quantities (called nondimensionalization), resulting in only dimensionless quantities remaining. In fact, physicists can choose their units so that the physical constants c, G, ħ = h/(2π), 4πε0, and kB take the value one, resulting in every physical quantity being normalized against its corresponding Planck unit. For that, it has been claimed that specifying the evolution of a dimensional quantity is meaningless and does not make sense. When Planck units are used and such equations of physical law are expressed in this nondimensionalized form, no dimensional physical constants such as c, G, ħ, ε0, nor kB remain, only dimensionless quantities, as predicted by the Buckingham π theorem. Short of their anthropometric unit dependence, there simply is no speed of light, gravitational constant, nor the Planck constant, remaining in mathematical expressions of physical reality to be subject to such hypothetical variation. For example, in the case of a hypothetically varying gravitational constant, G, the relevant dimensionless quantities that potentially vary ultimately become the ratios of the Planck mass to the masses of the fundamental particles. Some key dimensionless quantities (thought to be constant) that are related to the speed of light (among other dimensional quantities such as ħ, e, ε0), notably the fine-structure constant or the proton-to-electron mass ratio, could in principle have meaningful variance and their possible variation continues to be studied. General critique of varying c cosmologies From a very general point of view, G. F. R. Ellis and expressed concerns that a varying c would require a rewrite of much of modern physics to replace the current system which depends on a constant c. Ellis claimed that any varying c theory (1) must redefine distance measurements; (2) must provide an alternative expression for the metric tensor in general relativity; (3) might contradict Lorentz invariance; (4) must modify Maxwell's equations; and (5) must be done consistently with respect to all other physical theories. VSL cosmologies remain out of mainstream physics. References External links Is the speed of light constant? "Varying constants" Hypotheses Electromagnetic radiation Light Special relativity de:Physikalische Konstante#Konstanz der Naturkonstanten