String theory is a theory of the fundamental forces of nature. Since the mid-1980’s, physicists have developed many forms of the theory, including a group of superstring theories. However, the theory is still incomplete.
The key to string theory is its description of elementary particles, objects that are not made up of other objects. According to conventional theories of physics, these objects—which include electrons and quarks—are pointlike. But in string theory, they are tiny strings that can vibrate in various ways. Different patterns of vibration would appear to us as different particles.
A successful string theory would be the first single theory to describe all four of the known fundamental forces: (1) the electromagnetic force that underlies electricity and magnetism; (2) the strong nuclear force that binds together quarks in protons, neutrons, and other objects; (3) the weak nuclear force, responsible for the radioactive decay of atomic nuclei; and (4) gravitation, the attraction between material objects. Physicists have developed successful conventional theories of the four forces, but they have not combined those theories.
The conventional theories of the electromagnetic, strong, and weak forces are quantum theories—that is, they use the principles of the theory of quantum mechanics. According to that theory, particles transmit forces to one another by means of quanta, or “chunks” of energy. (Quanta is the plural of quantum.) For example, a quantum called the photon transmits electric and magnetic forces.
The gravitational theory is the theory of general relativity, developed by the German-born physicist Albert Einstein (see Relativity (General relativity)). The theory of relativity is not a quantum theory. Rather, the theory says, gravitation is an effect of a distortion of space and time by the presence of matter. A successful string theory would combine aspects of general relativity and quantum mechanics.
