"What if the world was so strange, we could never hope to understand it?" "And science was wasting it's time trying to do so." "It sounds like the sort of thing a mystic might say." "But this was a suggestion made three decades ago by the most famous scientist in the world:" "Stephen Hawking." "We were appalled by the idea, appalled not by Stephen having said it, but the possibility that it might be true." "The implications are about as profound as you can imagine." "Predictability goes out the window." "Cause and effect in some sense become unrelated." "After 30 years of debate, Stephen Hawking's legacy is being called into question as never before." "This is the story of his most controversial theory, and perhaps his greatest mistake." "He is one of the most famous people on the planet." "Perhaps the only true celebrity scientist." "Stephen Hawking's diary reads like that of a global personality, with appearances across the world." "Including Spain, where he recently launched a new edition of his best-selling book:" "A Brief History of Time." "It is not really any shorter and I hope it is easier to understand." "He's become a by-word for genius." "He's absolutely unique and I think he has been a very important person in both the intellectual and the cultural life of the past century." "Stephen has inspired more people than any physicist in recent times." "And therefore his impact on the public has been as great as any physicist, probably since Einstein." "But recently doubts have been expressed by some physicists, about Hawking's scientific reputation." "A debate has begun about exactly what legacy Stephen Hawking will leave behind." "There's a tremendous gulf between the public perception of the importance of Hawking, and the scientific evaluation of his contribution." "There are many many theoretical physicists who've made a big impact on physics, and a greater impact than Hawking has." "The doubts about Hawking have recently crystallized around an idea he proposed over 30 years ago." "He called it the information paradox." "It's an idea which has provoked one of the great intellectual battles of recent times, between the most famous scientist in the world, and those determined to prove him wrong." "It's a battle which ranges from the birth of the universe to deep inside the atom." "And it's roots are in Hawking's earliest work." "I have always been interested in how things work." "I used to take things apart to see what made them tick, but I wasn't so good at putting them back together again." "Understanding means that, in a sense, you are in control." "From the age of 14 I knew I wanted to do physics because it was the most fundamental of the sciences." "Stephen Hawking was born on the 8th January, 1942." "I was born 300 years to the day after the death of Galileo." "Like Galileo I have wanted to work out my own understanding of the universe." "For some time nothing seemed to mark Hawking out as extraordinary." "But then he arrived at Cambridge University as a young postgraduate student." "Cambridge in the 60s, which was when I began my research, was a very exciting place to work." "We felt everything was possible, and much of it was." "Hawking made his name almost immediately when he worked on a new theory:" "The big bang." "He showed how the vast universe around us had begun as an infinity small point, of infinite gravity, and infinite density." "It established him as a great scientist." "But there remained many mysteries about the big bang, so Hawking now turned his attention to something he hoped would shed light on them:" "the black hole." "A black hole begins with a momentous event." "The death of a star." "As a massive star dies it collapses in on itself." "Where the star had been, gravity now becomes infinitely powerful, and anything which approached it is sucked in, even light." "Light can't get out, no radiation can get out." "It will look like an absolutely pitch black sphere in space, and so it will really truly look like a black hole." "At the invisible heart of the black hole is a mysterious place, where the fabric of time and space are transformed, and the equations of physics break down." "If you follow the equations to their inevitable conclusion the matter would collapse to form a single point." "This single point is infinitely small and infinitely dense." "The centre of a black hole bears a startling resemblance to the beginning of the big bang." "Which is why Hawking was convinced black holes would help him reach a deeper understanding about the nature of the universe." "They would, but they would also in time lead inexorably to his most controversial idea." "Black holes are places where space and time come to an end, and matter is crushed out of existence." "If we could understand how time comes to an end in black holes, it might help us understand how time began in the big bang." "The attempt to understand black holes drew in physicists from all over the world, including someone who would become one of Hawking's closest friends:" "Kip Thorne." "The period from about 1965 to about 1980 is what I like to call the 'golden age' of research on the theory of black holes." "Enormous progress was made and Stephen was the dominant figure in that