0:00:04 > 0:00:08There's something deeply disturbing in deep space.
0:00:08 > 0:00:15Something so incredibly massive, it could swallow an entire star.
0:00:15 > 0:00:21People tend to be fascinated by things which are big and scary, like dinosaurs,
0:00:21 > 0:00:26and there's really nothing that's bigger and scarier than a black hole.
0:00:26 > 0:00:29Black holes are one of the most destructive forces in nature.
0:00:31 > 0:00:35But far from being monsters, scientists now believe
0:00:35 > 0:00:39they could hold the key to the greatest mystery of all...
0:00:40 > 0:00:42..where the universe came from.
0:00:46 > 0:00:53Black holes are the doorway to understanding the basic laws of the universe around us.
0:00:53 > 0:01:00The trouble is, they're practically invisible and billions of kilometres from Earth.
0:01:00 > 0:01:02We think right there is a black hole.
0:01:02 > 0:01:05Right there.
0:01:05 > 0:01:11The more we try to understand them, the stranger black holes become.
0:01:11 > 0:01:15Everything we know about common sense is thrown out the window.
0:01:15 > 0:01:18The equations no longer make any sense.
0:01:22 > 0:01:28Black holes could force us to abandon everything we thought we knew about the universe.
0:01:32 > 0:01:35There aren't questions much bigger than this.
0:01:35 > 0:01:39There's really a lot that we don't understand.
0:02:03 > 0:02:10We humans have evolved to make sense of planet Earth and, so far, we've made a pretty good stab at it.
0:02:11 > 0:02:15In the last century, we've made sense of the impossibly small...
0:02:17 > 0:02:19..and the unimaginably large.
0:02:22 > 0:02:26The enormity of space, and the microscopic behaviour of atoms.
0:02:28 > 0:02:33Yet there are some things that threaten to elude us completely.
0:02:34 > 0:02:37The harder we look, the more questions we uncover.
0:02:40 > 0:02:43Nowhere is this more true than for a black hole.
0:02:56 > 0:03:04I think of a black hole as the symbol of what it is we don't understand about the universe.
0:03:05 > 0:03:10Black holes are one of the most mysterious objects in the cosmos.
0:03:15 > 0:03:22- What are black holes made of? - Oh, OK. Already you've asked me a question that I can't answer.
0:03:24 > 0:03:29They fell out of Einstein's theory of relativity in 1916,
0:03:29 > 0:03:33and they've defied some of our greatest minds ever since.
0:03:36 > 0:03:39Are black holes made of anything?
0:03:46 > 0:03:48Black holes... Hmm.
0:03:48 > 0:03:51We don't really have any idea what's going on, so....
0:03:53 > 0:03:56I don't understand black holes. I love black holes.
0:03:56 > 0:04:00I love black holes because I don't understand them.
0:04:00 > 0:04:04There are many strange things in this universe, but I think I've picked
0:04:04 > 0:04:09the weirdest thing to actually study which is the black hole.
0:04:10 > 0:04:15Until recently, there wasn't much evidence they existed at all,
0:04:15 > 0:04:21because while we think they're out there, we can't see them.
0:04:21 > 0:04:25Black holes are, by definition, completely black.
0:04:27 > 0:04:30Nothing can escape it, even light,
0:04:30 > 0:04:36and that's why it's called a black hole, because light can't come out of it.
0:04:41 > 0:04:47Black holes, totally mysterious, billions of kilometres away
0:04:47 > 0:04:49and practically impossible to see.
0:04:50 > 0:04:53Not that that's stopped astronomers trying.
0:05:09 > 0:05:12Doug Leonard even thinks he's seen one,
0:05:12 > 0:05:15or at least seen one form.
0:05:15 > 0:05:19It took two years and the Hubble space telescope.
0:05:19 > 0:05:24It was only possible at all because we think black holes
0:05:24 > 0:05:28begin their lives as something we've all seen in space - stars.
0:05:29 > 0:05:35Stars, like our sun, are essentially big, hot balls of gas
0:05:35 > 0:05:40that have nuclear generators in their core, that create all the heat and light that we see shining.
0:05:40 > 0:05:43Stars are enormous.
0:05:43 > 0:05:50You could fit a million Earths inside the sun, and the sun is not even an abnormally large star.
0:05:53 > 0:05:56But the most fascinating thing to me about stars is that they die.
0:05:59 > 0:06:02The theory is, black holes are born when nature's most massive stars
0:06:02 > 0:06:07burn off all their fuel and violently collapse.
0:06:07 > 0:06:11The cores of these massive stars implode in less than a second.
0:06:11 > 0:06:14They go from something about the size of the Earth,
0:06:14 > 0:06:17down to something about the size of a small city.
0:06:17 > 0:06:23And they don't stop there, they continue imploding all the way down to a point.
0:06:23 > 0:06:28That point is what we believe becomes a black hole.
0:06:30 > 0:06:34And it's this process that Doug Leonard believes he's spotted
0:06:34 > 0:06:39when a massive explosion, supernova, signalled the death of a star
0:06:39 > 0:06:43in a remote galaxy billions and billions of kilometres from Earth.
0:06:52 > 0:06:56This is a picture of a galaxy 215 million light years away
0:06:56 > 0:06:59and, indicated by the arrow, this is the supernova,
0:06:59 > 0:07:07a single star that exploded that, for a short period of time, is as bright as the entire galaxy that it's in.
0:07:07 > 0:07:09And that big blob there is the galaxy?
0:07:09 > 0:07:13This big blob here is the combined light
0:07:13 > 0:07:17of tens of billions of ordinary stars.
0:07:17 > 0:07:21This is a close-up, an extreme close-up, of the supernova
0:07:21 > 0:07:25while it was still very, very bright. Once the supernova was discovered,
0:07:25 > 0:07:28we trawled the Hubble space telescope archives
0:07:28 > 0:07:33and found a picture of this exact spot taken eight years earlier,
0:07:33 > 0:07:37and what we found at the location of the supernova
0:07:37 > 0:07:41was this object, which is actually an extremely bright star.
0:07:41 > 0:07:44So what we did next was wait.
0:07:44 > 0:07:51For two years, we waited for all the fire arcs of this supernova explosion to disappear and go out,
0:07:51 > 0:07:54and we went back and took another picture
0:07:54 > 0:07:58of that exact spot in the sky, and what we found was nothing.
