  Dilute granular flows are routinely described by collisional kinetic theory,
but dense flows require a fundamentally different approach, due to
long-lasting, many-body contacts. In the case of silo drainage, many continuum
models have been developed for the mean flow, but no realistic statistical
theory is available. Here, we propose that particles undergo cooperative
displacements in response to diffusing ``spots'' of free volume. The typical
spot size is several particle diameters, so cages of nearest neighbors tend to
remain intact over large distances. The spot hypothesis relates diffusion and
cage-breaking to volume fluctuations and spatial velocity correlations, in
agreement with new experimental data. It also predicts density waves caused by
weak spot interactions. Spots enable fast, multiscale simulations of dense
flows, in which a small, internal relaxation enforces packing constraints
during spot-induced motion. In the continuum limit of the model, tracer
diffusion is described by a new stochastic differential equation, where the
drift velocity and diffusion tensor are coupled non-locally to the spot
density. The same mathematical formalism may also find applications to glassy
relaxation, as a compelling alternative to void (or hole) random walks.
