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'Much of string theory is yet to be understood and discovered'

Print edition : Feb 03, 2001 T+T-
Interview with Prof. David Gross.

Professor David Gross is one of the leaders of a generation of theoretical high-energy physicists 'who had the good fortune', as he put it, to contribute to the development of the standard model of particle interactions. David Gross, and his then student Frank Wilczek, made a seminal contribution to the development of the theory of strong nuclear interactions when they showed in 1973 that qantum chromodynamics (or QCD) as the gauge theory of strong nuclear interactions is known, predicts that the se interactions actually become weak at high energies, a phenomenon referred to by physicists as asymptotic freedom.

This has enabled remarkable experimental confirmation of QCD and its eventual acceptance as the theory of strong nuclear interactions. Gross, one of the leading proponents of string theory, was a co-discoverer of the so-called hetorotic string in 1985, a n important early step in the "first superstring revolution" of the second half of the 1980s.

Currently Director of the Institute of Theoretical Physics, Santa Barbara, in the United States, after having been for several years Professor at the Department of Physics of Princeton University, he is widely regarded in the world of physics as a leadin g candidate for the award of the Nobel Prize in Physics for his joint discovery of asymptotic freedom. Prof. Gross spoke to T. Jayaraman on a wide variety of issues ranging from the current status of string theory to the economic importance of ba sic science research. Excerpts:

At the press conference (in Mumbai) you spoke of string theory being in the middle of an ongoing revolution. And that the best is yet to come. What did you mean by that?

We have been exploring string theory now for three decades. We are stumbling in the dark to a large extent, exploring those avenues of string theory that we can get to with the tools that we have. I personally think that the story won't be over, we will not have completed our understanding of the theory and we will not be able to control it enough to really confront it with reality to make predictions until we make some further breaks with the way we conceptualise nature. So I think there are many revol utionary developments that are still ahead of us. The steps that have been taken so far have been very dramatic, but perhaps even more dramatic things remain.

If you say that in some respects we are still groping for the real structure, what makes string theory so compelling in your opinion?

It's not that we're on a treadmill. We're actually making progress continuously and sometimes the progress is very fast and very dramatic. At other times it is more normal, but progress is always being made. We learn an awful lot every year. In some year s in fact the field is totally changed and our understanding of it.

For instance, about five years ago the whole development of so-called dualities showed that all the different forms of string theory that were thought to be separate theories were actually just different parts of the same theory. There were precise relat ionships or dualities between one kind of string theory and another kind of string theory. And those relationships allowed one to solve easily what appeared to be very hard questions in one formulation of the theory by translating them into easy question s in another formulation of the theory. And that development was quite revolutionary to begin with and has given us enormous power to answer lots of interesting and very important questions, some quite profound. For example, having to do with the propert ies of black holes. And there are many other questions of that type that we would love to answer, such as for example the nature of cosmological singularities.

There again it seems to me very likely that string theory will provide an answer to the paradoxes that occur when we extrapolate general relativity back to the earliest times in the history of the universe and encounter singularities and physics breaks d own.

Then of course there are the absolutely crucial problems of understanding what in string theory picks out the world that we see around us. If string theory is to be successful as a description of nature, we have to understand the dynamics that produces i n principle the world at large distances and low energies that we observe. And so far we don't know how to do that either, and because of that we are prevented from doing the kind of thing that physicists love to do, which is to make predictions about th e real world or to calculate properties of the real world that can be measured or have already been measured but not explained.

So there are many good things ahead to do, plenty of good problems. In fact, young people coming into the field should be delighted that we're still confused, and that we're still halfway towards the end because it means that 'they' can make important co ntributions. Much of string theory is yet to be understood and discovered. It is a wide open field for bright, young people.

What are the prospects for the experimental verification of string theory, or the prospects for just further experimental evidence that would tell us that we are headed in the right direction, even if not necessarily a direct test or confirmation of t he theory?

The most likely direct experimental evidence that will help support string theory and encourage it is of course the discovery of supersymmetry which would be one of the great discoveries in physics of all time if it... when it happens.

Supersymmetry is not necessarily a consequence of string theory, but it is a necessary part of string theory. One could imagine having supersymmetry without string theory. On the other hand, its discovery would give an enormous encouragement to string th eory which relies so heavily on supersymmetry.

It would, as we understand it, solve many problems of particle physics quite directly and it would open a new world of experimental discovery that would reveal how this fundamental symmetry of nature is broken and is not manifest around us. That will be extremely useful for people who want to connect string theory to the real world.

One of the major obstacles to doing so is our lack of understanding of how that supersymmetry is broken in string theory and it would always help to have hints from nature. And the discovery of supersymmetry many people, not just string theorists, believ e is very likely at the next generation of accelerators, at the LHC (Large Hadron Collider) which would be completed in five years at CERN (the French acronym for the European Centre for Nuclear Research in Geneva), or even at Fermilab, in the U.S., whic h is going to run with much higher beam intensity in March.

