Tenet: How time reversal and inversion works in the new Christopher Nolan film

But how does this “inversion” work? And does it have any basis in real science? The answers, inevitably, are fairly complicated.

“The movie challenges our traditional ways of interpreting time, interpreting what we perceive is real, our learned behaviours,” star John David Washington says in the film’s production notes.

“There’s a lot more going on. I had never read or seen anything like this before. Nobody has. Chris deals head on with how we understand the physics of time, all through the lens of this character. I don’t know what his fascination with time is, but I love how he deals with it in his movies.”

Early in the film, Clemence Poésy’s scientist explains this new understanding to The Protagonist (Washington) with the example of inverted bullets – in other words, bullets that travel backwards from our perspective.

Cause and effect are how we understand time, she explains – but for these bullets it’s reversed. The effect is there, but the cause hasn’t happened yet. Consequently, they can fly backwards into a gun, or be caught in a hand – but only if the person handling them makes themselves remember dropping or firing them, even though they haven’t yet.

And notably, these inverted bullets are said to be more deadly - “an inverted weapon might be able to affect our past as well,” she informs him.

Of course this sounds fantastical, but according to director Christopher Nolan it’s not entirely outlandish, playing off a general rule of physics – that a lot of the laws work in whatever direction time works – that applies to almost every law bar one.

“Every law of physics is symmetrical—it can run forwards or backwards in time and be the same—except for entropy,” Nolan said of his brain-melting concept.

In this context, entropy refers to the transfer of energy characterised by the deterioration or decline of something – in other words, how we, objects around us and the universe as a whole change over time, which from our perspective looks like gradual destruction, but from a scientific perspective is more of a redistribution of energy.

“The theory being,” Noland continued, “that if you could invert the flow of entropy for an object, you could reverse the flow of time for that object, so the story is grounded in credible physics.”

Of course that’s a big if, but don’t take his word for it – Nolan also got some advice from physicist Kip Thorne (who helped create the similarly time-twisting Interstellar) on whether his idea of reversing entropy could equate to reversing time.

“I did have (physicist) Kip Thorne read the script and he helped me out with some of the concepts, though we’re not going to make any case for this being scientifically accurate. But it is based roughly on actual science.”

So far, so relatively understandable. But where were these inverted bullets made? Well actually, the bullets were discovered, not made – because they haven’t been invented yet. One day in the future someone will invent them, and because they travel backwards through time, they’ve arrived in the present day.

Or, in Poésy’s words: “Someone’s making them in the future – they’re streaming back to us.”

So there you have it – inverted bullets. But the time reversal technology doesn’t stop there. As the film continues, The Protagonist faces strange foes with oxygen masks who appear to be moving backwards – these are revealed to be inverted people, who have used the future technology (also sent back, like the bullets) to reverse their own timeline.

Later, The Protagonist gets to experience this for himself by going through a Temporal Stile – a sort of sideways, metal revolving door that reverses his timeline – and lives through inverted time.

And there are immediately new rules to understand. One is that you can’t enter the stile unless you see your other self – inverted or normal – returning at the same time.

More practically, the Protagonist is also informed that “regular air won’t pass through inverted lungs” – hence the need for his own oxygen supply, which he can invert by bringing a tank with him through the stile. Other physical processes are similarly reversed – air resistance moves behind you while you run, the heat of a fire turns to ice on your skin – though some, like gravity, apply more or less the same.

This in turn leads to some uniquely staggering action setpieces with characters moving forwards and backwards at the same time, cars driving in reverse, explosions receding back into their source and much, much more. And according to Nolan, this all required “a ruleset that was not as simple as reversing the camera or things going backwards.

“There is an interaction between the direction of time and the environment we’re in: how things move around us and even the air we breathe,” he added. “The notion of inversion is an asymmetrical one, so the ruleset was complicated and had to be addressed in more complicated ways.”

From the perspective of a “normal” person moving from time the inverted people run backwards, heal from wounds and perform impossible feats – from the perspective of the inverted, it’s the “normal” people doing this. In Tenet, sometimes this means we get a double perspective of the same scene at different points in the story, adding new layers as the film continues.

At this point, things again seem fairly clear. You can create objects, like bullets that move back in time rather than forwards – you can also put yourself in that position. But as the film continues, characters are also able to use the technology to travel in time. The technique is simple – invert time, wait for however long you need to (say, two weeks in a shipping container, out of the way) then re-invert yourself to normal time.

Rather than popping back to where you left off, you’ll be re-posited in the correct chronal direction where you are. In this example, you’re now two weeks in your past, moving forwards, while your past self is now in your future, inverted from your perspective. Your other past self, moving forwards, is also knocking about.

Again, it’s a lot to get your head around – and Tenet still doesn’t stop there. A couple of times the film introduces the idea of a Temporal Pincer Movement. The idea is that during an operation, one half of your team moves through it in normal time – the other half, after the fact, is inverted and lives through it backwards. This way, one half can warn the other about what’s coming and make sure the mission is a success, while also accomplishing things the other team can’t.

Of course, this in itself is a bit of a paradox – the inverted people are already there when the normal ones travel through the events, even though they’re technically only going to experience them after they happen – but to be honest, in a film this complicated you have to more or less go with the flow.

There’s more to inversion that could be explained here that delves into more spoilery territory – all we’ll say is, imagine the concept applied to the entire world, not just individuals and objects – but generally speaking, Nolan’s big move is to create a way of time running backwards for specific people and things, that can be used imaginatively for espionage.

“We’re all a little bit obsessed with time, aren’t we?” producer Emma Thomas said.

“It’s something that, whoever you are, wherever you’re from, whatever your life experience is, you know you can’t do anything about it. It rules you. I can’t really speak for Chris, but that’s my perspective on it.

“It’s interesting because, given the fact that time is universal, it’s also something that you feel very subjectively: you know, kids feel time very differently from adults. I feel like it’s speeding up immeasurably. And then, during this pandemic, our perception of time has been a whole other thing…days have felt like weeks and months have felt like minutes. It’s been very weird.”

Certainly, for both the characters and audience of Tenet, the times are a-changin'.

Tenet is in UK cinemas now. Want something to watch? Check out our TV guide