0:00:02 > 0:00:07Just two months ago, a major scientific discovery was announced.
0:00:07 > 0:00:12Ladies and gentlemen, we have detected gravitational waves.
0:00:12 > 0:00:14- APPLAUSE - We did it!
0:00:16 > 0:00:21This detection was solid evidence of something Albert Einstein predicted
0:00:21 > 0:00:23100 years ago.
0:00:23 > 0:00:26But it was also the most direct observation ever
0:00:26 > 0:00:28of black holes.
0:00:28 > 0:00:32The discovery of gravitational waves has launched a new era
0:00:32 > 0:00:35in the study of perhaps the most captivating
0:00:35 > 0:00:38and powerful objects in the universe.
0:00:38 > 0:00:42In science fiction, spaceships often go through a black hole
0:00:42 > 0:00:44to another universe,
0:00:44 > 0:00:47or another part of our universe.
0:00:48 > 0:00:51But black holes are stranger than anything
0:00:51 > 0:00:53dreamt up by science-fiction writers.
0:00:54 > 0:00:57Tonight, we're joined by Stephen Hawking
0:00:57 > 0:01:02to take a mind-blowing journey into the enigmatic world of black holes.
0:01:02 > 0:01:05We're going to find out how the astonishing discovery
0:01:05 > 0:01:08of gravitational waves, made just a few weeks ago,
0:01:08 > 0:01:12is already helping us understand the fundamental nature of them.
0:01:41 > 0:01:44In a truly extraordinary life, Professor Stephen Hawking
0:01:44 > 0:01:47has become the world's most celebrated scientist.
0:01:49 > 0:01:54And for over 40 years, he's wrestled with the toughest of questions.
0:01:54 > 0:01:57What happens inside a black hole?
0:01:57 > 0:02:01Black holes are formed by the collapse of massive stars
0:02:01 > 0:02:04when they have exhausted their nuclear fuel...
0:02:08 > 0:02:11..and can no longer support themselves against
0:02:11 > 0:02:12their own gravity.
0:02:17 > 0:02:22They are quite literally holes in space that stuff can fall into,
0:02:22 > 0:02:24but not get out of.
0:02:24 > 0:02:27They are places where the gravitational field is
0:02:27 > 0:02:32so strong that nothing, not even light, can get away.
0:02:34 > 0:02:38This basic picture of black holes is very well-known.
0:02:38 > 0:02:41But, unfortunately, it is also far from complete.
0:02:42 > 0:02:45At the heart of the mystery of black holes
0:02:45 > 0:02:47lie two fundamental problems.
0:02:47 > 0:02:50Firstly, theorists have found it hard to understand what's happening
0:02:50 > 0:02:54inside a black hole. In fact, the more they crunch the numbers,
0:02:54 > 0:02:57the more it seems that black holes defy the laws of physics.
0:02:57 > 0:03:01In particular, it throws up a mind-boggling conundrum
0:03:01 > 0:03:03known as the information paradox.
0:03:03 > 0:03:07Secondly, astronomers trying to observe these unusual objects
0:03:07 > 0:03:10and find physical evidence with which to test these theories,
0:03:10 > 0:03:13hit an apparently insurmountable problem -
0:03:13 > 0:03:15you can't see inside a black hole.
0:03:15 > 0:03:17The maths don't add up, and no-one's ever
0:03:17 > 0:03:21seen inside a black hole, so how do we know they even exist?
0:03:21 > 0:03:23Also, what kind of discovery can hope to answer
0:03:23 > 0:03:25all of these mysteries?
0:03:25 > 0:03:27Well, with the help of Professor Hawking,
0:03:27 > 0:03:29we're going to try and answer all of those questions.
0:03:29 > 0:03:32So, we've come here to the University of Cambridge,
0:03:32 > 0:03:34home of Stephen Hawking and, arguably,
0:03:34 > 0:03:36the birthplace of black hole science.
0:03:38 > 0:03:43In tonight's programme, we'll hear from Stephen Hawking himself,
0:03:43 > 0:03:46how he transformed our view of black holes.
0:03:47 > 0:03:51Black holes were thought to be completely black,
0:03:51 > 0:03:54until I discovered that they glow like hot bodies.
0:03:57 > 0:04:00I'll be finding out from cosmologist Andrew Pontzen
0:04:00 > 0:04:03why Hawking's black hole theories created one of the
0:04:03 > 0:04:05biggest conundrums in astrophysics.
