 On behalf of the Institute of Physics, it gives me great pleasure to present the Isaac Newton Medal, the Institute's international medal to Professor Edward Whitten for your insight, your creativity, your mathematical powers, which have led to numerous and profound transforming contributions in areas of particle theory, quantum field theory and general relativity. Our many congratulations to you. Thank you so much Chancellor. It's quite an honour to be chosen for the Newton Medal by the Institute of Physics. I only hope that I'm able to live up to this honour. I grew up in Baltimore, Maryland and when I was quite young I was actually interested in math and science a lot. I remember as a kid I was especially interested in astronomy. You have to remember that those were the days of the space race, so in a sense everybody was interested in astronomy, but I wanted to be an astronomer although I was afraid that being an astronomer would mean having to go into space to do observations and that seemed scary. My father certainly encouraged my interest in astronomy and then later math and physics while I was growing up. He himself was a theoretical physicist working mostly in general relativity by the time I was a teenager. When I was growing up my father sometimes encouraged me by giving me books and explaining things to me. I didn't really go into his office to visit. The only time we wrote a paper together actually was once in the 1980s. It was an unusual experience because growing up you're a kid and your father explains everything and gives you these very advanced books, but later on ten or fifteen years after I got into my PhD we worked together as colleagues really. It was an interesting experience to get to collaborate professionally with my father. So apart from physics one of my lifelong passions has actually been peace in the Middle East. I guess I got interested in it because we lived in Israel for a year as a child. You actually might see a photo of me on a camel in Bergevo that was taken that year in 1964 with my mother Lorraine Witten and on the sides are my sister Celia and my brother Matt. I'm sensibly attached to the Jewish tradition which I was raised in but I don't take seriously the truth value of my own tradition or of other religious traditions. So my wife Karen Appi, she is an Italian physicist. We actually met when I was a graduate student at a physics summer school so we were married a few years later and we have three children whose ages range from thirty to nineteen. My wife and I work in similar areas and we've occasionally collaborated scientifically but not usually. None of our children are physicists but they've all got scientific interests. The oldest works in a neuroscience lab. The second has just gotten a PhD in statistics and our son is interested in computers and applied math. As the children were growing up my wife and I encouraged them to learn about science certainly but we encouraged them to learn about all kinds of other things. We took lots of exotic trips to places like the Galapagos, Belize and Jordan among others partly because it was fun but we also wanted to give them a chance to be interested in different things. They've all gotten interested in science of one form or another as they've grown up but you know children have to find their own way. So growing up until the age of about fifteen or sixteen I was really interested in astronomy, math and physics but I didn't study them at as an advanced level at that age as other young people interested in those subjects might do. In fact in hindsight I might have been a little bit better off if I'd gone a little bit farther but as it was I didn't I think in part my parents didn't believe in as they saw it pushing children to get too far too fast with that sort of interest. And at the age of about sixteen or seventeen I developed other interests in history, linguistics, journalism all kinds of things actually for a few years. My degree is in history it's factually correct but at my university to have a degree that was labeled history degree required a very modest number of history courses so what I actually had was a wide smattering of all kinds of courses actually the subject I was most intensely interested in as an undergraduate at least for a while was linguistics. After I got my degree though I was out of the university for a year or two for example for half a year I worked on the McGovern campaign but during that period I had the sense to realize that math and physics were what would suit my talents best and I actually took home a bunch of books from the library or perhaps borrowed some from my sister who was then a math student actually and decided what I wanted to do based on those books and reached the conclusion that it was theoretical physics. I was lucky enough to be able to get into graduate school originally in applied math and then changing into physics. That was at Princeton University where I got my PhD in 1976. As a student I was studying the standard model of particle physics. My education was fairly standard at that time for a theoretical physicist with interest in elementary particles and the fundamental laws of nature meaning I studied quantum theory and relativistic quantum theory, thermodynamics, quantum field theory, Einstein's theory of gravity which we call general relativity and all those nice subjects. I didn't take anything you could possibly call a math course. I worked successfully as a student on some questions involving the standard model which could be treated by small but sometimes fun extensions of known methods and then there were bigger questions about the standard model