We woul know now. (Score 1)

I'm guessing if it was possible to go back in time, then we would already be aware of people that have done it, as they would have already visited. There is a simple way to work out if time travel is possible. Set up a meeting and determine that the first time traveller will go back to the time and place of the meeting. If by the time the meeting ends and no=ones turned up, then you're pretty sure its not going to happen.