forum; There are actually two questions. 1. Why is the propagation speed of light constant? I am not aware of any unanimous answer. If Maxwell used permittivity and permeability, em properties of space/vacuum to calculate the propagation speed, it would seem reasonable that space has a structure, since a void/nothing having properties is nonsensical. That structure could determine the propagation speed, just as a medium does for other forms of energy. 2. Why is the measured speed of light constant? Special Relativity predicts motion induced phenomena of time dilation and length contraction, which alters perception and measurement. The two complementary effects result in a constant measure of light speed relative to the observer. Since measurement is the verification tool of science, it is the experimental basis for a constant independent light speed. I would question whether photons have energy vs photons are energy.