0:04:05 > 0:04:08With Hawking radiation, very slowly, a black hole
0:04:08 > 0:04:11gradually shrinks away until it's completely gone.
0:04:11 > 0:04:13There's no trace of the Earth, no evidence
0:04:13 > 0:04:16that it ever really existed, let alone what it was made up of.
0:04:18 > 0:04:21And I'll be meeting one of the team behind the wonderful discovery
0:04:21 > 0:04:25of gravitational waves, exploring what they tell us
0:04:25 > 0:04:26about black holes.
0:04:27 > 0:04:30We know its mass, we also know that it's spinning.
0:04:30 > 0:04:32It's spinning about 100 times a second.
0:04:32 > 0:04:35We know it's about the size of Iceland,
0:04:35 > 0:04:38it's not spherical, it's actually an ovoid.
0:04:41 > 0:04:44But we'll start with that astonishing discovery of
0:04:44 > 0:04:45gravitational waves.
0:04:46 > 0:04:49Unravelling the secrets of the universe,
0:04:49 > 0:04:53the most important scientific discovery for a generation.
0:04:53 > 0:04:56Scientists in the United States have announced they have discovered
0:04:56 > 0:04:57gravitational waves.
0:04:57 > 0:05:00Einstein was right after all.
0:05:00 > 0:05:03Gravitational waves ripple through space and time.
0:05:07 > 0:05:11The discovery of these elusive waves is the end of a search
0:05:11 > 0:05:15that began with Einstein's work 100 years ago.
0:05:17 > 0:05:20But it's also the beginning of a new way of seeing
0:05:20 > 0:05:23what had previously been invisible in the universe,
0:05:23 > 0:05:26revealing remarkable objects, like black holes.
0:05:29 > 0:05:32The idea that black holes might exist was suggested
0:05:32 > 0:05:35here in Cambridge as far back as 1784.
0:05:36 > 0:05:38The Queen's College Don, John Michell,
0:05:38 > 0:05:42was fascinated by the idea of extreme gravity,
0:05:42 > 0:05:45and he imagined that a big enough star might generate
0:05:45 > 0:05:48a gravitational pull that was so intense,
0:05:48 > 0:05:50not even light could escape.
0:05:54 > 0:05:57John Michell called these objects "dark stars"
0:05:57 > 0:06:00and he realised that we couldn't ever see them directly.
0:06:00 > 0:06:02But he thought we might be able to detect them
0:06:02 > 0:06:05by watching for their gravitational effects on objects around them.
0:06:05 > 0:06:09Sadly, his 18th-century colleagues ignored him and dark stars
0:06:09 > 0:06:12were lost to the world for the best part of 200 years.
0:06:14 > 0:06:17That is until the arrival of Hawking and a new wave
0:06:17 > 0:06:19of black hole physicists.
0:06:20 > 0:06:24They were intent on understanding some of the surprising implications
0:06:24 > 0:06:27of Einstein's theory of general relativity.
0:06:28 > 0:06:31When physicists began to explore Einstein's theory of
0:06:31 > 0:06:34general relativity, they found that it made all sorts of
0:06:34 > 0:06:37bizarre predictions, and one of the weirdest
0:06:37 > 0:06:39was the existence of black holes.
0:06:40 > 0:06:43To see how black holes were predicted by general relativity,
0:06:43 > 0:06:46you need get to grips with the concept at the heart
0:06:46 > 0:06:49of Einstein's theory - space-time.
0:06:52 > 0:06:53Einstein insisted that time
0:06:53 > 0:06:57and three-dimensional space weren't actually separate at all,
0:06:57 > 0:07:01but they were woven together into the four dimensions of space-time.
0:07:01 > 0:07:06What's more, space-time was distorted by mass, just as this ball
0:07:06 > 0:07:10distorts my sheet, causing a curved dip.
0:07:12 > 0:07:16Just as our dip causes a second ball to roll towards it,
0:07:16 > 0:07:20distortions in space-time cause objects to fall together.
0:07:20 > 0:07:23And that is what we feel as the pull of gravity.
0:07:26 > 0:07:30This curving of space-time elegantly explains how planets
0:07:30 > 0:07:33and spaceships orbit around a large mass.
