Twenty years of string theory

Print edition : January 14, 2005

VIVEK BENDRE

Interview with Prof. Andrew Strominger.

String theory, which considers that the fundamental building blocks of nature are strings rather than point-like objects or particles as has been believed hitherto, has become a major theme of research in the discipline of theoretical high-energy physics. Hailed by its practitioners as the `Theory of Everything' because of its ability to provide a framework to unify all the fundamental forces of nature, the string theory has both fascinated and mystified physicists. When one particular account of the theory, The Elegant Universe by scientist Brian Greene, became a bestseller a few years ago, it was clear that string theory had, like the theories of Big Bang and black holes, captured the public imagination to some extent.

Prof. Andrew Strominger, a Professor of Physics at Harvard University, United States, is a leading scientist in string theory research. He was in India recently to participate in an international conference, the Indian Strings Meeting 2004 (ISM04), at Khajuraho from December 15 to 23. Dr. T. Jayaraman, a theoretical physicist at the Institute of Mathematical Sciences, Chennai, interviewed him when he visited the Tata Institute of Fundamental Research (TIFR), Mumbai, on the current status of the theory, 20 years after its initial breakthrough in the world of fundamental physics. Strominger also provided an interesting glimpse of the blend of differing moods - of adventure and excitement at the unexpected advance of the theory, of caution over unsolved problems and of optimism based on experience - that characterise fundamental science in the making.

Excerpts from the interview:

Prof. Strominger, 20 years after the dramatic result of Michael Green and John Schwarz in 1984, which set off what string theorists like to call the first revolution in string theory, to what extent has the initial promise of string theory been realised?

Well, I think that the way the subject has developed in the last 20 years would not have been predicted by anyone. In some ways, the development has been more exciting and interesting than we had anticipated but in other ways it has been disappointing.

Let me start with the way in which we have been disappointed. First of all, I should say that string theory is a very promising, exciting and interesting proposal for developing our understanding of the laws of nature beyond where they currently stand. What happened is that we discovered that string theory contains within it particles and forces that look very, very much like the particles and forces that we see in the world around us. The door was opened to the possibility that string theory really is a theory of nature. The initial progress in 1984 and 1985 was so rapid and dramatic that many people had the feeling that we would push it all the way to the finish line within a few years or even months. That hasn't happened. Twenty years later the jury is still out. We don't know if string theory is the correct theory of nature or not. Taking that last step to getting some kind of direct evidence, making a prediction that is then experimentally verified, has been more difficult than many people had anticipated back in 1984.

On the other hand, I think that the richness and beauty of the theory and the way the theoretical structure has expanded outward and made contacts with other areas of physics and mathematics has been beyond anybody's wildest imagination. The connections that string theory would make with the black holes theory, with Donaldson theory [a subject of study in mathematics], and with the theory of strong interactions and all these different problems that people were studying in mathematics and physics - nobody would have guessed these in 1984. And that has been extremely wonderful and gratifying, and gives us a lot of encouragement that we are on the right track with string theory. But it may take a long time.

When you referred to developing our understanding of the laws of nature beyond where they currently stand, were you referring to the programme of unification of all fundamental forces including gravity?

There are two major problems that have arisen in theoretical physics in the last half of the 20th century. The first is the very basic problem that the laws of physics as we know them are not self-consistent. Quantum mechanics is not consistent with Einstein's theory of general relativity. So we don't have any choice but to go on. We have a theory where we can ignore various inconsistencies and make some predictions but ultimately if we were to try and make detailed enough predictions, those inconsistencies would prevent us from getting correct results. So we are not allowed to stop and be satisfied with where we are, and with what we have. We know that what we have is only a piece of the story and that's a problem that has to be solved.

