What we've discovered in the last several years is that string theory has an incredible diversity—a tremendous number of solutions—and allows different kinds of environments. A lot of the practitioners of this kind of mathematical theory have been in a state of denial about it. They didn't want to recognize it. They want to believe the universe is an elegant universe—and it's not so elegant. It's different over here. It's that over here. It's a Rube Goldberg machine over here. And this has created a sort of sense of denial about the facts about the theory. The theory is going to win, and physicists who are trying to deny what's going on are going to lose.

A Talk with Leonard Susskind

The Reality Club: Responses by Paul Steinhardt, Lee Smolin, Kevin Kelly, Alexander Vilenkin, Lenny Susskind, Steve Giddings, Lee Smolin, Gino Segre, Lenny Susskind, Gerard 't Hooft, Lenny Susskind, Maria Spiropulu



or some people, the universe is eternal. For me, it's breaking news.

Recently I sat down to talk with Lenny Susskind, the discoverer of string theory. After he left, I realized I had become so caught up in his story-telling that I forgot to ask him "what's new in the universe?" So I sent him an email. Here's his response...


"The beginning of the 21st century is a watershed in modern science, a time that will forever change our understanding of the universe. Something is happening which is far more than the discovery of new facts or new equations. This is one of those rare moments when our entire outlook, our framework for thinking, and the whole epistemology of physics and cosmology are suddenly undergoing real upheaval. The narrow 20th-century view of a unique universe, about ten billion years old and ten billion light years across with a unique set of physical laws, is giving way to something far bigger and pregnant with new possibilities.
"Gradually physicists and cosmologists are coming to see our ten billion light years as an infinitesimal pocket of a stupendous megaverse. At the same time theoretical physicists are proposing theories which demote our ordinary laws of nature to a tiny corner of a gigantic landscape of mathematical possibilities.

"This landscape of possibilities is a mathematical space representing all of the possible environments that theory allows. Each possible environment has its own laws of physics, elementary particles and constants of nature. Some environments are similar to our own corner of the landscape but slightly different. They may have electrons, quarks and all the usual particles, but gravity might be a billion times stronger. Others have gravity like ours but electrons that are heavier than atomic nuclei. Others may resemble our world except for a violent repulsive force (called the cosmological constant) that tears apart atoms, molecules and even galaxies. Not even the dimensionality of space is sacred. Regions of the landscape describe worlds of 5,6…11 dimensions. The old 20th century question, 'What can you find in the universe?' is giving way to 'What can you not find?'

"The diversity of the landscape is paralleled by a corresponding diversity in ordinary space. Our best theory of cosmology called inflationary cosmology is leading us, sometimes unwillingly, to a concept of a megaverse, filled with what Alan Guth, the father of inflation, calls 'pocket universes.' Some pockets are small and never get big. Others are big like ours but totally empty. And each lies in its own little valley of the landscape.

"Man’s place in the universe is also being reexamined and challenged. A megaverse that diverse is unlikely to be able to support intelligent life in any but a tiny fraction of its expanse. Many of the questions that we are used to asking such as 'Why is a certain constant of nature one number instead of another?' will have very different answers than what physicists had hoped for. No unique value will be picked out by mathematical consistency, because the landscape permits an enormous variety of possible values. Instead the answer will be 'Somewhere in the megaverse the constant is this number, and somewhere else it is that. And we live in one tiny pocket where the value of the constant is consistent with our kind of life. That’s it! There is no other answer to that question.'

"The kind of answer that this or that is true because if it were not true there would be nobody to ask the question is called the anthropic principle. Most physicists hate the anthropic principle. It is said to represent surrender, a giving up of the noble quest for answers. But because of unprecedented new developments in physics, astronomy and cosmology these same physicists are being forced to reevaluate their prejudices about anthropic reasoning. There are four principal developments driving this sea change. Two come from theoretical physics, and two are experimental or observational.

"On the theoretical side, an outgrowth of inflationary theory called eternal inflation is demanding that the world be a megaverse full of pocket universes that have bubbled up out of inflating space like bubbles in an uncorked bottle of Champagne. At the same time string theory, our best hope for a unified theory, is producing a landscape of enormous proportions. The best estimates of theorists are that 10500 distinct kinds of environments are possible.

"Very recent astronomical discoveries exactly parallel the theoretical advances. The newest astronomical data about the size and shape of the universe convincingly confirm that inflation is the right theory of the early universe. There is very little doubt that our universe is embedded in a vastly bigger megaverse.

