**‘Albert Einstein, the icon of genius.**

‘His theory of **General relativity**(1915) is one of the greatest ‘feats of thinking about nature to come from a single mind. **‘It is now 100 years old.’**

Can you kill the lights, fellas? How do you even study the universe? How can you study everything, all of this mass, all the stuff, all the energy in the universe at one time?

It turns out that you actually can do that with *Einstein’s theory of general relativity*. One mathematical sentence, and from it you can derive an understanding of the entire universe on a larger scale.

And that is beautiful. How did a theory that explains so much come from one person? Einstein had a magical talent. He could take a hard physical problem and boil it down to a powerful visual image, the thought experiment. This is the story of how a young Albert Einstein imagined a series of thought experiments that fundamentally altered our view of reality.

The problems are formulated simply but it turns out that the answers revolutionize the whole of science. The seeds for Einstein s key thoughts were planted when he was just a child. He grew up in a small house in Munich in southern Germany. His unique personality was evident early on.

Like many great innovators, Einstein was a rebel, a loner, but deeply curious. He was slow in learning to speak as a child. So slow that his parents consulted a doctor, but he later said that that s maybe why he thought in visual thought experiments. His sister remembers him building little card towers using playing cards.

He was a **daydreamer but he was deeply persistent**. Einstein’s father,** Herman**, manufactured electrical equipment. He nurtured his son’s interest in science. On one occasion, he brought him a compass.

This needle kicks and points you where to go but you can t see how or why. And that kind of puzzlement is very characteristic of young scientists. You and I maybe remember getting a compass when we were kids and we re like, “Oh, look, the needle twitches and points north,” but then we’re onto something else, like, “Oh, look, there s a dead squirrel.” But for Einstein, after getting that compass, he developed a lifelong devotion to understanding how things can be forced to move, even though nothing is touching them.

The young Einstein became gripped by a desire to understand the underlying laws of nature. He developed a unique way of thinking about the physical world, inspired by his favorite book. The book Einstein loved told little stories like, what it would be like to travel through space or go through an electrical wire. It made Einstein think visually.

These imagined situations, which we often call thought experiments, became a defining feature of Einstein’s thinking. One of the critical thought experiments that Einstein began to play with, very young, at around the age of 16, was trying to imagine what would happen if he could catch up with a light wave.

It’s one thing to imagine a light wave zooming past him at some seemingly impossible speed. But what if he could somehow just propel himself really quickly?

What would it look like if he could catch up with that light wave? What would he see?

He said it caused him to walk around in such anxiety his palms would sweat. Now, you and I may remember what was causing our palms to sweat at age 16 and it was not a light beam. But that s why he’s Einstein. This dreamlike thought about the nature of light was Einstein’s first step on the path to his great theory.

It stayed with him throughout his time at school and college. He was extremely gifted in science and math as a young person and very bad at other classes, mostly because he kept cutting class and being very rude to his teachers. Many teachers from his high school days on were convinced he’d never amount to anything. He was a discipline problem and…

He applies to the second best university in Zurich, the Zurich Polytech, and gets rejected. I d love to meet the admissions director who rejected Albert Einstein. But eventually, he gets in and he does moderately well, but not good enough to get a teaching fellowship and so he ends up at the **Bern Swiss Patent Office** as a **third-class examiner**.

Einstein’started work at the patent office in **1902, aged 23**. Here, his job was to assess the originality of new devices. He was immersed in the kinds of technical details that he’d been fascinated by as a very young kid. And here he was sitting in the kind of wave of the modern age.

This was the era of electrification. So all the latest clever ideas for switching technology, for coordinating clocks, in particular, were all passing through his office. Time zones had recently been introduced in Central Europe and accurately synchronizing clocks within regions was a major challenge of the day.

Switzerland was a world leader in time technology. Dozens of patents to link clocks passed through Einstein’s office. He could whip through these patent applications and then out of his drawer he d pull his physics notes and his boss was very indulgent. He would sort of turn a blind eye as Einstein was doing his theories in his spare time. It s really important to remember that theoretical physics was new when Einstein was a young man.