some of which remain unsolved to this day which fascinated me at the time. Then and especially later during my postdoctoral years after I'd received the PhD which was in 1976. Gradually it turned out that both trying to understand the standard model better and trying to go beyond it we were led to ask new questions that involved more sophisticated mathematical methods. So bit by bit I started learning and applying mathematical techniques that originally were not part of the education of a theoretical physicist back in the days that I was a student and the perspective changed because things kept changing. A new idea called supersymmetry emerged largely during the period when I was a student although I didn't really learn very much about it until just after I'd graduated. Supersymmetry is a kind of updating of quantum theory to include part of Einstein's work special relativity although not the deeper part of his work history of gravity or general relativity. Supersymmetry would involve new elementary particles like a non-magnetic cousin of the electron. Then and to this day supersymmetry has been one of the most interesting ideas for going beyond the standard model and as I and others showed in the late 70s and afterwards it enables us to get a lot of insights about understanding the standard model better. Einstein had pursued the dream of finding a unified theory of gravity and other interactions. The most interesting idea he had and the only one that influenced later physicists was to assume that nature has not just the four dimensions of ordinary experience the three space dimensions in time but also a fifth dimension a little tiny one that we don't usually notice and then that would lead to a unified theory of gravity and octromagnism. Gravity and octromagnism were the only forces Einstein knew about but nowadays we know about other forces the weak nuclear force and the strong nuclear force and to modern physicists it looked like Einstein's attempt to unify the forces of nature was premature because he didn't know about most of them. Still the idea of extra dimensions as a method of unification was interesting and I was one of many physicists who worked on that in the late 70s and early 80s and I contributed to making it more realistic but I also linked it up with my interest in the question of whether energy was positive in Einstein's theory. I actually showed that if you assume one more extra dimension and you don't have supersymmetry the catastrophe that people had speculated about with negative energy actually happens and the world does explode and come to an end. So it was a nice illustration of the potential importance of supersymmetry and also positive energy in explaining why the real world is still here. Meanwhile some of my colleagues were pursuing a more ambitious theory even than supersymmetry which aimed to unify gravity with the other forces at the quantum mechanical level. 20th century physics is broadly based on two big theories. There's general relativity which is Einstein's theory of gravity and there's quantum theory which is the framework in which we understand all the other forces. If you try to put the two together you run to seemingly intractable difficulties. The origin of the difficulties is that according to Newton the gravitational force becomes infinitely strolling when two particles approach each other. It's not a problem for the moon going around the earth because the moon always stays at a safe distance but if you consider two elementary particles you've got a problem they can really go all the way into zero distance. So even for electromagnetism this was a problem and solving it there had led to quantum theory but it turned out for gravity it really didn't work because of the nonlinear mathematics in Einstein's theory. And now when thinking about this problem directly during the 70 or 80 years since it was first raised has ever had a good idea I would say. The only good idea about it came rather indirectly as a byproduct of something that originally had quite a different motivation which we know of as string theory. In string theory a little elementary particle is replaced by a small vibrating string. It can vibrate in many ways just like a piano string vibrates in many ways giving us middle C and its higher overtones. So one of these strings can vibrate in different ways giving us electrons, muons and other elementary particles. So the richness and beauty of music comes from the interplay of the higher harmonics. A tuning fork produces approximately a pure tone but it sounds ugly to the human ear. The sound of a violin string or a piano string is far more beautiful because of the interplay of the higher harmonics. In the case of these strings they vibrate in many different ways and the richness of elementary particles comes from the different shapes of the possible vibrations. Now physicists developing this idea weren't trying to describe gravity and when features arrived in their equations that in hindsight were hints of gravity they didn't like it they tried to get rid of it and that's why we eventually became confident that if you try to do string theory you're stuck with gravity. String theory is a quantum theory so you're actually stuck with quantum gravity in other words you're stuck doing what physicists otherwise have been unable to do which is to reconcile the two big theories of 20th century physics gravity and quantum mechanics. String theory was developed in a way that made it possible to have theories of gravity and quantum mechanics with gravity unified with elementary particle forces but I realized a key point actually which was that there was a