Variable speed of light find out that it doesn't go only at the speed of light! It may surprise you that there is an amplitude for a photon to go at speeds faster or slower than the conventional speed, "c"." These virtual photons, however, do not violate causality or special relativity, as they are not directly observable and information cannot be transmitted acausally in the theory. Feynman diagrams and virtual photons are usually interpreted not as a physical picture of what is taking place, but rather as a convenient calculation tool (which, in some cases, happen to involve faster-than-light velocity vectors). In 1937, Paul
Speed of light systems of natural units of measurement or the geometrized unit system where . Using these units, does not appear explicitly because multiplication or division by 1 does not affect the result. The speed at which light waves propagate in vacuum is independent both of the motion of the wave source and of the inertial frame of reference of the observer. This invariance of the speed of light was postulated by Einstein in 1905, after being motivated by Maxwell's theory of electromagnetism and the lack of evidence for the luminiferous aether; it has since been consistently confirmed by many experiments. It