Number 200 (Story #2), October 26, 1994 by Phillip F. Schewe and Ben Stein 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.)