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,
One-way speed of light Thus the measured value of the average one-way speed is dependent on the method used to synchronize the start and finish clocks. This is a matter of convention. The Lorentz transformation is defined such that the one-way speed of light will be measured to be independent of the inertial frame chosen. Some authors such as Mansouri and Sexl (1977) as well as Will (1992) argued that this problem doesn't affect measurements of the isotropy of the one-way speed of light, for instance, due to direction dependent changes relative to a "preferred" (aether) frame Σ. They based their analysis on a