Last time we started looking at the rather complex time travel movie Primer, and the mistake its time travelers make right at the beginning, trying not to change history before that history has been written instead of after they have made their trip. The problem of that approach is brought into stark relief when Aaron gets a call on his cell phone.
Abe, who discovered that the machine the two of them had built would enable them to travel back in time, had used his first trip to buy a stock he knew would be a strong advancer during the day, and to explain the discovery to Aaron. On the second day, they both did what Abe did on the first day: drive out to Russelfield and take a hotel room for the day, so that they would be completely out of touch with the world until late afternoon, when they could determine what were the best investments for the day before traveling back to the morning to make their purchase, then spend the day doing what they want. (It appears that they are not in the Eastern time zone, as they can get stock market closing prices early.) The problem with this approach, discussed last time, hits them this time, because while they are in the hotel in Russelfield, Aaron's wife calls him. Completely uncertain what to do but having to make a decision quickly, he answers his phone and makes up an excuse for why he cannot join her for lunch.
That's not a big deal, really. The problem is that when the duo returns to the morning and are spending the day downtown instead of sequestered in the hotel, right on schedule Aaron's wife calls him, and his cell phone rings. He apologizes that he did not turn it off, and they again discuss the problem. They conclude that if his cell phone is ringing here downtown, it is not also ringing in Russelfield. That means that his other self, out at the hotel, cannot answer it. Worried too late about changing history, Aaron does not answer his wife's call, which therefore goes unanswered.
The problem is that Aaron has just changed himself. There is a version of Aaron in the hotel in Russelfield who has not spoken to his wife today because his cell phone never rang, and there is this version of him who has spoken to her from the hotel but ignored her call when he was downtown. Presumably the version of Aaron who is staying at the hotel will tonight travel back to this morning, and it will be he who is downtown when his cell phone rings. He, though, does not have the memory of having answered the call at the hotel, nor of making up that excuse; it is likely, then, that he will answer the call, and will invent an excuse for not meeting his wife for lunch, which may or may not be the same excuse, or might decide that meeting her for lunch is a good idea.
The problem exists at the other end as well. When Aaron gets home tonight, will his wife ask him about the work that kept him from joining her for lunch, or will she ask why he didn't answer her call, or will she talk about whatever came from him answered the call downtown? In at least three different versions of the day, at least three different things happened to her at that moment, so she has three distinct memories; yet she can only have one memory of that moment.
The logical solution (a solution alluded to by the narrator later in the film in relation to a different problem) is that whatever happens in the last version of history is what she remembers; it is also, then, what Aaron remembers. The Aaron we have followed cannot be the Aaron who goes home that night--he has to be the Aaron whose cell phone did not ring at the hotel but did ring downtown. Because he was not worried about changing history on his second pass but only on his first, he will assume that his predecessor answered the phone and will do the same without giving it a thought.
Fortunately, there is every reason to think that any future version of Aaron will do the same: the phone will ring downtown, not at the hotel, and he will answer it when it rings. Time thus stabilizes into that version of history, and those versions of Aaron and his wife are the ones who make it to the next day. We have what the replacement theory calls a sawtooth snap with an N-jump termination*--that is, we will repeat history a couple times but ultimately get something internally self consistent, and so the future will be built on that version of history.
Next time we'll take a look at the peculiar incident of the shotgun at the party.
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