0:00:02 > 0:00:04Are we ready?
0:00:04 > 0:00:07Dr Goldstein will make the presentation.
0:00:07 > 0:00:08Dr Belkin will.
0:00:08 > 0:00:10Oh, Dr Belkin?
0:00:10 > 0:00:11HUBBUB OF VOICES
0:00:12 > 0:00:15Will you do it very briefly, sir?
0:00:15 > 0:00:17- OK?- Say when.
0:00:17 > 0:00:20'Albert Einstein, the icon of genius.
0:00:22 > 0:00:25'His theory of general relativity is one of the greatest
0:00:25 > 0:00:30'feats of thinking about nature to come from a single mind.
0:00:30 > 0:00:32'It is now 100 years old.'
0:00:33 > 0:00:34Can you kill the lights, fellas?
0:00:41 > 0:00:43'How do you even study the universe?'
0:00:43 > 0:00:45How can you study everything,
0:00:45 > 0:00:47all of this mass, all the stuff,
0:00:47 > 0:00:49all the energy in the universe at one time?
0:00:51 > 0:00:54It turns out that you actually can do that with Einstein's
0:00:54 > 0:00:56theory of general relativity.
0:00:56 > 0:01:00One mathematical sentence, and from it you can derive
0:01:00 > 0:01:04the understanding of the entire universe on the larger scales.
0:01:04 > 0:01:05And that is beautiful.
0:01:07 > 0:01:12How did a theory that explains so much come from one person?
0:01:12 > 0:01:15Einstein had a magical talent.
0:01:15 > 0:01:17He could take a hard physical problem
0:01:17 > 0:01:21and boil it down to a powerful visual image,
0:01:21 > 0:01:23the thought experiment.
0:01:23 > 0:01:27This is the story of how a young Albert Einstein imagined
0:01:27 > 0:01:30a series of thought experiments that fundamentally
0:01:30 > 0:01:32altered our view of reality.
0:01:34 > 0:01:36The problems are formulated simply
0:01:36 > 0:01:40but it turns out that the answers revolutionise the whole of science.
0:01:54 > 0:01:57The seeds for Einstein's key thoughts were planted
0:01:57 > 0:01:59when he was just a child.
0:02:02 > 0:02:06He grew up in a small house in Munich in southern Germany.
0:02:07 > 0:02:10His unique personality was evident early on.
0:02:14 > 0:02:18Like many great innovators, Einstein was a rebel, a loner,
0:02:18 > 0:02:19but deeply curious.
0:02:20 > 0:02:23He was slow in learning to speak as a child.
0:02:23 > 0:02:27So slow that his parents consulted a doctor, but he later said that
0:02:27 > 0:02:30that's maybe why he thought in visual thought experiments.
0:02:35 > 0:02:36His sister remembers him
0:02:36 > 0:02:39building little card towers using playing cards.
0:02:40 > 0:02:44He was a daydreamer but he was deeply persistent.
0:02:44 > 0:02:49Einstein's father, Herman, manufactured electrical equipment.
0:02:49 > 0:02:51He nurtured his son's interest in science.
0:02:53 > 0:02:55On one occasion, he brought him a compass.
0:03:00 > 0:03:04This needle kicks and points you where to go
0:03:04 > 0:03:06but you can't see how or why.
0:03:07 > 0:03:10And that kind of puzzlement
0:03:10 > 0:03:14is very characteristic of young scientists.
0:03:14 > 0:03:16You and I maybe remember getting a compass when we were kids
0:03:16 > 0:03:19and we're like, "Oh, look, the needle twitches and points north,"
0:03:19 > 0:03:21but then we're onto something else, like,
0:03:21 > 0:03:22"Oh, look, there's a dead squirrel."
0:03:22 > 0:03:25But for Einstein, after getting that compass,
0:03:25 > 0:03:29he developed a lifelong devotion to understanding how things can be
0:03:29 > 0:03:32forced to move, even though nothing is touching them.
0:03:38 > 0:03:41The young Einstein became gripped by a desire to understand
0:03:41 > 0:03:43the underlying laws of nature.
0:03:46 > 0:03:49He developed a unique way of thinking about the physical world,
0:03:49 > 0:03:50inspired by his favourite book.
0:03:52 > 0:03:55The book Einstein loved told little stories like, what it would
0:03:55 > 0:03:59be like to travel through space or go through an electrical wire.
0:03:59 > 0:04:01It made Einstein think visually.
0:04:04 > 0:04:08These imagined situations, that we often call thought experiments,
0:04:08 > 0:04:12became a defining feature of Einstein's thinking.
0:04:12 > 0:04:15One of the critical thought experiments that Einstein
0:04:15 > 0:04:18began to play with, very young, at around the age of 16,
0:04:18 > 0:04:20was trying to imagine what would happen
0:04:20 > 0:04:22if he could catch up with a light wave.
0:04:22 > 0:04:24It's one thing to imagine a light wave zooming past him at some
0:04:24 > 0:04:27seemingly impossible speed.
0:04:28 > 0:04:32But what if he could somehow just propel himself really quickly.
0:04:32 > 0:04:35What would it look like if he could catch up with that light wave?
0:04:35 > 0:04:36What would he see?
0:04:42 > 0:04:45He said it caused him to walk around in such anxiety
0:04:45 > 0:04:47his palms would sweat.
0:04:47 > 0:04:49Now, you and I may remember what was causing our palms
0:04:49 > 0:04:51to sweat at age 16 and it was not a light beam.
0:04:53 > 0:04:54But that's why he's Einstein.
0:05:01 > 0:05:04This dreamlike thought about the nature of light
0:05:04 > 0:05:07was Einstein's first step on the path to his great theory.
0:05:10 > 0:05:14It stayed with him throughout his time at school and college.
0:05:16 > 0:05:18He was extremely gifted in science and math as a young person
0:05:18 > 0:05:20and very bad at other classes,
0:05:20 > 0:05:22mostly because he kept cutting class
0:05:22 > 0:05:24and being very rude to his teachers.
0:05:24 > 0:05:27Many teachers from his high school days on were convinced he'd
0:05:27 > 0:05:28never amount to anything.
0:05:28 > 0:05:31He was a discipline problem and... He was bad news.
0:05:31 > 0:05:34He applies to the second best university in Zurich,
0:05:34 > 0:05:37the Zurich Polytech, and gets rejected.
0:05:37 > 0:05:41I'd love to meet the admissions director who rejected Albert Einstein.
0:05:41 > 0:05:44But eventually he gets in and he does moderately well,
0:05:44 > 0:05:47but not good enough to get a teaching fellowship
0:05:47 > 0:05:50and so he ends up at the Bern Swiss Patent Office
0:05:50 > 0:05:52as a third-class examiner.
0:05:56 > 0:06:02Einstein started work at the patent office in 1902, aged 23.
0:06:06 > 0:06:10Here, his job was to assess the originality of new devices.
0:06:11 > 0:06:15He was immersed in the kinds of technical details that he'd
0:06:15 > 0:06:17been fascinated by as a very young kid.
0:06:18 > 0:06:23And here he was sitting in the kind of wave of the modern age.
0:06:23 > 0:06:26This was the era of electrification.
0:06:26 > 0:06:29So all the latest clever ideas for switching technology,
0:06:29 > 0:06:32for coordinating clocks, in particular,
0:06:32 > 0:06:35those were all passing through his office.
0:06:35 > 0:06:38Time zones had recently been introduced in Central Europe
0:06:38 > 0:06:41and accurately synchronising clocks within regions was a major
0:06:41 > 0:06:42challenge of the day.