flaw in the models of elementary particles which is that in that framework as it existed then it was impossible for the weak interactions to violate the symmetry between left and right in the way that we actually see in nature. So this was a puzzle to some point. I did some peripheral calculations sharpening the problem a little bit but then in the summer of 1984 there was a wonderful breakthrough achieved by Green and Schwartz where this problem was overcame and it became possible for models of elementary particle physics unified with gravity via string theory to be far more realistic. Now when that happened I regarded it as a kind of signal from heaven because I believe that a miracle like that making the theory vastly more realistic wouldn't happen unless there was a reason for it and it was actually on the right track. If you actually look at the history of string theory there's been a whole chain of remarkable discoveries that had been made long before this period starting in 1968 when the theory had its roots. Some of them may have been just as miraculous as what Green and Schwartz discovered in 1984 but their discovery had the biggest impact on me because it was the first miracle that was discovered in the field while I was closely watching. So I did regard it as a kind of signal from heaven and since then string theory and matters associated to it have been my primary interest. The first thing that happened after the work of Green and Schwartz is that it did become possible for models of elementary particle physics derived from string theory to be far more realistic. The perspective of the mid-80s was that string theory introduced a new ingredient modifying Einstein's ideas of geometry and spacetime that was just as fundamental as quantum theory but different and most of us string theorists spent the decade after 1984 studying this new dimension in geometry while thinking of quantum effects as being something we would take into account later. We treated the quantum effects as small and we looked at this new dimension in geometry. From that point of view there were five possible string theories which was a big advance over what we had in physics before but it did raise the question of who needed four extra theories. So in the mid-90s there was a new synthesis where we didn't treat the quantum effects as small. We took them into account more comprehensively and we learned that in the quantum world the five different string theories were just five different limiting cases of one bigger theory. I coined a name for that bigger theory which was M theory but that really was just giving a name to the unknown. M theory is our candidate for super unification. If this general approach is right we've got one theory which includes gravity and quantum theory and the elementary particles and it's our candidate for a deeper understanding of laws of nature. We've been studying it in different ways now for at least 40 years but we're far from understanding it properly. It's conceivable that we'll fall short in our dream of understanding what string theory means and how it applies to the universe but the fact is that once we've discovered a unique theory that's a candidate for unifying gravity, quantum mechanics and all the forces it's inevitable to try to understand it as well as we can. There's never any guarantee in life of what you can achieve but when you have this opportunity it's inevitable to think about it and I'm sure that this effort will continue. A lot of the most fun I've had personally since the mid 90s has been in trying to use string theory and M theory ideas to shed light on established but not fully understood physical theories. So the standard model of particle physics is based on what we call quantum gauge theories. So we know the right because we can calculate a lot of stuff and compare with what happens in the lab but there's an awful lot of stuff that's also important that we actually can't calculate even with the help of computers even in physical theories that are well established. So sometimes we can use string or M theory methods to understand these theories better for example explaining why we'd ever see an individual quark in the theory of strong interactions. That's very satisfying when we're able to do it. It's been part of my interests in the late 90s and in this last decade. String theory has given us a lot of new insights about how known physical theories behave. These range from the proof of positive energy, my own earliest contribution in that spirit, to later insights about questions like why quarks are trapped inside atomic nuclei, the behavior of heavy ion collisions, quantum critical behavior, quantum black holes. There are many instances where questions were asked in the context of established physical theories and often these are simply questions about known but intractable equations of standard physical theories where re-asking these questions in the light of string theory has given a lot more insight about the answers. So one of the things that gives me confidence that string theory is on the right track is that it shed a lot of light on the behavior of established physical theories. Another thing that convinces me that string theory is on the right track is that in a strikingly elegant and beautiful way, it gives a good rough draft of the real world of elementary particles as we see it unified with gravity. It's true that we haven't really gotten past a rough draft but a lot of astonishing discoveries have gone into the fact that that rough draft is possible. I don't believe that such a chain of astonishing discoveries has happened by coincidence. I think it's