"But the biggest news is that in our pocket the notorious cosmological constant is not quite zero, as it was thought to be. This is a cataclysm and the only way that we know how to make any sense of it is through the reviled and despised anthropic principle.

"I don’t know what strange and unimaginable twists our view of the universe will undergo while exploring the vastness of the landscape. But I would bet that at the turn of the 22nd century, philosophers and physicists will look back nostalgically at the present and recall a golden age in which the narrow provincial 20th century concept of the universe gave way to a bigger better megaverse, populating a landscape of mind-boggling proportions."


Below is a wide ranging discussion with Lenny. "To this day," he says, "the only real physics problem that has been solved by string theory is the problem of black holes. It led to some extremely revolutionary and strange ideas."

"Up to now string theory has had nothing to say about cosmology. Nobody has understood the relationship between string theory and the Big Bang, inflation, and other aspects of cosmology. I frequently go to conferences that often have string theorists and cosmologists, and usually the string theory talks consist of apologizing for the fact that they haven't got anything interesting to tell the cosmologists. This is going to change very rapidly now because people have recognized the enormous diversity of the theory."

Read on...


LEONARD SUSSKIND, the discoverer of string theory, is the Felix Bloch Professor in theoretical physics at Stanford University. His contributions to physics include the discovery of string theory, the string theory of black hole entropy, the principle of "black hole complementarity", the holographic principle, the matrix description of M-theory, the introduction of holographic entropy bounds in cosmology, the idea of an anthropic string theory "landscape".


(LEONARD SUSSKIND:) What I mostly think about is how the world got to be the way it is. There are a lot of puzzles in physics. Some of them are very, very deep, some of them are very, very strange, and I want to understand them. I want to understand what makes the world tick. Einstein said he wanted to know what was on God's mind when he made the world. I don't think he was a religious man, but I know what he means.

The thing right now that I want to understand is why the universe was made in such a way as to be just right for people to live in it. This is a very strange story. The question is why certain quantities that go into our physical laws of nature are exactly what they are, and if this is just an accident. Is it an accident that they are finely tuned, precisely, sometimes on a knife's edge, just so that the world could accommodate us?

For example, there is a constant in nature called the cosmological constant, and it's a certain number. If that number differed by the tiniest amount from what it really is, the universe could not have been born with galaxies, stars, planets, and so forth. Is it an accident that the number was exactly right to be able to form the universe as we see it? Or is it some feature of the way the universe works that makes it necessarily create life? It sounds crazy and most physicists think such thoughts are hogwash, but I'll give you an example.

Suppose we lived on a planet and we couldn't see out because there was too much fog and too many clouds. Suppose we wanted to know why the temperature on this planet is precisely right for us to be able to live without getting cooked and without getting frozen. Is it an accident, or is there a design involved? Most people, knowing the answer, would say that if you look out far away into the cosmos you see all kinds of planets, stars, empty regions and so forth. Some of them are much too hot to live on, some of them are much too cold to live on, and some of them are in between but don't have water; there are all kinds of planets are out there.

The answer is, we simply live on the planet that we can live on because the conditions are exactly right. It's an environmental fact that conditions are exactly right, so it's no accident that we happen to find ourselves in an environment which is finely tuned, and which is precisely made so that we can live in it. It's not that there's any law of nature that says that every planet has to be livable, it's just that there are so many different things out there—roughly 1022 planets in the known universe, which is a huge number—and surely among them there will be a small number which will be at the right temperature, the right pressure, and will have enough water, and so forth. And that's where we live. We can't live anywhere else.

The question is whether our environment in a bigger sense—in terms of the laws of nature that we have, the elementary particles, the forces between them, and all those kinds of things—are environmental things which are contingent in our particular region of the universe, or are exactly the same throughout the whole universe. If they're contingent, that means that they may vary from place to place, or they may vary from one thing to another thing to another thing. If that were the case then we would answer some subset of the questions that we're interested in by saying things are the way they are because if they were any other way we couldn't live here. The environment has to be right for us to exist.

On the other hand, if everything is the same, all across the universe from beginning to end, then we don't understand why things are tuned in the way that allows us, with knife-edge precision, to be in an environment that supports life. This is a big controversy that's beginning to brew in physics: whether the laws of nature as we know them are simply derivable from some mathematical theory and could not be any other way, or if they might vary from place to place. This is the question that I would like to know the answer to.

