The study of string theory has helped to advance the frontiers of mathematics and theoretical physics. One of its most distinguished proponents examines the 'unifying' road ahead.

Strings Strung

John Schwarz

We think of particle physics addressing the microscopic extreme and cosmology addressing the macroscopic extreme. However, string theory, through its explorations of the behaviour of gravity in both scales, allows us to consider particle physics and cosmology together. In fact, string theory offers deep connections between the two.
Proponents of string theory have contended that the theory requires extra spatial dimensions, which have generally been assumed to form a compact space of six or seven dimensions. Recent investigations, however, suggest that the extra dimensions in which the strings exist could be much larger than previously envisioned--perhaps large enough to be experimentally observable.
I find the idea of large extra dimensions to be implausible because it sacrifices some of the successes of supersymmetric grand unification, such as the unification of the couplings and suppression of proton decay. Nevertheless, I agree that it is worthwhile to explore these ideas. Who knows what might turn up?
I do not expect string theory to be completely understood by pure thought alone. The more traditional back and forth between theory and experiment surely will be important in this quest. The most immediate experimental question is whether supersymmetry particles will be discovered this decade. Such a discovery would have profound theoretical and experimental consequences, helping to set the agenda for experimental particle physics for several decades to come.
The discovery of supersymmetry would make it clear that the abstract mathematical musings of the past 30 years can be connected to experimental science. Experimental findings about supersymmetry at the electroweak scale, in turn, would provide crucial guidance in the quest to understand how to connect the underlying theory to the real world.
Through the study of string theory, the last third of the 20th century witnessed the construction of an amazing mathematical edifice that we are still struggling to understand. I am convinced that the theory is unique, and I am optimistic that a deeper formulation will be found. The theory could well lay the intellectual foundation for uncovering solutions accounting for the observations both of particle physics and cosmology.
The subject of string theory has involved a host of new and bizarre concepts, much as quantum mechanics did in the first half of the 20th century. To get it right we will need help from both our mathematical and experimental friends. But that should come as no surprise. In fact, it is only fitting that a theory that seeks to unify particles and forces at extreme micro- and macro-scales may wind up unifying disciplines as well.

John Schwarz
ICTP Dirac Medallist
California Institute of Technology, Pasadena, California, USA

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