Who's Afraid of a Big Black Hole? Horizon

There's something deeply disturbing in deep space.

Something so incredibly massive, it could swallow an entire star.

People tend to be fascinated by things which are big and scary, like dinosaurs,

and there's really nothing that's bigger and scarier than a black hole.

Black holes are one of the most destructive forces in nature.

But far from being monsters, scientists now believe

they could hold the key to the greatest mystery of all...

..where the universe came from.

Black holes are the doorway to understanding the basic laws of the universe around us.

The trouble is, they're practically invisible and billions of kilometres from Earth.

We think right there is a black hole.

Right there.

The more we try to understand them, the stranger black holes become.

Everything we know about common sense is thrown out the window.

The equations no longer make any sense.

Black holes could force us to abandon everything we thought we knew about the universe.

There aren't questions much bigger than this.

There's really a lot that we don't understand.

We humans have evolved to make sense of planet Earth and, so far, we've made a pretty good stab at it.

In the last century, we've made sense of the impossibly small...

..and the unimaginably large.

The enormity of space, and the microscopic behaviour of atoms.

Yet there are some things that threaten to elude us completely.

The harder we look, the more questions we uncover.

Nowhere is this more true than for a black hole.

I think of a black hole as the symbol of what it is we don't understand about the universe.

Black holes are one of the most mysterious objects in the cosmos.

-What are black holes made of?

-Oh, OK. Already you've asked me a question that I can't answer.

They fell out of Einstein's theory of relativity in 1916,

and they've defied some of our greatest minds ever since.

Are black holes made of anything?

Black holes... Hmm.

We don't really have any idea what's going on, so....

I don't understand black holes. I love black holes.

I love black holes because I don't understand them.

There are many strange things in this universe, but I think I've picked

the weirdest thing to actually study which is the black hole.

Until recently, there wasn't much evidence they existed at all,

because while we think they're out there, we can't see them.

Black holes are, by definition, completely black.

Nothing can escape it, even light,

and that's why it's called a black hole, because light can't come out of it.

Black holes, totally mysterious, billions of kilometres away

and practically impossible to see.

Not that that's stopped astronomers trying.

Doug Leonard even thinks he's seen one,

or at least seen one form.

It took two years and the Hubble space telescope.

It was only possible at all because we think black holes

begin their lives as something we've all seen in space - stars.

Stars, like our sun, are essentially big, hot balls of gas

that have nuclear generators in their core, that create all the heat and light that we see shining.

Stars are enormous.

You could fit a million Earths inside the sun, and the sun is not even an abnormally large star.

But the most fascinating thing to me about stars is that they die.

The theory is, black holes are born when nature's most massive stars

burn off all their fuel and violently collapse.

The cores of these massive stars implode in less than a second.

They go from something about the size of the Earth,

down to something about the size of a small city.

And they don't stop there, they continue imploding all the way down to a point.

That point is what we believe becomes a black hole.

And it's this process that Doug Leonard believes he's spotted

when a massive explosion, supernova, signalled the death of a star

in a remote galaxy billions and billions of kilometres from Earth.

This is a picture of a galaxy 215 million light years away

and, indicated by the arrow, this is the supernova,

a single star that exploded that, for a short period of time, is as bright as the entire galaxy that it's in.

And that big blob there is the galaxy?

This big blob here is the combined light

of tens of billions of ordinary stars.

This is a close-up, an extreme close-up, of the supernova

while it was still very, very bright. Once the supernova was discovered,

we trawled the Hubble space telescope archives

and found a picture of this exact spot taken eight years earlier,

and what we found at the location of the supernova

was this object, which is actually an extremely bright star.

So what we did next was wait.

For two years, we waited for all the fire arcs of this supernova explosion to disappear and go out,

and we went back and took another picture

of that exact spot in the sky, and what we found was nothing.

The star was gone. It exploded as a supernova and had now disappeared.

And we think right there is a black hole.

Right there.

But I can't ever be 100% sure about that.

Is that because you can't see it?

Seeing nothing in black hole science is a great thing.

You don't expect to see anything when you're looking at a black hole.

As images of black holes go, these few dark pixels are about as good as it gets.

Without the death of a star, there'd be no reason to suspect there was a black hole there at all.

In fact, black holes are so hard to see, most of what we know about them hasn't come from those observing

the universe but from another group of scientists - the theorists.

