Original subtitles

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.