I asked a member of our forum to explain Primer to me, because he seems smart and I do not.

This was the email I got in reply. -Teague


This is a fucking novel. If you read the whole thing I will be legitimately astounded.

Okay, so the awesome thing about "Primer" is that there are at least two whole movies going on that we never get to see, and maybe three. The events depicted on screen comprise no more than a third, and maybe less, of the events that transpire during the course of the story.

First, a word about the time machine and how it works. I'm gonna talk about this because the movie actually treats the time machine in two completely different and incompatible ways.

Before we dive into it, go to Google and search for "Rotating Cylinders and the Possibility of Global Causality Violation." That'll get you to a 1973 paper by a physicist named Frank Tipler. The short-bus version for those wimps out there who can't follow covariant tensor calculus: the Einstein field equations describe the curvature of spacetime in the presence of matter and energy. According to some solutions of those equations, it's possible for there to exist closed timelike curves through spacetime. Traversing a closed timelike curve is a lot like going for a walk around the block. When you walk around the block, you start out at a point in time and space, then you move, then you return to the same point in space at a later point in time. But if you move along a closed timelike curve, you return to the same point in space AND time.

Nobody knows whether closed timelike curves are a real phenomenon, or just an artifact of the Einstein field equations. But at least according to the current understanding of modern physics, they're not explicitly impossible. (They do appear only to form around massive rotating bodies, however, so it's not like "Primer" is TOTALLY science-fact. But whatever. It's just a movie.)

I bring this up because time travel in "Primer" is based around the idea of a closed timelike curve.

Let's imagine a time traveler, Alice. Alice has a time machine in her garage. It's pretty simple, just a box with a button. When Alice presses the button, the machine turns on; this takes a few seconds, because the machine has to "warm up," so to speak. If she presses the button again, the machine turns off, again taking a few seconds to "cool down."

The sole complication is that Alice has rigged her machine up to a timer. The need for this will become apparent shortly.

We'll start by describing Alice's day from her point of view. She wakes up in the morning on March 1, 2010. She goes out to the garage and starts the timer on her machine at exactly 7:45. Then she goes inside and has a coffee, then goes on about her day. For sake of argument, let's say she locks herself in her spare bedroom and stays there. At 8:00 a.m. exactly, the timer she started goes off and the machine starts to power itself on. Again, it takes a few seconds for it to warm up, but soon it's humming along nicely.

Alice spends her whole day locked in her spare bedroom. Maybe she reads a book or something, whatever.

At 6:00 p.m. still on March 1, the same day she leaves her spare bedroom and goes to the garage, where she pushes the button that turns off the time machine. The machine has been running for exactly ten hours at this point. After a few seconds it powers down, and just when it does, she climbs inside. When she does, she hears the machine power itself up again (let's assume the machine hums or something). Then she waits there for ten hours.

By Alice's wristwatch, it was 6:00 p.m. when she climbed into the box. After ten hours, by her wristwatch it's 4:00 a.m. on March 2, 2010. Let's call the time according to Alice's wristwatch AST Alice Standard Time. At 4:00 a.m. March 2 AST, she hears the box start to power itself off again. A few seconds later, when the machine is fully off, she gets out of the box.

The clock on the wall of Alice's garage reads 8:00 a.m. on March 1. She has gone back in time. (We're going to call the time according to the wall clock UST, for Universe Standard Time.)

Now Alice can do whatever she wants. She can go to work, or watch TV, or do anything at all she cares to do. Just for sake of giving her something to do, let's say she goes to the movies, runs some errands and hangs out at the beach. Whatever she does with her time, let's stipulate that she doesn't get back home for twelve hours 4:00 p.m. March 2 AST, 8 p.m. March 1 UST. She has a light supper and goes to bed, exhausted from the fact that it's been more than 32 hours, subjectively, since she got up that morning but otherwise totally normal.

Now let's describe Alice's day from an imaginary "objective" point of view, say that of an observer in Alice's house. Alice gets up on the morning of March 1, starts the machine at 7:45 and locks herself in her spare bedroom. At 8:00, Alice gets out of the machine.