In the United States the cosmologists don't like the idea of the anthropic principle at all. In England they love it. I was very surprised to find out when I started talking about this that the physicists, like myself, people who are interested in theoretical, mathematical questions in physics, are rather open to it in the United States, but the cosmologists are not. This idea originated to a large extent among British cosmologists—Martin Rees being one of them, John Barrow being another one. There's also Andrei Linde, who is a Russian but of course lives in the United States, who was one of them, as was Alexander Vilenkin. But that's not the crowd that I'm addressing my remarks to.

The crowd that I'm addressing are the high-energy physicists, the string theorists, and includes the Brian Greenes, the Ed Wittens, the David Grosses and so forth. The reason is because over the last couple of years we've begun to find that string theory permits this incredible diversity of environments. It's a theory which simply has solutions which are so diverse that it's hard to imagine what picked one of them in the universe. More likely, the string theory universe is one with many different little patches of space that Alan Guth has called pocket universes. Of course they're big, but there are little patches of space with one environment, little patches of space with another environment, etc.

Mostly physicists have hated the idea of the anthropic principle; they all hoped that the constants of nature could be derived from the beautiful symmetry of some mathematical theory. And now what people like Joe Polchinski and I are telling them is that it's contingent on the environment. It's different over there, it's different over there, and you will never derive the fact that there's an electron, a proton, a neutron, whatever, with exactly the right properties. You will never derive it because it's not true in other parts of the universe.

Physicists always wanted to believe that the answer was unique. Somehow there was something very special about the answer, but the myth of uniqueness is one that I think is a fool's errand. That is, some believe that there is some very fundamental, powerful, simple theory which, when you understand it and solve its equations, will uniquely determine what the electron mass is, what the proton mass is, and what all the constants of nature are.
If that were to be true, then every place would have to have exactly the same constants of nature. If there were some fundamental equation which, when you solved it, said that the world is exactly the way we see it, then it would be the same everywhere.

On the other hand you could have a theory which permitted many different environments, and a theory which permitted many different environments would be one in which you would expect that it would vary from place to place. What we've discovered in the last several years is that string theory has an incredible diversity—a tremendous number of solutions—and allows different kinds of environments. A lot of the practitioners of this kind of mathematical theory have been in a state of denial about it. They didn't want to recognize it. They want to believe the universe is an elegant universe—and it's not so elegant. It's different over here. It's that over here. It's a Rube Goldberg machine over here. And this has created a sort of sense of denial about the facts about the theory. The theory is going to win, and physicists who are trying to deny what's going on are going to lose.

These people are all very serious people. Davis Gross, for example, is very harshly against this kind of view of diversity. He wants the world to be unique, and he wants string theorists to calculate everything and find out that the world is very special with very unique properties that are all derivable from equations. David considers this anthropic idea to be giving up the hope for uniqueness, and he quotes Winston Churchill when he's with young people, and he says, "Nevah nevah, nevah, nevah give up."

Ed Witten dislikes this idea intensely, but I'm told he's very nervous that it might be right. He's not happy about it, but I think he knows that things are going in that direction. Joe Polchinski, who is one of the really great physicists in the world, was one of the people who started this idea. In the context of string theory he was one of the first to realize that all this diversity was there, and he's fully on board. Everybody at Stanford is going in this direction. I think Brian Greene is thinking about it. Brian moved to some extent from hardcore string theory into thinking about cosmology. He's a very good physicist. There were some ideas out there that Brian investigated and found that they didn't work. They were other kinds of ideas, not this diversity idea, and they didn't work. I don't know what he's up to now. I haven't spoken to him for all of a month. Paul Steinhardt hates the idea. Alan Guth is certainly very susceptible. He's the one who coined the term "pocket universes."

The reason that there is so much diversity in string theory is because the theory has an enormous number of what I call moving parts, things you can tinker with. When you build yourself an example of string theory, as in Brian's book, it involves the geometry of these internal compact spaces that Brian became famous for studying. There are a lot of variables in fixing one of them, and a lot of variables to tinker around with. There are so many variables that this creates an enormous amount of diversity.


String theory started out, a long time ago, not as the theory of everything, the theory of quantum gravity, or the theory of gravitation. It started out as an attempt to understand hadrons. Hadrons are protons, neutrons, and mesons—mesons are the particles that fly back and forth between protons to make forces between them—just rather ordinary particles that are found in the laboratory that were being experimented on at that time.

