:: 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