It is a well-known relation that x=ct is used in Minkowski space. We know that c is a constant. The big question is about photon mass. If we accept that a photon mass is equal to 1 in some arbitrary units we obtain that fundamental physics is calibrated in a natural system of units. We have E=mc2, but when m=1 for photons we obtain that E=c2. In classical mechanics it is well known that time is homogeneous and discontinuous, but in quantum mechanics we have the uncertainly principle, Et h, which means that time can be inhomogeneous. Photons have the velocity of light and at large scales they do not interact with matter and the trajectory is homogeneous, but at the atomic scale, they interact with elementary particles such as electrons and their energy has discreet values. The Fermi principle is well known in optics, which correlates with functional action in theoretical mechanics.