Time travel in Primer

Primer is an exceedingly complex and sophisticated movie and much exposition was omitted from the final cut for the sake of brevity. I do intend to create a writeup which explains the full sequence of events in the movie, but here, first, is some prerequisite knowledge: how time travel works.

The channel

The time travel boxes used in the movie work like this. When the box is switched on, a "channel" begins in spacetime. That is point A. Call it 12:00pm. Over the next few seconds or minutes (depending on the box size) an electromagnetic field builds up gradually in a parabolic form until it plateaus.

Later, the box is turned off. This causes the field to diminish parabolically to zero. This closes the channel at point B, let us say at 12:01pm.

A weeble placed within the confines of the channel - that is, within the boundaries of the box and between the times of 12:00pm and 12:01pm - does not move through time in the normal sense, because time does not flow within the channel in the same way that it does in reality. Instead, the weeble wanders from point A to point B and back again, backwards and forwards, over and over, in an indeterminate state, gradually accumulating time from its point of view.

This continues until the weeble collapses and it is removed - either at point A or point B. Every time the weeble reaches end A or end B of the channel, there is a finite probability that the waveform will collapse and the weeble will exit, as opposed to continuing on another trip along the channel. Actual hard figures about this are fairly sketchy in the movie, but it seems like the probability of exiting the channel is about one in ten thousand. After roughly 1300 trips from A to B, the probability is better than even that the weeble will exit. So, the amount of time that the weeble spends in the channel (from the weeble's point of view) is variable, but usually works out to roughly 1300 minutes.

Collapsing the waveform is the tricky bit. As in the famous Schrodinger's Cat experiment, the waveform is only collapsed when the box is opened. Of necessity, when the object involved is an inanimate weeble, the box can only be opened from the outside, by Aaron and Abe, at point B, at 12:01pm. Therefore, the weeble always exits at 12:01pm.

  • If the weeble is put in at point A, the amount of time experienced by the weeble (or their watches, when they try it) is always an odd number of minutes because the object has to make an odd number of trips to end up at point B. In the movie they get 1347 minutes on the one time they try it.
  • If the weeble is put in at point B, while the box is powering down, and then removed again at point B, the weeble must make an even number of trips. In the movie they get 1334 minutes on the one time that they try it.

Almost immediately once they discover this, Aaron and Abe reason that if they could create a device which had some degree of intelligence, like a programmable miniature robot or something, they could program the object to measure the amount of time that has passed and then spontaneously trigger the collapse of its own waveform from inside the box, thus selectively exiting the box at point B or point A. From this they imagine a box where the robot is inserted at 12:01pm, point B, waits for one minute, climbs out of the box at point A (12:00pm), and thereby travels backwards in time.

Aaron and Abe skip this step entirely and jump straight to building coffin-size boxes so that they can travel through time themselves.

Operating the box

When the box is turned on the field builds gradually. During this build-up there is a period during which the box is still turned on but the field inside it is still weak enough that an object can exit or enter it, and thereby drop into the indeterminate channel. This narrow window of a few minutes is point A. The channel persists until the box is switched off later. When this is done, the field diminishes and again there is a window of a few minutes in which an object can enter or exit the box safely. This is point B.

Operating the box becomes a matter of timing and preparation. The operator, let's say Aaron, turns on the box at the time he wishes to exit and then walks away so that he is not present for window A. Later, Aaron returns to the box and switches it off. As the machine powers down, window B opens, and he climbs inside. Time passes and the machine switches off entirely. At this point, instead of exiting, Aaron "bounces" off the far end of the channel and begins to loop around backwards in time instead of forwards. Because time is now running backwards, the box appears to power up again from Aaron's point of view. Aaron waits until the box appears to power down again (in reality, this is point A, where it powered up), and then exits the box through window A in a timely fashion. He is now Aaron-2.

Aaron-2 must avoid Aaron-1 for the rest of the day. Why? Because if they interact with one another, or the course of events of the day is altered in any way, then it would be possible that Aaron-1 would never get into the box at window B. This would not rupture the timeline or anything stupid like that, but it would mean that there are two Aarons in this timeline. Permanently. And only one identity between them.

While inside the box, the operator has no way of knowing how strongly the field is actually running. Exiting the box during one of the safe windows, then, requires advance calculation of the precise amount of time that the box was running, and you have to take a stopwatch into the box with you to make sure you stay in for the right amount of time. In theory, you could do what the weeble does and stay in the box for months or years, making multiple round trips before exiting. Indeed, if you died inside the box, then eventually your dessicated corpse would emerge at point B, having spent subjective years going around and around the loop before exiting. However, this is not explored in the movie.

