Research Interests

The goal of elementary particle physics is to achieve a unified understanding of fundamental forces (gravitational, electromagnetic and nuclear) and elementary particles in terms of concise and beautiful mathematical principles. This program was pioneered by Einstein, who lacked sufficient experimental information to achieve a unified field theory. Building on the lessons of the last 25 years we may now be on the verge of realizing Einstein's dream. Curiously, superstring theory, the prime candidate for the unified theory, was initially developed to solve a completely different problem.

The strong nuclear force that binds quarks together inside protons, neutrons and other hadrons was not yet understood in the 1960s. During that decade, theorists faced the challenge of finding a simple explanation for the wealth of data that the experimentalists were producing with their large accelerators. I was a student in Berkeley, where Professors Geoffrey Chew, Stanley Mandelstam and others were developing ideas such as the "bootstrap hypothesis" and "Regge pole theory". These approaches were not fully successful, but by a remarkable sequence of events they led to superstring theory.

Around 1970 (when I was a postdoc at Princeton), Gabriele Veneziano, Yoichiro Nambu and others developed the "dual resonance model", later interpreted as the theory of a relativistic string. This model incorporates the bootstrap and Regge ideas in a specific mathematical framework and thus was able to describe many qualitative features of hadron physics. In 1971 a second (somewhat better) dual model was discovered by Pierre Ramond, André Neveu and me. Both models shared certain defects, however: They required more than four dimensions for space-time and predicted the existence of massless particles, which do not exist in the hadron spectrum. Various tricks were developed for dealing with the extra dimensions, but no amount of cleverness could get rid of the massless particles.

The final nail was driven into the coffin of string theory in 1973-74, when "quantum chromodynamics" (QCD) emerged as a theory of the strong nuclear force. Its successes were immediate and convincing. String Theory, a very active area of research for almost five years, dried up practically overnight.

In 1972 I moved to Caltech, and in 1974 I arranged a visit by Joël Scherk, a French physicist with whom I had worked earlier in Princeton. We felt strongly that string theory was too beautiful a mathematical structure to be completely irrelevant to nature. We were convinced of the essential correctness of QCD, but still thought that string theory deserved a last look before being abandoned. Soon we realized that its defects could be turned into virtues if it was used for a completely different purpose than that for which it was originally developed.

Massless particles do occur in nature: The quanta of light (photons) and of gravity (gravitons) are examples. These particles are not hadrons, however. Indeed, all consistent versions of the string theories we knew about contained a massless particle with exactly the properties of a graviton. Its interactions at low energy were shown to agree precisely with Einstein's general theory of relativity. (This result was obtained independently by the Japanese physicist Tamiaka Yoneya.) Also, it was known since the work of Kaluza and Klein in the 1920s that extra dimensions of space can play a useful role in gravitation theories, where the geometry of space-time is dynamical.

Since my training was as an elementary particle physicist, gravity was far from my mind in early 1974. Traditionally, elementary particle physicists ignored the gravitational force, which is entirely negligible under ordinary circumstances. For example, the gravitational attraction between an electron and proton in a hydrogen atom is about 1038 times weaker than the electric attraction. General relativists, physicists who specialize in the study of gravity, generally study the largest things in the universe (even the universe itself) and have traditionally had no use for subnuclear physics. They attended different meetings, read different journals, and (until recently) had no need for serious communication with particle physicists, just as particle physicists (until recently) felt they had no need for galaxies, black holes and the early universe in their quest to understand elementary particles.

For these reasons, even when Scherk and I realized that string theory had mathematical features suggestive of gravity, we were not predisposed at first to interpret it as a physical theory of gravity. Fortunately, after a few weeks of intense deliberations, we were ready to take the plunge.

Thus Scherk and I proposed reinterpreting string theory as a candidate for a unified theory of gravity and the other fundamental forces. This was a radical change in viewpoint that required, among other things, supposing that the size of a string is approximately equal to the Planck length 10-33 cm) in order for the gravitational force to have the correct Newtonian strength. This is 20 orders of magnitude smaller than what was envisioned when strings were being used to describe hadrons, whose typical size is 10-13 cm.

In addition to incorporating gravity in a unified theory there was another bonus. All previous attempts to include gravity in the framework of quantum field theory had led to formulas that were plagued by meaningless infinities. We knew that string theories have a much "softer" short-distance behavior, and we were therefore optimistic that this problem would not occur. Recent studies support this conclusion.

Scherk and I were very excited by the possibility that string theory could be the Holy Grail of unified field theory, overcoming the problems that had stymied other approaches. In addition to publishing our work in scholarly journals we gave numerous lectures at conferences and physics departments all over the world. We even submitted a paper to the 1975 essay competition of the Gravity Research Foundation. For the most part our work was politely received --- as far as I know, no one accused us of being crackpots. Yet, for a decade, almost none of the experts took the proposal seriously. I suspect that very few physicists even remember having heard of the proposal in 1974 or 1975.

In 1980, Michael Green and I began collaborating on the further development of superstring theory. Each year we made discoveries that we felt would convince other physicists of the virtue of string theory. This did not happen until after a discovery we made in the summer of 1984 while working at the Aspen Center for Physics, which showed how certain apparent inconsistencies, called anomalies, could be avoided. The subject suddenly became very fashionable and is now one of the most active areas of research in theoretical physics.