You could do quite a lot of this work by reading a relatively small number of science journals and making the calculations yourself.

Einstein’s world in 1905 was dominated by two kinds of physics. One was about 200 years old, founded by Isaac Newton, the British natural philosopher.

For Newton, all there is in the world is matter moving.

Newton showed that the motion of falling apples and orbiting planets are governed by the same force, gravity. His equations are so effective, that we still use them today to send probes to the farthest reaches of the solar system. The other important theory of Einstein’s day covered electricity and magnetism. That branch of physics had been revolutionized in **1855 by the Scottish physicist James Clerk Maxwell.**

Maxwell’s theory describes light as an

electromagnetic wavethat travels at a fixed speed. This prediction that the speed of light will be an absolute fixed value, never faster, never slower, never stopping, is so surprising, as ordinary things don t have a prediction from the fundamental law of what their speed is. A basketball can be fast or slow or it can stop. There is no fundamental fact about the speed of a basketball.

This notion of a fixed speed of light captivates Einstein. He visualizes it in a brilliant thought experiment. He imagines a man standing on a railway platform. A lamp turns on.

A beam of light rushes past him and he observes the speed. Then he imagines a train traveling at close to the speed of light. A lady on board sees the same beam. Einstein visualizes that in Newton s world, because the lady is moving at close to the speed of light, she would see the beam pass her train window relatively slowly.

But in Maxwell’s world, the speed of light must be the same for both the man and the lady. Einstein could see immediately that there s a contradiction between Newton and Maxwell. They just don t fit together. The reason they don t fit together is that if Newton is right, then, if you measure the speed of light, it will be different depending on how you’re moving.

And what the Maxwellians were saying was the speed of light is always going to turn out the same.

**Those two things cannot simultaneously be true**. And one of the things Einstein hated – hated – was the **contradiction**. If there s one kind of physics that says this, and another kind of physics that says that and they’re different, that s a sign that something s gone wrong, and it needs fixing.

For months, Einstein wrestles with the problem. Eventually, he makes his breakthrough. He focuses on a key element of speed – time. He realized that, in a statement about time, it’s simply a question about what is simultaneous. For example, if you say the train arrives at seven, that simply means that it gets to the platform simultaneously with the clock going to seven.

He feels this crucial notion of things happening at the same moment should depend on how you’re moving. And that would mean the flow of time might not be the same for everyone. He explores this radical idea in another thought experiment. Again, he imagines the man standing on the platform.

This time, two bolts of lightning strike on either side of him. The man is standing exactly halfway between them. And the light from each strike reaches his eyes at exactly the same moment. For him, the two strikes are simultaneous.

Then Einstein imagines the lady on the fast-moving train. At close to the speed of light, what would SHE see? As the light travels out from the strikes, the train is moving towards one, and away from the other. Light from the front strike reaches her eyes first. For the lady, time elapses between the two strikes.

For the man on the platform, there is no time between the strikes. This simple thought has mind-blowing significance. If different observers can t agree on what s simultaneous, then they can t agree on the flow of time itself. If there s no such thing as simultaneity, then there s no such thing as absolute time everywhere throughout the universe, and Isaac Newton was wrong.

The lady on the fast moving train does measure the speed of light to be the same as the man. Because relative to him, her time runs slower. This concept, that time and space are flexible, depending on how you re moving, became known as special relativity. It led to remarkable results.

Such as the famous equation relating energy to mass. Einstein published his article in 1905 to exactly no acclaim. Most people ignored it. This was not setting the world on fire.

Two years go by before a very eminent physicist, Johannes Stark, invites Einstein to write a review article on Einstein’s own work precisely because no-one was paying attention. And he begins thinking about ways to generalise and to push his own results from 1905. What if he considers not only a train moving at a fixed speed past the platform. What if that train begins to speed up or slow down? What if there s acceleration? Adding acceleration to the equations was his first task.

