Except in science fiction films, it is established as a marble table of the laws of nature that time can only flow in one direction: from the past to the future. Well, this certainty has just been shaken by an international team of physicists who have discovered that, under particular conditions, time can flow backwards. Let's try to understand how and why.
Drom time immemorial, time has been a riddle to man. Quid est tempus? What is time?" St. Augustine wondered. And he answered thus in his Confessions : " When no one asks me, I know. If someone asks me about it, I'm going to explain, and I don't know... ». Saint Augustine was right. We all talk about time, we all talk about hours, about time past, about time to come. And everyone understands. And yet, when you think about it, nothing is more obscure. Our only certainty is that the time arrow has only one direction. It always goes from the past to the future. We remember what happened yesterday; we can't remember what happened tomorrow.
Scientists accept this certainty, but do not understand why a cause should necessarily come before its effect. The further down to the level of matter, the more this question deserves to be asked.
Entropy or nothing
Physicists provide answers to define time by focusing on a major characteristic, the movement of energy. They thus refer to entropy. In a system such as the universe, which cannot receive external energy, the physical elements necessarily move from an ordered state to a disordered state. From order to chaos. If we want to understand the phenomenon on our scale, it is like saying that a glass of hot water placed in a cold room will never warm up. It can only get colder. This is one of the laws of thermodynamics. Like time, we cannot reverse this reality.
Even at the quantum level, particles behave according to the initial starting conditions. They always go in one direction only, forward.
This functioning seems to be established as irreversible. But today, a flaw has been introduced into this certainty. Researchers have established, on a very small scale, that this arrow can be reversed and that time can be turned back. Basically, the glass of hot water became hotter and hotter, even in a cold environment.
The hidden mysteries of chloroform
An international team of researchers from Germany, Singapore, New York, England, etc., have just published an article on the specialized site arXiv.org. This publication (see the study in pdf - English) is currently being validated for publication in major scientific journals. It is therefore still necessary to be cautious about the validity of the results. However, this study arouses enormous interest because, if it were proven, it would be a revolution in physics, with applications that we would not dare to imagine today. Going back in time is man's dream and perhaps the key to space travel.
The scientists had an original idea. They set out to examine a very well-known product: chloroform. This molecule is composed of a carbon atom, connected to a hydrogen atom and three chlorine atoms. They used a strong magnetic field to align the carbon and hydrogen atoms and placed them in suspension in acetone. They then used nuclear magnetic resonance to slowly heat the nuclei of the atoms. And they observed what happened.
Logically, when a core is heated, it should transfer its heat, i.e. its random movements, to the colder particles so that they have the same temperature as it does. That's exactly what happened in the experiment. The researchers observed changes in the energy state of their guinea pig particles. Under normal conditions, not surprisingly, the entropy mechanism works. But they had a big surprise when they observed an unexpected phenomenon: when the particles are correlated, nothing happens as expected.
To understand, one must enter the field of quantum physics. Don't worry, we'll only be standing on the doorstep. When particles have interacted, they have probabilities that lock them together at a distance. It's a less complex version of quantum entanglement. This notion seems something magical to a philistine: two particles have identical quantum states, no matter how far apart they are. We can imagine one particle lying on Earth and another lying on Mars. If they are intertwined, we can modify the Earth particle and immediately its twin sister based on Mars will be modified identically. Difficult to conceive, and fraught with philosophical implications, this dimension of quantum physics calls into question Albert Einstein's principle of local realism and opens up avenues of application in information transmission, cryptography, quantum computing and even teleportation.
Let's get back to our chloroform molecules. Certain correlated particles change the way energy is shared between bodies. Scientists have observed that heated hydrogen particles become even hotter while their carbon partners become colder. The opposite of what was expected. The researchers observed, on a very small scale, the thermodynamic equivalent of time inversion.
This study promises to be just the beginning of a long story. In addition to the consequences in the field of industrial applications such as heat conduction, the study opens the way for further investigations to find out whether these phenomena, observed in the infinitely small, can be found on the scale of systems as large and complex as the Universe.
Science has the art of providing answers that immediately raise billions of questions. To be continued.