Just two months ago, a major scientific discovery was announced.
Ladies and gentlemen, we have detected gravitational waves.
APPLAUSE We did it!
This detection was solid evidence of something Albert Einstein predicted
100 years ago.
But it was also the most direct observation ever
of black holes.
The discovery of gravitational waves has launched a new era
in the study of perhaps the most captivating
and powerful objects in the universe.
In science fiction, spaceships often go through a black hole
to another universe,
or another part of our universe.
But black holes are stranger than anything
dreamt up by science-fiction writers.
Tonight, we're joined by Stephen Hawking
to take a mind-blowing journey into the enigmatic world of black holes.
We're going to find out how the astonishing discovery
of gravitational waves, made just a few weeks ago,
is already helping us understand the fundamental nature of them.
In a truly extraordinary life, Professor Stephen Hawking
has become the world's most celebrated scientist.
And for over 40 years, he's wrestled with the toughest of questions.
What happens inside a black hole?
Black holes are formed by the collapse of massive stars
when they have exhausted their nuclear fuel...
..and can no longer support themselves against
their own gravity.
They are quite literally holes in space that stuff can fall into,
but not get out of.
They are places where the gravitational field is
so strong that nothing, not even light, can get away.
This basic picture of black holes is very well-known.
But, unfortunately, it is also far from complete.
At the heart of the mystery of black holes
lie two fundamental problems.
Firstly, theorists have found it hard to understand what's happening
inside a black hole. In fact, the more they crunch the numbers,
the more it seems that black holes defy the laws of physics.
In particular, it throws up a mind-boggling conundrum
known as the information paradox.
Secondly, astronomers trying to observe these unusual objects
and find physical evidence with which to test these theories,
hit an apparently insurmountable problem -
you can't see inside a black hole.
The maths don't add up, and no-one's ever
seen inside a black hole, so how do we know they even exist?
Also, what kind of discovery can hope to answer
all of these mysteries?
Well, with the help of Professor Hawking,
we're going to try and answer all of those questions.
So, we've come here to the University of Cambridge,
home of Stephen Hawking and, arguably,
the birthplace of black hole science.
In tonight's programme, we'll hear from Stephen Hawking himself,
how he transformed our view of black holes.
Black holes were thought to be completely black,
until I discovered that they glow like hot bodies.
I'll be finding out from cosmologist Andrew Pontzen
why Hawking's black hole theories created one of the
biggest conundrums in astrophysics.
With Hawking radiation, very slowly, a black hole
gradually shrinks away until it's completely gone.
There's no trace of the Earth, no evidence
that it ever really existed, let alone what it was made up of.
And I'll be meeting one of the team behind the wonderful discovery
of gravitational waves, exploring what they tell us
about black holes.
We know its mass, we also know that it's spinning.
It's spinning about 100 times a second.
We know it's about the size of Iceland,
it's not spherical, it's actually an ovoid.
But we'll start with that astonishing discovery of
gravitational waves.
Unravelling the secrets of the universe,
the most important scientific discovery for a generation.
Scientists in the United States have announced they have discovered
gravitational waves.
Einstein was right after all.
Gravitational waves ripple through space and time.
The discovery of these elusive waves is the end of a search
that began with Einstein's work 100 years ago.
But it's also the beginning of a new way of seeing
what had previously been invisible in the universe,
revealing remarkable objects, like black holes.
The idea that black holes might exist was suggested
here in Cambridge as far back as 1784.
The Queen's College Don, John Michell,
was fascinated by the idea of extreme gravity,
and he imagined that a big enough star might generate
a gravitational pull that was so intense,
not even light could escape.
John Michell called these objects "dark stars"
and he realised that we couldn't ever see them directly.
But he thought we might be able to detect them
by watching for their gravitational effects on objects around them.
Sadly, his 18th-century colleagues ignored him and dark stars
were lost to the world for the best part of 200 years.
That is until the arrival of Hawking and a new wave
of black hole physicists.
They were intent on understanding some of the surprising implications
of Einstein's theory of general relativity.
When physicists began to explore Einstein's theory of
general relativity, they found that it made all sorts of
bizarre predictions, and one of the weirdest
was the existence of black holes.
To see how black holes were predicted by general relativity,
you need get to grips with the concept at the heart
of Einstein's theory - space-time.
Einstein insisted that time
and three-dimensional space weren't actually separate at all,
but they were woven together into the four dimensions of space-time.
What's more, space-time was distorted by mass, just as this ball
distorts my sheet, causing a curved dip.
Just as our dip causes a second ball to roll towards it,
distortions in space-time cause objects to fall together.
And that is what we feel as the pull of gravity.
This curving of space-time elegantly explains how planets
and spaceships orbit around a large mass.