progress." "As well as Hawking's scientific triumphs, he was also to witness first hand the decline in his health." "At the age of 22 Hawking was diagnosed with ALS, a type of motor neurone disease." "His doctors' gave him two years to live." "It was a prediction he chose to ignore." "Stephen's ALS has progressed much more slowly that with almost anybody else." "We don't know for sure why, but it has been much more slow." "By the 1970s Hawking had lost the power of his limbs, and his speech was slurred." "All this might have destroyed his career." "Instead, astonishingly, he turned it to his advantage." "So Stephen will basically be talking about infinity." "He had to develop a whole new way, different from the rest of us, for working with the mathematics of Einstein's relativity." "He learned to do it entirely in his brain without the benefit of writing things down." "Now it just so happens that we happen to have the universe here." "It's upside down." "He developed a way of doing it that involved manipulating images of the shapes of objects." "The shapes of curves, the shapes of surfaces" "Not three dimensional space, but in four dimensional space plus time." "It's a kind of intuition he has about spatial relationships." "He has the ability to visualize things that's very uncommon and that's one of the things that makes him so effective at what he does." "It has made him unique among all physicists in his ability to do a wide range of calculations, far better than he would have been able to do if he not had ALS." "This unique way that Hawking was forced to work and think about the world would now allow him to understand a black hole better than anyone else." "Before Hawking began to tackle the black hole, it had been shrouded in mystery." "But over a decade and a half, he developed a series of laws and equations which began to explore some of its many secrets." "He described the swirling edge of the black hole, the event horizon, where gravity became an irresistible force." "He described how time slowed and eventually stopped around a black hole." "And how, once matter crossed the event horizon, it could never return." "But Hawking wanted more." "He wanted a complete mathematical description of a black hole." "It was an extraordinarily difficult task." "Bernard Carr was Hawking's research student during this time." "Stephen is always wrapped up in the problems he's working on." "He thinks very deeply about things and inevitably one is wrapped up, but this time I think he was particularly wrapped up in this, because it was such an important problem." "The conditions in and around a black hole were so varied and extreme they called on many different types of physics, some of which were incompatible and had never been brought together before." "It was now that Hawking's new way of working came into its own." "He imagined a series of elaborate interactions between a black hole and the different forces of nature." "From gravity to those which govern the quantum world of subatomic particles." "The equations which governed all these different interactions were long and complex." "But in one of the great insights of modern physics he managed now to boil them all down into one single equation." "I remember he was very puzzled, I remember talking to him over tea and he saying he was doing calculations and getting this strange result." "In just one elegant equation were symbols which represented all the major branches of physics which might affect a black hole." "There was 'G' for gravity, 'c' from the work of Einstein and his E=mc^2, 'h' from the baffling world of quantum physics, and 'S' from thermodynamics." "From particle physics to Newton, everything Hawking knew about a black hole was now brought together in a small but audacious piece of mathematical brilliance." "Sometimes in physics you get a result which is so beautiful it sort of almost has to be correct, and this result was beautiful." "It's a classic." "The logic that goes into deriving it is very complicated but it's a beautifully simple formula." "He was able, just by visualizing the process in his mind, to get to the right answer." "It's a marvellous thing." "It was a triumph, and it confirmed Hawking's reputation as a genius." "Results like that are so elegant, they must be right." "It shows there is a deep underlying unity in nature." "But it soon became clear that this enigmatic equation had a powerful dark side to it." "Hawking discovered that at its heart was a paradox which could undermine the whole of physics." "And this claim would lead him to confront his colleagues, in a battle that would last 30 years." "The man who had come to understand the disturbing implications of Hawking's work and become his chief opponent, was Leonard Susskind." "My relationship with Stephen Hawking has been what you would call adversarial." "But we are friends, there's no question we are friends." "It's an intellectually adversarial relationship." "Two people who believe two different things, arguing as strenuously as we can for our own position but with a great deal of respect for each other." "Until 1980 Susskind had spent most of his time working in an unrelated area of physics from that of Hawking." "And he might never have come