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
[9]
The idea is that the 11th added dimension is imediatly
rolled up so the membrane acts as a ten dimensional string
M-theory