It also reveals they can only overcome that pull
and escape from orbit if they can travel fast enough
to make it all the way up the side of the hole.
Pretty straightforward so far.
But things get interesting when you consider really dense objects -
the sort of thing that cosmologists consider result from
the death of a giant star.
If it was massive enough, a star like this would collapse
under its own gravity and, according to general relativity,
form a singularity.
A speck of infinite density
at the centre of a bottomless pit in space-time.
In order to escape the steeply sided curve of a hole like this,
you needed to be travelling faster than the speed of light.
Now, nothing can travel that fast, so nothing - not even light -
can escape a hole like this...
a black hole.
But even though general relativity implied that black holes should
exist in theory, most physicists didn't much like the idea.
It raised awkward questions,
like - what would happen to things once they'd fallen in?
And besides, the sort of dense, collapsed star that you need to
produce a black hole had never been observed.
But then in 1967, astronomers here in Cambridge,
discovered the first pulsar - an object powered by
a collapsed, dense star, the first hint that such objects
really existed and, suddenly, black holes were back on the agenda.
Here at Gonville and Caius College, a young Stephen Hawking took up
the challenge to take a fresh look at black hole theory...
..and right from the start, he managed to make waves.
Black holes were thought to be completely black,
until I discovered that they glow like hot bodies,
with a temperature that is higher the smaller the black hole.
This points to a deep and unexpected connection between black holes
and thermodynamics, the science of heat.
The trouble was, how could heat be coming from a black hole?
Nothing should be able to escape a black hole.
Hawking suggested a new form of radiation,
consisting of strange particles that exist in the bizarre world
of quantum theory.
Quantum mechanics implies that the whole of space is filled with
pairs of virtual particles and antiparticles,
that are constantly materialising in pairs,
and then annihilating each other.
Now in the presence of a black hole,
one member of a pair may fall into the hole.
The other particle may fall after its partner,
but it may also escape to infinity,
where it appears to be radiation emitted by the black hole.
This mind-boggling concept is now known as Hawking radiation.
And with it, Stephen Hawking had solved the problem
of how energy can escape from a black hole.
The trouble is, that in solving one problem, it created another,
bigger one - the information paradox.
In Einstein's most famous equation, E = mc2,
he showed that energy and mass are intertwined.
So a black hole losing energy must also be losing mass,
albeit very slowly.
The radiation will carry away energy from the black hole.
The black hole will lose mass and eventually disappear.
This creates a paradox,
because the information about what fell into the black hole
appears to be lost,
but the laws of physics say that information can never be lost.
"Information" is tricky - it's not things like names and stories
that can't be lost.
Maggie met up with cosmologist Andrew Pontzen to find out
what kind of information does cause the paradox.
Information is a critical thing in physics.
It's telling us where we are and how we are sitting,
what we're made out of.
We think of all that stuff as being information.
So these are the parameters governing the atoms of the universe?
Exactly. The idea is that if you know everything about the universe
today then you should be able to predict what will
happen in the future, or work through the equations backwards
and predict what happened in the past, but if you want to be
able to do that, then you need this idea of preserving information.
The problem is, if you imagine something falling into a black hole,
you or me, or maybe just put the whole Earth into a black hole -
in fact, I can do that for you now, if I take my...
Phew, it's just a piece of paper!
Not the real Earth, just a piece of paper.
Pop it into my black hole over here, for which I'm using a shredder.
Now, in principle, if you looked carefully enough inside
the black hole, you can imagine, you've got some bits and pieces
in there... So we could recreate this, and put it back together,
with a long time and a lot of Sellotape.
It would be boring but you could do it in principle,
so the information is still there.
However, with Hawking radiation,
very slowly, a black hole reduces its mass so over time, it gradually
shrinks away until it's completely gone -
there's no trace of the Earth, no evidence that it ever existed,
let alone what it's made up of.
And you can't retrieve that information in any way?
Exactly.
It seems like you wouldn't be able to get that information at all,
so there is no way you could work backwards
and work out that the Earth used to exist, it's just gone and lost.
So is there any way to resolve this?
We hope there is a way but right now, it's fair to say nobody knows.
People are trying to resolve it in lots of different ways.
Despite 40 years of effort,
the information paradox is still unresolved.
If Hawking radiation exists,
then we have to solve the information paradox -
we have to work out what happens to the information as it falls
into a black hole, and how they can evaporate without destroying it.
If on the other hand Hawking radiation doesn't exist,
then something's fundamentally wrong with our understanding
of black holes and perhaps even quantum theory.
The problem is that theorists have raced ahead with these ideas
about what black holes MIGHT be like,
but how do we observe that they exist at all?
The trouble is, we can never see them directly.
What we can see is some of the evidence that suggests
black holes are lurking out there.
In fact, some of it is surprisingly easy to spot with a telescope.
