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.

  • 1
    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, 2017 at 14:06
  • How do I physically get in the box? One leg at a time, same as everyone else. Jul 26, 2022 at 16:09

3 Answers 3


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.


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.

Annotated flow diagram entitled "Time Travel Method in PRIMER" It depicts a person before entering the box and turning it on ("Original person lives on undisturbed timeline and could decide to time travel").  Next the person exists in the period (between points labelled "A" and "B") when their double also exists ("Original person prepares for entry into box, existing alongside his time-travelling double").  After "B" the person is in the box, looping back to "A" ("Original person has an ambiguous temporal existence inside the box").  From "A" the person exits the box on to the new timeline labelled "C" ("Double emerges from box, and has 6 hours of causal influence on both his new timeline and the original person's timeline.")  Finally, beyond the point "B" at which the original person is in the box, the double continues ("The double creates an altered future for the double alone.  The original person loses his existence as such in the loop, because his future is only to step into the box.")

  • 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, 2017 at 3:16
  • 2
    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, 2017 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, 2017 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, 2017 at 12:46

I am adding an answer to this obscure question 5 years after it was asked because I too was reminded of this movie and had the same question.

Some background

First, to go over some of my understandings from re-watching it and reading a couple of explanations online. The box in Primer is meant to create a time-loop between two points in time, A and B:

  • Point A: The instant the time field is created - which is the instant the machine is turned On.
  • Point B: The instant the time field completely collapses - which only happens after machine has been switched Off - since it takes some small time (seconds/minutes?) for the field to collapse completely.

Though the box may be entered and exited at any point, it's meant to be more dangerous while the field is in full intensity. The two "safer windows" to enter/exit the box occur:

  • Window 1: While the field is building up, shortly after being turned On (aka shortly after Point A).
  • Window 2: While the field is dying down, shortly after being turned Off (aka shortly before Point B).

To travel backwards in time relatively "safer" a person must enter the machine during Window 2 and exit it during Window 1. This is what is usually illustrated in diagrams (such as the one posted in the current top answer).

The questions

The question which I had, and if I understood you correctly, this is your question well: What's inside the box while it's running? Because if it's a person, then as you said, you cannot enter it, as you're already inside it.

Additionally to this question, I was wondering what would be in the box in the original timeline once the field completely dies off.

Question 1

Q: What's inside the box while it's running?

To answer this, let's consider one thing: If you continue to wait inside the box without exiting, you would in theory loop indefinitely (barring some talk about rising probability of quantum collapse which limits how much time you can spend inside). While you remain in the box you bounce back and forth between points A and B.

Yet, while you are bouncing relative to the outside time, you (or any object inside) continue to experience time linearly. Meaning your proper time just runs in a straight line. Think about this: if you don't exit at point A (when the machine is turned on) then you start heading forward in time to point B again - but, this is the important part, you don't suddenly find yourself co-existing with your self that just travelled from B to A. At least to my understanding.

So a way to think about it is that inside the box there isn't a loop of time, but rather a straight line of time that starts at point A, and possibly continues indefinitely. While points of this linear time inside the box periodically correspond to points A and B, the actual time passing inside is linear (and questionably never ending, though you lose access to it from your timeline once the time field completely stops).

That was a lot to explain, so here's my rough paint drawing of the above explanation:
[Editor's note: This drawing has vanished and was apparently never archived. It is most likely lost unless the author comes back and replaces it.]

This is backwards time-travel, which of course is not real, so any explanation may fall short - but in this scenario, multiple sections of a linear timeline inside the box correspond to one section of the timeline outside the box.

So to provide an answer: Since infinitely(?) many of the timeline-inside-the-box sections are empty (in the above case, all but 1 of the zig-zagging lines are empty), when you open the box at any time it's very plausible that you would see an empty box. This is why you can get in just fine - when you open it, it's exceedingly unlikely that you'll collapse the internal timeline to the one which you are currently travelling backwards in time.

This also makes the diagram perhaps a bit inaccurate since the insertion point doesn't have to be exactly on the first section ("zag" backwards from B to A) of the timeline-inside-the-box, but can be on any of the following (from internal points of view) "zags".

Question 2

A question I had, hopefully with a much shorter answer following from the above conclusions.

Q: If someone else standing and watching a person enter the box at point B before the field completely collapses, and then opens the box after the field has collapsed what do they see?

Well, considering the timeline inside the box doesn't "exist" at the time on the main timeline, then the answer is that it's an empty box with no special properties, just normal time inside. This in theory means that if you had a transparent box and something inside it, the moment the field collapses, the thing inside it would just vanish.


On the transparent box: The question of what we would, in theory, see if we had a transparent box is a good way to explain exactly what you were asking. However by the explanations discussed in the film and online, a transparent box would break the entire process of time travel. This is because observation = interaction in the quantum world, so any observation would collapse the timeline and the observer would only see 1.

Does that mean that the inside of the box needs to be completely un-entangled with the outside world? Yeah - I would guess this is why they talked about using Argon inside the box, though, that really isn't enough, and true un-entangled state on such macroscopic scale would be probably impossible to achieve. But this is science fiction of course.

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