Tenet is the latest science fiction film by Christopher Nolan – who has already impressed us with films that are very well documented from a scientific point of view, such as Interstellar – that presents us with an amazing proposal of time travel, with concepts as incredible as bullets that come out of the body and go back into the gun, people driving backwards … This time, what the director asks us has entropy as its protagonist, a concept of physics a bit complicated to understand, but what, at the same time, it alludes to our common sense when it comes to imagining how objects should be arranged in the universe or, rather, how systems should manifest themselves in their different possible states. What is entropy, then? What does it have to do with time travel?
Despite the fact that the RAE defines entropy as “the measure of the disorder of a system”, any physicist who is reading these lines will surely ‘squeak’ this description; on the contrary, entropy has more to do with the concept of probability . This was stated by the particle physicist Javier Santaolalla and the theoretical physicist José Luis Crespo, two popularizers who have tried to explain entropy in an understandable way on their YouTube channels.
To understand entropy, let’s take an example available to all of us: let’s imagine that we have a box with a gas inside it. What is more likely? That the gas appears scattered throughout the available space, or that, instead, all its atoms are limited to a small space on one side of the box?
Entropy is the law by which processes that we consider irreversible are governed, but not because there is a physical force that forces the particles to behave in that way; but because it is the most likely to happen.
The idea of entropy would come to provide a mathematical way to encode the intuitive notion of which processes are impossible, but without violating the fundamental law of conservation of energy. That is to say, a glass vase still on a table will tend to preserve its state, unless some force falls on it. If we throw the vase on the ground, it will break, and the pieces will scatter. If we group all these pieces together and throw them back on the ground, what is the probability that they will return to the shape of the vase? By the entropy principle, they most likely adopt any other distribution (which we subjectively consider to be chaotic or disordered). There is very little chance that the pieces will rebuild the vase, but there is no physical law to prevent it. It is simply extremely unlikely. For this reason, relating disorder or chaos with entropy is not exact from a strict point of view, since the order is subjective.
Therefore, the most exact definition of entropy would be the following: “Entropy is a quantity that measures the number of microstates for the same macrostate of a system.” This definition links with the second law of thermodynamics, which states that the spontaneous change of an irreversible process in an isolated system (which is not subjected to any force or energy exchange with the environment) always proceeds in the direction of increasing entropy . In other words, a system always evolves to its most probable configuration, which is, at the same time, the one with the most microstates, and the one with the most entropy.
Now let’s go back to our example. How many microstates does the ‘vase’ system have in pieces? A huge amount. That, spontaneously, the pieces return to adopt the original shape of the vase (unless we put it back together with glue), would be so improbable that, mathematically, it could not occur even in the entire age of the universe. Meanwhile, the system will tend to configure itself in the most probable way.
The ‘arrow of time’
And finally, what does this have to do with Tenet’s time travel? According to physicists, entropy is useful to us because it allows us to establish ‘the arrow of time’. By the law of entropy, we can also extract that the universe always evolves to a state of less information or of information destruction. In other words, it is easier to destroy than to build; and this allows us to discern the past from the future.
