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Just two months ago, a major scientific discovery was announced. | 0:00:02 | 0:00:07 | |
Ladies and gentlemen, we have detected gravitational waves. | 0:00:07 | 0:00:12 | |
-APPLAUSE -We did it! | 0:00:12 | 0:00:14 | |
This detection was solid evidence of something Albert Einstein predicted | 0:00:16 | 0:00:21 | |
100 years ago. | 0:00:21 | 0:00:23 | |
But it was also the most direct observation ever | 0:00:23 | 0:00:26 | |
of black holes. | 0:00:26 | 0:00:28 | |
The discovery of gravitational waves has launched a new era | 0:00:28 | 0:00:32 | |
in the study of perhaps the most captivating | 0:00:32 | 0:00:35 | |
and powerful objects in the universe. | 0:00:35 | 0:00:38 | |
In science fiction, spaceships often go through a black hole | 0:00:38 | 0:00:42 | |
to another universe, | 0:00:42 | 0:00:44 | |
or another part of our universe. | 0:00:44 | 0:00:47 | |
But black holes are stranger than anything | 0:00:48 | 0:00:51 | |
dreamt up by science-fiction writers. | 0:00:51 | 0:00:53 | |
Tonight, we're joined by Stephen Hawking | 0:00:54 | 0:00:57 | |
to take a mind-blowing journey into the enigmatic world of black holes. | 0:00:57 | 0:01:02 | |
We're going to find out how the astonishing discovery | 0:01:02 | 0:01:05 | |
of gravitational waves, made just a few weeks ago, | 0:01:05 | 0:01:08 | |
is already helping us understand the fundamental nature of them. | 0:01:08 | 0:01:12 | |
In a truly extraordinary life, Professor Stephen Hawking | 0:01:41 | 0:01:44 | |
has become the world's most celebrated scientist. | 0:01:44 | 0:01:47 | |
And for over 40 years, he's wrestled with the toughest of questions. | 0:01:49 | 0:01:54 | |
What happens inside a black hole? | 0:01:54 | 0:01:57 | |
Black holes are formed by the collapse of massive stars | 0:01:57 | 0:02:01 | |
when they have exhausted their nuclear fuel... | 0:02:01 | 0:02:04 | |
..and can no longer support themselves against | 0:02:08 | 0:02:11 | |
their own gravity. | 0:02:11 | 0:02:12 | |
They are quite literally holes in space that stuff can fall into, | 0:02:17 | 0:02:22 | |
but not get out of. | 0:02:22 | 0:02:24 | |
They are places where the gravitational field is | 0:02:24 | 0:02:27 | |
so strong that nothing, not even light, can get away. | 0:02:27 | 0:02:32 | |
This basic picture of black holes is very well-known. | 0:02:34 | 0:02:38 | |
But, unfortunately, it is also far from complete. | 0:02:38 | 0:02:41 | |
At the heart of the mystery of black holes | 0:02:42 | 0:02:45 | |
lie two fundamental problems. | 0:02:45 | 0:02:47 | |
Firstly, theorists have found it hard to understand what's happening | 0:02:47 | 0:02:50 | |
inside a black hole. In fact, the more they crunch the numbers, | 0:02:50 | 0:02:54 | |
the more it seems that black holes defy the laws of physics. | 0:02:54 | 0:02:57 | |
In particular, it throws up a mind-boggling conundrum | 0:02:57 | 0:03:01 | |
known as the information paradox. | 0:03:01 | 0:03:03 | |
Secondly, astronomers trying to observe these unusual objects | 0:03:03 | 0:03:07 | |
and find physical evidence with which to test these theories, | 0:03:07 | 0:03:10 | |
hit an apparently insurmountable problem - | 0:03:10 | 0:03:13 | |
you can't see inside a black hole. | 0:03:13 | 0:03:15 | |
The maths don't add up, and no-one's ever | 0:03:15 | 0:03:17 | |
seen inside a black hole, so how do we know they even exist? | 0:03:17 | 0:03:21 | |
Also, what kind of discovery can hope to answer | 0:03:21 | 0:03:23 | |
all of these mysteries? | 0:03:23 | 0:03:25 | |
Well, with the help of Professor Hawking, | 0:03:25 | 0:03:27 | |
we're going to try and answer all of those questions. | 0:03:27 | 0:03:29 | |
So, we've come here to the University of Cambridge, | 0:03:29 | 0:03:32 | |
home of Stephen Hawking and, arguably, | 0:03:32 | 0:03:34 | |
the birthplace of black hole science. | 0:03:34 | 0:03:36 | |
In tonight's programme, we'll hear from Stephen Hawking himself, | 0:03:38 | 0:03:43 | |
how he transformed our view of black holes. | 0:03:43 | 0:03:46 | |
Black holes were thought to be completely black, | 0:03:47 | 0:03:51 | |
until I discovered that they glow like hot bodies. | 0:03:51 | 0:03:54 | |
I'll be finding out from cosmologist Andrew Pontzen | 0:03:57 | 0:04:00 | |
why Hawking's black hole theories created one of the | 0:04:00 | 0:04:03 | |
