It took string theory 23 controversial years and two revolutions to get where it is today, and the question still remains: where exactly is it? The theory has always offered the promise of a quantum theory of gravity, uniting quantum mechanics with Einstein’s general relativity, but does so by postulating 10-dimensional universes—perhaps an infinite number of them—while providing no experimental predictions and no method of distinguishing one of those universes as the one in which we happen to be living. String theorists will argue that they’ve made great progress understanding the nuances and boundaries of the theory. Critics have questioned how "progress" can be defined if string theorists are no closer than ever in making predictions that can be tested in experiment.

"Right now, string theory is the only consistent framework we have that incorporates gravity and all the fundamental interactions," says Ashoke Sen of the Harish-Chandra Research Institute, Allahabad, India.

Despite the continuing controversy, string theory has come to dominate the theoretical end of high-energy physics. Among those physicists who have been dominant players in the field since the earliest days is Ashoke Sen of the Harish-Chandra Research Institute in Allahabad, India. Sen, who was recently recognized by Thomson Scientific as one of India’s Citation Laureates, has more than 12 papers in the last decade alone that have each been cited more than 100 times. Two of his papers, both on the role of so-called "tachyons" in string theory, have been cited more than 300 times, a remarkable number for the relatively small field of high-energy theoretical physics.

Sen, now 50, received his bachelor’s of science degree in 1975 from Calcutta University, and his master’s three years later from the Indian Institute of Technology Kanpur. He did his doctoral work in physics at SUNY Stony Brook, where he graduated in 1982, subsequently spending the next three years as a post-doc at Fermilab and another two and a half at the Stanford Linear Accelerator Center (SLAC). In March 1988, he moved back to India and the Tata Institute of Fundamental Research. Since 1995 he has been a full professor at the Harish-Chandra Research Institute. Between 1998 and 2003, Sen visited the Isaac Newton Institute in Cambridge, U.K., as Rothschild Visiting Professor, and, between 2004 and 2005, was at MIT as Morningstar Visiting Professor.

Sen spoke to Science Watch from Pasadena, California, where he was visiting and lecturing at Caltech.

You began working on string theory in 1985, when you were still a post-doc and it was a brand-new idea. Weren’t you concerned that it might be risky for a post-doc without a permanent position to pursue such a theory?

To me, it just seemed like an exciting theory, one that can achieve the unification we have always been seeking. That was my reason for getting into it. I didn’t really think much, in any career sense, about the risk.

What had you been working on before string theory?

I did my Ph.D. in field theory, the more formal aspects of what is known as perturbative field theory. I was trying to understand the behavior of the theory at high energies. I then worked on grand unified theories, monopoles, supersymmetry, and eventually landed in string theory, which is where I’ve stayed.

Would you describe yourself as having been optimistic in the 1980s that string theory would provide a so-called "theory of everything"?

There was always one obvious problem from the very beginning: there was more than one string theory, and each string theory had more than one vacuum, which means more than one potential universe. That was always quite bothersome. The hope was that we would find out that most of these theories were inconsistent and so get a unique string theory with a unique vacuum—a unique universe. It hasn’t happened.

How did you personally approach the problem?

I started out trying to fit string theory into grand unification. My take was always more phenomenological. I was trying to understand how to get the known hierarchy of particles and forces from string theory. Then I moved into more formal aspects—for example, looking at how strings propagate in different backgrounds.

In your CV, you mention six areas where you’ve made major contributions to string theory and another half dozen in which you’ve contributed technical aspects. When you’re working on a field with so little connection with the known universe, what drives your next research project?

That’s actually hard to say. Of course, the eventual goal that we always must have in mind is to derive particle physics from string theory. This has always been my interest, and this is what I mean by my take being more phenomenological. But my choice of problems has been typically driven by what looked like a logical question to ask at each stage. The long-term goal was always to eventually get quantum mechanics and particle physics out of string theory, but I didn’t necessarily have that in mind each time I took on a new problem.

As a scientist based in India, do you find that being geographically isolated from your peers in this pursuit tends to work for you or against you?

I think it makes a difference, but it depends on the circumstances. On the one hand, I’m not so directly influenced by what’s considered the hot subject of the moment. That can be an advantage when the subject is somewhat stuck in its tracks. And being far from the crowd allows you to think independently—you’re not immediately influenced by what everyone is doing. But when a lot is going on and the field is evolving quickly, it can be a disadvantage to be far away. It’s harder to keep up.

What do you see as the biggest challenge to string theorists today?

Highly Cited Papers by Ashoke Sen, Published Since 1998 (Ranked by total citations) Rank Paper Citations 1 A. Sen, "Rolling tachyon," J. High Energy Phys., 4: 048, 2002. 377 2 A. Sen, "Tachyon condensation on the brane antibrane system," J. High Energy Phys., 8: 012, 1998. 327 3 A. Sen, "Tachyon matter," J. High Energy Phys., 7: 065, 2002. 264 4 A. Sen, "Universality of the tachyon potential," J. High Energy Phys., 12: 027, 1999. 235 5 A. Sen, "Descent relations among bosonic D-branes,", Int. J. Mod. Phys., 14(25): 4061-77, 1999. 222 SOURCE: Thomson Scientific Web of Science

The biggest challenge is still the obvious one: trying to understand particle physics as we know it from string theory. We now have this one scenario, known as the "landscape" idea, that gives one framework by which particle physics might be realized in string theory. I don’t find it particularly desirable. I would have liked to get a more unique answer from string theory, but you cannot dictate to nature what it should do—if that’s the answer, then that’s the answer. At the moment, we need a better, more theoretical understanding of this landscape to really see what’s going on.