We pause here to make an important point. It's tempting to say that there are two Alices, the one in her spare bedroom and the one in the garage. This is incorrect, and will just lead to confusion. Instead, remember that there's only ONE Alice, but she happens to be in two places at the same time. Remember, when we went through her day from her point of view, there were no branches or divergences. She never cloned herself at any point. Instead, she experienced everything in totally ordinary, boring, linear fashion. What we're seeing as "objective" observers that is, observers moving along an inertial trajectory through spacetime is Alice at two different points in her own personal history.

I could go off on a tangent here about the concept of simultaneity and how it works in a universe where the speed of light is a constant in all inertial reference frames, but hell, we all took high school physics. Maybe we don't all remember the math, but in this context it's enough to say that events which APPEAR to be simultaneous in one reference frame will not necessarily APPEAR to be simultaneous in all reference frames. In the reference frame of the comoving observer, it APPEARS that Alice is both in her spare bedroom and climbing out of the box simultaneously, but this is just an illusion. From Alice's point of view, one of those events followed the other by several hours.

Remember: there are no privileged reference frames. There's no objectively "true" sequence of events in a relativistic universe.

Anyway. Back to Alice's day. It's 8:00 a.m. on March 1, and we see Alice in her spare bedroom, and also in the garage getting out of the box. Alice-in-the-garage leaves the house; we do not see her again for twelve hours. Alice-in-the-spare-bedroom waits until 6:00 p.m., then goes to the garage and turns off the box and climbs inside, and apparently never gets out of it. If we were to follow her into the garage, wait until she gets into the box and then peek inside, we'd find it both off (because she turned it off) and empty (because she got out of it at 8:00 that morning, ten hours earlier).

We dick around Alice's house for another couple hours, just to see what happens, and at 8:00 p.m. we see Alice return home from her day out. Maybe she's looking a little ragged, like she recently pulled an all-nighter, but other than that, she's just totally normal Alice.

That's the FIRST way time travel is handled in "Primer." Maybe it's a little confusing if you don't have a good grasp of relativistic physics, but really it's very straightforward. Time is linear and entirely conventional both for Alice and for a notional objective observer. It's just that events which appear to be sequential to Alice appear to be simultaneous to the rest of the universe.

This is the paradigm Abe and Aaron follow when they pull the stock-market trick. To them, time is purely sequential; they start the machines, wait, get into the machines, wait, get out of the machines, go on with their day. To the comoving observer, two Aaron-and-Abe sets exist simultaneously, but again, that's just an artifact of relativity.

A digression now to talk about exactly how the stock-market trick worked. Abe and Aaron are changing events, right? They're watching the market, gathering information about it, then going back in time and using that information to change it, aren't they? Well not necessarily. See, Carruth's choice of the stock market was either wise or lucky, because it leaves enough ambiguity for the story to work either way. Yes, maybe Abe and Aaron are changing history with their actions, but it's equally possible in fact, a more likely and consistent explanation that they're really not. See, Abe and Aaron look, at the end of the day, for stocks on which significant profits were taken. They have no information about WHO took those profits; that information is not available to them. Then they go back to the beginning of the day and take profits on those stocks. What they were actually observing at the end of the day (earlier in their reference frame, later in the universe's reference frame) is the effect of their own trading. The trick wouldn't work or at least wouldn't fit the story if they looked to see who won the lottery, got their lotto numbers and then went back and played those numbers. They'd obviously be changing the outcome in that case. But in the case of the stock market, there's only one set of events, but Abe and Aaron experience that set of events in a different order than the rest of the universe. No rules are being broken with the stock-market trick.

Now. If the movie stopped there, it would still be way interesting. But it doesn't. It goes to a different place. And to explain that, we need to ask ourselves two questions: Why the timers, and why the hotel rooms?

Remember Alice? Alice went to the garage and started a timer, then locked herself in her spare bedroom. Why? Why didn't she just mash the button that would turn the machine on? The answer is causality, and more specifically Abe and Aaron's lack of understanding of causality.

When Alice gets into the box at 6:00 p.m. on March 1, it's turning itself off. Once inside, she perceives it powering itself back up again. What's really happening, though, is that Alice is experiencing the box powering itself DOWN, only "played back in reverse," because she's now moving backwards in time relative to the universe. After five hours inside the box, Alice's wristwatch reads 11:00 p.m. on March 1, but the clock on the wall of her garage reads 1:00 p.m. on March 1. A minute later, her watch says 11:01 p.m., but the wall clock says 12:59 p.m. She's moving forwards in time in her own reference frame as everyone always does, obviously but backwards in time in the universe's reference frame. (Put the other way around, when Alice is inside the box, the universe is moving backwards in time relative to her reference frame. Same difference; there are no privileged points of view.)

