n. (Physics) A mathematical theory for describing the properties of fundamental particles, which represents the particles as one-dimensional string-like objects, which exist in the normal four dimensions of space-time plus additional dimensions, the total dimensions being ten, eleven, or twenty-six depending on the version of the theory. The properties of fundamental particles in string theory and their manner of interaction with each other depend upon the modes of vibration of the strings{ 17 }. The attractiveness of this theory rests in part on its ability to provide a unified treatment of gravity as well as the three other basic forces of nature, in a manner consistent with quantum mechanics. The great difficulty of doing the calculations required by the theory, however, has thus far (1999) made it impossible to calculate the observable properties, such as the mass, of known particles, such as the electron, proton, mesons, quarks, and neutron; thus there is as yet no experimental verification for the theory. The most popular version of the theory depends on a mathematical property called supersymmetry, and the theory derived form this principle is properly called superstring theory, a term which is often used interchangeably with string theory. See also string{ 17 }. [ PJC ]