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In the film Primer, past time travel is achieved by means of powered boxes: you switch the box on at the "destination time", switch it off some time later, enter, wait for the same amount of time, and exit the box to emerge at the past destination time.

Initially the characters state that the box is "single use" in that sense. The box mustn't be disturbed during the time it's active, since during that time it contains the future occupant "travelling backwards". But later on, it is revealed that the boxes may be collapsed and taken along inside another box, and that they are thus "recyclable". What does that mean? How can you take advantage of packing a box inside another box in order to travel back further or more often than once per box activation? If you take a folded-up box into the past, you won't have the box anymore in the future, so it seems that nothing is really gained or lost in the big picture.

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The simple answer (hah!) is that they aren't recyclable in the sense of being able to be used multiple times, but that a powered-on "time travel box" can be carried into a time loop, allowing the creation of a sub-loop (or cycle) that sits inside or outside of the new loop.

Primer Box

The picture above shows a graphic of the operation of a normal box. When you enter a box, you travel backwards in time the same amount that you spend in the box, up to a maximum time when you originally powered the box. The box itself can be powered on, then folded up as long as it's unfolded at the time you intend to emerge.

The confusion seems to come when someone carries a second (folded-up but powered) box, into the original loop, creating a second "loop within a loop". As the participant emerges from the second loop, they now have the choice of continuing to the end of the loop or returning to the sub-loop without creating a paradox. We see this when the protagonists repeatedly attempt to influence the outcome of the party.

Time Travel in Primer

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  • Interesting. So does the inner box continue "building up backwards time" as it's being transported? Say I have two boxes, run the first one as normal, but just when I enter the first box, I switch on the second one, fold it up and take it with me. When I get out and enter the second box, where does that take me?
    – Kerrek SB
    Feb 9, 2014 at 17:44
  • And for the other extreme: Suppose I switch on both boxes at once, and when I'm ready to enter the first, I fold up the second and take it along. Where will it take me?
    – Kerrek SB
    Feb 9, 2014 at 17:45
  • @KerrekSB - In answer to your second question, you could easily end up with a nested dolls paradox where you're inside both the smaller box AND the larger box simultaneously. In-universe, the situation would resolve itself when you finally caught up with your original timeline and entered the first box. I like to think of it as a programming loop. The first box is a "goto 10" whereas the subsequent boxes are gosub loops.
    – Valorum
    Feb 9, 2014 at 18:11
  • In fact, if the occupant of the box experiences normal forward time, then you could "age" the second box while inside the box, and thus go back indefinitely: Go into the box with the second, folded-up and running box, emerge, pack up the box you emerged from, unpack the box you brought along, and enter that box with the folded-up original box. Repeat.
    – Kerrek SB
    Feb 9, 2014 at 19:56
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    Wait what? Folding a powered on box? I thought you'd just carry a folded up and deactivated box (or even just parts that you can assemble very quickly), and then unfold and activate it very soon after you exit the box.
    – b_jonas
    May 4, 2016 at 23:34
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The Boxes can be duplicated just as Abe and Aaron are duplicated. If you bring a disassembled Box back in time, you create another timeline where the Box is still there. Now you have two Boxes. You can assemble the Box you brought with you somewhere else without anyone else being aware there's an extra Box, and now you have a contingency plan in case what you wanted to change doesn't turn out right.

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The boxes are "recyclable" in the sense that the same "box activation" (i.e. turning a box on) can be used to travel backwards multiple times without requiring the box to be reset. This is in specific contrast to the operating model assumed by Abe at the start of the story, in which he describes the boxes as "one time use only" because he believes that there is a single, unified timeline.

For example, if you were to set the timer for a box so that it powers on at 8:00 AM on Monday, then turn it off and get into it at 9:00 AM, so that you could travel back one hour, you would emerge at 8:00 AM, as you expected. However, you are now in an "offshoot" timeline that is different from the one from which you just traveled back. If, at 8:05 AM, you are looking at the powered-on box that you just came out of five minutes ago, it is "empty" in the sense that it doesn't, at that moment and from your perspective, contain your past you coming backwards through time. (This does not agree with the assumptions behind the rules for time travel developed by Abe, but there is plenty of evidence in the movie to demonstrate that there are multiple independent timelines.)

As an example: If you wait around until 9:00 AM, there will be another you (per the movie's lingo, your "double") showing up to get into the box. If you're planning to stick around in this timeline, it's preferable to avoid this encounter and to allow your double to do so, since then your double will exit this timeline and there will be only you left here. However, it's not required that you avoid this encounter. If you stayed around until 9:00 AM, you could probably prevent your double from entering the box. Congratulations! You have just created what seems like a paradox. But, let's ignore the philosophical implications and look at the box again. It's past 9:00 AM, and nobody has turned it off yet. There's your proof that this is a different timeline from the one that you left when you yourself traveled back to get here.

Suppose that, after your encounter, your double wanders off reflect on the novelty of meeting oneself... or suppose instead that your double has been conveniently drugged and locked in an attic by your actions. The box is still on at 9:30 AM. If you wait another half hour, it's still on at 10:00 AM. What happens if you turn it off at that point and get in yourself? Two hours later (from your standpoint), you'll be back at 8:00 AM, in yet another offshoot timeline. There will be a "double" of you here, as well, who is expecting to travel back at 9:00 AM -- just like the double that was there the last time you came through, and really just like you yourself were expecting 15 minutes after you first set the timer.

What if you wait half an hour, turn off the box at 8:30 AM and get in yourself? Your double will probably wander in at 9:00 AM and be surprised to find the box is not even on. But who cares? You can never get back to a timeline you've left, so it's all theoretical, anyway. From your standpoint, you've just gone back to 8:00 AM on Monday morning for a third time.

You've only ever turned on the box once. It's the "same" box in all three timelines. (This is where we discover that the definition of the word "same" is insufficient for dealing with the rules of reality as implied by the box.) But you've already used it three times from that single initial activation. The box will always be "empty" in any offshoot timeline, so it can be used to travel backwards again and again with just one box activation, by someone so inclined.

In the movie, Aaron and Abe are both struggling to understand and express their ideas of how the box works and its implications, so "recyclable" is the best way Aaron can describe the above. I don't think it's directly related to the fact that Aaron takes another box back with him when he discovers and uses Abe's failsafe -- this is just a way Aaron figures out to "smuggle" a working box back to a time before Abe has completed (or at least before he has turned on) his second box, which Abe intends to use for his own first trip back.

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