The Physics Behind Tenet: Explained
By: Rich Mazzola
Tags: Christopher Nolan, Physics, Tenet 2020

Introduction

Warning: This post contains spoilers for Christopher Nolan’s ‘Tenet’. The goal of this post is to provide a framework to understand the physics that Tenet is based on, thus allowing you to analyze scenes for yourself. I don’t represent that all of the applications of the physics in Tenet are scientifically accurate.

Complaints of Christoper Nolan are that his films are quixotic. That criticism misses the mark; the garadiose nature of his films are meant to inspire deep feeling, reflection and multiple viewings. Each viewing reveals more to the spectator. Like a relationship; the more you put into the film’s experience, the more you get out of it.

It’s only the best of pop culture that works this way. It’s what made infinite jest the darling of literary circles for the last 20 years. It’s not that Fast in the Furious 5 doesn’t ‘kick ass’, it’s just that you don’t need a research budget and a joint to figure out what Vin Diesel’s motives were.

Christopher Nolan gave his loyal audiences this type of introspection and more in his most recent release ‘Tenet’. Part of what Nolan gifts to the viewer is the creation of a new world, governed by new laws of Physics. However, the laws are based on established physical theories. It takes multiple viewings and reflection on the base physics to fully understand what was achieved in Tenet. This post sets out to shed light on the physics that Tenet’s world was based on, and how it relates to the characters’ experiences.

Entropy / Reversing Entropy

In the opening scene of Nolan’s 2000 release ‘Memento’, Guy Pierce is shown shaking a polaroid as the picture moves from clear to opaque, signaling that time is moving in reverse. This is confirmed when moments later a bullet moves backwards through the head of the victim, showing the scene play out in reverse.

The opening scene of Memento shows the narrative in reverse to the audience. This is where the idea for Tenet began more than 20 years ago

In Memento, the story is told to the audience in reverse, and forward, meeting in the middle. Nolan plays on this storytelling technique in Tenet, by making a variation. Instead of scenes being shown in reverse to the audience, the characters are aware the scene is occurring in reverse. Furthermore, they can take part in that reversal. What physical laws need to change in order to enable characters to reverse the flow of time? Entropy is where we’ll start.

What is Entropy?

A detailed description of entropy can be found here. To summarize, Entropy is a measure of how things move from order to disorder. That definition doesn’t tell the whole story though. The best way to think of entropy is via example. Entropy is why sand castles tend to become sand piles over time (more ways to arrange the sand as a pile than as a castle).

Entropy always increases, which means there was a lower entropy in the past. This makes entropy and time indistinguishable. Both time and entropy march in one direction. This is why entropy gets the moniker ‘the arrow of time’.

Entropy in Tenet

An important feature of entropy is that it’s probabilistic. That means that it’s not impossible that the sand pile could blow into the exact shape of the sand castle from which it came, it’s just overwhelmingly likely that it wouldn’t. In Tenet, the creation of the algorithm means that the probabilities can be controlled, thus weaponizing entropy for the protagonist and Andrei Sator. The best way to think of the ‘turnstiles’ in Tenet are as probability shifters.

With the probabilistic outcomes of systems being controlled, it means that it is no longer overwhelmingly likely that order → will move to disorder. It can be reversed. Nolan names this new feature of entropy ‘inversion’. This allows any character who can access a turnstile to reduce the disorder of a system locally¹, effectively reversing the arrow of time in that system.

Linear time as we experience it only exists because entropy is always increasing. So when you begin to mess with entropy, you mess with time. One of the nuances of Tenet is that characters can’t time travel to change the past. As Neil states, “whatever’s happened has happened”. Nolan finds a new take on time travel at the confluence of the military strategy ‘pincer’, where troops attack from both sides, and inversion, where entropy is reversed. This means that the arrow of time can only be reversed to gain information. Then the information can be shared with un-inverted characters to change the future. In this way, all warfare in Tenet is informational and physical.

I’ve detailed how characters can use turnstiles to complete a temporal pincer attack, moving both forwards and backwards through time:

The ontological paradox and reversing entropy turns the arrow of time into a loop

As you can see, this gives rise to two paradoxical ideas:

If the past can’t be changed, how does Event A become Event B? If entropy gives rise to the arrow of time, if entropy is reversed, what happens to the arrow?

Nolan addresses these concerns by invoking the ‘ontological paradox’², where you can alter the past but only because it’s already been altered, because you always altered it and are thus playing out your part in the timeline, completing the causal loop³. In summary, Event A can only be changed to Event B because it already was changed to Event B. So it’s not changing the past, it’s fulfilling the future which would always happen⁴. A second nuance is that there is no ‘time travel’ in Tenet, but time reversal. This is why the character can’t go months or years into the past, but must reverse time and experience the reversal to the point at which they wish to gain new information.

So in the end of Tenet, when Neil inverts himself to save the protagonist by taking a bullet, he doesn’t change the past. He always inverted himself, to travel back to the event, to save the protagonist. The ending of Tenet shows the ontological paradox off to viewers. Neil is stuck in the loop of changing the past, which he’ll always change, because it’s already been changed.

This ending reveals Nolan’s cleverness and the depth of what he’s created. He’s invoked standard physical laws e.g the past cannot be changed, AS WELL as invoking the ontological paradox that states the past can be changed because it already was. Fucking brilliant.

