How's this for a theory: Travelling back in time will never be possible. That is to say, humans will never discover, or never use, the ability to do so. If it were possible to travel back in time, it stands to reason that somebody would do it, and probably bring the technology back with them. Therefore it would already be possible.

The X-Files makes me think too much about too many things I don't really understand. Hence the half-arsed theory above.