After ten hours in the box, Alice perceives the machine powering down. This is the machine powering UP, "played back in reverse," because Alice's trajectory through spacetime is backwards relative to the universe. When, from Alice's point of view, the machine is fully powered down, she gets out. It's now 8:00 a.m., and the timer just went off and the machine is just starting to power up.

Now the need for the timer becomes obvious: If Alice just went into the garage and mashed the button at 8:00, she'd immediately be greeted by HERSELF emerging from the box. Abe and Aaron want to avoid this, because they simply don't know what would happen. Their heads are filled with sci-fi fantasies of paradoxes and antimatter explosions and god knows what else, and they just want no part of it. So they avoid the question entirely by rigging up the boxes on a delay timer, giving them an opportunity to vacate the premises before anything happens.

Of course, the more interesting question is what if they pushed the button on the box and nothing came out of it? What would that mean? Again, that falls into the category of shit-they-want-no-part-of, so Abe and Aaron never try to find out.

The other thing to ponder is the matter of Alice's spare bedroom. After she starts the timer, she locks herself in her spare bedroom for the day. (And Abe and Aaron lock themselves in a hotel room.) Why is that? What's the point of having a time machine, after all, if you have to bolt yourself in a room and avoid interacting with the world for your first trip through your day? The answer is that Abe and Aaron don't want to fuck with their own personal histories. The reason they're able to emerge from the box in the morning is because they entered the box in the afternoon; entering the box is part of their own personal histories. But from the viewpoint of a comoving observer, Abe and Aaron have NOT entered the box yet at the time they emerge from it. From THEIR point of view, entering the box is in the past; it's done. But from the comoving point of view, that event hasn't happened yet AND MAY NOT.

Once again, Abe and Aaron are scaredy-cats. They don't even want to know what might happen if they fail to get into the boxes "after" (in the comoving reference frame) having emerged from them. A moment's thought on this subject reveals that it's really not even worth worrying about. Again, there aren't really TWO Abes or two Aarons; they haven't been CLONED. They only appear to be in two places at the same time because of an artifact of relativity. In fact, by the time Abe and Aaron emerge from the boxes, they've already gotten into the boxes; this happened in their own personal histories. It's fact, irrevocable and true, and cannot be changed. THERE ARE NO PRIVILEGED REFERENCE FRAMES. The fact that Abe and Aaron appear to exist simultaneously to an outside observer doesn't trump the fact that they exist in a purely linear, continuous fashion just like everybody else.

Except no.

Now at this point, you can either criticize the movie for breaking its own rules, or admire it for having the balls to say that what the characters (and the audience) THOUGHT the rules were weren't actually the rules at all. "Primer," for all its mind-bending nonlinearity, sticks relentlessly to the point of view of its two main characters. The film sets up rules for time travel that make perfect sense (if you have an advanced degree in physics) because those are the rules Abe and Aaron figure out. Except Abe and Aaron are wrong. Time travel in "Primer" doesn't work at all like what I described above. In fact, while time travel in "Primer" resembles a logical method that's consistent with relativity, the way it actually works flies in the face of not just relativity but even the idea of conservation of energy.

It starts with the phone call.

On one of their "trips," Abe and Aaron are in the hotel room when Aaron's cell phone rings. He was supposed to leave it at home, but he forgot. The guys, after a moment of panic, decide that everything's okay, as long as Aaron does not take the phone back to that morning with him. He lets the call go to voicemail, and they go on with their day.

Except he DOES take the phone back with him, and later that day (in Aaron's reference frame) it rings again. The guys debate whether BOTH phones are ringing, or just the one in Aaron's pocket right then (in their reference frame). They don't know the answer and according to the commentary track on the DVD, the companies that make cell phones aren't even clear on what would happen in that situation but Aaron answers the call anyway. It's totally mundane, but it still raises the possibility: What if they changed their own personal histories? In their histories, Aaron's phone rang when they were in the hotel room. But if the phone exists in two places at once, and if it DOESN'T ring in both places, then the fact that Aaron's phone rang later (in his reference frame) means it didn't ring before, except it clearly did in his own history. So what? Are they dealing with an Einsteinian universe where causality is fixed but simultaneity is an illusion? Or is it something more mysterious? They don't know but they're no longer sure they understand what's going on.

