I know that this question may sound stupid, but what I mean is that photons have some energy and no mass, yet the mass and energy are said to be equivalent (or maybe I got that part wrong). In an earlier question I got an answer that gravity is dependent on stress-energy tensor and not simply mass, so that a photon should also create a very small gravity influence. Then how do we know that photons are moving at $c$ and not let's say 99.999999% of $c$?
I'm asking this because of time dilation, because then a photon moving at slightly below $c$ would only experience a massive time slowdown, but still not a complete halt, right?
EDIT: Note that in the hypothetical case of light being a bit slower than $c$ it would still be a limit of speed used for relativistic calculations but no longer speed of light, so the question isn't exactly "why the light moves with the speed of light?".