It's commonly known among physicists that the speed of light has a constant value in SR. Except in GR, but I'm not so sure. I THINK, intuitively, it doesn't. I guess I could have done some research on this topic about which various questions are asked, but I didn't make the effort to do so because I don't think it's of real importance to my question.
If the speed of light had a different value as it actually has, would that make an essential difference for Special or General Relativity?
