I think it would be too early to call FTL travel and time travel impossible (the two beeing closly connected). There are interesting theories for FTL (like the Alcubierre drive http://en.wikipedia.org/wiki/Alcubierre_drive) and although this does require negative matter as parent noted this does not mean it is impossible. The Casimir effect for example (a well known quantum effect) produces a mass-negative region of space.