0:07:36 > 0:07:39It also reveals they can only overcome that pull
0:07:39 > 0:07:42and escape from orbit if they can travel fast enough
0:07:42 > 0:07:45to make it all the way up the side of the hole.
0:07:47 > 0:07:49Pretty straightforward so far.
0:07:49 > 0:07:53But things get interesting when you consider really dense objects -
0:07:53 > 0:07:56the sort of thing that cosmologists consider result from
0:07:56 > 0:07:57the death of a giant star.
0:08:00 > 0:08:04If it was massive enough, a star like this would collapse
0:08:04 > 0:08:08under its own gravity and, according to general relativity,
0:08:08 > 0:08:10form a singularity.
0:08:11 > 0:08:13A speck of infinite density
0:08:13 > 0:08:16at the centre of a bottomless pit in space-time.
0:08:20 > 0:08:23In order to escape the steeply sided curve of a hole like this,
0:08:23 > 0:08:27you needed to be travelling faster than the speed of light.
0:08:27 > 0:08:30Now, nothing can travel that fast, so nothing - not even light -
0:08:30 > 0:08:33can escape a hole like this...
0:08:33 > 0:08:34a black hole.
0:08:39 > 0:08:42But even though general relativity implied that black holes should
0:08:42 > 0:08:46exist in theory, most physicists didn't much like the idea.
0:08:49 > 0:08:51It raised awkward questions,
0:08:51 > 0:08:55like - what would happen to things once they'd fallen in?
0:08:58 > 0:09:01And besides, the sort of dense, collapsed star that you need to
0:09:01 > 0:09:04produce a black hole had never been observed.
0:09:06 > 0:09:10But then in 1967, astronomers here in Cambridge,
0:09:10 > 0:09:13discovered the first pulsar - an object powered by
0:09:13 > 0:09:17a collapsed, dense star, the first hint that such objects
0:09:17 > 0:09:21really existed and, suddenly, black holes were back on the agenda.
0:09:24 > 0:09:28Here at Gonville and Caius College, a young Stephen Hawking took up
0:09:28 > 0:09:31the challenge to take a fresh look at black hole theory...
0:09:33 > 0:09:36..and right from the start, he managed to make waves.
0:09:38 > 0:09:41Black holes were thought to be completely black,
0:09:41 > 0:09:45until I discovered that they glow like hot bodies,
0:09:45 > 0:09:49with a temperature that is higher the smaller the black hole.
0:09:51 > 0:09:55This points to a deep and unexpected connection between black holes
0:09:55 > 0:09:58and thermodynamics, the science of heat.
0:09:59 > 0:10:03The trouble was, how could heat be coming from a black hole?
0:10:03 > 0:10:06Nothing should be able to escape a black hole.
0:10:08 > 0:10:11Hawking suggested a new form of radiation,
0:10:11 > 0:10:15consisting of strange particles that exist in the bizarre world
0:10:15 > 0:10:17of quantum theory.
0:10:18 > 0:10:22Quantum mechanics implies that the whole of space is filled with
0:10:22 > 0:10:26pairs of virtual particles and antiparticles,
0:10:26 > 0:10:29that are constantly materialising in pairs,
0:10:29 > 0:10:32and then annihilating each other.
0:10:32 > 0:10:34Now in the presence of a black hole,
0:10:34 > 0:10:38one member of a pair may fall into the hole.
0:10:38 > 0:10:41The other particle may fall after its partner,
0:10:41 > 0:10:44but it may also escape to infinity,
0:10:44 > 0:10:48where it appears to be radiation emitted by the black hole.
0:10:48 > 0:10:53This mind-boggling concept is now known as Hawking radiation.
0:10:53 > 0:10:57And with it, Stephen Hawking had solved the problem
0:10:57 > 0:11:01of how energy can escape from a black hole.
0:11:01 > 0:11:05The trouble is, that in solving one problem, it created another,
0:11:05 > 0:11:07bigger one - the information paradox.
0:11:10 > 0:11:14In Einstein's most famous equation, E = mc2,
0:11:14 > 0:11:18he showed that energy and mass are intertwined.
0:11:19 > 0:11:22So a black hole losing energy must also be losing mass,
0:11:22 > 0:11:24albeit very slowly.
0:11:26 > 0:11:30The radiation will carry away energy from the black hole.
0:11:30 > 0:11:35The black hole will lose mass and eventually disappear.
