[Movie explained] The Science Behind “TENET”: Time Reversal and Antimatter (Director Christopher Nolan, Starring John David Washington, Supervisor Kip Thorne)

I wrote this article in Japanese and translated it into English using ChatGPT. I also used ChatGPT to create the English article title. I did my best to correct any translation mistakes, but please let me know if you find any errors. By the way, I did not use ChatGPT when writing the Japanese article. The entire article was written from scratch by me, Saikawa Goto.

Introduction

Movies and books covered in this article

I will write an article about this movie/book

Three takeaways from this article

This is a thought-provoking movie supervised by Kip Thorne, a Nobel Prize-winning scientist!

Self-introduction article

Please refer to the self-introduction article above to learn about the person writing this article. Be sure to check out the Kindle book linked below as well.

The quotes used in this article are based on notes taken at the movie theater from movies in Japanese and are not direct quotes from the foreign language original movies, even if they exist.

“TENET” is Full of Knowledge of Quantum Mechanics Such as “Antimatter” and “Measurement Problem”

This is an Article that Explains “TENET” From the Perspective of Quantum Mechanics

In this article, I will touch upon the content of “TENET”, but my main focus is on how this movie depicts quantum mechanics.

I have watched “TENET” twice. The first time, I went in with no prior knowledge, and the second time, I read various websites before watching it. Both times were very interesting, and as someone who already likes science, it was a movie that stimulated my intellectual curiosity from that perspective.

In this article, I will write about from the hard-to-understand “revolving door” setting to my personal interpretation of the work, from the quantum mechanics perspective.

Although I attended a well-known private university’s science department, I dropped out before entering the specialized course, so my knowledge of quantum mechanics comes only from reading books for the general public. I don’t think there are any fatal errors, but I’m not an expert, so there may be incorrect descriptions in the details. I hope you can read it as “the opinion of an individual who loves movies”.

“Positron,” “Antimatter,” and “Time Inversion”

In the movie, there is a scene where the word “positron” appears. It might be completely incomprehensible to those in the humanities, and even people in science who are not interested in quantum mechanics may have few opportunities to encounter this word.

My main argument in this book can be summarized as follows:

The revolving door in “TENET” is a “matter/antimatter reversal device”.

And “positron” is the “antimatter” that humanity first became aware of. So let’s start with an explanation of “positron”.

“Positron” has almost the same properties as “electron”, but the charge is positive instead of negative. Initially, it was called “antielectron” when it was discovered, but now it is called “positron”.

Anderson, a scientist, discovered the “antielectron (positron),” but there was someone who predicted its existence. That person was a genius scientist named Dirac. However, he did not initially believe in something like “positron,” which ultimately became his prediction. So how did Dirac “discover” “positron”?

At that time, Dirac was tackling a difficult problem of “incorporating quantum mechanics into Special relativity” (even if you don’t understand what Special relativity or quantum mechanics are, it’s okay for now). Both Special relativity and quantum mechanics were known as excellent theories that produced remarkable predictions and results independently. However, when both Special relativity and quantum mechanics had to be used at the same time, unfortunately, they were completely useless.

So at the time, scientists were hoping for an equation that could apply both Special relativity and quantum mechanics at the same time. And it was the genius Dirac who completed the long-awaited equation.

This equation became known as the Dirac equation, and everyone was impressed by its perfect fusion. However, there was one problem. When solving the equation, two solutions would come up, but one of them was a mystery. For example, if one of the answers was “electron”, the other answer was something with the same properties as an electron, but with a different electric charge. At first, no one knew what this was.

Dirac himself couldn’t believe in the existence of “a being that is identical to an electron except for its charge,” which is one of the solutions of his own equation. However a few years later, Anderson discovered the positron. It often takes a long time for scientific theories to be confirmed, so it was quite early that Dirac’s prediction came true in just a few years. Thanks to this, Dirac was recognized as a “prophet of the positron” during his lifetime. I would say he was very lucky.

He spoke self-deprecatingly, saying that his theory was “smarter than him,” and that he couldn’t believe in it himself.

“Antimatter” is the term used to describe substances whose charges only is opposite to a particular substance, such as “positron”. Interestingly, “antimatter” actually moves backward through time. What does this mean?