Then there was that mysterious Newtonian force of gravity to contend with. In Newton s theory, gravity is a force that acts instantaneously. But special relativity says that s impossible. Nothing can travel faster than light.

What Newton s theory tells you is that, suppose the Sun were to disappear, the orbit of the Earth should change at that very moment. But the notion of at that very moment in two different places has told you isn t a good physics notion. So, you ve now got this challenge of trying to work out how to take the success of Newton s theory of gravity, but fit it into this new special relativistic picture. Einstein begins to think about how objects fall.

One of the major features that gravity has was pointed out by Galileo, that everything falls at the same rate in a gravitational field, if you can ignore the effects of air resistance, even heavy objects and light objects, they all fall the same way when gravity pulls on them. This bowling ball and feather inside the airless environment of a vacuum chamber, fall in perfect unison. Einstein figured that if everything falls at the same rate in a gravitational field, then imagine all you re allowed to do is look at the things around you. You re not allowed to look at the wider world.

Then you wouldn t even be able to tell that you were falling in a gravitational field because everything would be doing the same thing, whatever that thing was. So Einstein’said, well, that s a very strange feature for a force of nature to have. That things that are next to each other can t even tell that that force is there. And, again, being Einstein, he started to think, well, what kind of force of nature would have that property? What would it mean for a force of nature to act on absolutely everything in the same way? Einstein feels that there must be an important link between gravity and acceleration.

We all know that when we are accelerated, and of course now we have cars and aeroplanes to give us the physical feeling. If you are in an aeroplane and it s taking off, you are pushed back in your chair. You feel a kind of a force pushing you back. Which feels very similar to the force of gravity.

But you need the brilliance of Einstein to explain why they are related. We have another moment here where Einstein is looking at something familiar but then seeing it in a different way. And concluding some remarkable new principles about it. Suddenly, he hits upon what he describes as the happiest thought of his life – what if gravity and acceleration are really the same thing? Again, he examines the idea in a beautiful thought experiment.

He imagines a man in a box, floating weightlessly in a distant region of space, in zero gravity. Suddenly, the man stops floating and finds himself on the floor. What has happened? Either the box is now close to a planet…

..and the force of gravity has pulled the man downwards…

..or…

someone has attached a rope and the box is now being pulled and accelerated upwards. So, which is it? Gravity? Or acceleration? Without being able to see outside, the man can t tell why he s on the floor. Einstein realised there s no way to tell the difference between sitting in a gravitational field and being accelerated. These are equivalent situations.

The fact that these two effects give the same result, means that gravity IS acceleration. It s not just like acceleration. It s the same thing. It s a big breakthrough.

By extending his theory of special relativity to include acceleration, he could begin to formulate a new theory of gravity. By 1912, Einstein is living in Zurich with his wife, Mileva, and two young sons, Hans and Eduard. The academic world had realised the importance of special relativity, and his career had taken off. He s now a professor at the esteemed Swiss Federal Institute of Technology.

But spends as much time as possible working on his theory. He needs to describe how objects move in space and time. And soon realises that the best tool for the job is a strange but powerful concept called space-time. If I think of space, I know that I can find anything if I know where it is.

North-south, east-west, and up-down. Three points. But that doesn t mean I can find it cos I also have to know where it is in time. So if we start to think, to know everything about an event in the universe, I have to know not just its spatial coordinates but also its time coordinate, I can begin to think about where it is in space-time.

Imagine a camera filming an action, capturing each moment in time as a single frame. Einstein basically tells us think of the movie reel. So we have all these little pictures. Now, cut them apart, one by one, and stack them on top of each other.

You get this pile. And if you go up in the pile, you go up in time. And now glue them all together into one big block. And that block has both space and time.