0:11:37 > 0:11:39This creates a paradox,
0:11:39 > 0:11:43because the information about what fell into the black hole
0:11:43 > 0:11:45appears to be lost,
0:11:45 > 0:11:49but the laws of physics say that information can never be lost.
0:11:52 > 0:11:55"Information" is tricky - it's not things like names and stories
0:11:55 > 0:11:57that can't be lost.
0:11:57 > 0:12:00Maggie met up with cosmologist Andrew Pontzen to find out
0:12:00 > 0:12:04what kind of information does cause the paradox.
0:12:04 > 0:12:06Information is a critical thing in physics.
0:12:06 > 0:12:09It's telling us where we are and how we are sitting,
0:12:09 > 0:12:10what we're made out of.
0:12:10 > 0:12:13We think of all that stuff as being information.
0:12:13 > 0:12:16So these are the parameters governing the atoms of the universe?
0:12:16 > 0:12:19Exactly. The idea is that if you know everything about the universe
0:12:19 > 0:12:22today then you should be able to predict what will
0:12:22 > 0:12:25happen in the future, or work through the equations backwards
0:12:25 > 0:12:28and predict what happened in the past, but if you want to be
0:12:28 > 0:12:33able to do that, then you need this idea of preserving information.
0:12:33 > 0:12:36The problem is, if you imagine something falling into a black hole,
0:12:36 > 0:12:39you or me, or maybe just put the whole Earth into a black hole -
0:12:39 > 0:12:42in fact, I can do that for you now, if I take my...
0:12:42 > 0:12:44Phew, it's just a piece of paper!
0:12:44 > 0:12:47Not the real Earth, just a piece of paper.
0:12:47 > 0:12:50Pop it into my black hole over here, for which I'm using a shredder.
0:12:54 > 0:12:57Now, in principle, if you looked carefully enough inside
0:12:57 > 0:12:59the black hole, you can imagine, you've got some bits and pieces
0:12:59 > 0:13:02- in there...- So we could recreate this, and put it back together,
0:13:02 > 0:13:04with a long time and a lot of Sellotape.
0:13:04 > 0:13:07It would be boring but you could do it in principle,
0:13:07 > 0:13:08so the information is still there.
0:13:08 > 0:13:11However, with Hawking radiation,
0:13:11 > 0:13:16very slowly, a black hole reduces its mass so over time, it gradually
0:13:16 > 0:13:18shrinks away until it's completely gone -
0:13:18 > 0:13:21there's no trace of the Earth, no evidence that it ever existed,
0:13:21 > 0:13:22let alone what it's made up of.
0:13:22 > 0:13:25And you can't retrieve that information in any way?
0:13:25 > 0:13:27Exactly.
0:13:27 > 0:13:30It seems like you wouldn't be able to get that information at all,
0:13:30 > 0:13:32so there is no way you could work backwards
0:13:32 > 0:13:36and work out that the Earth used to exist, it's just gone and lost.
0:13:36 > 0:13:38So is there any way to resolve this?
0:13:38 > 0:13:43We hope there is a way but right now, it's fair to say nobody knows.
0:13:43 > 0:13:46People are trying to resolve it in lots of different ways.
0:13:46 > 0:13:48Despite 40 years of effort,
0:13:48 > 0:13:52the information paradox is still unresolved.
0:13:54 > 0:13:56If Hawking radiation exists,
0:13:56 > 0:13:59then we have to solve the information paradox -
0:13:59 > 0:14:02we have to work out what happens to the information as it falls
0:14:02 > 0:14:06into a black hole, and how they can evaporate without destroying it.
0:14:06 > 0:14:08If on the other hand Hawking radiation doesn't exist,
0:14:08 > 0:14:11then something's fundamentally wrong with our understanding
0:14:11 > 0:14:14of black holes and perhaps even quantum theory.
0:14:14 > 0:14:18The problem is that theorists have raced ahead with these ideas
0:14:18 > 0:14:20about what black holes MIGHT be like,
0:14:20 > 0:14:23but how do we observe that they exist at all?
0:14:23 > 0:14:26The trouble is, we can never see them directly.
0:14:29 > 0:14:33What we can see is some of the evidence that suggests
0:14:33 > 0:14:35black holes are lurking out there.
0:14:36 > 0:14:40In fact, some of it is surprisingly easy to spot with a telescope.
