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.
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 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.
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".
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.