0:07:58 > 0:08:05The star was gone. It exploded as a supernova and had now disappeared.
0:08:05 > 0:08:07And we think right there is a black hole.
0:08:07 > 0:08:10Right there.
0:08:10 > 0:08:12But I can't ever be 100% sure about that.
0:08:12 > 0:08:15Is that because you can't see it?
0:08:17 > 0:08:21Seeing nothing in black hole science is a great thing.
0:08:21 > 0:08:25You don't expect to see anything when you're looking at a black hole.
0:08:27 > 0:08:34As images of black holes go, these few dark pixels are about as good as it gets.
0:08:34 > 0:08:39Without the death of a star, there'd be no reason to suspect there was a black hole there at all.
0:08:46 > 0:08:53In fact, black holes are so hard to see, most of what we know about them hasn't come from those observing
0:08:53 > 0:08:57the universe but from another group of scientists - the theorists.
0:08:57 > 0:09:00And the universe they study is in their heads.
0:09:05 > 0:09:07I think of theoretical physics
0:09:07 > 0:09:11really as a great detective story that you get to be part of.
0:09:11 > 0:09:16The clues look so few and scant that it seems like a hopeless case,
0:09:16 > 0:09:22but if you work really hard at it, often you can discover amazing stuff.
0:09:24 > 0:09:30So it's amazing to me how much one can actually learn about reality just by detective work.
0:09:30 > 0:09:36Black holes have existed in theorists' minds and notebooks for almost a century,
0:09:36 > 0:09:41most notably in the mind and notebook of Albert Einstein.
0:09:44 > 0:09:49In 1916, Einstein changed the way we see our world.
0:09:49 > 0:09:53Purely by the power of thought, and some clever mathematics,
0:09:53 > 0:09:57he explained something we all take for granted - gravity.
0:09:59 > 0:10:04Gravity is the universal force which holds everything together.
0:10:04 > 0:10:09If you were to shut off gravity right now, the sun would explode,
0:10:09 > 0:10:15the Earth would fall apart, and we'd be flung into outer space at a thousand miles per hour.
0:10:15 > 0:10:18So it's gravity that keeps us rooted onto the Earth
0:10:18 > 0:10:22and holds and binds the galaxy and the solar system together.
0:10:25 > 0:10:31Scientists had been able to calculate the effects of gravity for centuries.
0:10:32 > 0:10:36But until Einstein, what caused it had remained a mystery.
0:10:40 > 0:10:43The answer was stranger than anyone had imagined.
0:10:49 > 0:10:56Einstein's great insight was to realise that gravity is caused by the bending of space and time.
0:10:56 > 0:11:04So gravity is not really pulling me down to the ground, it is space that is pushing me down.
0:11:04 > 0:11:07Einstein called his theory general relativity.
0:11:11 > 0:11:16The theory of relativity is infamous for its difficulty.
0:11:16 > 0:11:22I want to show that there's nothing peculiarly difficult about it.
0:11:24 > 0:11:26Space isn't simply an empty void,
0:11:26 > 0:11:30it can be bent and stretched.
0:11:30 > 0:11:32Let me illustrate this one example.
0:11:32 > 0:11:39Let's imagine that this piece of jelly is the space, then the presence of matter is to distort the space.
0:11:39 > 0:11:45All massive objects like stars and planets bend the space and time around them.
0:11:47 > 0:11:53Any object that passes through that warped space time will move as if being pulled by a force,
0:11:53 > 0:11:57and this is what we experience as gravity.
0:11:57 > 0:12:02Einstein's theory of relativity does lead us into very strange and unfamiliar paths.
0:12:05 > 0:12:10Relativity is perfectly intelligible to anybody who is willing to think.
0:12:17 > 0:12:22Einstein's theory has withstood the test of time for almost a century.
0:12:22 > 0:12:27If there is one data point out of place, we would have to throw the entire theory out.
0:12:27 > 0:12:32Everywhere we look in the heavens, Einstein's theory comes right on the spot.
0:12:35 > 0:12:39But less than a year after it was published, theorists realised
0:12:39 > 0:12:42general relativity predicted something so profoundly troubling,
0:12:42 > 0:12:47many believed it couldn't exist in the real world.
0:12:47 > 0:12:53Anything very heavy and very small would create such a strong gravitational field
0:12:53 > 0:12:57that space and time would be bent and twisted to breaking point.
0:13:01 > 0:13:04General relativity had predicted the existence of black holes.
0:13:11 > 0:13:13And it didn't just say that they would exist...
0:13:13 > 0:13:19general relativity allows us to imagine what it would be like to travel into one.
0:13:48 > 0:13:52There's a beautiful analogy between black holes and waterfalls
0:13:52 > 0:13:56which actually lets us calculate all properties of black holes exactly.
0:14:01 > 0:14:05When you approach a waterfall, the river flows faster and faster.
0:14:05 > 0:14:11When you approach a black hole, it's not the water that flows faster, it's space itself.
0:14:14 > 0:14:21The structure of a black hole is similar to the relentless flow of water over a waterfall.
0:14:21 > 0:14:27It's an analogy that follows the water from the river above to the rocks below
0:14:27 > 0:14:31and allows us to journey into the very heart of a black hole.
0:14:34 > 0:14:37If you're swimming upstream from a waterfall,
0:14:37 > 0:14:41there is an invisible line where the water flows as fast as you can swim,
0:14:41 > 0:14:45and if you cross that line, it's the point of no return.
0:14:45 > 0:14:48You wouldn't feel anything special, but no matter how hard you struggle,
0:14:48 > 0:14:53you can never escape getting sucked all the way down.
0:14:55 > 0:15:00For a black hole, the point of no return is called the event horizon.
0:15:02 > 0:15:06Past it, space is travelling inwards faster than the speed of light.
0:15:19 > 0:15:25Even if I can only swim at a maximum speed, the water can obviously fall much faster than that.
0:15:25 > 0:15:31In the same way, even though I can never go faster than the speed of light through space,
0:15:31 > 0:15:35space itself is allowed, in the black hole, to fall as fast as it wants,
0:15:35 > 0:15:40which means that everything that's there, even a particle of light trying to go upward,
0:15:40 > 0:15:44will be sucked inexorably downwards towards the centre.