For example, as previously explained, “positron” is a type of electron with a positive charge. To be more accurate, it is an electron that moves forward in time with a positive charge. And it seems, mathematically, that it is the same as an electron with a negative charge that moves backward in time.

I wrote “seems to be” because I can’t explain this area in detail. Quantum mechanics is full of stories that are far from everyday intuition, so it’s best not to think too much about it. If you want to know more, you may be able to get some kind of understanding of it, if you look up “Feynman diagram” on the internet. (The “Feynman diagram” is a groundbreaking tool for explaining quantum mechanics, but it’s still quite difficult for me.)

In short, the point is that “antimatter” can be considered an entity that is “traveling back in time from the future to the past” in theory.

This is why the word “positron” appears in the movie “TENET.”

Is “Time Inversion” Scientifically Acceptable?

Up until now, I started with the word “positron” and showed that “antimatter travels back in time.” However, you might wonder if it is scientifically acceptable for something to “travel back in time.”

In conclusion, it’s not a problem. There is no fundamental constraint in physical law that says time must flow from the past to the future. Even if time flowed from the future to the past, physical laws would still hold true. Of course, the phenomena themselves would appear to run in reverse, but we could still use the same physical laws to make predictions and derive results.

It is also said that the famous Einstein had seriously considered time travel. Einstein had pondered that “if something could move faster than the speed of light, it should be possible to travel back in time.”

However, in Einstein’s theory of “Special relativity” (I touched on this a bit in the Dirac section), there is a rule called “constancy of the speed of light” which means that nothing can move faster than light. So, is time travel Einstein had thought possible theoretically?

Actually, it turns out that incorporating ideas from quantum mechanics may allow for the possibility of surpassing the speed of light. The key idea here is the “Uncertainty principle”.

Let’s talk about magicians here. Magicians deceive the audience using various techniques. For example, it’s a piece of cake for them to make a coin that was supposed to be in their right hand move to their left hand. The audience doesn’t know when, how, or even if the coin moved. However, in reality, the coin is moving from the right hand to the left hand.

In this case, it would be appropriate to say that “the audience could not observe what was happening while the coin was moving from the right hand to the left hand.”

Similar things happen in the world of quantum mechanics.

In quantum mechanics, there is an extremely short amount of time called the “Planck time,” which is about 5.391 x 10 to the power of negative 44 seconds. You would be able to roughly understand that it is very short. And there are no theoretical constraints on things that happen in shorter time periods than this.

To help you understand this, let’s use the example of the magician from earlier. Imagine that the magician is really throwing the coin from their right hand to their left hand (in short, not using any tricks like having two coins). And let’s say that a human eye can’t detect a coin moving from the right hand to the left hand if it takes less than 0.1 seconds.

In this case, if the magician takes 0.2 seconds to move the coin, the audience will see that they “threw the coin from their right hand to their left hand.” However, if the magician moves the coin in less than 0.1 seconds, the audience won’t be able to see it.

Similar things happen in the world of quantum mechanics. Things that happen in longer time periods than the Planck time are constrained by physical laws, such as the speed of light limit imposed by special relativity. However, things that happen in shorter time periods than the Planck time are not constrained by physical laws, which means they can move faster than the speed of light without breaking the rules of science.

I won’t go into too much detail here, but if you search for things like “virtual particle” or “tunnelling effect,” you’ll understand that there are phenomena that can occur in less time than the Planck time.

So, while Special relativity forbids anything from moving faster than the speed of light, it’s okay to do so for periods of time shorter than the Planck time. And if you could move faster than the speed of light, you’d be traveling back in time.

While time travel may be difficult for humans to achieve, it’s not forbidden by the laws of physics, and those laws still hold even if time were to run backwards.

So why do we experience time as flowing from past to future? Scientists are still studying this, but one idea that’s been around for a while is that it has to do with entropy. But I’ll touch more on that later.

Is the “Revolving Door” a Device that Converts Matter to Antimatter?

In movies, a time reversal device like the “revolving door” appears, but the details of how it works are not explained. Most people watching the movie would understand that “time reverses when they pass through that revolving door.”

However, it is probably not the case. Because in the movie, there is a scene where the protagonist receives the following warning:

Do not come into contact with your other self in this world, as it will cause annihilation and a big explosion.

So, what exactly is “annihilation”?

