  For the past twenty-five years, nearly all analyses of accretion disk
dynamics have assumed that stress inside the disk is locally proportional to
pressure (the "alpha-model") and that this stress goes to zero at the
marginally stable orbit. Recently, it has been demonstrated that MHD turbulence
accounts for the bulk of internal disk stress. In contradiction with the
traditional view, the stress from this MHD turbulence does not diminish near
the marginally stable orbit, and the ratio of magnetic stress to pressure rises
sharply there. Examples of the consequences include: an increase in accretion
efficiency that may also be time- and circumstance-dependent; a decrease in the
rate of black hole spin-up by accretion; and generation of disk luminosity
fluctuations. Preliminary results from numerical simulations lend support to
analytic estimates of these effects.