0:14:40 > 0:14:44Pete Lawrence has spent a night on the hunt for black holes.
0:14:49 > 0:14:52Various clues have been spotted over the years that seem to
0:14:52 > 0:14:54signal the presence of black holes.
0:14:55 > 0:14:58You just need to know where to look.
0:14:58 > 0:15:03One area of interest lies in the constellation of Cygnus, the swan.
0:15:03 > 0:15:06Here, there are several pieces of evidence which may suggest
0:15:06 > 0:15:07the presence of black holes.
0:15:07 > 0:15:11You can currently find Cygnus low in the north-east part of the sky
0:15:11 > 0:15:12at about 1am.
0:15:18 > 0:15:22In 1964, one of the earliest space telescopes -
0:15:22 > 0:15:25a suborbital rocket fitted with a Geiger counter -
0:15:25 > 0:15:29detected a flood of radiation coming from this part of the sky.
0:15:29 > 0:15:34They narrowed the source down to about there...
0:15:34 > 0:15:36and called it "Cygnus X-1".
0:15:40 > 0:15:41On closer inspection,
0:15:41 > 0:15:44it appeared that this was something incredibly small
0:15:44 > 0:15:48and compact, producing an extraordinary amount of energy.
0:15:50 > 0:15:54Astronomers put forward a theory to explain what they were seeing.
0:15:54 > 0:15:58What initially looked like a single star actually turned out to be
0:15:58 > 0:16:02a binary system - a large blue star with an invisible companion.
0:16:04 > 0:16:06Could this be a black hole?
0:16:09 > 0:16:12One explanation for all that energy being released,
0:16:12 > 0:16:14was that gas from the visible star
0:16:14 > 0:16:18was being sucked towards a black hole, creating immense friction
0:16:18 > 0:16:21as it spiralled into what is called an accretion disk.
0:16:23 > 0:16:27As it's heated, that gas would release a colossal amount of energy
0:16:27 > 0:16:29and even bright X-ray flashes.
0:16:32 > 0:16:36There are signs of much bigger black holes to look for too.
0:16:38 > 0:16:42If we look back at Cygnus, there's an unremarkable patch of sky
0:16:42 > 0:16:44in the western wing of the swan.
0:16:44 > 0:16:47There's not much to see here visually.
0:16:47 > 0:16:51But astronomers were astounded to see a bright radio source
0:16:51 > 0:16:53emanating from this region.
0:16:54 > 0:16:58The bright signals came from two jets of material spewing out
0:16:58 > 0:17:02from either side of a distant galaxy, at tremendous speeds.
0:17:03 > 0:17:07Such jets should take huge amounts of energy to produce -
0:17:07 > 0:17:12like converting a million times the mass of the sun to pure energy,
0:17:12 > 0:17:16more than the nuclear fusion that drives stars could ever produce.
0:17:18 > 0:17:21The only known phenomenon that could convert matter to energy
0:17:21 > 0:17:24that efficiently was what astronomers had seen
0:17:24 > 0:17:28in the Cygnus X-1 system - accretion.
0:17:28 > 0:17:31But this must be on a much, much larger scale -
0:17:31 > 0:17:34so there must a massive, unseen black hole
0:17:34 > 0:17:36at the centre of the galaxy.
0:17:37 > 0:17:41Bright jets and accretion disks give us enough evidence
0:17:41 > 0:17:45to suggest similar "supermassive" black holes exist
0:17:45 > 0:17:47at the centre of almost all galaxies.
0:17:50 > 0:17:54But if we look towards the heart of our own galaxy,
0:17:54 > 0:17:56there's even more compelling evidence.
0:17:59 > 0:18:01The centre of the Milky Way galaxy
0:18:01 > 0:18:03is in the constellation of Sagittarius,
0:18:03 > 0:18:06which, at the moment, is more or less behind me.
0:18:06 > 0:18:09If you're out on a clear summer evening,
0:18:09 > 0:18:13then this region of sky can be seen low down in the south.
0:18:15 > 0:18:18Now, of course, we can't see the supermassive black hole
0:18:18 > 0:18:19at the centre of our galaxy,
0:18:19 > 0:18:23but we can see the effect it's having on the stars around it.
0:18:24 > 0:18:28For 25 years, astronomers have tracked the motion of stars
0:18:28 > 0:18:30at the heart of the galaxy.