biggest conundrums in astrophysics. | 0:04:03 | 0:04:05 | |
With Hawking radiation, very slowly, a black hole | 0:04:05 | 0:04:08 | |
gradually shrinks away until it's completely gone. | 0:04:08 | 0:04:11 | |
There's no trace of the Earth, no evidence | 0:04:11 | 0:04:13 | |
that it ever really existed, let alone what it was made up of. | 0:04:13 | 0:04:16 | |
And I'll be meeting one of the team behind the wonderful discovery | 0:04:18 | 0:04:21 | |
of gravitational waves, exploring what they tell us | 0:04:21 | 0:04:25 | |
about black holes. | 0:04:25 | 0:04:26 | |
We know its mass, we also know that it's spinning. | 0:04:27 | 0:04:30 | |
It's spinning about 100 times a second. | 0:04:30 | 0:04:32 | |
We know it's about the size of Iceland, | 0:04:32 | 0:04:35 | |
it's not spherical, it's actually an ovoid. | 0:04:35 | 0:04:38 | |
But we'll start with that astonishing discovery of | 0:04:41 | 0:04:44 | |
gravitational waves. | 0:04:44 | 0:04:45 | |
Unravelling the secrets of the universe, | 0:04:46 | 0:04:49 | |
the most important scientific discovery for a generation. | 0:04:49 | 0:04:53 | |
Scientists in the United States have announced they have discovered | 0:04:53 | 0:04:56 | |
gravitational waves. | 0:04:56 | 0:04:57 | |
Einstein was right after all. | 0:04:57 | 0:05:00 | |
Gravitational waves ripple through space and time. | 0:05:00 | 0:05:03 | |
The discovery of these elusive waves is the end of a search | 0:05:07 | 0:05:11 | |
that began with Einstein's work 100 years ago. | 0:05:11 | 0:05:15 | |
But it's also the beginning of a new way of seeing | 0:05:17 | 0:05:20 | |
what had previously been invisible in the universe, | 0:05:20 | 0:05:23 | |
revealing remarkable objects, like black holes. | 0:05:23 | 0:05:26 | |
The idea that black holes might exist was suggested | 0:05:29 | 0:05:32 | |
here in Cambridge as far back as 1784. | 0:05:32 | 0:05:35 | |
The Queen's College Don, John Michell, | 0:05:36 | 0:05:38 | |
was fascinated by the idea of extreme gravity, | 0:05:38 | 0:05:42 | |
and he imagined that a big enough star might generate | 0:05:42 | 0:05:45 | |
a gravitational pull that was so intense, | 0:05:45 | 0:05:48 | |
not even light could escape. | 0:05:48 | 0:05:50 | |
John Michell called these objects "dark stars" | 0:05:54 | 0:05:57 | |
and he realised that we couldn't ever see them directly. | 0:05:57 | 0:06:00 | |
But he thought we might be able to detect them | 0:06:00 | 0:06:02 | |
by watching for their gravitational effects on objects around them. | 0:06:02 | 0:06:05 | |
Sadly, his 18th-century colleagues ignored him and dark stars | 0:06:05 | 0:06:09 | |
were lost to the world for the best part of 200 years. | 0:06:09 | 0:06:12 | |
That is until the arrival of Hawking and a new wave | 0:06:14 | 0:06:17 | |
of black hole physicists. | 0:06:17 | 0:06:19 | |
They were intent on understanding some of the surprising implications | 0:06:20 | 0:06:24 | |
of Einstein's theory of general relativity. | 0:06:24 | 0:06:27 | |
When physicists began to explore Einstein's theory of | 0:06:28 | 0:06:31 | |
general relativity, they found that it made all sorts of | 0:06:31 | 0:06:34 | |
bizarre predictions, and one of the weirdest | 0:06:34 | 0:06:37 | |
was the existence of black holes. | 0:06:37 | 0:06:39 | |
To see how black holes were predicted by general relativity, | 0:06:40 | 0:06:43 | |
you need get to grips with the concept at the heart | 0:06:43 | 0:06:46 | |
of Einstein's theory - space-time. | 0:06:46 | 0:06:49 | |
Einstein insisted that time | 0:06:52 | 0:06:53 | |
and three-dimensional space weren't actually separate at all, | 0:06:53 | 0:06:57 | |
but they were woven together into the four dimensions of space-time. | 0:06:57 | 0:07:01 | |
What's more, space-time was distorted by mass, just as this ball | 0:07:01 | 0:07:06 | |
distorts my sheet, causing a curved dip. | 0:07:06 | 0:07:10 | |
Just as our dip causes a second ball to roll towards it, | 0:07:12 | 0:07:16 | |
distortions in space-time cause objects to fall together. | 0:07:16 | 0:07:20 | |
And that is what we feel as the pull of gravity. | 0:07:20 | 0:07:23 | |
This curving of space-time elegantly explains how planets | 0:07:26 | 0:07:30 | |
and spaceships orbit around a large mass. | 0:07:30 | 0:07:33 | |
It also reveals they can only overcome that pull | 0:07:36 | 0:07:39 | |
