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.
Previous |Next
| Contents | Resolving Contradictions | Supersymmetry | A Brief History of Superstings |
| Basic Ideas of Superstring Theory | Superstring Revolution, part deux |