Terminator Time Travel

In the Terminator films, Skynet, a computer program that controls nearly the whole world in the future, sends a machine to the past in order to kill John Connor, the future leader of the human resistance, at different points of his life: once before he is conceived (by killing his mother, Sarah Connor). In The Terminator, the machines send the T-101 and the humans send Kyle Reese: the first will give the people in the past the necessary components that will end up starting the Skynet project, and Kyle will be John Connor's father (that is, if the time travel hadn't happened then Skynet wouldn't have been created and John Connor wouldn't have been born).

A predestination paradox (also called causal loop, causality loop, and (less frequently) closed loop or closed time loop) is a paradox of time travel. It exists when a time traveler is caught in a loop of events that "predestines" or "predates" them to travel back in time. Because of the possibility of influencing the past while time traveling, one way of explaining why history does not change is by saying that whatever has happened must happen. A time traveler attempting to alter the past in this model, intentionally or not, would only be fulfilling their role in creating history as we know it, not changing it. Or that the time-traveler's personal knowledge of history already includes their future travels to their own experience of the past.

Terminator 2: Judgment Day, commonly abbreviated as T2, is set ten years after the events of The Terminator, it follows Sarah Connor, her 10-year-old son John, and a reprogrammed Terminator from the future as they defend themselves from a T-1000 and attempt to prevent Judgment Day, a future event in which machines will begin to exterminate humanity. The surviving arm and CPU chip of the original Terminator was analyzed and found that the technology was so advanced, they (humans) would have never invented the technology themselves and was used to create Skynet in the first place, suggesting a second predestination paradox.

Terminator 3: Rise of the Machines does somewhat negate any paradox by showing that the American military have secretly developed the technology independently, suggesting that Skynet was always developed this way and not from the arm of the T-101.

The predestination paradox of the Terminator movies neatly resolve the grandfather paradox by suggesting that time lines inevitably collapse into one sustainable sequence. Unlike a grandfather paradox, where each event negates the other, in a predestination paradox each event is dependent on the other. This avoids the logical pitfalls of the grandfather paradox because once both events occur, the timeline is complete.

An interesting aspect of the "loop" is that the initial events that gave rise to the loop need not be in the final loop. For instance, maybe the original Terminator timeline had no John Connor at all--instead Earth's savior was a man named "Ron Steadman" and initially the Terminators were sent back to kill him. The humans send Kyle Reese back to save Ron, and along the way he impregnates Sarah Connor and she gives birth to John Connor--who now becomes a greater hero than Ron and the target of Skynet's attack in the new timeline. Thus, Kyle Reese is now sent back to save John instead of Ron (who is forgotton) and the 'movie timeline' is born despite its seemingly trigerless series of events.

Another interesting possibility is that somethings have to happen--like the birth of Skynet. Since Skynet can manipulate time, it can send parts of itself back in time and thus will its own creation into a past where it otherwise would not exist. The terrifying implication of this is that if Skynet exists in one timeline it can put itself in all timelines; making it a truly unbeatable opponent. Alternatively, T3 suggests that Skynet must exist not because it sent itself back in time but because its creation is a necessary product of human nature--an invention as inevitable as the wheel. Either way, The Terminator series suggests a creative solution to the grandfather paradox: a 'guided' time travel where some events must or must not occur to create a cohesive timeline.