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.

When the weeble is being experimented on, it is possible to put the weeble in the box, switch the machine on from cold (point A), switch it all the way off again (point B), and then remove the weeble afterwards. This way, the weeble enters and exits the channel when the field is at precisely zero and so there is no danger to travelling through in time in this way. However, this is impossible when travelling backwards in time. You have to enter the channel before it has completely switched off and exit it after it has begun switching on. Thus, travelling back in time is inherently more dangerous than travelling forwards in time.

The box has to be flooded with argon, an inert, harmless noble gas, to operate, which is why Aaron and Abe have to use oxygen tanks on their trips. Because the boxes are small, dark and quiet, they also function like conventional isolation tanks, which is why Aaron and Abe dream inside and feel weird afterwards. This is probably also due to the sedatives they take before entering. Either way, it has nothing to do with the fact that they are travelling backwards through time.

Structure of the Primer timeline

The topology of the timeline is a straightforward forking model. Each trip through time creates a new timeline, divergent from all previous timelines, in which different things can happen. However, Abe's precautions of non-interference are still a good idea, as this ensures that a time-traveller's past self enters the box and departs the timeline on schedule. If they do not, there are two of the same person in the same timeline, which raises substantial practical problems.

Exactly what would occur if you shut off a working coffin while you knew somebody was inside it (i.e. because you watched them climb out at point A), is not remotely clear.

Next: what actually happens?

Back to Time
Back to Things Of Interest

Discussion (26)

2009-06-26 23:10:11 by YarKramer:

Do you think this "guide to Primer" you're making would be more helpful before/as preparation for watching the movie, or afterward/as explanation?

2009-06-27 00:48:18 by JeremyBowers:

On the one hand, I would suggest that you watch it first, or the explanations aren't going to make much sense.

On the other hand, I read a timeline once before seeing the movie, and to be honest it didn't spoil it much either.

Whether this is the sign of a great movie or a terrible movie, I'll have to leave in the mind of the beholder. I'm not sure I know either. :)

(Well, I guess I certainly can't call it terrible. If you're a time travel fan, it's certainly one of the definitive time travel movies you should see, no question.)

2009-06-27 02:51:21 by Randall:

I've seen a timeline somewhere where someone claims to have mapped out the whole movie, but I can neither confirm nor deny whether it's accurate; honestly, the last 10 minutes of the movie only showed maybe 1/4 of the plot-relevant events that would be necessary to know what the hell was happening, and the rest needed to be inferred.

2009-06-27 04:18:33 by Ross:

I believe this is the chart you are talking about:
http [colon slash slash] neuwanstein.fw.hu [slash] primer_timeline.html

2009-06-27 13:39:35 by Raphfrk:

I came to the same conclusion as you when watching it, they should have used an automatic device + timer to test out the effects, rather than using it for themselves. Granted, had they done that, there would have been less action in the movie.

All they would need is to set up the channel so that a toy car can drive in an out of the channel via a door under timer control. It might be worth having 2 doors.

The process would be

1) Power up field
2) timer in car is reset to zero

4) begin powering down field
5) Car records current time and drives into the channel

6) Field completes power-down and "past car" has disappeared

The car should have some way of carrying things. For example, it might be able to carry a sealed envelop (or a USB stick). The USB stick would allow them send back detailed info.

I would then start experimenting (which should always happen *before* human trials)

Experiment 1 - non-paradox version

I write a letter and and place the envelop in the past car at the start of the experiment.

Expected result:
At the end of the run, the past car will have disappeared and only the future car will remain.

The envelop in the future car contains what appears to be the letter. I can even read it before the past car disappears. The letter matches.

However, if I run the experiment lots of times, then sometimes no future car appears (future car didn't "bounce"). Also, sometimes the past car doesn't disappear (past car doesn't bounce).

Both of these events have the same probability, but don't have to happen in the same run.

The conclusion from this is the multi-timeline scenario.

If the past car doesn't disappear, then there will be no future car in the "next" timeline. Similarly, if there is no future car, then the past car didn't disappear in the "previous" timeline.

Experiment 2 - paradox version

The multi-timeline results (sometimes you end the experiment with 2 cars) indicate that paradoxs won't destroy the unverse, so this experiment attempts to force a paradox.

I write a letter. If the future car comes back with no letter, then I place the letter in the past car. Otherwise, I leave the past car empty.

Expected result:

If the future car brings a letter, then it will match the one I wrote. If it brings back nothing, then the experiment will end and I will have no letters at all. Also, after multiple runs, half of the time I end up with 2 letters and half of the time I end up with none (plus the low probability 1 in 1300 results).

