Variable speed of light

A variable speed of light (VSL) is a feature of a family of hypotheses stating that the speed of light in vacuum, usually denoted by c, may in some way not be constant, e.g. varying in space or time, or depending on frequency. A variable speed of light occurs in some situations of classical physics as equivalent formulations of accepted theories, but also in various alternative theories of gravitation and cosmology, many of them non-mainstream.

Notable attempts to incorporate a variable speed of light into physics have been made by Einstein in 1911, by Robert Dicke in 1957, and by several researchers starting from the late 1980s.

The speed of light in vacuum instead is considered a constant, and defined by the SI as 299792458 m/s. Variability of the speed of light is therefore equivalent with a variability of the SI meter and/or the SI second.

VSL should not be confused with faster than light theories; nor should it be confused with the fact that the speed of light in a medium is slower than the speed of light in vacuum depending on the medium's refractive index.

While Einstein first mentioned a variable speed of light in 1907, he reconsidered the idea more thoroughly in 1911. In analogy to the situation in a medium, where a shorter wavelength ${\displaystyle \lambda }$, by means of ${\displaystyle c=\nu \lambda }$, leads to a lower speed of light, Einstein assumed that clocks in a gravitational field run slower, whereby the corresponding frequencies ${\displaystyle \nu }$ are influenced by the gravitational potential (eq.2, p. 903):

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