The speed of light in a vacuum is constant because it participates in no inertial reference frame. If I am moving at 300m per second and shoot a beam of light ahead of me that beam travels at 300,000m per second irrespective of my speed or my motion. If I increase my speed, time slows down such that the speed of light from my perspective never changes. I assume that the speed of light is constant from my perspective if I am traveling forward and a beam of light from far behind me is shot in my direction. If I am moving at 300m per second, the beam fired from behind me must seem to make up that speed in order to remain constant. Of course, that beam doesn’t actually increase its speed; there must be a variation in time. Does time speed up so the speed of light remains constant?