http://phys.org/news/2015-03-einstei...time-foam.html
"One hundred years after Albert Einstein formulated the general theory of relativity, an international team has proposed another experimental proof. In a paper published today in Nature Physics, researchers from the Hebrew University of Jerusalem, the Open University of Israel, Sapienza University of Rome, and University of Montpellier in France, describe a proof for one of the theory's basic assumptions: the idea that all light particles, or photons, propagate at exactly the same speed."

This is an assumption of Newton's emission theory of light:

http://en.wikipedia.org/wiki/Emission_theory
"Emission theory, also called emitter theory or ballistic theory of light, was a competing theory for the special theory of relativity, explaining the results of the Michelson-Morley experiment. Emission theories obey the principle of relativity by having no preferred frame for light transmission, but say that light is emitted at speed "c" relative to its source instead of applying the invariance postulate."

Pentcho Valev