and escape from orbit if they can travel fast enough | 0:07:39 | 0:07:42 | |
to make it all the way up the side of the hole. | 0:07:42 | 0:07:45 | |
Pretty straightforward so far. | 0:07:47 | 0:07:49 | |
But things get interesting when you consider really dense objects - | 0:07:49 | 0:07:53 | |
the sort of thing that cosmologists consider result from | 0:07:53 | 0:07:56 | |
the death of a giant star. | 0:07:56 | 0:07:57 | |
If it was massive enough, a star like this would collapse | 0:08:00 | 0:08:04 | |
under its own gravity and, according to general relativity, | 0:08:04 | 0:08:08 | |
form a singularity. | 0:08:08 | 0:08:10 | |
A speck of infinite density | 0:08:11 | 0:08:13 | |
at the centre of a bottomless pit in space-time. | 0:08:13 | 0:08:16 | |
In order to escape the steeply sided curve of a hole like this, | 0:08:20 | 0:08:23 | |
you needed to be travelling faster than the speed of light. | 0:08:23 | 0:08:27 | |
Now, nothing can travel that fast, so nothing - not even light - | 0:08:27 | 0:08:30 | |
can escape a hole like this... | 0:08:30 | 0:08:33 | |
a black hole. | 0:08:33 | 0:08:34 | |
But even though general relativity implied that black holes should | 0:08:39 | 0:08:42 | |
exist in theory, most physicists didn't much like the idea. | 0:08:42 | 0:08:46 | |
It raised awkward questions, | 0:08:49 | 0:08:51 | |
like - what would happen to things once they'd fallen in? | 0:08:51 | 0:08:55 | |
And besides, the sort of dense, collapsed star that you need to | 0:08:58 | 0:09:01 | |
produce a black hole had never been observed. | 0:09:01 | 0:09:04 | |
But then in 1967, astronomers here in Cambridge, | 0:09:06 | 0:09:10 | |
discovered the first pulsar - an object powered by | 0:09:10 | 0:09:13 | |
a collapsed, dense star, the first hint that such objects | 0:09:13 | 0:09:17 | |
really existed and, suddenly, black holes were back on the agenda. | 0:09:17 | 0:09:21 | |
Here at Gonville and Caius College, a young Stephen Hawking took up | 0:09:24 | 0:09:28 | |
the challenge to take a fresh look at black hole theory... | 0:09:28 | 0:09:31 | |
..and right from the start, he managed to make waves. | 0:09:33 | 0:09:36 | |
Black holes were thought to be completely black, | 0:09:38 | 0:09:41 | |
until I discovered that they glow like hot bodies, | 0:09:41 | 0:09:45 | |
with a temperature that is higher the smaller the black hole. | 0:09:45 | 0:09:49 | |
This points to a deep and unexpected connection between black holes | 0:09:51 | 0:09:55 | |
and thermodynamics, the science of heat. | 0:09:55 | 0:09:58 | |
The trouble was, how could heat be coming from a black hole? | 0:09:59 | 0:10:03 | |
Nothing should be able to escape a black hole. | 0:10:03 | 0:10:06 | |
Hawking suggested a new form of radiation, | 0:10:08 | 0:10:11 | |
consisting of strange particles that exist in the bizarre world | 0:10:11 | 0:10:15 | |
of quantum theory. | 0:10:15 | 0:10:17 | |
Quantum mechanics implies that the whole of space is filled with | 0:10:18 | 0:10:22 | |
pairs of virtual particles and antiparticles, | 0:10:22 | 0:10:26 | |
that are constantly materialising in pairs, | 0:10:26 | 0:10:29 | |
and then annihilating each other. | 0:10:29 | 0:10:32 | |
Now in the presence of a black hole, | 0:10:32 | 0:10:34 | |
one member of a pair may fall into the hole. | 0:10:34 | 0:10:38 | |
The other particle may fall after its partner, | 0:10:38 | 0:10:41 | |
but it may also escape to infinity, | 0:10:41 | 0:10:44 | |
where it appears to be radiation emitted by the black hole. | 0:10:44 | 0:10:48 | |
This mind-boggling concept is now known as Hawking radiation. | 0:10:48 | 0:10:53 | |
And with it, Stephen Hawking had solved the problem | 0:10:53 | 0:10:57 | |
of how energy can escape from a black hole. | 0:10:57 | 0:11:01 | |
The trouble is, that in solving one problem, it created another, | 0:11:01 | 0:11:05 | |
bigger one - the information paradox. | 0:11:05 | 0:11:07 | |
In Einstein's most famous equation, E = mc2, | 0:11:10 | 0:11:14 | |
he showed that energy and mass are intertwined. | 0:11:14 | 0:11:18 | |
So a black hole losing energy must also be losing mass, | 0:11:19 | 0:11:22 | |
albeit very slowly. | 0:11:22 | 0:11:24 | |
The radiation will carry away energy from the black hole. | 0:11:26 | 0:11:30 | |
The black hole will lose mass and eventually disappear. | 0:11:30 | 0:11:35 | |
This creates a paradox, | 0:11:37 | 0:11:39 | |
