The Physics of Tenet Is Shaky, but It Still Kicks Arse

The Physics of Tenet Is Shaky, but It Still Kicks Arse
Image: Warner Bros. Pictures
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“Don’t try to understand it,” a scientist tells the protagonist of Tenet, as she briefly explains the physics of Christopher Nolan’s $US205 ($271)-million, time-travelling spy thriller. Sure, the physics is often unrealistic and confusing, but it’s fascinating. And with its many Easter eggs, Tenet sets up some nice jumping-off points for Wikipedia rabbit holes. (Sator square? T.S. Eliot’s “The Hollow Men”? Soviet Closed Cities? Good stuff.)

I am not a physicist, though I do explain physics concepts professionally, and I found the fantastical ways the movie attempts to tackle some real-world science impressive. It’s a worthy addition to the list of movies that people who think they’re smart like to recommend to each other (like Memento, Inception, and Interstellar). I had a blast.

Illustration: Gizmodo Illustration: Gizmodo

Tenet is, at its core, a cat-and-mouse jaunt through space and time. John David Washington plays the protagonist, potentially a CIA agent, who with the help of agent Neil (Robert Pattinson) has to stop a Soviet-born time-travelling villain named Andrei Sator (Kenneth Branagh) from committing nefarious deeds. The story unfolds in typical Christopher Nolan style, with ample foreshadowing and details simmering up slowly such that you don’t know what’s going on until a sizable chunk of the movie has passed.

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But rather than just travelling back in time to stop the bad guys, characters pay occasional visits to “turnstiles” that reverse the flow of time. Action scenes will involve different characters travelling both forward and backward in time, with bullets travelling in and out of guns and bombs exploding and… other bombs exploding backward simultaneously. The key concept at the core of the movie’s conflicts, and where all the amazing special effects happen, are “temporal pincer movements.” The regular pincer movement is a military strategy that involves trapping the enemy from the front and back. The film’s temporal pincer movements instead feature characters trying to outsmart the enemy by attacking from both forward, from the present, and backward, from the future.

As for the physics, well, the technology is extremely hand-wavy. We don’t learn more about how they develop this time travel technology in the future, other than that its creator really didn’t like that she developed it. The turnstile is more or less a spinning room, and we hear lots of characters reminding the protagonist, and us, not to worry about it. I honestly prefer it that way, because it allows viewers who just want to enjoy really cool fight scenes to do so, but more importantly, it leaves lots of room to talk about some of the nifty physics theories.

Most important to the movie’s curious physics is the flow of time. Time is interesting among the universe’s properties, in that we experience it in only one way, unlike space, though which we can travel in many directions. The laws of motion don’t actually forbid us from travelling backward through time; Albert Einstein’s theory of special relativity treats time as another dimension of space and as a property that depends on the person experiencing it. Someone theoretically travelling at nearly the speed of light would experience time as usual, but if they observed a stationary person, the stationary person would appear to age much quicker. Special relativity’s effects on time have led to plenty of creative mind games and paradoxes.

In the real world, we only experience time as moving forward, in part due to entropy.

Entropy is a property of matter that defines how much energy is not available to make things (like a chemical reaction, for example) happen. If you want specific things to happen in any physical system, this requires available energy acting in an ordered way, so more entropy means more unavailable energy and more disordered randomness. The second law of thermodynamics says that the entropy of an isolated system always increases with time. I like to think of it as a Jenga tower: If you seal off a Jenga tower in a room, it will always tend toward a pile of blocks. If you un-isolate the system, you can temporarily decrease its entropy by introducing hands to rebuild the tower. However, if we treat the entire universe as an isolated system, then, overall, things will tend toward a pile of cosmic rubble in the far-distant future.

The second law of thermodynamics is a standout among physical laws; almost all of these laws work the same forward as they do backward, but the fact that entropy never decreases with time is a one-way rule. Physicists posit that this law, in real life, allows us to perceive the forward movement of time — since entropy is always spontaneously increasing, time is moving forward. Therefore, by assuming that reversing entropy would reverse the flow of time, or maybe more accurately, by voiding the second law of thermodynamics, Nolan finds a way to explore some of those wacky questions that physicists grapple with when they try to apply the mathematics of special relativity to the real, human-scale world. This entropy-reversing part isn’t how things work in real life, though.

Tenet scientist Laura, who was assigned to explain this entropy inversion to our protagonist, doesn’t get into the details but says that it’s got something to do with radiation and antimatter. Early in their training, real-life particle physicists learn about antimatter, stuff that is identical to regular matter except it’s a mirror image with the opposite charge. But the mathematics of antimatter also allows physicists to interpret it as regular matter moving backward in time, as illustrated by Richard Feynman’s famous diagrams.

A Feynman diagram showing the radiation of a gluon when an electron and positron are annihilated. (Image: Public Domain) A Feynman diagram showing the radiation of a gluon when an electron and positron are annihilated. (Image: Public Domain)

Most physicists don’t think that antimatter is actually time-travelling matter — the maths just works out this way — but it’s fun to think about what it might mean if reactions where particles meet their antiparticles and annihilate are actually just the result of a particle switching direction from forward to backward in time. The film’s plot hints that our backwards-travelling characters are basically antimatter versions of themselves, by saying that the characters will annihilate themselves if they come in contact with themself…s.

The characters aren’t really made of antimatter, since if they were, all of their atoms would find anti-atoms to annihilate rather quickly out of the turnstile. Some of the ways that the film deals with these “inverted” time travellers interacting with the real world, like bringing breathing apparatuses with them through the turnstiles so they can breathe inverted air while they travel backwards through time, make sense, physicist Claudia De Rham told the Los Angeles Times. Others, like fires causing them to freeze, are a little bunk.

Wonky science aside, the film is a fun intro to thinking about some of the big questions that physicists face when they try to apply the strange behaviour of subatomic particles to the universe we live in. If physics allows for time travel, then what if you went back in time to kill your grandfather, for example? If you did, according to Neil, you might enter into a parallel universe (again, the protagonist, and the audience, are asked not to think about it too much), a reference to the Many Worlds interpretation of quantum mechanics. The Many Worlds theory more or less says that whenever a quantum system has multiple possible outcomes (like an electron that can be in one of two quantum states), all of the possible outcomes occur simultaneously in parallel universes, and the observer happens to exist in the universe where the choice they measured exists. The film also nods to some of the first people to ask these big questions and develop the theory behind them, such as Robert Oppenheimer, John Wheeler, Richard Feyman, and others. There really are researchers in quantum computing exploring creating quantum states where they reverse the time for half of the system, but this is more of a mathematical technicality that looks like reversing the time, rather than actually sending information backward in time.

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Guillaume Verdon, quantum physicist at X, told me he wasn’t too peeved by the inaccuracies and enjoyed the film for the same reasons that anyone might. “I’m really a sucker for these Christopher Nolan movies. The cinematography is great, and the soundtrack is great. It gets you thinking, and it gets you hooked into trying to piece it together.” He liked Nolan’s attempt at writing a story using the same paradoxes that physicists think about when they try to apply certain rules to contexts where they don’t usually appear, like applying interpretations of antiparticle behaviour to people.

As the characters repeat throughout the movie, it’s really not worth thinking about it too much, and it doesn’t make you a genius if you get it (nor does it make you stupid if you don’t get it). Just enjoy the web of the story and the special effects, and then go read up on the science later.