It’s been two years since tenet was released in cinemas, with once again, games on the notion of time. After Interstellar Where Mementofor which time is not or does not seem linear either, Nolan is playing on something else this time to distort temporality:entropy. But other ways of time travel exist in science fiction. Some play on universe parallels, others on the idea of a destiny that includes all time travel carried out. Others, finally, experience paradoxes!
“Tenet”: time inversion, which uses entropy
Let’s start with one of the most special: tenet. Released on August 26, 2020, it is based on the principle of time inversion: rather than creating a time travelscientists have created, or rather will create a way to reverse the entropy of matter. Thus, when a person by a device consisting of a turnstile, then a part whose we don’t know what it contains, sees its entropy reversed, everything works backwards for it, including time.
But first, let’s define theentropy. It is a quantity used in thermodynamicsdenoted S, and defined in Joule by Kelvin (J/K). It characterizes the degree of disorder of a system, or disorganization. Also referred to as dispersion of theenergy : the more it is dispersed, the greater the entropy. However, a system always tends towards a situation of equilibrium, therefore a situation where the energy is very dispersed.
Thus, over time, the second law of thermodynamics states that the entropy of an isolated system can only increase or remain constant, but not decrease. When this system interacts with the outside, its entropy can decrease, but at the cost of increasing that of the outside. Thus, we can define as a arrow of thermodynamic time », which imposes the direction of time according to the variation of entropy: the smaller value is always before the larger value.
In tenet, everything rests on the inversion of this entropy: instead of increasing, it decreases. Thus, time, which follows entropy, also flows in the opposite direction. But other elements physical find themselves changed: the flames create cold, life is moving in the wrong direction, breathing is done in reverse too, hence the need to breathe via bottles ofoxygen. The film then stages fights with armies operating in time inversion: past and future coexist and come together in the present. This results in a convoluted time line (or timeline) for people using this device, but which remains coherent with the surrounding world. Christopher Nolan also adds in tenet the absence of free will: each character, although he has the impression of deciding, will always be led to perform future actions that have influenced his past. Thanks to this, the film frees itself from the many paradoxes that a real time travel.
???? SPOILER ALERT????
You did see #TENET but you did not understand anything? Do not panic, follow the plan to find your way through the maze concocted by #ChristopherNolan ⬇️ pic.twitter.com/jtdY7cTZO7
— Tom February (@TomFevrier) August 30, 2020
The temporal paradoxes: resolved by the multiverses?
Because the concept of time travel causes several paradoxes, the best known of which remains the grandfather paradox : a time traveler travels back in time to meet his grandfather, and kills him before he can conceive his children. The traveler then should not have been born, since his grandfather never gave birth to one of his parents. But in this case, he could not have killed his grandfather, if he had not been born, and therefore he should have been born… we arrive at a paradox! Another example is that of the writer, or of predestination: the inventor of the travel machine in the time uses it to send the blueprints of the machine to his past self.
This paradox is similar to that which one might encounter in Interstellarif we suppose that the beings having sent the wormhole are in fact the Earthlings of the future. If so, how could they have sent that wormhole without being able to save themselves from Earth first, knowing that their rescue initially required sending the wormhole? It is still present in relation to the coordinates of the Nasa, sent by Cooper at the end of the film, from the tesseract: he could only send these coordinates to his daughter Murphy because initially he had received them! This is the same problem that is encountered in the third installment of the saga Harry Potter : he survives against the dementors thanks to a savior who sends them away, then goes back in time and becomes the savior himself. Its survival implying the savior, but the savior implying its survival, we arrive at an insoluble loop of causality.
For this, the theory of multiple worlds comes to solve the problem! At least in some cases. Established by physicist and mathematician Hugues Everett to respond to the superposition of states of Quantum mechanics, it postulates that each value of a quantum state would exist in a world. The resulting multiple parallel worlds would reside within a single quantum universe governed by a single solution to Schrodinger’s equation: the universal wave function. In this case, no more paradox: each time a timeline is modified, it is in fact… in another world! This idea is used in several films, notably in the saga avengers. However, this does not solve the problem of time loops.
Or rather, for that, we can appeal to the principle of self-consistency, also called the principle of coherence, established by the physicist Igor Novikov, which stipulates more or less that what is has always been. In other words, the past is impossible to change, whatever has to happen will happen. Whatever attempts are made to prevent an event, they will only rush fate towards it. It is also this idea that is present in tenetand which removes free will, because everything is already written in the timeline when the characters cross it!
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