The conclusion here is that half of the timelines now have 2 letters and half have none. In effect, mass is being moved between timelines.

Experiment 3 - timeline counting

I start the experiment. If the future car is empty (or no future car), then I write a zero and place it in the past car. Otherwise, I read the number from the future car and increment it by 1 and place it in the past car.

Expected result:

I should get a random number generator. Numbers 1 to 1300 should be reasonably equal probability but after 1300, it should start to drop.

In effect, the experiment counts timelines since the last time a bounce failed.

Experiment 4 - "Time loop logic" computer

This system could be used to factor numbers using the time travel computer effect. If there was no "bounce-failures", then you could just use the counting method. To factor N, you just need to follow the rule "assuming that the number received is A, if A is a factor, then send back A, otherwise, send back A+1 (or 2 if A = N-1)". If N was prime, the result would be a random number. Even without bounces, there could be issues with people messing up the transfer. The odds of a mis-copy would need to be much lower than 1/N.

Even with bounces, I think it should be possible to still get it to work by using multiple machines, maybe one per digit.

The MSB channel would be powered up first and LSB last, and then the channels would be powered down from LSB to MSB channel.

In the middle, you have all the numbers from the "future" cars, but haven't decided what to send back in the past cars yet. You would work out if you have a factor, if not, increment the number by 1. I think this works as long as the odds of each channel failing to bounce is much lower than 1 in 10.

2009-06-27 13:42:57 by raphfrk:

I never seem to post properly to this site.

Anyway, I had included steps in html-like brackets and they were stripped.

This is what it should have said.

The process would be

1) Power up field
2) timer in car is reset to zero
("Future" car emerges)
4) begin powering down field
5) Car records current time and drives into the channel
("Past car" timer set up so that it leaves channel at time A)
6) Field completes power-down and "past car" has disappeared

2009-06-28 03:29:36 by RGB:

The problem with using an automated device is that it would not work under their assumptions. The weeble coming from the future would have to come from another, prior, timeline. They, however, think that they are in the original timeline (which is later shown to not be true).

2009-06-30 06:55:58 by Anon:

Odd, I saw this movie posted on /b/ a couple of days ago on a post for mindf$ck flicks, and here it is. Hello fellow Anon?

2009-07-08 21:49:22 by JonnyAxehandle:

Sam an Anon? Wouldn't be too much of a stretch.

Also Primer is a great movie and this guide won't spoil it. Even WITH a guide the movie is pretty hard to grasp with just one watch.

2009-07-20 15:27:58 by Colin:

I'm in the UK and have never heard of this movie. I shall find it though, as it sounds intriguing.

2009-07-23 07:53:34 by Anonymous:

Raphfrk talked about what I see is a Primer Computer. I think this is (besides a Zero Point Module of sorts) one of the most interesting devices one could build in the Primer universe, a computer that would stay in the loop until the program required user input or had any output to show, thus reducing the computation time to a mere instant.

I have been asked the same "captcha" question both times I have posted here btw.

2011-05-26 19:08:31 by Benny:

That 'Primer computer' is actually very similar to the idea of what a quantum computer would do.

I think it was Hans Moravec's book that had something relevant in it as well - a device that takes one second to compute AND goes back one second in time, so it can take however (near infinitely, if necessary) long it needs to do a calculation, looping back over the same second over and over as many times as it takes, but will spit out the answer immediately when the switch is flipped. If anything like that were possible, it would be the end of everything in a very short time... the (inevitable!) problems that plagued Abe and Aaron in Primer would look tame compared to the changes such a device would bring.

Lest that sound too irrelevant, physicist John Kramer's Transactional Interpretation of Quantum Mechanics explains away all of the weird stuff in QM (like Schroedinger's Cat, and the experiment with two slits, etc) by incorporating waves that travel back in time that *may* actually exist... the solutions to Einstein's (and Schroedinger's) wave equations have always had two answers (as Abe points out, in the garage in the Weeble scene), one positive and one negative, but the negative one has always been disregarded because going backwards in time seems absurd. Only lately have people realized that maybe the math has been telling us something important all along.

There's also some relevant theoretical discussion in the comic book 'Planetary' by Warren Ellis... they are discussing turning on a device that is essentially a time machine, and one guys says that the second you turn that on, that's the end of history:

"Look, if you're in the future, and you've got a time machine, and you're interested in history, what's the first thing you'd go and look at?"
"God, I don't know. Dinosaurs. The crucifixion. The great flood."
"Sure, but you can't. The furthest back you can go is the point where the first time machine was switched on."
"So I'd go back and look at that."
"You and everybody else. Everyone from the entirety of future history arriving at once, the second after you flipped the switch. Therefore, the whole of the future can be said to have happened at once. And you can't change it, because it's already happened... That's planetary apocalypse condition, really. What's the point of anything it it's all already happened? It's Schroedinger's Cat writ large... the future currently exists as a mass of probability waves, collapsing into choices and events one at a time as we move forward. Turning on a time machine collapses them all at once."