And the universe they study is in their heads.

I think of theoretical physics

really as a great detective story that you get to be part of.

The clues look so few and scant that it seems like a hopeless case,

but if you work really hard at it, often you can discover amazing stuff.

So it's amazing to me how much one can actually learn about reality just by detective work.

Black holes have existed in theorists' minds and notebooks for almost a century,

most notably in the mind and notebook of Albert Einstein.

In 1916, Einstein changed the way we see our world.

Purely by the power of thought, and some clever mathematics,

he explained something we all take for granted - gravity.

Gravity is the universal force which holds everything together.

If you were to shut off gravity right now, the sun would explode,

the Earth would fall apart, and we'd be flung into outer space at a thousand miles per hour.

So it's gravity that keeps us rooted onto the Earth

and holds and binds the galaxy and the solar system together.

Scientists had been able to calculate the effects of gravity for centuries.

But until Einstein, what caused it had remained a mystery.

The answer was stranger than anyone had imagined.

Einstein's great insight was to realise that gravity is caused by the bending of space and time.

So gravity is not really pulling me down to the ground, it is space that is pushing me down.

Einstein called his theory general relativity.

The theory of relativity is infamous for its difficulty.

I want to show that there's nothing peculiarly difficult about it.

Space isn't simply an empty void,

it can be bent and stretched.

Let me illustrate this one example.

Let's imagine that this piece of jelly is the space, then the presence of matter is to distort the space.

All massive objects like stars and planets bend the space and time around them.

Any object that passes through that warped space time will move as if being pulled by a force,

and this is what we experience as gravity.

Einstein's theory of relativity does lead us into very strange and unfamiliar paths.

Relativity is perfectly intelligible to anybody who is willing to think.

Einstein's theory has withstood the test of time for almost a century.

If there is one data point out of place, we would have to throw the entire theory out.

Everywhere we look in the heavens, Einstein's theory comes right on the spot.

But less than a year after it was published, theorists realised

general relativity predicted something so profoundly troubling,

many believed it couldn't exist in the real world.

Anything very heavy and very small would create such a strong gravitational field

that space and time would be bent and twisted to breaking point.

General relativity had predicted the existence of black holes.

And it didn't just say that they would exist...

general relativity allows us to imagine what it would be like to travel into one.

There's a beautiful analogy between black holes and waterfalls

which actually lets us calculate all properties of black holes exactly.

When you approach a waterfall, the river flows faster and faster.

When you approach a black hole, it's not the water that flows faster, it's space itself.

The structure of a black hole is similar to the relentless flow of water over a waterfall.

It's an analogy that follows the water from the river above to the rocks below

and allows us to journey into the very heart of a black hole.

If you're swimming upstream from a waterfall,

there is an invisible line where the water flows as fast as you can swim,

and if you cross that line, it's the point of no return.

You wouldn't feel anything special, but no matter how hard you struggle,

you can never escape getting sucked all the way down.

For a black hole, the point of no return is called the event horizon.

Past it, space is travelling inwards faster than the speed of light.

Even if I can only swim at a maximum speed, the water can obviously fall much faster than that.

In the same way, even though I can never go faster than the speed of light through space,

space itself is allowed, in the black hole, to fall as fast as it wants,

which means that everything that's there, even a particle of light trying to go upward,

will be sucked inexorably downwards towards the centre.

Assuming your body withstood the intense gravity,

leaving the universe forever could be remarkably uneventful.

People used to think that you would die at the event horizon, but we now understand that for big black holes,

it's perfectly possible to still be alive at this stage, you just have no choice but to continue downward.

Everything would feel just normal to you, you wouldn't even know necessarily that you're doomed.

The only thing is that there's no way you can ever get out again.

As you approach the centre of the black hole, you reach the inner horizon,

where everything falling in meets matter being pushed out by the hole's rotation,

similar to where the torrent flowing over the falls hits water rebounding back up.

Eventually, the inward flow actually slows down to become slower than the speed of light,

because the rotation of the black hole causes a sort of repulsion.

At that point, you have things colliding together near the speed of light,

creating these ridiculously high temperatures, much hotter than inside of a star.

So hot that it would vaporise me and any ordinary matter.

So that makes an ordinary traffic accident seem tame in comparison,

now you're being hit by a truck going almost 300,000km per second.

It's not a place where I would wanna be.

The inner horizon is one of the most extreme environments in the universe.

