Yeah my feeling about time travel to the past is impossible.  I just don't think the universe would allow it because of the affect it would have on the present.  That is of course if there is only one timeline we exist in.  If every time we make a decision and every possible outcome happens on different but parallel timelines maybe screwing up the past doesn't matter as much. This is just me talking out of my ass as I'm sure my understanding of these theory's of time travel and parallel time lines is poor and best.