This article is taken from the monthly journal Sciences et Avenir – La Recherche #905-906 July-August 2022.
“O time, stop your flight! And you, auspicious hours, stop your course! Let’s enjoy the quick pleasures of the most beautiful of our days! pleaded with Alphonse de Lamartine in Le Lac. Alas, we know that time never stops. And it does not retreat, despite the creative efforts of many science fiction writers, starting with Herbert George Wells (1866-1946) and his novel The Time Machine (1895).
However, on closer examination, some laws of physics turn out to be not so irreconcilable with the arrow of time … Let’s take two billiard balls thrown at each other. They collide and then separate. Let’s play the movie upside down, as if the passage of time had actually been reversed. Two balls move towards each other, collide, return. An observer watching the two films will not be able to tell what is going on in the “good sense” of time… Since no physical laws are violated, nothing strange appears.
“This is due to the fact that the laws of Newtonian mechanics that govern the motion of two balls are reversible,” said Eduard Kirlik, director of the Faculty of Physics at the Sorbonne University. – In equations, you can replace “t” with ‘-t’ without problems. This quickly gets more complicated if you multiply the number of balls. Let’s imagine a pool table with 1023 balls: this is the number of atoms present in a liter of air. You watch them collide with each other, then you change the course of time. This time you will never return to the original state. Why? Because the number of collisions is too high, there are too many configurations. A tiny difference in speed between the two balls will result in different trajectories. And one thing leading to another, the balls, returning back, will be far from their original location.
The film is irreversible. As if time suddenly took on a direction. What happened ? “In going from two balls to a very large number, we left mechanics for thermodynamics. And the laws of thermodynamics are irreversible.”
An irresistible example of an ice cube in a glass of water
To understand, let’s take another example: an ice cube immersed in a glass of water. As you know, heat always passes from a hot body to a cold one. The water will transfer energy to the ice cube, causing it to melt as it cools. What happens if the movie is played in reverse order? Then the heat will move from the cold body (ice cube) to the hot body (water). In other words, the ice cube will regenerate when the liquid is heated. Absurd! Unlike the two billiard ball example, time reversal would indeed lead to a physical impossibility.
Behind these two cases lies the same fundamental concept of thermodynamics: entropy, the source of the arrow of time. This value corresponds to the measure of disorder. This is estimated by the amount of information needed to fully describe the system. For example, it is relatively easy to depict a vase for someone who has never seen it. On the other hand, if the vase is broken, its description will be much more delicate. It will be necessary to make an inventory of each fragment: its shape, size, position. Conclusion: a broken vase has more entropy than a whole vase. However, “the second principle of thermodynamics says that the entropy of an isolated system only increases,” continues Eduard Kirlik. That is why a vase can evolve towards a higher entropy under the influence of mechanical action. the reverse, namely parts of a vase joining together to form an intact vase, is not possible, as this would result in a decrease in its entropy.”
What if we glue it together? “Its entropy does decrease, but in this case there is a complex external interference. The second principle applies only if the system is isolated, that is, it receives neither work nor heat. In fact, you can always lower the entropy. locally. Only the operations required to repair the vase will correspond to an increase in entropy in the system external to the vase. And finally, the global entropy will increase. Because the entropy of the Universe has only increased since the Big Bang and there are no exceptions to this rule.
Try to go straight to the point in the past
Therefore, it is really impossible to reverse the course of time, even to correct a mistake… This spontaneous increase in entropy also affects an ice cube in a glass: the entropy of the “ice cube + liquid water” system is less (since the water molecules in ice are well ordered) than that of a melted ice cube. an ice cube in water, where all the molecules vibrate randomly in a liquid state.
But if you can’t just go back, can’t you go straight to a point in the past, like in Doctor Who, where the main characters travel through time aboard a spaceship disguised as a telephone booth? This idea is not so ridiculous (apart from a telephone booth), if you take into account the general theory of relativity of Albert Einstein (1915). According to this theory, the space-time that makes up the universe is shaped by matter and the energy it contains. For example, the Earth “hollows out” the space-time around itself. Any body passing nearby will be attracted by it, but not by a force acting at a distance, as described by Isaac Newton. According to Albert Einstein, he falls into a gravity well dug by the Earth around him.
The idea behind traveling into the past is to warp spacetime enough to create a time loop that is equivalent to a space loop. Explanation: On Earth, a traveler who always goes straight will eventually return to his starting point: since the Earth is round, he necessarily describes a loop. In the same way, if we were able to create a time loop, the individual caught in it would one day end up in their past and be on the road again, as in Day Without a Fin (1993). where Bill Murray relives the same day over and over and uses that situation to try and seduce Andie MacDowell in a thousand ways.
But how to warp space like this? The mathematician Kurt Gödel demonstrated in 1949 that such time loops could exist in a universe revolving around itself. What we don’t have. Another hope: black holes. In 1963, New Zealand mathematician Roy Kerr calculated that a rapidly spinning black hole could create time loops. However, we now know that most black holes rotate. The trouble, of course, is that in order to take advantage of this rewind, one would first have to approach the black hole without disappearing inside. It’s clearly more risky than going to a phone booth! Finally, unlike “Un Jour sans fin”, the time traveler will relive the same events indefinitely. He will not correct the error, but will reproduce it forever. It takes a lot of interest in time travel…