0:15:47 > 0:15:50Assuming your body withstood the intense gravity,
0:15:50 > 0:15:54leaving the universe forever could be remarkably uneventful.
0:16:00 > 0:16:06People used to think that you would die at the event horizon, but we now understand that for big black holes,
0:16:06 > 0:16:12it's perfectly possible to still be alive at this stage, you just have no choice but to continue downward.
0:16:16 > 0:16:22Everything would feel just normal to you, you wouldn't even know necessarily that you're doomed.
0:16:22 > 0:16:25The only thing is that there's no way you can ever get out again.
0:16:25 > 0:16:31As you approach the centre of the black hole, you reach the inner horizon,
0:16:31 > 0:16:37where everything falling in meets matter being pushed out by the hole's rotation,
0:16:37 > 0:16:43similar to where the torrent flowing over the falls hits water rebounding back up.
0:16:43 > 0:16:49Eventually, the inward flow actually slows down to become slower than the speed of light,
0:16:49 > 0:16:54because the rotation of the black hole causes a sort of repulsion.
0:16:54 > 0:17:00At that point, you have things colliding together near the speed of light,
0:17:00 > 0:17:05creating these ridiculously high temperatures, much hotter than inside of a star.
0:17:05 > 0:17:09So hot that it would vaporise me and any ordinary matter.
0:17:09 > 0:17:14So that makes an ordinary traffic accident seem tame in comparison,
0:17:14 > 0:17:19now you're being hit by a truck going almost 300,000km per second.
0:17:19 > 0:17:22It's not a place where I would wanna be.
0:17:23 > 0:17:28The inner horizon is one of the most extreme environments in the universe.
0:17:28 > 0:17:31According to general relativity,
0:17:31 > 0:17:37the only place more extreme is what lies beyond it.
0:17:49 > 0:17:51Let me gather my thoughts for a moment.
0:18:05 > 0:18:08It's remarkably difficult for us
0:18:08 > 0:18:14to actually calculate with Einstein's equations what happens inside the inner horizon.
0:18:16 > 0:18:20But if I jumped into a black hole, that's probably as far down as I would get.
0:18:25 > 0:18:28At the centre of a black hole,
0:18:28 > 0:18:31the equations predict something so strange,
0:18:31 > 0:18:35it blows Einstein's greatest achievement out of the water
0:18:35 > 0:18:39and forces us to question our understanding of the universe.
0:19:01 > 0:19:07Einstein hoped that general relativity would form the framework for a new understanding of nature.
0:19:09 > 0:19:12But at the heart of its description of a black hole,
0:19:12 > 0:19:16theorists found a problem with Einstein's mathematics.
0:19:16 > 0:19:21Something so disturbing, his theory breaks down completely.
0:19:43 > 0:19:48Einstein's equations of general relativity simply say the following -
0:19:48 > 0:19:53the Ricci curvature tensor minus one half the metric tensor,
0:19:53 > 0:19:58times the contracted curvature tensor is proportional to the stress energy tensor.
0:19:58 > 0:20:04All this says that if I start with a star, a black hole, or even a universe,
0:20:04 > 0:20:09that determines the curvature that surrounds that concentration of matter and energy.
0:20:09 > 0:20:13But inside these equations, there's a monster.
0:20:13 > 0:20:16In the extreme gravity of the core of a black hole,
0:20:16 > 0:20:21Einstein's equations spiral wildly out of control.
0:20:23 > 0:20:31After every long tedious calculation, I mostly get zeros but the non-zero term is given as follows...
0:20:31 > 0:20:38M is the mass of the black hole, R describes the distance from the black hole...
0:20:38 > 0:20:43Here is the problem, right there... when R is equal 0...
0:20:44 > 0:20:48The point at which physics itself breaks down.
0:20:48 > 0:20:55So one over R equals one over 0 equals infinity.
0:20:55 > 0:20:59To a mathematician, infinity is simply a number without limit.
0:20:59 > 0:21:02To a physicist, it's a monstrosity.
0:21:02 > 0:21:08It means that gravity is infinite at the centre of a black hole, that time stops. And what does that mean?
0:21:08 > 0:21:14Space makes no sense, it means the collapse of everything we know about the physical universe.
0:21:14 > 0:21:19In the real world, there's no such thing as infinity,
0:21:19 > 0:21:25therefore there is a fundamental flaw in the formulation of Einstein's theory.
0:21:25 > 0:21:30According to Einstein then, all the mass of the black hole is contained
0:21:30 > 0:21:38within an infinitely small point that takes up precisely no space at all.
0:21:38 > 0:21:45This impossible object of infinite density and infinite gravity is called the singularity.
0:21:45 > 0:21:48We know what a singularity is.
0:21:48 > 0:21:52A singularity is when we don't know what to do.
0:21:55 > 0:21:58To me what's so embarrassing about a singularity
0:21:58 > 0:22:02is that we can't predict anything about what's gonna come out of it.
0:22:02 > 0:22:07I could have a singularity and - boom - out comes a pink elephant with purple stripes.
0:22:07 > 0:22:13And that's consistent with what the laws of physics predicts, because they don't predict anything.
0:22:14 > 0:22:22A singularity is when our understanding of nature breaks down, that's what a singularity is.
0:22:31 > 0:22:37Einstein realised there was a problem when he was shown this infinity,
0:22:37 > 0:22:43but he thought that black holes could never physically form, therefore it was an academic question.
0:22:43 > 0:22:49Sure, there was a problem, but it didn't matter because mother nature could never create a black hole.
0:22:51 > 0:22:54In 1939, Einstein even wrote a paper
0:22:54 > 0:22:59that appeared to prove black holes would never be found in the real world.
0:22:59 > 0:23:04He hoped that there'd be some physical mechanism that would stop them from actually being produced.
0:23:04 > 0:23:06And he really wanted to ask the question
0:23:06 > 0:23:12could they physically form? I think he wanted to show the answer was no.
0:23:12 > 0:23:17Given the physics known at the time, his assumptions were reasonable,
0:23:17 > 0:23:22but we've learned a lot of physics since then so therefore we know that his reasoning was incomplete.
0:23:24 > 0:23:29At the time, no-one had seen anything to suggest Einstein was wrong.
0:23:29 > 0:23:31For years, theorists were happy
0:23:31 > 0:23:37that general relativity was a complete understanding of gravity in our universe.
