Friction in the vacuum?
(PHYSorg.com Physics)

(Phys.org)-When three physicists first discovered through their calculations that a decaying atom moving through the vacuum experiences a friction-like force, they were highly suspicious. The results seemed to go against the laws of physics: The vacuum, by definition, is completely empty space and does not exert friction on objects within it. Further, if true, the results would contradict the principle of relativity, since they would imply that observers in two different reference frames would see the atom moving at different speeds (most observers would see the atom slow down due to friction, but an observer moving with the atom would not).