Editing Variable speed of light (section)

=== Dicke's proposal (1957) ===
[[Robert Dicke]], in 1957, developed a VSL theory of gravity, a theory in which (unlike general relativity) the speed of light measured locally by a free-falling observer could vary.<ref name="Dicke">{{cite journal |author=Dicke |first=Robert |year=1957 |title=Gravitation without a Principle of Equivalence |journal=Reviews of Modern Physics |volume=29 |issue=3 |pages=363–376 |bibcode=1957RvMP...29..363D |doi=10.1103/RevModPhys.29.363}}</ref> Dicke assumed that both frequencies and wavelengths could vary, which since <math> c = \nu \lambda </math> resulted in a relative change of ''c''. Dicke assumed a refractive index <math> n= \frac{c}{c_0} = 1+\frac{2 GM}{r c^2} </math> (eqn.&nbsp;5) and proved it to be consistent with the observed value for light deflection. In a comment related to [[Mach's principle]], Dicke suggested that, while the right part of the term in eq. 5 is small, the left part, 1, could have "its origin in the remainder of the matter in the universe".

Given that in a universe with an increasing horizon more and more masses contribute to the above refractive index, Dicke considered a cosmology where ''c'' decreased in time, providing an alternative explanation to the [[Hubble's law|cosmological redshift]].<ref name="Dicke"/>{{rp|374}}