That s the space-time continuum. It s almost looking at a movie not frame by frame but seeing the whole movie at once. There will now be two strands going up in space and time, and they will be spaghetti strands. In fact, we all are spaghetti strands moving in this space-time.

Einstein feels that space-time is the natural arena in which his theory of relativity should play out. But now he needs sophisticated mathematics. By your standard or mine, Einstein was good at math. He was Einstein.

But he was not really a mathematician, per se. He didn t prove theorems, he didn t pour over math books. He was a physicist. He did thought experiments, he thought of very tangible, concrete situations and what would happen.

So, when it came time for him to really bear down to the absolute cutting-edge mathematics of his day, he required help. He has to have a better grasp of how to describe paths of objects as they move through space-time. He needs new mathematics. And he doesn t have it at his fingertips, so he has to go and look for it.

At university, Einstein had skipped the geometry classes, letting his friend Marcel Grossmann take notes for him. Grossmann had excelled in geometry, and was now chairman of the maths department. He suggests Einstein uses advanced mathematics, in which the shape of space and time could be curved. Because space-time has a geometry, he thinks to himself, well, maybe it s the actual shape of space-time itself that is giving rise to gravity.

After months of work, Einstein has an extraordinary idea. What if space-time is shaped by matter? And that s what we feel as gravity. In struggling to figure out what causes gravity, then, Einstein has this great insight. It is simply that a mass distorts the shape of space-time around it.

So, we get rid of this force of gravity and instead we have curvature of space-time. In Einstein’s universe, then, if space were empty, it would be flat, there d be nothing going on. But as soon as you put objects down, they warp the space and time around them, and that causes a deviation of the geometry so that now things start moving. Everything wants to move as simple as possible through space and time.

But Einstein tells us that mass sculpts space and time, and it s the curved motion through this sculpture that s the force of gravity. We have this feeling that the reason I can feel pressure on the soles of my feet, the reason things are going to drop when I throw them, are because there s a force attracting us down to the centre of the earth. What general relativity tells you is that s not the right way to think about what s going on there. What s really going on is that your natural path in space-time would take you to the centre of the earth.

What s actually happening is the floor is getting in the way. It s pushing you upwards. We look at it, we go, “Ah! “The force of gravity!” But Einstein’says, “No, no, no. “The curvature of space-time.”

It s a stunning insight. Just as an ant might feel forces pulling it left and right as it walks over crumpled paper, when it s simply the shape of a surface dictating its path…

.. Einstein’saw that what we feel as the force of gravity is, in fact, the shape of the space-time we re moving through. That move to stop treating gravity as something spooky and inexplicable, and start thinking of it as something that s absolutely to do with the very geometry of the world, that then allowed him to begin to complete a general – a universal – theory of relativity.

Einstein now has everything he needs to formulate his final theory of gravity. But he makes a critical mistake. He misinterprets one of his equations. And, unaware of his error, continues working on incorrect ideas.

The point at which Einstein is going to give THE most essential equations of the theory, Einstein considers something like them and then says, “Ah, but these don t work,” and then writes down the wrong equations. What follows are alternations of confidence and despair as he convinces himself that everything is fine with this theory, and then he realises that things aren t so good with the theory. It is a long, dark period for Einstein as he struggles to reconcile himself with a theory that is just not working. Two years later, Einstein is in Berlin.

At just 36 years old, he has one of the most prestigious positions in physics. But he is still struggling with his theory. By 1915, he d reached the pinnacle of the profession. He was in the Prussian Academy, and a professor at the University of Berlin.

But his marriage has fallen apart, his wife and two kids have moved back to Switzerland. So, he s pacing around almost all alone in this apartment in Berlin. He changes fundamentally the way he does his physics. He had relied on physical intuition all the way through here, and he d let the mathematics take a back seat.

He decided that that was a mistake, that he should have listened to the natural mathematics first. And Einstein adopted that new method, and started to write down not the equations that he thought were physically the most plausible, but the equations that were mathematically the most natural. But now he has a competitor. Einstein had enthusiastically shared his ideas with the brilliant mathematician David Hilbert.

