  The fact that macroscopic systems approach thermal equilibrium may seem
puzzling, for example, because it may seem to conflict with the
time-reversibility of the microscopic dynamics. We here prove that in a
macroscopic quantum system for a typical choice of "nonequilibrium subspace",
any initial state indeed thermalizes, and in fact does so very quickly, on the
order of the Boltzmann time $\tau_\mathrm{B}:=h/(k_\mathrm{B}T)$. Therefore
what needs to be explained is, not that macroscopic systems approach
equilibrium, but that they do so slowly.