because the information about what fell into the black hole | 0:11:39 | 0:11:43 | |
appears to be lost, | 0:11:43 | 0:11:45 | |
but the laws of physics say that information can never be lost. | 0:11:45 | 0:11:49 | |
"Information" is tricky - it's not things like names and stories | 0:11:52 | 0:11:55 | |
that can't be lost. | 0:11:55 | 0:11:57 | |
Maggie met up with cosmologist Andrew Pontzen to find out | 0:11:57 | 0:12:00 | |
what kind of information does cause the paradox. | 0:12:00 | 0:12:04 | |
Information is a critical thing in physics. | 0:12:04 | 0:12:06 | |
It's telling us where we are and how we are sitting, | 0:12:06 | 0:12:09 | |
what we're made out of. | 0:12:09 | 0:12:10 | |
We think of all that stuff as being information. | 0:12:10 | 0:12:13 | |
So these are the parameters governing the atoms of the universe? | 0:12:13 | 0:12:16 | |
Exactly. The idea is that if you know everything about the universe | 0:12:16 | 0:12:19 | |
today then you should be able to predict what will | 0:12:19 | 0:12:22 | |
happen in the future, or work through the equations backwards | 0:12:22 | 0:12:25 | |
and predict what happened in the past, but if you want to be | 0:12:25 | 0:12:28 | |
able to do that, then you need this idea of preserving information. | 0:12:28 | 0:12:33 | |
The problem is, if you imagine something falling into a black hole, | 0:12:33 | 0:12:36 | |
you or me, or maybe just put the whole Earth into a black hole - | 0:12:36 | 0:12:39 | |
in fact, I can do that for you now, if I take my... | 0:12:39 | 0:12:42 | |
Phew, it's just a piece of paper! | 0:12:42 | 0:12:44 | |
Not the real Earth, just a piece of paper. | 0:12:44 | 0:12:47 | |
Pop it into my black hole over here, for which I'm using a shredder. | 0:12:47 | 0:12:50 | |
Now, in principle, if you looked carefully enough inside | 0:12:54 | 0:12:57 | |
the black hole, you can imagine, you've got some bits and pieces | 0:12:57 | 0:12:59 | |
-in there... -So we could recreate this, and put it back together, | 0:12:59 | 0:13:02 | |
with a long time and a lot of Sellotape. | 0:13:02 | 0:13:04 | |
It would be boring but you could do it in principle, | 0:13:04 | 0:13:07 | |
so the information is still there. | 0:13:07 | 0:13:08 | |
However, with Hawking radiation, | 0:13:08 | 0:13:11 | |
very slowly, a black hole reduces its mass so over time, it gradually | 0:13:11 | 0:13:16 | |
shrinks away until it's completely gone - | 0:13:16 | 0:13:18 | |
there's no trace of the Earth, no evidence that it ever existed, | 0:13:18 | 0:13:21 | |
let alone what it's made up of. | 0:13:21 | 0:13:22 | |
And you can't retrieve that information in any way? | 0:13:22 | 0:13:25 | |
Exactly. | 0:13:25 | 0:13:27 | |
It seems like you wouldn't be able to get that information at all, | 0:13:27 | 0:13:30 | |
so there is no way you could work backwards | 0:13:30 | 0:13:32 | |
and work out that the Earth used to exist, it's just gone and lost. | 0:13:32 | 0:13:36 | |
So is there any way to resolve this? | 0:13:36 | 0:13:38 | |
We hope there is a way but right now, it's fair to say nobody knows. | 0:13:38 | 0:13:43 | |
People are trying to resolve it in lots of different ways. | 0:13:43 | 0:13:46 | |
Despite 40 years of effort, | 0:13:46 | 0:13:48 | |
the information paradox is still unresolved. | 0:13:48 | 0:13:52 | |
If Hawking radiation exists, | 0:13:54 | 0:13:56 | |
then we have to solve the information paradox - | 0:13:56 | 0:13:59 | |
we have to work out what happens to the information as it falls | 0:13:59 | 0:14:02 | |
into a black hole, and how they can evaporate without destroying it. | 0:14:02 | 0:14:06 | |
If on the other hand Hawking radiation doesn't exist, | 0:14:06 | 0:14:08 | |
then something's fundamentally wrong with our understanding | 0:14:08 | 0:14:11 | |
of black holes and perhaps even quantum theory. | 0:14:11 | 0:14:14 | |
The problem is that theorists have raced ahead with these ideas | 0:14:14 | 0:14:18 | |
about what black holes MIGHT be like, | 0:14:18 | 0:14:20 | |
but how do we observe that they exist at all? | 0:14:20 | 0:14:23 | |
The trouble is, we can never see them directly. | 0:14:23 | 0:14:26 | |
What we can see is some of the evidence that suggests | 0:14:29 | 0:14:33 | |
black holes are lurking out there. | 0:14:33 | 0:14:35 | |
In fact, some of it is surprisingly easy to spot with a telescope. | 0:14:36 | 0:14:40 | |