In fact, this is a phenomenon that occurs when “matter” and “antimatter” come into contact. For example, when an “electron” collides with a “positron”, “annihilation” occurs, and not only do the “electron” and “positron” disappear, but a big explosion also occurs.

In this movie, the actual physical laws are ignored and it is suggested that “annihilation” occurs when the “self made of matter” and the “self made of antimatter” meet. In reality, if “something made of matter” comes into contact with “something made of antimatter”, “annihilation” should occur. However, if this were strictly applied, the “antimatter version of the protagonist” would only be able to touch things made of “antimatter”, which would cause problems for the development of the movie. That would be why this constraint was relaxed.

Anyway, the word “annihilation” is used, so I think it’s based on the idea that “matter passing through a revolving door is converted to antimatter.”

However, even with this understanding, there are still problems. Before explaining that, let me tell you a story about how I couldn’t understand the basic function of the “revolving door” when I watched it for the first time.

“TENET” has too much information, and I couldn’t understand what was going on at all for the first time. At least, I understood that “time reverses when passing through the revolving door,” but I had no idea what they were doing by using that revolving door.

And after watching it for the first time and thinking about it on the way home from the theater, I finally understood that “they were using this revolving door to go back in time.”

In other words, when they pass through the “revolving door,” time reverses, which means that if they spend one week in that state, they can go back one week in time. I couldn’t understand even this setting during being watching it for the first time. Therefore, I had to think about it a lot after watching the movie to understand what they were trying to do with Kat at the exploding airport and why they were on the ship which seagulls flying backwards through the window were looked.

This is too difficult.

My Hypothesis is “the Direction of the Light Entering the Eye Changes”

However, even if the revolving door is a matter/antimatter reversal device, it doesn’t solve the problem. Because in that case, only things that have passed through the revolving door can travel back in time.

However, the protagonist who went through the revolving door experienced visual phenomena such as birds flying backwards and winds blowing in reverse. Since the birds and atmosphere did not pass through the revolving door, they should be “matter” instead of “antimatter.” So why is time reversing?

After thinking about this point for a while, I came up with my own hypothesis. It is that “the direction of light entering the eye changes.” What does that mean?

If passing through the revolving door turns them into antimatter, then their eyes must also turn into antimatter. Since it is possible to go back in time by turning into antimatter, normally, light would enter the eye from the outside, but in the case of an antimatter eye, light might leave the eye from the inside. That’s how I thought about it.

Of course, “seeing” means that the retina catches the light coming in from outside, so if the light is coming out from the inside of the eye, it might mean “not seeing anything”. But let’s put aside that scientific strictness. If we assume that “light entering from the outside of the eye” means “phenomena in the world appearing in the forward direction”, then “light exiting from the inside of the eye” might mean “phenomena in the world appearing in reverse”, I thought.

In other words, the birds are not actually flying backwards, but to the protagonist with antimatter eyes, it appears as if they are.

This is an important point to understand the movie “TENET.” Because in this movie, both forward and backward movements occur within the same world; there are no parallel universes.

For example, let’s say I’m walking through Shibuya Crossing on January 1st. Then, if I enter a revolving door on January 7th and wait for a week before going back to Shibuya, I will see myself walking through Shibuya Crossing. This means that “me on January 1st” and “me who went back to January 1st by entering the revolving door on January 7th” exist in the same world.

Reciprocal phenomena (“birds flying forward” and “the same birds flying backward”) cannot occur at the same time in the same world. So it would be strange if it were not a situation that “birds take one action, but they look different depending on the viewpoint.” Therefore, I think my idea that “in the case of the antimatter eye, light exiting from the inside of the eye” is not completely off base.

However, there is still a problem with this interpretation. It is the existence of the “inverted bullet” that appeared in the facility that seemed to be a laboratory, which was shown at the beginning of the movie.

My Hypothesis Regarding the Relationship Between “What Happens for the First Time to Something Made of Antimatter” and “Entropy”

There is a scene like this. In the lab, a bullet placed on a desk mysteriously fall up in the protagonist’s hand by itself (if this is recorded on video and played in reverse, it appears as though the protagonist placed the bullet on the desk).

At this point in the research facility, the protagonist has not yet passed through the revolving door. In other words, he has not become an “antimatter eye,” so the above hypothesis does not apply. Nevertheless, the bullets show a reverse movement.

