Superstring theory is a type of theory of particle physics that treats elementary particles as extended one-dimensional "string-like" objects rather than as the dimensionless points in space-time used in other theories. Superstring theories became popular during the 1980s when Michael Green of Queen Mary College, London, and John Schwarz of the California Institute of Technology showed that certain types of such theories might provide a fully self-consistent quantum theory that describes gravity as well as the weak, strong, and electromagnetic forces. The development of such a unified quantum theory is a major goal in theoretical particle physics, but usually the inclusion of gravity has led to intractable problems with infinite quantities in the calculations.
The basic entities in superstring theory are one-dimensional massless strings only 10-33 cm long. (This distance is the so-called Planck length, at which quantum effects in gravity can no longer be ignored.) The strings vibrate, and each different mode of vibration corresponds to a different particle. The strings can also interact in ways that correspond to the observed interactions of particles.
String theories of elementary particles were first introduced in the early 1970s in attempts to describe the strong force. Although quantum chromodynamics was soon accepted as the correct theory of the strong force, string theory was given new life with the incorporation of supersymmetry, the symmetry between fermions (particles with half-integral values of spin) and bosons (particles with integral values of spin). Not only did the resulting superstring theory successfully encompass all the fundamental forces, but only a limited type of the many possible versions seemed to be properly self-consistent, making it a leading candidate for a fully unified theory of particles and forces.
At first sight this particular type of superstring theory might seem to have an insurmountable drawback in that it deals with a space-time of ten dimensions instead of the three dimensions of space and one of time that are perceived in the everyday world. It seems, however, that six of the ten dimensions may be "compactified," or "curled up"--i.e., so small that they are unnoticeable. Other problems remain, however. The theory is still a long way from explaining the masses of the known particles. It also implies the existence of new particles in a form of "shadow matter," with which normal matter can interact only through gravity.
Excerpt from the Encyclopedia Britannica without permission.