How the Film Tenet Explores Entropy, Information, and Maxwell’s Demon


By: Hannah Pell

“If we conceive a being whose faculties are so sharpened that he can follow every molecule in its course, such a being, whose attributes are still as essentially finite as our own, would be able to do what is impossible to us,” wrote James Clerk Maxwell in his Theory of Heat (1871). With this sentence, Maxwell cast considerable doubt on the second law of thermodynamics, which states that the entropy of an isolated system left to spontaneous evolution cannot decrease. (Think of how air always flows from hot to cold, eventually reaching thermal equilibrium). Such a “being” was later characterized as a “demon” by William Thomson in an 1874 article published in the journal Nature, because of its “far-reaching subversive effects on the natural order of things.” Thus, Maxwell’s demon was born, outlining a paradox that remained unresolved for 115 years.

Maxwell envisioned the thought experiment as follows: Picture two adjacent rooms — A and B — containing gases at equal temperature and pressures 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 through from room A to B, not vice versa. By contrast, slow-moving molecules pass from room B to A, thereby cooling room A. In principle, this arrangement would violate the second law, as the work done by the demon would be negligible (although physicist Leo Szilard later pointed out that the demon’s acquisition of particle speed would require energy). The demon is effectively instigating a decrease in entropy, a physical implausibility.

Image Credit: Plenio & Vitelli, “The physics of forgetting: Landauer’s erasure principle and information theory,” (2010).

Further developments linked thermodynamic principles to information. In 1948, mathematician Claude Shannon interpreted the entropy of a random variable as the average level of information or uncertainty inherent in its potential values. Later, in 1961, physicist Rolf Landauer showed that erasing information corresponds to an increase in entropy, which physicists now interpret as a measure of hidden information about the system in question. This idea is referred to as Landauer’s principle.

Such physical concepts are woven into the universe of the science fiction movie Tenet. Before its release in August 2020, director Christopher Nolan told EW that the film is not about time travel but rather deals with different ways that time can function. “Not to get into a physics lesson, but inversion is this idea of the material that has had its entropy inverted, so it’s running backward through time, relative to us.” Nolan hired physics Nobel laureate Kip Thorne to consult on the script, but caveats that the producers were “not going to make any case for this being scientifically accurate.”

So is the main character — referred to simply as “Protagonist” — a modern Maxwell’s demon? Let’s watch to find out. (Spoilers ahead!)

We learn early on that a strange class of materials is being amassed in what appears to be a top-secret storage facility. Described as “inverted,” they’ve supposedly been manufactured in the future and behave as if they move backward in time from the characters’ present perspective; the Protagonist catches a bullet, and “Entropy runs backward,” the scientist explains. “Don’t try to understand it.”(If you look closely, Maxwell’s demon appears on the whiteboard in the background).


Later, chaos unfolds at the Freeport. “What happened here?” Neil asks. “It hasn’t happened yet,” the Protagonist replies. And we watch the scene unfold as if on rewind. Dropped weapons — the “inverted materials” — fly off the ground. Even the “antagonists” are inverted, fighting in reverse. Cars drive backward on the highway, and Estonian is heard backward on the radio. A negotiation with Andrei Sator, the main antagonist, becomes an exercise in discerning what information has been shared in some version of the past. Along the way we hear about a “positron moving backward in time,” the Grandfather paradox, plutonium, and theories of parallel worlds. We are finally told of the “temporal pincer,” a time-bending mission technique in which half the team of good guys travels forward in time and the other half travels backward (ten minutes exactly, asymmetry reflected in “tenet”). The time-reversal is governed by an algorithm, which Sator aims to control in order to rewrite history.

“Does being here now mean it never happened?” Protagonist asks.

Such nonlinear temporality is reflected in how the film unfolds. As the audience, we’re given very few clues as to the dynamics of the conflict, creating a perception of heightened disorder, yet characters make decisions as if they already know what will happen. It turns out, however, that Protagonist did know the whole time or at least a version of him did. The “tenet” was devised by a Protagonist of the future; we eventually see him literally fighting himself.

The physics throughout Tenet indeed constructs a “twilight world.” Although the film is not particularly scientific, who knows? Maybe the future looks different — only time will tell. “What’s happened has happened, which is an expression of faith in the mechanics of the world,” Neil proclaims.

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