0:06:46 > 0:06:49Switzerland was a world leader in time technology.
0:06:50 > 0:06:55Dozens of patents to link clocks passed through Einstein's office.
0:06:55 > 0:06:58He could whip through these patent applications and then out of his
0:06:58 > 0:07:02drawer he'd pull his physics notes and his boss was very indulgent.
0:07:02 > 0:07:05He would sort of turn a blind eye as Einstein was
0:07:05 > 0:07:07doing his theories in his spare time.
0:07:09 > 0:07:12It's really important to remember that theoretical physics was new
0:07:12 > 0:07:15when Einstein was a young man.
0:07:15 > 0:07:19You could do quite a lot of this work by reading a relatively small
0:07:19 > 0:07:21number of science journals
0:07:21 > 0:07:25and making the calculations yourself.
0:07:27 > 0:07:32Einstein's world in 1905 was dominated by
0:07:32 > 0:07:34two kinds of physics.
0:07:34 > 0:07:39One was about 200 years old,
0:07:39 > 0:07:41founded by Isaac Newton,
0:07:41 > 0:07:43the British natural philosopher.
0:07:43 > 0:07:47For Newton, all there is in the world is matter moving.
0:07:52 > 0:07:54Newton showed that the motion of falling apples
0:07:54 > 0:07:58and orbiting planets are governed by the same force,
0:07:58 > 0:07:59gravity.
0:08:02 > 0:08:04His equations are so effective,
0:08:04 > 0:08:06we still use them today to send probes
0:08:06 > 0:08:08to the farthest reaches of the solar system.
0:08:11 > 0:08:14The other important theory of Einstein's day covered
0:08:14 > 0:08:16electricity and magnetism.
0:08:17 > 0:08:20That branch of physics had been revolutionised
0:08:20 > 0:08:25in 1855 by the Scottish physicist James Clerk Maxwell.
0:08:27 > 0:08:32Maxwell's theory describes light as an electromagnetic wave
0:08:32 > 0:08:34that travels at a fixed speed.
0:08:38 > 0:08:41This prediction that the speed of light will be an absolute
0:08:41 > 0:08:45fixed value, never faster, never slower, never stopping,
0:08:45 > 0:08:47that is so surprising,
0:08:47 > 0:08:51as ordinary things don't have a prediction from fundamental law
0:08:51 > 0:08:52what their speed is.
0:08:52 > 0:08:55A basketball can be fast or slow or it can stop.
0:08:55 > 0:08:59There is no fundamental fact of the speed of a basketball.
0:09:01 > 0:09:05This notion of a fixed speed of light captivates Einstein.
0:09:06 > 0:09:09He visualises it in a brilliant thought experiment.
0:09:20 > 0:09:24He imagines a man standing on a railway platform.
0:09:24 > 0:09:25A lamp turns on.
0:09:27 > 0:09:31A beam of light rushes past him and he observes the speed.
0:09:32 > 0:09:36Then he imagines a train travelling at close to the speed of light.
0:09:41 > 0:09:43A lady on board sees the same beam.
0:09:47 > 0:09:50Einstein visualises that in Newton's world,
0:09:50 > 0:09:53because the lady is moving at close to the speed of light,
0:09:53 > 0:09:56she would see the beam pass her train window relatively slowly.
0:10:04 > 0:10:06But in Maxwell's world,
0:10:06 > 0:10:09the speed of light must be the same
0:10:09 > 0:10:10for both the man...
0:10:12 > 0:10:14..and the lady.
0:10:19 > 0:10:22Einstein could see immediately that there's a contradiction
0:10:22 > 0:10:26between Newton and Maxwell.
0:10:26 > 0:10:29They just don't fit together.
0:10:30 > 0:10:35The reason they don't fit together is that if Newton is right,
0:10:35 > 0:10:38then, if you measure the speed of light,
0:10:38 > 0:10:43it will be different depending on how you're moving.
0:10:43 > 0:10:48And what the Maxwellians were saying was the speed of light is
0:10:48 > 0:10:51always going to turn out the same.
0:10:51 > 0:10:56Those two things cannot simultaneously be true.
0:10:56 > 0:11:02And one of the things Einstein hated - hated - was contradiction.
0:11:02 > 0:11:06If there's one kind of physics that says this,
0:11:06 > 0:11:09and another kind of physics that says that, and they're different,
0:11:09 > 0:11:14that's a sign that something's gone wrong, and it needs fixing.
0:11:19 > 0:11:23For months, Einstein wrestles with the problem.
0:11:24 > 0:11:28Eventually, he makes his breakthrough.
0:11:28 > 0:11:31He focuses on a key element of speed -
0:11:31 > 0:11:32time.
0:11:34 > 0:11:37He realised that, in a statement about time,
0:11:37 > 0:11:40it's simply a question about what is simultaneous?
0:11:40 > 0:11:43For example, if you say the train arrives at seven,
0:11:43 > 0:11:47that simply means that it gets to the platform simultaneous
0:11:47 > 0:11:49with the clock going to seven.
0:11:51 > 0:11:55He feels this crucial notion of things happening at the same moment
0:11:55 > 0:11:58should depend on how you're moving.
0:11:59 > 0:12:04And that would mean the flow of time might not be the same for everyone.
0:12:08 > 0:12:12He explores this radical idea in another thought experiment.
0:12:16 > 0:12:20Again, he imagines the man standing on the platform.
0:12:22 > 0:12:25This time, two bolts of lightning strike on either side of him.
0:12:28 > 0:12:32The man is standing exactly halfway between them.
0:12:32 > 0:12:36And the light from each strike reaches his eyes at exactly
0:12:36 > 0:12:41the same moment. For him, the two strikes are simultaneous.
0:12:51 > 0:12:55Then Einstein imagines the lady on the fast moving train.
0:12:58 > 0:13:02At close to the speed of light, what would SHE see?
0:13:07 > 0:13:10As the light travels out from the strikes,
0:13:10 > 0:13:15the train is moving towards one, and away from the other.
0:13:16 > 0:13:19Light from the front strike reaches her eyes first.
0:13:22 > 0:13:24For the lady,
0:13:24 > 0:13:27time elapses between the two strikes.
0:13:30 > 0:13:34For the man on the platform, there is no time between the strikes.
0:13:40 > 0:13:44This simple thought has mind-blowing significance.
0:13:44 > 0:13:48If different observers can't agree on what's simultaneous,
0:13:48 > 0:13:52then they can't agree on the flow of time itself.
0:13:52 > 0:13:54If there's no such thing as simultaneity,
0:13:54 > 0:13:57then there's no such thing as absolute time everywhere
0:13:57 > 0:13:59throughout the universe, and Isaac Newton was wrong.
0:14:01 > 0:14:05The lady on the fast moving train does measure the speed of light
0:14:05 > 0:14:07to be the same as the man.
0:14:09 > 0:14:13Because relative to him, her time runs slower.
0:14:16 > 0:14:20This concept, that time and space are flexible,
0:14:20 > 0:14:25depending on how you're moving, became known as special relativity.
0:14:26 > 0:14:28It led to remarkable results.
0:14:29 > 0:14:33Such as the famous equation relating energy to mass.
0:14:33 > 0:14:37Einstein published his article in 1905 to exactly no acclaim.
0:14:37 > 0:14:41Most people ignored it. This was not setting the world on fire.