This is where the movie starts to get complicated. No, seriously. Stop laughing.

It's at this point that the Platt-punching idea comes back up. Earlier in their personal histories, Abe and Aaron (and Aaron's wife) were talking about what they would do if they could act without consequences. Aaron says that he'd punch someone named Platt right in the face. It's never made clear just who this Platt was, but it's obvious from context that he's somebody who wronged Aaron on some level. What Aaron imagines is this: He turns on a machine, later enters it, goes back, finds Platt and punches him, then subsequently stops his earlier (from his reference frame) self from getting in the machine in the first place, ensuring that the trip back never occurs and Platt never gets punched.

Abe tells Aaron they can't do that. It's not clear whether he means they LITERALLY can't, or that they mustn't. It's possible that Abe believes, at this point, that what Aaron proposes is literally impossible, that their time machines don't work that way. (In other words, he's speaking under the assumption that they live in an Einsteinian universe where simultaneity is an illusion but causality is real and fixed; things can appear to happen out of order depending on your reference frame, but effect always follows cause and the past [ANY past, regardless of reference frame] cannot be undone.)

But later, after the phone call incident, it's not at all clear that that's the case. If ONLY later-Aaron's phone rang the second time (in Abe and Aaron's reference frame), then the first call (still in Abe and Aaron's reference frame) never happened; they inadvertently changed their own histories. But the thing is, they don't KNOW that's what happened. Aaron never answered the phone the first time (in his reference frame) it rang, so he doesn't know whether the call was also, simultaneously (in the universal reference frame) ringing his phone later (in his reference frame).

That's when Abe gets the idea to do an experiment. After one of their stock-trading adventures, he goes back to the U-Haul and turns on both boxes. He does this specifically so they can do an experiment with causality. He comes to Aaron's house in the middle of the night and tells Aaron that the boxes are running, and they decide what the hell, to give it a shot. They leave to go to the U-Haul place and screw around with time.

Except they never get there, because they see Granger. Granger, a wealthy businessman and Abe's sort-of girlfriend's father, found one of the boxes that Abe started and uses it to go back in time. Remember how I said the stuff we see on screen is just a fraction of the events that happen in the story? We NEVER see this, or even any hint of it, in the film. We have absolutely no idea how Granger found the box, or when. It could have happened at virtually any point in the indefinite future, as long as the box Granger used was running continuously from the time Abe turned it on. (It's not even entirely clear just WHICH box Granger found, which I'll get to in a minute.) We also are never told WHY Granger used the box but hints are provided that lead to a pretty comfortable assumption which I'll explain later.

Anyway, Abe and Aaron, on their way to the U-Haul, see a Granger that's obviously traveled in time. They chase him, but he falls into a coma for no apparent reason. It seems clear that Granger didn't stay in his box until it turned itself off; rather, he got out early (in his reference frame), while the box was still running. This has an unspecified but detrimental effect on anybody who does it; this is established earlier in the film when on Aaron's first trip he messes up the timing slightly and gets hurt by it. So now there are two Grangers simultaneously (in the universe's reference frame), and one of them is in a coma, perhaps permanently.

It's at this point that Abe says enough.

I'm gonna talk now about the failsafe box.

Again, what happens on screen is just a fraction of the events of the story. What we see is that Abe figures out how the machine works, builds a larger version and uses it, then spends his second pass through his day giving Aaron his big demo. What actually happens is that, unseen and unmentioned until later in the film, Abe constructs another box and sets it up in another unit in the storage facility and turns it on. This is his failsafe box. Once that box has been turned on, at any point in the future Abe can go back to it, turn it off and climb inside, emerging at the moment the box was originally turned on. He turns this failsafe box on before he does anything else related to time travel, theoretically giving him the chance to go back to before-the-beginning and change events if anything bad should happen.

Even at the beginning, Abe knows or at least suspects that all that stuff I said before about illusory simultaneity and fixed causality everything Einstein ever assumed, in other words is bullshit. At the very least, he wants to be prepared just in case.