According to general relativity,

the only place more extreme is what lies beyond it.

Let me gather my thoughts for a moment.

It's remarkably difficult for us

to actually calculate with Einstein's equations what happens inside the inner horizon.

But if I jumped into a black hole, that's probably as far down as I would get.

At the centre of a black hole,

the equations predict something so strange,

it blows Einstein's greatest achievement out of the water

and forces us to question our understanding of the universe.

Einstein hoped that general relativity would form the framework for a new understanding of nature.

But at the heart of its description of a black hole,

theorists found a problem with Einstein's mathematics.

Something so disturbing, his theory breaks down completely.

Einstein's equations of general relativity simply say the following -

the Ricci curvature tensor minus one half the metric tensor,

times the contracted curvature tensor is proportional to the stress energy tensor.

All this says that if I start with a star, a black hole, or even a universe,

that determines the curvature that surrounds that concentration of matter and energy.

But inside these equations, there's a monster.

In the extreme gravity of the core of a black hole,

Einstein's equations spiral wildly out of control.

After every long tedious calculation, I mostly get zeros but the non-zero term is given as follows...

M is the mass of the black hole, R describes the distance from the black hole...

Here is the problem, right there... when R is equal 0...

The point at which physics itself breaks down.

So one over R equals one over 0 equals infinity.

To a mathematician, infinity is simply a number without limit.

To a physicist, it's a monstrosity.

It means that gravity is infinite at the centre of a black hole, that time stops. And what does that mean?

Space makes no sense, it means the collapse of everything we know about the physical universe.

In the real world, there's no such thing as infinity,

therefore there is a fundamental flaw in the formulation of Einstein's theory.

According to Einstein then, all the mass of the black hole is contained

within an infinitely small point that takes up precisely no space at all.

This impossible object of infinite density and infinite gravity is called the singularity.

We know what a singularity is.

A singularity is when we don't know what to do.

To me what's so embarrassing about a singularity

is that we can't predict anything about what's gonna come out of it.

I could have a singularity and - boom - out comes a pink elephant with purple stripes.

And that's consistent with what the laws of physics predicts, because they don't predict anything.

A singularity is when our understanding of nature breaks down, that's what a singularity is.

Einstein realised there was a problem when he was shown this infinity,

but he thought that black holes could never physically form, therefore it was an academic question.

Sure, there was a problem, but it didn't matter because mother nature could never create a black hole.

In 1939, Einstein even wrote a paper

that appeared to prove black holes would never be found in the real world.

He hoped that there'd be some physical mechanism that would stop them from actually being produced.

And he really wanted to ask the question

could they physically form? I think he wanted to show the answer was no.

Given the physics known at the time, his assumptions were reasonable,

but we've learned a lot of physics since then so therefore we know that his reasoning was incomplete.

At the time, no-one had seen anything to suggest Einstein was wrong.

For years, theorists were happy

that general relativity was a complete understanding of gravity in our universe.

Then, in the early 1970s, astronomers made a breakthrough.

X-rays revealed hot gas falling into objects

that were both extremely massive and invisible to normal light.

For some, these images could only be caused by black holes.

Material on the way into the black hole can become very hot.

So hot that it becomes a million degrees or even ten million degrees, and that makes x-rays.

And just before this lump of material disappears in the black hole,

it becomes a bright flash of x-ray radiation.

Professor Reinhard Genzel is Director of the Max-Planck Institute for Extraterrestrial Physics.

He's spent the last 25 years looking for proof of the existence of one particular black hole.

While we can't see black holes as such, we can see that they're there and what they are

through their interaction

with visible objects like stars, like gas in their vicinity.

Using radio telescopes,

astronomers had also seen objects at the centres of galaxies they suspected were black holes.

But to prove it, they'd need to make more precise measurements.

Unfortunately, the nearest one was 25,000 light years away

and totally obscured by dust.

It was at the centre of our own galaxy.

It took Genzel and his team nearly ten years

to develop an infrared telescope capable of seeing enough detail

through the clouds of dust and gas surrounding the galactic centre.

It took them a further 13 years of painstaking observations before they saw the thing they were looking for.

A star orbiting exceptionally close to the centre.

Genzel knew that measuring the star's orbit could tell him about whatever it was orbiting.

So what we are seeing are the innermost stars.

This green cross, that's the centre of the Milky Way, Sagittarius A star.

