Suppose now that a pair
of theories A and B are S-dual. This means that if
f denotes any physical observable and l
denotes the coupling constant, then
(The expansion parameter a introduced
earlier corresponds to l).
This duality, whose recognition
was the first step in the current revolution, 
generalizes the electric-magnetic symmetry of Maxwell theory. Since the
Dirac quantization condition implies that the basic unit of magnetic charge
is inversely proportional to the unit of electric charge, their interchange
amounts to an inversion of the charge (which is the coupling constant).
S duality relates
the type I theory to the HO theory and the IIB theory
to itself. This explains the strong coupling behavior of those three theories.
| Contents |
Resolving Contradictions | Supersymmetry
| A Brief History of Superstings |
| Basic Ideas of Superstring Theory
| Superstring Revolution, part deux |