0:23:38 > 0:23:42Then, in the early 1970s, astronomers made a breakthrough.
0:23:49 > 0:23:52X-rays revealed hot gas falling into objects
0:23:52 > 0:23:56that were both extremely massive and invisible to normal light.
0:23:58 > 0:24:02For some, these images could only be caused by black holes.
0:24:04 > 0:24:09Material on the way into the black hole can become very hot.
0:24:09 > 0:24:14So hot that it becomes a million degrees or even ten million degrees, and that makes x-rays.
0:24:14 > 0:24:17And just before this lump of material disappears in the black hole,
0:24:17 > 0:24:21it becomes a bright flash of x-ray radiation.
0:24:28 > 0:24:34Professor Reinhard Genzel is Director of the Max-Planck Institute for Extraterrestrial Physics.
0:24:36 > 0:24:43He's spent the last 25 years looking for proof of the existence of one particular black hole.
0:24:44 > 0:24:50While we can't see black holes as such, we can see that they're there and what they are
0:24:50 > 0:24:52through their interaction
0:24:52 > 0:24:56with visible objects like stars, like gas in their vicinity.
0:24:59 > 0:25:02Using radio telescopes,
0:25:02 > 0:25:08astronomers had also seen objects at the centres of galaxies they suspected were black holes.
0:25:08 > 0:25:12But to prove it, they'd need to make more precise measurements.
0:25:14 > 0:25:18Unfortunately, the nearest one was 25,000 light years away
0:25:18 > 0:25:21and totally obscured by dust.
0:25:23 > 0:25:27It was at the centre of our own galaxy.
0:25:31 > 0:25:34It took Genzel and his team nearly ten years
0:25:34 > 0:25:38to develop an infrared telescope capable of seeing enough detail
0:25:38 > 0:25:43through the clouds of dust and gas surrounding the galactic centre.
0:25:44 > 0:25:52It took them a further 13 years of painstaking observations before they saw the thing they were looking for.
0:25:52 > 0:25:57A star orbiting exceptionally close to the centre.
0:25:57 > 0:26:04Genzel knew that measuring the star's orbit could tell him about whatever it was orbiting.
0:26:07 > 0:26:11So what we are seeing are the innermost stars.
0:26:11 > 0:26:17This green cross, that's the centre of the Milky Way, Sagittarius A star.
0:26:17 > 0:26:21So in 2002, this star here was very close to this
0:26:21 > 0:26:25and the next year, it has moved quite a substantial distance.
0:26:25 > 0:26:29Because the galactic centre is so far away,
0:26:29 > 0:26:33this minute change means the star is moving incredibly fast.
0:26:33 > 0:26:41The separation which you see is quite an enormous distance, these are several light weeks.
0:26:41 > 0:26:43And how far is that in kilometres?
0:26:44 > 0:26:47OK...
0:26:47 > 0:26:53So we have an hour, and we have a day, and then take a week,
0:26:53 > 0:26:55then we have the speed of light...
0:26:55 > 0:26:58and so in kilometres, OK...
0:26:58 > 0:27:03Wow, is that a big number - 180 billion kilometres.
0:27:03 > 0:27:05Let me just check this so...
0:27:08 > 0:27:13Yeah, a 180...180 billion kilometres.
0:27:13 > 0:27:16I can't deal with that number.
0:27:16 > 0:27:20It's hard to imagine what a 180 billion kilometres is.
0:27:20 > 0:27:25Once you know the size of a star's orbit and the time it takes to go round,
0:27:25 > 0:27:31it's a relatively simple calculation to work out the mass of the object it's orbiting.
0:27:31 > 0:27:38Although tracking a single star would be enough to measure the mass of the central object,
0:27:38 > 0:27:44Professor Genzal has mapped the orbits of the 30 stars closest to the galactic centre.
0:27:44 > 0:27:47Here we have the innermost stars.
0:27:49 > 0:27:55And these orbits we determine uniquely from the motion we have tracked over the years.
0:27:55 > 0:27:58So it takes S2, this innermost star,
0:27:58 > 0:28:0215 years to move once around the centre of the Milky Way here.
0:28:02 > 0:28:08The other stars are slower, some of them take several hundred years to move around.
0:28:08 > 0:28:13From the size of each of these orbits and the speed the stars were travelling,
0:28:13 > 0:28:19Professor Genzal calculated the mass of the central object and it was truly astronomical.
0:28:19 > 0:28:25From these two numbers, you already can determine uniquely the central mass,
0:28:25 > 0:28:28and we can do this for each of these stars,
0:28:28 > 0:28:31and we find that the mass is always the same.
0:28:31 > 0:28:34It's four million times the mass of the sun.
0:28:36 > 0:28:40Because the closest stars pass so near to the centre,
0:28:40 > 0:28:47this extraordinary mass, four million times heavier than the sun, must be in a very small space.
0:28:47 > 0:28:51That really clinches this. Because nothing fits in there,
0:28:51 > 0:28:56into this relatively small volume other than the massive black hole.
0:28:56 > 0:29:03Even a schoolchild can analyse the data and will come to the same conclusion, it's very clear.
0:29:05 > 0:29:08What Genzel had found at the centre of our galaxy
0:29:08 > 0:29:13was so heavy and so small, it had to be a black hole,
0:29:13 > 0:29:17but it was far too big to have formed from the collapse of a single star.
0:29:19 > 0:29:24The black hole at the centre of our galaxy is an object
0:29:24 > 0:29:27which is much more massive than the stellar black holes.
0:29:28 > 0:29:32It's about four million times the mass of the sun.
0:29:32 > 0:29:36So we would call these super massive black holes.
0:29:37 > 0:29:40Although Professor Genzel hadn't seen a black hole,
0:29:40 > 0:29:43the indirect evidence was so compelling
0:29:43 > 0:29:46there could be little doubt black holes were real
0:29:46 > 0:29:50and it won him the 2008 Shaw Prize for Astronomy.
0:29:52 > 0:29:56So the prize, the Shaw prize, is a fairly large amount of money,
0:29:56 > 0:29:58actually a million dollars,
0:29:58 > 0:30:02which was given to me and with no strings attached.
0:30:03 > 0:30:07So I've given some of it away to my colleagues,
0:30:07 > 0:30:12some of it I kept myself and, you know, people have convinced me
0:30:12 > 0:30:16I should use some of that to buy a new car.