Pete Lawrence has spent a night on the hunt for black holes. | 0:14:40 | 0:14:44 | |
Various clues have been spotted over the years that seem to | 0:14:49 | 0:14:52 | |
signal the presence of black holes. | 0:14:52 | 0:14:54 | |
You just need to know where to look. | 0:14:55 | 0:14:58 | |
One area of interest lies in the constellation of Cygnus, the swan. | 0:14:58 | 0:15:03 | |
Here, there are several pieces of evidence which may suggest | 0:15:03 | 0:15:06 | |
the presence of black holes. | 0:15:06 | 0:15:07 | |
You can currently find Cygnus low in the north-east part of the sky | 0:15:07 | 0:15:11 | |
at about 1am. | 0:15:11 | 0:15:12 | |
In 1964, one of the earliest space telescopes - | 0:15:18 | 0:15:22 | |
a suborbital rocket fitted with a Geiger counter - | 0:15:22 | 0:15:25 | |
detected a flood of radiation coming from this part of the sky. | 0:15:25 | 0:15:29 | |
They narrowed the source down to about there... | 0:15:29 | 0:15:34 | |
and called it "Cygnus X-1". | 0:15:34 | 0:15:36 | |
On closer inspection, | 0:15:40 | 0:15:41 | |
it appeared that this was something incredibly small | 0:15:41 | 0:15:44 | |
and compact, producing an extraordinary amount of energy. | 0:15:44 | 0:15:48 | |
Astronomers put forward a theory to explain what they were seeing. | 0:15:50 | 0:15:54 | |
What initially looked like a single star actually turned out to be | 0:15:54 | 0:15:58 | |
a binary system - a large blue star with an invisible companion. | 0:15:58 | 0:16:02 | |
Could this be a black hole? | 0:16:04 | 0:16:06 | |
One explanation for all that energy being released, | 0:16:09 | 0:16:12 | |
was that gas from the visible star | 0:16:12 | 0:16:14 | |
was being sucked towards a black hole, creating immense friction | 0:16:14 | 0:16:18 | |
as it spiralled into what is called an accretion disk. | 0:16:18 | 0:16:21 | |
As it's heated, that gas would release a colossal amount of energy | 0:16:23 | 0:16:27 | |
and even bright X-ray flashes. | 0:16:27 | 0:16:29 | |
There are signs of much bigger black holes to look for too. | 0:16:32 | 0:16:36 | |
If we look back at Cygnus, there's an unremarkable patch of sky | 0:16:38 | 0:16:42 | |
in the western wing of the swan. | 0:16:42 | 0:16:44 | |
There's not much to see here visually. | 0:16:44 | 0:16:47 | |
But astronomers were astounded to see a bright radio source | 0:16:47 | 0:16:51 | |
emanating from this region. | 0:16:51 | 0:16:53 | |
The bright signals came from two jets of material spewing out | 0:16:54 | 0:16:58 | |
from either side of a distant galaxy, at tremendous speeds. | 0:16:58 | 0:17:02 | |
Such jets should take huge amounts of energy to produce - | 0:17:03 | 0:17:07 | |
like converting a million times the mass of the sun to pure energy, | 0:17:07 | 0:17:12 | |
more than the nuclear fusion that drives stars could ever produce. | 0:17:12 | 0:17:16 | |
The only known phenomenon that could convert matter to energy | 0:17:18 | 0:17:21 | |
that efficiently was what astronomers had seen | 0:17:21 | 0:17:24 | |
in the Cygnus X-1 system - accretion. | 0:17:24 | 0:17:28 | |
But this must be on a much, much larger scale - | 0:17:28 | 0:17:31 | |
so there must a massive, unseen black hole | 0:17:31 | 0:17:34 | |
at the centre of the galaxy. | 0:17:34 | 0:17:36 | |
Bright jets and accretion disks give us enough evidence | 0:17:37 | 0:17:41 | |
to suggest similar "supermassive" black holes exist | 0:17:41 | 0:17:45 | |
at the centre of almost all galaxies. | 0:17:45 | 0:17:47 | |
But if we look towards the heart of our own galaxy, | 0:17:50 | 0:17:54 | |
there's even more compelling evidence. | 0:17:54 | 0:17:56 | |
The centre of the Milky Way galaxy | 0:17:59 | 0:18:01 | |
is in the constellation of Sagittarius, | 0:18:01 | 0:18:03 | |
which, at the moment, is more or less behind me. | 0:18:03 | 0:18:06 | |
If you're out on a clear summer evening, | 0:18:06 | 0:18:09 | |
then this region of sky can be seen low down in the south. | 0:18:09 | 0:18:13 | |
Now, of course, we can't see the supermassive black hole | 0:18:15 | 0:18:18 | |
at the centre of our galaxy, | 0:18:18 | 0:18:19 | |
but we can see the effect it's having on the stars around it. | 0:18:19 | 0:18:23 | |
For 25 years, astronomers have tracked the motion of stars | 0:18:24 | 0:18:28 | |
at the heart of the galaxy. | 0:18:28 | 0:18:30 | |
And it's quite clear they're orbiting around something. | 0:18:30 | 0:18:34 | |
