  We know that in empty space there is no preferred state of rest. This is true
both in special relativity but also in Newtonian mechanics with its associated
Galilean relativity. It comes as something of a surprise, therefore, to
discover the existence a friction force associated with spontaneous emission.
he resolution of this paradox relies on a central idea from special relativity
even though our derivation of it is non-relativistic. We examine the
possibility that the physics underlying this effect might be explored in an ion
trap, via the observation of a superposition of different mass states.