“The Arrow of Time dictates that as each moment passes, things change, and once these changes have happened, they are never undone. Permanence is a fundamental part of being human. We all age as the years pass by — people are born, they live, and they die. I suppose it’s part of the joy and tragedy of our lives, but out there in the universe, those grand and epic cycles peer eternal and unchanging. But that’s an illusion. See, in the life of the universe, just as in our lives, everything is irreversibly changing.”

— Brian Cox

Second Law of thermodynamics

The second law of thermodynamics is responsible for the principles of entropy that were reviewed in the previous section e.g. that the entropy of a system always increases.

However, the entropy of an object can also be a measure of the amount of energy which is unavailable to do work.

The reason that the definition says ‘unavailable to do work’ is because of the nuanced difference between thermal energy and heat. The difference is:

Thermal energy is not in the process of being transferred; it’s being stored. So under current conditions it is unavailable to do work.

Heat is thermal energy that’s moving. This is energy that’s available to do work.

Put simply, heat is the transfer of thermal energy.

Adding heat to a system will always increase the entropy of a system, because you’ll increase the number of potential arrangements that the molecules can have.

Take the example below:

System 1: cold, low entropy. When molecules are cold, they move slower. This is why ice forms. Ice represents the inability of molecules to move.

System 2: hot, high entropy. When molecules heat up the velocity increases and the likelihood that the molecules will bounce into one another increases.

Thus, heat increases entropy.

Now, when Sator blows up the car we see a combustion (chemistry governs the details of the combustion, if you’re interested, check here) where entropy of the system is increased. Using the diagram above, system 1 (car functioning properly) becomes system 2 (car exploding).

Sator ‘blowing up’ the protagonist’s car, causing it to freeze. Source: Warner Brothers

When the scene is shown from the protagonist’s perspective, who’s invented along with his car, the entropy is actually being reversed. This means we’re moving from system 2 (hot, molecules moving fast) to system 1 (cold, molecules moving slow) as shown in the diagram above. This is why the protagonist freezes in the car when the ‘explosion’ occurs. This process of moving from system 2 to system 1 highlights the nuanced difference between time travel and the reversal of entropy.

In typical sci-fi, time travel means jumping a certain distance into the past. With Nolan’s inversion, the entropy of systems are being reversed. Since entropy gives rise to the arrow of time, technically time is being reversed, but characters aren’t time traveling. It’s likely you’ll need to read that sentence a few times, it took a while to write.

Antimatter

At the beginning of the universe, photons created in the big bang converted their energy into matter and antimatter. When balanced, they would annihilate each other and nothing would exist.

To help contextualize the relationship between matter and antimatter, Famed physicist Richard Feynman created the ‘Feynman Diagram’:

One intriguing feature of Feynman diagrams is that antiparticles are represented as ordinary matter particles moving backward in time — that is, with the arrow head reversed on the lines that depict them. For example, in another typical interaction (shown in the figure), an electron collides with its antiparticle, a positron (e+), and both are annihilated. A photon is created by the collision, and it subsequently forms two new particles in space: a muon (μ−) and its antiparticle, an antimuon (μ+). In the diagram of this interaction, both antiparticles (e+ and μ+) are represented as their corresponding particles moving backward in time (toward the past)⁵.

Nolan has taken the idea that a particle / anti particle could actually be the same particle moving forwards and backwards through spacetime and allowed the characters to interact with that. We’re shown the protagonist (particle) and inverted protagonist (anti particle) fighting one another. At first viewing of the scene from the protagonists perspective the ‘particles’ are different. On second viewing, we know the ‘particles’ are the same, but moving in different directions of time.

Conclusion

Christopher Nolan has developed a cult like following by creating a 3 step process that transforms hard science into an immersive world in his films:

Deeply understand a physical law e.g. Entropy of physical objects give rise to the arrow of time. Find nuances in that physical law e.g. technically, entropy is probabilistic, so physics allows for the entropy of objects to decrease thus reversing the arrow of time. Take the nuance and explore what would happen if that applied to the physical state e.g. if physical objects didn’t adhere to entropy and the arrow of time, what would the implications be? What decisions could characters now make with this knowledge. In this framing, it’s easy to see how Tenet came to be.

The loudest criticism of Tenet is: This doesn’t make sense, this wasn’t thought out, not enough attention to detail was paid to character development. For anyone for who thinks a lack of thought was what plagued Tenet I would like to share the following:

Sator: Main Villain

Arepo: The painting

Opera: Opening scene

Rotas: Sators Company Name

The most fair criticism is Nolan expects too much out of his audience. From where I’m sitting that’s a feature; not a bug.

¹ Locally meaning each character can do this

² Source: https://www.empireonline.com/movies/features/tenet-explained-inversion-temporal-pincer-movements-timeline-plot/

³ Note that a critical part of the ontological paradox is that the arrow of time becomes a circle.

⁴ You can conclude that free will doesn’t exist, and I believe this is a correct interpretation https://www.psychologytoday.com/us/blog/plato-pop/202009/tenet-nolans-masterpiece-fatalism-and-free-will

⁵ Direct quote from Encyclopedia Britanica 2007