  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.
