Tenet, the latest trippy sci-fi movie from Inception and Instellar director Christopher Nolan, offers a unique take on the well-worn genre of time travel movies - but how solid is the science in the movie? Nolan once again recruited theoretical physicist Kip Thorne, who served as a consultant on Interstellar, to advise on the script and ensure that Tenet was anchored in the real laws of physics and time, while also taking some creative liberties with them.

John David Washington stars as the Protagonist, a CIA agent who is recruited to a mysterious organization called Tenet. He learns that a war is being waged from the future, where technology has been invented that allows objects and people to "invert," reversing the flow of their entropy so that they travel backwards in time instead of forwards. Tenet was also created in the future, and its goal is to stop the antagonists from setting off a doomsday weapon that will wipe out both the past and the present.

Related: Tenet's Timeline & Time Travel Rules Explained

To help moviegoers wrap their brains around the concepts in Tenet, Screen Rant spoke to Dr. Lucian Harland-Lang, a theoretical particle physicist at the University of Oxford. Specifically he works in the field of phenomenology, applying theoretical physics to the real data being collected at the Large Hadron Collider. And while Tenet might not appear to be a movie about particle physics, the behavior of particles actually gets to the very heart of its time travel mechanics.

What Is Entropy?

Tenet Movie Green Glove

Entropy is a term in thermodynamics that is most simply defined as the measure of disorder. The more disordered particles are, the higher their entropy. Liquids have higher entropy than solids, and gases have higher entropy than liquids, and the universe is constantly becoming more chaotic over time. Mathematician James R. Newman called this "the general trend of the universe toward death and disorder," while physicist Arthur Eddington coined the rather poetic term "the arrow of time." Harland-Lang describes entropy as a "probability argument." The entropy of a closed system can only increase over time, never decrease.

Think of an egg dropped on the ground. So long as time is moving forward the egg can shatter (becoming more disordered), but it will never reform as a whole egg (becoming less disordered). The flow of entropy is the only thing preventing this from happening, since all the other relevant laws of physics are symmetrical: anything that can happen forwards can also happen backwards. "You don't see eggs reforming on the ground and jumping back up, but taken to a logical conclusion, physically speaking that would be allowed by the laws of physics," Harland-Lang explains.

Because of this symmetry there is actually a non-zero possibility that the egg could repair itself. For this to happen would require that the movement of all of the air and ground molecules - through which the sound and heat energy of the egg were carried when it originally cracked on the ground - occur in the reverse direction. But for every molecule in the egg and its surrounding environment there are very few ways for the egg to reform, and billions upon billions upon billions of ways for it to remain shattered. The probability weighs so heavily in favor of remaining smashed and against reforming that you will never see a broken egg reform. For the same reason you'll never see a bullet pull itself out of a wall and return cleanly back into a gun, or see a car perfectly repair itself after a collision. "It is in effect a zero probability that that's going to happen," says Harland-Lang. "But it isn't zero. So that's how you get around this paradox that, technically speaking, all of these things you're not used to seeing could happen. They just wouldn't."

In the production notes for Tenet, Nolan says that the movie is based around the idea "that if you could invert the flow of entropy for an object, you could reverse the flow of time for that object." Though he doesn't claim that the movie is scientifically accurate, he says that it is grounded in "credible physics." Having seen the film, Harland-Lang more or less agrees with that assessment: "It's not 100 percent grounded in science. It's sort of inspired by it or an analogy to it." If you could invert the flow of entropy for an egg, the egg would not literally start travelling backwards in time. But entropy and time are so strongly linked that if people saw a smashed egg reassemble itself, jump up off the ground and return to the kitchen counter, it would look like time was moving backwards for the egg.

Related: Tenet Cast Guide: Where You Recognize The Actors From

Theoretical Physics In Tenet

Tenet's central idea of people and objects having their time inverted is based on a theory by physicists Richard Feynman and John Wheeler. They're actually name-dropped in the movie when Neil is musing about the meaning of inversion and what the turnstiles do, though it's a throwaway line that's easy to miss. Specifically, Neil refers to Feynman and Wheeler's idea that positrons could be electrons that are moving backwards in time.

Electrons are particles that hold a negative charge, and positrons are antiparticles that have the same mass as electrons, with an equal-but-opposite positive charge. There are other types of antiparticles that mirror other types of particles, like antineutrons and antiprotons. Collectively these antiparticles are known as antimatter. Positrons can be found in natural phenomena like cosmic rays, or created in a particle accelerator like the Large Hadron Collider. Structurally they are the mirror image of electrons, and the Feynman-Wheeler theory posits that if you were able to force time's arrow to march backwards for an electron, it would look like a positron. Moving backwards through time it could co-exist alongside its former self, or even collide with itself.

Tenet wisely doesn't get too bogged down in explaining all of this. Instead it uses the idea of a time war and machines that allow people to reverse direction in time as an analogy for Feynman and Wheeler's model. This is particularly seen in the two versions of the Oslo Freeport fight scene, where the Protagonist fights a mysterious inverted man who is later revealed to be himself. In showing the fight scene twice, Tenet changes our understanding of what we're seeing. The first time around, the audience thinks that the Protagonist and the man he's fighting are two different people. The second time, we realize that we're seeing the same person in different states of being. This is what Feynman and Wheeler proposed: that what we perceive as a particle and an antiparticle could actually be the same particle moving both forwards and backwards in time.