The moment when Abe turns on his failsafe box represents a fixed point in time and space in the film. We have no idea when it happens it's never depicted but we know it happens before Abe tells Aaron about the machine's properties. This box is the first one that ever gets turned on the first human-scale one, that is. It is, therefore, the earliest point in time to which anyone can ever go back.

So Abe, freaked out by the Granger incident and (perhaps more so) by how close he and Aaron came to deliberately fucking with causality, decides to use the failsafe box, go back in time to the morning of the day he first told Aaron about the machine's properties, and reset everything.

His plan is to go back, find his earlier self (relative to his reference frame), knock him out with gas to, then take his place and meet Aaron in the park and NOT tell him about the machine. It almost works, too. Except when Abe gets to the park, he's completely exhausted, having spend DAYS in the failsafe box "riding" back. By the time he meets Aaron, he can't carry on, and he collapses.

And that's when the REAL twist comes: We learn that Aaron has also somehow traveled back in time. He's been listening to recordings of previous conversations on his earpiece.

Let's switch gears and talk about Aaron's story, because no, seriously it's the most complicated of all.

Abe tells Aaron about the machine's properties, just as we see in the film. He shows Aaron the box he'd used to go back in time. He does NOT show Aaron the failsafe box. Sometime later we have no idea when, relative to any reference frame Aaron discovers that Abe had rented two storage units at the U-Haul. He goes into the second, secret one and discovers the failsafe box, running. He figures out that Abe put it in place in order to have a way to go back to before the beginning.

Now, this part we know: At some point prior to Abe's use of his failsafe, Aaron uses Abe's failsafe box. What we don't know is when or (entirely) why. But again, we're given enough clues to suss it out, although they're presented so tangentially as to be practically baffling at first. It's all related to the party, Rachel, Will, the shotgun, and maybe even somehow the Granger incident.

I'm gonna tell this from Aaron's reference frame. When I use words like "before" and "after," I'm speaking in terms of Aaron's subjective experience.

We know that three events took place, but we have no clues as to the order of those events (in any reference frame) or the causality of them. We know that Aaron discovered Abe's failsafe box; this is one of the few turning-point events in the story that's actually shown on screen, God bless America. We know that Aaron uses Abe's failsafe machine to go back and establish his own, separate failsafe machine. And we know that something bad happens at a party.

Aaron's use of the failsafe machine looks like this: He builds and collapses two boxes and enters Abe's failsafe box with both of them. He emerges from Abe's failsafe box when Abe turned it on, back before the beginning. Abe's failsafe box is now not usable again; once the box has been turned off (in the objective future), the timelike curve is closed, and it can't be turned on again without creating a new timelike curve. So Abe's failsafe box is, for all intents and purposes, destroyed.

Aaron sets up one of the two boxes he brought back with him and puts it in place of Abe's failsafe box. He sets up the other box somewhere else; that box becomes Aaron's private failsafe box. He turns on his own private failsafe box first (because he wants to be able to go back in time farther than anyone else), and then minutes or hours later turns on the new box that replaced Abe's failsafe box. This new box is the one Abe takes back later, believing it to be his own failsafe box that he set up.

Aaron now goes to his own house and spikes the milk he'll later use on his cereal with propofol. He waits around until his earlier self has breakfast, at which point he (his earlier self) passes out. Aaron then hides his earlier self in the attic and takes his place. He wears an earpiece under the pretense that he's listening to March Madness games, but he's actually recording the day's conversations. Aaron is thinking ahead: he knows that he might need to go back and relive these events at some point in his personal future.

The upshot? The Aaron that Abe meets in the park for the first time (in Abe's reference frame) has already traveled in time at least once. And possibly many, many times.

Now, about this party. The party is an oddity in the film; it's barely depicted at all, and only discussed directly a couple of times. But it remains the crux of the whole story.

Everything I've said so far I'm fairly, not totally but fairly, confident about. It's all more-or-less explicitly supported by the events we see on screen. But this part I'm basically making up. I think it fits the story, but I don't think much of it is EXPLICITLY supported by the film itself.

I think the party happens for the "first" time (relative to any of our time-traveling characters) before Aaron first uses Abe's failsafe box. It's not clear to me whether it happens before or after Aaron DISCOVERS Abe's failsafe box, but I think it happens before he actually USES the failsafe box. In fact, I think Aaron uses Abe's failsafe box because of the party.