across the information paradox, if it wasn't for an unlikely meeting." "The first time we met was in San Francisco in a mansion owned by a gentleman by the name of Werner Erhard." "Werner Erhard started his career as a car salesman, but he wasn't content with cars." "He moved into therapy." "And soon inspired a worldwide movement of psychological reprogramming." "Werner Erhard was a very very wealthy man who had made his money in something called 'EST', Erhard Seminar Training." "And this was a pop-psychology movement." "Go." "Do it." "If I push harder than you do I'm going to squash you." "So you better push fast now, hard, do it." "He would put people in a room, keep them there for 16 hours, not let them go to the bathroom, harangue them, harass them" "And as a consequence of this they would come out of the room completely different, or so they thought." "Erhard liked to claim that his outlandish training sessions were somehow scientific." "And he befriended some of the best known scientists in the world." "He had a thing for physics, he loved physics and he loved physicists." "We convinced him to hold some small conferences up in his mansion." "We wanted to draw together a group of people who are interested in common problems." "Problems of very very frontlines of theoretical physics, and we thought there would be no place greater than in Warner's mansion." "It was during one of those conferences that Susskind first met Stephen Hawking." "A meeting which was to change the course of his career." "Hawking had been asked to give a presentation about his latest work on black holes." "And it soon became clear that his new theory was astonishing." "For Hawking said he could prove that bits of the universe were disappearing." "This was a bomb shell." "This was something that violated all of the principals of physics." "Everything that we knew about Physics up to that point." "We stood at the blackboard in this electric moment of stunning confusion." "Leonard Susskind got very upset." "I think he was the only one in the room who fully appreciated the implications of what I had said." "Hawking's arguments were related to his brilliant equation, and in particular, the symbol S, from the study of thermodynamics." "Until Hawking it had been assumed that black holes were dark, cold places that lasted forever." "But S suggested an effect that the black hole was radiating heat, and burning up." "That had important consequences." "What happens if you follow that to its logical conclusion, is that the black hole will radiate all of its mass away, and go 'poof'." "Hawking was saying that a black hole would eventually disappear, and it was this that caused the problem." "Throughout its life a black hole would suck in anything that came near it, from astronauts to light." "But when the black hole was gone, what happened to all that stuff?" "Hawking claimed it just disappeared too." "But scientists hated this idea, for it wasn't just inconvenient, it was scientific heresy." "Because it went against one of the fundamental principals of physics." "According to physicists, everything in the universe is made up of particles." "It's long been one of their most deeply held beliefs that the information encoded in these particles can never be lost." "Information never disappears." "It can be scrambled." "You can get, you know, chopped up like chopped liver or whatever, but it is a very very basic principle of physics that information is never destroyed." "According to the strange laws of quantum physics, whatever you do to an object, you can always retrieve the information in the particles that make it up." "This means in theory, that if you collected all the information about an object, you could reconstruct it, almost magically, from scratch." "And it was this fundamental principal that Hawking was now challenging." "Once you admit that this kind of thing can happen, once you admit that information can be lost, you can't quarantine it and say it only happens in black holes." "You're going to find it happening in all kinds of contexts." "And it will have the most incredibly dramatic effects in the ordinary world." "If the information paradox was right then, effectively, bits of the universe were missing." "The implications are about as profound as you can imagine." "Because what it really means is that predictability goes out the window." "Cause and effect become de-coupled." "And that means nothing science knows, not even our memories, could be trusted." "The universe would be completely chaotic." "At its most extreme scale what it means is everything you've come to know and love would ultimately disappear." "Because anything you know and love involves information in a sense." "Those things go away." "There's nothing that remains sensible and is preserved over time." "Scientists usually assume various unique correspondence between the past and the future, cause and effect." "But if information is lost, this is not the case." "One wouldn't be able to predict the future with certainty." "And one couldn't be sure what happened in the past." "This was an idea that struck at the very nature of reality." "But as Susskind listened, Hawking insisted it was true." "And what's more, his mathematics seemed