Then there's another problem. The laws of physics as we currently understand are kind of a laundry list of equations and masses for various particles and strengths of various forces and there doesn't seem to be any unifying theme or any order behind this list. Of course that's not an inconsistency but I think that almost every physicist feels that there should be some unifying framework behind the laws of physics and not just a seemingly random laundry list of equations. So those are the two main problems - the problem of unification, and the problem of consistently putting together quantum mechanics and general relativity.

Prof. Stephen Hawking in Mumbai when he came to participate in an international conference on string theory in 2001.-REUTERS/PRASHANT NADKAR

It is interesting that both of those problems have only one viable proposed solution, and that is string theory. String theory is the only proposal on the table for a theory that consistently puts together quantum mechanics and general relativity. And it is also the only proposal on the table for a theory that unifies all the forces and particles of nature into one single framework. When you solve two problems, potentially solve them with one proposal; that's one of the main reasons you feel that you are on the right track.

You mentioned earlier that one of the achievements of string theory was the connection that research in string theory made with the study of black holes. One of your own major contributions in string theory has been to this area of research. What was the contribution of string theory to the study of black holes?

Well, there was a famous paradox that was brought about by Stephen Hawking's work 30 years ago on the quantum theory of black holes. This had to do with what happens when you put together the theory of quantum mechanics and Einstein's theory of gravity, which predicts the existence of black holes. There is no question that the world around us is described by quantum mechanics. There has been all sorts of experimental verification of that. And in the last 10 or 20 years, there has been increasingly convincing astronomical evidence, I would say nearly irrefutable at this point, that there are black holes up there in the sky. Black holes are here in our universe and we have to understand them.

One of the features of a black hole, in the classical world, is that once something crosses into the interior of a black hole it can't get out. But Hawking showed that, in the quantum world, light and other things can slowly boil off the surface of a black hole. And what Hawking argued was that information is lost in a black hole. A black hole is like a hole in space and when you throw something into a black hole it disappears from the universe forever and the information is gone. And this is a crisis for physics because all the laws of physics are based on determinism. They are based on the idea that nothing is ever lost and that with exact knowledge of the present the future can be exactly predicted and the past can be exactly reconstructed. And Hawking called this into question. The problem that faced us was how to account for the information that falls into a black hole. Is it really just a feature of this hole in space or is there some structure there?

Now of course in this discussion we have to take into account both quantum mechanics and general relativity and we don't have a fully consistent theory that incorporates both. But string theory is a proposal for a fully consistent reconciliation of quantum mechanics with general relativity. So it should resolve this puzzle that Hawking raised. It should explain to us where is the information that Hawking claims is just falling into a hole that takes it out of our universe. I think it was clear 20 years ago that string theory should solve Hawking's conundrum if it was a correct theory. But 20 years ago we didn't understand the theory well enough to analyse this question. But as the years went on, we began to understand it better and better. Finally in the mid-1990s Prof. Cumrun Vafa [of Harvard University] and I were able to show that the strings that make up the black hole encode all the information about what was thrown into the black hole, thereby in large part resolving Hawking's conundrum.

This was a very satisfying development. String theory wasn't invented to solve this problem. String theory was discovered to solve other problems. But this was an example of how string theory provided a solution of a problem that was posed by other people working in other areas of physics. And so that was a very reassuring signpost - that we are on the right track with string theory, though very far from a proof that string theory is correct.

This summer Hawking agreed that he had lost his bet with his colleague that there would be no way around the paradox of information being lost in the black hole, within the framework of quantum mechanics. Was string theory part of the reason he conceded the bet?

Of course you have to ask Hawking that question. But certainly it is the case that the string theory picture has given a very compelling and beautiful explanation of Hawking's paradox. It's a sort of an exhilarating journey. We started by studying string theory and that leads us to the solution of a puzzle concerning black holes. And in the process of trying to understand the behaviour of black holes we were led into solutions of other problems in theoretical physics. So I think that it is such a beautiful and compelling interconnected web of ideas that physicists have, by and large, been persuaded to believe that the information is not lost in the black hole though I think it has not been proven yet.