The first time (relative to any of our characters) the party happens, the sequence of events goes like this: Aaron invites Will to the party. Will shows up with a shotgun. Something bad happens, maybe even something fatal. Aaron, horribly guilty because he invited Will to the party in the first place, decides to use Abe's failsafe box.

Aaron goes back, does all the stuff I described above, then goes to the party again, this time with foreknowledge. He tries to create a different outcome.

And here's the good part: We have absolutely no idea how many times Aaron does this. He can use the fold-up-a-box-and-take-it-back trick basically indefinitely, each time giving himself another chance to loop through events again. There's even a line in the film that alludes to this.

Eventually Aaron loops through the party events a sufficient number of times to become the hero. But the outcome isn't optimal. In any case, for reasons never explained, Aaron stops looping through the events of the party maybe he just grows weary of it and goes back to the stock-market stuff with Abe. Right up to the Granger incident.

After Abe panics and uses what he thinks is his original failsafe machine in actuality, the original failsafe machine is long gone, used up by Aaron and replaced the first time he went back to correct the events of the party he encounters an Aaron who's on at least his third pass through the events of that day. (The first time through, he experienced events for the first time; the second time, he recorded his conversations. Since he's listening to playback of that recording when Abe meets him post-failsafe, we know it's at least his third time through that day, and maybe more. He's been time-traveling an indeterminate number of times in order to change the Rachel incident.)

Because Abe failsafed back to before the events of the party, Aaron has no choice but to traverse those events again. This time he inducts Abe into his conspiracy, and they concoct a way to get the shells out of Will's shotgun before he takes it into the party. They succeed, Will goes to jail.

Now there are two Abes and two Aarons; there's the Abe and Aaron who went to the party, and there's the Abe that's locked in his apartment, and the Aaron who's locked in his attic. It's at this point that Abe and Aaron have the airport conversation we see toward the end of the movie. Aaron decides to leave; we're never told where he's bound. Abe stays to prevent the original versions of the guys from ever building the machine in the first place.

Movie ends.

Except that's not it. I've left out one thing, and it's fucking huge.

Let's talk about Aaron again. Aaron finds Abe's failsafe box, is inspired (I think by the Rachel incident) to use it. He goes back, drugs his earlier self and hides his earlier self in the attic. But then another Aaron shows up! What the hell? Where did this Aaron come from? This Aaron used his OWN failsafe after Abe decides to use (what he thinks is) his original failsafe.

For sake of clarity, I'm going to call this Aaron "older Aaron," because he's literally older, subjectively; he has experienced more time. I'll call the Aaron who drugs the milk "younger Aaron," because he is. The Aaron who drinks the milk we don't care about, because he gets locked in the attic for the rest of the story.

So why did older Aaron use his failsafe? This might actually be explained in-film, but if so I've never caught it. Maybe he thinks Abe might somehow "erase" him by changing the past, even though we've seen no evidence that time works that way. Whatever his reason, older Aaron decides, after Abe panics, to use his failsafe to go back to the beginning.

Younger Aaron (who used Abe's original failsafe box) goes back and spikes the milk, with the intention of taking his earlier self's place. Older Aaron (who used his own failsafe box, which was set up and turned on by younger Aaron) catches younger Aaron just after the act. They struggle. Younger Aaron subdues older Aaron, but after they talk, older Aaron convinces younger Aaron to leave and let older Aaron impersonate his earlier self. Older Aaron has already experienced all the events of the story so far; older Aaron went back in Abe's original failsafe box, older Aaron looped through the Rachel incident, older Aaron recorded all his conversations, all that stuff. Younger Aaron hasn't done any of that yet. Younger Aaron has just gotten out of Abe's original failsafe box, and has no firsthand knowledge of any of the events that older Aaron experienced subsequent to his own first trip back in Abe's failsafe.

Older Aaron convinces younger Aaron to leave and it's younger Aaron who becomes the film's narrator. He makes a phone call to Abe which Abe, exactly? I'm not certain, but I've got a theory I'll get to shortly and that phone call makes up the film's narration.