Regarding this point, I have come up with a hypothesis that “when something made of antimatter experiences events that happened for the first time in that world, it is observed as a phenomenon where entropy decreases.”

Explanation of “entropy” will be provided later. First, let’s explain “happened for the first time.”

The phenomenon captured by “antimatter eyes” should be “something that has already happened” in that world. At the moment of passing through the revolving door, the character has “antimatter eyes,” and from there, time is reversed, so it means that “antimatter eyes” “capture the phenomenon that has already happened in the past in reverse.” In other words, the above hypothesis of “antimatter eyes” can be applied to “something that has already happened.”

However, if an antimatter object that has passed through a revolving door exists in front of them, that object will experience its first event too. The phenomenon of the bullet returning from the desk to the palm of the hand that was mentioned earlier is also the first time it has occurred in this world due to the protagonist’s arrival at the laboratory.

My hypothesis is that when such a first event occurs with an antimatter-made object, it results in a decrease in “entropy.”

I briefly mentioned “entropy” earlier in the explanation of why humans sense the flow of time.

Entropy is an indicator of disorder, and it’s known as the second law of thermodynamics that entropy (disorder) increases with time.

Now, let’s think about a child’s room. At first, the mother tidies it up neatly, but as the child plays, the room becomes messy (disordered). This is a state where “entropy has increased.” And entropy doesn’t decrease on its own. In the case of a child’s room, the room will not “become clean (a state of low entropy)” without the effort (energy) of “mom’s tidying up.”

Also, it is sometimes interpreted that “the direction in which entropy increases is the direction of the arrow of time.” However, this is not sufficient as a reason for the existence of the flow of time. Certainly, there may be some relationship between “entropy” and “the flow of time,” but it is unclear whether “the flow of time exists because entropy increases.”

However, one thing that can be said for certain is that “the direction of decreasing entropy is the direction of time reversal.”

When time is moving forward, events only occur from the direction of low entropy to high entropy. Even the action of dropping a bullet on the floor from one’s palm is a movement from a low entropy state to a high entropy state (It would be easier to understand which state has a higher entropy in terms of whether or not the opposite phenomenon naturally occurs).

And, in the case of the first event occurring in something made of antimatter, my hypothesis is that the reverse will happen, namely a movement from a high entropy state to a low entropy state. This is exactly what “decreasing entropy” means, and it is also consistent with the idea of antimatter going back in time.

By the way, there’s a scene in this movie where “entropy” is emphasized the most. It’s the scene where “hypothermia caused by gasoline explosion” occurs. Normally, if a gasoline explosion occurs, “heat” is generated and they get “burned”. However, in this scene, the movement is occuring from a high entropy state to a low entropy state, so the opposite of “heat generation”, which is “cooling”, occurs, causing the character to suffer from “hypothermia”. I was amazed at the idea.

By considering “entropy,” the scene where “bullets return to the gun in the palm” at the laboratory can be resolved, and the scene where “bullets return to the gun from the wall where they were fired” can also be largely resolved. If both the gun and the bullets are made of antimatter, then the reverse movement of “the bullet fired from the gun hitting the wall” which means “the bullet returning to the gun from the wall where it was fired” would be a movement of “decreasing entropy.”

However, there is still a mystery about “the bullet inside the wall.” The problem is “where did the bullets driven into the wall come from?” I think that “if I think about it normally, the bullet must have been in a different place before being fired into the wall, so I should be able to ask where it was.”

And regarding this, the concept of the “measurement problem” in quantum mechanics might be applicable.

Considering “the Bullet Inside the Wall” Mystery From the Perspective of the “Measurement Problem” in Quantum Mechanics

The basis for interpreting this section is not my original idea, but something that came up in a discussion with someone else about “TENET.”

Quantum mechanics has a very famous experiment called the “Double-slit experiment.” I encourage you to search for more details, but suffice it to say that it is one of the most mysterious experiments in the history of science.

And from this experiment, a very incomprehensible explanation ended up emerging, which is that “by humans observing an event, it is determined whether it will occur or not.” In other words, it’s like saying “the person who opened the door did so because I observed them,” rather than “I observed the person who opened the door” (it’s a bit different, but that’s the general idea). It’s a very strange conclusion, but in the world of quantum mechanics, there are phenomena that can’t be understood without this kind of explanation.

