Why can’t we reverse the course of time? – Science and future


This article is taken from the monthly Sciences et Avenir – La Recherche n°905-906, July-August 2022. It is more beautiful these days!” pleaded Alphonse de Lamartine in the poem “Le Lac”. Unfortunately, we know that time never stops its course, nor does it go backwards, despite the imaginative efforts of many science fiction writers, beginning with Herbert George Wells (1866-1946) and his novel The Time Machine (1895) .However, on closer inspection, certain laws of physics are not so intransigent with the arrow of time… Take two billiard balls thrown at each other.They collide and then go their separate ways. Let’s play the film upside down, as if the course of time were being reversed. The two balls move towards each other, collide, bounce back. An observer watching both films would be unable to tell what happens in the “good sense” of time… Since no physical laws are violated, nothing strange appears. “This is e xplica by the fact that the laws of Newtonian mechanics, which govern the movement of the two balls, are reversible, points out Édouard Kierlik, director of the UFR of physics at the Sorbonne University. In equations, you can change ‘t’ to ‘-t’ without problem. This gets complicated quickly if you multiply the number of balls. Imagine a pool table with 1023 balls: this is the number of atoms present in one liter of air. Watch them collide with each other, then you reverse the direction of the flow of time. This time, you will never go back to the starting state. Because? Because the number of collisions is too large, the configurations are too numerous. A small difference in speed between two balls will cause different trajectories. And one thing leads to another, the bouncing balls will find themselves far from their initial location. The film is no longer reversible. As if time had suddenly acquired an orientation. What happened ? “By going from two balls to a very large number, we have left mechanics to thermodynamics. And the laws of thermodynamics are not reversible.” The unstoppable example of an ice cube in a glass of water To understand this, let’s take another example: an ice cube submerged in a glass of water. As everyone has experienced, heat always moves from a hot body to a cold body. The water will transfer energy to the ice cube, causing it to melt as it cools. What if the movie was played backwards? Heat would then transfer from the cold body (the ice cube) to the hot body (the water). In other words, the ice cube would reconstitute itself as the liquid is heated. Absurd! Contrary to the example of the two billiard balls, the reversal of time would effectively lead to a physical impossibility. Behind these two cases hides the same fundamental concept of thermodynamics: entropy, at the origin of the arrow of time. This amount corresponds to a measure of disorder. This is evaluated by the amount of information needed to fully describe a system. For example, it is relatively easy to portray a vase to 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 inventory each fragment: its shape, its size, its position. Conclusion: The broken vase has more entropy than the intact vase. However, “the second principle of thermodynamics says that the entropy of an isolated system only increases, continues Édouard Kierlik. This is why the vase can evolve into a state of greater entropy under the effect of a mechanical action . On the other hand, the reverse, that is, the pieces of the vase that come together to form the intact vase, is impossible because this would lead to a reduction in its entropy.” What if we stick it back?” Its entropy does decrease but in this case there is a complex external intervention. The second principle only applies if the system is isolated, that is, it receives neither work nor heat. In fact, it is always possible to 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. Since the entropy of the Universe has only grown since Big Bang and there is no exception to this rule.” Trying to go back directly to a point in the past It is therefore impossible to reverse the course of time, even to correct a mistake… This spontaneous increase in entropy also acts for the ice cube in the glass: the entropy of the “ice cube + liquid water” system is lower (because the water molecules in ice are well ordered) than that of the ice cube in water, where all molecules move erratically in the liquid state. But if you can’t go back, you can’t go straight back to a point in the past, like in the series “Doctor Who” where the protagonists travel through time aboard a ship space disguised as a phone booth? This idea is not so ridiculous (phone booth aside), if t taking into account the general relativity of Albert Einstein (1915). According to this theory, the space-time that makes up the Universe is sculpted by the matter and energy it contains. For example, the Earth “empties” the space-time around it. Any body passing by will be attracted to it, but not under the effect of a force acting at a distance as described by Isaac Newton. According to Albert Einstein, it falls into the gravitational well dug by the Earth around it. The idea of ​​traveling to the past would be to bend space-time enough to create a time loop equivalent to a space loop. Explanation: On Earth, a traveler always walking straight will eventually return to his starting point: the Earth being round, it necessarily describes a loop. In the same way, if we manage to create a time loop, an individual trapped in it would find himself one day in his past, and he would go out for a walk again, like in the movie “Un Jour sans fin” (1993) . where Bill Murray relives the same day over and over and uses this situation to try to seduce Andie MacDowell in a thousand ways. But how does space curve like that? Mathematician Kurt Gödel, in 1949, showed that such time loops could exist in a globally rotating universe. Which a priori does not happen with ours. Another hope: black holes. In 1963, New Zealand mathematician Roy Kerr calculated that a rapidly rotating black hole could generate time loops. However, we now know that most black holes rotate. The problem, of course, is that to benefit from this recoil, one would first have to get close to the black hole without disappearing into it. It’s clearly riskier than walking into a phone booth! Finally, unlike “Un Jour sans fin”, the time traveler would relive the exact same events, ad infinitum. Far from repairing a mistake, it would reproduce it for eternity. This takes a lot of interest out of time travel…
#reverse #time #Science #future


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