  Violation of the action-reaction principle is shown to occur in the van der
Waals interaction between two atoms, one of which is excited. It is accompanied
by the transfer of linear momentum to the electromagnetic vacuum. The vacuum
momentum results from the asymmetric interference of the virtual photons
scattered off each atom along the interatomic direction, which is in itself a
manifestation of the optical theorem. This momentum, of equal strength and
opposite direction to the momentum gained by the two-atom system, is ultimately
released through directional spontaneous emission. A quantitative prediction of
this phenomenon is made in a two-alkali atom system. It is conjectured that
action-reaction violation takes place in any asymmetrically excited system.
