  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.