watertight." "Stephen had this 'Stephen' look on his face, a little smile which says you may not believe it but I'm right and make no mistake about it." "We were absolutely sure Stephen was wrong, but we couldn't see why." "I had a 40 minute drive back to Palo Alto and I couldn't concentrate on the way back." "Every time the traffic slowed down all I could think about was drawing this picture of the black hole on my windshield." "I would draw the picture and stare at it in the midst of all of this traffic." "I was probably a menace on the way home, and I couldn't stop thinking about it." "I was just completely trapped and obsessed with this question that Stephen had asked." "It now seemed as if Hawking's great equation threatened science and its ability to understand the world." "He named this problem:" "the Information Paradox." "It scandalised and perplexed scientists in equal measure." "But Hawking stuck to his guns and challenged the physicists who scorned him to prove it wrong." "I would describe Stephen as being the most stubborn individual in the entire universe." "Somewhere deep down inside himself I'm sure there are questions and uncertainties, but they don't show on the outside at all, even to his closest friends." "He is dogged about pushing his view of how nature is, and he thereby challenges all of his colleagues to prove him wrong." "Now my first guest tonight is a truly remarkable man, regarded by many as a genius, arguably the greatest theoretical physicist since Einstein." "If the information paradox had been put forward by any other scientist, perhaps it would have been easier to dismiss." "but its announcement coincided with an extraordinary change in Hawking's status." "Now the nature of the universe is an unlikely subject for a best-seller." "From someone known mainly in academic circles, he became a global celebrity, best selling author." "Has about sold 8 million copies now." "The most famous physicist and the face of science itself." "The man who said science should read the mind of God." "When he speaks with this curious electronic voice it's almost like some kind of oracle pronouncing on it, and that kind of also puts him in that kind of curiously mystical role and I really think a great many people see him as that kind of" "strange incomprehensible figure who can communicate with the universe." "As his status grew, Hawking continued to maintain that the information paradox was true." "Which was worrying because the threat of black holes was growing too." "It wasn't until 1972 that astronomers were actually able to see a black hole." "Had there been only one, then perhaps physics could have found a way around the information paradox, it could have become a unique exception, but there wasn't just one black hole." "Soon astronomers had found thousands, then there were millions" "Within our universe there are at least a hundred thousand million black holes." "And they weren't all the same either." "Astronomers found an enormous super massive black hole at the centre of our galaxy." "Then it was even suggested that we could be surrounded by tiny subatomic black holes." "And they could be all through the universe including in this room or indeed in my head, and we wouldn't know anything about them." "If the information paradox was true, then all of these black holes were machines eating up information." "And so across the world physicists were increasingly keen to solve it." "No-one more so than Leonard Susskind." "For over a decade Susskind studied all the literature on black holes, looking for a way around Hawking's reasoning." "In time it would lead him to a remarkable idea which was all about images, and projections." "His breakthrough came when he looked back at early work done by Hawking and others in the 1970s." "This concerned an inexplicable contradiction that seemed to occur at the edge of the black hole, the event horizon." "The theory stated that if an observer watched someone falling in towards the event horizon, they would see something extraordinary happening to that person." "As that person gets closer and closer to the black hole, they will be evaporated, ionised, all the terrible things that will happen from extremely high temperatures will happen to them." "and eventually they would just be converted to electrons, protons, neutrons." "Just completely destroyed." "But the very same equation said that the person falling in would themselves experience something rather different." "The really bizarre thing is that the same laws of nature tell you that from the perspective of somebody falling into the black hole they would see nothing special at the horizon, they would just freely fall through it." "No hot temperatures, no destructive influences, and would cleanly pass through the horizon with absolutely nothing interesting happening." "The same equations were saying that someone could both be dead, and alive." "Even for physics this was a pretty bizarre contradiction." "And at first there seemed no way around it." "Susskind's inspired move was to find a way to explain how both could be equally correct." "While it seems extremely weird that both of these stories could be true, as far as we can tell now, both stories are true." "What