... or, as Abe found out, maybe you *can* change things, but there will be consequences. And you'll never be safe from anything again. (That comic [#27] actually has a lot of relevance to Primer, and it's a brilliant story worth checking out for other reasons. The issues are collected into a graphic novel but the final issue, #27, took years to come out and as far as I know isn't collected in a book yet, so you'd need to get the graphic novel and #27.)

2013-01-20 09:45:39 by dragonofdrakness:

Hi guys, there is something that I do not understand about the time travel model in Primer. When they speak of the object inside the device traveling from point A to point B, shouldn't they really be talking about the PROBABILITY of the object appearing at point A or point B. If we are talking about collapsing waves shouldn't it be based on wave probability, since that is how particles theoretically travel. If the object is in an indetermined state, does this not mean that we cannot tell where it is, only the probability of where it could be. So when they talk about 1300 trips, really they mean that the probability of the most likely place of the object moves from point A to point B. In addition, I thought that the trip from point A to Point B was an exponent of the total amount of time the device was on for. Finally, I thought the reason they entered/exited the device at specific times, was because the time within the object was traveling, such that even the gas was displaced in time, so entering at any other time would be equivalent to entering an elevator shaft, when the elevator was on a different floor.

2013-03-03 11:43:17 by DanKaplan:

Extra fun note for your timeline which rocks:

When you noted Aaron exits the time machine/box the first time in the movie, you noted the given explanation in the movie as to why he felt sick. That is because he exited too soon/late. Interestingly, we believe this because he's inexperienced, unlike Abe. Except for the fact that this inexperienced Aaron is actually very experienced (relatively speaking) time traveler Aaron3! (As we almost never see Aaron1 during this movie at all...he's just the rat sound in the attic.)

When Aaron3 leaves the box early/late, he is actually taking another blow purposefully repeating his mistaken early exit again presumably to keep everything the same per his crib notes.

2013-10-19 04:48:30 by mwein:

I appreciate the explanation, but the majority of it was merely a summarization of the movie as opposed to really explaining anything.

2013-12-19 07:55:26 by Rich:

I'm late to the party, but I'll contribute, seeing at the OP posted five years after the film's release. Relative to my timeline, the movie is still 2-days fresh.

"Aaron-2 must avoid Aaron-1 for the rest of the day. Why? Because if they interact with one another, or the course of events of the day is altered in any way, then it would be possible that Aaron-1 would never get into the box at window B. This would not rupture the timeline or anything stupid like that, but it would mean that there are two Aarons in this timeline. Permanently. And only one identity between them."

Given the logic of the boxes presented in the film, an Aaron-2 ONLY exists because 6 hours in the future Aaron-1 got into the box to move back in time to become Aaron-2. That's the whole premise. Aaron-1 collects the information about the future that gives Aaron-2 more information than Aaron-1 had at the moment the box is first turned on. Maybe it was a cautious move for Aaron-1 (or Abe-1) to step away from the box after turning it on, but would he have stayed to observe, both versions of Aaron could have confirmed that Aaron-1 did indeed enter the box in the future when Aaron-2 steps out. If Aaron-1 had stayed near the box when he turned it on, and never saw Aaron-2 step out, then he can presume that he himself (as Aaron-1) never enters the box (or at least that box).

The film seems to violate its own logic -- version-1 must exit time in the future, by entering the box, for his double (version-2) to exist. Hence why it's a challenge to figure out how to interpret the clues that there are permanent multiple versions of people when the logic presented by the film doesn't seem to account for that.

Having said that, the filmmaker's theme of the dissolution of trust, and the corrupting influence of a powerful invention, is still valid. It was very interesting to watch the different ethics played out by the two characters, even Abe's failsafe built to fix an unanticipated outcome of surfing time, becoming a tool of manipulation and control in Aaron's hands. (Abe's no angel, but his ambition is different.)

But the permanent extra versions of Aaron & Abe seemed a bit of a sci-fi gimmick. The idea of being able to leap further into the past than another time-traveler to gain the advantage, with the attendant complications of having to deal with your temporary multiples during the timeline overlaps, was compelling enough on its own to advance the story.

2014-05-19 03:45:49 by Marcus:

Random question: What would happen if you brought a smaller box into a larger box an used both at the same time. Could you go back any farther then when the original box was turned on?

