Optical dispersion relations as a fundamental tool for the superluminal relativistic limit

The superluminal relativistic limit (SRL), which states that the velocity of light in vacuum (c) cannot be passed by any object, is wrongly considered a postulate of the special theory of relativity (STR) even if Einstein himself never claimed something like that. We derive the SRL as a direct consequence of the classical causality principle and of the optical dispersion relations. Attention is paid to the case of vacuum with gravitational fields where the SRL can lose its validity in agreement with the quantum electrodynamics.