0:18:30 > 0:18:34And it's quite clear they're orbiting around something.
0:18:34 > 0:18:38That something must be over four million times
0:18:38 > 0:18:43the mass of our sun, squeezed into just 17 times its size.
0:18:44 > 0:18:48And that's what we now call "Sagittarius A Star" -
0:18:48 > 0:18:50our very own supermassive black hole.
0:18:53 > 0:18:56With all the evidence we've seen over the years,
0:18:56 > 0:18:59we're pretty convinced that black holes do actually exist.
0:18:59 > 0:19:03But the next challenge would be to detect one directly.
0:19:07 > 0:19:09Until this year, astronomers have relied
0:19:09 > 0:19:12on this circumstantial evidence to deduce
0:19:12 > 0:19:15almost everything that we know about black holes.
0:19:15 > 0:19:18But all that changed just a few weeks ago,
0:19:18 > 0:19:23with the first direct detection of not one, but two black holes.
0:19:23 > 0:19:25It was these black holes that caused
0:19:25 > 0:19:29the gravitational waves in February's big announcement.
0:19:31 > 0:19:36'Professor Sheila Rowan is one of the team behind the discovery.
0:19:36 > 0:19:38'I joined her to find out more
0:19:38 > 0:19:41'and to see what it all means for black hole science.'
0:19:42 > 0:19:45It is a very exciting time to be talking about
0:19:45 > 0:19:48gravitational waves, but what exactly have we seen?
0:19:48 > 0:19:54What we have seen is space vibrating, space shaking,
0:19:54 > 0:19:58as picked up by the LIGO observatories in the United States
0:19:58 > 0:20:01with a device that is about 4km long,
0:20:01 > 0:20:05measuring motions 1/10,000th of the size
0:20:05 > 0:20:07of a proton in the nucleus of an atom, so...
0:20:07 > 0:20:11- Quite phenomenal. - Wow. Incredibly small.
0:20:11 > 0:20:13This is the signal that we observed,
0:20:13 > 0:20:19and what we saw was those vibrations speeding up, vibrating faster
0:20:19 > 0:20:23and faster to a peak, and then the vibrations died down slightly.
0:20:23 > 0:20:28And all of this detail, all of this vibration happens very quickly.
0:20:28 > 0:20:32It does, it happens in about 0.2 of a second, but amazingly,
0:20:32 > 0:20:35encoded in the vibrations, in that 0.2 of a second,
0:20:35 > 0:20:39is information about the source of those vibrations.
0:20:39 > 0:20:45Two black holes colliding about 1.3 billion light years away,
0:20:45 > 0:20:46so 1.3 billion years ago,
0:20:46 > 0:20:49and arriving with us here on Earth last September.
0:20:49 > 0:20:52And that speeding up is actually the two black holes
0:20:52 > 0:20:56spiralling in faster and faster until they eventually collide
0:20:56 > 0:21:00to form a new black hole that then wobbles at a particular frequency,
0:21:00 > 0:21:03and we can use that again to tell us about the properties
0:21:03 > 0:21:05of the new black hole that has been formed.
0:21:05 > 0:21:07See, this is, to me, as an astronomer,
0:21:07 > 0:21:10the really exciting thing, to be able to make an observation
0:21:10 > 0:21:13that tells you about the properties of the black hole itself.
0:21:13 > 0:21:16Not about stuff falling into it, but actually about the black hole.
0:21:16 > 0:21:19So what do we know about the two that merged
0:21:19 > 0:21:21and what do we know about the state of the system now?
0:21:21 > 0:21:24We can tell the masses of the two black holes that merged,
0:21:24 > 0:21:28and one of them was about 36 times the mass of our sun.
0:21:28 > 0:21:31The other one was about 29 times the mass of our sun.
0:21:31 > 0:21:36The mass of that final black hole is about 62 times the mass of our sun.
0:21:36 > 0:21:40If you add up the initial masses of the two black holes
0:21:40 > 0:21:43that merged, in that final mass, you will discover there is
0:21:43 > 0:21:46energy equivalent to three times the mass of our sun
0:21:46 > 0:21:48that has gone somewhere,
0:21:48 > 0:21:51- and where it has gone is into gravitational waves.- Wow.
0:21:51 > 0:21:54In a short period, a huge amount of energy is produced,
0:21:54 > 0:21:58more than the luminosity, the light power, of all the stars
0:21:58 > 0:22:00and galaxies in the observable universe.