Pete Lawrence has spent a night on the hunt for black holes.
Various clues have been spotted over the years that seem to
signal the presence of black holes.
You just need to know where to look.
One area of interest lies in the constellation of Cygnus, the swan.
Here, there are several pieces of evidence which may suggest
the presence of black holes.
You can currently find Cygnus low in the north-east part of the sky
at about 1am.
In 1964, one of the earliest space telescopes -
a suborbital rocket fitted with a Geiger counter -
detected a flood of radiation coming from this part of the sky.
They narrowed the source down to about there...
and called it "Cygnus X-1".
On closer inspection,
it appeared that this was something incredibly small
and compact, producing an extraordinary amount of energy.
Astronomers put forward a theory to explain what they were seeing.
What initially looked like a single star actually turned out to be
a binary system - a large blue star with an invisible companion.
Could this be a black hole?
One explanation for all that energy being released,
was that gas from the visible star
was being sucked towards a black hole, creating immense friction
as it spiralled into what is called an accretion disk.
As it's heated, that gas would release a colossal amount of energy
and even bright X-ray flashes.
There are signs of much bigger black holes to look for too.
If we look back at Cygnus, there's an unremarkable patch of sky
in the western wing of the swan.
There's not much to see here visually.
But astronomers were astounded to see a bright radio source
emanating from this region.
The bright signals came from two jets of material spewing out
from either side of a distant galaxy, at tremendous speeds.
Such jets should take huge amounts of energy to produce -
like converting a million times the mass of the sun to pure energy,
more than the nuclear fusion that drives stars could ever produce.
The only known phenomenon that could convert matter to energy
that efficiently was what astronomers had seen
in the Cygnus X-1 system - accretion.
But this must be on a much, much larger scale -
so there must a massive, unseen black hole
at the centre of the galaxy.
Bright jets and accretion disks give us enough evidence
to suggest similar "supermassive" black holes exist
at the centre of almost all galaxies.
But if we look towards the heart of our own galaxy,
there's even more compelling evidence.
The centre of the Milky Way galaxy
is in the constellation of Sagittarius,
which, at the moment, is more or less behind me.
If you're out on a clear summer evening,
then this region of sky can be seen low down in the south.
Now, of course, we can't see the supermassive black hole
at the centre of our galaxy,
but we can see the effect it's having on the stars around it.
For 25 years, astronomers have tracked the motion of stars
at the heart of the galaxy.
And it's quite clear they're orbiting around something.
That something must be over four million times
the mass of our sun, squeezed into just 17 times its size.
And that's what we now call "Sagittarius A Star" -
our very own supermassive black hole.
With all the evidence we've seen over the years,
we're pretty convinced that black holes do actually exist.
But the next challenge would be to detect one directly.
Until this year, astronomers have relied
on this circumstantial evidence to deduce
almost everything that we know about black holes.
But all that changed just a few weeks ago,
with the first direct detection of not one, but two black holes.
It was these black holes that caused
the gravitational waves in February's big announcement.
'Professor Sheila Rowan is one of the team behind the discovery.
'I joined her to find out more
'and to see what it all means for black hole science.'
It is a very exciting time to be talking about
gravitational waves, but what exactly have we seen?
What we have seen is space vibrating, space shaking,
as picked up by the LIGO observatories in the United States
with a device that is about 4km long,
measuring motions 1/10,000th of the size
of a proton in the nucleus of an atom, so...
Quite phenomenal. Wow. Incredibly small.
This is the signal that we observed,
and what we saw was those vibrations speeding up, vibrating faster
and faster to a peak, and then the vibrations died down slightly.
And all of this detail, all of this vibration happens very quickly.
It does, it happens in about 0.2 of a second, but amazingly,
encoded in the vibrations, in that 0.2 of a second,
is information about the source of those vibrations.
Two black holes colliding about 1.3 billion light years away,
so 1.3 billion years ago,
and arriving with us here on Earth last September.
And that speeding up is actually the two black holes
spiralling in faster and faster until they eventually collide
to form a new black hole that then wobbles at a particular frequency,
and we can use that again to tell us about the properties
of the new black hole that has been formed.
See, this is, to me, as an astronomer,
the really exciting thing, to be able to make an observation
that tells you about the properties of the black hole itself.
Not about stuff falling into it, but actually about the black hole.
So what do we know about the two that merged
and what do we know about the state of the system now?
We can tell the masses of the two black holes that merged,
and one of them was about 36 times the mass of our sun.
The other one was about 29 times the mass of our sun.
The mass of that final black hole is about 62 times the mass of our sun.
If you add up the initial masses of the two black holes
that merged, in that final mass, you will discover there is
energy equivalent to three times the mass of our sun
that has gone somewhere,
and where it has gone is into gravitational waves. Wow.
