It works pretty well under the "divergent timelines" approach to time travel.
Right, that's my recollection, too. As I say above, a "divergent timelines" approach can indeed resolve the logical contradiction of the grandfather paradox.
The thing is, when you take such an approach, you have to be careful not to mix it with a single timeline. This is a common mistake a lot of films and TV shows make, including the Back to the Future series. Each change to the past has to create its own tangent timeline, rather than erasing the original. That's how you avoid the paradox.
But, when you have a bunch of copies of characters from erased futures existing simultaneously in the same timeline, it's usually indication there's only one timeline that's being repeatedly rewritten. If there were multiple timelines, most of these copies would exist in different futures, rather than the same one like in Primer.
As I say, I'm too lazy to go back and work out all the loops and resets. I'm not even sure it can be done since so much of the time travel takes place offscreen. But the existence of multiple copies of a main character at the end strikes me as at least prima facie indication the film mixes divergent and single timeline approaches.
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carl_johnson-1
9 years ago
I really don't see a huge difference if old timelines get erased or still exist.
Say I build my time machine, get in and go back in time to before I get in the machine. I stop myself from entering. Now there's 2 of me.
With divergent timelines, the original timeline just has me get into the machine and disappear. It might as well be a disintegrator from that timeline's perspective. Life goes on without me.
With the new timeline, there's two of me. And we timeshare whatever we can, play tricks on people, and/or can't stand each other and it ends in murder, who knows?
From this new timeline's perspective, it doesn't matter if the original timeline gets erased or not. If it stays, no problems. It it goes, and we're dealing with a single-timeline that's changeable, I still think there are no problems.
If the old timeline gets erased, that shouldn't erase the me that arrived in the new timeline. I'm there already, and there's no physical force connecting me to the original timeline anymore. Sure it looks like my existence blows up "Cause and Effect" and "Conservation of Energy/Matter", but what are they going to do about it? :)
With divergence, the two timelines don't interact anymore after the arrival point, so either timeline may as well consider that the other doesn't exist. And if the original timeline doesn't exist, well, it also can't interact with the new timeline anymore. It doesn't exist, so can't interact.
It's why I really see no difference between the two approaches. Even with a single changeable timeline, you can just consider that the other timelines may still exist, even if they don't.
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thebigmouth
9 years ago
I really don't see a huge difference if old timelines get erased or still exist.
Say I build my time machine, get in and go back in time to before I get in the machine. I stop myself from entering. Now there's 2 of me.
That's actually the essence of the grandfather paradox. If you never time traveled, then how are you there to stop yourself from entering the time machine? Your presence in the past is an effect that no longer has any prior cause.
Physicists disagree whether time travel is theoretically possible. But even proponents of the possibility take it as a given that causality violations like the grandfather paradox aren't allowed. Kip Thorne actually co-authored a paper addressing the same basic scenario you describe (i.e., shooting a billiard ball through a time machine such that it deflects itself before entering).
http://authors.library.caltech.edu/6469/Divergent timelines aren't super scientific either, but at least there's no causality violation. If two timelines exist simultaneously, then there's still a reality in which you time traveled into the past to stop yourself. The chain of cause and effect is unbroken.
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carl_johnson-1
9 years ago
And we really don't know if Primer contends that prior timelines still exist or not. From the point of view of the "final" timeline (3 Aarons + 2 Abes), whether they do or do not shouldn't matter.
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thebigmouth
9 years ago
We don't know for sure, no. But like I explained above:
when you have a bunch of copies of characters from erased futures existing simultaneously in the same timeline, it's usually indication there's only one timeline that's being repeatedly rewritten. If there were multiple timelines, most of these copies would exist in different futures, rather than the same one like in Primer.
I think if you make an effort to actually map out a simple time travel hypothetical where each change to the past creates a new timeline, you'll see what I mean. There's an inherent limit to the number of "copies" of yourself that can co-exist in a given timeline "branch" in such a scenario.
BTW, another movie that I think we'd agree gets the logic of time travel correct is Interstellar.
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