Let us hypothesize that travelling back (or forth) in time is possible, and even frequent in our time. If I'd want to go back and change something in my life, why couldn't I do it?
Simple: if I could, I'd already done it. I wouldn't have gone back, but I would have changed it already because I'll go back in the future. Confusing? Perhaps.
Quick example:
It's the year 2012. But in 2020, I'll want to travel back to 1990 and change what I ate for dinner on the 13th of January. Well, since I've already passed 1990, because I'm in 2012, whatever it was that I ate on that lovely Saturday night is already gone, and can't be changed. If I ate chicken, it was because it was chicken in the first place, or because, 8 years from now, I'll go back in time, toss out the beef and serve me a nice piece of chicken. Either way, I can't change the fact that I ate chicken, and therefore I can't go back and change it to beef. Actually I can, and that would explain why I'd go back later and change it to chicken...
So, broadly speaking, it's hard to imagine going back in time and changing the world, because it would already be changed...
I'm starting to think that the problem lies not just in Physics, but in perception...
