Before the 1990s, string theorists believed there were five distinct superstring theories: type I, type IIA, type IIB, and the two flavors of heterotic string theory (SO(32) and E8×E8). The thinking was that out of these five candidate theories, only one was the actual correct theory of everything, and that theory was the one whose low energy limit, with ten spacetime dimensions compactified down to four, matched the physics observed in our world today. It is now believed that this picture was incorrect and that the five superstring theories are related to one another by the dualities described above. The existence of these dualities suggests that the five string theories are in fact special cases of a more fundamental theory called M-theory.
|Bosonic||26||Only bosons, no fermions, meaning only forces, no matter, with both open and closed strings; major flaw: a particle with imaginary mass, called the tachyon, representing an instability in the theory.|
|I||10||Supersymmetry between forces and matter, with both open and closed strings; no tachyon; gauge group is SO(32)|
|IIA||10||Supersymmetry between forces and matter, with only closed strings; no tachyon; massless fermions are non-chiral|
|IIB||10||Supersymmetry between forces and matter, with only closed strings; no tachyon; massless fermions are chiral|
|HO||10||Supersymmetry between forces and matter, with closed strings only; no tachyon; heterotic, meaning right moving and left moving strings differ; gauge group is SO(32)|
|HE||10||Supersymmetry between forces and matter, with closed strings only; no tachyon; heterotic; gauge group is E8×E8|