There was a group of mathematically-minded physicists who constructed a formula. It's a formula for something that's known as a scattering amplitude, which governs the probability for various things to happen when two particles collide. Physicists study particles in a rather stupid way; somebody described it as saying that if you want to find out what's inside a watch you hit it as hard as you can with a hammer and see what comes flying out. That's what physicists do to see what's inside elementary particles. But you have to have some idea of how a certain structure of particles might manifest itself in the things that come flying out.

And so in 1968 Gabrielli Veneziano, who was a very young physicist, concocted this mathematical formula that describes the likelihood for different things to come out in different directions when two particles collide. It was a mathematical formula that was just based on mathematical properties with no physical picture, no idea of what this thing might be describing. It was just pure mathematical formula.

At that time I was a very young professor in New York, and I was not an elementary particle physicist. I tended to work on things like quantum optics and other things, just whatever I happened to be interested in. A fellow by the name of Hector Rubinstein came to visit me and my friend, Yakir Aharonov, and he was wildly excited. He said, "The whole thing is done! We've figured out everything!"

I said, "What are you talking about, Hector?"

And jumping up and down like a maniac, he finally wrote this formula on the blackboard.

I looked at the formula and I said, "Gee, this thing is not so complicated. If that's all there is to it I can figure out what this is. I don't have to worry about all the particle physics that everybody had ever done in the past. I can just say what this formula is in nice, little, simple mathematics."

I worked on it for a long time, fiddled around with it, and began to realize that it was describing what happens when two little loops of string come together, join, oscillate a little bit, and then go flying off. That's a physics problem that you can solve. You can solve exactly for the probabilities for different things to happen, and they exactly match what Veneziano had written down. This was incredibly exciting.

I felt, here I was, unique in the world, the only person to know this in the whole wide world! Of course, that lasted for two days. I then found that Yoichiro Nambu, a physicist at Chicago, had exactly the same idea, and that we had more or less by accident come on exactly the same idea on practically the same day. There was no string theory at that time. In fact, I didn't call them strings—I called them rubber bands.

I was just incredibly excited. I figured, "Okay, here I am. I'm going to be a famous physicist. I'm going to be Einstein, I'm going to be Bohr, and everybody's going to pay great attention to me," so I wrote up the manuscript.

In those days we didn't have computers and we didn't have e-mail, so you hand-wrote your manuscript and gave it to a secretary. A secretary typed it, and then you went through the equations that the secretary had mauled and corrected them, and this would take two weeks to get a paper ready, even after all the research had been done and all you had to do was write it up. Then you put it in an envelope and you mailed it by snail mail to the editor of the Physical Review Letters. Now the Physical Review Letters was a very pompous journal. They said they would only publish the very, very best. What usually happens when people start getting that kind of way is they wind up publishing the very worst, because when standards get very, very high like that nobody wants to bother with them, so they just send it to someplace where it's easy to publish.

I sent it to the Physical Review Letters, and you understand, weeks had gone by in which I was preparing it, and having it typed, and I was getting more and more nervous, thinking somebody was going to find out about it. I was telling my friends about it, and finally I sent the manuscript off. In those days it went to the journal, the journal would have to mail it, again by snail mail, to referees. The referees might sit on it for a period, and then send it back. All of this could take months—and it did take months.

And how did it came back? Well, they said, "This paper is not terribly important, and it doesn't predict any new experimental results, and I don't think it's publishable in the Physical Review."

Boom! I felt like I had gotten hit over the head with a trashcan, and I was very, very deeply upset. The story I told Brian Greene for his television program was correct: I went home, I was very nervous, and very upset. My wife had tranquilizers around the house for some reason and she said, "Take one of these and go to sleep." So I took one and I went to sleep, and then I woke up, and a couple of friends came over and we had a couple of drinks, and this did not mix. I not only got drunk but I passed out and one of my physicist friends had to pick me up off the floor and take me to bed. That was tough. It was not a nice experience.

Of course I wasn't going to leave it at rest that way; I sent it back to them and said, "Get another referee." They sent it back to me and said, "We don't get more referees." I sent it back saying, "You have to get more referees. This is important." They sent it back, saying "No we don't," and finally I sent it to another journal, which accepted it instantly. It was Physical Review, which is different than Physical Review Letters.