That something must be over four million times | 0:18:34 | 0:18:38 | |
the mass of our sun, squeezed into just 17 times its size. | 0:18:38 | 0:18:43 | |
And that's what we now call "Sagittarius A Star" - | 0:18:44 | 0:18:48 | |
our very own supermassive black hole. | 0:18:48 | 0:18:50 | |
With all the evidence we've seen over the years, | 0:18:53 | 0:18:56 | |
we're pretty convinced that black holes do actually exist. | 0:18:56 | 0:18:59 | |
But the next challenge would be to detect one directly. | 0:18:59 | 0:19:03 | |
Until this year, astronomers have relied | 0:19:07 | 0:19:09 | |
on this circumstantial evidence to deduce | 0:19:09 | 0:19:12 | |
almost everything that we know about black holes. | 0:19:12 | 0:19:15 | |
But all that changed just a few weeks ago, | 0:19:15 | 0:19:18 | |
with the first direct detection of not one, but two black holes. | 0:19:18 | 0:19:23 | |
It was these black holes that caused | 0:19:23 | 0:19:25 | |
the gravitational waves in February's big announcement. | 0:19:25 | 0:19:29 | |
'Professor Sheila Rowan is one of the team behind the discovery. | 0:19:31 | 0:19:36 | |
'I joined her to find out more | 0:19:36 | 0:19:38 | |
'and to see what it all means for black hole science.' | 0:19:38 | 0:19:41 | |
It is a very exciting time to be talking about | 0:19:42 | 0:19:45 | |
gravitational waves, but what exactly have we seen? | 0:19:45 | 0:19:48 | |
What we have seen is space vibrating, space shaking, | 0:19:48 | 0:19:54 | |
as picked up by the LIGO observatories in the United States | 0:19:54 | 0:19:58 | |
with a device that is about 4km long, | 0:19:58 | 0:20:01 | |
measuring motions 1/10,000th of the size | 0:20:01 | 0:20:05 | |
of a proton in the nucleus of an atom, so... | 0:20:05 | 0:20:07 | |
-Quite phenomenal. -Wow. Incredibly small. | 0:20:07 | 0:20:11 | |
This is the signal that we observed, | 0:20:11 | 0:20:13 | |
and what we saw was those vibrations speeding up, vibrating faster | 0:20:13 | 0:20:19 | |
and faster to a peak, and then the vibrations died down slightly. | 0:20:19 | 0:20:23 | |
And all of this detail, all of this vibration happens very quickly. | 0:20:23 | 0:20:28 | |
It does, it happens in about 0.2 of a second, but amazingly, | 0:20:28 | 0:20:32 | |
encoded in the vibrations, in that 0.2 of a second, | 0:20:32 | 0:20:35 | |
is information about the source of those vibrations. | 0:20:35 | 0:20:39 | |
Two black holes colliding about 1.3 billion light years away, | 0:20:39 | 0:20:45 | |
so 1.3 billion years ago, | 0:20:45 | 0:20:46 | |
and arriving with us here on Earth last September. | 0:20:46 | 0:20:49 | |
And that speeding up is actually the two black holes | 0:20:49 | 0:20:52 | |
spiralling in faster and faster until they eventually collide | 0:20:52 | 0:20:56 | |
to form a new black hole that then wobbles at a particular frequency, | 0:20:56 | 0:21:00 | |
and we can use that again to tell us about the properties | 0:21:00 | 0:21:03 | |
of the new black hole that has been formed. | 0:21:03 | 0:21:05 | |
See, this is, to me, as an astronomer, | 0:21:05 | 0:21:07 | |
the really exciting thing, to be able to make an observation | 0:21:07 | 0:21:10 | |
that tells you about the properties of the black hole itself. | 0:21:10 | 0:21:13 | |
Not about stuff falling into it, but actually about the black hole. | 0:21:13 | 0:21:16 | |
So what do we know about the two that merged | 0:21:16 | 0:21:19 | |
and what do we know about the state of the system now? | 0:21:19 | 0:21:21 | |
We can tell the masses of the two black holes that merged, | 0:21:21 | 0:21:24 | |
and one of them was about 36 times the mass of our sun. | 0:21:24 | 0:21:28 | |
The other one was about 29 times the mass of our sun. | 0:21:28 | 0:21:31 | |
The mass of that final black hole is about 62 times the mass of our sun. | 0:21:31 | 0:21:36 | |
If you add up the initial masses of the two black holes | 0:21:36 | 0:21:40 | |
that merged, in that final mass, you will discover there is | 0:21:40 | 0:21:43 | |
energy equivalent to three times the mass of our sun | 0:21:43 | 0:21:46 | |
that has gone somewhere, | 0:21:46 | 0:21:48 | |
-and where it has gone is into gravitational waves. -Wow. | 0:21:48 | 0:21:51 | |
In a short period, a huge amount of energy is produced, | 0:21:51 | 0:21:54 | |
more than the luminosity, the light power, of all the stars | 0:21:54 | 0:21:58 | |
