For quite literally years I’ve been fascinated by the changes the film Arrival makes to the plot of Story of Your Life, the Ted Chiang story it’s based on. I’m going to thoroughly spoil both (which should be fine since they came out in 1998 and 2016) because I finally found the angle to explain that fascination. Here we go, buckle up.

Both plots concern first contact with aliens, and a linguist, Louise, who learns to communicate with them. Their language is based on a perception of spacetime very different to ours: they perceive entire spacetime histories, which as we perceive it corresponds to memories of the future being available in the present. Learning this language modifies Louise’s consciousness, giving her the same acausal view of the universe.

But the story and the film disagree on the metaphysics of this perceptual shift, and we see that disagreement in two major differences in plot: a personal tragedy that Louise remembers from her future, and the political resolution of the tension caused by first contact. It’s the metaphysics I find fascinating, but to understand them we need to study the plot differences.

First the political. In the film Louise uses her atemporal perspective to avert catastrophe, convincing a politician to choose peace by giving him a personal message based on a future memory. In the story, instead, her atemporal perspective makes her aware of the impending resolution of the contact situation (the aliens leave as abruptly and inexplicably as they arrived), but she tells no one and takes no action. This alteration makes perfect sense for a film: it increases the tension, it gives the protagonist something to protag. But it establishes as canon that information from the future can have an impact on the present.

And that means the film cannot take the story’s approach to Louise’s personal tragedy. In the story, her daughter dies in a climbing accident that Louise knew (by future memory) was coming, and that she could have prevented simply by telling her daughter not to go on that particular trip. In Chiang’s metaphysics the atemporal perspective brings with it a compulsion to act to bring about the future memory, which overwhelms any motivation Louise might have had to make future-informed choices in the present. Obviously that’s not going to fly in the film’s version, when she’s done exactly that in the political sphere, so the daughter’s death is rewritten as the result of an incurable disease: equally foreseen, but no longer avoidable. Instead it is framed explicitly as her choice to have a child, in full foreknowledge of the grief to come from losing that child.

So there’s the enormous gaping chasm between the themes of film and story. The film is about the choice to love-and-lose, made more pointed by the clear knowledge of when and how the loss will occur, but also more intense by the clear knowledge of how much love will come in the meantime. We might say it’s about external determinism: how events in the world will happen, whether we want them to or not, and how we have little influence to bring to bear on them, but also how we must still choose how we will engage with them. Which is a grand theme, all power to them, although it seems a bit unnecessary to bring in aliens and whacko causality: it’s an equally fraught choice to have a child in poverty, or in time of war, or in the face of climate catastrophe, for instance.

The story, on the other hand, is about internal determinism. Its theme is that we do not make choices. Once Louise knows her daughter died in a climbing accident, she also knows she didn’t warn her off: she no longer has the capacity to choose to warn her off, even though the moment at which she might do so is still in the future.

This is a much trickier concept than the films, and I’m not really surprised they softened it for the screenplay: not just because their version allows the political tension and protagonism, but also because it’s very hard to convey what’s really intended here. I have enormous respect for Chiang for coming up with the idea, and his attempt is quite heroic, but it’s also at root quite incoherent.

Louise narrates her periods of atemporal perception as feeling “like the carefully bland exchanges of spies who meet in public, but never break cover”. From the perspective of the narration, then, what she experiences at that moment is unlike what she would have experienced without the atemporal perception; but what she does is constrained to match what she would have done if perceiving and remembering only her past. Unless Chiang is a mind/body dualist, that makes no kind of sense!

And I mean, these causal loops are pretty tricky anyway. In the movie’s metaphysics, where choices can be made based on atemporal perception, could Louise have a future-memory of simply letting her child die in a climbing accident? Seems unlikely. But what future-memory should she have instead? If she warns her off, there’s no future in which she died, so the warning wouldn’t be based on anything: that’s incoherent. Chiang’s metaphor of Fermat’s principle of least time suggests that there should be a “most coherent” history-line, which I suppose might include Louise coincidentally suggesting an alternative, or (more tragically) might have her trying to warn her daughter but failing to convince her to stay home. Chiang’s version, though, goes all the way to Louise behaving exactly as she would have if she had no memory of the future at all: I can’t see any way to make that consistent with the narration saying that her internal experience of events in the moment is affected by her atemporal perception.

That’s where I think the movie’s version of how things work might actually be more appropriate than Chiang’s. Sure, remembering the future is probably inconsistent with the phenomenology of “choosing”; maybe even the phenomenology is, as Chiang suggests, “a sense of obligation to act precisely as she knew she would” (I’m sceptical about obligation, but let’s run with it); still it seems like what she will do (and knows she will do, and feels obligated etc) should be influenced by what she knows, which can include other information from the future. The weirdness of Chiang’s setup is his insistence that what she knows she will do also must be precisely what she would have done without foreknowledge.

Much though I respect Chiang, I think he missed a trick here. He almost showed us what an atemporal perspective could look like, and he argues convincingly that it’s incompatible with the experience of free will, but he messed up the details trying to make Louise’s history coherent from both perspectives at the same time. If he had allowed her actions to look prescient to an outside observer, I think he would have had to work harder for his tragedy but he could still have had it, Cassandra-style.

But then the film, which perhaps gets this retroactive causality thing slightly better, chose instead to avoid the question entirely by making the tragedy unavoidable. And in the process both trivialised the matter (from “free will is an illusion” to ”it’s worth choosing love even when it brings pain”) and made it, quite frankly, a bit unclear why they bothered with the sci-fi setting. On the other hand the heptapod scenes are utterly gorgeous, so I really shouldn’t complain.

In short: the story is a brilliant concept, the film is an excellent cinematic treatment of it, and neither of them gets it right. But if you put together some pieces from each, you can just about see the shape of what “getting it right” might have been. Which, honestly, is pretty impressive for something so bonkers. Hats off to them all.