I have recently finished watching the Netflix series “Dark”, a fascinating and addicting sci-fi thriller whose plot involves a sequence of convoluted and intertwined time travels (no spoilers from here on).
The fascination of this show is the strictly logical and consequential way these time travels unfold and concatenate. There is no room for the grandfather paradoxes that abound in many sci-fi and super-hero movies, where characters travel back in time to change a past they don’t like and succeed (Terminator, Star Trek, The Avengers…). In “Dark” you do find characters driven by this motivation, but they inevitably fail (and in doing so, they often prompt the sequence of events they wanted to prevent in the first place). Most of the twists in the series revolve around this simple principle.
A frustrating Deus Ex Machina
What I found really interesting is the way this consequential approach to time travel — that I rationally recognise as more “correct” because it is logical and paradox-free — instinctively felt wrong and unnatural to me. “Why would you do that?! Can’t you see this would ruin your plan?” is what you will find yourself shouting at the characters over and over again. Some of their actions feel nonsensical and designed by the screenwriters with the only purpose to make them fail and fit the consequential approach. You sense the continuous presence of a Deus Ex Machina that just doesn’t feel realistic.
And yet, I would argue that this sense of dissonance is not Dark’s screenwriters’ problem. It is ours. It is not the result of far-fetched screenwriting; it is the result of our illusory sense of free will. And thinking through this “it doesn’t feel right” sense can give us some powerful insight into the illusion of free will.
Are you so different from a tennis ball?
Let me explain my point by starting with a simple example. Imagine a time travel that only involves inanimate objects: let’s say that a tennis ball travels 10 seconds back in time. Now imagine that, once the ball is in the past, it starts rolling down (because it had ended up on top of a ramp) until it hits the replica of itself that exists in this time line (i.e. the same ball, just 10 seconds younger). As a consequence of the impact the second ball ends up in a time travel machine, which immediately activates (let’s say there is an automatic activation system) and teleports the second ball 10 seconds back, on top of the ramp¹. The ball rolls down, hits its past self… and so on (hopefully the image below makes the whole thing intelligible).
If we assume that things like “time machines” are physically possible, then the whole architecture I described is consistent and logical. It is a toy replica of what I called the “consequential approach” in “Dark”: the future is a cause of what happens in the past, and the past never changes its unfolding as that would otherwise change the future that caused it.
But crucially, and unlike “Dark”, when looking at this example you don’t think “why doesn’t the ball decide to turn left and not hit the past self?”. You don’t ask that because you know that the ball does not have any choice but to follow the laws of physics. But now my question is: what’s so different when humans are involved in the chain of events?
An uncomfortable (but old) truth
Our brains are incredibly complex systems, but they are undeniably physical systems that obey the deterministic laws of physics². In this respect, they are no different from a tennis ball rolling down a ramp. If we ended up in a similar time travel loop, we wouldn’t be any more able than a ball to break the eternal sequence of travels. And this would happen even if the guy trapped in the eternal loop is trying really hard not to hit his past self: all his plans and thoughts are only sophisticated tennis balls rolling down a ramp, accidental perpetrators of the future which will cause them. There is indeed a Deus Ex Machina here, it is called physics.
“Dark” is a great show because it depicts this inescapable dynamic at a huge scale. What we feel like far-fetched screenwriting is the result of our illusory belief that our brains are somehow above the law of physics. A belief that we have known to be wrong for at least a century. Time travel might be impossible, but it forces us to face this illusion.
 Yes, this time machine is also a teleport machine… not the cleanest thought experiment, I grant you, but there is no loss of generality here (i.e. one can easily imagine variants of this example without teleportation).
 Quantum mechanics cannot rescue our free will here. For a number of reasons, but the main one is that our cognitive processes are 100% “classical physics”: they can be fully described using the classical theory of electromagnetism.