Looking for the next breakthrough

Print edition : January 20, 2001

The Strings 2001 conference held in Mumbai was dominated by the themes and directions that had been initiated over the last four or five years, considered the period of "second superstring revolution".

"WILL we hear the announcement of a new breakthrough at this meeting?" was the question that most string theorists asked, as they gathered from all over the world for the Strings 2001 conference held at the Tata Institute of Fundamental Research, Mumbai, from January 5 to 10. Progress in string theory, with its ambitious programme of unifying all the fundamental forces of nature, including gravity, has sometimes been made with almost manic intensity. But the enormously high goals that string theory has set for itself, make even these leaps inadequate in the context of the problems that still need to be resolved. Thus, as ideas run their course, exhausting the results that could be gleaned from the fresh insights and leaving behind the inevitable pile o f research publications in their wake, progress slows down. It is then time to look for new directions as the world-wide community of string theorists restlessly searches for the next breakthrough.

General relativist and cosmologist Stephen Hawking in Mumbai.-SHERWIN CRASTO/AP

In the event, Strings 2001 in Mumbai was not to be the setting for the announcement of any such dramatic advance. That, of course, the organisers of the conference could not arrange. It was perhaps the only lacuna, if indeed it can be called that, of an otherwise near-flawlessly organised conference, whose co-ordinators were Spenta Wadia, Sunil Mukhi and Atish Dabholkar, all leading string theorists from the TIFR. Backed by the infrastructure facilities of their institute, which are impressive even by i nternational standards, and run by a remarkably efficient programme committee (comprising all string theorists working at the TIFR) and secretariat, together with the extra touch of hospitality that international conferences held in India invariably have , the conference was universally acknowledged by participants to be one of the best organised in the series.

The presence of Stephen Hawking as one of the participants also generated unprecedented media interest in the conference, an interest that otherwise would most certainly not have been of this scale. India proved to be in no way immune to the magic of Haw king's name but the media hype provided as a corollary an unprecedented level of media coverage of this branch of science, which is virtually unknown to the Indian reading public. It also served to draw attention to the fact that India has a small but th riving and active research community in this new and exciting frontier area of fundamental science, whose contributions, some of which have been pioneering in character, are respected and well-regarded on the global scene.

The conference itself was dominated by the themes and directions that had been initiated over the last four or five years, the period of what string theorists refer to, somewhat grandiosely, as the "second superstring revolution". One of the key ingredie nts in the making of this "revolution" was the discovery that, just as in a wide variety of other physical systems, string theory also admits collective excitations known as solitons.

Solitons are ubiquitous objects in physics. These are excitations of a physical system that do not dissipate away their energy and relapse to the ground state of the system. The typical excitation of a physical system is like a ripple in water, that even tually dies down. Solitons on the contrary do not die away and indeed the first examples of solitons were waves in water that lasted for a far longer time than the typical ripple. Solitons in fundamental theories point to the existence of objects that ha ve almost the same status as the fundamental entities of the theory. In string theory in particular, this led to the appearance of not only strings, but also point-like particles, two-dimensional surfaces, and in fact objects of every possible dimension up to the maximal number of ten! Indeed it is possible that in the final formulation of string theory, whose basic organising principle remains as yet unknown, the geometric notion of a one-dimensional object, namely the string, might itself disappear, r e-appearing only for some limiting values of the parameters of the theory.

These solitons, particularly those that are referred to by string theorists as D-branes, have played a fundamental role in the realisation that the five string theories which were shown to exist in ten-dimensional space-time were actually intimately rela ted to one another. The study of these solitons continues to remain centre stage in string theory, as was evident at the conference. One surprise from these studies was the possibility that D-branes provided the first clues as to how the fundamental noti ons of space and time might be modified for energies high enough where the quantum nature of gravitational forces is too important to be ignored.

Professor Edward Witten (left), the leading figure of the string theory revolution, with Professors Michael Green (centre) and John Schwarz, whose remarkable work in 1984 brought String Theory to the centre stage of modern theoretical high-energy phys ics.-VIVEK BENDRE

One underlying assumption of the usual notions of space and time is that the coordinates of space and time could be multiplied in the same fashion that we normally multiply numbers in ordinary arithmetic. It has long been speculated that indeed in the qu antum domain this assumption might break down, and that space and time would become, as the mathematicians would say, non-commutative (ordinary multiplication is commutative). D-branes provided the first clues that this might indeed be true and the study of how physical systems, particularly those similar to the standard model of particle interactions, might behave on such non-commutative space-times has been a major theme in string theory over the last couple of years.

String theory has fostered a qualitatively new level of interaction between theoretical physics and mathematics. The study of novel properties of space and time was obviously no exception and the work of string theorists in this direction has drawn heavi ly on ideas from mathematics, particularly from the work of the French mathematician, Alain Connes.

BUT while in the short term progress continues along established lines of research, what does the future hold for string theorists? String theory has had spectacular success in providing a partial answer to the question of whether the entropy of black ho les might be explained just as the entropy of ordinary hot objects can be explained as a measure of the number of internal states of the system. This has come from string theory's ability to count the internal states of at least a class of black holes by regarding them as an assembly of D-branes.

Could string theory repeat this spectacular success in some other area? The summary talk at the conference delivered by John Schwarz, Professor at the California Institute of Technology in the United States, and together with Michael Green, one of the ar chitects of the dramatic increase in the level of attention grabbed by string theory in the realm of theoretical physics in 1984, was cautious in tone and made no predictions.

But perhaps string theorists are seeking to repeat their success with black holes in the arena of cosmology. Recent astronomical data suggesting that the universe has a small amount of vacuum energy, raises several problems. Perhaps string theory could s hed light on this issue, assuming that the experimental findings do not change. This was the theme of some of the speeches, notably that of Edward Witten, Professor at the Institute of Advanced Study, Princeton, and in many ways the leading figure of the string theory revolution.

Stephen Hawking's speech at Strings 2001 was on a closely related issue. At this conference, Hawking continued his critical engagement with string theory. From being a sharp critic in earlier years, Hawking has moved towards partial acceptance of the fac t that string theory has made important contributions to the understanding of black holes.

String theorists clearly would love to have him on their side, but it is unlikely that they would be deeply persuaded by his current criticisms of string theory.

Hawking maintained an active presence at the conference, wheeling into conference sessions or continuing scientific dialogues with other participants. To younger physicists enquiring cautiously when Hawking would be free for a discussion, the ready answe r of his assistant was always,"Any time. He is here for work." The media hype was cheerfully ignored by most of the scientists present, with veterans of other conferences where Hawking was in attendance relating amusing stories about media coverage on th ose occasions.

Hawking is treated like any other well-known scientist in the discipline. But there is also an underlying current of admiration for the courage of this remarkable man, whose pursuit of his scientific work continues undeterred by his ailment and consequen t disabilities. And this admiration is expressed in a manner that is wholly characteristic of science. His ideas are subject to the same unsparing criticism that is accorded to anyone else's ideas. Hawking himself, one is sure, would expect no less.

Dr. T. Jayaraman is a theoretical physicist at the Institute of Mathematical Sciences, Chennai.

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