So I've watched a couple videos and I think I have a pretty firm handle on the plot structure of Primer, but I'm still confused about some of the details about time travel.

Suppose it's 4:59pm and you're planning to enter the box at 5pm with the intent of getting out at 9am. You have a double (subjectively, you two minutes from now) in the box, travelling backward. How do you get in the box? Your double won't "vanish" from the outside perspective until after you get in, since every instant you're outside the box, you have to relive while inside it, just travelling backwards. This seems to me to cause problems for the actual physical act of entering the chamber -- you'd need to somehow physically merge with your double, which sounds like it'd probably be fatal for both versions of you. This is never mentioned in the film.

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    I think I understand what you mean. Just as an impulse : Primer uses a mutable timeline model. So perhaps it's just this : In the "original timeline", the box is empty. You can enter it without hitting a double. This causes a change in the timeline so that you now are in the box "all the time" (traveling backwards) and leave it in the past. So we have moved the problem to this new timeline, when later (if you don't prevent it) your other self tries to enter the box. But perhaps leaving the box in the past causes another timeline change, leaving the box empty again.
    – Hothie
    Jul 13 '17 at 14:06

Your confusion is twofold:

  1. The double exits when the machine is turned on, not when the other person enters, and they use a delayed switch to avoid being there.
  2. There is only ever one person in the box, the one who entered it, and exits when the machine is turned on.

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  • I gather that the double exits when the machine is turned on, but the double must vanish (not exit, but rather cease to exist in that timeline) after (in external time) the original enters the box. That is, if you opened the box between 9am and 5pm, you would find the double inside, but if you did so after 5pm, the box would be empty, no? That is my question -- the moment that the double vanishes cannot occur earlier than the original's entry to the box. Since it is impossible to enter a box instantaneously, what would the person entering see/feel?
    – SciFan
    Jul 5 '17 at 3:16
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    To pose my question another way, suppose the box was transparent. What would someone standing in the room see as someone entered the box? Before entry, they should see both the double traveling backwards (i.e., in reverse motion) and the original preparing to enter. What would they see the double doing as the original entered the box? Presumably, entering the box in "reverse", but at some point both the double and the original cease to exist. In the diagram, this corresponds to how there are three simultaneous stretches (i.e., copies of the traveler) between A and B, but only one after B.
    – SciFan
    Jul 5 '17 at 3:22
  • One of the ways in which Primer remains self consistent with the time travel is that, when someone exits the time machine that had been turned on, that is now the timeline. Before they enter the box, it's empty, as seen in how they can disassemble the box and take it with them.
    – FuzzyBoots
    Jul 5 '17 at 12:37
  • It is theorized that if you refuse to enter at the "end" of the loop, you sicken and die, as per Thomas Granger, but it's never proven, and might have just been the result of too long in the box without food or oxygen.
    – FuzzyBoots
    Jul 5 '17 at 12:46

Your double won't "vanish" from the outside perspective until after you get in, since every instant you're outside the box, you have to relive while inside it, just travelling backwards.

No, because you don't enter the box while it is running. You turn it off, then quickly get in while the field is still sustaining itself in the feedback loop mentioned early in the film.

This is, from my understanding, the real reason they time their entrance into the box (the reason they time the exit is of course so they don't loop back forward in time again). My write-up is a bit speculative, but is supported somewhat by events in the film.

When the box is turned on, the field inside gradually builds up; as an audible cue, the humming sound raises in pitch and intensity. This is the one end of the "parabola" of the time loop that Aaron draws on a piece of paper early in the movie when he and Abe are figuring out how it works.

When the box is turned off, the field sustains itself in a feedback loop for a little while (exemplified early in the movie when Aaron disconnects the batteries from the prototype) and the humming sound similarly gradually dies down after a little while.

The time traveler inside the box is only traveling backwards in the case of all the travels in the movie, but in theory he is looping back and forth between the start time and the end time on the parabola, like the weebler in the experiment early in the film. The time traveler exists in the box while the field is active; when it is starting up or dying down (which is the same thing for the traveler¹), the traveler "bounces back" to loop the other way around. Therefore, while the machine is turned off but the field subsiding, the box is empty; while the machine is on and the field is stable, it is occupied.

The time traveler must time his entrance into the box to within this window where the field gradually dies down. That's when the field is still sustained, and can loop back, but the box is switched off and so the field will eventually die and the chronological endpoint (as seen from the outside of the box) of the field will be reached.

Like I said, this is my interpretation – it runs a little counter to this well-known explanation of how the box works, which states the following:

Actually, windows A and B are arbitrary. It is possible to enter or exit the box at any time. However, doing this is dangerous. The more strongly the field is running, the more dangerous it is to do this. This is why Aaron and Abe only do it when the field is weak, i.e. when the box has just started powering up or is just about to finish powering off, and why Aaron feels seriously ill after his first trip-- he exits very slightly too early/late. In addition, entering or exiting the box is never truly safe because the field is always active to some extent. There is always a small static shock, and Aaron and Abe experience cumulative ill effects even though they always use the boxes carefully.

I don't see any evidence for this in the film, however, and this explanation would mean that the box is physically empty to the outside observer even when there is a person traveling backwards in it, which contradicts Abe and Aaron's earlier experiments with the weebler (which they observed using a camera inside the box; conceptually the same thing as the transparent box you suggest).

The physically "ill effects", such as the bleeding ear, hand tremors and (in Granger's case) inability to stay conscious near Aaron, instead seem to me to be caused by the timelines diverging, as alluded to in this question/answer. It's hard to know for sure, though.

¹The coming time traveler turns off the running box, and while the field is shutting down and the sound decreases, he quickly (as both Aaron and Abe do on their first trip together) gets into the box. Inside the box, he will start traveling backwards in time, and the humming sound will also reverse, increasing in intensity. When it becomes time to exit the box, the sound dies down subjectively for the traveler, but that is of course just the reversed sound of the field starting up to an outside observer.

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