Hilbert was so impressed, he decided to work on the theory himself. Einstein is now in a race to the finish with one of the world s best mathematicians. This is unfolding in a remarkably dramatic period in history. World War I has begun to ravage Central Europe.

Einstein is not just toiling in the abstract. He s toiling as the world seems to fall apart. That affects whom he can send letters to. It affects what journals from other countries he can even receive in the midst of the blockade.

The world is, certainly from the point of view of the middle of Germany, is not looking like a bright, happy place. Einstein’s feeling dejected because his work is not going well. He s concerned he s now in a race with a remarkably gifted colleague. His family life is not particularly happy.

And the headlines every day scream war, devastation and carnage. By November, 1915, Einstein is scheduled to present his work in a series of four weekly lectures at the esteemed Prussian Academy. But he s struggling to formulate his ideas. In the midst of this ordeal, letters arrive from his wife in Zurich pressing the issue of his financial obligations to his family and discussing contact with his sons.

As his lectures begin, his theory is still far from complete. The pressure on Einstein is huge. He would give a lecture, revise it, give it again. Spot mistakes, correct them, get up on the podium, explain what was wrong in the previous week s lecture, correct it, and then move on.

And then do that again and again for four weeks running. His work to convince them of the truth of this absolutely radical new theory of relativity that he was proposing is one of the most intense periods of work in the history of science. Somehow, he s able to focus on his theory with an incredible intensity. And he makes his breakthrough.

He tests his equations on a problem that Newton s theory of gravity couldn t solve. The orbit of Mercury. Mercury s path around the Sun has an anomaly that Newton s theory can t explain. It deviates slightly each time it goes around.

Einstein calculates the orbit with his new equations. The answer is correct. Exactly what astronomers had observed. He d found the final equations for his general theory of relativity.

You have to think about the hubris of being Albert Einstein. He had already thrown out Newtonian mechanics with special relativity, and then he d gone off on his personal quest to incorporate gravity, and, at the end of the day, he boils it down to a prediction for a number that had been observed. The procession of the orbit of Mercury. Miraculously, when the pages of algebra work out to their end, you get the right answer.

And suddenly it s not just playing with equations any more. He realises this is how the world works. All this abstract nonsense is the correct theory of reality. Einstein is at last able to present a successful theory.

That s a triumphant moment, one of the great moments in the history of physics. And, for Einstein, a victory very much against the odds. And he d won. On 25th November, 1915, Einstein lays out his findings in his climactic fourth lecture at the Prussian Academy.

He presents general relativity. The theory can be written as a single equation. It condenses sprawling complexities into a beautifully compact set of symbols. So the formula is really simple…

G, for the shape of space-time. And T for the distribution of mass and energy. So, this very simple formula captures all of Einstein’s general theory of relativity. It s a beautiful, simple equation, but it s a lot of work to unpack the symbols, the mathematical symbols, and see how, in this very simple formula, the whole geometry of the universe is hidden.

It s kind of an acquired taste to see the beauty. It s also a signature formula for Einstein. The true mark of his genius is that he combines two elements that actually live in different universes. The left-hand side lives in the world of geometry, of mathematics.

The right-hand side lives in a world of physics, of matter and movement. And, so, perhaps the most powerful ingredient of the equation is this very simple equals sign here, these two lines that actually are connecting the two worlds. And it s quite appropriate there are two lines because it s two-way traffic. Matter tells space and time to curve.

Space and time tells matter to move. The idea that gravity is the curving of space and time is completely alien to most of us. It s hard to imagine that time itself can be warped. But it s real.

We can measure it. The earth s gravity, the distortion of space and time, reduces the further you are from the mass of the planet. Time flows quicker at altitude. To put this to the test, a team of specialists has placed a highly accurate atomic clock at the top of New Hampshire s Mount Sunapee, 2,700 feet above sea level.