0:14:41 > 0:14:45Two years go by before a very eminent physicist,
0:14:45 > 0:14:49Johannes Stark, invites Einstein to write a review article
0:14:49 > 0:14:52on Einstein's own work precisely because no-one was paying attention.
0:14:52 > 0:14:55And he begins thinking about ways to generalise
0:14:55 > 0:14:57and to push his own results from 1905.
0:14:57 > 0:15:01What if he considers not only a train moving at a fixed speed past
0:15:01 > 0:15:04the platform. What if that train begins to speed up or slow down?
0:15:04 > 0:15:07What if there's acceleration?
0:15:07 > 0:15:10Adding acceleration to the equations
0:15:10 > 0:15:13was his first task. Then there was that mysterious
0:15:13 > 0:15:16Newtonian force of gravity to contend with.
0:15:17 > 0:15:22In Newton's theory, gravity is a force that acts instantaneously.
0:15:23 > 0:15:27But special relativity says that's impossible.
0:15:27 > 0:15:29Nothing can travel faster than light.
0:15:31 > 0:15:33What Newton's theory tells you is that, suppose the Sun were to
0:15:33 > 0:15:38disappear, the orbit of the Earth should change at that very moment.
0:15:38 > 0:15:42But the notion of at that very moment in two different places
0:15:42 > 0:15:44is exactly one of these notions that special relativity
0:15:44 > 0:15:47has told you isn't a good physics notion.
0:15:47 > 0:15:50So, you've now got this challenge of trying to work out how to
0:15:50 > 0:15:54take the success of Newton's theory of gravity,
0:15:54 > 0:15:56but fit it into this new special relativistic picture.
0:15:58 > 0:16:02Einstein begins to think about how objects fall.
0:16:07 > 0:16:10One of the major features that gravity has
0:16:10 > 0:16:12was pointed out by Galileo,
0:16:12 > 0:16:16that everything falls at the same rate in a gravitational field,
0:16:16 > 0:16:19if you can ignore the effects of air resistance, even heavy objects
0:16:19 > 0:16:23and light objects, they all fall the same way when gravity pulls on them.
0:16:26 > 0:16:29This bowling ball and feather inside the airless
0:16:29 > 0:16:34environment of a vacuum chamber, fall in perfect unison.
0:16:34 > 0:16:36Einstein figured that
0:16:36 > 0:16:39if everything falls at the same rate in a gravitational field,
0:16:39 > 0:16:43then imagine all you're allowed to do is look at the things around you.
0:16:43 > 0:16:45You're not allowed to look at the wider world.
0:16:45 > 0:16:48Then you wouldn't even be able to tell that you were falling
0:16:48 > 0:16:51in a gravitational field because everything would be
0:16:51 > 0:16:54doing the same thing, whatever that thing was.
0:16:54 > 0:16:55So Einstein said, well,
0:16:55 > 0:17:00that's a very strange feature for a force of nature to have.
0:17:00 > 0:17:03That things that are next to each other can't even tell
0:17:03 > 0:17:04that that force is there.
0:17:04 > 0:17:07And, again, being Einstein, he started to think, well,
0:17:07 > 0:17:10what kind of force of nature would have that property?
0:17:10 > 0:17:13What would it mean for a force of nature to act
0:17:13 > 0:17:16on absolutely everything in the same way?
0:17:23 > 0:17:25Einstein feels that there must be an important link
0:17:25 > 0:17:28between gravity and acceleration.
0:17:33 > 0:17:36We all know that when we are accelerated,
0:17:36 > 0:17:40and of course now we have cars and aeroplanes to give us
0:17:40 > 0:17:44the physical feeling. If you are in an aeroplane
0:17:44 > 0:17:47and it's taking off, you are pushed back in your chair.
0:17:47 > 0:17:49You feel a kind of a force pushing you back.
0:17:49 > 0:17:53Which feels very similar to the force of gravity.
0:17:53 > 0:17:56But you need the brilliance of Einstein to explain
0:17:56 > 0:17:58why they are related.
0:17:58 > 0:18:00We have another moment here where Einstein is
0:18:00 > 0:18:04looking at something familiar but then seeing it in a different way.
0:18:04 > 0:18:08And concluding some remarkable new principles about it.
0:18:10 > 0:18:13Suddenly, he hits upon what he describes
0:18:13 > 0:18:16as the happiest thought of his life -
0:18:16 > 0:18:21what if gravity and acceleration are really the same thing?
0:18:25 > 0:18:30Again, he examines the idea in a beautiful thought experiment.
0:18:31 > 0:18:33He imagines a man in a box,
0:18:33 > 0:18:38floating weightlessly in a distant region of space, in zero gravity.
0:18:41 > 0:18:44Suddenly, the man stops floating
0:18:44 > 0:18:46and finds himself on the floor.
0:18:49 > 0:18:51What has happened?
0:18:54 > 0:18:57Either the box is now close to a planet...
0:18:58 > 0:19:01..and the force of gravity has pulled the man downwards...
0:19:03 > 0:19:05..or...
0:19:05 > 0:19:08someone has attached a rope and the box is now being pulled
0:19:08 > 0:19:11and accelerated upwards.
0:19:16 > 0:19:19So, which is it?
0:19:19 > 0:19:20Gravity?
0:19:22 > 0:19:23Or acceleration?
0:19:26 > 0:19:28Without being able to see outside,
0:19:28 > 0:19:31the man can't tell why he's on the floor.
0:19:34 > 0:19:37Einstein realised there's no way to tell the difference
0:19:37 > 0:19:40between sitting in a gravitational field and being accelerated.
0:19:40 > 0:19:43These are equivalent situations.
0:19:43 > 0:19:46The fact that these two effects give the same result, means that
0:19:46 > 0:19:49gravity IS acceleration.
0:19:49 > 0:19:52It's not just like acceleration.
0:19:52 > 0:19:53It's the same thing.
0:19:55 > 0:19:56It's a big breakthrough.
0:19:59 > 0:20:02By extending his theory of special relativity
0:20:02 > 0:20:05to include acceleration,
0:20:05 > 0:20:09he could begin to formulate a new theory of gravity.
0:20:29 > 0:20:34By 1912, Einstein is living in Zurich with his wife, Mileva,
0:20:34 > 0:20:37and two young sons, Hans and Eduard.
0:20:39 > 0:20:43The academic world had realised the importance of special relativity,
0:20:43 > 0:20:45and his career had taken off.
0:20:46 > 0:20:47He's now a professor
0:20:47 > 0:20:51at the esteemed Swiss Federal Institute of Technology.
0:20:51 > 0:20:54But spends as much time as possible working on his theory.
0:20:56 > 0:21:00He needs to describe how objects move in space and time.
0:21:00 > 0:21:04And soon realises that the best tool for the job is a strange
0:21:04 > 0:21:07but powerful concept called space-time.
0:21:11 > 0:21:15If I think of space, I know that I can find anything
0:21:15 > 0:21:19if I know where it is. North-south, east-west, and up-down.
0:21:19 > 0:21:22Three points. But that doesn't mean I can find it
0:21:22 > 0:21:25cos I also have to know where it is in time.
0:21:25 > 0:21:28So if we start to think, to know everything about an event
0:21:28 > 0:21:32in the universe, I have to know not just its spatial coordinates
0:21:32 > 0:21:34but also its time coordinate,
0:21:34 > 0:21:37I can begin to think about where it is in space-time.
0:21:40 > 0:21:43Imagine a camera filming an action,
0:21:43 > 0:21:47capturing each moment in time as a single frame.
