The author considers speed of light being the absolute, irrevocable certainty, but how does he know that? Does he know _why_ c is constant? Had he mastered gen

The author considers speed of light being the absolute, irrevocable certainty, but how does he know that? Does he know _why_ c is constant? Had he mastered general relativity enough to be 100% sure that it can't vary in time or space? Or that our current understanding of physics is total and there absolutely can't be something that we don't know about speed of light? https://en.m.wikipedia.org/wiki/Time-variation_of_fundamental_constants https://en.m.wikipedia.org/wiki/Time-variation_of_fundamenta... https://en.m.wikipedia.org/wiki/Variable_speed_of_light https://en.m.wikipedia.org/wiki/Variable_speed_of_light