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.
Categories: hep-th astro-ph.CO gr-qc hep-ph quant-ph