Superstrings from the 1990's

By the 1990' there was emabarisingly six healthy string theories
 
Dualities between the theories begin to be discovered. This points toward the idea that the six seperate theories are just different perturbative expansions of a single                       underlying theory about five different points
     
               Three kinds of dualities, called S,T, and U, have been identified.

               
               It can sometimes happen that theory A with a large strength of interaction (or `strong coupling') is equivalent to theory B at weak coupling, in which case                           they are said to be S dual.
        
               Similarly, if theory A compactified on a space of large volume is equivalent to theory B compactified on a space of small volume, then they are called
                       T dual
.


               Combining these ideas, if theory A compactified on a space of large (or small) volume is equivalent to theory B at strong (or weak) coupling, they are
                          called U dual. If theories A and B are the same, then the duality becomes a self-duality, and it can be viewed as a kind of symmetry.

As a result of these dualities the different theories are a part of one underlying theory --- M theory
duality
[9]



The idea is that the 11th added dimension is imediatly rolled up so the membrane acts as a ten dimensional string


M-theory