THE SUPERSTRING THEORY, sometimes referred to as the "theory of everything" since it apparently unifies all four known forces into a single framework, has been criticized on the grounds that it is not testable. Now theorists Dimitri Nanopoulos of Texas A&M University and John Ellis of the CERN laboratory in Geneva, Switzerland assert that their version of the theory is open to experimental verification. In particular, they claim that soon they may be able to calculate the mass of neutrinos, or the lifetime of protons; they may be able to predict the existence of new particles not yet observed, including a new class of non-radiating particles, "cryptons," which, if they exist, might account for some of the "missing mass" in the universe. (Science News, October 13, 1990; Nanopoulos: 409-845-7790.)

SUPERSTRING THEORY has hit some snags. The notion that the universe consists of tiny one-dimensional strings vibrating and interacting to produce particles and forces was an attractive one, because, when combined with several mathematical postulates such as "supersymmetry," it showed promise of combining the laws of quantum mechanics and those of general relativity within a single framework, a feat that would unify all four known forces in physics. However, theorists now believe a more fundamental understanding of the nature of particles and forces--and a better grasp of the mathematical implications of superstrings--will be necessary before a successful theory can be constructed. One troubling aspect of the theory is that it tolerates many solutions, some of which are nonsensical, and others which, although approximating the real world, require the existence of extra dimensions and exotic particles presently unsubstantiated by experiment. Nevertheless, theorists are not abandoning the concept of superstrings because, for example, the theory can directly produce the equations of general relativity and more recently has yielded simplified sets of equations describing black holes. (Science, 12 June 1992)

THE THEORY OF SUPERSTRINGS seeks to account for all four of the known physical forces, including gravity. It holds that space has ten dimensions and that all matter, including the elementary particles recognized by the Standard Model---quarks and leptons---are really no more than tiny strings with a characteristic length of 10**-35 m, a size so small that it has a special name, the Planck length. Investigating matter on that level requires a powerful microscope, one in which the probe particles would have an energy of 10**19 GeV (the Planck energy), an energy so far beyond present or foreseeable accelerators as to preclude all thought of direct experimentation. Indeed, Texas physicist Steven Weinberg believes that the "intellectual investment now being made in string theory without the slightest encouragement from experiment is unprecedented in the history of science." (Scientific American, Oct. 1994.) Although it has stood up well to experimental tests, the Standard Model remains unsatisfactory as a "theory of everything," since for one thing is leaves out gravity and, for another, it requires the use of 19 different input parameters whose values must be derived from measurements. Superstring theory, if it could ever be made to work, would surmount these problems; it would include gravity and have no input parameters. But that the task has proven difficult. According to Lance Dixon of SLAC, the superstring framework is not so much a theory as it is a theory of theories. Dixon believes that some physicists are going back to the drawing board of general principles and giving up their work on specific string models. (Beamline, Summer 1994.)

IS SPACE DISCONTINUOUS? Several theories attempt to combine quantum mechanics, which reigns supreme in the microcosmic world of atoms, and gravity, which governs the macrocosmos of planets and galaxies. One quantum gravity theory, the superstring model, holds that all substance in the universe consists of the ceaseless quantum interactions of tiny strings. In this theory nothing is said of the background space in which the strings move. In another theory, one introduced by Oxford scientist Roger Penrose, space itself is quantized into discrete volumes (each consisting of a spinning loop) with a characteristic size about the same as that for superstrings, 10**-35 m. Although these theories do not make predictions that will be tested in the lab anytime soon (the energy needed to explore so small a size domain is greater than accelerators can muster) theoretical progress continues. For example, Lee Smolin of Penn State and Carlo Rovelli of the University of Pittsburgh have extended the work of Penrose and others. Without making any assumptions about the nature of space, they discover that space is indeed lumpy. Furthermore, they are able to calculate the range, or "spectrum," of allowable lump sizes. In other words, just as quantum mechanics obliges atoms to exist in only certain energy states, so the combination of quantum mechanics and gravity (at least in this particular theory) results in space itself being quantized. (Nuclear Physics B, 29 May 1995.)