Naturally, any theory of time travel is just that: a theory. For the purposes of this FAQ, the best we can do is try to apply one or more of these theories, while still maintaining internal consistency with the info presented in the films. There are many theories of time travel in science fiction and comics. However, most discussions of time travel focus on two theories of 'real-world' physics: classic Newtonian and quantum mechanical physics. For a good introduction to the application of these theories to time travel, see the article, "The quantum physics of time travel" in the March, 1994 issue of Scientific American.
The classical theory states that there is one existence, and thus a single timeline. According to this view, changing an event in the past could theoretically retroactively change history from the time traveler's POV. This theory is plagued by problems of "temporal paradoxes". For example, what happens if you go back in time and prevent your parents from meeting? (According to the movie _Back to the Future_, you will 'fade from existence'!)
The quantum view is that time travel is possible along distortions in space-time called closed timelike curves; also, reality exists as a multiverse of infinite possibilities. Thus, if you travel back in time and prevent your parents from meeting, there's no paradox. Your parents still meet and conceive you in the timeline you came from (after all, you must have come from somewhere!). However, a 'version' of you will not be born in the timeline you traveled to.
bcw3s@fermi.clas.virginia.edu (Brian Christopher Weaver) writes: "The 'many-universes' interpretation of quantum mechanics solves a lot of time travel paradoxes. A time traveler can make any change in the past he/she/it wants to without endangering their existence because they came from a different universe whose timeline is untouched by their meddling.
Therefore, there really is no paradox in the Terminator movies. The Terminators and Kyle Reese came from a universe where the war actually occurred, but by the end of T2 a universe had been created where John and Sarah Connor lived with no global thermonuclear war. The original timeline still exists, however, in a parallel universe."
T2 implies that its world is of one existence and a single timeline. Certainly, it would seem to be futile to send someone back to change the past in a multi-universe existence -- unless one is very altruistic! Consider this: T2 implied that Judgment Day never occurred due to manipulation of the past. But it all depends which timeline one looks at:
1995 2029 -----|----------------|--(existence with nuclear war) (A) \_______________|__(alternate peaceful existence) (B)Assuming the existence of multiple parallel timelines, if a time traveler could change an event in 1995 (such as destroying the CPU chip), all that would result is another existence (B) branching off from 1995. Note that the nuclear war still happens in existence (A), even if an event in the past is changed!
What can we conclude? Quantum physics can explain the events of T1/T2 well. However, it does not make for a good story. Although saving humanity in a single timeline out of an infinity is better than none at all, this situation would likely not have been accepted by the moviegoing public. Assuming Judgment Day does not occur (as per the 'lost ending' of T2), JC wanted to show that there is "no fate" but what we make of it. This philosophy is reinforced by the 'single-timeline' approach.
So there you have it: good physics and a watered-down story, or a ream of paradoxes and a strong story. Anyone care to posit a hybrid? ;-)