From: Hannah Pell
“If we imagine a being whose abilities are so sharpened that it can follow every molecule in its course, then such a being, whose properties are essentially as finite as our own, could do the impossible to us,” wrote James Clerk Maxwell in his Theory of Heat (1871). With this theorem Maxwell raised considerable doubts about the second law of thermodynamics, which says that the entropy of an isolated system that is left to spontaneous evolution cannot decrease. (Imagine how air always flows from hot to cold and eventually reaches thermal equilibrium). Such a “being” was later referred to by William Thomson in an article published in Nature in 1874 as a “demon” because of its “far-reaching subversive effects on the natural order of things.” This is how Maxwell’s demon was born, who outlined a paradox that remained unsolved for 115 years.
Maxwell envisioned the thought experiment as follows: Imagine two adjacent rooms – A and B – containing gases of the same temperature and pressure, separated by a microscopic hole in the wall. The demon can control whether the hole is opened or closed and only allows fast moving (hotter) particles to pass from room A to B, not the other way around. On the other hand, slow molecules migrate from room B to A and thereby cool room A. In principle, this arrangement would violate the second law, since the work of the demon would be negligible (although the physicist Leo Szilard later pointed out that the detection of the particle speed by the demon would require energy). The demon effectively instigates a decrease in entropy, a physical implausibility.
|Image source: Plenio & Vitelli, “The physics of forgetting: Landauer’s principle of extinction and information theory”,(2010).|
Image source: Plenio & Vitelli, “The Physics of Forgetting: Landauer’s principle of extinction and information theory” (2010).
Further developments linked thermodynamic principles with information. In 1948, mathematician Claude Shannon interpreted the entropy of a random variable as the average information content or uncertainty inherent in its potential values. Later, in 1961, the physicist Rolf Landauer showed that the erasure of information corresponds to an increase in entropy, which physicists today interpret as a measure of hidden information about the system in question. This idea is known as Landauer’s principle.
Such physical concepts are woven into the universe of the science fiction film Tenet. Prior to its August 2020 release, director Christopher Nolan told EW that the film is not about time travel, but rather about different ways that time can work. “Not to get into a physics class, but inversion is that idea of the material that has its entropy inverted, so it goes backwards through time relative to us.” Nolan hired Nobel laureate in physics, Kip Thorne, to advise on the script, but the producers would “make no argument that this is scientifically correct”.
Is the main character – simply referred to as the “protagonist” – a modern Maxwell demon? Let’s take a look to find out. (Spoilers ahead!)
We learn early on that a strange class of material is being accumulated in what appears to be a top secret warehouse. Labeled “inverted”, they were supposedly made in the future and behave as if they were moving back in time from the characters’ current perspective; the protagonist catches a ball and “entropy runs backwards,” explains the scientist. “Don’t try to understand.” (If you look closely, Maxwell’s demon appears on the whiteboard in the background).
Later, chaos unfolds in the Freeport. “What happened here?” asks Neil. “It hasn’t happened yet,” replies the protagonist. And we see the scene unfold as if it were rewinding. Discarded weapons – the “inverted materials” – fly off the ground. Even the “antagonists” are reversed and fight the other way around. Cars drive backwards on the motorway and you can hear Estonian backwards on the radio. Negotiating with Andrei Sator, the main antagonist, becomes an exercise in finding out what information was shared in any version of the past. On the way we hear of a “positron moving backwards in time”, the grandfather paradox, plutonium and theories of parallel worlds. We are finally told about the “temporal forceps”, a time-bending mission technique in which half of the team of good guys travels forward in time and the other half backwards (exactly ten minutes, asymmetry is reflected in “Tenet”). The time reversal is controlled by an algorithm that Sator wants to control in order to rewrite history.
“Does it mean being here now that it never happened?” Protagonist asks.
This non-linear temporality is reflected in the development of the film. As viewers, we get very little clues about the dynamics of the conflict, which creates a perception of increased disorder, but the characters make decisions as if they already know what is going to happen. However, it turns out that Protagonist knew all along, or at least knew some version of him. The “doctrine” was developed by a protagonist of the future; we finally see him literally struggling against himself.
The physics in Tenet actually constructs a “twilight world”. Even though the film isn’t particularly scientific, who knows? Maybe the future looks different – time will tell. “What happened, happened, which is an expression of belief in the mechanics of the world,” proclaims Neil.