Will a decaying atom feel a friction force?
  We show how a simple calculation leads to the surprising result that an excited two-level atom moving through vacuum sees a tiny friction force of first order in v/c. At first sight this seems to be in obvious contradiction to other calculations showing that the interaction with the vacuum does not change the velocity of an atom. It is yet more surprising that this change in the atom's momentum turns out to be a necessary result of energy and momentum conservation in special relativity. 
Matthias Sonnleitner, Nils Trautmann and Stephen M. Barnett
quant-ph physics.atom-ph