Susskind said is that as the person went into the black hole the information contained in their body was smeared on the event horizon." "So they appear to outsiders to disintegrate, but the person themselves would still think they were intact." "He said it was almost as if the black hole was acting like a giant cosmic projector." "Taking a three dimensional person and turning them into a two dimensional surface on its edge." "If you look in incredible detail, far more detail than we can really imagine, at everything taking place on the surface of the black hole, we could reconstruct everything taking place on the inside." "In effect, the information remains stretched on the edge of the black hole." "And that means information is not destroyed, it's all, theoretically, able to be retrieved." "The information paradox, according to Susskind, was solved." "Now, across the world, physicists raced to confirm it." "Finally after 10 years of hundreds of attempts, a paper emerged which vindicated Susskind." "It was a rigorous mathematical proof and seemed to clinch the debate." "Information was not lost in black holes." "This was enough to pretty much convince most of the community of physicists that information could never be lost from a black hole." "The tide was turning against Hawking." "The growing consensus was that he just had to be wrong." "Cause and effect were related after all." "Our memories were safe." "But one man begged to differ." "Shall I turn the page?" "Well, sweetheart, you head is falling forward." "Come on." "Don't you say no to me." "By the year 2000 Hawking had defied a series of doctors' dire predictions." "Do you want me to scan down?" "But his health had, nonetheless, declined." "And he was forced to spend increasing amounts of time at home." "To stay alive he relied on a staff of nurses and his wife, Elaine." "Are you going to have some of this?" "And you've got one of these left." "He was inevitably less active in the everyday world of the physics community." "And for a new generation of physicists, he was no longer the giant he once seemed." "When the journal Physics World held a poll on the eve of the millennium asking who was the most influential physicist, Stephen Hawking came last out of 20 with one vote." "I've done this with quite a lot of my colleagues in physics, said, well you know, if you were to draw up your top 10 list of physicists of the 20th century, say," "I've yet to find one who would put Hawking in there." "But they do mention people who are not known in the public at all." "People like Dirac, Feynman, Weinberg," "Ed Witten, who actually is not known outside of the field of physics, but has probably had a bigger impact on science than Hawking has." "What few people realized was that Stephen Hawking was planning a come back." "He was determined for to show that he had been right all along about the information paradox." "Even though the challenge was now greater than ever before." "In recent years Stephen has gradually become more slow in controlling his computer by clicking a 'clicker' in his hand." "And that had begun to impede his communication with his colleagues." "It was no longer possible for Hawking to work alone." "So to help him he called on a young research student, Christophe Galfard." "I guess it's a dream for many people to work with such an icon in science." "But working with Stephen doesn't mean working with equations." "It means working with words and with concepts and that's how he leads my research." "Hawking would direct Christophe, suggesting new ways to attack the problem." "Christophe would then work on the detailed mathematics, to see if Hawking's insights panned out." "For the first year and a half every sentence of Stephen's took me about six months to understand." "I was six months behind what he was telling me and catching up little bit by little bit." "It was hard." "After reviewing all the recent material on the Information Paradox," "Hawking decided to tackle directly the paper which had convinced people Susskind was right, and he was wrong." "Stephen asked me to have a look at that paper, so I took a little while to read it." "A little while being about a year and a half." "It took me a while to understand it." "It was an arduous task." "A 22 year old student, guided by the aging Hawking, was trying to over turn a paper that was at the very forefront of modern physics." "Few people thought he would succeed, and most especially, Leonard Susskind." "He was quite certain it was wrong and he set out to prove it." "In fact we met at one occasion and he gave off with all of his ideas about what was wrong with it." "I was certain, absolutely certain, and I told him so, that he was not going to disprove it, that it was correct." "For 18 months Christophe patiently tried to decipher the thoughts of his professor." "But in spite of all this work, Hawking and his student, were getting nowhere." "It was still very very hard to have a definite answer to whether the information was lost or not." "And then, disaster struck." "On December 1st 2003 Hawking was rushed to hospital." "For days, doctors had been increasingly concerned about a dramatic decline in Hawking's health." "Now they feared for his life." "He was