And using the concept of the “measurement problem,” it may be possible to solve the mystery of “the bullet inside the wall.”

To review the problem of “the bullet inside the wall,” it’s “where did the bullets driven into the wall come from?” However, according to the concept of the “measurement problem,” this question itself is wrong. In fact, we should reverse the logic and interpret it as “since I observed the bullet fired into the wall, it can return to the gun.”

In other words, the question “where did the bullets driven into the wall come from?” is a “meaningless question with no answer” (it is a question like “Which way is north of the North Pole?”). And, we should think that “because we observed the bullets driven into the wall, the event that ‘the bullet was fired from the gun to the wall’ was determined, and that’s why the bullet can go back from the wall to the gun.”

Although there are still parts that don’t quite make sense, there are many scenes that can be understood with this interpretation. For example, in a combat situation at an airport, by observing the “glass with bullet holes”, the fact that the protagonist fired a gun is determined, or in the final combat scene, by observing the “already bombed building”, the fact that the combatants destroy the building is determined. Although the cause and effect are twisted, accepting the conclusion suggested by the “Double-slit experiment” that “observing the result is what determines that the cause occurred” makes many scenes easier to understand.

Resolving the Mystery of the “Revolving Door” Using the “Measurement Problem”

Now, in the revolving door scene of “TENET”, strange phenomena occur that cannot be understood in normal terms, such as “both people who enter disappear” or “the same person comes out two at the same time.” These correspond to the quantum mechanics phenomenon of “pair production and annihilation,” and are exactly related to the “Feynman diagram” mentioned a bit earlier.

Regarding this “disappearing two people” and “emerging two people” phenomenon, even after reading various websites, I couldn’t understand it at all. However, I finally understood when someone wrote, “It’s about Feynman diagrams” (although I only felt like I understood it since I don’t fully understand Feynman diagrams).

I won’t explain “pair production/annihilation” in detail here, but just know that such phenomena exist in the world of quantum mechanics. Occurring “pair production” and “annihilation” in the “revolving door” make it appear as though two people disappear or reappear.

Now, there was something else I couldn’t quite understand while watching the movie. It was the reason why the revolving door needed two doors. If the “revolving door” is a matter/antimatter reversal device, as I hypothesize, then I think that only one door is needed. However, this can also be understood through the concept of “pair production/annihilation”.

Or perhaps it is better to understand that if “pair production/annihilation” does not occur, “antimatter” will not be created. Although this is not an accurate expression, the rough image is that “two people come out” by the occurrence of “pair production” and are converted into “antimatter”, and “two people disappear” by the occurrence of “annihilation” and return to “matter”. In any case, since the events must occur simultaneously for “two people”, two doors are also necessary.

Before entering this revolving door, they were warned to “visually confirm if someone is entering the other door.” This was also a point that I did not understand even after reading some websites, but I was able to understand it from the explanation of the “measurement problem” perspective. In other words, by “I observing that I am entering the other door”, “the event of I entering this door is determined”. It can be said this is also due to the strangeness of the reverse order of cause and effect, which leads to “incomprehensibility”.

The Mystery of the Revolving Door Itself

Let’s talk about the mystery of the revolving door itself, as the final doubt regarding topics like the revolving door and time inversion.

First of all, why does this revolving door exist in the “present” day? The basic reasoning is not difficult to understand. If this “revolving door component” is passed through another revolving door and is antimatterized, the antimatter can theoretically be sent to the “past.” Then, by assembling the antimatter-ized parts, it is theoretically possible to send the revolving door to the “present day.”

However, is it really possible to do such a thing?

In the movie, it wasn’t mentioned explicitly, but I think it’s people from a very distant future who created this revolving door. They made it because of the sense of crisis caused by the earth becoming uninhabitable due to environmental destruction and other factors (in short, trying to deal with reality by rewinding time). And it would take a fairly high level of technology to create this revolving door. I estimate that it was made in the future, at least about 100 years from now, at the earliest.

The mechanism behind this revolving door is that they can go back in time by the amount of time spent. For example, if the revolving door is developed 100 years from now and a plan is made to send something to the past (which would be “now” for us), it would take a long time, about 100 years, to make it happen.

