Early in the study of string theory, physicists learned that the theory does not make sense if space-time is made up of the three space dimensions plus one time dimension of ordinary experience. String theory also requires extra space dimensions that must be curled up to a very small size to be consistent with our never having seen them. As a string moves about, oscillating as it travels, the geometrical form of the extra dimensions plays a critical role in determining resonant patterns of vibration. Because the patterns of string vibrations appear to us as the masses and charges of the elementary particles, we conclude that these fundamental properties of the universe are determined, in large measure, by the geometrical size and shape of the extra dimensions. That's one of the most far-reaching insights of string theory.
String theory is still a work in progress.
In fact, the equations of string theory seem so complicated that physicists have managed to write down only approximate versions. Edward Witten of the Institute for Advanced Study in Princeton, N.J., urges that string theory already can claim experimental confirmation for its prediction of gravity. One reason that string theory appeals so powerfully to theoretical physicists is that it is the most symmetrical theory ever devised. String theory encompasses these symmetries and more. Supersymmetry relates the two quantum-mechanical categories of elementary particles, so that each of the known fundamental particles must have a superpartner whose spin differs by half a unit.