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.
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
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...
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."
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".
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.
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.
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.
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.
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.
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.
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."
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.
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.
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.
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
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.
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.
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.
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".
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.
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."
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
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
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.
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.
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