The discovery of string theory is usually credited to myself and Nambu. There was another version of it that was a little bit different but the guy had the right idea, although it was a little bit less developed. His name was Holger Nielsen. He was a Dane at the Niels Bohr Institute, and he was very familiar with these kinds of ideas. A little bit later he sent me a letter explaining his view of how it all worked, and it was a very similar idea.

After the paper came out, it was not accepted. People are very conservative about thinking pictorially like that, building models of things. They just wanted equations. They didn't like the idea that there was a physical system that you could picture behind the whole thing. It was a little bit alien to the way people were thinking at that time. This was five years before the standard model came along in '74 or ‘75.

The first thing that happened is that I immediately realized that this could not be a theory of hadrons. I understood why, but I also knew that the mathematics of it was too extraordinary not to mean something. It did turn out that it was not exactly the right theory of hadrons, although it's very closely related to the right theory. The idea was around for two or three years during which it was thought that it was the theory of hadrons, exactly in that form. I knew better, but I wasn't about to go tell people because I had my fish to fry, and I was thinking about things. I was not taken seriously at all. I was a real outsider, not embraced by the community at all.

I'll tell you the story about how I first got some credit for these things.

The already legendary Murray Gell-Mann, gave a talk in Coral Gables at a big conference, and I was there. His talk had nothing to do with these things. After his talk we both went back to the motel, which had several stories to it. We got on the elevator, and sure enough the elevator got stuck with only me and Murray on it.

Murray says to me, "What do you do?"

So I said, "I'm working on this theory that hadrons are like rubber bands, these one-dimensional stringy things."

And he starts to laugh...and laugh. And I start too feel like, well, my grandmother used to say, "poopwasser".

I was so crushed by the great man's comments that I couldn't continue the conversation, so I said, "What are you working on, Murray?" And of course he said, "Didn't you hear my lecture?" Fortunately at that point the elevator started to go.

I didn't see Murray again for two years. Then, there was a very big conference at FermiLab, and a thousand people were there. And me, I'm still a relative nobody. And Murray is in constant competition with his colleague Richard Feynmann over who is the world's greatest physicist.

As I'm standing there talking to a group of my friends, Murray walks by and in an instant turns my career and my life around.

He interrupts the conversation, and, in front of all my friends and closest colleagues, says "I want to apologize to you." I didn't know he remembered me, so I said, "What for?" He said, "For laughing at you in the elevator that time. The stuff you're doing is the greatest stuff in the world. It's just absolutely fantastic, and in my concluding talk at the conference I'm going to talk about nothing but your stuff. We've got to sit down during the conference and talk about it. You've got to explain it to me carefully, so that I get it right."

Something unimaginable had just happened to me and I was suddenly on a cloud. So for the next three or four days at the conference, I trailed Murray around, and I would say, "Now, Murray?" And Murray would say, "No, I have to talk to somebody important."

At some point there was a long line at the conference for people trying to talk to the travel agent. I was going to go to Israel and I had to change my ticket. It took about 45 minutes to get to the front of the line, and when I'm two people from the front of the line, you can imagine what happened. Murray comes over and plucks me out of the line and says, "Now I want to talk. Let's talk now." Of course, I was not going to turn Murray down, so I say, "Okay, let's talk," and he says, "I have 15 minutes. Can you explain to me in 15 minutes what this is all about?" I said okay, and we sat down, and for 14 minutes we played a little game: He says to me, "Can you explain it to me in terms of quantum field theory?" And I said, "Okay, I'll try. I'll explain it to you in terms of partons." Around 1968 Feynman proposed that protons, neutrons, and hadrons, were made of little point particles. He didn't know very much about them, but he could see in the data, correctly, that there were elements that made you think that a proton was made up out of little point particles. When you scatter protons off electrons, electrons come out. When you look at the rubbish that comes out, it tends to look as if you've struck a whole bunch of little tiny dots. Those he called partons. He didn't know what they were. That was just his name for them. Parts of protons.