After four days, they collect their clock. And take it down the mountain to their lab. There, they compare it to a second atomic clock that has remained just a few feet above sea level. Put that one into channel A.

And the master clock in channel B. You guys ready? This is it right here. The time interval counter s going to show us the time difference between these two clocks. 20 nanoseconds.

You can see the time difference between them represented here graphically. The clock that was up at the mountain for four days and our master clock. Since gravity is weaker at altitude, while the test clock was up the mountain, time ticked faster. It s now 20 nanoseconds – 20 billionths of a second – ahead of the sea-level clock, just as general relativity predicts.

This is really awesome. But, back in 1915, atomic clocks weren t available. Einstein needed a way to show the world the bizarre features of his theory. The general theory of relativity made predictions of things which looked really strange.

For example, the idea that light bends when it passes near a very heavy body. No-one had ever looked for that. No-one had ever observed it. Einstein was desperate, desperate to get astronomers to make that test.

Einstein’s theory predicts that when light from a distant star travels close to the Sun, the warped space around the Sun bends the light s path. In May, 1919, the English astronomer Arthur Eddington travelled to the African island of Principe to record images that would show this phenomenon. Right at the end of the war, Arthur Eddington, very much keen to work with Einstein, first because he took the general theory of relativity very seriously, he was a huge admirer of Einstein, but also because he was a Quaker, a pacifist. He wanted as quickly as possible to, as he put it, “solve the wounds of war”.

To bring the British and the Germans back together. What Eddington had been able to do was take photographs of stars during a total eclipse of the Sun, so the moon blocked most of the brightness of the Sun, and little pin pricks of light could be seen around the Sun that would otherwise be lost in the glare. And Eddington and his colleagues were able to measure that the appearance of those stars had been shifted compared to where they would have been had that big mass of the Sun not been deflecting that light from far away. Eddington is able to show in November of 1919, just one year to the day after the end of World War I, that Einstein’s general relativity theory is right and a revolution in science has been accomplished.

When the eclipse experiments prove Einstein’s theory right, he rockets to fame, not just because he s explained a new way of looking at the universe, but at the end of World War I you had the predictions of a German scientist be proven right by some British astronomers and it becomes headlines across the world. The New York Times says, “Lights all askew in the heavens. “Men of science more or less agog.” This is back when newspapers knew how to write great headlines, but Einstein kind of loves this fact, that he is now an icon of science.

Einstein becomes a worldwide celebrity, the icon of genius we still recognise today. The only person who was more widely known was Charlie Chaplin. And they got on like a house on fire. Chaplin said, “The reason they all love me is “cos they understand everything I do “and the reason they love you is that they don t understand anything you do.

“Can you explain that?” And Einstein’said… But in 1930s Berlin, the Nazi party is gaining power.

As a Jewish scientist, Einstein becomes increasingly caught up in the political unrest. Einstein’s theories became a target. They were deemed aesthetically repugnant to a kind of Aryan sensibility, so people attacked not just Einstein the Jewish scientist, but they would actually have people denouncing general relativity. In January, Nobel Prize mathematician Albert Einstein visited California.

He begins to make trips to America, where he is welcomed with open arms. Germany s loss, America s gain. And in 1933, he settles in Princeton, taking up a position at the Institute for Advanced Study. Today, the Institute is headed by Professor Robert Dijkgraaf.

He basically was still very much by himself, just actually as he was in Berlin, just concentrating on his deep ideas and struggling with understanding the universe. Of course, his office was here. At the Institute, Einstein worked to unify his theory of gravity with the other laws of physics. With Einstein you see this phenomena you see with many great scientists.

That they climb this very high mountain and instead of celebrating their success they re privileged to see a much wider landscape and they see all these mountains behind it and I think he was very much aware how much, still, there was to be done. Until the very last days of his life he was trying to push these equations and find a description of nature, all of nature, in terms of the geometry of space and time. But general relativity was fading from mainstream science. Physics was now focused on the quantum theory of atoms and tiny particles.

