  Inspired by patterns observed in mixtures of microtubules and molecular
motors, we propose continuum equations for the evolution of motor density, and
microtubule orientation. The chief ingredients are the transport of motors
along tubules, and the alignment of tubules in the process. The macroscopic
equations lead to aster and vortex patterns in qualitative agreement with
experiments. While the early stages of evolution of tubules are similar to
coarsening of spins following a quench, the rearrangement of motors leads to
arrested coarsening at low densities. Even in one dimension, the equations
exhibit a variety of interesting behaviors, such as symmetry breaking, moving
fronts, and motor localization.