So in 2002, this star here was very close to this

and the next year, it has moved quite a substantial distance.

Because the galactic centre is so far away,

this minute change means the star is moving incredibly fast.

The separation which you see is quite an enormous distance, these are several light weeks.

And how far is that in kilometres?

OK...

So we have an hour, and we have a day, and then take a week,

then we have the speed of light...

and so in kilometres, OK...

Wow, is that a big number - 180 billion kilometres.

Let me just check this so...

Yeah, a 180...180 billion kilometres.

I can't deal with that number.

It's hard to imagine what a 180 billion kilometres is.

Once you know the size of a star's orbit and the time it takes to go round,

it's a relatively simple calculation to work out the mass of the object it's orbiting.

Although tracking a single star would be enough to measure the mass of the central object,

Professor Genzal has mapped the orbits of the 30 stars closest to the galactic centre.

Here we have the innermost stars.

And these orbits we determine uniquely from the motion we have tracked over the years.

So it takes S2, this innermost star,

15 years to move once around the centre of the Milky Way here.

The other stars are slower, some of them take several hundred years to move around.

From the size of each of these orbits and the speed the stars were travelling,

Professor Genzal calculated the mass of the central object and it was truly astronomical.

From these two numbers, you already can determine uniquely the central mass,

and we can do this for each of these stars,

and we find that the mass is always the same.

It's four million times the mass of the sun.

Because the closest stars pass so near to the centre,

this extraordinary mass, four million times heavier than the sun, must be in a very small space.

That really clinches this. Because nothing fits in there,

into this relatively small volume other than the massive black hole.

Even a schoolchild can analyse the data and will come to the same conclusion, it's very clear.

What Genzel had found at the centre of our galaxy

was so heavy and so small, it had to be a black hole,

but it was far too big to have formed from the collapse of a single star.

The black hole at the centre of our galaxy is an object

which is much more massive than the stellar black holes.

It's about four million times the mass of the sun.

So we would call these super massive black holes.

Although Professor Genzel hadn't seen a black hole,

the indirect evidence was so compelling

there could be little doubt black holes were real

and it won him the 2008 Shaw Prize for Astronomy.

So the prize, the Shaw prize, is a fairly large amount of money,

actually a million dollars,

which was given to me and with no strings attached.

So I've given some of it away to my colleagues,

some of it I kept myself and, you know, people have convinced me

I should use some of that to buy a new car.

Everything in our galaxy, the Earth, the sun, a million million stars,

are all spinning around the super massive black hole at the centre.

And ours isn't even particularly impressive.

The super massive black hole at the centre of our galaxy is quite small relative

to other super massive black holes that we know about.

There are galaxies, not very far from ours,

in which we have seen super massive black holes

up to a thousand times more massive, several billion solar masses.

It now appears there's a super massive black hole at the centre of almost every galaxy.

And it could be that these black holes aren't simply agents of destruction,

because scientists have discovered a unique relationship they share with their parent galaxy.

So the mass of the super massive black hole

is related to the mass of the parent galaxy in a very simple way,

so I can show this with a graph here.

So let me say, along one axis, I'll show the mass of the black hole.

And I will measure this mass in terms of the mass of the sun.

So let's say down here it is a million times the mass of the sun.

Ten million, 100 million, billion times the mass of the sun,

so that's the range of black hole masses we have seen.

Along this axis, let me just show you the mass of the galaxy.

Let me start with a billion times the mass of the sun...

ten billion, 100 billion, a million million solar masses.

Basically, when people measure these two masses for a large number of galaxies,

what they find is different galaxies may come different places here on this diagram.

And the miraculous thing is that all these points seem to lie

more or less on a straight line in this plot.

So there seems to be a... some relation between the mass of the black hole and the galaxy.

Roughly, the black hole seems to be approximately

a thousand times less massive than the galaxy in which it lives.

The existence of this kind of a relation is rather surprising, because what it means is

somehow the black hole is able to influence the entire galaxy

and is actually modifying perhaps how the galaxy forms and evolves.

This is the surprise in this business.

In the last century, black holes have gone

from being mathematical curiosities to real objects in the cosmos,

millions of times the mass of the sun and seemingly crucial to the formation of galaxies.

I think black holes have got maybe a little bit of a bad rap

as being the ultimate bad guys in the universe.

