Suppose now that a pair of theories A and B are
S-dual. This means that if f denotes any physical
observable and
denotes the coupling constant, then
.
(The expansion parameter
introduced earlier corresponds to
). This duality, whose recognition
was the first step in the current revolution, generalizes the
electric-magnetic symmetry of Maxwell theory. The point is that 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.
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