0:22:00 > 0:22:02- Wow. - It's kind of amazing.- OK.
0:22:02 > 0:22:05So what else do we know about this newly-formed black hole?
0:22:05 > 0:22:07We also know that it is spinning.
0:22:07 > 0:22:10It's spinning about 100 times a second.
0:22:10 > 0:22:13We know it is about the size of Iceland.
0:22:13 > 0:22:16It is not spherical, it is actually an ovoid,
0:22:16 > 0:22:18it's kind of squished in one direction
0:22:18 > 0:22:20and stretched in the other slightly,
0:22:20 > 0:22:24and spinning away, and all of that we can get from
0:22:24 > 0:22:27this signal that we detect in gravitational waves.
0:22:27 > 0:22:30And this is the first time we have been able to do anything like that.
0:22:30 > 0:22:33I think the most remarkable thing is not just the signal,
0:22:33 > 0:22:35but the fact that it represents
0:22:35 > 0:22:37a huge amount of effort by a lot of people,
0:22:37 > 0:22:41most of whom laboured for years without ever detecting anything.
0:22:41 > 0:22:44How does it feel to be sitting here describing a real signal?
0:22:44 > 0:22:46It feels amazing, and it is the first time
0:22:46 > 0:22:48we have seen these binary systems.
0:22:48 > 0:22:50They might never have existed.
0:22:50 > 0:22:55And the black holes merging is the birth of a new black hole,
0:22:55 > 0:22:58that is a unique signature that we see,
0:22:58 > 0:23:00that we really couldn't see any other way.
0:23:00 > 0:23:02So that is really fantastic.
0:23:02 > 0:23:05This is incredibly exciting, but it is just one detection,
0:23:05 > 0:23:08- so what is next? - It is just one detection,
0:23:08 > 0:23:14but we can combine that with our models for how the universe is,
0:23:14 > 0:23:19and that lets us calculate possible rates of these events,
0:23:19 > 0:23:21how many we might expect.
0:23:21 > 0:23:24It could be anything from a few a month to one a day,
0:23:24 > 0:23:27and so that is phenomenally exciting,
0:23:27 > 0:23:30that we may see a whole population of these events out in the universe.
0:23:30 > 0:23:32And that's just from these two detectors.
0:23:32 > 0:23:35That's just from these two detectors, that's right.
0:23:35 > 0:23:39These are just part of a whole potentially new astronomy
0:23:39 > 0:23:42using a whole set of different instruments to detect
0:23:42 > 0:23:45gravitational waves across a wide range of frequencies,
0:23:45 > 0:23:46and that is very exciting.
0:23:46 > 0:23:48- So more data coming? - More data coming.
0:23:49 > 0:23:542016 is going to go down in history as a big year for black holes.
0:23:54 > 0:23:58But now we know that they exist and can even detect them directly,
0:23:58 > 0:24:00what does that mean for the information paradox
0:24:00 > 0:24:02and Hawking radiation?
0:24:03 > 0:24:06Critically, it's an opportunity for Stephen Hawking
0:24:06 > 0:24:08to test his theories at last.
0:24:09 > 0:24:13Especially his extraordinary idea that when black holes combine,
0:24:13 > 0:24:16they make one new one, with more surface area
0:24:16 > 0:24:18than the first two put together.
0:24:20 > 0:24:23The signal LIGO detected came from the collision
0:24:23 > 0:24:28and merger of two black holes in a black hole binary.
0:24:28 > 0:24:31This should make it possible to experimentally test
0:24:31 > 0:24:36my prediction the area of the horizon of the final black hole
0:24:36 > 0:24:41is greater than the sum of the areas of the original holes.
0:24:41 > 0:24:42This prediction is crucial
0:24:42 > 0:24:47to our understanding of the thermodynamics of black holes.
0:24:47 > 0:24:50By making sense of their thermodynamics,
0:24:50 > 0:24:54LIGO and its successors could provide the first
0:24:54 > 0:24:56experimental evidence that black holes DO glow
0:24:56 > 0:24:58with Hawking radiation.
0:24:58 > 0:25:00But could there be more direct evidence,
0:25:00 > 0:25:04out at the very edge of the observable universe?
0:25:04 > 0:25:06The cosmological horizon.
