Terence McKenna had some wonderful analysis of time travel.
He said that the day it began history would end—because everything would happen at once!
Think about it a while and it makes sense...
One theory of time is that everything has happened at once - no time travel needed - we only perceive it as moving in a certain direction, but that direction could go from the future to the past as easily as from the past to the future - there is simply no way for us humans to tell.