broken arrow of time: the three flavors of time travel (part 2 of 2)

On Tuesday, I began this post about the three flavors of time travel. The gist is that time travel films can be categorized based on their answer to the question ‘If I go back in time and try to kill my grandfather before he makes my existence possible, what will happen?’ I left off with ‘closed loop’ time travel, which is basically time travel in a universe where you cannot affect the future. Today I’m moving on to ‘open loop’ and ‘continuous-ridiculous’ universes.

photo by Bob Owen

open loop

In an open loop universe, if you go back to the past, you may in fact be successful at killing your grandfather. (‘Successful’ here is a purely clinical word.) But instead of going into the same future that produced you, you will continue in a new timeline that has split off, one in which you are never born. Time traveler you sticks around, beginning from whenever you popped into this point in the past, but now you can never go back to the future world you came from, because you’ve made it cease to exist; there’s no future with a you in it, only the present with the anachronistic you. In this case, the thread of your narrative that is pulled back to intersect with the past does cross paths for exactly a moment with the timeline that will eventually produce you. But it’s an ‘open loop’ because even as your narrative crosses that timeline, you create a new branch that goes off on its own trajectory.

It’s not a sure thing that you won’t exist in this new universe you’ve just created — since this whole thing is just a thought experiment — but the further back you go, the more likely it is that you’ll change the course of history because of the butterfly effect (surely one of the more famous effects). One sign of a poorly conceived time travel yarn is that the person designated an expert is always warning the protagonist not to do anything to change the course of history or create ‘anomalies.’ Apparently, it’s especially bad if you interact with your past self. This kind of stuff only makes sense in an open loop universe, since in a closed loop universe no one can change the future anyway. Even in an open loop situation, it’s kind of a waste of time trying not to change anything, but for the opposite reason: It’s almost inevitable that your very presence will alter the future. You’ve already screwed it up, so why worry about it?


This is the stinker. This is the one that explains at least half of the kookiness that is time-travel cinema.

The ‘ridiculous’ aspect of this ought to become clear as I explain the ‘continuous’ bit. I’ll take the example that for me epitomizes nonsensical time travel logic: In Back to the Future, Marty McFly, your average teenager from 1985, is running around 1955 in his hometown, trying to scheme how to get back to the future and avoid the amorous intentions of a younger version of his mother. (I mean, if you thought killing your grandfather was bad…)

This all sounds fine as far as time-travel movies go. What really kills it is the family photo from the future that Marty carries around. It shows him with his brother and sister; when Marty does something ‘wrong’ — i.e., interferes with the unspooling of events that lead to a future with him and his siblings in it — the photograph changes in front of his eyes to reflect this new version of the future.

Before you object that we can’t know what time travel would be like, I will point out that the problem with continuous-ridiculous loops is not that they are highly improbable, like open or closed loops. No, it’s that they’re just completely incoherent.

The problem is simple: If Marty has a photo that is now missing his brother, the Marty who time traveled to the past probably never had a brother to miss. The changes Marty causes are not happening continuously with the future, they are happening well before that future.

This plays on our prejudices as an audience, where we assume that we ourselves would not change along with everything else affected, because well, we’re too important. But we’re not. The future doesn’t respond to what we’re doing right now in some kind of real time, preserving only us. Or more to the point, even if it did, you wouldn’t realize it.

some advice to time travelers (or, “hey, dum-dum, just think about it for a minute”)

Marty’s first question after learning about time travel ought to be what the new rules of causality are: Is he living in a closed loop universe or an open loop one? If you learn nothing else from this post, it should be that if you are ever in the kind of situation where you are travelling through time to the past, your first concern should be to determine, if possible, whether you are capable of royally effing up the future you came from. I mean, think about the children, even if they don’t exist yet.

It is practically a convention in time travel films, however, that no one ever proclaims, ‘Say, this is an incredibly confusing situation we now find ourselves in with possibly universe-destroying potential. Maybe we ought to sit down for several minutes of exposition about the consequences of this new technology.’ In fact, Looper looks, for a moment, like it may do this, but Bruce Willis complains that if he explains the principle of time travel, ‘we’re going to be here all day,’ as if understanding the mysteries of the universe were a homework problem with no practical impact.

At least the younger version of himself is curious about how this whole thing works. Usually characters tend to just roll with the punches and take direction from the most convenient mad-scientist sort. You see, that’s the problem with people today: They are too blindly obedient to authority to question the structure of space-time.

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