Causality, light speed constancy and local action principle

In 1905 A. Einstein, from the experiments of Michelson and Morley in 1887, enunciates the light speed constancy principle in the inertial frames of reference. However, this principle was pointed by the equations of the electromagnetism of J. C. Maxwell in 1865. The prediction of the Maxwell equations is due to in the most basic principle of the science, the principle of causality, is the reason of the light speed constancy. This article shown how, only with the causality principle, we can deduce the properties of the light. At the end of the article we used the causality relativistic definition to approach the local action principle, as well as to solve the paradox that implies the EPR theorem.