That argument assumes that our understanding of physics is correct and complete. If FTL is possible, that is an incorrect assumption. But yes, any theory including FTL travel must explain how nature behaves if this happens.
No, it's much weaker than that... that assumes that our understanding of relativity is even remotely correct. You can obtain the FTL/causality problem from even a brutally simplified version of general relativity, such that one can literally draw the problem on a piece of paper. For FTL not to break causality essentially requires that A: you can only FTL jump once and only once in the entire history of the universe as long as you don't leap into your own past light cone, B: FTL is impossible or C: Causality is not absolute.
I've often thought that the announcement that we've developed FTL would actually be a very bad thing, as fun as it sounds at first. Sort of like this story, where it turns out a proof of P = NP is just about the worst possible thing: http://www.antipope.org/charlie/blog-static/fiction/toast/to...
I've often thought that the announcement that we've developed FTL would actually be a very bad thing, as fun as it sounds at first. Sort of like this story, where it turns out a proof of P = NP is just about the worst possible thing: http://www.antipope.org/charlie/blog-static/fiction/toast/to...