It might well be that the monster black holes in the middle of galaxies

actually helped the galaxies form and therefore helped life come on the scene.

As well as super massive black holes,

astronomers believe there are also billions of smaller stellar black holes all over the cosmos.

-How many black holes are there?

-Roughly every galaxy has got one big black hole in the middle,

super massive black hole, and millions and millions of smaller black holes.

Black holes are common, they're a very common occurrence

in nature, fantastic thing. Would we have thought it? No.

Think of all the galaxies, each one with a raging black hole in the centre.

Each one with perhaps thousands of stellar black holes in them

and then you begin to realise that black holes represent

one of the dominant forces in the evolution of the universe.

Black holes, it turns out, are everywhere.

And that means millions upon millions of places where Einstein's equations break down.

But physicists have always known that relativity is an incomplete theory of nature.

Although it beautifully describes how gravity influences the motions of planets, stars and galaxies,

it can never describe the world at the smallest possible scale.

The realm of atoms and the tiny particles that form them.

To do that, they use a separate theory.

A theory called quantum mechanics.

You might wonder why we'd wanna apply quantum mechanics

to something as large as a massive black hole,

when quantum mechanics deals with the very small.

And that's because, ultimately, at the heart of a large black hole is a singularity.

Whatever a singularity really is, one thing we do know is it must be very, very small.

It seems quite likely that, in order to really

understand what goes inside a black hole, we will need quantum mechanics,

that the final story of how a black hole works

and what happens at the singularity

can only be understood when quantum mechanics is included.

This subatomic world quantum mechanics describes is nothing like the world we experience.

Quantum mechanics tells us how the world works at a fundamental level

and it is stranger than you can imagine.

In the quantum world, the mere act of observing changes what you see.

You can't say where something is, only where it's likely to be

and anything that is possible, no matter how unlikely, happens all the time.

All of our notions about how things behave change.

For example, an object has a known location,

"I'm here, you're there," but at a quantum mechanical scale,

objects can be in many different places at the same time, literally.

Yet as strange as quantum mechanics is, theorists

believe the world it describes is the true nature of reality.

Quantum mechanics is so weird, it may sound like science fiction,

but it's not science fiction, it's science fact,

and it's done better than any other idea in physics.

It allows us to make the best predictions we've ever made,

so like it or not, it describes the world.

Quantum mechanics describes everything, there's no escaping quantum mechanics.

Every object is a quantum mechanical object subject to the laws of quantum mechanics.

And the world that we live in,

in the ultimate reality, is a quantum world.

So there's no question that there's some great truth in quantum mechanics.

But there's one thing quantum mechanics can't describe -

gravity.

And it's not normally a problem, because atoms are so light, the effect of gravity is irrelevant.

Most of the time, quantum mechanics and gravity leave each other in peace.

But there's one arena in which they're both important,

and that arena is when things are both very small

and the force of gravity is very large.

And that's what happens inside a black hole.

The singularity at the heart of a black hole is both astronomically heavy and infinitesimally small.

To understand it, quantum mechanics alone wasn't enough.

It needed to be extended to describe gravity.

A theory called quantum gravity.

The most obvious way to create such a theory

was to make a quantum version of Einstein's theory of relativity.

Proof of its success would be a new understanding of black holes

that explained what really happens in a singularity.

When physicists tried to combine the two theories, they encountered a familiar problem.

I insert this into the probability

that gravity will move from one point to another point.

When I actually do this calculation, I get yet another integral,

and when you do this integral,

you get something which makes no sense whatsoever -

an infinity.

Total nonsense!

In fact, you get an infinite sequence of infinities,

infinitely worse than the divergences of Einstein's original theory.

This is a nightmare beyond comprehension.

The search for a theory of quantum gravity had fallen apart,

because quantum mechanics and general relativity proved to be totally incompatible.

I think the most embarrassing problem we have in theoretical physics is that

we have these two different theories which won't talk to each other.

We have Einstein's theory of gravity, which beautifully describes the very big and the very fast,

and then we have quantum physics, which very successfully describes

the very small and yet, clearly, nature has one unique way

of operating, it's not schizophrenic,

and we humans just don't seem to be able to find that way.

The failure of these two great theories to understand black holes

means they are, at best, an approximation to the laws governing the universe.

The equations no longer make any sense

and nobody knows exactly what we're supposed to do about that.

Well, it's awful.

It means that physics is having a nervous breakdown.

