On The Origins Of Superstring Theory
String theory was initially invented to try and explain the strong nuclear force. This approach was only abandoned because; firstly it seemed to predict a particle that shouldn't exist; secondly the theory was ten dimensional, at odds with the traditional 4 dimensional picture of the universe; and lastly because the better theory of quantum chromodynamics (QCD) came along whilst these problems were being fixed!
When the theory was developed, between 1968 and 1973, it took two years before anybody realised that the equations involved depicted not the usual '0' dimensional point particles, but 1 dimensional 'strings'.
In 1971 it was shown that fermions could be incorporated into this new theory, if the theory had the property of supersymmetry. In fact it came to be realised that all string theories had to be supersymmetric, if fermions were to be included.
Then in 1974 Prof. John H. Schwarz and Joel Scherk showed that if the theory was extended, a grand unified theory might emerge, rather than being just a theory used to explain hadron (which experience the strong nuclear force) interaction. One result that fell out straight away was the rogue massless spin 2 particle predicted by the theory, was in fact the graviton. This is a nice result, as it makes gravity necessary, rather than impossible with conventional quantum field theory. Also as the 'strings' were not infinitely small, the infinities that crop up when you try and use classical quantum theory at very short distances (around the Planck length), do not exist. This had always been one of the biggest stumbling blocks when you try and marry gravity to quantum physics. The extra dimensions needed didn't really present a problem, gravity itself warps 4 dimensional space, so the strings could warp space in such a way that the 6 extra dimensions were curled up into a volume about the Planck length in size. These can be ignored as they have no effect on day to day physics experiments. To look down to scales this size, you'd need energies vastly larger than we could ever produce.(To resolve smaller distances you need smaller wavelengths, which means more energy)
The First Superstring Revolution
This novel approach to unification didn't really progress very far over the next ten years, until some key developments in 1984-1985. These revolutionised the theory, and suddenly it was the hot field to be working in. From the vast amount of papers produced it appeared that there were at least five self consistent superstring theories, and that none of them gave a complete picture of the universe! All have 9 spatial and 1 time dimensions, all have varying degrees of supersymmetry and are :-
E8 X E8 heterotic (HE, for short)
SO(32) heterotic (HO, for short)
(11D Supergravity is sometimes included here, as it too can fall under the m-theory umbrella)
Of these it looked like E8 x E8 (HE) would be the most likely, that with further work would turn out to be the best. In this theory, the extra dimensions are curled up into Calabi-Yau spaces the exact topology of which determines how the strings can wrap around the holes, which in turn relates to how many families of fundamental particle there are. Unfortunately there may be a large number of Calabi-Yau spaces that fit the conditions needed to result in the universe we see, and there's no clear reason to pick one over another. The seeming ease with which these theories could be constructed lead to a loss of faith from many researchers. How could one fundamental theory ever be found, if investigation only allows more of the same to be constructed?
The Second Superstring Revolution
This came about in 1994 when Edward Witten realised that the five separate theories above, could be related through the concept of dualities. Three of these dualities have been found S, T and U.
S duality refers to theory 'A' at strong coupling being equivalent to theory 'B' at weak coupling. Type I theory has S duality to the HO theory, and IIB is S dual to itself!
T duality means that 'A' compactified onto a large dimension is equivalent to 'B' compactified onto a small dimension. IIA and IIB theories and the HE and HO theories are T dual.
U duality is a combination of S and T, if theory A compactified on a large (or small) dimension is equivalent to B at strong or weak coupling
Investigation of how the IIA and HE might be dual to one another lead to the postulation of an 11 dimensional theory, which came to be known as M-theory. The greater richness of M-theory, which relies not on 1 dimensional strings, but on multi-dimensional' branes' allows the many superstring theories to be brought under one umbrella. This uniting theory again provides the hope that a single theory to explain it all can be found.