and galaxies in the observable universe. | 0:21:58 | 0:22:00 | |
-Wow. -It's kind of amazing. -OK. | 0:22:00 | 0:22:02 | |
So what else do we know about this newly-formed black hole? | 0:22:02 | 0:22:05 | |
We also know that it is spinning. | 0:22:05 | 0:22:07 | |
It's spinning about 100 times a second. | 0:22:07 | 0:22:10 | |
We know it is about the size of Iceland. | 0:22:10 | 0:22:13 | |
It is not spherical, it is actually an ovoid, | 0:22:13 | 0:22:16 | |
it's kind of squished in one direction | 0:22:16 | 0:22:18 | |
and stretched in the other slightly, | 0:22:18 | 0:22:20 | |
and spinning away, and all of that we can get from | 0:22:20 | 0:22:24 | |
this signal that we detect in gravitational waves. | 0:22:24 | 0:22:27 | |
And this is the first time we have been able to do anything like that. | 0:22:27 | 0:22:30 | |
I think the most remarkable thing is not just the signal, | 0:22:30 | 0:22:33 | |
but the fact that it represents | 0:22:33 | 0:22:35 | |
a huge amount of effort by a lot of people, | 0:22:35 | 0:22:37 | |
most of whom laboured for years without ever detecting anything. | 0:22:37 | 0:22:41 | |
How does it feel to be sitting here describing a real signal? | 0:22:41 | 0:22:44 | |
It feels amazing, and it is the first time | 0:22:44 | 0:22:46 | |
we have seen these binary systems. | 0:22:46 | 0:22:48 | |
They might never have existed. | 0:22:48 | 0:22:50 | |
And the black holes merging is the birth of a new black hole, | 0:22:50 | 0:22:55 | |
that is a unique signature that we see, | 0:22:55 | 0:22:58 | |
that we really couldn't see any other way. | 0:22:58 | 0:23:00 | |
So that is really fantastic. | 0:23:00 | 0:23:02 | |
This is incredibly exciting, but it is just one detection, | 0:23:02 | 0:23:05 | |
-so what is next? -It is just one detection, | 0:23:05 | 0:23:08 | |
but we can combine that with our models for how the universe is, | 0:23:08 | 0:23:14 | |
and that lets us calculate possible rates of these events, | 0:23:14 | 0:23:19 | |
how many we might expect. | 0:23:19 | 0:23:21 | |
It could be anything from a few a month to one a day, | 0:23:21 | 0:23:24 | |
and so that is phenomenally exciting, | 0:23:24 | 0:23:27 | |
that we may see a whole population of these events out in the universe. | 0:23:27 | 0:23:30 | |
And that's just from these two detectors. | 0:23:30 | 0:23:32 | |
That's just from these two detectors, that's right. | 0:23:32 | 0:23:35 | |
These are just part of a whole potentially new astronomy | 0:23:35 | 0:23:39 | |
using a whole set of different instruments to detect | 0:23:39 | 0:23:42 | |
gravitational waves across a wide range of frequencies, | 0:23:42 | 0:23:45 | |
and that is very exciting. | 0:23:45 | 0:23:46 | |
-So more data coming? -More data coming. | 0:23:46 | 0:23:48 | |
2016 is going to go down in history as a big year for black holes. | 0:23:49 | 0:23:54 | |
But now we know that they exist and can even detect them directly, | 0:23:54 | 0:23:58 | |
what does that mean for the information paradox | 0:23:58 | 0:24:00 | |
and Hawking radiation? | 0:24:00 | 0:24:02 | |
Critically, it's an opportunity for Stephen Hawking | 0:24:03 | 0:24:06 | |
to test his theories at last. | 0:24:06 | 0:24:08 | |
Especially his extraordinary idea that when black holes combine, | 0:24:09 | 0:24:13 | |
they make one new one, with more surface area | 0:24:13 | 0:24:16 | |
than the first two put together. | 0:24:16 | 0:24:18 | |
The signal LIGO detected came from the collision | 0:24:20 | 0:24:23 | |
and merger of two black holes in a black hole binary. | 0:24:23 | 0:24:28 | |
This should make it possible to experimentally test | 0:24:28 | 0:24:31 | |
my prediction the area of the horizon of the final black hole | 0:24:31 | 0:24:36 | |
is greater than the sum of the areas of the original holes. | 0:24:36 | 0:24:41 | |
This prediction is crucial | 0:24:41 | 0:24:42 | |
to our understanding of the thermodynamics of black holes. | 0:24:42 | 0:24:47 | |
By making sense of their thermodynamics, | 0:24:47 | 0:24:50 | |
LIGO and its successors could provide the first | 0:24:50 | 0:24:54 | |
experimental evidence that black holes DO glow | 0:24:54 | 0:24:56 | |
with Hawking radiation. | 0:24:56 | 0:24:58 | |
But could there be more direct evidence, | 0:24:58 | 0:25:00 | |
out at the very edge of the observable universe? | 0:25:00 | 0:25:04 | |
