Superstring theory

Superstring theory is an attempt to explain all of the particles and fundamental forces 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.

Number of superstring theories

Theoretical physicists were troubled by the existence of five separate superstring theories. This has been solved by the second superstring revolution in the 1990s during which the five superstring theories were discovered to be different limits of a single underlying theory: M-theory.

Related Topics:
Second superstring revolution - M-theory

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

The five consistent superstring theories are:

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

• The type I string has one supersymmetry in the ten-dimensional sense (16 supercharges). This theory is special in the sense that it is based on unoriented open and closed strings, while the rest are based on oriented closed strings.
• The type II string theories have two supersymmetries in the ten-dimensional sense (32 supercharges). There are actually two kinds of type II strings called type IIA and type IIB. They differ mainly in the fact that the IIA theory is non-chiral (parity conserving) while the IIB theory is chiral (parity violating).
• The heterotic string theories are based on a peculiar hybrid of a type I superstring and a bosonic string. There are two kinds of heterotic strings differing in their ten-dimensional gauge groups: the heterotic E₈×E₈ string and the heterotic SO(32) string. (The name heterotic SO(32) is slightly inaccurate since among the SO(32) Lie groups, string theory singles out a different Spin(32)/Z₂.)

Chiral gauge theories can be inconsistent due to anomalies. This happens when certain one-loop Feynman diagrams cause a quantum mechanical breakdown of the gauge symmetry. Having anomalies cancel puts a severe constraint on possible superstring theories.

Related Topics:
Gauge theories - Anomalies - Feynman diagram

~ ~ ~ ~ ~ ~ ~ ~ ~ ~