Variable speed of light in general relativity (3) might contradict Lorentz invariance (4) must modify Maxwell's equations (5) must be done consistently with respect to all other physical theories. Whether these concerns apply to the proposals of Einstein (1911) and Dicke (1957) is a matter of debate, though VSL cosmologies remain out of mainstream physics. 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
One-way speed of light Even though this theory is experimentally indistinguishable from special relativity, Lorentz's theory is no longer used for reasons of philosophical preference and because of the development of general relativity. A sychronisation scheme proposed by Reichenbach and Grünbaum, which they called ε-synchronization, was further developed by authors such as Edwards (1963), Winnie (1970), Anderson and Stedman (1977), who reformulated the Lorentz transformation without changing its physical predictions. For instance, Edwards replaced Einstein's postulate that the one-way speed of light is constant when measured in an inertial frame with the postulate: So the average speed for the round trip remains the experimentally