  Modern ether-drift experiments in vacuum could in principle detect the tiny
refractive index that, in a flat-space picture of gravity, is appropriate for
an apparatus placed on the Earth's surface. In this picture, in fact, if there
were a preferred reference frame, light on the Earth would exhibit a slight
anisotropy with definite quantitative differences from General Relativity. By
re-analyzing the data published by two modern experiments with rotating optical
resonators, and concentrating on the part of the signal that should be free of
spurious systematic effects, we have found evidences that would support the
flat-space scenario.
