Tenet and time traveling
Christopher Nolan’s latest film has raised the issue of the different types of time travel that both science and fiction have conceived. What are these four types and why would only one of them be impossible?
It’s not easy to talk about Tenet without falling into a spoiler, which is really a shame because a lot of the appeal of the movie lies precisely in the surprise factor. Unfortunately, the title of this post already represents a spoiler in itself, and what I’m going to develop next aggravates its degree of spoiling. So if you haven’t seen the movie yet, I recommend you not to continue reading until you do.
In case you have chosen not to follow my previous suggestion, I have good news for you: the spoiler I’m going to do is something we found out about in the 14th minute of the movie. Considering that this film is approximately 150 minutes long, it is something that happens after less than 10% of its plot. Therefore, surprises will still happen during that remaining 90%.
Having said that, let’s get down to business. In my opinion, there are four different types of time travel, which I explain below:
1.- Ordinary travel: As disappointing as it may seem, the first type of time travel is nothing more than the constant movement we make towards the future at the speed of one hour per hour. Obviously there is nothing extraordinary about this type of travel, however it becomes more interesting when we consider what Einstein called the twin paradox, and which Christopher Nolan himself used in his film Interstellar, to show how time passes more quickly in the vicinity of a black hole.
2.- Time Machine Travel: This is the type of travel that H. G. Wells invented and that Marty McFly does in Robert Zemeckis’ famous movie. This type of travel departs from physics as we know it today, so it would be impossible, since to make it happen you would need to leave space-time to return to another time and/or place. In this same category we find trips of the type we saw in the movie “Groundhog’s Day” or in the Netflix series “Russian Doll”, since the “resetting” of time to a given moment corresponds to taking the traveler out of space-time and putting her/him back in a moment where he/she was already.
3.- Wormhole travel: Considering that space-time is curved, as demonstrated by Albert Einstein, a possible type of time travel is no different from any other type of travel, except that it makes use of the temporal circularity of the path. What would happen if, making use of this mechanism, I manage to kill my own grandfather before my father is conceived, as the famous paradox says? Would the entire Universe collapse because of that contradiction? Nobody knows!
4.- Inversion of the arrow of time: This is the type of time travel that is used in the film Tenet and that I will explain below.
Although in principle Thermodynamics refers to the study of the relationship between heat and energy, the application of its laws is universal in physics and its second law plays a fundamental role in our fourth type of time travel.
Basically, what the second law of thermodynamics holds is that a physical quantity, called entropy, always increases. A very graphic way to explain this law is to imagine that to a freshly boiled water we add ice cubes. Obviously what will happen is that the ice will melt and the water will cool. Now, if it were not for this law, nothing would impede a glass of water to become spontaneously a glass of hot water with ice in it, which — obviously — never happens. However, in 2017 a team of scientists managed to observe the reversal of the usual heat exchange from hot objects to cold objects, and concluded that the second law of thermodynamics is not an absolute and definitive concept, but can be broken, thus laying the scientific foundation for Nolan’s film.
So far, all experiments in which an entropy inversion has been observed have been done on an atomic scale. For example, in the original experiment, physicists used a molecule of trichloromethane, which is composed of hydrogen and carbon, and made the nucleus of the hydrogen atom to heat up and the nucleus of the carbon atom to cool down. By correlating particles — something similar to quantum entanglement — they produced the exact opposite of what would happen according to the second law of thermodynamics. Does this mean they managed to reverse time? Even though many media published the news with that title, it does not necessarily mean that they managed to tame time, but simply that they successfully did something that the second law prohibits, but that the rest of physics allows, similar to what it would be to obtain ice and hot water from warm water.
In Tenet we do not see water becoming hot water with ice, but we do see other examples of situations in which the arrow of entropy is reversed, typically bullets that are anti-shooted towards the gun, objects that anti-fall upwards or buildings that anti-fall down, building themselves in seconds from their ruins and dust. Furthermore, from the perspective of a person whose arrow of time is reversed, everything goes backwards.
When I was a kid, I asked my mother why we couldn’t remember the future. She answered me that it was obvious, because the future had not yet happened. Her answer did not satisfy me. Years later I read a book by Stephen Hawking in which the famous physicist asked himself the same question, and his answer had to do precisely with the entropy and arrow of time. I then learned that this concept was first used by the British astronomer Arthur Eddington at the beginning of the last century.
Clearly entropy and time are related. Perhaps the experiments in which the second law is broken are laying the foundation for something that, who knows, we may one day achieve, and reverse the entropy of objects and even living things, including people. But I don’t think it will happen soon or that it will be within reach to travel to the past, because if it did I would have visited myself to give me some good advices.