taken immediately to the intensive care unit and placed on a life support machine." "Hawking was diagnosed with pneumonia." "For weeks his condition was critical." "We didn't know whether we would see him again, we were very very concerned." "Hawking's body was now seriously weakened." "But his mind was elsewhere." "Throughout his career Hawking had wrestled with the infinite." "The vastness of the universe, the beginning of time itself." "But now, on what appeared to be his death bed, he returned to his favourite theme:" "the black hole." "For Hawking, this ultimate destructive force of nature was familiar territory." "He had circled its horizon and plumbed its depths." "But this time as he contemplated it, he felt he could see something new." "For the first time in 30 years he could see a fresh way to consider the black holes greatest puzzle:" "the information paradox." "A new idea which, if true, would confirm himself as the world's foremost expert on black holes." "Once again Hawking defied his doctors' predictions, and within three months he was discharged from hospital." "And working on his new idea." "Soon after Stephen came back from hospital, he asked me to work with double speed." "We spend hours and hours writing at night, late nights, weekends, days, nights, all the time." "Then Hawking made a dramatic announcement." "He wanted to address the scientific community at one of the most prestigious conferences in physics." "Shortly before the meeting we got word from Hawking that he wanted to speak." "Sure, when someone of Stephen's stature asks to speak you try to accommodate him." "Hawking's paper was a late entry." "The schedule was cleared and he was found a slot." "The conference got under way, but what everyone was waiting for was Hawking's new idea." "Many in the audience were expecting another defiant performance." "A proof that Hawking was right and that information was lost in a black hole." "But Hawking had a surprise." "He wasn't here to defend the information paradox." "He was here to bury it." "Can you hear me?" "I want to report that I think I have solved a major problem in theoretical physics that has been around since 30 years ago." "Hawking's speech turned out to be one of the great U-turns of science." "For information, he now admitted, was not lost in black holes after all." "The idea he defended for 30 years had been wrong all along." "But after the shock there was a twist in the tale." "Hawking claimed Susskind wasn't right either." "And instead he now had his own solution, one which was to leave his audience largely bemused." "It was based on a familiar theory that the universe we live in might be only one of an infinite number of universes each with its own different history in some a black hole would exist, in others it would not." "To understand the real effect of a black hole you had to combine all of the parallel universes, together." "One therefore has to sum over alternative histories with, and without, a black hole." "Information is lost in the black hole histories but information is preserved in histories without a black hole." "In effect, those universes where black holes existed would be cancelled out by those where they didn't." "And that meant information didn't disappear because there would be no black hole for it to become trapped in, in the first place." "If one waits long enough, only the histories without a black hole will be significant." "So in the end, information is preserved." "If there is no black hole, then there is no problem basically." "The man who had spent most of his career trying to understand black holes had now come up with a proof that made tem disappear." "And in the process effectively admitted that for 30 years he'd been wrong." "My reaction to Stephen's announcement was mild amusement and to say to myself it's about time." "The response from the conference itself was muted." "His paper struck many as big on claims but short on maths." "He claimed to have solved this paradox and it was all over the newspapers and television." "But actually physicists were not really that impressed." "It hadn't been subjected to scrutiny, it was just an announcement, and it turned out to be not really all that convincing." "So will this latest announcement of Hawking's turn out to be a fitting coda for a lifetime of achievement?" "That will depend on the work he is doing right now." "He is trying to flesh out his idea with a mathematical proof." "If it is to convince his critics, it will have to be just as brilliant as his best work." "Progress is tortuously slow." "Hawking now finds it hard to spell out words" "And so Christophe tried to anticipate his professor's thoughts." "Hawking guides his with small movements of his face." "I don't know the details of his proposal and so I can't really comment on how successful it will be." "I can't say at the moment whether Stephen's new ideas will materialize into anything interesting." "Stephen posed this incredible problem and what could be better after having done that, than to bring it full circle and solve the very problem he posed." "I have no intention of stopping anytime soon." "I want to understand the universe and answer the big questions." "That is what keeps me going."