2014-06-12 14:30:46 by Aratak:

To Richs post:

"Given the logic of the boxes presented in the film, an Aaron-2 ONLY exists because 6 hours in the future Aaron-1 got into the box to move back in time to become Aaron-2. That's the whole premise. ..."

This type of paradoxon problem arises in timemodels where there is only one single timeline.
In this movie whenever someone leaves at an A event coming form an B event he arrives at an alternate timeline.
There won't be consequences for the traveler no matter what he does to his younger duplicate.

The reasoning for avoiding their younger past and the box is to:

- make sure the younger version will leave the timeline so that the both of them don't have to share one identity after the B event.
- not taking the risk of a paradoxon problem like you are hinting at.

The second reasoning only applies for a traveller that doesn't know yet that a timemodel with alternate timelines is the correct model (Abe after his first travel).

The movie shows that the alternate timeline model is correct. For instance travellers drug and gas the original versions of themselves without consequences for the travellers happening.

I hope that helps.

2015-02-13 21:17:34 by Juk:

Sorry, there is something I don't get. When you say "If the weeble is put in at point B, while the box is powering down, and then removed again at point B, the weeble must make an even number of trips. In the movie they get 1334 minutes on the one time that they try it.", how is it possible for the weeble to be put in at point B and then removed again at point B? Shouldn't it be removed at point B plus some time? Thanks.

2015-08-12 07:05:11 by Question. How explain alternate timelane model something like this: there is only one timeline for every one where you cant change anything in her past. The first one. There is happy Aaron time maschins are in sf movis only . Abe? Who is that ? If i:

2015-08-12 07:26:03 by In this timeline not exist even one atom from his body. How cant be in any alternative timeline. If they are somewhere. .:

Plain text only. Line breaks become "<br/>"

2015-09-10 23:48:14 by Cal:

One thing that is never explained by the film is why two brilliant people with unlimited time on their hands decide that the best use of it is to play a zero sum game against each other and rig the outcome of a party. I guess a story of world domination would have needed a bigger budget.

2016-01-07 21:11:42 by PK:

When they are talking at the end does Abe know something that Aaron does not when he says, "you have no idea what I'm capable of."? Has he figured out how to travel forwards in time or possibly go back further in time? Could he have multiple Abe's in the timelines as Aaron does?

Also, the Thomas Granger aspect of the movie is very cool and whish they could have went a little deeper into the story with their association. Could Granger have finally been their financial backer when they are in the warehouse seen? I also thought that they were attempting to build a larger- warehouse sized box so they could farther back in time although the theory above that he was building multiple boxes is pretty cool to think about.

I wish they had also explored the fact that Granger did show up. Why didn't they search the storage container records for further evidence that someone else or another version of them was using a different container somewhere in the facility. Perhaps though a box could have been used that wasn't at that facility anyways.

Overall the movie is one of the coolest I've ever seen. I've tried watching from the beginning and trying to determine if Abe or Aaron or both were time traveling up to that point. For some reason I feel like the new refrigerator is overlooked plot point. Maybe not. Thanks

2016-05-21 00:18:52 by RooMonster :

As much as I like this movie, and its interesting interpretation of time travel, there's not a way to have two of the same person at the same time. It's physically and physics-ly impossible. If Aaron never goes into the box at all, then there is only Aaron Prime. If he does go into the box, then Aaron2 comes out, and there is Aaron Prime in self seclusion at the hotel, and Aaron2 free to do whatever he wants. However, there's no way Aaron2 can interfere with Aaron Prime going into the box, because if Aaron Prime doesn't actually go into the box, there would never be an Aaron2. See?
Like I said, interesting concept, works well in theoretical sense, (as observed by the many clever explanation comments) and as a fantasy movie, but not in reality.
Just sayin. :-)

2016-07-07 17:29:09 by Marus Bolarious:

What was being built in France was explained by the comment made by Aaron in the airport: "You can look for me but will never find me", not the exact quote but close enough.

Aaron was building more "boxes" so should whomever he made the phone call to come looking for him he could simply slip away.

Thats my take on it anyways.

2017-01-01 13:17:07 by Chocnilla:


Considering time travel doesn't exist in our "reality", your definition is just as absurd as any other one presented. The fact that it holds up theoretically as you mention is enough. A number of people have already mentioned the solution in that when someone goes back, it creates an entire new timeline. Whatever happens to Aaron Prime here does not affect the copy, as they discover by the extra cell phone in the movie. You are the one who is not "seeing" because you are using a common misconception from just about every other time travel movie. The writer of this movie has presented a different take on time travel and that's what makes Primer so interesting and it actually works from a logical standpoint.