  The notion of spectral flow has given new insight into the motion of vortices
in superfluids and superconductors. For a BCS superconductor the spectrum of
low energy vortex core states is largely determined by the geometric optics
limit of Andreev reflection. We use this to follow the evolution of the states
when a stationary vortex is immersed in a transport supercurrent. If the core
spectrum were continuous, spectral flow would convert the momentum flowing into
the core via the Magnus effect into unbound quasiparticles --- thus allowing
the vortex to remain stationary without a pinning potential or other sink for
the inflowing momentum. The discrete nature of the states, however, leads to
Bloch oscillations which thwart the spectral flow. The momentum can escape only
via relaxation processes. Taking these into account permits a physically
transparent derivation of the mutual friction coefficients.
