It’s the reciprocal of the square root of the product of the permittivity and permeability of free space. Interestingly, neither of these properties require the existance of the photon and can be measured in the laboratory with modest equipment. Now as the article suggests, that these ‘constants’ aren’t really constant over vast periods of time, then due to things like the wave equation, light would ‘go along for the ride’ and adjust its speed accordingly, presumably over vast expanses of cosmological time. But for our little snapshot, c is for all intents and purposes, rock solid constant as measured from in all inertial reference frames. As an aside, since we don’t know the nature of conciousness and subjective time, and being essentially chemical meat computers, it makes one wonder if we’d even be aware of such a change in c.
It's fantasy, of course, because as you point out, an our-world guy could not exist in that world. And that-world guys would probably move so slowly that they also would never perceive the relativistic effects.
What color is a single photon? Are all photons the same color?
How does a single photon create the red shift if the speed of photons is constant regardless of the direction the source is moving?
Photon is a particle. It makes more sense if you treat it as a wave (TEM) and Maxwells equations?
It has been a while for me though.