The superluminal limit as a consequence of the classical causality principle

The superluminal limit states that no physical object can overpass the velocity of the light in vacuum. This limit is usually considered a consequence of the special theory of relativity (STR). In contrast with the basic hypotheses chosen by Albert Einstein, many authors wrongly consider it a basic assumption of the STR. Here, we revise this limit by showing that it can be derived as a consequence of the classical causality principle without invoking any additional hypotheses. It will be demonstrated to hold true in any kind of medium even in those exhibiting either a phase or group velocity of the light greater than that of light in vacuum. The behavior of some neutral particles (e.g. neutrinos, photons) is shown to be fully consistent with this model. The validity of our discussion rests on the hypotheses of a gravitationless vacuum.