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At present, the deepest problem in theoretical physics is harmonizing the theory of general relativity, which describes gravitation and applies to large-scale structures (stars, planets, galaxies), with quantum mechanics which describes the other three fundamental forces acting on the microscopic 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 the electromagnetic, strong nuclear and weak nuclear forces, but not gravity. Thus the development of a quantum theory of gravity must come about by different means than were used for the other forces.
The basic idea is that the fundamental constituents of reality are strings of the Planck length (about 10−33 cm) which vibrate at resonant frequencies. 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 crunches" 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.
Our physical space is observed to have only four large dimensionAbstract algebra Algebra Linear algebra Dimension (from Latin "measured out") is, in essence, the number of degrees of freedom available for movement in a space. In common usage, the dimensions of an object are the measurements that define its shape and ss, and a physical theory must take this into account. But nothing prevents one from having more than 4 dimensions per se. In the case of string theory, consistencyConsistency has three technical meanings: In mathematics and logic, as well as in theoretical physics, it refers to the proposition that a formal theory or a physical theory contains no contradictions. See consistency proof. In statistics, consistency (st requires spacetimeIn special relativity and general relativity, time and three-dimensional space are treated together as a single four-dimensional manifold called spacetime . A point in spacetime may be referred to as an event . Each event has four coordinates t x y z ; or to have 10, 11 or 26 dimensions. The conflict between observation and theory is resolved by making the unobserved dimensions compact dimensionIn string theory, a model used in theoretical physics, a compact dimension is curled up in itself and very small ( Planck length). Anything moving along this dimension's direction would return to its starting point almost instantaneously, and the fact thas.
Our minds have a hard time visualizing higher dimensions because we can only move in three spatial dimensions. And even then, we only see in 2+1 dimensions; vision in 3 dimensions would allow one to see all sides of an object simultaneously. One way of dealing with this limitation is to not try to visualize higher dimensions at all but to just 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 (that must show, ultimately, different results in 1, 2, or 2+1 dimensions to a human scientist). This, in turn, opens the question whether models that rely on such abstract modelling (and potentially impossibly huge experimental apparatus) can be considered 'scientific'.
Superstring theory is not the first theory to propose extra spatial dimensions; see Kaluza-Klein theoryKaluza-Klein theory (or KK theory for short) is a model which sought to unify classical gravity and electromagnetism. It was discovered by the mathematician Theodor Kaluza that if general relativity is extended to a five-dimensional spacetime, the equatio. 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.
See also: Why does consistency require 10 dimensionsIn our personal human experiences, we seem to exist in a universe with three spatial dimensions. Some theories in physics, including string theory, include the idea that there are additional spatial dimensions. Such theories suggest that there may be a sp?