It's also younger Aaron, I think, who we see in the movie's last shot, somewhere in France, building a box the size of a room for purposes unknown. A bigger box could carry more people and things, obviously, but it could also make longer trips back more practical. It's left entirely open-ended just what younger Aaron plans to do. Remember, this Aaron hasn't experienced any of the bad aspects of time travel yet; he may in fact have made only a single trip back, the one time he used Abe's original failsafe. He's a complete loose cannon; there's no way to guess what he's planning.

It's also at this point that we can finally see clearly how time travel in this story works. We have two Aarons now, existing simultaneously from the point of view of the non-time-traveling universe. Except they have divergent personal histories. When Abe and Aaron first started using the boxes, they kept their personal histories strictly linear; whenever two Abes or two Aarons existed simultaneously (in the reference frame of the universe) one of those Abes or Aarons existed in the other Abe or Aaron's past; the Abe in the hotel room was in the immediate and linear past of the Abe trading stocks. But our two Aarons aren't like that at all. One of them used Abe's failsafe, went back, impersonated his younger self and had many time-travel-related adventures. The other used Abe's failsafe, got his ass kicked by the other one, and decided to leave and go to France. The two Aarons are entirely divergent now, in a way that makes ABSOLUTELY no sense in the context of time travel as I first described it, and as the characters first thought they understood it.

And for the physics nerds in the audience, this is where we can most clearly see the flagrant disregard for conservation of energy. In the first, simpler time-travel paradigm, energy was still conserved despite the illusion of simultaneity. Each time traveler had a straight and linear world line in his own reference frame; he only appeared to be in two places at the same time in the universe's comoving reference frame. But in the more complex time-travel paradigm, we end up with two Abes and three Aarons all existing simultaneously, all with divergent personal histories, none in the linear subjective past of any of the others. In other words, there's no reference frame Abe's, Aaron's, Aaron's wife's, Granger's, nobody's in which the personal histories of the characters are linear. There's no coordinate transform we can make, to use math lingo, that would result in only a single Abe or Aaron existing continuously in flat spacetime.

But you know what? Even this isn't TOTALLY crazy, in context of modern physics. The conservation of energy is a theory, one that's generally agreed to hold at the macro scale. But physics is riddled with what appear to be violations of this principle. Hawking radiation is a prime example. Everywhere in space, all the time, particle-antiparticle pairs are constantly popping into existence and annihilating each other. When this happens in the vicinity of a black hole's event horizon, sometimes either a particle or an antiparticle crosses the event horizon (thus disappearing from the universe at large) while its partner scatters off into space. This is how black holes can emit radiation, and it's also a local violation of conservation of energy. General relativity throws the whole notion of conservation of energy into disarray, and the implications are still being worked out to this day.

So from a physics-nerd point of view, we can imagine our two Abes and our three Aarons as being analogous to particles "emitted" by a black hole through Hawking radiation. (This is just a metaphor; it makes no sense literally.) Even though time travelers who appear to exist simultaneously in the frame of reference of the universe SHOULD "collapse" back into a single worldline, they don't have to, and when they don't, another complete individual comes into existence, with his own unique past worldline and his own independent future worldline.

So that's "Primer" in a nutshell. Two guys invent a method of time travel that appears to conform to known and proven interpretations of relativity, but in fact it doesn't. It has its own rules, inconsistent with the theory of closed timelike curves. The two guys discover this the hard way, and in the end three instances of one character exist and two instances of the other. At the end of the movie, the older instance of one character vows to stop the "original" two characters (the two subjectively youngest) from inventing the time machine in the first place, while the second-oldest instance of the other character goes off to build another, bigger time machine for reasons never elaborated upon.

Now, I've attached a purty pitcher. Lemme walk you through it. (Available for download here.)

Follow the blue arrows. That's Abe's worldline. His first trip through time is when he does his "demo day" for Aaron; he gets into box 1 at the end of the loop, travels back to the start of the loop, exits the box and then spends the day showing Aaron how the boxes work.

Then he and Aaron do three days of stock trading, each time with Abe in box 1 and Aaron in box 2.