0:30:23 > 0:30:28Everything in our galaxy, the Earth, the sun, a million million stars,
0:30:28 > 0:30:32are all spinning around the super massive black hole at the centre.
0:30:39 > 0:30:42And ours isn't even particularly impressive.
0:30:45 > 0:30:50The super massive black hole at the centre of our galaxy is quite small relative
0:30:50 > 0:30:53to other super massive black holes that we know about.
0:30:53 > 0:30:55There are galaxies, not very far from ours,
0:30:55 > 0:30:59in which we have seen super massive black holes
0:30:59 > 0:31:04up to a thousand times more massive, several billion solar masses.
0:31:10 > 0:31:16It now appears there's a super massive black hole at the centre of almost every galaxy.
0:31:17 > 0:31:22And it could be that these black holes aren't simply agents of destruction,
0:31:22 > 0:31:27because scientists have discovered a unique relationship they share with their parent galaxy.
0:31:30 > 0:31:32So the mass of the super massive black hole
0:31:32 > 0:31:36is related to the mass of the parent galaxy in a very simple way,
0:31:36 > 0:31:39so I can show this with a graph here.
0:31:39 > 0:31:45So let me say, along one axis, I'll show the mass of the black hole.
0:31:45 > 0:31:49And I will measure this mass in terms of the mass of the sun.
0:31:49 > 0:31:54So let's say down here it is a million times the mass of the sun.
0:31:54 > 0:32:00Ten million, 100 million, billion times the mass of the sun,
0:32:00 > 0:32:03so that's the range of black hole masses we have seen.
0:32:03 > 0:32:09Along this axis, let me just show you the mass of the galaxy.
0:32:09 > 0:32:13Let me start with a billion times the mass of the sun...
0:32:13 > 0:32:20ten billion, 100 billion, a million million solar masses.
0:32:20 > 0:32:24Basically, when people measure these two masses for a large number of galaxies,
0:32:24 > 0:32:29what they find is different galaxies may come different places here on this diagram.
0:32:29 > 0:32:33And the miraculous thing is that all these points seem to lie
0:32:33 > 0:32:36more or less on a straight line in this plot.
0:32:38 > 0:32:45So there seems to be a... some relation between the mass of the black hole and the galaxy.
0:32:45 > 0:32:47Roughly, the black hole seems to be approximately
0:32:47 > 0:32:52a thousand times less massive than the galaxy in which it lives.
0:32:52 > 0:32:58The existence of this kind of a relation is rather surprising, because what it means is
0:32:58 > 0:33:02somehow the black hole is able to influence the entire galaxy
0:33:02 > 0:33:06and is actually modifying perhaps how the galaxy forms and evolves.
0:33:06 > 0:33:09This is the surprise in this business.
0:33:13 > 0:33:16In the last century, black holes have gone
0:33:16 > 0:33:20from being mathematical curiosities to real objects in the cosmos,
0:33:20 > 0:33:26millions of times the mass of the sun and seemingly crucial to the formation of galaxies.
0:33:31 > 0:33:35I think black holes have got maybe a little bit of a bad rap
0:33:35 > 0:33:38as being the ultimate bad guys in the universe.
0:33:38 > 0:33:42It might well be that the monster black holes in the middle of galaxies
0:33:42 > 0:33:47actually helped the galaxies form and therefore helped life come on the scene.
0:33:53 > 0:33:56As well as super massive black holes,
0:33:56 > 0:34:01astronomers believe there are also billions of smaller stellar black holes all over the cosmos.
0:34:10 > 0:34:15- How many black holes are there? - Roughly every galaxy has got one big black hole in the middle,
0:34:15 > 0:34:21super massive black hole, and millions and millions of smaller black holes.
0:34:22 > 0:34:25Black holes are common, they're a very common occurrence
0:34:25 > 0:34:29in nature, fantastic thing. Would we have thought it? No.
0:34:29 > 0:34:34Think of all the galaxies, each one with a raging black hole in the centre.
0:34:34 > 0:34:37Each one with perhaps thousands of stellar black holes in them
0:34:37 > 0:34:41and then you begin to realise that black holes represent
0:34:41 > 0:34:45one of the dominant forces in the evolution of the universe.
0:34:47 > 0:34:51Black holes, it turns out, are everywhere.
0:34:53 > 0:34:58And that means millions upon millions of places where Einstein's equations break down.
0:35:15 > 0:35:20But physicists have always known that relativity is an incomplete theory of nature.
0:35:25 > 0:35:32Although it beautifully describes how gravity influences the motions of planets, stars and galaxies,
0:35:32 > 0:35:36it can never describe the world at the smallest possible scale.
0:35:40 > 0:35:45The realm of atoms and the tiny particles that form them.
0:35:48 > 0:35:51To do that, they use a separate theory.
0:35:53 > 0:35:55A theory called quantum mechanics.
0:36:05 > 0:36:08You might wonder why we'd wanna apply quantum mechanics
0:36:08 > 0:36:11to something as large as a massive black hole,
0:36:11 > 0:36:15when quantum mechanics deals with the very small.
0:36:17 > 0:36:24And that's because, ultimately, at the heart of a large black hole is a singularity.
0:36:28 > 0:36:35Whatever a singularity really is, one thing we do know is it must be very, very small.
0:36:40 > 0:36:45It seems quite likely that, in order to really
0:36:45 > 0:36:50understand what goes inside a black hole, we will need quantum mechanics,
0:36:50 > 0:36:55that the final story of how a black hole works
0:36:55 > 0:36:59and what happens at the singularity
0:36:59 > 0:37:03can only be understood when quantum mechanics is included.
0:37:05 > 0:37:11This subatomic world quantum mechanics describes is nothing like the world we experience.
0:37:13 > 0:37:18Quantum mechanics tells us how the world works at a fundamental level
0:37:18 > 0:37:21and it is stranger than you can imagine.
0:37:22 > 0:37:28In the quantum world, the mere act of observing changes what you see.
0:37:28 > 0:37:32You can't say where something is, only where it's likely to be
0:37:32 > 0:37:39and anything that is possible, no matter how unlikely, happens all the time.
0:37:42 > 0:37:46All of our notions about how things behave change.