0:21:57 > 0:22:00Einstein basically tells us think of the movie reel.
0:22:02 > 0:22:04So we have all these little pictures.
0:22:06 > 0:22:11Now, cut them apart, one by one, and stack them on top of each other.
0:22:11 > 0:22:15You get this pile. And if you go up in the pile, you go up in time.
0:22:16 > 0:22:21And now glue them all together into one big block.
0:22:22 > 0:22:24And that block has both space and time.
0:22:27 > 0:22:29That's the space-time continuum.
0:22:31 > 0:22:35It's almost looking at a movie not frame by frame
0:22:35 > 0:22:37but seeing the whole movie at once.
0:22:39 > 0:22:43There will now be two strands going up in space and time,
0:22:43 > 0:22:46and they will be spaghetti strands.
0:22:47 > 0:22:51In fact, we all are spaghetti strands moving in this space-time.
0:22:55 > 0:22:59Einstein feels that space-time is the natural arena
0:22:59 > 0:23:02in which his theory of relativity should play out.
0:23:03 > 0:23:07But now he needs sophisticated mathematics.
0:23:08 > 0:23:12By your standard or mine, Einstein was good at math. He was Einstein.
0:23:12 > 0:23:15But he was not really a mathematician, per se.
0:23:15 > 0:23:17He didn't prove theorems, he didn't pour over math books.
0:23:17 > 0:23:20He was a physicist. He did thought experiments,
0:23:20 > 0:23:21he thought of very tangible,
0:23:21 > 0:23:24concrete situations and what would happen.
0:23:24 > 0:23:26So, when it came time for him to really bear down
0:23:26 > 0:23:29to the absolute cutting-edge mathematics of his day,
0:23:29 > 0:23:31he required help.
0:23:31 > 0:23:33He has to have a better grasp of how to describe
0:23:33 > 0:23:36paths of objects as they move through space-time.
0:23:38 > 0:23:39He needs new mathematics.
0:23:39 > 0:23:41And he doesn't have it at his fingertips,
0:23:41 > 0:23:43so he has to go and look for it.
0:23:47 > 0:23:51At university, Einstein had skipped the geometry classes,
0:23:51 > 0:23:55letting his friend Marcel Grossmann take notes for him.
0:23:56 > 0:23:58Grossmann had excelled in geometry,
0:23:58 > 0:24:01and was now chairman of the maths department.
0:24:03 > 0:24:06He suggests Einstein uses advanced mathematics,
0:24:06 > 0:24:10in which the shape of space and time could be curved.
0:24:12 > 0:24:15Because space-time has a geometry, he thinks to himself, well,
0:24:15 > 0:24:19maybe it's the actual shape of space-time itself that is
0:24:19 > 0:24:20giving rise to gravity.
0:24:22 > 0:24:27After months of work, Einstein has an extraordinary idea.
0:24:30 > 0:24:34What if space-time is shaped by matter?
0:24:36 > 0:24:39And that's what we feel as gravity.
0:24:41 > 0:24:43In struggling to figure out what causes gravity, then,
0:24:43 > 0:24:46Einstein has this great insight.
0:24:46 > 0:24:52It is simply that a mass distorts the shape of space-time around it.
0:24:52 > 0:24:54So, we get rid of this force of gravity
0:24:54 > 0:24:57and instead we have curvature of space-time.
0:24:58 > 0:25:02In Einstein's universe, then, if space were empty,
0:25:02 > 0:25:05it would be flat, there'd be nothing going on.
0:25:05 > 0:25:08But as soon as you put objects down, they warp the space and time
0:25:08 > 0:25:13around them, and that causes a deviation of the geometry
0:25:13 > 0:25:15so that now things start moving.
0:25:20 > 0:25:25Everything wants to move as simple as possible through space and time.
0:25:26 > 0:25:30But Einstein tells us that mass sculpts space and time,
0:25:30 > 0:25:35and it's the curved motion through this sculpture
0:25:35 > 0:25:37that's the force of gravity.
0:25:39 > 0:25:41We have this feeling that the reason I can feel
0:25:41 > 0:25:43pressure on the soles of my feet,
0:25:43 > 0:25:45the reason things are going to drop when I throw them, are
0:25:45 > 0:25:49because there's a force attracting us down to the centre of the earth.
0:25:49 > 0:25:52What general relativity tells you is that's not the right way
0:25:52 > 0:25:54to think about what's going on there.
0:25:54 > 0:25:58What's really going on is that your natural path in space-time
0:25:58 > 0:26:00would take you to the centre of the earth.
0:26:00 > 0:26:03What's actually happening is the floor is getting in the way.
0:26:03 > 0:26:05It's pushing you upwards.
0:26:06 > 0:26:08We look at it, we go, "Ah!
0:26:08 > 0:26:11"The force of gravity!" But Einstein says, "No, no, no.
0:26:11 > 0:26:13"The curvature of space-time."
0:26:19 > 0:26:21It's a stunning insight.
0:26:22 > 0:26:26Just as an ant might feel forces pulling it left and right
0:26:26 > 0:26:28as it walks over crumpled paper,
0:26:28 > 0:26:31when it's simply the shape of a surface dictating its path...
0:26:33 > 0:26:38..Einstein saw that what we feel as the force of gravity is, in fact,
0:26:38 > 0:26:41the shape of the space-time we're moving through.
0:26:45 > 0:26:50That move to stop treating gravity as something spooky
0:26:50 > 0:26:52and inexplicable,
0:26:52 > 0:26:55and start thinking of it as something that's absolutely
0:26:55 > 0:26:59to do with the very geometry of the world,
0:26:59 > 0:27:03that then allowed him to begin to complete a general -
0:27:03 > 0:27:06a universal - theory of relativity.
0:27:09 > 0:27:13Einstein now has everything he needs to formulate his final
0:27:13 > 0:27:14theory of gravity.
0:27:14 > 0:27:17But he makes a critical mistake.
0:27:18 > 0:27:21He misinterprets one of his equations.
0:27:21 > 0:27:26And, unaware of his error, continues working on incorrect ideas.
0:27:29 > 0:27:33The point at which Einstein is going to give THE most essential equations
0:27:33 > 0:27:37of the theory, Einstein considers something like them and then says,
0:27:37 > 0:27:41"Ah, but these don't work," and then writes down the wrong equations.
0:27:45 > 0:27:50What follows are alternations of confidence and despair
0:27:50 > 0:27:54as he convinces himself that everything is fine with this theory,
0:27:54 > 0:27:57and then he realises that things aren't so good with the theory.
0:28:01 > 0:28:05It is a long, dark period for Einstein as he struggles to
0:28:05 > 0:28:09reconcile himself with a theory that is just not working.
0:28:15 > 0:28:18Two years later, Einstein is in Berlin.
0:28:19 > 0:28:21At just 36 years old,
0:28:21 > 0:28:25he has one of the most prestigious positions in physics.
0:28:25 > 0:28:28But he is still struggling with his theory.
0:28:35 > 0:28:39By 1915, he'd reached the pinnacle of the profession.
0:28:39 > 0:28:41He was in the Prussian Academy,
0:28:41 > 0:28:43and a professor at the University of Berlin.
0:28:43 > 0:28:46But his marriage has fallen apart, his wife and two kids
0:28:46 > 0:28:48have moved back to Switzerland.
0:28:48 > 0:28:50So, he's pacing around
0:28:50 > 0:28:53almost all alone in this apartment in Berlin.
