Gravity's Rainbow: Limits for the applicability of the equivalence   principle
  Limits for the applicability of the equivalence principle are considered in the context of low-energy effective field theories. In particular, we find a class of higher-derivative interactions for the gravitational and electromagnetic fields which produce dispersive photon propagation. The latter is illustrated by calculating the energy-dependent contribution to the deflection of light rays. 
Rene Lafrance and Robert C. Myers