It means the collapse of physics as we know it, you know?

Something is fundamentally wrong.

Nature is smarter than we are.

If we want to understand the universe,

we must understand how quantum mechanics and gravity

can live together and so that's our challenge.

So it's quite a big question?

It's a huge question.

There aren't questions much bigger than this.

We don't understand.

For nearly 100 years, physics has been able to explain the universe around us.

General relativity perfectly describes the motions of stars and galaxies.

And the world of atoms is beautifully explained by quantum mechanics.

Yet the discovery of black holes means we don't fully understand anything.

But far from being a problem,

black holes represent one of the greatest opportunities in physics.

Black holes are the key to... taking the next step,

the doorway to our next step

in understanding the basic laws of the universe around us.

Unlocking the mysteries of black holes could provide

the answer to the biggest question every posed by the human mind.

Because there's one other place where our current laws of nature

fail as dramatically as they do in a black hole.

Any direction you look up from the Earth at distant galaxies,

every single one of them is moving away from us.

And the only way to make sense of that is to think of the entire universe just expanding.

This much we know and have known for 80 years.

But then, there is an immediate very profound implication.

If the universe is expanding, long ago it was much more compact.

Nearly 14 billion years ago, Einstein's theory says the universe began in the Big Bang.

So just to get an idea of the scale of the universe,

let's start with the Earth, which is a pretty big object.

The sun is about a million times more massive than the Earth

and most stars that we see in the sky are about the size of the sun

and our galaxy has roughly a million million of these stars.

And then the universe has roughly a million million galaxies.

So that's a huge amount of stuff and all that started from a singularity.

A point from which an initial explosion got the expansion going. That's the Big Bang.

For me, it's a weird concept, as weird a concept

as it would be to any person who's hearing about it for the first time.

But nature is doing it, so that's what makes this exciting.

The singularity, the impossible object found at the heart of every black hole,

is the same impossible object found at the very beginning of time.

The whole universe came out of a singularity, all of us are the product of a big singularity.

And so these singularities are very, very interesting for many reasons.

There are two places in nature where there apparently are singularities.

One is at the centre of a black hole

and the other is at the beginning of time itself at the Big Bang.

So it's quite likely, if we understood the singularity associated with the black hole,

we might resolve the question of how the universe began and where we came from.

Black holes could hold the key to understanding what there was before the universe existed.

But while we might seem tantalisingly close,

black holes and the theory that explains them remain just out of reach.

Quantum gravity is the name that we give to the solution to this problem.

We don't really know what quantum gravity is.

What's frustrating with quantum gravity is that previous revolutions in physics,

like quantum mechanics, relativity theory,

were all brought on by a lot of clues from nature

and, for quantum gravity, we have almost no clues at all.

Right now, we're mostly stuck with having to figure this out

with pencil and paper just from theory.

The trouble is, although we know black holes are everywhere,

we've never seen a single one directly.

-Have you ever seen a black hole?

-No.

-Have you ever seen a black hole?

-No.

No-one has ever seen a black hole directly.

Here is an object in outer space that is beyond our mathematics,

beyond our physical theories, demanding a theory beyond Einstein.

And, ironically, we can't see them.

But according to general relativity, a black hole won't just create a dark shadow in space,

this shadow would be surrounded by a bright halo.

A black hole's immense gravity warps the space around it,

focusing the starlight coming from behind into a ring.

And, in theory at least, we might even be able to see it.

You can see how they warp with the space around them.

Shep Doeleman is aiming to do just that.

He's devoted his career to making the first direct observations of a black hole.

I happen to really like making the observations,

getting things done, that there's a real joy

in assembling a new kind of telescope.

There's a real joy in making a new kind of measurement that no-one has ever made before.

I guess that theoreticians feel the same way when they think of an idea that nobody has thought of before.

Shep is attempting to take a picture of a shadow cast in space

by the super massive black hole at the centre of our galaxy.

Directly observing how and where general relativity fails

could provide vital clues for the theory that replaces it.

Our observations are aimed squarely at testing general relativity

in one of the most extreme environments in the universe -

the event horizon of a black hole. And it's there

that Einstein's theories may break down.

For quantum gravity, seeing the shadow exactly as predicted by Einstein would be of little use.

If we see something that is not consistent with general relativity,

the theorists will be extremely interested and will want to know everything about that

and that will point them in a new direction for a theory of gravity.

