Photon acceleration in vacuum
  A new process associated with the nonlinear optical properties of the electromagnetic vacuum, as predicted by quantum electrodynamics, is described. This can be called photon acceleration in vacuum, and corresponds to the frequency shift that takes place when a given test photon interacts with an intense beam of background radiation. 
J. T. Mendonca, M. Marklund, P. K. Shukla, and G. Brodin