Now you have to understand how competitive Murray and Dick Feynman were. So Murray says to me, "Partons? Partons? Putons! Putons! You're putting me on!" And I thought, "What's going on here?" I had really said the wrong word. And finally he says, "What do these partons have?" I said, "Well, they have momentum. They have an electric charge." And he says, "Do they have SU(3)?" SU(3) was just a property of particles, like the electric charge is a property, or like their spin. Another property was their SU(3)-ness, which is a property that distinguishes proton from neutron. It's the thing that distinguished different particles which are otherwise very similar. Murray Gell-Mann and Yuval Ne'Eman had discovered it in the early '60s, and it was what Murray became most famous for, and it led directly to the quark idea. I said, "Yeah, they can have SU-3," and he says, "Oh, you mean a quark!" So for 15 minutes he had played this power game with me. He wanted me to say quark, which was his idea, and not partons, which was Dick's idea. 14 of the 15 minutes had gone by, and he lets me start talking, and I explained to him everything in one minute, and he looks at his watch and says, "Excuse me, but I have to talk to somebody important."

So I'm on a rollercoaster. I had gone up, down, up, down, and now I'm really down. I thought to myself, "Murray didn't understand a word I said. He's not interested. He's not going to spend his time in his lecture talking about my work," and then off in a corner somewhere I hear Murray holding forth to about 15 people, and he's just spouting everything I told him and giving me all the credit I could hope for: "Susskind says this. Susskind says that. We have to listen to Susskind". And indeed, his talk at the end of the conference was all "Susskind this, Susskind that". And that was the start of my career. I owe Murray a lot. He's is a man of tremendous integrity, and he cares about the truth, and he certainly has an interesting personality.

That jump start to my career happened around 1971. I was teaching at the Belfer Graduate School of Science, which was part of Yeshiva University, way uptown. It was an extraordinary place for a brief period of time, and it had some of the greatest theoretical physics in the world; it was outstanding. The place closed up. It went broke and I had to move to Stanford. When I went to Stanford I was an elementary particle physicist. I was only interested in the mathematical structure of this thing. I became interested in elementary particles through it. Other people began to recognize that this was not the exactly right theory of hadrons, although it's closely related to the right theory.

I should go back a step. There were many things wrong with this theory—not wrong with the mathematical theory, but wrong in trying to compare it with nature, and to compare it with hadrons. Some of them were fixed up very beautifully by John Schwarz and Andre Neveu and a whole group of very mathematically-minded string theorists, who concocted all kinds of new versions of it, and these new versions were incidentally the start of the process of discovering this incredible diversity. Each of the new versions was a little bit different, and it was always hoped that one of the new versions would look exactly like protons, neutrons, mesons, and so forth. It never happened. There were some fatal flaws.

The first was that the theory only made sense in a ridiculous number of dimensions—ten dimensions. That's not a good thing for people who live in four dimensions. That got fixed and turned out not to be so bad. The other problem was that when the theory was solved it included forces between particles that were like gravitational forces. This theory was not behaving like nuclear physics—like it was supposed to behave. It was behaving like Newtonian gravity. Particles were having forces between them that were not the kind of forces that hold a proton and neutron together, but the kind of forces that hold the solar system together.

I lost a little bit of interest in it, because I was not interested at that time in gravity; John Schwarz and a number of others, including Joel Sherk realized that this was a great opportunity. They said don't think of it as a theory of hadrons, think of it as a theory of gravity. So out of a debacle, they turned it into a theory of gravitation instead of a theory or protons and neutrons. I wasn't interested at that point in gravity; I didn't know very much about gravity, and so I continued doing elementary particle physics. Elementary particle physicists at that time were not interested in gravity. They had no interest in gravity at all. There were people who were interested in gravity but they had no interest in string theory. So a small, isolated group of people—John Schwarz, Michael Green, Pierre Ramond and few others—carried the field on.

I became interested in it again because I became interested in black holes. Hawking had studied black holes, discovered that they radiate, that they have a temperature, that they glow, and that they give off light. I met Hawking and Gerard ‘t Hooft in the attic of Werner Erhard's house in San Francisco. Erhard was a fan of Sidney Coleman. Dick Feynman, myself, and David Finkelstein were his gurus. And of course we didn't give a damn about his silly business, but we loved his cigars, we loved his liquor, we loved the food that we got from him, and he was fun. He was very, very smart.

Hawking came and told us his ideas about black holes, and one of the things he told us was that things which fall into the black hole disappear from the universe completely and can never been returned, even in some scrambled form. Now, information is not supposed to be lost. It's a dictum of physics that information is preserved. What that means is that in principle you can always take a sufficiently precise look at things and figure out what happened in the past—infinitely accurately—by running them backwards.