There are other extraordinarily exciting speculations by people in the last few years based directly on string theory which have shown us that our previous ideas were a little too conservative.

It turns out that with some of these speculations, that the extra dimensions of space that occur in string theory are actually much bigger than we previously imagined.

If you really push these speculations, they seem to be consistent with the possibility that at the next round of accelerators like the LHC as we go to the next energy scale we could actually get to the string scale, actually see stringy excitations. We c ould actually produce black holes in the laboratory by colliding protons and protons, and see very dramatic signatures of quantum gravity and string theory in particle physics laboratories.

Now while these speculations as far as we know are consistent with string theories and suggested by certain ideas in string theory, there is no reason why the energy scale should be as low as what we are just beginning to explore. But it is a possibility . And then one would be in the wonderful situation of having direct experimental study of stringy states and quantum gravity in the laboratory.

In this context, how do you assess the current state of health of funding for high-energy physics, especially experimental work, in the world today?

It's not bad. Europe of course continued in its own plans to build a hadron collider and is now starting construction of the LHC. The U.S. has joined, as has India, in that effort, so it's become much more of an international machine and it will be built . At the same time, the support of the U.S. for high-energy physics has continued and is strong and Fermilab has just upgraded its accelerator which is still the highest energy accelerator in the world. At the moment the situation I think is very healthy and in the next 10 years we have a lot to expect.

The harder question has to do with the future. The exploration of fundamental physics at shorter and shorter distances at higher and higher energies is becoming more and more expensive. This was one of the reasons the SSC (Superconducting Super Collider) was killed.

And planning to go beyond present or under construction accelerators is very difficult. The time scale required for planning and constructing an experimental facility like this is now measured in decades. The cost is measured in billions. The number of p eople working on an experiment is now in their thousands. In other words, it is becoming not just "big science" but "immense science". And it gets harder and harder to mobilise the communities within a given nation and then the nations to engage in such collaborations. At the moment various high-energy physics communities throughout the world are struggling with the idea of constructing a very high-energy electron-positron linear collider which would be the natural next step and complement the LHC. I ha ve lots of hope that this will come through but it's not easy.

There has been the impression that maybe the enthusiasm for fundamental science is going down especially among the political leaderships, particularly in the U.S. and in western Europe. Do you think that is true? Is there a question mark over the futu re of fundamental science?

If there was such a perception a while back, I think it has changed. Science is regarded, as you know very well, by all politicians as important for economic development, though of course what they normally mean by science is technology. And so in the U. S. especially there is enormous investment in technology from the Federal government and from the State governments. But there is also the understanding that you cannot have a healthy, technologically innovative society in which basic science is impoveri shed.

And for many reasons. Young people are still attracted to science because of their scientific curiosity and their desire to understand how the world works. And of course fundamental science feeds new technologies either at a very rapid pace, as in some a reas of science such as condensed matter physics, or as in other fundamental sciences at a slower pace. But if you cut off the head, if you destroy basic science and focus only on technology and applications, the one thing that you absolutely ensure is t hat your technology will be old technology.

In the U.S. in the last few years, partly because the economy was so vibrant, there was so much money at the Federal level, the budgets for basic science has increased steadily. The NSF (National Science Foundation) just this year got a 20 per cent incre ase in its overall budget as part of a plan and there is a consensus in Washington that the NSF budget will double in five years. The technological strength of a modern country is in the end based on scientific power. And that goes for the most abstract aspect of science as well. If you destroy those, it has a demoralising effect, all the more in applied areas of science both for young people and for the community as a whole. So basic science which is ultimately very cheap compared to what governments s pend their money on I think is much more valued now than it was a decade ago.

You have been here in India before and you know many Indian physicists. Your impressions about physics in India?

I was last here ten years ago. Coming back, Mumbai looks like a different city. It's quite amazing. As far as physics goes, the changes are also remarkable. At the ITP I see many scientists from abroad who attend our programmes every year, and so I have a good feeling for the level of Indian physics. In condensed matter physics, in astrophysics and in high-energy physics Indian physics is very strong and we always have excellent participants from India attending our programmes. I think in string theory, which I know best, the group at the Tata Institute of Fundamental Research (TIFR) has a lot to be proud of. It is an extremely strong group, and it has produced a lot of good students who have gone abroad. Some have come back, have moved out to other in stitutions in India and have made India one of the important centres in this exciting field. And continue to make important contributions to the field.

So in those areas in particular that I am aware of, in astrophysics, in condensed matter physics and high-energy physics, especially string theory, India is comparable to any country in the world. Much better than a lot of very highly advanced European c ountries that I could think of. Certainly not a Third World country as far as science goes in any way and I think that is a big change from the way I used to think of India scientifically 20 years ago.