In a short period, a huge amount of energy is produced,
more than the luminosity, the light power, of all the stars
and galaxies in the observable universe.
Wow. It's kind of amazing. OK.
So what else do we know about this newly-formed black hole?
We also know that it is spinning.
It's spinning about 100 times a second.
We know it is about the size of Iceland.
It is not spherical, it is actually an ovoid,
it's kind of squished in one direction
and stretched in the other slightly,
and spinning away, and all of that we can get from
this signal that we detect in gravitational waves.
And this is the first time we have been able to do anything like that.
I think the most remarkable thing is not just the signal,
but the fact that it represents
a huge amount of effort by a lot of people,
most of whom laboured for years without ever detecting anything.
How does it feel to be sitting here describing a real signal?
It feels amazing, and it is the first time
we have seen these binary systems.
They might never have existed.
And the black holes merging is the birth of a new black hole,
that is a unique signature that we see,
that we really couldn't see any other way.
So that is really fantastic.
This is incredibly exciting, but it is just one detection,
so what is next? It is just one detection,
but we can combine that with our models for how the universe is,
and that lets us calculate possible rates of these events,
how many we might expect.
It could be anything from a few a month to one a day,
and so that is phenomenally exciting,
that we may see a whole population of these events out in the universe.
And that's just from these two detectors.
That's just from these two detectors, that's right.
These are just part of a whole potentially new astronomy
using a whole set of different instruments to detect
gravitational waves across a wide range of frequencies,
and that is very exciting.
So more data coming? More data coming.
2016 is going to go down in history as a big year for black holes.
But now we know that they exist and can even detect them directly,
what does that mean for the information paradox
and Hawking radiation?
Critically, it's an opportunity for Stephen Hawking
to test his theories at last.
Especially his extraordinary idea that when black holes combine,
they make one new one, with more surface area
than the first two put together.
The signal LIGO detected came from the collision
and merger of two black holes in a black hole binary.
This should make it possible to experimentally test
my prediction the area of the horizon of the final black hole
is greater than the sum of the areas of the original holes.
This prediction is crucial
to our understanding of the thermodynamics of black holes.
By making sense of their thermodynamics,
LIGO and its successors could provide the first
experimental evidence that black holes DO glow
with Hawking radiation.
But could there be more direct evidence,
out at the very edge of the observable universe?
The cosmological horizon.
LIGO is not sensitive to the wavelengths at which there is
appreciable Hawking radiation from black holes.
However, there is likely to be another kind of
Hawking radiation of much longer wavelength
coming from the cosmological horizon
which might be detected by radio telescopes.
Longwave radiation like this would have to come from a type
of black hole formed right after the big bang,
in the early primordial universe.
It would be a unique kind of gravitational radiation.
I hope that radio telescopes detect primordial gravitational radiation
from the cosmological horizon.
That would mean black holes almost certainly emit radiation
and would get me a Nobel Prize.
If Hawking radiation can be proved to exist,
then there must be a solution to the information paradox.
And Professor Hawking thinks he has found that too.
He believes that objects falling in to a black hole
leave the information they carry behind.
It's stored on the hole's surface, which becomes turbulent,
a process called "super translation".
Last year, I realised that a black hole can store
the information in what is called super translations of the horizon.
I am now working with my colleagues Malcolm Perry at Cambridge
and Andrew Strominger at Harvard
on whether this can resolve the paradox.
If he is right,
Stephen Hawking has solved one of the greatest mysteries in cosmology.
But does that mean you can escape from a black hole after all,
or would it still be bad news to fall in one?
Definitely bad news.
If it were a stellar-mass black hole,
you would be made into spaghetti before reaching the horizon.
On the other hand, if it were a supermassive black hole,
you would cross the horizon with ease,
but be crushed out of existence at the singularity.
BLAST
Well, there's no doubt in my mind that with the detection
of gravitational waves, the future of black hole science looks bright.
And after 50 years of theoretical debates, Stephen Hawking might get
the experimental evidence he wants
to test his ideas about black holes.
And there is another exciting project on the horizon too -
the Event Horizon Telescope,
a worldwide network of radio telescopes that next year
will team up to try and capture the first image of the shadow cast
by the enormous black hole at the Milky Way's centre.
This would have been unimaginable
when Stephen Hawking started grappling with black holes
over 40 years ago, but it shows this is another chapter
in the history of black hole science.
Next month, we will be previewing
one of the astronomical highlights
of the year, the transit of Mercury,
visible across the UK on May 9th.
And we will be using the opportunity to take a closer look
at Mercury, one of the strangest planets in the solar system.
To find out how to catch a glimpse of it before then,
check out the website for Pete's April star guide.
In the meanwhile, get outside and... get looking up. Good night.