This post started out as a review of the movie Looper. I was going to explain that I almost always find time travel movies to be a little disappointing because it usually isn’t very difficult to find a contradiction in the movie that reduces the whole thing to nonsense. I was going to use Back to the Future as an example of this, but I ended up writing six paragraphs on Back to the Future and, ironically, never getting around to Looper. Oh well.
The problem with allowing characters to change the past is that it almost inevitably results in a contradiction: for example, in Back to the Future, Marty radically changes his own past: in the new reality created by Marty’s adventure in the 1950s, his parents are happy and successful, his sister is popular and Marty himself wears different clothes and drives a shiny new truck. Marty has in fact created a new timeline which diverges from the original timeline at the Enchantment under the Sea dance in 1955. As a result of this divergence, there are now essentially two Martys: Marty1 and Marty2. Marty1 is the “original” Marty who we met at the beginning of the movie and Marty2 is the Marty who was produced as a result of Marty1’s messing with the past.
At the end of the movie, we see Marty1 observing Marty2 at the mall parking lot with Doc Brown. Marty1 witnesses Doc Brown getting shot and sees Marty2 escape from the Libyans in the DeLorean, disappearing into the past as he reaches 88 miles per hour. So it appears to Marty1 that everything in this scene happens the same way it did the first time (except that he now knows that Doc Brown was wearing a bulletproof vest). And of course it logically has to happen the same way it did the first time around, because if Marty2 doesn’t travel to 1955 the exact same way Marty1 did at the beginning of the movie, then nothing that Marty1 did in 1955 ever happened, in which case he would not be at the mall observing himself traveling to 1955.
Here’s the problem: The first time around, it was Marty1 traveling from 1985 to 1955. The second time around, it’s Marty2 traveling from 1985 to 1955. Why is this a problem? Because Marty2 is a different person with different memories than Marty1. Marty2 has no memory of his dad being a loser or of Biff being a bully, for example. It’s very unlikely that Marty2 would have acted exactly the same as Marty1 did in 1955, which means that what we just witnessed never actually happened. Even more troubling: if the outcome of Marty2’s actions are different in any way from that of Marty1’s, then Marty2 would effectively be creating a third alternate timeline, resulting in Marty3. Marty2 then would observe Marty3 traveling to 1955, where he would in all likelihood create Marty4. Marty3 would observe Marty4 traveling to 1955, where he would in all likelihood create Marty5. You get the idea.
It’s clear that Marty1 and Marty2 exist in two separate realities from the fact that the name of the mall changes between the Doc Brown’s first meeting with the Libyans and his “reenactment” of that meeting, as a result of Marty1 running over one of the pine trees with the DeLorean. H/T Back to the Future Wiki.
The problem is that if all of these events occur in the same timeline, all possible Martys eventually have to collapse into one actual Marty. This Marty can be Marty1 or Marty2, but not both. The existence of Marty2 logically requires that Marty1 never existed. But Marty1 created the timeline that caused Marty2 to exist. Thus the existence of Marty2 logically requires both (a) Marty1 must exist; and (b) Marty1 must not exist. Paradox.
One way around this paradox is to propose that Marty1 didn’t actually change the past but rather created a divergent timeline, so that now both Marty1 and Marty2 exist, but in different realities. The problem with this notion is that it doesn’t explain why, if there are two extant timelines, Marty1 went “back” to Marty2’s future. Did Marty1’s creation of the timeline that resulted in Marty2 somehow link Marty1 to Marty2’s timeline? Assuming it did, then Marty2 would presumably be returned to Marty3’s future, and Marty3 would be returned to Marty4’s timeline, etc. This is a little creepy, because it means that each Marty is being shifted over one reality, replacing the Marty that actually belongs there. Not only that, but the original timeline, the one that produced Marty1, is now Martyless. One day Marty1 just disappears out of his reality and is never heard from again. In this case, the title of the movie is wrong: Marty never goes “back” at all; he just shifts from one reality to another.
Other than arbitrarily severing the relationship between cause and effect at some point, the only other way to eliminate the paradox is to postulate that Marty1 merges with Marty2 at the moment of time travel, so that Marty2’s existence comes to an end before he ever appears in 1955. The Marty that appears in 1955, then, is always Marty1. This is a bit troubling as well, because Marty2 is no less “real” than Marty1. We can just as easily imagine the movie beginning with Marty2 as with Marty1. If we were to do that, though, we would see happy, well-adjusted Marty inexplicably morph into a similar but different Marty at the moment the DeLorean hits 88 miles per hour. Essentially, we are severing cause and effect: Marty2 suddenly becomes Marty1 for no apparent reason. Is Marty1 is somehow more “real” than Marty2? If so, then why doesn’t Marty2’s reality collapse along with him? In fact, the opposite happens: Marty1’s reality collapses and he finds himself in Marty2’s reality.
The movie seems to make sense because it relies on a sort of slight of hand, swapping out Marty2 for Marty1 at the critical moment, but in fact it’s incoherent. Applying the rules of time travel as they are presented in the movie, there is no way for what appears to happen to actually happen. Either Marty2 inexplicably ceased to exist at some point or Marty wasn’t actually time traveling but shifting from one reality to another. Neither is a very satisfying solution.
Speaking of time, it’s possible that I have too much on my hands.