Post on 2025/8/3

By Frank Kan

Edited by Fiona Qi

“We live in a twilight world,” declares the opening line of Tenet, a movie directed by Christopher Nolan. For more than a code, the utterance depicts the landscape of the movie: a world caught between the forward march of time and its reversal. Tenet operates within a universe where foundational principles of physics—entropy, temporality, and causality—are actively reinterpreted. Central to its premise is the notion of temporal inversion, a process whereby objects and subjects are able to traverse time in reverse through the manipulation of their thermodynamic properties.

At first glance, such a premise may appear to disregard established science. However, Tenet achieves a remarkable level of verisimilitude by grounding its temporal mechanics in genuine scientific ideas. Rather than obliterating the laws of physics, the film extrapolates from them. To that end, it builds its world around the second law of thermodynamics, which asserts that entropy always tends to increase in an isolated system over time. In other words, it states that energy spreads out over time.

In Tenet, Objects and individuals can invert their entropy, enabling them to move backward through time relative to the normal world. Thus, time does not erase entropy. Rather than ignoring the second law, Nolan chooses to reinterpret it. Entropy gets reversed for specific entities, altering their thermodynamic trajectory. This distinction allows the film to explore time not as an absolute but as something conditional that depends on the observer’s entropy flow.

The film's adherence to physical constraints deepens its ethos. Inverted characters must wear oxygen masks because their lungs can no longer process normal air; the direction of entropy affects chemical reactions and even respiration. Inversion also alters how vision, momentum, and combustion work. For instance, bullets “un-fire” because the entropy of their motion is reversed, but this does not happen arbitrarily. Entropy inversion is not a spontaneous event; it requires deliberate, technologically mediated intervention. Specifically, objects must pass through a turnstile device, a machine that alters the thermodynamic direction of the system by inverting its entropy. Therefore, a bullet “un-firing” into a gun is not the reversal of a past event, but a preconditioned outcome governed by the inverted timeline of the bullet's motion. The action appears reversed to a normal observer because the bullet is causally moving from the future toward the past. Another example is combustion. The protagonist exposed to flames while inverted does not burn; instead, they experience hypothermic damage, as the fire appears to contract and absorb heat. From the perspective of the inverted individual, the thermodynamic process is reversed: heat appears to flow inward, consistent with decreasing entropy, instead of chemical bonds breaking and releasing heat like normal fire.

These phenomena inevitably induce Maxwell's demon thought experiment explored in physics since the nineteenth century, which imagines a tiny being (demon) who can sort molecules in a gas, lowering entropy seemingly without expending energy and violating the second law. Though ultimately resolved through information theory—the demon must acquire, store, and erase information, which restores overall entropy—Maxwell’s demon remains a metaphor for entropy disorder through intelligent intervention.

In Tenet, the inversion algorithm operates as a kind of cinematic Maxwell’s demon, a future intelligence capable of acting on physical systems to reverse entropy flow. This narrative maintains the integrity of thermodynamic laws, yet making the phenomenon mechanistically plausible.

Ultimately, as a milestone in the sci-fi movie, Tenet builds its world with speculative extrapolation from real scientific ideas such as entropy, thermodynamic irreversibility, and informational paradoxes like Maxwell’s demon. Its commitment to internal coherence and physical plausibility makes it an unusually rigorous entry in the science fiction genre.