Tenet and time: The facts behind science fiction

Earlier this year, Christopher Nolan – best known for The Dark Knight – released his latest groundbreaking film, Tenet. Now you may be asking: why, when The Mancunion has a (fantastic) film section, is a science writer talking about a film?

Well, whilst this film has brilliant acting, lighting and other film funk, this is not why I love it. These things only serve to amplify the best aspect of this film: Physics.

Tenet is not Nolan’s first flirtation with science, as shown in the 2008 film Inception. This introduced the niche concepts of ‘multi-tiered dreams’, ‘shared dreaming’ and ‘the origin of inspiration’, which were all interesting in the plot’s context and added weight to the film. However, Tenet dwarfs this contribution to the genre.

The film centres on an organisation which is devoted to researching and preventing a ‘temporal war’ – a war related to time. To achieves this, it uses ‘inverted’ technology and battle strategy, which involves going back in time. This idea all begins with the second law of thermodynamics.

The simplest way to state this law is that; ‘The Entropy of an Isolated System must always increase.’ Entropy is the measure of disorder within a system. The entropy of an object relates to the number of ways it can be arranged and disordered. For example, sand is a high entropy object, as the grains can take many forms. However, to use Brian Cox’s analogy, a sandcastle, which arranges the sand in a more orderly way, has less entropy. Eventually, the sandcastle will crumble, and its entropy will increase. The second law of thermodynamics explains how the amount of ‘disorder’ in the universe must always increase.

Relativity, Einstein’s big idea, describes how the concept of ‘when’ and ‘where’ something has happened is not fixed, it depends on the observer. Nolan dabbles with this concept in the film Interstellar. Entropy, however, helps us to create an ‘arrow of time’, helping us to distinguish the past from the future.

It’s this rule that Tenet chooses to abandon. Nolan explores the idea that if you could reverse entropy, you could travel back in time. In forward-moving time, a gun shooting a bullet causes a hole in a wall. If time is in reverse, we see the hole before we see the gun that causes it. It’s not hard to see why this creates huge problems in the world of physics. The event would appear to have no cause.

In terms of the scale of the very small, where quantum mechanics governs the universe, measuring a condition changes that condition. The ‘Shrodinger’s Cat’ thought experiment describes how a cat in a box is both alive and dead until you open the box. These measurements set boundaries where time becomes irreversible, defining a sense of past and future.

Some scientists have extended this idea to the ‘Many Worlds’ theory, which states that every one of those possible measurements has happened and led to a different world in which this new condition is met. Obviously, in this new world, another ‘set’ of quantum measurements is made, splitting this world into more worlds.

To put it into a situation relatively easy for us to understand, if, right now you’re drinking cup of coffee, there may be another world in which you’re drinking a cup of tea. These worlds branched out from the moment you could have put coffee grains or a teabag into the cup.

In Tenet, this theory is assumed to be true, with a key exception; these branches overlap and interact with each other.  This leads to a key scene in the film, where John David Washington battles his inverted self.

All of these concepts are confusing, for physicists as well as for the general public. Attempting to describe time invokes many paradoxes, and many theories that contradict each other. We do not yet have any defined, universally accepted idea as to what time is.

Nolan makes a lot of invalidated assumptions in Tenet, but the way he manipulates theories is incredibly interesting. I’m sure it will give many physicists the world over some good conversation starters.