After the Granger incident, Abe decides to take his failsafe box back to the beginning. Unbeknownst to him, Aaron has already replaced his failsafe box (box 0) with a new box (box 4). He takes box 4 back to the start of its loop, gases himself and impersonates his younger self. At this point, Abe's personal history changes; the older Abe was never gassed and stuffed into a closet. So now there are two Abes, each with independent personal histories. One goes on about his life none the wiser, except for having been knocked out and locked in a closet inexplicably. The other one (who is our original Abe, from the beginning of the story) apparently works to prevent the time machines from ever being used, or at least that's what he says during the airport scene.

Abe's worldline is by far the simpler one.

Now trace the red one; that's Aaron's. He doesn't travel in time for the first time until after Abe already has. He takes his first trip the first day he and Abe trade stocks. Sometime after this first day-trading adventure shown here between the first and second trading days, but the exact sequence is indeterminate in the film Aaron discovers Abe's failsafe box, box 0. He takes it back to the beginning, bringing boxes 3 and 4 along with him. He sets them up, then goes to drug himself. This is the first of two events in which Aaron's history diverges. Aaron hides his unconscious younger self in the attic and impersonates him.

At this point, things get really INCREDIBLY complicated.

We're now at the spacetime event on the diagram labeled "Aaron convinces his younger self to leave town." At this point in Aaron's personal history, he has just taken box 0 back to the beginning of everything and drugged his younger self. But now he meets another, older version of himself, one who's just taken box 3 back. This version of Aaron is in our Aaron's subjective future; he has not yet become this Aaron. But there he is. The two Aarons fight, then talk. The older Aaron (the one who emerged from box 3) convinces the younger Aaron (from box 0) to leave town. The younger Aaron does, going to France where we see him at the end of the film. But in affecting this sequence of events, the older Aaron (from box 3) manages to alter his own past. Because he WAS the younger Aaron from box 0; he remembers drugging his younger self and impersonating him. Those events transpired in his subjective past; they're part of his personal history. Only now those events are prevented, because he has just talked younger Aaron from box 0 into leaving the country.

Now remember, there is no indication in the movie that there's anything like "meta-time." It makes no sense to refer to spacetime events happening more than once. We can imagine that the "first time" Aaron passes through this event there is no older Aaron there, and he goes through with his plan for impersonating his younger self, then later goes back to that event a "second time" and changes it. But this is simply nonsense; time doesn't work that way either in the movie or (as far as we have any reason to believe) in real life.

Some folks on the Internet have gotten around this by subscribing to the many-worlds interpretation of quantum mechanics. Whenever anyone uses one of the boxes to travel through time, a parallel universe is created where the same events can have different outcomes. This is lazy, to my mind. No, the proper solution to this puzzle is to embrace ambiguity. The spacetime event labeled "Aaron convinces his younger self to leave town" must by the rules of the film be a superposition of different outcomes. One outcome is that the Aaron from box 0 never appears, and the Aaron who's never traveled in time goes on to meet Abe and learn about the boxes. Another outcome is that the time-virgin Aaron is drugged by the Aaron from box 0, and the Aaron from box 0 goes on to meet with Abe for what is for him the second time. A third outcome is that the Aaron from box 0 is interrupted by the Aaron from box 3, who convinces the Aaron from box 0 to leave town.

All three of these things must result from the spacetime event labeled "Aaron convinces his younger self to leave town" on the diagram. All three things must happen. Which one a subjective individual experiences when traversing that spacetime event is a matter of probability, and of which events lie in that individual's subjective past. If Aaron in that spacetime event has none of the other events in his subjective past, the overwhelming probability is that he will not be drugged, and will not encounter any other instances of himself. But if Aaron in that spacetime event has box 3 in his subjective past, the overwhelming probability is that he'll meet Aaron-from-box-0 and talk him into leaving. It's baffling, but it's consistent with both the story we see on screen, and also quantum mechanics.

What would an objective observer have seen at that point in space and time? Well, obviously the superposition of all three outcomes. You'd see Aaron-from-box-0 spike the milk, then you'd see time-virgin-Aaron collapse, then you'd see Aaron-from-box-3 appear and talk Aaron-from-box-0 into leaving. Not coincidentally, this is exactly what we see depicted in the film; in fact, this is the only version of events we see. Which makes sense, because this is the only thing that "really happened," from the reference frame of an outside observer. It is, in a sense, the "final version" of events; the other two outcomes are just first drafts.

So anyway. That's "Primer." At least I think so.

(I didn't do dick today. Obviously.)