Variable speed of light contribute to the above refractive index, Dicke considered a cosmology where "c" decreased in time, providing an alternative explanation to the cosmological redshift (p. 374). Dicke's theory is compatible with the SI definition of c since the time and length units second and meter can vary in concert, keeping the value of c fixed (p. 366). Spatial variation of the speed of light is implicitly present in general relativity, occurring in the coordinate space description.: "note that the photon speed is ... formula_9." (formula_10 denoting the gravitational potential −"GM"/"r") Based on this observation, variable speed of light models, including Dicke's,
Speed of light his whole system of philosophy might be demolished. In Descartes' derivation of Snell's law, he assumed that even though the speed of light was instantaneous, the denser the medium, the faster was light's speed. Pierre de Fermat derived Snell's law using the opposing assumption, the denser the medium the slower light traveled. Fermat also argued in support of a finite speed of light. In 1629, Isaac Beeckman proposed an experiment in which a person observes the flash of a cannon reflecting off a mirror about one mile (1.6 km) away. In 1638, Galileo Galilei proposed an experiment, with an apparent