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Superstring theory is an attempt to explain all of the Particles and Fundamental Force s of nature in one theory by modeling them as vibrations of tiny Supersymmetric strings. It is considered one of the most promising candidate theories of Quantum Gravity . Superstring theory is a shorthand for '''supersymmetric string theory''' because unlike Bosonic String Theory , it is the version of String Theory that incorporates Fermions and Supersymmetry . BACKGROUND The deepest problem in Theoretical Physics is harmonizing the theory of General Relativity , which describes gravitation and applies to large-scale structures (stars, galaxies, super clusters), with Quantum Mechanics , which describes the other three Fundamental Forces acting on the atomic scale. The development of a Quantum Field Theory of a force invariably results in infinite (and therefore useless) probabilities. Physicists have developed mathematical techniques ( Renormalization ) to eliminate these infinities which work for three of the four fundamental forces - electromagnetic, strong nuclear and weak nuclear forces - but not for gravity. The development of a quantum theory of gravity must therefore come about by different means than those used for the other forces. The basic idea is that the fundamental constituents of reality are strings of the Planck Length (about 10−35 m) which vibrate at Resonant frequencies. Every string in theory has a unique resonance, or harmonic. Different harmonics determine different fundamental forces. The tension in a string is on the order of the Planck Force (1044 Newton s). The Graviton (the proposed messenger particle of the gravitational force), for example, is predicted by the theory to be a string with wave amplitude zero. Another key insight provided by the theory is that no measurable differences can be detected between strings that wrap around dimensions smaller than themselves and those that move along larger dimensions (i.e., effects in a dimension of size R equal those whose size is 1/R). Singularities are avoided because the observed consequences of " Big Crunch es" never reach zero size. In fact, should the universe begin a "big crunch" sort of process, string theory dictates that the universe could never be smaller than the size of a string, at which point it would actually begin expanding. EXTRA DIMENSIONS See also: Why does consistency require 10 Dimensions ? Our Physical Space is observed to have only three large Dimension s — and taken together with time as the fourth dimension — a physical theory must take this into account. However, nothing prevents a theory from including more than 4 dimensions, per se. In the case of String Theory , Consistency requires Spacetime to have 10, 11 or 26 dimensions. The conflict between observation and theory is resolved by making the unobserved dimensions Compactified . Our minds have difficulty visualizing higher dimensions because we can only move in three spatial dimensions. One way of dealing with this limitation is not to try to visualize higher dimensions at all, but just to think of them as extra numbers in the equations that describe the way the world works. This opens the question of whether these 'extra numbers' can be investigated directly in any experiment (which must show different results in 1, 2, or 2+1 dimensions to a human scientist). This, in turn, raises the question of whether models that rely on such abstract modeling (and potentially impossibly huge experimental apparatus) can be considered 'scientific.' Six-dimensional Calabi-Yau shapes can account for the additional dimensions required by superstring theory. Superstring theory is not the first theory to propose extra spatial dimensions; see Kaluza-Klein Theory . Modern string theory relies on the mathematics of folds, knots, and Topology , which was largely developed after Kaluza and Klein, and has made physical theories relying on extra dimensions much more credible. NUMBER OF SUPERSTRING THEORIES Theoretical physicists were troubled by the existence of five separate superstring theories. This has been solved by the . The five consistent superstring theories are:
Chiral Gauge Theories can be inconsistent due to Anomalies . This happens when certain one-loop Feynman Diagram s cause a quantum mechanical breakdown of the gauge symmetry. Having anomalies cancel puts a severe constraint on possible superstring theories. INTEGRATING GENERAL RELATIVITY AND QUANTUM MECHANICS General Relativity typically deals with situations involving large mass objects in fairly large regions of Spacetime whereas Quantum Mechanics is generally reserved for scenarios at the atomic scale (small spacetime regions). The two are very rarely used together, and the most common case in which they are combined is in the study of Black Hole s. Having "peak density", or the maximum amount of matter possible in a space, and very small area, the two must be used in synchrony in order to predict conditions in such places; yet, when used together, the equations fall apart, spitting out impossible answers, such as imaginary distances and less than one dimension. The major problem with their congruence is that, at sub-Planck (an extremely small unit of length) lengths, general relativity predicts a smooth, flowing surface, while quantum mechanics predicts a random, warped surface, neither of which are anywhere near compatible. Superstring theory resolves this issue, replacing the classical idea of point particles with loops. These loops have an average diameter of the Planck length, with extremely small variances, which completely ignores the quantum mechanical predictions of sub-Planck length dimensional warping, there being no matter that is of sub-Planck length. SEE ALSO
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