The Science of Tenet Time-Inversion

Entropy and the arrow of time. Bonus-fun: Replace the concept of entropy with the concept of happiness.

Lenka Otap
Sep 20, 2020 · 10 min read
Image by Anton Kufer from Pixabay

‘Don’t try to understand it. Feel it.’

The above is a quote from the movie, Tenet. While we tried to feel it in the movie theater, in this article, we will try to understand it. The arrow of time, entropy, reversed universes, and the science behind it.

Bonus in the end: A silly analogy of entropy that leads to maximizing happiness. Based on the same analogy in my university thermal physics book, and expanded a bit.

Last, but not least: If you are curious about the time concept, and would enjoy a free challenge to change your perception of time through movie examples, then join my 5-day FB challenge: Time in the Movies. (Starts October 5th)

Now, let’s get on with the movie Tenet and the arrow of time.

The official trailer for the movie Tenet. Warner Bros.

Entropy and the arrow of time

There is nothing in the fundamental physics equations, that says anything about which way time should flows. Even when time is part of the equations, it doesn’t matter if the change is positive or negative. There is no difference between the past and the future in a physical 4D ‘block-universe’, where every point in the timespace is equally valid and ‘now’ is physically nothing special. This point of view is called eternalism and is often used in physics.

Only one equation in physics deals with a specific direction of time, and this is the 2nd law of thermodynamics. It deals with entropy, which is the physical term that Tenet is based around.

The 2nd law of thermodynamics says, that without any intervenience, the configuration of a system (that is anything that consists of a large enough number of particles) will always become more disordered in time and will require less information to describe.

Since entropy is the only physical property, that is dependent on the direction of time, we call entropy “The arrow of time”.

When we only perceive one direction of time, it’s due to the entropy only be able to increase in a closed system. Smoke does not go back into the cigarette, eggs don’t unscramble, and we don’t see someone catching a bullet with their gun as it happened in the movie Tenet when the arrow of time was reversed.

But physically a negative entropy change doesn’t violate any fundamental equations and thus is not impossible. It is just extremely unlikely. Extremely in the order of statistically happening once in some extreme number of times the age of the current universe. The reason why eggs don’t unscramble is a statistical probability that it just won’t happen.

Entropy is what gives us the arrow of time on the macrolevel because the scrambled mess that can be described with many different combinations on the microlevel is more likely than an ordered egg with a very specific configuration.

If we zoom into the particle-level of our world, the microscopic world, the direction of reactions doesn’t matter. If you reverse a film of particles moving and vibrating, it will not look weird to us, no matter which direction we played it. Not until we zoomed enough out, to see the bigger picture.

Thus entropy is an emergent concept when we look at our world. Something isn’t a property of the particles themselves, just as temperature, which also isn’t a property of one single particle, but only shows as the average energy of a large number of particles interacting in a local system. And thus time itself might be an emergent concept on a macro-level.

Can the arrow of time be reversed?

Living things actually do decrease entropy locally, and this is one of the better definitions for what life is. As living things, we can do some work and create order. But our muscles and metabolism require energy in form of light or food (other living things) we make the world a messier place, no matter how many structures we create. Living things increases the total entropy in the Universe, although they (we) can decrease it locally.

If life decreases entropy locally, does it mean that we can reverse time locally? No, unfortunately, since the total entropy is still increasing, it only counts if entropy can be decreased in a closed system. Our system includes the energy that we need and the carbon dioxide that we exhale, and the entropy change is always positive, in our perceived direction of time.

As mentioned, there is nothing that theoretically forbids us to reverse the arrow of time and go towards a smaller entropy, except that we just don’t see it, because of the statistical probabilities. But let’s imagine that it somehow could be possible, just for the mental exercise.

If we could reverse entropy so that it would decrease, would time run backward, just as it did in Tenet? And how could it be possible?

Let’s consider the case of inverted anti-particles in theoretical physics and a bonus case of a mirror universe, somehow colliding with our own.

Wheeler-Feynman’s time-reversed electron world

The physicists, Richard Feynman and John Wheeler, once proposed that a positron (the anti-particle of an electron, that is an electron with a positive charge) is an electron moving backward in time.

A Feynman diagram is used in particle physics to describe the interactions of fundamental particles and forces. In this diagram, an electron and a positron (left) annihilate each other with the result of a photon (middle) which then results in a quark-anti-quark pair and a gluon (right). When interpreted to include retrocausality, the electron was not destroyed, instead, it becomes a positron and moves backward in time.

The Feynman diagram shows the annihilation of an electron and a positron (anti-electron, e+) producing a photon. Time is moving in the right direction, and hence it can be interpreted that there is no annihilation of particles and anti-particles, but the electron instead becomes the positron (e+) and moves backward in time.

This interpretation is as accepted as any other nowadays. Feynman even boldly suggested that all electrons are the same one and single electron, moving back and forth in time creating the complex world that we live in.

Every particle that we know has an anti-particle, which essentially is the same particle with all properties, but with the opposite charge. All matter has anti-matter, although the Universe that we know consists mainly of matter.

Tenet is probably based on this idea, and it moves the quantum-level theory into our macro-size. When people go through the time-reversal machine, they walk into a world where the Universe is made of backward-moving antimatter.

Just like the electron is the same particle as the backward-moving positron, the protagonist in Tenet is fighting himself when he moves backward in time. We see this scene twice. The first time, we (and the protagonist) don’t know that the opponent is himself, and the second time we realize that he is fighting himself in an inverted version when we see the fight scene again from the inverted point of view.

In this case, we revert the particles of the people who go through the machine, so that they become antimatter moving backward.

