Editing Variable speed of light (section)

{{short description|Non-mainstream theory in physics}}

A '''variable speed of light''' ('''VSL''') is a feature of a family of hypotheses stating that the [[speed of light]] may in some way not be [[Physical constant|constant]], for example, that it varies in space or time, or depending on [[frequency]]. Accepted [[Classical physics|classical theories of physics]], and in particular [[general relativity]], predict a constant speed of light in any [[Principle of locality|local]] [[frame of reference]] and in some situations these predict apparent variations of the speed of light depending on frame of reference, but this article does not refer to this as a variable speed of light. Various alternative theories of [[gravitation]] and [[cosmology]], many of them non-mainstream, incorporate variations in the local speed of light.

Attempts to incorporate a variable speed of light into physics were made by [[Robert Dicke]] in 1957, and by several researchers starting from the late 1980s.

VSL should not be confused with [[faster than light]] theories, its dependence on a [[condensed matter physics|medium]]'s [[refractive index]] or its measurement in a remote observer's frame of reference in a [[gravitational potential]]. In this context, the "speed of light" refers to the limiting speed ''c'' of the theory rather than to the velocity of propagation of [[photon]]s.