All about strings: An interview with Leonard Susskind

Leonard Susskind
Leonard Susskind

Leonard Susskind is considered one of the fathers of string theory in physics. He is the Felix Bloch Professor of Theoretical Physics at Stanford University and Director of the Stanford Institute of Theoretical Physics.This is an edited transcript of an interview with Leonard Susskind (Stanford 13/04/2004)

GZ: What are the big questions in physics today?
LS: The connection between cosmology, gravitation, quantum mechanics and string theory (if it turns out to be the right theory which is probably). For me these are the central questions. Of course there are questions which divide the universe into before year 2000 and after year 2000. There are questions left over from the twentieth century. The questions from the twentieth century are how we understand the pattern of elementary particles and so forth, how we understand what’ s called the Standard Model, how does it fit into something bigger and more complete.
Under 21st century physics I would classify questions that have to do with the structure of universe, as well as the origin of our laws of nature, our laws of physics.

GZ: Could string theory be the “theory of everything” and give all the answers?
LS: I dislike the term “theory of everything” and I would never use it myself and if knew who had said it first I would shoot him. It’s an inflammatory term and all kinds of people correctly say that it is not a theory of everything. It doesn’t explain how the brain works and so it’s a term which I would not use. If it is a theory which can, at some point, explain the origin of the universe and the spectrum of elementary particles and so forth, it remains to be seen. My feeling is that there’s probably only one quantum theory of gravity and string theory appears to be a part of that theory of gravity.
What we are discovering about string theory is very different from what we had expected and hoped for. The original hope of string theory was that it would provide an absolutely unique set of answers to the questions such as: what is the particles’ spectrum, what are the masses of particles. It would have been a very elegant answer, a beautiful mathematical answer and extremely unique. Unique in that we would find that, basically, the world could not be any other way that the way it is. That was the hope. The reality is extremely different. The reality is that the more we study of the theory, the more possible kinds of things we discover it can describe. We discover it’s a theory with a vast number of solutions. We simply find that there are enormous numbers of possible worlds that string theory can describe.

GZ:String theory has often been called a “revolution in physics”…
LS: The word revolution has been tremendously overused. Super and revolution are the most overused words in physics. Everything is a revolution. Is string theory a revolution? We don’t know yet. I think we don’t know what string theory is yet. I think we’ve made very wrong guesses about what string theory will do for us. I think we got it completely wrong. We thought it would give us a unique theory of the elementary particles. Instead it’s giving us perhaps as many as 10500 different possibilities of what the universe could be like. This is very puzzling. What do we make out of it? Do we just randomly pick one of these possible universes? Or all of them are important? What’s going on? My own view for some time now, is that in an inflationary context you could have a patch of this universe, a patch of that, a patch of whatever else is possible. In string theory it looks like 10500 possibilities are possible, each with its own set of particles, set of interactions. My guess is that the universe is just exceedingly big, full of tremendous amount of diversity. All these different possibilities materialize at some place. We simply live where is possible to live, in that part of this giant structure which is not totally hostile or lethal to our existence.

GZ: String theory has captured the public imagination because it refers to hidden dimensions. Of course, science fiction stories have made a lot of hidden dimensions. Why do we need so many extra dimensions to explain nature?
LS: Wish I could give you a simple mathematical explanation, for I’m afraid nobody can explain it simply otherwise. It‘s a very complicated theory which fits together in a consistent way only if the number of dimensions are ten or eleven. Why does physics need them? Elementary particles in the ordinary view of things are point particles. A point can’t have many, many properties. A point is too simple to have properties. However, we know that elementary particles have a lot of properties. They have spin, they have electric charge, they have something called isotopic spin, they have a quantum number called color – it’s not got anything to do with ordinary color – they have generations that they belong to, there are whole catalogs of different kinds of quantum numbers, of different kinds of properties that quarks, electrons, netrinos, or photons have. It sounds unreasonable for a point to have that structure. So the feeling most of us have is that, at some level, if you look deeply enough into things, you‘ll discover that particles aren’t points. That they must have all kinds of internal machinery that gives them these properties. One of those machineries, one of the ingredients into understanding what the quantum numbers of particles are, is the idea of higher dimensions. I‘ll give you an example. The simplest and oldest theory of higher dimensions is called the Kaluza theory. It was invented by Kaluza in 1917. Einstein liked it very much. It postulated one extra dimension, i.e. a particle in the extra dimension could be regarded as a little circular dimension. The idea of Kaluza theory is that the particle can move not only in the usual three directions of space but it can also move around in this extra dimension. Well, the particle which moves around in the extra dimension is different than one that moves differently in the extra dimension. The amount of speed that is going in the extra dimension as well as the direction it goes matter a lot. What is this new option corresponds to? It corresponds to the electric charge of the particle in Kaluza’s theory. So, electric charge becomes motion in another direction, in a new direction. What’s going on now is that these extra directions – all of them – correspond in various kinds of ways to the extra properties that these points have. So I wouldn’t say that we needed the extra dimensions, but we needed the kind of structure, the kind of complexity in space that could explain why these other degrees of freedom are there.

