String Theory encountered its first major split when it was required to account for both fermions and bosons, the two types of particles in nature. Bosons, which trasmit forces, are accounted for in both String Theories, but when the theory was required to account for fermions, particles which make up matter, as well, there occurred a great divide. Not much is said, anymore, about the theory that went the way without fermions, as it required 26 spacetime dimensions and made use of the tachyon, a particle with imaginary mass that supposedly traveled beyond the speed of light and backwards in time…most physicists prefer matter to exist, it seems. To include fermions, however, String Theory had to introduce the concept of supersymmetry, which means that for every fermion there is a boson chaperone. Theorists account for the utter lack of observed supersymmetric partners by claiming that the partners are simply too massive to be detected at current accelerators (while strings themselves, of course, are too small). From superstring theory (String Theory which incorporates supersymmetry), five kinds have evolved, all of which require 9 spatial dimensions and one temporal:

Type I- both open and closed strings

Type IIA- closed strings, fermions spin both ways

Type IIB- closed strings, fermions spin one way

Type HO-closed strings, right moving and left

moving strings differ, group symmetry is SO(32)

Type HE-closed strings, right moving and left moving strings differ, group symmetry is E₈xE₈

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