It's been proven rather thoroughly though. Look at Kip Thorne's body of work. His book: Black Holes and Time Warps is accessible. c0 is the speed of light on

It's been proven rather thoroughly though. Look at Kip Thorne's body of work. His book: Black Holes and Time Warps is accessible. c0 is the speed of light only incidentally. It is the speed of causality first. You can warp spacetime but you can't change the speed limit. You might think there would be a hypothetical way to cheat it from some unknown phenomenon, but then any occurrence of that phenomenon would destroy itself in zero time.