A theory incompatible with Einstein’s ideas. But one that could be tested in a lab. Most of general relativity was then beyond the reach of experiment. When Einstein died in 1955, aged 76, the wider scientific community presumed his theory had reached a dead end.

But they couldn t have been more mistaken. The best theories in physics always take us to places where the people who invented them didn t imagine. And a truly wonderful theory like general relativity predicts all sorts of things that Einstein didn t conceive of. The theory has a life of its own.

We understand general relativity much better right now than Albert Einstein ever did. Today, huge telescopes peer deep into the universe. It is general relativity that allows us to make sense of what they see. And there s one prediction of Einstein’s theory this technology has allowed us to explore that is straight out of science fiction.

A black hole. Everything that we re familiar with in ordinary life is made from matter. But not black holes. Black holes are made from warped space and time.

And nothing else. A black hole is an object that is spherical, like a star or like the earth, with a sharp boundary called the horizon, through which nothing can come out. So it casts a shadow on whatever is behind it. It s just a black, black shadow.

Unbelievably black. This simulation shows the distortion of starlight around the black hole. Even though Einstein knew his theory predicted black holes, he found it hard to believe they would really exist in nature. In the 1960s, Professor Kip Thorne worked on a mathematical concept of black holes.

The idea made sense on paper, and he began to feel that these science fiction-like objects might actually be real. It must be here somewhere. It s in one of these piles…

Kip made a bet with fellow physicist Stephen Hawking about whether or not a strong source of X-rays, known as Cygnus X1, was in fact a black hole. Yeah, here we go. Relativist stars and black holes. Yeah, there it is.

So, that is a copy of the famous bet. The bet says, “Whereas Stephen Hawking has such a large “investment in general relativity and black holes “and desires an insurance policy and whereas Kip Thorne likes to “live dangerously, without an insurance policy”. That s a good characterisation of myself, much to my wife s chagrin! “Therefore, be it resolved that Stephen Hawking “bets one year s subscription to Penthouse magazine “as against Kip Thorne s wager of a four-year subscription “to a political magazine called Private Eye, that Cygnus X1 “does not contain a black hole of mass above the Chandrasekhar Limit.” It s written as this 10th day of December 1974, while Stephen is at Caltech with me.

We made that bet under circumstances where there was mounting evidence that Cygnus X1 really is a black hole. Stephen Hawking had a terribly deep investment in it actually being a black hole and so he made the bet against himself as an insurance policy that at least he would get something out of it if Cygnus X1 turned out not to be a black hole. And the evidence mounted thereafter, over the period of the 70s and 80s and in June 1990, Stephen snuck into my office and signed off on the bet that finally the evidence was absolutely overwhelming that Cygnus X1 really is a black hole. And Penthouse magazine arrived.

He sent me the British version of Penthouse, which was ever so much more raunchy than the American Penthouse. Actually enough to turn my face red when I received it at first. Today, thanks to concepts built on Einstein’s theory, we have evidence suggesting there are millions of black holes in our galaxy alone. And his general relativity tells us more.

Just as a collision of two objects produces sound waves, the collision of two black holes generates waves in space-time. There are huge things in the universe happening, like black holes colliding or stars exploding. And they create these gravitational waves, the waves in the shape of space and time that travel through the universe at the speed of light. And so right now, the space around me is being squeezed and stretched by gravitational waves just getting here from, let s say, two black holes colliding a billion light years away.

But the squeezing and stretching is so minute, I absolutely could not personally detect it and so what we re trying to do is build an instrument that can. In Louisiana and Washington State, a vast experiment called LIGO is in the final phases of calibration. It s hoped that laser beams travelling 4km between precisely aligned mirrors will measure the squeezing of space caused by gravitational waves. The experiment is able to measure the difference between two mirrors at 4km and two mirrors at 4km plus or minus a ten-thousandth of the nucleus of an atom.

