  We point out that time's arrow is naturally induced by quantum mechanical
evolution, whenever the systems have a very large number ${\cal N}$ of
non-degenerate states and a Hamiltonian bounded from below. When ${\cal N}$ is
finite, the arrow is imperfect, since evolution can resurrect past states. In
the limit ${\cal N} \rightarrow \infty$ the arrow is fixed by the "tooth of
time": the decay of excited states induced by {\it spontaneous emission} to the
ground state, mediated by interactions and a large number of decay products
which carry energy and information to infinity. This applies to individual
isolated atoms, and does not require a coupling to a separate large heath bath.
