The duality era:

This started with an idea of Ashok Sen’s i.e. that N=4 Super-Yang-Mills has a weak-strong coupling, electric-magnetic duality. I.e., parameters have the symmetry SL(2,C).

Harvey shows picture of how values of parameters that label different theories lie in the fundamental region of SL(2,C). Can always map strong coupling to weak coupling.

This was quickly extended to SUSY string theories and led to results about Calabi-Yau spaces. Harvey thinks that the dualities should be understood as different descriptions of one underlying object.

Quotes Rickles as drawing an analogy between dualities and gauge symmetries. Harvey adds that there are examples where dualities really are the same as gauge symmetries (Duality=Gauge).