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Is there any reason given either in the movie, or by any of its many analysts, why Abe and Aaron must make the 6 hour trip backwards themselves?

Could they not have designed an intelligent device, as discussed in this article, that would allow them to just send a piece of paper backwards in time at 2:30PM with the stock market information from the day, which they would have picked up at 8:30AM when they power up the machine?

Here's the relevant paragraph from the article I linked above:

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."

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3 Answers 3

up vote 5 down vote accepted

Several difficulties/problems come to mind:

Given the time setting of the movie (based on the type of computers and cell phones they use), they don't have the same technology we currently have. It would have been hard to design something that autonomous that still worked inside the storage unit. I don't think looking up the stock info from inside the storage unit was possible (why else would they go to the library?).

If they wanted to design something like that, it would need software to program all its actions. Abe and Aaron are not that skilled at programming (I remember them saying they didn't need Robert/Philip because the device didn't need software). They are better in physics and mechanical engineering so they build a bigger box instead of programming a robot.

Finally, going back in time yourself would be the next logical step because it offers so much more possibilities (like them engineering the party event). They're engineers looking for profit and publishing. They would have inevitably taken their research to the next level.

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As they begin their exploitation of the stock market, both characters are afraid of bringing about any possible "paradox" situation. They sequester themselves in a hotel room. They do not communicate with anyone until they go "back" (second time through the same day), except to check the stock market. Only when the forgotten cell phone rings when they are not "back" do they allow themselves to play with that dynamic.

Therefore, any concept of sending a machine back in time to make the stock transaction for them would have been contrary to their (initial) conception of how to use this technology as safely as possible. Such a plan would force them to confront the Grandfather Paradox and related issues, which they did not have to do if they simply go back in time themselves (as long as they strictly follow their intended sequestration discipline).

In short, with the information they have at the beginning of the film, there is no proof that using a machine to do it for them would even work - maybe the machine makes them a bunch of money in a parallel universe, not in their own. The way in which both characters eventually seek to answer this question for themselves, lose trust in each other, and then turn on each other, makes the second half of the film so interesting (and confusing)!

Besides, there is the additional burden of designing a machine to extract itself from the time capsule mechanically after the correct delay, so that it does not "loop" forward and backward through time, only to be removed at the time the capsule is powered off.

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Along with the answer given by another, it would create a paradox given the way that time travel works in Primer. The alteration of one's past self creates a paradox self, which is why, later in the film, one pair of Abe and Aaron lose the ability to write. With them going back in time, it allows them to take the full benefit of time travel without the risk of altering their past selves and creating a paradox.

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(spoiler!) In the film's theory of time travel, there is no paradox. Abe and Aaron both murder their past selves, with their bare hands, in the film in their struggle for power over the other. But the characters initially fear that a paradox would occur (rightly so in my opinion), so they take care to avoid this. –  wberry Mar 11 at 14:44
If the reason for losing handwriting is explained in the film, I missed it. My guess is neurological damage incurred from being in the machines for extended periods of time. Note that in the scene when they first test their prototype in the garage, one of them places his hand over it and recoils in apparent pain or discomfort. –  wberry Mar 11 at 14:51

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