All language subtitles for 3C85D716B01B172A710C04F78C823E2F_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.

A

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 and may

even have influenced the history of our
planet.

We can imagine 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, was released
from the shuttle cargo bay.

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 galactic core.

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 kilometres 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 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
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 as 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,
and yet 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.

We are 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.

Einstein taught us that 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.

So 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.

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.

More offerings to feast on.

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 stripped 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're looking for reasons why life not
only began here on Earth but was able to

prosper for 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 center of our

galaxy and say that's
one of the reasons why.

But we're 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 ejected 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's 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.

Imagine if there was some
giant star that had formed

close by that exploded
in a supernova explosion.

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 feel.

Nothing 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 has been
pieced together over the last 20 years by

observational data from telescopes such as
Chandra and Fermi.

But there is 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 orbit around Sagittarius A
star.

Some 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.

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 if 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.

This thing here, H-bar,
is Planck's constant and

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 because it's got a
pi in it.

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 die.

And 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'll 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, certainly over the last 100 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 ain't as black as they are
painted.

They are not the eternal prisons they were
once called.

Things can get out
of 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.

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.

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 invisible.

It's 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 is close 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 mysteries 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 are black holes.

And so I suppose it's not surprising that
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.

Baby, you understand me now If sometimes
you see that I'm mad Baby, you understand

me now But I'm just a
soul Whose intentions are

good Oh Lord, please don't
let me be misunderstood

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
with a crew of five.

I

think the 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 two million pounds

of fuel and weighs half
of what it did at launch.

It's huge.

Chandra is about the size of a school bus.

It's the largest telescope to ever be
launched by the space shuttle.

SRV separation is confirmed.

You're 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's cut off,
we're in zero G.

We separate the tank, and we're orbiting
the Earth.

And when we're sure that everyone is ready
at mission control, then I go ahead and

pull the switch mark,
deploy, and you're 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'd 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.

And there will be 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's been able to witness that as
well.

Chandra has witnessed the destruction of a
star by a black hole itself, right?

So this poor star wanders into the black
hole.

Then it rips the star apart.

It shears it, right?

And then the corpse of that star,
this sort of spaghettified matter that

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 5,000
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.