Time travel... Interesting subject. Possible? Maybe. There's just too much variability in the theoretical process. Maybe we have had visitors already and they've been integrated into our history... Or stopping something from happening. Or maybe our history is already full of 'best case scenarios'... Maybe no matter what happened, Hitler was the most favorable outcome for someone to lead Germany...
I personally think time travel is possible, but no one would possibly want to mess up history that much... The so called 'butterfly effect'.
Gah, my brain hurts thinking about it.