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#REDIRECT'''Superstring theory''', is an attempt to unify all the [[fundamental force|fundamental forces]] and particles in one theory by modelling them as tiny vibrating "strings". It is currently the most promising theory to replace [[quantum mechanics]], which has been unable to incorporate [[gravity]] into its descriptions of the universe.

The deepest problem at present 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 theory of a force invariably results in infinite (and therefore useless) answers. Physicists have developed techniques to eliminate these infinities that work on for the electromagnetic, strong nuclear and weak nuclear forces. But these techniques do not work when applied to general relativity; thus the development of a quantum theory of gravity must come about by different means than were used for the other fundamental forces.

One fact about the universe is that in order to locate a point in [[physical space]], one must use three numbers, and to locate an event one must add another number which is the time that the event occurred.  Physical theories about the world must take into account this fact.  However, there is nothing to prevent one from creating a physical theory that a point in space must require more than four numbers to locate.  In the case of string theory, there are five mathematically possible superstring theories, and in order to be consistent, string theory requires [[spacetime]] to have either 10, 11 or 26  [[dimension|dimensions]].  In other words to locate a point in space governed by string theory, one would have to specify 10, 11, or 26 independent numbers rather than three.  This is a problem since it seems to contradict reality.

This requirement can be reconciled with the observation that the universe seems to have only three spatial dimensions, by making the universe measured along these other dimensions subatomic in size. Imagine an ant on a flat surface.  The location of the ant on the surface requires two numbers to specify its location.  Now imagine the flat surface curled up really, really tightly.  If you curl the plane tightly enough, the ant will start to think that it is living on a one dimensional line rather than a two dimensional plane.  It is assumed that the extra dimensions that exist are curled up so that we can't see them.

Our minds have a hard time visualizing extra dimensions because it is only the three spatial dimensions we normally live
with.  One way of dealing with this is to not to try to visualize them at all and just think of the extra dimensions as
extra numbers in the equations that describe the way the world works.  In our every day world, we need three numbers to
locate a point in space, but in some other universe, we need 11.

Superstring theory is not the first theory to propose extra spatial dimensions; see [[Kaluza-Klein theory]].

Theoretical physicists were troubled by the existence of five separate superstring theories. This has been solved in the [[Second Superstring Revolution]] by the development of [[M-theory]]; the five superstring theories have been found to be simply different limits of M-theory.

The five consistent superstring theories are:
<B>type I</B>, <B>type IIA</B>, <B>type IIB</B>, <B>Heterotic E8 X E8</B> ( or <B>HE</B>t), and <B>Heterotic SO(32)</B> (or <B>HO</B>t).

* The <B>type II</B> theories have two supersymmetries in the ten-dimensional sense, the rest just one.
* The <b>type I</b> theory is special in that it is based on unoriented open and closed strings.
* The other four are based on oriented closed strings.
* The <B>IIA</B> theory is special because it is non-chiral (parity conserving).
* The other four are chiral (parity violating).

'''Further reading:'''
* ''The Elegant Universe'' - Brian Greene
* The Official String Theory Website - String Theory Basics, online at http://superstringtheory.com/basics/index.html

See also: [[String theory]]