Photon-particle interactions, both ‘classical,’ such as Compton Scattering, and ‘speculative,’ such as ones associated with ‘tired light’ theories of the cosmological red-shift, are explained with the assumption of the constancy of the speed of light. For classical interactions, reciprocal variations in light frequency and wavelength occur under the constraint that their product always equals constant speed ‘c.’ Proponents of a cosmological red-shift claim that the universe is expanding due to expansion of space (or space-time) itself as light, traveling at ‘c,’ is stretched as the distance between the source and observer increases due to this space (or space-time) expansion. Counter arguments to this interpretation often fall into the realm of ‘tired light,’ dismissed by mainstream physicists for various reasons, but still advocated by various ‘dissident’ physicists, since the term was first coined by Zwicky in 1929. In this paper, I examine a classical photon-particle interaction, Compton Scattering, and one of the more popular ‘tired light’ theories to show that the assumption of a constant speed of light is unnecessary, i.e., similar results evolve from assuming a variable light speed.
