Invariants of diffeomorphisms of four-manifolds
More gauge theory coming at you. This post is based on Ruberman's paper "An obstruction to smooth isotopy in dimension 4", and inspired by a conversation with John Baldwin. I want to talk about an idea in gauge theory that is slightly less advertised, not too complicated to describe, and still really cool. Let's say we have a closed four-manifold $X$ (simply-connected, smooth, oriented, connected, large $b^+_2$, all the good things). A natural question is how to distinguish self-diffeomorphisms of $X$ up to isotopy? Freedman and Quinn proved that if two diffeomorphisms of $X$ induce the same map on homology, then they are isotopic through homeomorphisms. Totally cool, but they may not be isotopic through diffeomorphisms! It turns out we can use gauge theory to make this distinction. Get pumped! First, here's the basic idea. If we fix a metric $g$ on $X$ and an $SO(3)$-bundle $E$ with non-trivial $w_2$, then recall that we ge...