Folks, allow an aged old nuclear curmudgeon to speak concerning the basic laws of physics:

C is a relative constant for the speed of light through 4-d space, which is still under discussion in some circles: Guth and Feinberg would argue with a stop sign but they raise an important issue about light:

C is a relative term and is subject to the density of space through which it travels.  Counterintuitively, in some instances, lighter space transmits light slower than we see it move in 4-d.  The math involves multiple and compound infinities, so I hesitate to disclose it here for fear of inducing nervous breakdowns in my colleagues who don't like the way I express differentials.

Please defer to my 1954 "Snake in a sandbox" analogy: the snake (Crotalus Velocitans) needs something to slither against, so he uses sand to propel himself forward (or backward) through sand dependent on the energy he has available.  So, what happens when C. Velocitans hits the board that contains the sand?  He bounces off, imparting some of his energy to the box, some to the sand, and re-gains some from other slithering serpents in the box. 

Note that the outside of the sandbox is supported by air.  The back-pressure from the serpent hitting the board only has so much force, resulting in the snake losing a few scales, but the snake isn't that bright so he continues moving at C regardless that he has lost energy.

In the dried-out terminology of physicists, we call that madness red-shift.  That is because the snake is slithering at C through the sandbox we call space. 

I could go on, but I would probably bore you.