0:37:47 > 0:37:50For example, an object has a known location,
0:37:50 > 0:37:54"I'm here, you're there," but at a quantum mechanical scale,
0:37:54 > 0:37:57objects can be in many different places at the same time, literally.
0:37:59 > 0:38:02Yet as strange as quantum mechanics is, theorists
0:38:02 > 0:38:07believe the world it describes is the true nature of reality.
0:38:07 > 0:38:13Quantum mechanics is so weird, it may sound like science fiction,
0:38:13 > 0:38:16but it's not science fiction, it's science fact,
0:38:16 > 0:38:20and it's done better than any other idea in physics.
0:38:20 > 0:38:24It allows us to make the best predictions we've ever made,
0:38:24 > 0:38:28so like it or not, it describes the world.
0:38:28 > 0:38:35Quantum mechanics describes everything, there's no escaping quantum mechanics.
0:38:35 > 0:38:42Every object is a quantum mechanical object subject to the laws of quantum mechanics.
0:38:42 > 0:38:46And the world that we live in,
0:38:46 > 0:38:49in the ultimate reality, is a quantum world.
0:38:52 > 0:38:56So there's no question that there's some great truth in quantum mechanics.
0:38:59 > 0:39:02But there's one thing quantum mechanics can't describe -
0:39:02 > 0:39:04gravity.
0:39:04 > 0:39:09And it's not normally a problem, because atoms are so light, the effect of gravity is irrelevant.
0:39:13 > 0:39:20Most of the time, quantum mechanics and gravity leave each other in peace.
0:39:21 > 0:39:28But there's one arena in which they're both important,
0:39:28 > 0:39:34and that arena is when things are both very small
0:39:34 > 0:39:38and the force of gravity is very large.
0:39:38 > 0:39:41And that's what happens inside a black hole.
0:39:47 > 0:39:54The singularity at the heart of a black hole is both astronomically heavy and infinitesimally small.
0:39:54 > 0:39:58To understand it, quantum mechanics alone wasn't enough.
0:39:58 > 0:40:02It needed to be extended to describe gravity.
0:40:02 > 0:40:05A theory called quantum gravity.
0:40:15 > 0:40:18The most obvious way to create such a theory
0:40:18 > 0:40:22was to make a quantum version of Einstein's theory of relativity.
0:40:22 > 0:40:26Proof of its success would be a new understanding of black holes
0:40:26 > 0:40:30that explained what really happens in a singularity.
0:40:41 > 0:40:47When physicists tried to combine the two theories, they encountered a familiar problem.
0:40:47 > 0:40:50I insert this into the probability
0:40:50 > 0:40:55that gravity will move from one point to another point.
0:40:55 > 0:41:00When I actually do this calculation, I get yet another integral,
0:41:00 > 0:41:03and when you do this integral,
0:41:03 > 0:41:08you get something which makes no sense whatsoever -
0:41:08 > 0:41:10an infinity.
0:41:10 > 0:41:13Total nonsense!
0:41:13 > 0:41:17In fact, you get an infinite sequence of infinities,
0:41:17 > 0:41:21infinitely worse than the divergences of Einstein's original theory.
0:41:21 > 0:41:25This is a nightmare beyond comprehension.
0:41:31 > 0:41:35The search for a theory of quantum gravity had fallen apart,
0:41:35 > 0:41:41because quantum mechanics and general relativity proved to be totally incompatible.
0:41:44 > 0:41:49I think the most embarrassing problem we have in theoretical physics is that
0:41:49 > 0:41:52we have these two different theories which won't talk to each other.
0:41:56 > 0:42:02We have Einstein's theory of gravity, which beautifully describes the very big and the very fast,
0:42:02 > 0:42:07and then we have quantum physics, which very successfully describes
0:42:07 > 0:42:11the very small and yet, clearly, nature has one unique way
0:42:11 > 0:42:14of operating, it's not schizophrenic,
0:42:14 > 0:42:19and we humans just don't seem to be able to find that way.
0:42:21 > 0:42:25The failure of these two great theories to understand black holes
0:42:25 > 0:42:30means they are, at best, an approximation to the laws governing the universe.
0:42:33 > 0:42:36The equations no longer make any sense
0:42:36 > 0:42:42and nobody knows exactly what we're supposed to do about that.
0:42:46 > 0:42:48Well, it's awful.
0:42:48 > 0:42:51It means that physics is having a nervous breakdown.
0:42:51 > 0:42:57It means the collapse of physics as we know it, you know?
0:42:57 > 0:43:00Something is fundamentally wrong.
0:43:03 > 0:43:05Nature is smarter than we are.
0:43:09 > 0:43:13If we want to understand the universe,
0:43:13 > 0:43:19we must understand how quantum mechanics and gravity
0:43:19 > 0:43:24can live together and so that's our challenge.
0:43:26 > 0:43:29So it's quite a big question?
0:43:29 > 0:43:32It's a huge question.
0:43:32 > 0:43:35There aren't questions much bigger than this.
0:43:39 > 0:43:42We don't understand.
0:43:48 > 0:43:54For nearly 100 years, physics has been able to explain the universe around us.
0:43:54 > 0:44:00General relativity perfectly describes the motions of stars and galaxies.
0:44:01 > 0:44:06And the world of atoms is beautifully explained by quantum mechanics.
0:44:07 > 0:44:12Yet the discovery of black holes means we don't fully understand anything.
0:44:16 > 0:44:18But far from being a problem,
0:44:18 > 0:44:23black holes represent one of the greatest opportunities in physics.
0:44:23 > 0:44:29Black holes are the key to... taking the next step,
0:44:29 > 0:44:33the doorway to our next step
0:44:33 > 0:44:38in understanding the basic laws of the universe around us.
0:44:46 > 0:44:49Unlocking the mysteries of black holes could provide
0:44:49 > 0:44:53the answer to the biggest question every posed by the human mind.
0:44:56 > 0:44:59Because there's one other place where our current laws of nature
0:44:59 > 0:45:02fail as dramatically as they do in a black hole.
0:45:09 > 0:45:14Any direction you look up from the Earth at distant galaxies,
0:45:14 > 0:45:17every single one of them is moving away from us.
0:45:19 > 0:45:24And the only way to make sense of that is to think of the entire universe just expanding.
0:45:24 > 0:45:28This much we know and have known for 80 years.