GZ: Skeptics say that string theory will forever remain outside the realm of real science, because it’s not experimentally falsifiable.

LS: I would simply dismiss these people for lack of imagination. There are all these people who are constantly pontificating of what science is and what science isn’t. These people lack of imagination. I also lack of imagination but I have a lot of imagination to know that I lack of imagination. We do not know what people can do in the future. We do not know what the human intelligence is capable of collectively. True, we are in a new course of exploration s studying extremely remote things which are very, very difficult to establish experimentally. But we should not abandon our course.GZ: Let me take you back to what we discussed before about string theory predicting up to 10500 different possible universes. Is this perhaps an answer to the paradox that we live in a universe so finely-tuned. Is this the answer to the Anthropic principle?
LS: That may be. My view is that the fine-tuning of the universe, particularly with regards to the cosmological constant, is so exceptional that we can no longer ignore it. Nevertheless, we have to ask whether the Anthropic principle is really serious business. Different people mean different things by the anthropic principle. Some give a religious depth to that thing, that the Almighty created the universe for no other purpose than people to live there. That’s one theory to which I don’t subscribe. I think it’s the duty of physicists and scientists to take that theory only as an absolutely last resort, when everything else fails. Other people think it’s part of the weirdness of quantum mechanics that somehow we live in the one place we can.
My view is a little different. It’s similar to asking why we live on a planet which is so finely-tuned. Our planet is at just the right distance from the sun so that we do not get boiled or frozen. That’s a very small window of opportunity, it’s a fine tuning. To find the reason why this is so, you need at least two things: a set of very large alternative possibilities and a cosmology which creates all of these different possibilities. So, it wouldn’t be enough to know that the “planet equation” – whatever it is – has many, many solutions with different values of, say, the “right temperature”. I also want to know that the surrounding universe grew and expanded creating lots of planets. Those two ingredients make sense out of this anthropic idea. One, that the theory, whatever it is, has so many solutions that even though it takes a very fine tuning for life, there will still be enough other solutions, so that statistically there will be one. And that, whatever the cosmology of the universe is, it creates always different possibilities some place.GZ: How about string theory then. Does it fit your two requirements for an explanation?
LS: I think that string theory provides us with a space of enormous possibilities. By involving so many mechanisms put together in various combinations the number of possible universe is 10500or something. We don’t what the number is, but it’s vast. The other thing we need is something like Linde’s and Lincoln’s theory of eternal inflation, where inflation takes place constantly and spins off different environments. Their ideas seem to me to be very, very reasonable, that the universe expands to something enormously big and it creates patches of all different kinds of what Alan Guth calls “pocket-universes”.GZ: Could we ever find if that is true?
LS: For the moment it looks impossible because of the horizon problem. Our world has a horizon that we can not see beyond. Presumably these other worlds are behind this horizon. One of the things we’ve learnt from thinking about black holes in the context of string theory, is that at quantum level the horizon is not really a barrier to knowledge. What goes on outside the horizon is also equally well described by the Hawking radiation of the black hole. I suspect that cosmic horizons are scrambled in complicated ways. Cosmic microwave background, which is light Hawking radiation, has this information in it. Can anybody ever extracted it? Certainly not with experimental tools currently available. But as I said never say never. We don’t know what the limits of imagination, or the limits of intelligence, are and that’s something for the future to do. Young smart physicists want to be explorers. They want to explore those things which everybody else says are impossible.GZ: The LHC is underway and will soon start experiments for the Higgs particle . Will they find it?
LS: I think so. I don’t see any other good alternative.

GZ: So the LHC is money worth spent!
LS: Well, yes, whether we find the Higg’s particle or not. If we find it’s there, that’s wonderful and confirms everything we knew. If it’s not there, it’s even more radical and money will have been even better spent. It will mean that we have been thinking wrong about physics for thirty years now.

GZ: Is time is an illusion?
LS:Space is an illusion. You are an illusion.

GZ:This sounds very Buddhist to me.
LS:Well, physicists don’t think that way because it’s not a useful way to think. We can measure time, just like as we can measure space, just like we can measure electrons. So why pick on time as being an illusion? Everything is an illusion in that view. But it’s not a useful view for a physicist. If you can measure it, if you can describe it, then we regard it as real.

GZ: Let me press this point of illusion a little further. Quantum mechanics introduces the observer into the very fabric in reality. Somehow if you take observers out, if you take consciousness out, “reality” ceases to exist. Does the universe exist when we do not observe it?
LS: I’m not a philosopher and I’m trying not to be philosophical. I’m trying to be more practical. Let’s see…Ask me the question again.

GZ: Is there something that we can call extended reality, a reality outside our perception? Or are we constantly creating reality through our measurements or our observations?
LS: We don’t really know how to understand the world of quantum mechanics. We know how to understand one special set of circumstances, where you can clearly separate the world in observer and system.
But the real world is not like that. We the observers are always part of the system. In the context of the laboratory we can usually make some separations. We cannot make that separation about cosmology of the universe. We are part of it. We influence it. So we do not really understand how to think about a system when we are a part of it, because of quantum mechanics. A very good friend goes so far as to say that he doesn’t think that quantum mechanics is complete because of this. And he thinks underlying the quantum mechanics is something much more deterministic. Most physicists think it’s a screwy idea. My answer is I don’t know.