What is the landscape idea, and where does it get you?

"Landscape" refers to the idea that string theory has many, many different vacua. From the four-dimensional perspective, some are what we call de Sitter spaces, some are anti-de Sitter, and some are Minkowski spaces. If these had been different theories with no way of going from one to the other, then we would be stuck again with that question of which is the vacuum we live in; who chooses the vacuum? However, in string theory they appear as different phases of the same underlying theory. Furthermore, if you have a phase with a large enough cosmological constant, it expands so fast that even as parts of that universe decay into more stable vacua, these parts to not merge with each other and remain separated by the intervening medium of a rapidly expanding phase of the universe. This way you can populate all the vacua in the theory, starting with a single de Sitter universe that has a large enough cosmological constant. What happens is that because the original de Sitter phase expands forever, this process goes on forever and eventually all possible vacua of string theory are populated in some part of the universe or another.

And we just happen to be living in a universe in which the various parameters are suitable for life?

That’s right.

It sounds like the kind of idea that would seriously bother those theorists who believe a viable theory should specify a single universe and particularly the universe we live in.

Yes.

How do you feel about it?

If this is the truth, then we have to accept it. It might not be the truth we prefer, but we still have to accept it. I would also add that it has not been established that this is the truth—there’s a lot to understand about this subject before we conclude anything one way or the other.

Your most influential research is on tachyons. What are tachyons, and how do they fit into string theory?

The name "tachyon" is a misnomer. If we try to quantize a theory with instability, it appears as if the theory has a particle with negative mass squared. That’s technically a tachyon, so that’s where the name comes from. In actuality, it’s just the presence of an instability in the system. It says we’re trying to quantize the theory around a maximum of potential energy, and that gives you the instability in the system.

What makes these tachyons, these instabilities, so important to other string theorists?

One reason is that these instabilities have been around in string theory since the beginning. The very first string theory that was discovered had these instabilities. And it’s important to understand what they really mean. The fact that a potential has a maximum, for instance, doesn’t mean it also has a minimum. So the first question we need to ask in a system with a tachyon is: does the potential have a minimum around which we can quantize the theory? If so, then what kind of properties does this quantum theory have? These are some of the questions one tries to answer in the context of the tachyons that appear in string theory.

More generally, what are you working on at the moment?

I have been trying to understand the relationship between black-hole thermodynamics and string theory. That would give us a better understanding of the structure of black holes. Black holes themselves are described as solutions of a theory of gravity. Because superstring theory is a consistent quantum theory of gravity, we should be able to use it to understand much more about black holes than we do at present.

The Large Hadron Collider at CERN is scheduled to go online in the next year. What would you hope or expect to see from research on the LHC?

The first thing I would want to see is the Higgs particle. That would confirm, at least, that the standard model is correct. Then it would be nice if supersymmetry would be found. Of course, the best thing would be if something entirely new appeared, something that surprises everyone.

Would any of these speak directly to the viability of string theory?

They could. Ultimately, if we’re going to make progress, it’s going to take some kind of mixture between the top-down approach and the bottom-up. We need information from both ends. The LHC will give us more leads from the bottom up.

Have any of the developments in string theory brought us any closer to reality—the universe as we know it—than was the case in 1984?

Possibly. This landscape idea, for example, wouldn’t really give us a satisfactory picture if it didn’t have all string theories unified into one single theory. With the landscape, it’s at least possible to conceive of a universe in which all possible vacuua are realized in one single universe. You can have different parts of the universe in different vacuua, and one of those vacuua could be the universe we live in. This concept alone would not have been possible if all string theories had not been unified into one.

One of the criticisms of string theory is that it makes no testable predictions and so is impossible to refute. This makes it questionable as a scientific endeavor. Is it possible to refute the hypothesis?

The point, of course, is that string theory does have a definite prediction: if you go to the Planck energy, you’ll start seeing strings. The problem is that, with currently available technology, we don’t know how to get to the Planck energy. So what we have to do is see if we can come up with other predictions of the theory that can be testable directly. Perhaps some special type of vacuum will be testable where string theory comes down to the TeV scale. There are some models in which that happens, and if our universe happens to be in one of these, then we might be able to test it, even with the LHC.

So if we’re lucky, then we can discover that the theory is true, but we can’t refute it if we’re not.

Unless we can figure out a way to get to the Planck energy. Otherwise we have to get lucky. And the Planck energy is not something we can achieve in the foreseeable future unless there is a major technological breakthrough.

Okay, so if the theory was simply wrong, how might we ever know?

One way that would happen, obviously, is if somebody else finds a theory that does better at describing particle physics, the universe we live in. Right now, string theory is the only consistent framework we have that incorporates gravity and all the fundamental interactions. With the landscape, at least we have a viable picture of the universe. Another way it could turn out to be wrong is if suddenly there should be some unexpected experimental phenomenon that would lead us to a different concept or which could not be explained by string theory. Although what such a development might be, I wouldn’t know. Certainly not now.

Download this article in an Adobe PDF file.