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 :: I have not seen any reference to Fermat's principal being the sacrosanct reference to a particle in gravity etc during any phase in history.. It was more of a ""if i put a block of glass light bends like this"" we have done those experiments in school. There is a debate if the velocity of light is a constant or is varying with wavelength / group velocity/phase velocity but again , I think it is a circular reference as mentioned by Spiel496.. -Alok "