Last time we raised the question of why SkyNet did not kill Kyle Reese when it had the chance, and suggested that it did not know he was John Conner's father, but only that he was someone Conner wanted to find. Certainly Skynet finds out the connection when it downloads the data from Marcus Wright during their direct interface; there is no evident way for it to have known before that.
Yet there might be a different reason for SkyNet to have hesitated, and it has to do with Niven's Law.
One of the great paradoxes in time travel discussions, commonly called a grandfather paradox (although there are actually two distinct anomalies under that name), asks what happens if a time traveler kills his own grandfather before his father is conceived. In such grand terms it becomes the basis for great debates. However, the popular interpretation of Niven's Law assumes that once a change has been made in the past it remains changed, even if the cause in the future is thereby undone. Applied to our current story, it means that once John Conner is born in the past, that cannot be prevented by killing Kyle Reese in the future.
If at this point you are thinking that this is absurd, you are in ample company; John Conner obviously believes that the death of his father before he becomes his father would be fatal to him, even if that death happened in the future. Yet this challenges our fundamental notions about causality and time travel: would the death of Kyle Reese in the future matter to the birth of John Conner in the past, or not? More fundamentally, if the cause in the future of an effect in the past is undone, does that undo the effect as well?
Strict replacement theory would say yes, and Conner is working on that basis. Niven's Law provides a potential escape, but leads to this absurdity. If we think Kyle's survival matters, we do not believe Niven.
This is not the only place where this problem arises. If John Conner dies, he will never send Kyle Reese to the past; probably SkyNet will never send any terminator to the past if John dies here in the future, because he will have ceased to be a problem. Similarly, if the resistance wins the war here, now, then no one will be sent to the past and it will all be undone. These are the issues that the film raises. Without anyone traveling in time during this portion of the story, time travel threatens the history and future of the world. Any one of these changes could create an infinity loop, bringing about the end of time.
Fortunately that does not happen. Those who must survive--John, Kyle, Kate, and Skynet itself--all do. That, though, raises a perennial question about the entire Terminator series: is it fixed time? That is our question for next time.
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