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A proposal for a unified theory of all interactions, including gravity. At present, the strong, weak, and electromagnetic interactions are accounted for within the framework of the standard model. This model correctly describes experiments up to the highest energies performed so far, and gives a complete description of the elementary particles and their interactions down to distances of the order of 10-18 m. Nevertheless, it has serious limitations, and attempts to overcome them and to unify the forces of nature have been only partly successful. Moreover, these attempts have left standing fundamental difficulties in reconciling gravitation and the laws of quantum mechanics. Superstring theory represents an ambitious program to unify all of the interactions observed in nature, including gravitation, in a theory with no unexplained parameters. In other words, this theory, if successful, should be able to account for all of the particles observed in nature and their interactions. See Elementary particle, Fundamental interactions
In string theory, the fundamental objects are not point particles, as in standard theories of elementary particles, but one-dimensional extended objects, the open and closed strings. In such a theory, what are usually called the elementary particles are simply particular quantum states of the string. In superstring theories, space-time is ten-dimensional (space is nine-dimensional). If such theories are to describe nature, six dimensions must be “curled up” or “compact.” The main consequence of such extra dimensions is the existence of certain very massive particles. See Space-time
The essential features of string theories can be understood by analogy with the strings of a musical instrument. Such strings vibrate at a characteristic frequency, as well as any integer multiple of that frequency. Each of these modes of vibration (so-called normal modes) can be excited by plucking or striking the string. In classical physics, the amplitudes of vibration of each mode can take on a continuum of values. If there were a string of atomic dimensions, subject to the laws of quantum mechanics, the energies of this quantum string could take on only discrete values, corresponding to particular quantum states. See Quantum mechanics, Vibration
The strings of superstring theory are quite similar. The main difference is that they obey Einstein's principles of special relativity. As a result, since each quantum state has a particular energy, it has a definite mass. Thus, each state of the string behaves as a particle of definite mass. Because it is possible, in principle, to pump an arbitrarily large amount of energy into the string, the theory contains an infinity of different types of particles of arbitrarily large mass. The interactions of these particles are governed by the ways in which the strings themselves interact. To be consistent with the principles of relativity, a string can interact only by splitting into two strings or by joining together with another string to form a third string. As a result, the interactions of strings are nearly unique. This geometric picture of string interactions translates into a precise set of rules for calculating the interaction of individual string states, that is, particles. See Relativity
Obtaining a description of superstring theory analogous to quantum field theory is an active topic of research. However, even though the equations that describe this field theory are not completely known at present, it is known how to find classical solutions of these equations, and by various techniques, an enormous number of such solutions have been found. These include states in which space-time has any dimension between one and ten, and states with many bizarre symmetries and spectra. Each of these solutions then corresponds to a possible ground state of the system. The theories built around some of these states look very much like the real world. Not only are four dimensions flat while six are compact, but they possess gauge symmetries close to that of the standard model. Some have three or four generations of quarks and leptons, as well as light Higgs particles, which are of crucial importance in the standard model. Many of these solutions possess space-time supersymmetry. See Higgs boson, Lepton, Quarks
However, if the theory does describe nature, it must have some mechanism that chooses one of the possible ground states. Because the masses and couplings of the elementary particles depend only on the choice of ground state, determining this true ground state will yield a set of predictions for these quantities. If string theory is a correct theory, these predictions must agree with the experimental values.