Chiral gauge theory at the boundary between topological phases

I show how chiral fermions with an exact gauge symmetry in any representation can appear on the d-dimensional boundary of a finite volume (d+1)-dimensional manifold, without any light mirror partners. The condition for it to look like a local d-dimensional theory is that gauge anomalies cancel, and that the volume be large. This provides a new paradigm for the lattice regularization of chiral gauge theories. While most of the analysis is done in the continuum, very simple lattice calculations reveal the existence of Weyl fermions when appropriate boundary conditions are applied.