how do we know that c is the limit? and why is it the limit?
Actually, it's the principle of relativity (in SR) that is at odds with FTL travel and causality. If FTL had a special frame in which it works, there'd be no problems with the experiments. So, following JesseM: You can have exactly two of the following: FTL travel, relativity, causality.So in summary, if you want FTL, you have to give up causality, or you have to find something wrong with both the Michelson-Morley experiment and the Hafele-Keating experiment (as well as many others).