Quote Originally Posted by andruejaysin
String theory requires either 10 or 26 dimensions, for mathmatical reasons I don't begin to understand.
String theory strongly suggests the existence of ten or eleven (in M-theory) spacetime dimensions, as opposed to the usual four (three spatial and one temporal) used in relativity theory; however the theory can describe universes with four effective (observable) spacetime dimensions by a variety of methods.

(Wikipedia)