There would be two possible ways to send an object that has passed through a revolving door (an antimatter-ized object) from the future to the past. Those are that someone who has also passed through the revolving door carry, or someone bury somewhere.

If it takes 100 years, it would be difficult for humans to “carry” it even if it is relayed by several people. However, “burying” it is not so easy either. This is because they have to find a place where nothing has happened in that location for 100 years. For example, suppose that the “buried location” is a place where “landslides occurred during the past 100 years but measures were properly taken”. Let’s set it concretely to make it easy to understand.

Suppose that the parts were buried at point X in 2121 in order to transport the parts from 2121 to 2021. However, let us assume that at point X, a landslide occurred in 2025, and the site was then properly backfilled. In this case, if they did not know that a landslide had occurred, the buried object might be damaged. The reason is that from 2121 to 2026 we will be safe, but in 2025 a landslide (the opposite of it) will occur.

It would be quite difficult to find a place where they can bury something safely for 100 years without anything happening. Because, they would have to check 100 years of records for a particular location.

In principle, it is possible to send “things through a revolving door” to the past. However, it may be difficult in reality. Especially since there are multiple “revolving doors” in the “present”, the difficulty would probably double. Well, it’s just fiction, so we may not need to think too strictly about it.

Anecdotes Related to “Time Inversion”

That’s enough science talk. I’ll finish with two personal thoughts on the “inversion” in this movie.

First, there’s a contradiction that I noticed the second time around. This contradiction is probably intentional on the part of the filmmakers and not a mistake. I felt that they prioritized visual clarity.

It’s about “how does spoken sound sound during time inversion?”

First, let’s talk about the car chase scene. There is a scene where the “not time-invert protagonist” says, “Isn’t this Estonian?” However, it is later revealed that this is because the language spoken by the time-invert person sounds like it is played in reverse, so he thought it was Estonian.

In other words, in this car chase scene, it is set up that “when a time-invert person speaks, it sounds like a sound played in reverse.”

On the other hand, in the scene where Kat gets shot, the depiction is different. Here, when the “time-invert Sator” on the other side of the glass speaks, it is not played backwards, but is described as “the mouth moves after the voice is heard”.

So, even though it is the same scene where “time-invert people speak,” the depiction of “inversion” is different.

This, however, would be a matter of direction. There is no other way but to play it backward to convey that it is “inverted” on a phone where only sound is heard. On the other hand, in the scene where Kat is shot, it is meaningless if we cannot hear what Sator is saying. That’s probably why this difference exists.

But thinking about what would happen if time were actually inverted is fun. I probably think it would be played backwards. The “movement of opening the mouth” and the “act of producing sound” occur almost simultaneously, so I feel that there is something wrong with the deviation even if time is inverted. However, if to be played backwards is correct, it would mean that conversation would not be possible between normal and time-invert people, which would be a lonely situation.

Lastly, let’s talk about the points to watch for when I watched the movie for the second time. I paid attention to the “acting during time inversion.”

The director Christopher Nolan is known for using as little CG as possible. So the scene where “everything except for protagonist such as the birds and wind are moving in reverse” is actually “having the protagonist act in reverse and then reversing its footage.”

After watching the movie for the first time, I browsed various websites and also read interviews with the main character. It was written there that “blinking was the most difficult.” This means that it was difficult to act in a way that looks like natural blinking when it was reversed. I thought this was interesting, so for the second time, I paid particular attention to the scenes where the characters would be moving in reverse.

I felt some unnatural movement in scenes of intense fighting already at the moment of the first time I saw them, but I was surprised to see many scenes that did not feel at all out of place the even second time I saw them. The scene where he was doing chin-up on the ship was one of them. In the screen, birds are flying backwards, so it’s definitely a reversed footage, but the protagonist’s chin-up looked perfectly natural. I felt that this movie had such struggles throughout.

Conclusion

I’ve only watched this movie twice so far, but I would like to watch it again if I have the chance. I think there may be new discoveries every time I watch it.

Kip Thorne, a physicist who also supervised “Interstellar,” is involved in this movie. I knew only that information before watching the movie, but I was surprised that it was such a story packed with physics topics.

Christopher Nolan, who never fails to surprise us with new works, is someone to look forward to for his next movie.