How is "c" determined to be the maximum speed? A person I was discussing light with asked me this question: Is there evidence to support the contention that the maximum speed is 'c'? I must admit I couldn't answer the question. I happen to agree that 'c' is the maximum universal speed but I would like to ask my erudite readers if this has actually been proven. That the maximum is in fact the maximum or is it sort of a default statement based on the validity of the rest of the theory, such as because x and Y are true then Z must be the case? I don't wish to presuppose the answer so any help will be appreciated.