While the theory behind this is scientifically sound, Harland-Lang cautions that Feynman and Wheeler were not literally arguing that the universe is full of reverse time-traveling particles - just that it theoretically could be. "Fundamentally a positron is a thing moving forwards in time," he clarifies. "It's not like there's time travel happening every time we see a positron in the world." The theory is a device for interpreting what we see when we look at electrons and positrons, and Tenet translates that by replacing the particles and antiparticles with people and inverted people.

Related: Tenet Projected For Biggest U.S. Box Office Since Theaters Reopened

What The Turnstiles Do

The effect of the turnstiles in Tenet is not to individually invert every particle in a person's body. If they did that, the Protagonist would be converted into antimatter and he would explode on contact with the outside world. In a reaction known as annihilation, matter and antimatter colliding results in the destruction of both particles (an electron and a positron for example) and the release of energy. We've only ever seen this on an atomic level, but scaled up its destructive potential would be devastating.

Harland-Lang refers to the plot of Angels & Demons, in which a bomb containing just one-eighth of a gram of antimatter has enough firepower to blow up the Vatican. "I haven't sat down and done the maths," says Harland-Lang. "But I think the amount of energy from one person completely annihilating with the rest of the world would destroy the whole world." Indeed, if a 200lb man like the Protagonist was converted into antimatter, the resulting explosion would be equivalent to around 3800 megatons of TNT. To put that in perspective, the most powerful nuclear weapon ever created and tested - the USSR's Tsar Bomba, whose detonation shattered windows and caved in roofs hundreds of miles from the blast zone - had a yield of 50 megatons. Since Tenet doesn't end with the Protagonist exploding the moment he steps outside after being inverted, it's safe to say that the turnstiles don't create antimatter.

It appears that what they actually do is create a closed system where a person's body continues to experience normal entropy, but they are able to move backwards through time within the bubble of their closed system. Think of it like creating a small, Protagonist-sized universe where time flows in the opposite direction to the larger universe. If the Protagonist went through a turnstile and remained inverted for a very long time, from a normal perspective he would look like an old man miraculously defying the second law of thermodynamics by getting younger. Within his closed system, however, he would be ageing normally.

All of the inverted objects in Tenet share the properties of this closed system, which allows them to interact. The Protagonist has to be given an inverted oxygen tank containing inverted air for him to breathe, because it would be impossible for him to breathe air that is experiencing an opposite flow of time to him. It's also important that the turnstile's entrance and exit are in different places, because if they were in the same place then a person entering the turnstile would collide with their inverted self exiting it, and things would get messy. But what if instead of creating a closed system within the universe, you inverted the entire universe's flow of time and entropy? That's where Tenet's doomsday weapon comes in.

Related: Tenet: Inversion & Reverse Time Travel Explained

The Algorithm and Annihilation

John David Washington and Rich Ceraulo Ko in Tenet Movie

The first time that the Protagonist goes through a turnstile in Tenet, he is warned against interacting with his forward-moving counterpart because, like an electron and a positron colliding, the two of them touching would result in annihilation. Again, this is more of an analogy of science than it is scientifically accurate. If the Protagonist's body had been converted into antimatter then he would indeed annihilate if he touched his non-inverted self, but he would also annihilate if he touched any other kind of matter. As Harland-Lang explains, "Each electron is essentially identical, and any given electron if it meets any other given positron would annihilate." The electrons in the Protagonist's body are no different to the electrons in Neil's body, or in Sator's, or the electrons in the air all around us.

Annihilation is also at the heart of Tenet's MacGuffin: a chunky nine-part metal shape called the Algorithm. It's explained that the same scientist who created turnstiles in the future also discovered a way to invert the entire world's flow of entropy. Afraid of what her contemporaries would do with this information, she divided the Algorithm into nine parts, inverted them and sent them back in time. The effect of the Algorithm being activated would be mass annihilation. Every particle in the world would simultaneously be reflected back in time and bump into its past self coming the other way. All of those particle pairs would cease to exist and there would be a release of energy on an unfathomable scale - though, in theory, this explosion would be directed backwards in time, leaving the world after the Algorithm was activated untouched.

Is Tenet Scientifically Accurate?

Tenet movie Robert Pattinson John David Washington

The big question that audiences may have coming out of Tenet is whether or not the movie is scientifically accurate. Could you, by inverting the flow of entropy for an object or person, cause them to start moving backwards in time from the point where they were inverted? "I mean, the short answer is I don't think so, no," says Harland-Lang. But in imagining a world where you could do this, he explains, Tenet raises some interesting questions about time and our experience of it.

"It's definitely open to question whether... even though, as I've described, entropy gives us a natural conception of time intuitively, whether that's really so directly related to how we experience time. That's one question. And even if that is the case, even if those things are intimately tied, nonetheless time is there and it marches forward."

This brings us back to the idea of entropy being time's arrow. If you were lost in time and had no idea which way it was flowing, you could look at the entropy of a closed system. The direction in which the entropy of that system was increasing would be the direction in which time is moving forward. And because our signposts for time passing - people ageing, eggs breaking, mountains eroding - are the result of entropy, it's very difficult to separate our experience of time from our experience of entropy. To us, they seem like the same thing.

Just as Interstellar aimed to accurately portray what it would be like for a person to travel through a black hole, Tenet imagines what it would be like for a person to travel through time in reverse. This leads to some pretty mind-bending fight scenes, but it's also a fascinating gateway into some much bigger ideas.

More: Tenet's Ending Explained