Abstract
Following the ideas of Ll. Bel expressed at ERE's 91 we give a definition of rigid congruences in both General and Special Relativity, and we try to make the definition plausible. To this end we recall Fermat's principle in General Relativity and we show that this principle allows us to reinterpret the “quotient metric” as the quadratic form which defines the optical length in a gravitational field. We apply the definition to the Earth-Sun system in the post-Newtonian approximation. Furthermore we compute the Fermat tensor and the corresponding relative variation of the speed of light in a Michelson-Morley-like experiment performed on the Earth's surface. According to all measurements to date, this quantity is extremely small (10−13). Consistently with these results, negotiations regarding the repetition of the Michelson-Morley experiment, in its modern version (laser interferometry ), are being achieved in order to test these results.
Published Version
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