String theory, also called superstring theory, is, generally speaking, any physico-mathematical framework that describes fundamental physical reality in terms of superstrings Strings in this context should not be confused with cosmic strings, which are one-dimensional (string-like) regions of cosmic extent containing vacuum energy different from that of the true vacuum. The superstrings of string theory, in contrast, are extremely tiny loops, or possibly segments, that have been suggested as the most fundamental of all physical entities, and as the source of all other fields and particles.

Before the 1980s, the most fundamental entities were most often considered to be particles, which are zero-dimensional objects, but it has become clear that particle models do not provide a rich and flexible enough basis for fundamental quantum field theories; strings are much more suitable. More specifically, string theory provides promising candidates for an adequate quantum theory of gravity and, at the same time, for theories of the total unification of all four fundamental physical interactions (gravity, electromagnetism, and the strong and weak nuclear forces. Grand Unified Theory (GUT) will provide unification of the three nongravitational interactions.

Quantum mechanics (along with its extension to quantum field theory) and Albert Einstein's (1879–1955) theory of gravitation are two important pillars of contemporary physics. And yet, as they are presently formulated, they are deeply incompatible with one another. As of 2002, constructing a complete and adequate quantum theory of gravity has evaded the best efforts of theoreticians. Exciting and surprising work on superstrings since about 1984, however, has moved science much closer to achieving quantization of the gravitational field, thus resolving and healing this incompatibility. It is already clear that the leading string theory candidates yield general relativity as their low-energy limit. Essentially, this means that string theory, if successful, will become not only the quantum theory of gravity, but also the quantum theory of space and time, with crucial applications to early-universe cosmology.

It also appears likely that some version of string theory will at the same time unify all four fundamental physical interactions, including gravity, thus bringing to successful completion the much heralded quest for unification that motivated the physicist James Clark Maxwell (1831–1879), Einstein, and so many others. In order to accomplish this unification, the strings must manifest supersymmetry—they must be superstrings. Consider that all fundamental particles have either half-integral spin (½, , . . . ) or integral spin (0, 1, 2, . . . ). The half-integral spin particles are called fermions, and constitute the building blocks of matter; protons, neutrons, electrons, and quarks are all fermions. The integral spin particles are called bosons, and are the force-carriers between the fermions, mediating the electromagnetic, gravitational, and strong and weak interactions. Photons, W massive bosons, Z massive bosons, gluons, and gravitons are the bosons that mediate the electromagnetic, weak, strong, and gravitational interactions, respectively.

Fermions and bosons satisfy different statistics and symmetries, and have to be treated differently in standard quantum field theory. The first seriously considered string theories—studied for purposes other than those for which newer superstring theories are studied—were bosonic strings, which only incorporated the symmetries and statistics of bosons. Obviously, if a theory is going to unify all particles and fields, it will have to incorporate the symmetries of both fermions and bosons within the same framework; it will have be supersymmetric, and the strings will therefore have be superstrings.

Where would the superstring description of reality be needed? Certainly, it would provide a detailed and physically complete explanation of all the characteristics and parameters of material reality, including their deep interconnections and their origins in the vibrations and interactions of the fundamental superstrings. It would, at the same time, provide an adequate description of material reality at temperatures higher than $1032 K, where the general relativistic description of space, time, and mass-energy breaks down. There was a time in the very early universe, immediately after the Big Bang, when those temperatures obtained and during which the physics of the universe was that of a single unified fully quantitized superforce. This era is referred to as the Planck era, after the German theoretical physicist Max Planck (1858–1947). In fact, it is only in such terms that the Big Bang itself, as well as the emergence or origin of space, time, and matter, can really be characterized.

Superstring theories resolve a number of difficult anomalies and divergences in quantum theory. But they also lead to some features that are, at first sight, puzzling. One of these is that they almost always require higher dimensions—for example ten or twenty-six—rather than the three spatial dimensions and one time dimension that characterize the low-energy world. How then can these superstring theories be reconciled with reality as we know it? The answer is straightforward but surprising. At very high energies or temperatures, such as immediately after the Big Bang, reality will be ten dimensional or twenty-six dimensional, as described by superstring theory. But, as the universe exits the Planck era, and enters the classical domain where gravity is adequately described by Einstein's general relativity and is no longer unified with the other interactions, the extra dimensions compactify (curl up into infinitesimal knots) leaving only the four-dimensional spacetime with which we are familiar. Of course, if this is true, scientists should find some evidence of these extra curled-up dimensions. Such relics of the supersymmetric past would constitute powerful confirmation of superstring theories. This is an active area of research.

This answer is a complete copy from
http://www.enotes.com/science-religion-encyclopedia/string-theory
and
http://en.wikipedia.org/wiki/Graviton

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