Time Travel and Causal Loops in Dark

Taylor W. Cyr

***This post contains spoilers for the first season of Dark.***

If you have heard anything about the German Netflix series Dark, you probably know that time travel plays an important role in the story. Over the course of the show’s three seasons, Dark tells an inter-generational story of the residents of Winden, a small German town that many of its own residents despise. The show begins in roughly the present (the first episode debuted in 2017 but was set in 2019) but periodically jumps 33 years through time—following characters who travel through time. And, like any well-written time travel story, it turns out that certain crucial future events have influenced past events, which have themselves influenced those crucial future events, giving rise to what is sometimes called a “causal loop,” or—as it is called in Dark—the “bootstrap paradox.”

Dark raises a host of philosophical issues, from the various paradoxes of time travel to questions about free will and determinism, personal identity, death and non-existence, and many others as well. It is a show that sparks intriguing discussion among viewers and that rewards multiple viewings, even if only to keep track of who is related to whom (and how!). Here I will focus exclusively on the puzzle of causal loops, and I will only refer to certain events from the first season. (If you have not yet seen seasons two and three, go watch it! You will be glad I did not give anything away.) But before getting into the causal loop that I want to discuss, it is worth taking a step back and clarifying what we mean by time travel in the first place.

An ordinary traveler is someone who moves around in space. A time traveler, then, would seem to be someone who moves around in time. But, in one sense, all of us are moving through time—at a rate of roughly one minute per minute. So how is a time traveler different? The standard view among philosophers, going back to a very influential paper by David Lewis (1976), is to distinguish between two senses of time: there’s time as it is experienced by the person (or as measured by their watch), which is called personal time, and there’s time as we ordinarily think of it (time itself), which is called external time. When a person time travels, they experience moments of external time out of order, so to speak. In the case of time travel to the past (backward time travel), which will be our focus here, moments that are earlier in external time are experienced as later in the time traveler’s personal time.

Back to Dark. Throughout the first season, we see a middle-aged stranger lurking through Winden. At one point he travels from 2019 to 1986, via a passage in Winden’s caves. The stranger brings a suitcase containing a mysterious device to a clockmaker named H. G. Tannhaus, asking him to repair it. At the end of the season, the stranger returns to 1986, and to Tannhaus’s shop, to collect the repaired device. Upon entering, he sees two devices, one looking much newer than the other but otherwise remarkably similar. Tannhaus explains that the two devices are the same device—that one is an older stage of the other—and that he had begun constructing the device many years earlier. But, and here’s the paradox, Tannhaus says to the stranger, “If you hadn’t shown me what the device looks like in the future, I wouldn’t have been able to build it” (Season One, Episode Ten, Alpha and Omega). The stranger goes on to reveal that the device may be used to travel through time, and he takes the newer-looking iteration of the device with him.

If we were to chronicle the history of the mysterious device in its own “personal” time, we would begin with a much younger Tannhaus beginning to build the device at time t1 but not knowing how to complete it. At the next stage, t2, Tannhaus would complete the device, but only because he was able to copy the construction from a similar-looking but much older device, provided by the stranger. The completed device would then be collected by the stranger at t3 and used to travel through time. Eventually the device, now looking much older, would be brought to Tannhaus at the earlier time t2 and would be used to construct the newer-looking device. (We do not know what happens to the older iteration of the machine after t2, but as it was in disrepair we can suppose it was no longer used.)

What’s paradoxical about this device is that the only way for it to be built is for it to “already exist” in some sense, namely the sense in which it must be brought back from the future in order to be built in the first place. The device’s construction depends on the older iteration of the device, the existence of which itself depends on its being constructed. This is not to say (as the characters in Dark sometimes mistakenly put it) that the device goes on forever and ever, endlessly. Each stage of the device’s existence happens only once (that is, occurs at exactly one moment of external time), so no stage is repeated or has existed an infinite number of times. Still, the information used to build the device is not provided by anything outside of the loop, and that is puzzling.

