  We analyze the entanglement generated in a finite time between a pair of
space-like separated atoms, one of which emits a photon. As we show to order
$e^2$, the origin of entanglement can be traced back to the uncertainty about
which one of the atoms emitted the photon. We check this by comparing the time
behaviors of the emission processes allowed by energy conservation vs. those
forbidden by the same reason. No physical signal propagates between the atoms
in the processes considered, however an effective light cone separating
non-entangled from entangled regions in space-time emerges from our
calculations.
