  Flavor-dependent neutrino emission is critical to the evolution of a
supernova and its neutrino signal. In the dense anisotropic interior of the
star, neutrino-neutrino forward-scattering can lead to fast collective neutrino
oscillations, which has striking consequences. We present a theory of fast
flavor depolarization, explaining how neutrino flavor differences become
smaller, i.e., depolarize, due to diffusion to smaller angular scales. We show
that transverse relaxation determines the epoch of this irreversible
depolarization. We give a method to compute the depolarized fluxes, presenting
an explicit formula for simple initial conditions, which can be a crucial input
for supernova theory and neutrino phenomenology.
