Variable speed of light 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
Speed of light for interferometry measurements of the speed of light. However interferometric determination of wavelength becomes less precise with wavelength and the experiments were thus limited in precision by the long wavelength (~) of the radiowaves. The precision can be improved by using light with a shorter wavelength, but then it becomes difficult to directly measure the frequency of the light. One way around this problem is to start with a low frequency signal of which the frequency can be precisely measured, and from this signal progressively synthesize higher frequency signals whose frequency can then be linked to the original signal. A