We can distinguish between two puzzles about causal loops, which Ryan Wasserman (2018) calls the “bootstrapping paradox” and the “ex nihilo paradox,” respectively. First, causal loops seem to require self-causation. In the case of the mysterious device, there is a sense in which it is the cause of its own existence, for if it hadn’t “already” existed in the future and been brought back to the past, it could never have been created. It seems as though the device has brought itself into existence “by its own bootstraps”—a reference to a classic time travel story by Robert Heinlein (1959), “By His Bootstraps.” Second, causal loops seem to require that certain events be uncaused. In the case of the device, where did the information about how to build the device come from in the first place? There is a cause of the device’s existence at each stage of the loop, but there is no cause of the information about how to build the device from outside of the loop. So the existence of the information used to construct the device cannot be causally explained.

Now, we could regiment these puzzles about causal loops into arguments against the possibility of (backward) time travel (modified from Wasserman 2018, chapter 5):

(1) If time travel were possible, then causal loops would be possible.

(2) If causal loops were possible, then self-causation/uncaused events would be possible.

(3) Self-causation/uncaused events are impossible.

The case of the device from Dark highlights the plausibility of (2), and I won’t contest that premise here. Instead, I’ll briefly explain why one might reject the other premises, and I’ll take them in reverse order.

Clearly there’s something weird about self-causation and uncaused events, but perhaps that’s exactly what we should expect when we’re dealing with the extraordinary circumstances of time travel. Ordinarily we can’t (literally) pull ourselves up by our bootstraps, but here’s an atypical way to attempt the task that might succeed (Wasserman 2018: 157):

Step 1: Build a time machine and travel back to the days of your youth.

Step 2: Put your younger (and much lighter) self in a pair of boots.

Step 3: Pull him or her up.

And there’s nothing incoherent about this sort of bootstrapping, so perhaps the same applies to the bootstrapping involved in the construction of Dark’s mysterious device.

Similarly, it would be out of the ordinary for an event to be uncaused (at least, so many of us think), but perhaps the event of a causal loop simply isn’t like ordinary events. Consider how Lewis responds to the paradox:

The parts of the loop are explicable, the whole of it is not. Strange! But not impossible, and not too different from inexplicabilities we are already inured to. Almost everyone agrees that God, or the Big Bang, or the entire infinite past of the universe, or the decay of a tritium atom, is uncaused and inexplicable. Then if these are possible, why not also the inexplicable causal loops that arise in time travel? (Lewis 1976: 149)

Even if causal loops are bizarre, then, that is not by itself a reason to think that they (or the time travel that gives rise to them) are impossible.

So much for (3). But there’s reason to doubt (1) as well, for it may very well be that not all cases of time travel would generate a causal loop. Suppose you jump five seconds into the past but 10,000 miles from your point of departure; presumably this spatial distance would preclude your arrival in the past (only five seconds prior) from causally affecting any events that took place near your point of departure. (For a similar example, see Hanley 2004: 130.) Of course, this kind of time travel case would not be nearly as interesting as cases involving loops—indeed, Dark wouldn’t be the same without loops!—but the possibility of time travel does not seem to depend on the possibility of causal loops.

Without giving away anything from seasons two and three, I will conclude by noting that Dark’s mysterious device is not the show’s only paradoxical object, and the paradoxes introduced in the show are not limited to paradoxes about causal loops. (For a survey of some other paradoxes of time travel, see Cyr 2018.) Still, the use of the device in season one is an especially clear illustration of a causal loop, and one that plays a crucial and fascinating role in the story Dark tells.

Taylor W. Cyr is Assistant Professor of Philosophy at Samford University in Birmingham, Alabama and co-host of the podcast The Free Will Show. His philosophical interests include free will, moral responsibility, death, and time (especially time travel). His website is taylorwcyr.com, and he is on Twitter @taylorwcyr.

References:

Cyr, Taylor (2018). “Time Travel,” 1000-Word Philosophy. https://1000wordphilosophy.com/2018/05/23/time-travel/

Hanley, Richard (2004). “No End in Sight: Causal Loops in Philosophy, Physics, and Fiction,” Synthese 141: 123-152.

Heinlein, Robert (1959). “By His Bootstraps,” in R. Heinlein, The Menace from Earth. Hicksville, N.Y.: Gnome Press.

Lewis, David (1976). “The Paradoxes of Time Travel,” American Philosophical Quarterly 13: 145-152.

Wasserman, Ryan (2018). Paradoxes of Time Travel. New York: Oxford University Press.