Hi all. New to PFs. I deal in (exotic) human biology, which happens to have put me at the intersection of fields and, in this instance, conceptually more challenging physics. I'm presently involved in a comparative analysis of biological versus physical time contraction-dilation. I happen to have a question for the experts on relativistic time dilation, if I may cut-the-chase: Does the speed of light term in relativistic calculations strictly only refer to its speed in a vacuum or could it also be that in a substance, say, e.g., water, where it's speed happens to be some 25 % less? Thanks, much appreciated.