0:45:28 > 0:45:32But then, there is an immediate very profound implication.
0:45:32 > 0:45:36If the universe is expanding, long ago it was much more compact.
0:45:39 > 0:45:46Nearly 14 billion years ago, Einstein's theory says the universe began in the Big Bang.
0:45:58 > 0:46:01So just to get an idea of the scale of the universe,
0:46:01 > 0:46:04let's start with the Earth, which is a pretty big object.
0:46:04 > 0:46:10The sun is about a million times more massive than the Earth
0:46:10 > 0:46:14and most stars that we see in the sky are about the size of the sun
0:46:14 > 0:46:19and our galaxy has roughly a million million of these stars.
0:46:19 > 0:46:23And then the universe has roughly a million million galaxies.
0:46:23 > 0:46:27So that's a huge amount of stuff and all that started from a singularity.
0:46:27 > 0:46:34A point from which an initial explosion got the expansion going. That's the Big Bang.
0:46:35 > 0:46:39For me, it's a weird concept, as weird a concept
0:46:39 > 0:46:43as it would be to any person who's hearing about it for the first time.
0:46:43 > 0:46:48But nature is doing it, so that's what makes this exciting.
0:46:51 > 0:46:56The singularity, the impossible object found at the heart of every black hole,
0:46:56 > 0:47:01is the same impossible object found at the very beginning of time.
0:47:02 > 0:47:09The whole universe came out of a singularity, all of us are the product of a big singularity.
0:47:09 > 0:47:16And so these singularities are very, very interesting for many reasons.
0:47:16 > 0:47:21There are two places in nature where there apparently are singularities.
0:47:21 > 0:47:24One is at the centre of a black hole
0:47:24 > 0:47:29and the other is at the beginning of time itself at the Big Bang.
0:47:31 > 0:47:36So it's quite likely, if we understood the singularity associated with the black hole,
0:47:36 > 0:47:41we might resolve the question of how the universe began and where we came from.
0:47:43 > 0:47:48Black holes could hold the key to understanding what there was before the universe existed.
0:47:50 > 0:47:54But while we might seem tantalisingly close,
0:47:54 > 0:47:59black holes and the theory that explains them remain just out of reach.
0:48:05 > 0:48:11Quantum gravity is the name that we give to the solution to this problem.
0:48:11 > 0:48:15We don't really know what quantum gravity is.
0:48:15 > 0:48:19What's frustrating with quantum gravity is that previous revolutions in physics,
0:48:19 > 0:48:22like quantum mechanics, relativity theory,
0:48:22 > 0:48:25were all brought on by a lot of clues from nature
0:48:25 > 0:48:29and, for quantum gravity, we have almost no clues at all.
0:48:30 > 0:48:34Right now, we're mostly stuck with having to figure this out
0:48:34 > 0:48:37with pencil and paper just from theory.
0:48:41 > 0:48:45The trouble is, although we know black holes are everywhere,
0:48:45 > 0:48:49we've never seen a single one directly.
0:48:49 > 0:48:53- Have you ever seen a black hole? - No.
0:48:53 > 0:48:58- Have you ever seen a black hole? - No.
0:48:58 > 0:49:00No-one has ever seen a black hole directly.
0:49:02 > 0:49:07Here is an object in outer space that is beyond our mathematics,
0:49:07 > 0:49:13beyond our physical theories, demanding a theory beyond Einstein.
0:49:13 > 0:49:17And, ironically, we can't see them.
0:49:18 > 0:49:24But according to general relativity, a black hole won't just create a dark shadow in space,
0:49:24 > 0:49:28this shadow would be surrounded by a bright halo.
0:49:28 > 0:49:32A black hole's immense gravity warps the space around it,
0:49:32 > 0:49:37focusing the starlight coming from behind into a ring.
0:49:38 > 0:49:43And, in theory at least, we might even be able to see it.
0:49:44 > 0:49:48You can see how they warp with the space around them.
0:50:00 > 0:50:03Shep Doeleman is aiming to do just that.
0:50:04 > 0:50:09He's devoted his career to making the first direct observations of a black hole.
0:50:18 > 0:50:22I happen to really like making the observations,
0:50:22 > 0:50:25getting things done, that there's a real joy
0:50:25 > 0:50:28in assembling a new kind of telescope.
0:50:28 > 0:50:33There's a real joy in making a new kind of measurement that no-one has ever made before.
0:50:33 > 0:50:39I guess that theoreticians feel the same way when they think of an idea that nobody has thought of before.
0:50:40 > 0:50:44Shep is attempting to take a picture of a shadow cast in space
0:50:44 > 0:50:48by the super massive black hole at the centre of our galaxy.
0:50:48 > 0:50:53Directly observing how and where general relativity fails
0:50:53 > 0:50:56could provide vital clues for the theory that replaces it.
0:50:57 > 0:51:03Our observations are aimed squarely at testing general relativity
0:51:03 > 0:51:06in one of the most extreme environments in the universe -
0:51:06 > 0:51:09the event horizon of a black hole. And it's there
0:51:09 > 0:51:12that Einstein's theories may break down.
0:51:14 > 0:51:20For quantum gravity, seeing the shadow exactly as predicted by Einstein would be of little use.
0:51:23 > 0:51:28If we see something that is not consistent with general relativity,
0:51:28 > 0:51:32the theorists will be extremely interested and will want to know everything about that
0:51:32 > 0:51:36and that will point them in a new direction for a theory of gravity.
0:51:36 > 0:51:38We could look at the centre of our galaxy,
0:51:38 > 0:51:44see something completely unpredicted around this black hole that would send us back to the drawing board.
0:51:45 > 0:51:49Shep is an astronomer at the Haystack Observatory near Boston.
0:51:50 > 0:51:54But the 37-metre telescope here simply isn't big enough
0:51:54 > 0:51:58to photograph the black hole at the centre of our galaxy.
0:51:58 > 0:52:03To do that, Shep needs a telescope with 100,000 times the resolution.
0:52:03 > 0:52:09And that requires a dish 4,500 kilometres across,
0:52:09 > 0:52:12roughly the size of the continental United States.
0:52:16 > 0:52:20To observe the object we're after, we have to create a telescope
0:52:20 > 0:52:25that can see finer details than any other telescope in the history of astronomy.
