Discover Magazine has a great article on the real science and logic of the paradoxes of time travel. Just what is possible and what isn't? Prepare to have your brain at least partially melted.
Sure, it doesn't address anything pesky like parallel timelines, but it does talk about the impossibility of changing something that's already happened.
The nub of the problem is that you cannot have a consistent "arrow of time" in the presence of closed timelike curves. The arrow of time is simply the distinction between the past and the future. We can turn an egg into an omelet, but not an omelet into an egg; we remember yesterday, but not tomorrow; we are born, grow older, and die, never the reverse. Scientists explain all of these manifestations of the arrow of time in terms of entropy-loosely, the "disorderliness" of a system. A neatly stacked collection of papers has a low entropy, while the same collection scattered across a desktop has a high entropy. The entropy of any system left to its own devices will either increase with time or stay constant; that is the celebrated second law of thermodynamics. The arrow of time comes down to the fact that entropy increases toward the future and was lower in the past.
But what the hell is that temple all about, science?! [Discover]