String theory

In physics, string theory, also called superstring theory, is an attempt to unify all the 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.

There are five string mathematically possible superstring theories. In order to be consistent, string theory requires space to have either 10 or 26 dimensions. This requirement can be reconciled with the observation that the universe seems to have only four spatial dimensions, by making the other dimensions subatomic in size. Superstring theory is not the first theory to propose extra spatial dimensions; see Kaluza-Klein theory.

Theoretical physicists have been troubled by the existence of five separate superstring theories. This seems to be solved by the recent development of M-theory; the five superstring theories have been found to be simply different subsets of M-theory.

Further reading:

• The Elegant Universe - Brian Greene