Hawking was saying that when things fall into a black hole they're truly lost and you can never reconstruct what fell in. This violated a number of basic principles of quantum mechanics, and ‘t Hooft and I were stunned. Nobody else paid any attention, but we were both really stunned. I remember ‘t Hooft and myself were standing, glaring at the blackboard. We must have stood there for 15 minutes without saying a word when Hawking told us these things. I was sure that Hawking was wrong. ‘t Hooft was sure that Hawking was wrong. And Hawking was absolutely sure that he was right in saying that information was lost inside black holes.

For 13 years I thought about this—continuously, pretty much—and at the end of that 13 years I began to suspect that string theory had in its guts a solution to this problem. And so I became interested again in string theory. I didn't remember anything about it. I had to go back and read my own papers because I tried reading other people's papers, and I couldn't understand them.

In the intervening years powerful mathematics was brought to bear on the theory. I found it rather dry, since it was rather completely mathematics with very little of an intuitive, physical picture. The main things that happened were that, first of all, five versions of it were discovered. Tricks were discovered about how to get rid of the extra dimensions. You don't actually get rid of them, you curl them up into little dimensions. You can read all about that in Brian Greene's book, The Elegant Universe. That turned out to be a good thing.

John Schwarz and Michael Green, and a few other people, worked out the very difficult mathematics in great detail, and demonstrated that the theory was not inconsistent in the ways that people thought it might be. When they showed that the mathematics was firm, Ed Witten got very excited, and once Ed Witten walked into it, well, he's a real mathematical powerhouse, and dominated the field very strongly. Witten's written many famous papers, but one of his key papers, which may have been the most important one, was written in about 1990. He and collaborators around him worked out the beginnings of a mathematics of these Calabi-Yau manifolds, which are tiny, curled-up spaces that are very well explained in Brian Greene's book.

Ed is also a physicist, and he had a lot of interest in trying to make this into a real theory of elementary particles. He never quite succeeded, but discovered a lot of beautiful mathematics about it. I found a lot of it rather dry, because it was not addressing physics questions the way I enjoy addressing them. It was just a little too mathematical for my taste. My taste leans less toward the mathematical and more toward the pictorial. I think in terms of pictures.

I wasn't really following the subject too closely at that point. I was still interested in black holes, and it wasn't until about 1993 that I began to suspect that there were ingredients in string theory that could resolve this puzzle of Hawking's. So at that point I really got into it. I started to think about the connection between string theory and black holes.

String theory was a theory of gravity. When you have gravity you can have black holes, and so string theory had to have black holes in it, and it should have a resolution of this problem. Over a period of a couple of years it did have a resolution. It did, in fact, turn out that Hawking was wrong. That is to say, he was wrong in a great way. When a person puts a finger on a problem of that magnitude, independently of whether they got it right or they got it wrong, they have a tremendous impact on the subject. And he has had a tremendous impact.

I developed some simplified ways of thinking about it that demonstrated that black holes did not lose information, that things don't fall into the black hole and disappear, that they eventually come back out. They are all scrambled up, but nevertheless they come back out. I began writing papers on that, and my paper, which said that stuff does not get lost inside a black hole in string theory, stimulated the string community to start thinking about black holes. There was an eruption of papers—mine, Joe Polchinski's, Andy Strominger's, Cumrun Vafa's—that really nailed that problem down. And black holes have been solved. Black holes have been understood. To this day the only real physics problem that has been solved by string theory is the problem of black holes. It led to some extremely revolutionary and strange ideas.

Up to now string theory has had nothing to say about cosmology. Nobody has understood the relationship between string theory and the Big Bang, inflation, and other aspects of cosmology. I frequently go to conferences that often have string theorists and cosmologists, and usually the string theory talks consist of apologizing for the fact that they haven't got anything interesting to tell the cosmologists. This is going to change very rapidly now because people have recognized the enormous diversity of the theory.

People have been trying to do business the old way. With string theory they were trying to do the things that they would have done with the earlier theories, and it didn't make a lot of sense for them to do so. They should have been looking at what's really unique and different about string theory, not what looks similar to the old kind of theories. And the thing which is really unique and very, very special is that it has this diversity, that it gives rise to an incredibly wild number of different kinds of environments that physics can take place in.

Responses by Paul Steinhardt, Lee Smolin, Kevin Kelly, Alexander Vilenkin, Lenny Susskind, Steve Giddings, Lee Smolin, Gino Segre, Lenny Susskind, Gerard 't Hooft, Lenny Susskind, Maria Spiropulu

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