GZ: When we talk about physics, when we talk about reality, we usually talk about energy, talk about matter, interactions between particles and fields etc. Let me for a moment suggest to you that the universe is not like that at all, the universe is made out of bits and information. Wolfram wrote a book about cellular automata which made quite of sensation. Could you believe in Matrix world? Could the universe be made out of bits, at an elementary level?
LS: Yes I think that it is made out of bits. It is nothing but information .

GZ: Could this notion change our physics?
LS: No, it can explain our physics. With respect to Wolfram, I’ m a physicist who thinks Wolfram’s ideas are interesting. But keep in mind that Wolfram’s ideas have no place for quantum mechanics. And the world is quantum mechanical. Wolfram believes that the world is cellular automata. But he knows that cellular automata are not quantum mechanical. Quantum mechanics has to come from somewhere. Where does it come from I don’t know. So I would say, until you can understand why the world is quantum mechanical, to say that it’s made of cellular automata is servicing a point. No quantum mechanics no cigar.

GZ: Einstein once said that the most inexplicable thing about this universe is that we can explain it. Are we now reaching the limits of our cognitive abilities? Do you believe that there are cognitive limitations to the human mind?
LS: Sure there are limitations. The human mind can not have more bits of information than the whole universe has. There are limitations to the human abilities in general. I would have bet anything on that no human being can play six musical instruments at the same time. I would have bet very much that nobody can jump as high as Michael Jordan. It turns out that the ranges of people have in the very, very far parts of the distribution are so amazing that nobody would have guess that they were possible. When people say such things as we’re reaching the limits, what they really mean is that I ´m reaching my limit. When they say they can’t conceive of anybody solving a certain problem what they really mean is that they can conceive of themselves solving this problem. The limits are probably way beyond what we imagine. They always are. The danger in trying to predict the limits of human abilities is always going in the wrong direction. It’s much more likely to underestimate than to overestimate. As long as it’s physically possible, as long as it doesn’t violate the laws of nature, it means there’s a possibility that human beings can do it. Forget individuals. Collectively human beings have such a diversity of different kinds or ways of thinking, they have the flexibility to be able to bend their own way of thinking about new things. We simply don’t know, but I would guess that when we try to estimate these things we ‘re in much more danger of underestimating to what people can do than overestimate it.

GZ: As we expand our knowledge of the cosmos through our physics, if we ever reach points that we don’t really understand and we can not possibly falsify, then are we in a danger that science, physics can regresses to religion?
LS: There is such a danger. So, on one hand there is such a danger and in the other hand I also say that you are also in danger of underestimating what people will be able to do in the future. Will they be able to do experiments that now seem to be so completely out of this world that they seem totally impossible? I‘ll give you two examples. The first is the inflationary theory of the universe. Everybody who saw that the first time said “well, that’s very nice Alan Guth, but your own admission of the inflation of the universe wipes out any evidence of itself. Nobody will ever be able to make science out of it”. That’s what everybody said, including Alan. Nobody expected that within twenty-five years people would figure out how to confirm observationally that the inflation theory was right. But it happened. I’ll give you another example. I can easily imagine people telling Darwin “nice theory Charles, but the only way to confirm it will be to go back a billion years and see what’s going on, and that’s simply impossible”. Well, it took a hundred years to make science out of evolutionary theory. It took a hundred years for people to get enough knowledge about biochemistry and genetics, to watch microorganisms evolve. It took a hundred years but it happened. And as I said the two dangers are, falling into a trap of t faith-based physics, but also giving up because it looks too hard.

GZ: Apart from technological development making life more comfortable what is the role of science in the 21st century? With regards to politics, society and perhaps ideologically. Does science play a role in modern world of religious conflict, fanaticism and lack of rationality?
LS: To a certain extent I think physicists have been the keepers of the truth. A case of point would be the Soviet Union during the dark days when the keepers of the truth were physicists, people like Zakharof and Orlof. They were people who simply believed in the concept of the truth. You know what’s happened to the concept of the truth in American society? It’s been replaced by advertising! All kinds of things which tend to make irrelevant to what’s true and what’s not true. Scientists in general are people who recognize what it means for something to be true. They are people who will question when something it’s not true. And their whole mental make up, their whole ideological basis, has to do with finding the truth. That’s necessary for preserving society.

GZ: So you are not a postmodernist?

LS: Postmodernism has some truth but it has been carried too far. It is true that the way humans think about the laws of nature, the words that we use to explain things, are dependent on culture and so forth. When new scientific ideas come into the front a lot of the argument about them tends to be dominated about the language that we should use to describe them. But, eventually, through some filter, what comes out of the other end is pretty much independent of the specific mentalities of the people who discovered it. And so yes, I believe there is real truth in the bottom of all of it, and it’s also true that the language we use to describe things depends on culture. So, that was a good idea, it was an important idea but it got carried too far when it said that there’s no such thing as objective truth.