0:28:54 > 0:28:58He changes fundamentally the way he does his physics.
0:28:58 > 0:29:02He had relied on physical intuition all the way through here,
0:29:02 > 0:29:05and he'd let the mathematics take a back seat.
0:29:05 > 0:29:07He decided that that was a mistake,
0:29:07 > 0:29:11that he should have listened to the natural mathematics first.
0:29:11 > 0:29:14And Einstein adopted that new method,
0:29:14 > 0:29:16and started to write down not the equations
0:29:16 > 0:29:19that he thought were physically the most plausible,
0:29:19 > 0:29:23but the equations that were mathematically the most natural.
0:29:24 > 0:29:27But now he has a competitor.
0:29:29 > 0:29:32Einstein had enthusiastically shared his ideas
0:29:32 > 0:29:35with the brilliant mathematician David Hilbert.
0:29:36 > 0:29:40Hilbert was so impressed, he decided to work on the theory himself.
0:29:42 > 0:29:45Einstein is now in a race to the finish with one of the world's
0:29:45 > 0:29:47best mathematicians.
0:29:53 > 0:29:58This is unfolding in a remarkably dramatic period in history.
0:29:58 > 0:30:02World War I has begun to ravage Central Europe.
0:30:02 > 0:30:05Einstein is not just toiling in the abstract.
0:30:05 > 0:30:08He's toiling as the world seems to fall apart.
0:30:11 > 0:30:13That affects whom he can send letters to.
0:30:13 > 0:30:15It affects what journals from other countries
0:30:15 > 0:30:18he can even receive in the midst of the blockade.
0:30:18 > 0:30:21The world is, certainly from the point of view of the middle
0:30:21 > 0:30:24of Germany, is not looking like a bright, happy place.
0:30:24 > 0:30:28Einstein's feeling dejected because his work is not going well.
0:30:28 > 0:30:32He's concerned he's now in a race with a remarkably gifted colleague.
0:30:32 > 0:30:35His family life is not particularly happy.
0:30:35 > 0:30:40And the headlines every day scream war, devastation and carnage.
0:30:41 > 0:30:46By November, 1915, Einstein is scheduled to present his work
0:30:46 > 0:30:51in a series of four weekly lectures at the esteemed Prussian Academy.
0:30:51 > 0:30:54But he's struggling to formulate his ideas.
0:30:56 > 0:31:00In the midst of this ordeal, letters arrive from his wife in Zurich
0:31:00 > 0:31:04pressing the issue of his financial obligations to his family
0:31:04 > 0:31:07and discussing contact with his sons.
0:31:11 > 0:31:15As his lectures begin, his theory is still far from complete.
0:31:17 > 0:31:21The pressure on Einstein is huge.
0:31:23 > 0:31:27He would give a lecture, revise it, give it again.
0:31:27 > 0:31:32Spot mistakes, correct them, get up on the podium, explain what was
0:31:32 > 0:31:35wrong in the previous week's lecture,
0:31:35 > 0:31:37correct it, and then move on.
0:31:37 > 0:31:41And then do that again and again for four weeks running.
0:31:43 > 0:31:49His work to convince them of the truth of this absolutely
0:31:49 > 0:31:54radical new theory of relativity that he was proposing
0:31:54 > 0:31:58is one of the most intense periods of work in the history of science.
0:32:08 > 0:32:14Somehow, he's able to focus on his theory with an incredible intensity.
0:32:14 > 0:32:16And he makes his breakthrough.
0:32:18 > 0:32:22He tests his equations on a problem that Newton's theory of gravity
0:32:22 > 0:32:24couldn't solve.
0:32:24 > 0:32:26The orbit of Mercury.
0:32:28 > 0:32:30Mercury's path around the Sun
0:32:30 > 0:32:34has an anomaly that Newton's theory can't explain.
0:32:34 > 0:32:37It deviates slightly each time it goes around.
0:32:41 > 0:32:45Einstein calculates the orbit with his new equations.
0:32:45 > 0:32:47The answer is correct.
0:32:47 > 0:32:52Exactly what astronomers had observed.
0:32:52 > 0:32:57He'd found the final equations for his general theory of relativity.
0:33:08 > 0:33:11You have to think about the hubris of being Albert Einstein.
0:33:11 > 0:33:14He had already thrown out Newtonian mechanics with special relativity,
0:33:14 > 0:33:16and then he'd gone off on his personal quest to
0:33:16 > 0:33:19incorporate gravity, and, at the end of the day,
0:33:19 > 0:33:23he boils it down to a prediction for a number that had been observed.
0:33:23 > 0:33:25The procession of the orbit of Mercury.
0:33:25 > 0:33:29Miraculously, when the pages of algebra work out to their end,
0:33:29 > 0:33:30you get the right answer.
0:33:30 > 0:33:33And suddenly it's not just playing with equations any more.
0:33:33 > 0:33:36He realises this is how the world works.
0:33:36 > 0:33:39All this abstract nonsense is the correct theory of reality.
0:33:47 > 0:33:52Einstein is at last able to present a successful theory.
0:33:52 > 0:33:54That's a triumphant moment,
0:33:54 > 0:33:57one of the great moments in the history of physics.
0:33:57 > 0:34:03And, for Einstein, a victory very much against the odds.
0:34:03 > 0:34:05And he'd won.
0:34:10 > 0:34:15On 25th November, 1915, Einstein lays out his findings
0:34:15 > 0:34:18in his climactic fourth lecture at the Prussian Academy.
0:34:21 > 0:34:24He presents general relativity.
0:34:27 > 0:34:30The theory can be written as a single equation.
0:34:31 > 0:34:34It condenses sprawling complexities
0:34:34 > 0:34:37into a beautifully compact set of symbols.
0:34:40 > 0:34:42So the formula is really simple...
0:34:44 > 0:34:46G, for the shape of space-time.
0:34:47 > 0:34:51And T for the distribution of mass and energy.
0:34:51 > 0:34:53So, this very simple formula
0:34:53 > 0:34:57captures all of Einstein's general theory of relativity.
0:34:57 > 0:35:00It's a beautiful, simple equation, but it's a lot of work to
0:35:00 > 0:35:05unpack the symbols, the mathematical symbols, and see how, in this
0:35:05 > 0:35:10very simple formula, the whole geometry of the universe is hidden.
0:35:10 > 0:35:14It's kind of an acquired taste to see the beauty.
0:35:14 > 0:35:18It's also a signature formula for Einstein.
0:35:18 > 0:35:22The true mark of his genius is that he combines two elements
0:35:22 > 0:35:25that actually live in different universes.
0:35:25 > 0:35:28The left-hand side lives in the world of geometry, of mathematics.
0:35:28 > 0:35:33The right-hand side lives in a world of physics, of matter and movement.
0:35:33 > 0:35:38And, so, perhaps the most powerful ingredient of the equation
0:35:38 > 0:35:41is this very simple equals sign here,
0:35:41 > 0:35:45these two lines that actually are connecting the two worlds.
0:35:45 > 0:35:47And it's quite appropriate there are two lines
0:35:47 > 0:35:48because it's two-way traffic.
0:35:49 > 0:35:53Matter tells space and time to curve.
0:35:53 > 0:35:56Space and time tells matter to move.
0:36:05 > 0:36:09The idea that gravity is the curving of space and time
0:36:09 > 0:36:12is completely alien to most of us.
0:36:12 > 0:36:16It's hard to imagine that time itself can be warped.
0:36:16 > 0:36:19But it's real. We can measure it.