We could look at the centre of our galaxy,

see something completely unpredicted around this black hole that would send us back to the drawing board.

Shep is an astronomer at the Haystack Observatory near Boston.

But the 37-metre telescope here simply isn't big enough

to photograph the black hole at the centre of our galaxy.

To do that, Shep needs a telescope with 100,000 times the resolution.

And that requires a dish 4,500 kilometres across,

roughly the size of the continental United States.

To observe the object we're after, we have to create a telescope

that can see finer details than any other telescope in the history of astronomy.

The reason you haven't heard about this massive telescope is because it only exists in Shep's computer.

He hooked up radio telescopes from across the continent, effectively to product one giant virtual dish.

The way a normal telescope works is it focuses all the light

because of its particular shape into a single focal point.

When you link telescopes around the world together, we don't have a lens.

We have to do it in a super computer here in Massachusetts.

Shep's super computer, the correlator,

pieces together the raw data from all his separate telescopes

to build up a computer-generated dish the size of America.

The level of detail you can see with a single dish is limited by the size of that dish.

But when you link telescopes around the world together, something magic happens.

You create a virtual dish that's as big as the distance between those dishes,

and that gives a level of detail that's a thousand times finer than you can get with a single dish.

Instead of creating pictures, each of Shep's telescopes produces reams upon reams of data.

And this is where we keep all of the data when it comes back from the telescopes,

each of these contains eight very large hard disk drives and when you have two modules together,

that contains as much data as the US Library of Congress, the largest library in the world,

and we have on these shelves about 64 such libraries.

The amount of data is just staggering, really.

We've spent a lot of money in this project on disk drives.

There's so much data, processing just a few nights' observations takes months.

Hey, Mike, what's the latest from the correlator?

Ah, actually a lot of interesting things from last night.

You've got a full hour of direct detections on the galactic centre.

-These are great.

-Perfectly clear.

-These are great, looks like this is gonna be a great data set.

What about the other baselines? That's excellent, That is just excellent.

That's with zeroes, that's with no corrections.

That's beautiful, that is absolutely beautiful.

This gives me a lot of confidence we'll be able to do what we wanna do.

Despite producing all this data, Shep doesn't yet

have enough telescopes linked together to build up a full image.

Yeah, so this is a great data set.

This is... I'm very, very happy with this.

But this year, he might be able to detect our first glimpse

of something that has, until now, eluded us -

the shadow of the event horizon.

If someone said, "That's impossible, you can't do it,"

I would say, "That's our job to try and see things that can't be seen,

"to try to do things that are great challenges."

The reason that we're interested in this is, quite frankly, because it's hard.

And if you'd asked me five years ago if it was possible, I flatly would have said no.

Shep believes that, within ten years, his virtual telescope

will have the resolution to create an image of a black hole

and put relativity to the ultimate test.

That's very exciting for me to know that we're almost there

and that with just a little more effort, a little more ingenuity,

linking a few more telescopes together, we'll be able to see something extraordinary.

What would be the most exciting thing to see?

Would you rather be the guy who confirms Einstein's predictions or the guy who...?

Yeah. Well, look, nobody wants to be the person known as the one who disproved Einstein.

At the same time, it would be extremely exciting to be able to make some observations

that would speak directly to the validity of general relativity.

So either way, whether we see the shadow as the right size or we see the shadow as not the right size

would be incredibly exciting. I can't decide which would be the best.

Whether the breakthrough comes from a clue observed in the heavens or theoretical detective work,

most physicists believe we will eventually crack the question of quantum gravity

and produce a unified theory of everything.

A theory that could explain the singularities at the heart of a black hole

and may even provide the science to predict what happened before our universe existed.

I suspect that this is a case where we need

a new Einstein with a grand thought,

a completely new thought that suddenly makes sense of things.

Many people think it's never gonna happen, we humans just aren't smart enough.

If we one day succeed in finding this holy grail, these equations of everything,

that's when the real work begins to try and solve these equations and predict stuff

and that'll keep physicists out of harm's way for a long time, I think.

It doesn't dishearten me that we don't understand everything about the universe.

I find it wonderful and exciting.

It seems amazing that we can understand anything about the world around us.

It might seem as if it would be easier if things like black holes just went away,

-but then, where would the fun be?

-HE LAUGHS

We don't know what's out there.

People might give you an answer, but they'll probably be wrong.

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