The most unbelievable thing about time travel is the spacial aspect. If you travel back or forward in time, you'd have to have a machine that doesn't just travel through the fourth dimension, but through the third, because the Earth won't be in the same place as when you left. (At least the TARDIS gets points for that one.) But that does lead to my theory that UFOs aren't actually alien craft, but time travelers entering space before making their journeys in order to come out someplace where there won't be anything in their way.