0:36:22 > 0:36:26The earth's gravity, the distortion of space and time,
0:36:26 > 0:36:29reduces the further you are from the mass of the planet.
0:36:31 > 0:36:34Time flows quicker at altitude.
0:36:36 > 0:36:39To put this to the test,
0:36:39 > 0:36:43a team of specialists has placed a highly accurate atomic clock
0:36:43 > 0:36:48at the top of New Hampshire's Mount Sunapee, 2,700 feet above sea level.
0:36:51 > 0:36:54After four days, they collect their clock.
0:36:57 > 0:37:00And take it down the mountain to their lab.
0:37:13 > 0:37:17There, they compare it to a second atomic clock that has remained
0:37:17 > 0:37:21- just a few feet above sea level. - Put that one into channel A.
0:37:21 > 0:37:23And the master clock in channel B.
0:37:25 > 0:37:26You guys ready? This is it right here.
0:37:26 > 0:37:29The time interval counter's going to show us the time difference
0:37:29 > 0:37:31between these two clocks.
0:37:32 > 0:37:3420 nanoseconds.
0:37:34 > 0:37:36You can see the time difference between them
0:37:36 > 0:37:38represented here graphically.
0:37:38 > 0:37:41The clock that was up at the mountain for four days
0:37:41 > 0:37:42and our master clock.
0:37:44 > 0:37:47Since gravity is weaker at altitude,
0:37:47 > 0:37:52while the test clock was up the mountain, time ticked faster.
0:37:53 > 0:37:57It's now 20 nanoseconds - 20 billionths of a second -
0:37:57 > 0:38:02ahead of the sea-level clock, just as general relativity predicts.
0:38:02 > 0:38:04This is really awesome.
0:38:04 > 0:38:08But, back in 1915, atomic clocks weren't available.
0:38:10 > 0:38:13Einstein needed a way to show the world the bizarre
0:38:13 > 0:38:14features of his theory.
0:38:20 > 0:38:24The general theory of relativity made predictions of things
0:38:24 > 0:38:26which looked really strange.
0:38:29 > 0:38:31For example, the idea that light bends
0:38:31 > 0:38:35when it passes near a very heavy body.
0:38:35 > 0:38:37No-one had ever looked for that.
0:38:37 > 0:38:39No-one had ever observed it.
0:38:39 > 0:38:45Einstein was desperate, desperate to get astronomers to make that test.
0:38:45 > 0:38:47Einstein's theory predicts that
0:38:47 > 0:38:51when light from a distant star travels close to the Sun,
0:38:51 > 0:38:55the warped space around the Sun bends the light's path.
0:38:58 > 0:39:03In May, 1919, the English astronomer Arthur Eddington
0:39:03 > 0:39:07travelled to the African island of Principe to record images
0:39:07 > 0:39:09that would show this phenomenon.
0:39:10 > 0:39:13Right at the end of the war, Arthur Eddington,
0:39:13 > 0:39:19very much keen to work with Einstein, first because
0:39:19 > 0:39:22he took the general theory of relativity very seriously, he was
0:39:22 > 0:39:28a huge admirer of Einstein, but also because he was a Quaker, a pacifist.
0:39:28 > 0:39:32He wanted as quickly as possible to, as he put it,
0:39:32 > 0:39:35"solve the wounds of war".
0:39:35 > 0:39:38To bring the British and the Germans back together.
0:39:38 > 0:39:42What Eddington had been able to do was take photographs of stars
0:39:42 > 0:39:46during a total eclipse of the Sun, so the moon blocked most of the
0:39:46 > 0:39:49brightness of the Sun, and little pin pricks of light could be
0:39:49 > 0:39:53seen around the Sun that would otherwise be lost in the glare.
0:39:53 > 0:39:55And Eddington and his colleagues were able to measure
0:39:55 > 0:39:59that the appearance of those stars had been shifted compared to where
0:39:59 > 0:40:02they would have been had that big mass of the Sun not been
0:40:02 > 0:40:04deflecting that light from far away.
0:40:05 > 0:40:08Eddington is able to show
0:40:08 > 0:40:11in November of 1919,
0:40:11 > 0:40:15just one year to the day after the end of World War I,
0:40:15 > 0:40:19that Einstein's general relativity theory is right
0:40:19 > 0:40:23and a revolution in science has been accomplished.
0:40:32 > 0:40:36When the eclipse experiments prove Einstein's theory right,
0:40:36 > 0:40:37he rockets to fame,
0:40:37 > 0:40:41not just because he's explained a new way of looking at the universe,
0:40:41 > 0:40:45but at the end of World War I you had the predictions of
0:40:45 > 0:40:50a German scientist be proven right by some British astronomers
0:40:50 > 0:40:53and it becomes headlines across the world.
0:40:53 > 0:40:57The New York Times says, "Lights all askew in the heavens.
0:40:57 > 0:40:59"Men of science more or less agog."
0:40:59 > 0:41:03This is back when newspapers knew how to write great headlines,
0:41:03 > 0:41:05but Einstein kind of loves this fact,
0:41:05 > 0:41:08that he is now an icon of science.
0:41:12 > 0:41:15Einstein becomes a worldwide celebrity,
0:41:15 > 0:41:19the icon of genius we still recognise today.
0:41:22 > 0:41:27The only person who was more widely known was Charlie Chaplin.
0:41:27 > 0:41:30And they got on like a house on fire.
0:41:30 > 0:41:33Chaplin said, "The reason they all love me is
0:41:33 > 0:41:36"cos they understand everything I do
0:41:36 > 0:41:39"and the reason they love you is that they don't understand anything you do.
0:41:39 > 0:41:40"Can you explain that?"
0:41:40 > 0:41:41And Einstein said...
0:41:46 > 0:41:49But in 1930s Berlin,
0:41:49 > 0:41:51the Nazi party is gaining power.
0:41:53 > 0:41:54As a Jewish scientist,
0:41:54 > 0:41:58Einstein becomes increasingly caught up in the political unrest.
0:42:00 > 0:42:02Einstein's theories became a target.
0:42:02 > 0:42:06They were deemed aesthetically repugnant to a kind of Aryan
0:42:06 > 0:42:10sensibility, so people attacked not just Einstein the Jewish scientist,
0:42:10 > 0:42:14but they would actually have people denouncing general relativity.
0:42:17 > 0:42:21'In January, Nobel Prize mathematician Albert Einstein visited California.'
0:42:21 > 0:42:23He begins to make trips to America,
0:42:23 > 0:42:25where he is welcomed with open arms.
0:42:27 > 0:42:28'Germany's loss, America's gain.'
0:42:30 > 0:42:34And in 1933, he settles in Princeton,
0:42:34 > 0:42:38taking up a position at the Institute for Advanced Study.
0:42:42 > 0:42:46Today, the Institute is headed by Professor Robert Dijkgraaf.
0:42:47 > 0:42:51He basically was still very much by himself,
0:42:51 > 0:42:54just actually as he was in Berlin,
0:42:54 > 0:42:56just concentrating on his deep ideas
0:42:56 > 0:43:01and struggling with understanding the universe.
0:43:01 > 0:43:04Of course, his office was here.
0:43:04 > 0:43:09At the Institute, Einstein worked to unify his theory of gravity
0:43:09 > 0:43:11with the other laws of physics.
0:43:14 > 0:43:19With Einstein you see this phenomena you see with many great scientists.