The respiratory system in the anti-body would need anti-oxygen to breathe. But reversing the time-flow would mean that in the reverse-world you would breathe in anti-CO2 and breathe out anti-oxygen instead of the opposite process that we need. Therefore, reverse travelers wear an oxygen mask. The oxygen does get reversed as well of course, so it becomes anti-oxygen for the antimatter body.

The mask used for being able to breathe anti-oxygen in the anti-body in reversed time. Image Credits: Warner Bros.

“Any sufficiently advanced technology is indistinguishable from magic.” — Arthur C. Clark

Mirrored anti-universe and Janus Cosmological Model

In the Janus Cosmological Model (JCM), the Universe started as two mirrored universes with ordinary matter in one and anti-matter of opposite charge in the other. Those two Universes would have been created from the same Big Bang singularity with opposite arrows of time from t=0. In contrast to current theoretical physics, the antimatter in JCM has a negative mass.

Using this cosmological model as a theory behind the reversal of time for Tenet, it would imply that the reversed universe exists along with our own and that we somehow in the future found a connection to the reversed universe.

Our universe expands in one direction from the singularity known as the Big Bang, or Janus point, while our twin universe expands in the opposite direction.

Two mirrored universes expanding in opposite time direction from the big bang singularity, or Janus point. Image credits: NASA Spaceflight.

Stephen Hawking once theorized about the reversal of entropy if it reached its maximum state. This was later disproved by the physicist Andrei Sakharov, who instead argued that entropy could only increase, and only be reversed when it has a minimum value. He then argued for a “cold model” of the big bang, where the entropy was at its minimum and from there, two twin universes could have been created, with opposite directions of the arrow of time.

That’s the physics part of time and entropy. Now, let’s have some (more) fun!

Fun with entropy: Replace it with happiness

There is a “Silly analogy” chapter in my University Introduction to Thermal Physics book.

Imagine that there is a world, maybe not unlike our own, where the exchange value is money and the more money we get, the happier we are. So in this world:

Money is like Energy
Happiness is like Entropy
Generosity is like Temperature

We exchange energy (money) and more receiving more energy leads to higher entropy (happiness).

A rise in temperature usually follows with receiving more energy, but it doesn’t have to be so. This is for example seen in the fusion of stars, where an increase in energy goes to the gravitational potential and holding the star together while the internal pressure rises.

What temperature (generosity) really measures is the willingness of an object to give up energy (money).

In this world, overall happiness is constantly rising and the reason is money.

I’ll try this out on an even different kind of world:

Knowledge is like Energy
Happiness is like Entropy
Curiosity is like Temperature

Happiness will increase through exchanging knowledge (energy). The more knowledge people in a society will receive, the more overall happiness. That is why education is so important. And as they say, the more you know, the more you realize that you don’t know, and thus you also become more curious (temperature = willingness to give away energy).

With a little stretch, curiosity would be defined by the willingness of a person to share their knowledge. I know, I’m stretching it a bit now, because the opposite is more like the definition: A curious person wants to receive as much knowledge as possible. I will dare to assume, that a curious person also wants to share more of their new and exciting knowledge. Maybe we could also have used communication in our analogy. The combination of curiosity and communication will lead to an overall happier life for a society.

In Schroeder’s thermal physics book, he shows three graphs of a relationship between energy and entropy:

An Introduction to Thermal Physics, by Daniel V. Schroeder, First Edition, p. 90.

Let’s translate this into our new world:

  • Normal: Happiness increases with more knowledge. It naturally increases more in the beginning, but you keep being curious, learn more, and happier.
  • Miserly: Happiness increases potentially faster with just a little knowledge, so you quickly end up not needing more! You stop being curious because you are happy without further knowledge. Ignorance is bliss and you’re a happy idiot.
  • Enlightened: Happiness increases when you give away knowledge.

I think I like this world. I believe in this world. Let’s all learn more about the world and the Universe. About the little things and the big things. Let’s share our knowledge and let’s replace judgment, fear, and hate with curiosity on a path to an overall happier life.

Replace judgment, fear, and hate with curiosity.

Stay curious: Learn more about time here

Want to learn more about time in the movies and how to relate it to your personal time? Join my Facebook challenge and change your perception of time through movies during a 5-day challenge. It‘s about time: physical time, biological time, and neurological time. Join my free Time in the Movies challenge (Oct. 5th-9th) here.

→ Update: ←
The challenge is over (you are still welcome to join and watch my videos), but I have now moved to a more general group for science and curiosity, which you are vel much welcome to join!

Key points

  • Entropy is the measure of disorder.
  • Entropy is, just like temperature, an emergent property that only exists with a sufficiently large system of particles, not as a property of single particles.
  • Entropy always increased in a closed system and is the only physical property where the direction of time matters, hence it is called the arrow of time.
  • In Tenet the entropy is locally reversed due to a future technology that is not further explained.
  • The Janus Cosmological Model theorizes that the Big Bang created two mirrored universes, each with an opposite direction of time from t=0, where entropy would be reversed in our anti-universe.
  • Tenet could be a local collision with such a reversed anti-universe.
  • If entropy is happiness, energy is knowledge, and temperature is curiosity, then the more knowledge we get and share, and the more curious we become, the happier we will be as a society.
  • Stay curious, join my free and fun science and curiosity FB group.


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Lenka Otap

Written by

Computer scientist and almost-astrophysicist. Curious about life, the universe, and everything. Are you too?



where the future is written

Lenka Otap

Written by

Computer scientist and almost-astrophysicist. Curious about life, the universe, and everything. Are you too?



where the future is written

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