The cosmological horizon. | 0:25:04 | 0:25:06 | |
LIGO is not sensitive to the wavelengths at which there is | 0:25:07 | 0:25:11 | |
appreciable Hawking radiation from black holes. | 0:25:11 | 0:25:15 | |
However, there is likely to be another kind of | 0:25:15 | 0:25:19 | |
Hawking radiation of much longer wavelength | 0:25:19 | 0:25:22 | |
coming from the cosmological horizon | 0:25:22 | 0:25:25 | |
which might be detected by radio telescopes. | 0:25:25 | 0:25:28 | |
Longwave radiation like this would have to come from a type | 0:25:30 | 0:25:33 | |
of black hole formed right after the big bang, | 0:25:33 | 0:25:37 | |
in the early primordial universe. | 0:25:37 | 0:25:39 | |
It would be a unique kind of gravitational radiation. | 0:25:39 | 0:25:44 | |
I hope that radio telescopes detect primordial gravitational radiation | 0:25:46 | 0:25:51 | |
from the cosmological horizon. | 0:25:51 | 0:25:55 | |
That would mean black holes almost certainly emit radiation | 0:25:55 | 0:25:59 | |
and would get me a Nobel Prize. | 0:25:59 | 0:26:01 | |
If Hawking radiation can be proved to exist, | 0:26:04 | 0:26:07 | |
then there must be a solution to the information paradox. | 0:26:07 | 0:26:10 | |
And Professor Hawking thinks he has found that too. | 0:26:10 | 0:26:14 | |
He believes that objects falling in to a black hole | 0:26:14 | 0:26:18 | |
leave the information they carry behind. | 0:26:18 | 0:26:20 | |
It's stored on the hole's surface, which becomes turbulent, | 0:26:20 | 0:26:24 | |
a process called "super translation". | 0:26:24 | 0:26:27 | |
Last year, I realised that a black hole can store | 0:26:27 | 0:26:31 | |
the information in what is called super translations of the horizon. | 0:26:31 | 0:26:36 | |
I am now working with my colleagues Malcolm Perry at Cambridge | 0:26:36 | 0:26:40 | |
and Andrew Strominger at Harvard | 0:26:40 | 0:26:43 | |
on whether this can resolve the paradox. | 0:26:43 | 0:26:46 | |
If he is right, | 0:26:48 | 0:26:49 | |
Stephen Hawking has solved one of the greatest mysteries in cosmology. | 0:26:49 | 0:26:53 | |
But does that mean you can escape from a black hole after all, | 0:26:53 | 0:26:57 | |
or would it still be bad news to fall in one? | 0:26:57 | 0:26:59 | |
Definitely bad news. | 0:27:01 | 0:27:03 | |
If it were a stellar-mass black hole, | 0:27:03 | 0:27:05 | |
you would be made into spaghetti before reaching the horizon. | 0:27:05 | 0:27:10 | |
On the other hand, if it were a supermassive black hole, | 0:27:10 | 0:27:14 | |
you would cross the horizon with ease, | 0:27:14 | 0:27:17 | |
but be crushed out of existence at the singularity. | 0:27:17 | 0:27:20 | |
BLAST | 0:27:22 | 0:27:23 | |
Well, there's no doubt in my mind that with the detection | 0:27:27 | 0:27:30 | |
of gravitational waves, the future of black hole science looks bright. | 0:27:30 | 0:27:34 | |
And after 50 years of theoretical debates, Stephen Hawking might get | 0:27:34 | 0:27:38 | |
the experimental evidence he wants | 0:27:38 | 0:27:40 | |
to test his ideas about black holes. | 0:27:40 | 0:27:42 | |
And there is another exciting project on the horizon too - | 0:27:42 | 0:27:46 | |
the Event Horizon Telescope, | 0:27:46 | 0:27:47 | |
a worldwide network of radio telescopes that next year | 0:27:47 | 0:27:51 | |
will team up to try and capture the first image of the shadow cast | 0:27:51 | 0:27:55 | |
by the enormous black hole at the Milky Way's centre. | 0:27:55 | 0:27:58 | |
This would have been unimaginable | 0:27:58 | 0:28:00 | |
when Stephen Hawking started grappling with black holes | 0:28:00 | 0:28:03 | |
over 40 years ago, but it shows this is another chapter | 0:28:03 | 0:28:06 | |
in the history of black hole science. | 0:28:06 | 0:28:09 | |
Next month, we will be previewing | 0:28:09 | 0:28:10 | |
one of the astronomical highlights | 0:28:10 | 0:28:12 | |
of the year, the transit of Mercury, | 0:28:12 | 0:28:15 | |
visible across the UK on May 9th. | 0:28:15 | 0:28:17 | |
And we will be using the opportunity to take a closer look | 0:28:19 | 0:28:23 | |
at Mercury, one of the strangest planets in the solar system. | 0:28:23 | 0:28:26 | |
To find out how to catch a glimpse of it before then, | 0:28:28 | 0:28:30 | |
check out the website for Pete's April star guide. | 0:28:30 | 0:28:34 | |
-In the meanwhile, get outside and... get looking up. -Good night. | 0:28:37 | 0:28:41 |