0:43:19 > 0:43:22That they climb this very high mountain
0:43:22 > 0:43:26and instead of celebrating their success
0:43:26 > 0:43:30they're privileged to see a much wider landscape
0:43:30 > 0:43:32and they see all these mountains behind it
0:43:32 > 0:43:34and I think he was very much aware
0:43:34 > 0:43:36how much, still, there was to be done.
0:43:38 > 0:43:42Until the very last days of his life
0:43:42 > 0:43:44he was trying to push these equations
0:43:44 > 0:43:48and find a description of nature,
0:43:48 > 0:43:52all of nature, in terms of the geometry of space and time.
0:43:55 > 0:43:59But general relativity was fading from mainstream science.
0:44:01 > 0:44:04Physics was now focused on the quantum theory of atoms
0:44:04 > 0:44:05and tiny particles.
0:44:07 > 0:44:11A theory incompatible with Einstein's ideas.
0:44:11 > 0:44:14But one that could be tested in a lab.
0:44:15 > 0:44:19Most of general relativity was then beyond the reach of experiment.
0:44:21 > 0:44:26When Einstein died in 1955, aged 76,
0:44:26 > 0:44:28the wider scientific community
0:44:28 > 0:44:30presumed his theory had reached a dead end.
0:44:32 > 0:44:35But they couldn't have been more mistaken.
0:44:37 > 0:44:39The best theories in physics always take us
0:44:39 > 0:44:42to places where the people who invented them didn't imagine.
0:44:42 > 0:44:45And a truly wonderful theory like general relativity
0:44:45 > 0:44:48predicts all sorts of things that Einstein didn't conceive of.
0:44:48 > 0:44:50The theory has a life of its own.
0:44:50 > 0:44:53We understand general relativity much better right now
0:44:53 > 0:44:55than Albert Einstein ever did.
0:45:03 > 0:45:07Today, huge telescopes peer deep into the universe.
0:45:10 > 0:45:14It is general relativity that allows us to make sense of what they see.
0:45:19 > 0:45:23And there's one prediction of Einstein's theory this technology
0:45:23 > 0:45:28has allowed us to explore that is straight out of science fiction.
0:45:31 > 0:45:32A black hole.
0:45:37 > 0:45:40Everything that we're familiar with in ordinary life
0:45:40 > 0:45:41is made from matter.
0:45:43 > 0:45:44But not black holes.
0:45:44 > 0:45:48Black holes are made from warped space and time.
0:45:49 > 0:45:51And nothing else.
0:45:53 > 0:45:57A black hole is an object that is spherical,
0:45:57 > 0:46:01like a star or like the earth, with a sharp boundary
0:46:01 > 0:46:06called the horizon, through which nothing can come out.
0:46:06 > 0:46:10So it casts a shadow on whatever is behind it.
0:46:11 > 0:46:13It's just a black, black shadow.
0:46:13 > 0:46:15Unbelievably black.
0:46:18 > 0:46:23This simulation shows the distortion of starlight around the black hole.
0:46:24 > 0:46:28Even though Einstein knew his theory predicted black holes,
0:46:28 > 0:46:31he found it hard to believe they would really exist in nature.
0:46:34 > 0:46:38In the 1960s, Professor Kip Thorne worked on
0:46:38 > 0:46:40a mathematical concept of black holes.
0:46:42 > 0:46:45The idea made sense on paper, and he began to feel that these
0:46:45 > 0:46:49science fiction-like objects might actually be real.
0:46:51 > 0:46:52It must be here somewhere.
0:46:52 > 0:46:54It's in one of these piles...
0:46:54 > 0:46:57Kip made a bet with fellow physicist Stephen Hawking
0:46:57 > 0:47:00about whether or not a strong source of X-rays,
0:47:00 > 0:47:05known as Cygnus X1, was in fact a black hole.
0:47:07 > 0:47:11Yeah, here we go. Relativist stars and black holes. Yeah, there it is.
0:47:12 > 0:47:16So, that is a copy of the famous bet.
0:47:16 > 0:47:19The bet says, "Whereas Stephen Hawking has such a large
0:47:19 > 0:47:22"investment in general relativity and black holes
0:47:22 > 0:47:25"and desires an insurance policy and whereas Kip Thorne likes to
0:47:25 > 0:47:28"live dangerously, without an insurance policy".
0:47:28 > 0:47:32That's a good characterisation of myself, much to my wife's chagrin!
0:47:32 > 0:47:35"Therefore, be it resolved that Stephen Hawking
0:47:35 > 0:47:38"bets one year's subscription to Penthouse magazine
0:47:38 > 0:47:41"as against Kip Thorne's wager of a four-year subscription
0:47:41 > 0:47:45"to a political magazine called Private Eye, that Cygnus X1
0:47:45 > 0:47:49"does not contain a black hole of mass above the Chandrasekhar Limit."
0:47:49 > 0:47:53It's written as this 10th day of December 1974,
0:47:53 > 0:47:55while Stephen is at Caltech with me.
0:47:55 > 0:47:59We made that bet under circumstances where there was
0:47:59 > 0:48:03mounting evidence that Cygnus X1 really is a black hole.
0:48:03 > 0:48:07Stephen Hawking had a terribly deep investment in it
0:48:07 > 0:48:11actually being a black hole and so he made
0:48:11 > 0:48:16the bet against himself as an insurance policy that at least
0:48:16 > 0:48:21he would get something out of it if Cygnus X1 turned out not to be a black hole.
0:48:21 > 0:48:26And the evidence mounted thereafter, over the period of the '70s and '80s
0:48:26 > 0:48:31and in June 1990, Stephen snuck into my office
0:48:31 > 0:48:35and signed off on the bet that finally the evidence was
0:48:35 > 0:48:38absolutely overwhelming that Cygnus X1 really is a black hole.
0:48:39 > 0:48:42And Penthouse magazine arrived.
0:48:42 > 0:48:44He sent me the British version of Penthouse,
0:48:44 > 0:48:47which was ever so much more raunchy than the American Penthouse.
0:48:47 > 0:48:52Actually enough to turn my face red when I received it at first.
0:48:58 > 0:49:02Today, thanks to concepts built on Einstein's theory,
0:49:02 > 0:49:05we have evidence suggesting there are millions of black holes
0:49:05 > 0:49:07in our galaxy alone.
0:49:08 > 0:49:11And his general relativity tells us more.
0:49:12 > 0:49:16Just as a collision of two objects produces sound waves,
0:49:16 > 0:49:21the collision of two black holes generates waves in space-time.
0:49:25 > 0:49:27There are huge things in the universe happening,
0:49:27 > 0:49:30like black holes colliding or stars exploding.
0:49:30 > 0:49:33And they create these gravitational waves,
0:49:33 > 0:49:36the waves in the shape of space and time that travel through
0:49:36 > 0:49:38the universe at the speed of light.
0:49:38 > 0:49:42And so right now, the space around me is being squeezed
0:49:42 > 0:49:46and stretched by gravitational waves just getting here from, let's say,
0:49:46 > 0:49:49two black holes colliding a billion light years away.
0:49:53 > 0:49:56But the squeezing and stretching is so minute,
0:49:56 > 0:49:59I absolutely could not personally detect it
0:49:59 > 0:50:02and so what we're trying to do is build an instrument that can.
0:50:04 > 0:50:07In Louisiana and Washington State,
0:50:07 > 0:50:11a vast experiment called LIGO is in the final phases of calibration.
0:50:13 > 0:50:16It's hoped that laser beams travelling 4km
0:50:16 > 0:50:19between precisely aligned mirrors will measure
0:50:19 > 0:50:23the squeezing of space caused by gravitational waves.