0:52:25 > 0:52:31The reason you haven't heard about this massive telescope is because it only exists in Shep's computer.
0:52:31 > 0:52:38He hooked up radio telescopes from across the continent, effectively to product one giant virtual dish.
0:52:41 > 0:52:45The way a normal telescope works is it focuses all the light
0:52:45 > 0:52:48because of its particular shape into a single focal point.
0:52:48 > 0:52:53When you link telescopes around the world together, we don't have a lens.
0:52:53 > 0:52:57We have to do it in a super computer here in Massachusetts.
0:52:58 > 0:53:01Shep's super computer, the correlator,
0:53:01 > 0:53:05pieces together the raw data from all his separate telescopes
0:53:05 > 0:53:10to build up a computer-generated dish the size of America.
0:53:14 > 0:53:20The level of detail you can see with a single dish is limited by the size of that dish.
0:53:20 > 0:53:24But when you link telescopes around the world together, something magic happens.
0:53:24 > 0:53:28You create a virtual dish that's as big as the distance between those dishes,
0:53:28 > 0:53:33and that gives a level of detail that's a thousand times finer than you can get with a single dish.
0:53:35 > 0:53:41Instead of creating pictures, each of Shep's telescopes produces reams upon reams of data.
0:53:41 > 0:53:45And this is where we keep all of the data when it comes back from the telescopes,
0:53:45 > 0:53:52each of these contains eight very large hard disk drives and when you have two modules together,
0:53:52 > 0:53:57that contains as much data as the US Library of Congress, the largest library in the world,
0:53:57 > 0:54:01and we have on these shelves about 64 such libraries.
0:54:01 > 0:54:04The amount of data is just staggering, really.
0:54:04 > 0:54:09We've spent a lot of money in this project on disk drives.
0:54:11 > 0:54:16There's so much data, processing just a few nights' observations takes months.
0:54:22 > 0:54:25Hey, Mike, what's the latest from the correlator?
0:54:25 > 0:54:28Ah, actually a lot of interesting things from last night.
0:54:28 > 0:54:33You've got a full hour of direct detections on the galactic centre.
0:54:33 > 0:54:38- These are great.- Perfectly clear. - These are great, looks like this is gonna be a great data set.
0:54:38 > 0:54:43What about the other baselines? That's excellent, That is just excellent.
0:54:43 > 0:54:45That's with zeroes, that's with no corrections.
0:54:45 > 0:54:48That's beautiful, that is absolutely beautiful.
0:54:48 > 0:54:52This gives me a lot of confidence we'll be able to do what we wanna do.
0:54:52 > 0:54:55Despite producing all this data, Shep doesn't yet
0:54:55 > 0:54:59have enough telescopes linked together to build up a full image.
0:54:59 > 0:55:02Yeah, so this is a great data set.
0:55:02 > 0:55:06This is... I'm very, very happy with this.
0:55:06 > 0:55:11But this year, he might be able to detect our first glimpse
0:55:11 > 0:55:14of something that has, until now, eluded us -
0:55:14 > 0:55:17the shadow of the event horizon.
0:55:19 > 0:55:22If someone said, "That's impossible, you can't do it,"
0:55:22 > 0:55:26I would say, "That's our job to try and see things that can't be seen,
0:55:26 > 0:55:29"to try to do things that are great challenges."
0:55:29 > 0:55:33The reason that we're interested in this is, quite frankly, because it's hard.
0:55:33 > 0:55:39And if you'd asked me five years ago if it was possible, I flatly would have said no.
0:55:40 > 0:55:44Shep believes that, within ten years, his virtual telescope
0:55:44 > 0:55:47will have the resolution to create an image of a black hole
0:55:47 > 0:55:51and put relativity to the ultimate test.
0:55:51 > 0:55:54That's very exciting for me to know that we're almost there
0:55:54 > 0:55:58and that with just a little more effort, a little more ingenuity,
0:55:58 > 0:56:03linking a few more telescopes together, we'll be able to see something extraordinary.
0:56:03 > 0:56:05What would be the most exciting thing to see?
0:56:05 > 0:56:11Would you rather be the guy who confirms Einstein's predictions or the guy who...?
0:56:11 > 0:56:17Yeah. Well, look, nobody wants to be the person known as the one who disproved Einstein.
0:56:17 > 0:56:23At the same time, it would be extremely exciting to be able to make some observations
0:56:23 > 0:56:28that would speak directly to the validity of general relativity.
0:56:28 > 0:56:33So either way, whether we see the shadow as the right size or we see the shadow as not the right size
0:56:33 > 0:56:37would be incredibly exciting. I can't decide which would be the best.
0:56:46 > 0:56:53Whether the breakthrough comes from a clue observed in the heavens or theoretical detective work,
0:56:53 > 0:56:59most physicists believe we will eventually crack the question of quantum gravity
0:56:59 > 0:57:03and produce a unified theory of everything.
0:57:03 > 0:57:07A theory that could explain the singularities at the heart of a black hole
0:57:07 > 0:57:14and may even provide the science to predict what happened before our universe existed.
0:57:19 > 0:57:23I suspect that this is a case where we need
0:57:23 > 0:57:26a new Einstein with a grand thought,
0:57:26 > 0:57:30a completely new thought that suddenly makes sense of things.
0:57:33 > 0:57:38Many people think it's never gonna happen, we humans just aren't smart enough.
0:57:38 > 0:57:44If we one day succeed in finding this holy grail, these equations of everything,
0:57:44 > 0:57:48that's when the real work begins to try and solve these equations and predict stuff
0:57:48 > 0:57:53and that'll keep physicists out of harm's way for a long time, I think.
0:57:53 > 0:58:01It doesn't dishearten me that we don't understand everything about the universe.
0:58:01 > 0:58:06I find it wonderful and exciting.
0:58:06 > 0:58:12It seems amazing that we can understand anything about the world around us.
0:58:14 > 0:58:18It might seem as if it would be easier if things like black holes just went away,
0:58:18 > 0:58:23- but then, where would the fun be? - HE LAUGHS
0:58:25 > 0:58:27We don't know what's out there.
0:58:27 > 0:58:31People might give you an answer, but they'll probably be wrong.
0:58:44 > 0:58:47Subtitles by Red Bee Media Ltd
0:58:47 > 0:58:49E-mail: subtitling@bbc.co.uk