0:25:07 > 0:25:11LIGO is not sensitive to the wavelengths at which there is
0:25:11 > 0:25:15appreciable Hawking radiation from black holes.
0:25:15 > 0:25:19However, there is likely to be another kind of
0:25:19 > 0:25:22Hawking radiation of much longer wavelength
0:25:22 > 0:25:25coming from the cosmological horizon
0:25:25 > 0:25:28which might be detected by radio telescopes.
0:25:30 > 0:25:33Longwave radiation like this would have to come from a type
0:25:33 > 0:25:37of black hole formed right after the big bang,
0:25:37 > 0:25:39in the early primordial universe.
0:25:39 > 0:25:44It would be a unique kind of gravitational radiation.
0:25:46 > 0:25:51I hope that radio telescopes detect primordial gravitational radiation
0:25:51 > 0:25:55from the cosmological horizon.
0:25:55 > 0:25:59That would mean black holes almost certainly emit radiation
0:25:59 > 0:26:01and would get me a Nobel Prize.
0:26:04 > 0:26:07If Hawking radiation can be proved to exist,
0:26:07 > 0:26:10then there must be a solution to the information paradox.
0:26:10 > 0:26:14And Professor Hawking thinks he has found that too.
0:26:14 > 0:26:18He believes that objects falling in to a black hole
0:26:18 > 0:26:20leave the information they carry behind.
0:26:20 > 0:26:24It's stored on the hole's surface, which becomes turbulent,
0:26:24 > 0:26:27a process called "super translation".
0:26:27 > 0:26:31Last year, I realised that a black hole can store
0:26:31 > 0:26:36the information in what is called super translations of the horizon.
0:26:36 > 0:26:40I am now working with my colleagues Malcolm Perry at Cambridge
0:26:40 > 0:26:43and Andrew Strominger at Harvard
0:26:43 > 0:26:46on whether this can resolve the paradox.
0:26:48 > 0:26:49If he is right,
0:26:49 > 0:26:53Stephen Hawking has solved one of the greatest mysteries in cosmology.
0:26:53 > 0:26:57But does that mean you can escape from a black hole after all,
0:26:57 > 0:26:59or would it still be bad news to fall in one?
0:27:01 > 0:27:03Definitely bad news.
0:27:03 > 0:27:05If it were a stellar-mass black hole,
0:27:05 > 0:27:10you would be made into spaghetti before reaching the horizon.
0:27:10 > 0:27:14On the other hand, if it were a supermassive black hole,
0:27:14 > 0:27:17you would cross the horizon with ease,
0:27:17 > 0:27:20but be crushed out of existence at the singularity.
0:27:22 > 0:27:23BLAST
0:27:27 > 0:27:30Well, there's no doubt in my mind that with the detection
0:27:30 > 0:27:34of gravitational waves, the future of black hole science looks bright.
0:27:34 > 0:27:38And after 50 years of theoretical debates, Stephen Hawking might get
0:27:38 > 0:27:40the experimental evidence he wants
0:27:40 > 0:27:42to test his ideas about black holes.
0:27:42 > 0:27:46And there is another exciting project on the horizon too -
0:27:46 > 0:27:47the Event Horizon Telescope,
0:27:47 > 0:27:51a worldwide network of radio telescopes that next year
0:27:51 > 0:27:55will team up to try and capture the first image of the shadow cast
0:27:55 > 0:27:58by the enormous black hole at the Milky Way's centre.
0:27:58 > 0:28:00This would have been unimaginable
0:28:00 > 0:28:03when Stephen Hawking started grappling with black holes
0:28:03 > 0:28:06over 40 years ago, but it shows this is another chapter
0:28:06 > 0:28:09in the history of black hole science.
0:28:09 > 0:28:10Next month, we will be previewing
0:28:10 > 0:28:12one of the astronomical highlights
0:28:12 > 0:28:15of the year, the transit of Mercury,
0:28:15 > 0:28:17visible across the UK on May 9th.
0:28:19 > 0:28:23And we will be using the opportunity to take a closer look
0:28:23 > 0:28:26at Mercury, one of the strangest planets in the solar system.
0:28:28 > 0:28:30To find out how to catch a glimpse of it before then,
0:28:30 > 0:28:34check out the website for Pete's April star guide.
0:28:37 > 0:28:41- In the meanwhile, get outside and... get looking up.- Good night.