0:50:25 > 0:50:30The experiment is able to measure the difference between two mirrors
0:50:30 > 0:50:36at 4km and two mirrors at 4km plus or minus a ten-thousandth of the nucleus of an atom.
0:50:39 > 0:50:43Some time between today and a few years from now, we really
0:50:43 > 0:50:47expect to have made the first direct detection of gravitational waves
0:50:47 > 0:50:52to actually record the ringing of the shape of space and time.
0:50:52 > 0:50:55A direct measurement of pure gravitation.
0:50:55 > 0:50:58We're not collecting light, we're not talking about matter,
0:50:58 > 0:51:00we're not talking about anything,
0:51:00 > 0:51:03just measuring pure modulations in space and time.
0:51:05 > 0:51:07So it's pure general relativity.
0:51:14 > 0:51:18Of all of Einstein's theory's remarkable breakthroughs,
0:51:18 > 0:51:23the most profound is that our universe has a beginning.
0:51:29 > 0:51:33The discovery that distant galaxies are moving outwards
0:51:33 > 0:51:36and the detection of background radiation from the very start
0:51:36 > 0:51:40of the universe provided evidence for the big bang
0:51:40 > 0:51:42and a universe that's growing.
0:51:43 > 0:51:47With this picture of an expanding universe,
0:51:47 > 0:51:48there is natural questions.
0:51:48 > 0:51:52Is the universe slowing down as it expands?
0:51:52 > 0:51:56Is it so dense that someday it will come to a halt and collapse?
0:51:56 > 0:51:58Will the universe come to an end?
0:51:58 > 0:52:01These seem like good questions.
0:52:01 > 0:52:06To find answers, in the 1990s, Saul and his team studied
0:52:06 > 0:52:11exploding stars called supernovae to track the growth of the universe.
0:52:13 > 0:52:16When we made the measurement, we discovered that the universe
0:52:16 > 0:52:18isn't slowing down enough to come to a halt.
0:52:18 > 0:52:22In fact, it's not slowing at all. It's speeding up.
0:52:22 > 0:52:24The universe is expanding faster and faster.
0:52:27 > 0:52:30In order to explain the acceleration of the universe within
0:52:30 > 0:52:34Einstein's theory of general relativity, we're considering
0:52:34 > 0:52:37a energy spread throughout all of space that we've never seen before.
0:52:37 > 0:52:40We don't know what it is, we call it dark energy.
0:52:41 > 0:52:46And if so, it would require something like 70% of all the stuff
0:52:46 > 0:52:50of the universe to be in this form of previously unknown dark energy.
0:52:50 > 0:52:54So, this is a lot to swallow, and you might imagine that at that point
0:52:54 > 0:52:56you should go back and revisit your theory.
0:52:56 > 0:52:59The problem is that Einstein's theory is so elegant
0:52:59 > 0:53:03and it predicts many, many, many digits of precision
0:53:03 > 0:53:07that it's very, very difficult to come up with any other theory.
0:53:13 > 0:53:18For 100 years, general relativity has proven correct
0:53:18 > 0:53:19time and time again.
0:53:20 > 0:53:26But Einstein himself knew that his great theory had limits.
0:53:26 > 0:53:30The huge problem with theoretical physics now is to combine
0:53:30 > 0:53:34general relativity, our best theory of space, time and gravity,
0:53:34 > 0:53:37with quantum mechanics, our best theory of very small things.
0:53:37 > 0:53:42Two phenomenally successful theories that don't automatically jell with one another.
0:54:00 > 0:54:04Here at the Institute for Advanced Study where Einstein worked,
0:54:04 > 0:54:08the world's leading theoretical physicists are trying to solve
0:54:08 > 0:54:10the problem Einstein never could.
0:54:11 > 0:54:17Finding a single set of rules that applies to both the cosmic and atomic scales.
0:54:18 > 0:54:20A unified theory.
0:54:20 > 0:54:23The Holy Grail of physics.
0:54:25 > 0:54:29We are now in what at this time is the School of Physics,
0:54:29 > 0:54:31so here, our people are still struggling with
0:54:31 > 0:54:34many of the same issues that Einstein was struggling with
0:54:34 > 0:54:40and are still trying to capture the laws of the universe,
0:54:40 > 0:54:45from the very small to the very large, in a single equation.
0:54:45 > 0:54:50And it's still a blackboard that's the weapon of choice!
0:54:52 > 0:54:57The brightest minds of the world are coming here to work 24 hours,
0:54:57 > 0:55:00seven days a week, struggling to grasp
0:55:00 > 0:55:03the great mysteries of the universe.
0:55:03 > 0:55:07And I think we are still driven by the same dream
0:55:07 > 0:55:10that at some point, we can capture everything in elegant mathematics.
0:55:12 > 0:55:17100 years after Einstein transformed our understanding of nature,
0:55:17 > 0:55:20the stage is set for the next revolution.
0:55:22 > 0:55:24Quantum mechanics was very different than general relativity.
0:55:24 > 0:55:27It came about by many people stumbling into it,
0:55:27 > 0:55:29maybe that will be the way we do it next time.
0:55:29 > 0:55:32Einstein was this singular genius who managed to get gravity right.
0:55:32 > 0:55:34He didn't manage to get quantum mechanics right.
0:55:34 > 0:55:36When we finally move beyond Einstein,
0:55:36 > 0:55:39it might be another singular genius that comes along,
0:55:39 > 0:55:42someone struggling in a poor school in Kenya right now,
0:55:42 > 0:55:46that we don't know about, or it might be 20 different people with
0:55:46 > 0:55:4920 different points of view, gradually building brick-by-brick to
0:55:49 > 0:55:54finally figure out a more comprehensive view that includes general relativity in it.
0:56:01 > 0:56:04I think the most important thing that you learn from Einstein
0:56:04 > 0:56:06is just the power of an idea.
0:56:06 > 0:56:09If it's correct, you know, it's unstoppable.
0:56:11 > 0:56:14It's extremely encouraging that he was able
0:56:14 > 0:56:17with pure thought to solve the riddle of the universe.
0:56:22 > 0:56:26There are only a few moments in science history where we've
0:56:26 > 0:56:29had to completely rethink our picture of the world that we live in
0:56:29 > 0:56:31and this was one of those moments.
0:56:33 > 0:56:37The moment you enter the world of general relativity,
0:56:37 > 0:56:42you encounter claims, propositions, that are doing nothing less than
0:56:42 > 0:56:45calculating how much matter there is in the universe,
0:56:45 > 0:56:49whether the whole of space is curved,
0:56:49 > 0:56:53so that what we thought was in a way beyond experience
0:56:53 > 0:56:59becomes a system that can be described, can be tested.
0:56:59 > 0:57:02That still seems to me to be an absolutely amazing fact.
0:57:07 > 0:57:12You already have the huge universe and it obeys the laws of nature.
0:57:12 > 0:57:18But where in the universe are these laws actually discovered, where are they studied?
0:57:19 > 0:57:22And then you go to this tiny planet and there's this one individual,
0:57:22 > 0:57:24Einstein, who captures it.
0:57:26 > 0:57:29And now there's a small group of people walking in his footsteps
0:57:29 > 0:57:30and trying to push it further.
0:57:32 > 0:57:35And I often feel, well, you know, it's a small part of the universe
0:57:35 > 0:57:36that actually is reflecting upon itself,
0:57:36 > 0:57:38to try to understand itself.