Some time between today and a few years from now, we really expect to have made the first direct detection of gravitational waves to actually record the ringing of the shape of space and time. A direct measurement of pure gravitation. We re not collecting light, we re not talking about matter, we re not talking about anything, just measuring pure modulations in space and time. So it s pure general relativity.

Of all of Einstein’s theory s remarkable breakthroughs, the most profound is that our universe has a beginning. The discovery that distant galaxies are moving outwards and the detection of background radiation from the very start of the universe provided evidence for the big bang and a universe that s growing. With this picture of an expanding universe, there is natural questions. Is the universe slowing down as it expands? Is it so dense that someday it will come to a halt and collapse? Will the universe come to an end? These seem like good questions.

To find answers, in the 1990s, Saul and his team studied exploding stars called supernovae to track the growth of the universe. When we made the measurement, we discovered that the universe isn t slowing down enough to come to a halt. In fact, it s not slowing at all. It s speeding up.

The universe is expanding faster and faster. In order to explain the acceleration of the universe within Einstein’s theory of general relativity, we re considering a energy spread throughout all of space that we ve never seen before. We don t know what it is, we call it dark energy. And if so, it would require something like 70% of all the stuff of the universe to be in this form of previously unknown dark energy.

So, this is a lot to swallow, and you might imagine that at that point you should go back and revisit your theory. The problem is that Einstein’s theory is so elegant and it predicts many, many, many digits of precision that it s very, very difficult to come up with any other theory. For 100 years, general relativity has proven correct time and time again. But Einstein himself knew that his great theory had limits.

The huge problem with theoretical physics now is to combine general relativity, our best theory of space, time and gravity, with quantum mechanics, our best theory of very small things. Two phenomenally successful theories that don t automatically jell with one another. Here at the Institute for Advanced Study where Einstein worked, the world s leading theoretical physicists are trying to solve the problem Einstein never could. Finding a single set of rules that applies to both the cosmic and atomic scales.

A unified theory. The Holy Grail of physics. We are now in what at this time is the School of Physics, so here, our people are still struggling with many of the same issues that Einstein was struggling with and are still trying to capture the laws of the universe, from the very small to the very large, in a single equation. And it s still a blackboard that s the weapon of choice! The brightest minds of the world are coming here to work 24 hours, seven days a week, struggling to grasp the great mysteries of the universe.

And I think we are still driven by the same dream that at some point, we can capture everything in elegant mathematics. 100 years after Einstein transformed our understanding of nature, the stage is set for the next revolution. Quantum mechanics was very different than general relativity. It came about by many people stumbling into it, maybe that will be the way we do it next time.

Einstein was this singular genius who managed to get gravity right. He didn t manage to get quantum mechanics right. When we finally move beyond Einstein, it might be another singular genius that comes along, someone struggling in a poor school in Kenya right now, that we don t know about, or it might be 20 different people with 20 different points of view, gradually building brick-by-brick to finally figure out a more comprehensive view that includes general relativity in it. I think the most important thing that you learn from Einstein is just the power of an idea.

If it s correct, you know, it s unstoppable. It s extremely encouraging that he was able with pure thought to solve the riddle of the universe. There are only a few moments in science history where we ve had to completely rethink our picture of the world that we live in and this was one of those moments. The moment you enter the world of general relativity, you encounter claims, propositions, that are doing nothing less than calculating how much matter there is in the universe, whether the whole of space is curved, so that what we thought was in a way beyond experience becomes a system that can be described, can be tested.

That still seems to me to be an absolutely amazing fact. You already have the huge universe and it obeys the laws of nature. But where in the universe are these laws actually discovered, where are they studied? And then you go to this tiny planet and there s this one individual, Einstein, who captures it. And now there s a small group of people walking in his footsteps and trying to push it further.

And I often feel, well, you know, it s a small part of the universe that actually is reflecting upon itself, to try to underst and itself.