HEre's the thing. THere are multiple string theories/ For example:

"String theories

Type, Spacetime dimensions, Details

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; group symmetry is SO(32)

IIA, 10 ,Supersymmetry between forces and matter, with closed strings and open strings bound to D-branes; no tachyon; massless fermions spin both ways (achiral)

IIB, 10 ,Supersymmetry between forces and matter, with closed strings and open strings bound to D-branes; no tachyon; massless fermions only spin one way (chiral)

HO, 10 ,Supersymmetry between forces and matter, with closed strings only; no tachyon; heterotic, meaning right moving and left moving strings differ; group symmetry is SO(32)

HE, 10 ,Supersymmetry between forces and matter, with closed strings only; no tachyon; heterotic, meaning right moving and left moving strings differ; group symmetry is E8×E8 "

Tachyons are a bit imponderable to my way of thinking, but I don't know much, anyway. THere sure is a lot to clarify in all this stuff... sigh...


A