The eye of the beholder: how fast can atoms see each other?

Van der Waals forces between neutral, polarisable atoms are mediated by an exchange of two virtual photons. Surprisingly, a very fundamental question regarding such forces had remained unsolved until very recently: how does the finite speed of light affect the van der Waals interaction between an excited and a ground-state atom? Contradictory results were obtained in the past: Some studies found an oscillatory distance-dependence due to an in-terference of photons emitted by the excited atom and back-reflected by the ground-state atom. Others predicted a purely monotonous force.
I will show that a time-dependent analysis leads to surprising and Solomonic answer to this puzzle which appears to violate Newton’s action–reaction principle. The result has implica-tions for lateral dispersion forces and raises new questions regarding the distance-dependence of resonance energy transfer.