I recently saw Donnie Darko for like the twentieth time and i must admit I love movies/stories based on time travel like The Terminator Trilogy, Back To The Future Trilogy and the one I already named. These movies and a general interest in the science/philosophy behind time-travel made me think about how these story arcs come to effect and how nicely they are created and what makes them initially possible, if time-travel would be possible. Most of these arcs are build around the theory of the "ontological paradox" shortly explained as the time-traveler not changing the future but simply being part of history by creating a consistent loop. Like if the machines in Terminator wouldn't have travelled back in time to kill John Connor before he was born then Kyle Reese wouldn't have travelled to the eighties and would never impregnated Sarah Connor. So the machine traveling back isn't traveling back to change history just to play it's part in history, by not only helping a hand in the conceiving of John Connor, but also by providing the initial chip on which skynet is created. So lets put things together and add some science. Machine travels back creating the blueprint for Skynet and causing the time travel of Kyle Reese causing the birth of John Connor who rebels against Skynet causing the machine to travel back in time. Since beginning and end are the same we conclude that these events are part of a loop, a consistent loop that is. If time-traveling was possible the only loops created would be consistent because a time traveler attempting to alter the past in this model, intentionally or not, would only be fulfilling his role in creating history and not changing it. The Novikov self-consistency principle proposes that contradictory causal loops (like the son travelling back to kill his father before he was conceived) cannot form, but that consistent ones can. We can find similar examples in the other movies mentioned, Heroes, comicbooks or within the terminator trilogy itself. Think about it!
