## Just One Theory!The second superstring revolution (1994-??) has brought non-perturbative string physics within reach. The key discoveries were the recognition of amazing and surprising "dualities." They have taught us that what we viewed previously as five distinct superstring theories is in fact five different perturbative expansions of a single underlying theory about five different points.
It is now clear that there is a unique theory, though it may allow many different quantum mechanical solutions. For example, a sixth special quantum solution implies the existence of an 11-dimensional space-time. Another lesson we have learned is that, non-perturbatively, objects of more than one dimension (membranes and higher " p-branes") play a central role. In most respects they
appear just as fundamental as the strings (which can now be called one-branes),
except that a perturbation expansion cannot be based on p-branes with p >1.
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.
T duality, unlike S or U duality, can be understood perturbatively, and therefore it was discovered between the two string revolutions.
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| Contents | Resolving Contradictions | Supersymmetry | A Brief History of Superstings |

| Basic Ideas of Superstring Theory | Superstring Revolution, part deux |