Ever since I first watched Terminator, the central plot of future Kyle Reese coming back to the past to impregnate Sarah Connor thus giving birth to John Connor who goes on to become the resistance leader in a post-apocalyptic Earth against Skynet where future Kyle was sent back in the first place, is a paradox that I tried but failed to comprehend*. At each turn, there appears to be more questions than answers. For instance:
- How could John Connor be born in the first, first place when his father wasn't born yet?
- If humans were already by and large decimated on Earth, how would it be cost efficient for Skynet to go back to the future to kill off Sarah Connor before John was born, instead of just finishing the job? After all, they are not using humans like batteries like in Matrix.
- By killing Sarah and John, how does Skynet know that the time line won't be altered in such a way that would put them in greater jeopardy?
- How did future John know that Skynet is building, much less planning to use a time machine to kill his mother or himself?
- Why didn't Skynet send an entire army back to the past to get the job done?
- Why did they send T1000 to kill him as a teenager when they could have sent T1000 back to deal with Sarah Connor before John was born, and keep sending more to the same time line? Wouldn't it be easier to kill Sarah before she developed an awareness of how dangerous Skynet is and go on a crusade against the creation of Skynet?
- Why create a time machine when an easier way to wipe out humans would be to create a virus, or nuke the world some more so much that no one can survive the ensuing winter?
There are also other questions regarding the time travel itself:
a) why is it that only unclothed humanoid form could travel time?
b) if a human holds on to a gun, the gun will not make the trip. What if the gun is grafted on the human's arm?
c) it seems that a cyborg or even a robot in the case of T1000 could make the trip so long as they are in humanoid form, but what if a human has unusually long arms or legs? Will those "extended" limbs make it?
At a "scientific" level, the altering-the-past-time-travel-to-correct-the-future concept is premised upon a single stream of space-time continuum - i.e. if you reset the past the future will change. But there are other theories - multi-streams such as Back to the Future - where if you reset the past, the past will branch off another reality, with the "current" branch of reality continuing on and on. Which is more theoretically plausible?
* One possibility that I speculate is that in the original time line, there was no John Connor, but due to Skynet tampering with the infrastructure of physics itself, a temporal loop was created whereby a human was sent back in time, and by accident created a resistance leader. Skynet has been trying to correct this error ever since and we don't know the ending yet.