Original subtitles

NEIL ARMSTRONG: I can see everything quite clearly.

It has a stark beauty all its own.

EXPLOSION

MUSIC: Neptune by Foals

BUZZ ALDRIN: Magnificent desolation.

Beautiful! Beautiful! Isn't that something?

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*

and we believe it to be a black hole.

Sagittarius A* has played a major role in the evolution

of our galaxy 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*

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 prisms 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.

EXPLOSION

STEPHEN HAWKING: It is said that fact is sometimes

stranger than fiction, and nowhere is that more true

than in the case of black holes.

Black holes are stranger than anything dreamed up

by science fiction writer,

but they are firmly matters of science fact.

EILEEN COLLINS: And we copy. Go for deploy.

You look out and this thing is so big,

you certainly know that it's moving 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.

There is nothing as beautiful as Chandra sailing 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 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.

RUMBLING

Ripped apart and burning up in a fireball

300 times brighter than the Sun.

The culprit?

Sagittarius A*.

The asteroid's destruction was our galaxy's black hole

signalling its presence to the world.

20 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* 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*

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 burned 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* 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.

And in fact, this river provides a beautiful

and surprisingly accurate analogy.

You 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 could 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* is.

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

From the moment the black hole formed,

the seed of Sagittarius A* 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.

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 - curved and distorts

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* 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* 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* 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*,

they became locked in a gravitational embrace...

..spiralling towards each other, approaching half the speed of light.

Before colliding.

Sagittarius A* cannibalised its cousin...

..creating ripples in the fabric of the universe itself.

More 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* at its core.

Sagittarius A* 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* 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 gluttons period

that marks 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* 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*'s eruptions.

Engine start.

One, zero!

Lift off.

The Delta rocket 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*.

Think about that.

Sagittarius A* 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*'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 centre 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* 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*

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* 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

the 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* turned what is potentially a violent region

of our galaxy into a peaceful one.

The warm gases pushed out by Sagittarius A*

slowed the rate of star formation.

And, 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*,

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* is a sleeping giant...

..a brooding beast...

..operating on a slow simmer.

Sagittarius A*'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'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?

STEPHEN HAWKING: Black holes challenge the most basic principle

about the predictability of the universe

and the certainty of history.

Nothing can 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*.

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.

Inexorably, the singularity awaits.

The end of time,

where all paths terminate.

Over trillions of years, all the stars around Sagittarius A*

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.

BIRD CAWS

If nothing can ever escape from black holes, if Sagittarius A*

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, it's 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 timescales that are billions

and billions and billions and billions of times longer

than the current age of the universe,

Sagittarius A* will eventually evaporate away.

Very gradually, the Hawking radiation

will erode Sagittarius A*.

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

"some time 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 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?

STEPHEN HAWKING: Black holes ain't as black as they are painted.

They are not the eternal prisons they were once thought.

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.

If information somehow escapes from Sagittarius A*

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*

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*

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 aeons 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* 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

# 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,

and 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's 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 a night launch is particularly exciting.

You have a go for start.

We have booster ignition and 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. 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 engines 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

and I go ahead and pull the switch marked 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, if... 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* 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 may be 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 in to the black hole

that it rips the star apart, it shears it, right?

And then the corpse of that star,

this sort of spaghettified matter that starts spiralling 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.

Next time...

..we journey back to the cosmic dark ages,

exploring a time before the Big Bang...

..to answer the ultimate question.

How did the universe come to be?

Journey through the universe with the Open University

and learn more about stars, planets and galaxies with this free poster.

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# I'm just a soul whose intentions are good

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

# Don't let me be misunderstood

# I try so hard, so please, don't let me be misunderstood... #