And while I think the odds are on the side of string theory, I think it was a little premature for Hawking to have conceded the bet because there are still some issues there that we need to understand.

Black holes are fascinating for the non-scientist too. But what was the origin of your own scientific interest in black holes?

I would like to push our understanding of the fundamental laws of nature beyond where they are now. There are a number of little clues floating around out there about how we might go about doing that. And the clues are paradoxes. When you have a paradox, it is not a bad thing, it is a good thing. Because it is a crack that you can try to squeeze your way into, to see what's coming next. And so I thought very hard about all the paradoxes in the fundamental laws of nature and there aren't so many of them. There is the problem of the consistent reconciliation of quantum mechanics and gravity, of unification, of the cosmological constant, of the origin of the universe, and of Hawking's paradox with black holes. But I've always felt that some of these problems are very hard to think about in any concrete way and to make any progress on. So I think about all of them and try to work on all of them but the one I've made the most headway with is the black hole.

You would recall that in the early years of string theory, say in 1984-85, there were a lot of hostile reactions to it. For instance, Sheldon Glashow, a Nobel laureate in high-energy physics, described string theory as theology. Is there greater acceptance now for string theory in the community of physicists?

Shelly Glashow is my colleague at Harvard and I enjoy talking to him very much and the fact that they appointed me at Harvard is already some indication that Harvard is no longer opposing string theory [laughs]. The hostile reaction I think was just part of the natural quarrelling, the natural arguing, that goes on among scientists who tend to feel strongly about the things that they are working on.

But actually I think the tables have turned. My view now is that people are too enthusiastic about string theory, especially after the work of the mid-1990s on black holes and other developments. And I think they should be listening to Shelly Glashow more because we really don't know what the truth is. Let me quote to you what Shelly said to me once: "It is clear that string theory is going somewhere. Where it is going is not clear". I think that's a pretty good assessment of the field. We know we're moving but we can't see the end of the road. I think there is still a lot that we're missing.

If you were to speculate, scientifically speaking, where would you think possible experimental evidence for string theory would come from?

There are various proposals on the table for how we might get experimental evidence for string theory. One of them, may be the simplest of them, is that in the Large Hadron Collider (LHC), the big accelerator under construction at CERN [European Centre for Nuclear Research] in Geneva, you could literally make strings and you could see them. Now whether or not you could see strings at the LHC, depends on exactly how the theory is realised in nature. We have to be very, very lucky in order that it is realised in exactly the right way so that it would be observed. I wouldn't personally place any bets on it.

I think we need some luck to see some direct experimental evidence for string theory. But I don't think that after 2,000 years of trying to advance our knowledge of the fundamental laws of nature, on both the experimental and the theoretical front, we have somehow come to an end now. Sooner or later we'll figure it out but I can't predict when.

Some string theorists speak of our still missing an underlying principle in string theory. Where do you think the advances on the theoretical front would come from?

Well, there are many people working on string theory and they have many different styles. And I think it takes every different style. String theory is a very elegant but also a complex and intricate theory and it is too much for any one person to try to tackle, or even any one approach. My personal approach is to think about the various paradoxes, like the black hole paradox, and the physical puzzles. That's the kind of approach that was used for example by Einstein when he was puzzled about whether or not things could travel faster than the speed of light; he had some paradoxes that he contemplated and tried to solve. Many physicists have used this kind of route. Other people proceed mathematically. That's how Dirac worked. That's another kind of style. And really it will take all different approaches to move forward.

This is not your first visit to India. What are your impressions of the work on string theory in India and the contribution of Indian physicists?

It is an absolutely fantastic contribution. I gave a seminar yesterday and the level of intellectual exchange is equal to anywhere in the world, including

Princeton or Harvard or... . anywhere. It is quite remarkable. This [TIFR] is a very nice place to work. I think there is a very good intellectual atmosphere here. I've learned a lot about problems I've been thinking about and working on, talking to my colleagues here at TIFR.

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