because we're actually onto something. The last thing that convinces me that string theory is on the right track is that it's given rise to a lot of deep new insights about geometry. String theorists repeatedly have come up with things that are sort of inevitable consequences of trying to understand how the physical theory behaves that have led to new insights about geometry that have surprised the mathematicians and often have influenced mathematics at the frontier. This is certainly not at all what I intended to work about when I was a graduate student but in life you've got to take advantage of opportunities as they appear and this was an unexpected opportunity that did appear during the time that I've been in the field. The fact that we're able to come up with so many surprising insights about geometry is partly because we don't understand the theory. My guess is that if we understood the theory there might be a big new insight about geometry and then the geometers would go on and develop it for themselves. But because the foundations of the theory are not well understood and the problem is actually a hard one for mathematicians to appreciate. So really only physicists work on it. Physicists keep coming up with new insights about geometry that surprise the geometers. And I don't think that that's happening because physicists are brilliant geometers. I think it's happening because physicists have stumbled upon a very deep theory that we don't understand well that knows a lot of physical and mathematical secrets, most of which we don't fully grasp yet. What fascinates me most is what is the core new idea that string theory is based on. So Einstein based his theory of gravity on the notion that space-time was dynamical, that gravity resulted from the curvature of space-time and that the future of space-time can only be found by solving some equations that he eventually discovered. The core ideas in string theory are not known. String theory was invented in an incredible and, to my mind, rather lucky process of discovery where people stumbled upon a wonderful trail that started long before I was in the field, but which I've been privileged to contribute to. What fascinates me most would be to understand what it really means. But at the same time, a little modesty compels us to say that during the last 40 years, nobody's really had the conception or the ability to anticipate what was going to happen next. The theory has repeatedly forced upon us new turns that we didn't anticipate. And that's probably still true today. So while I'm most fascinated about the big questions, I don't think I'm likely to be able to answer them by direct assault. And I usually don't think about them actively. In fact, the hardest part of research is always to find a question that's big enough that it's worth answering, but little enough that you actually can't answer it. What I'm actually working on today is it's something that's fascinated me for roughly eight years since I learned about the question. I learned about something new that mathematicians, algebras, really had discovered starting with a man named Kovanov. Kovanov found a new twist on something that I and other physicists had done in the last 80s. It was a very interesting new twist, though. I felt it should be understood better using physics based methods. But I was for years completely baffled about how to do that. Even though actually some fellow physicists made some contributions in that direction, I still couldn't understand what to make of it. I mean, while the mathematicians working by their purely algebraic methods kept making progress. But in the last few months, I finally succeeded in finding a physics based viewpoint that I'm happy with. Now I've invested a lot of energy into that and there's a lot more still to come because just writing up what I've understood is actually a lot of work. And as is often the case, it's very hard to predict what the payoff may be. The short term is that the algebras will go on using the algebraic methods because they're better founded mathematically. And the insights are there, they can be understood by string theorists who want to, but only a few string theorists will want to. But I think it's interesting, so I've wanted to understand it. And sometimes virtue is its own reward. It doesn't come with the guarantee that understanding something really is important. I suspect it will be in this case, but it might take a long time. What's great about science is that the frontiers are still there. You're not going to be able to discover a new continent and climbing a mountain that hasn't been climbed before is getting harder all the time. But in science, even though the frontiers are different from what they were in the past because we know so much more, there are frontiers that are every bit as exciting as the frontiers of the past. So we're working on a theory whose fundamentals we don't understand, but the thing is that that's a mixed blessing. One would like to be able to understand what we're doing, but because we don't, there's ample room for fresh discoveries. I'd say that there's every prospect that a young student, maybe in high school or even elementary school listening to this interview, will have the chance to possibly grow up and be the one who will figure out what string theory means or at least important pieces of that puzzle. We are more like people who have stumbled upon some incredible underground treasure. We don't even really know what the treasure is, we just know that when we dig in the ground we find bits and pieces of buried treasure, and we don't yet understand where the main load comes from.