of the
universe.

It seems that space-time wormholes.

Open up.

Between the interior of the black hole.

And those distant.

Parts of the universe.

And it's that that allows us to read.

The information inside.

Now that is supposed.

To sound weird.

And I should say that nobody.

Really agrees on the physical.

Picture of what's happening.

But what everybody agrees on.

Is this.

The black holes are telling us.

That our intuitive picture of reality.

Of space and time.

Is wrong.

The idea that this place.

To this place.

And that time ticks along.

Is wrong.

There is a deeper picture of reality.

In which space and time.

Do not exist.

Our attempt to answer a seemingly simple
question.

About the fate of objects.

That fall into black holes.

Has led us to a profound.

And quite unsettling conclusion.

Space and time.

Concepts so foundational.

To how we experience the world.

Are not.

Fundamental properties of nature.

They emerge.

From a deeper reality.

In which neither.

Exist.

The thing about.

Black holes.

Is that nobody.

Really understands.

Them.

So.

Don't worry.

If you don't understand.

What I'm talking about.

Because I don't understand.

What I'm talking about.

And nobody else does either.

We're still a long way.

From fully comprehending.

The secrets.

Of black holes.

But we are beginning.

To lift the veil.

Far from being.

A mere.

Cosmic aberration.

Sagittarius A star.

Is a part of our history.

And.

Of our future.

Our black hole.

Not only made us.

Who we are today.

It's our teacher.

Slowly revealing.

The deepest.

Mistress.

Of the universe.

Secrets.

Sealed away.

For so long.

Inside a place.

Beyond forever.

The moral of the story.

Is this.

Understanding the book of nature.

Is hard.

And so the more of nature we observe.

The more chance we have.

Of finishing the book.

Now the strangest objects in nature.

By far.

By peering over the horizon.

And into the darkness.

We have caught a glimpse.

Of something deeply hidden.

The underlying structure.

Of reality.

Itself.

So if we want to understand.

The meaning of it all.

We can't restrict ourselves.

To the intellectually safe.

Confines of our planet.

We have to look.

Out there.

To the universe beyond.

So of the seemingly endless.

Zoo of objects.

In our universe.

From clouds of gas.

To planets.

To stars.

Galaxies.

What have you.

Black holes.

Are probably one of the most.

Fundamentally important.

Singular class of objects.

That we can study.

A place where the laws of physics.

Literally break down.

We have theories.

But we can't really know what's happening.

So black holes present.

This remarkable invitation.

To physicists, mathematicians,
astronomers.

One of the best tools we have.

To study these exotic phenomena.

Is the Chandra telescope.

Chandra is kind of like a black hole
hunter.

Finding them near and far.

Throughout the galaxy and the universe.

Just a few minutes away.

From the 26th flight of the shuttle
Columbia.

To a crew of five.

I

Think a night launch.

Is particularly exciting.

We have a goal for stars.

We have booster ignition.

Liftoff of Columbia.

You just see fire.

It lights up the night sky.

In the most beautiful way.

Columbia now has burned.

More than 2 million pounds of fuel.

And weighs half of what it did at launch.

It's huge.

It's the largest telescope.

To ever be launched.

By the space shuttle.

SRV separation is confirmed.

You are stressed.

About the astronauts on board.

That are literally risking their lives.

To help us get a better view.

Of the universe.

When the main engine is cut off.

We are in zero G.

We separate the tank.

And we are orbiting the earth.

And when we are sure.

That everyone is ready.

Then I go ahead.

And pull the switch mark deploy.

And you are looking at the deploy.

Of the chandra x-ray observatory.

Chandra is an x-ray telescope.

Which means that it can see.

The most energetic light coming at us.

From the universe.

The telescope has to be.

Outside the earth's atmosphere.

Because the earth's atmosphere.

Thankfully for us.

Blocks out x-rays.

Otherwise we just get fried.

The chandra satellite.

By rising above the atmosphere.

Has a much clearer view.

Why is that so interesting.

For studying things like black holes.

Black holes can excite matter.

In their vicinity.

To very high energies.

They can get atoms.

And parts of atoms.

Whipping around.

And by revving up those particles.

To very very high energies.

They will radiate.

So we can gather.

Enormous amounts of information.

About the immediate vicinity.

Of a black hole.

When we look at Sagittarius.

A star today.

It's quiet and not doing very much.

But when we look at other supermassive
black holes.

In the universe.

They're active.

So you can take chandra.

And watch a black hole have a small snack.

Maybe like a human might have.

A little biscuit in the afternoon.

And it's something like an asteroid.

A small sort of x-ray signature.

From that event.

But you can also see a black hole.

Have a really really big snack.

And chandra has been able to witness that
as well.

Chandra has witnessed.

The destruction of a star.

By a black hole itself.

So this poor star.

Wanders into the black hole.

That it rips the star apart.

It shears it.

And then the corpse of that star.

This sort of spaghettified matter.

That starts to form.

And starts spiraling around the event
horizon.

Lights up in x-rays.

So to be witness to that.

To observe the destruction of a star.

You know in time.

Was just extraordinary.

It's just unbelievable.

There's this one.

Pretty famous image that chandra took.

Called the chandra deep field south.

And it's the deepest x-ray image ever.

In that one data set.

There's thousands of black holes.

Like maybe 5000 of them.

If not more.

And it just kind of helps show you that.

They're all over the place.

And there's so much more to discover.