Physicists unravel the mystery of the ‘arrow of time’

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The arrow of time, first mentioned in 1928 by astrophysicist Arthur Eddington, describes the flow of time from the past to the future. However, it remains a mystery what links this phenomenon to the microscopic interactions between particles and cells. Theoretical physicists at the City University of New York Graduate Center are now providing new answers.

Time always flows in the same direction, and it would be difficult for us to imagine the opposite. But how do all the particles and cells that make up matter give rise to this large-scale dynamic? It’s a mystery that physicists have been trying to solve for decades. This “arrow of time” also raises several questions, due to the asymmetry that characterizes it: the past is not a strict reflection of the future and it is possible to act on the future, but not on the past. Time is the only physical dimension that seems to be characterized by irreversibility.

The arrow of time is derived from the second law of thermodynamics (also known as Carnot’s principle), according to which the microscopic arrangements of physical systems tend to randomly increase from order to disorder (resulting in an increase in entropy). of the system). The more a system becomes disordered, the more difficult it is for it to return to its initial (ordered) state, and the stronger the arrow of time. ” In short, the Universe’s tendency toward disorder is the fundamental reason we find that time flows in only one direction. “, summarize the researchers in a press release.

Where does the irreversibility of time come from?

The arrow of time is a purely macroscopic notion and defines our way of seeing the world. A scene that has just unfolded before our eyes cannot be reversed without the result appearing completely unreal. On the atomic scale, on the other hand, there is no distinction between the past and the future: absolutely all the phenomena that we observe present, on the microscopic scale, a perfect symmetry between the past and the future; in other words, they are reversible. But then, where does the irreversibility of time come from?

To shed light on the arrow of time theory, the team – which includes Christopher Lynn, a postdoc Initiative for Theoretical Sciences from the CUNY Graduate Center, attempted to answer two specific questions. First, it was a question of considering a particular system and seeing if it was possible to quantify the strength of its arrow of time. Then, it was about determining how this force emerges from the microscopic scale -in which cells and neurons interact- to finally apply it to the system as a whole.

To answer these questions, the researchers attempted to break down a local arrow of time, looking at specific parts of a system and their interactions. ” In a system with many degrees of freedom, there is a term that arises from the irreversible dynamics of the individual variables, and then a series of nonnegative terms that contribute to the correlations between pairs, triplets, and combinations of higher order variables. “, they write in their article.

Clearly, the “parts” in question here could be, for example, the neurons located in a retina (the system). At a given moment, the arrow of time can break down into several elements: those produced by parts that work individually, in pairs, in triplets or in more complex configurations. On this basis, the researchers analyzed the results of previous experiments on the response of neurons in the retina of a salamander to the viewing of different films.

Irreversibility based on pairwise dynamics

One of the movies used showed a single object moving randomly across the screen. Another showed more complex scenes, usually found in nature. For both movies, the researchers found that the arrow of time was the result of simple interactions between pairs of neurons (and not more complex neural interactions).

They explain that evidence for the local arrow of time accumulates from the behavior of individual degrees of freedom and their interactions. Each of the progressively higher order dynamics makes a non-negative contribution, adding to the local irreversibility. ” Our results are the first step in understanding how the arrow of time that we experience in everyday life emerges from these more microscopic details. », emphasizes the team.

Unexpectedly, they also noticed that the retina showed a stronger arrow of time (hence stronger irreversibility) when looking at random motion than when looking at a more natural scene. According to Lynn, this latest finding raises questions about how our internal perception of the arrow of time aligns with the outside world. ” These findings may be of particular interest to neuroscientists. he said, adding that they might suggest that the arrow of time works differently in the brains of neuroatypical people.

Despite these large differences in the strength of the local time arrow in response to different inputs, the way large-scale irreversibility is constructed from small-scale dynamics is consistent, with a dominant role played by correlations between pairs of neurons. This emergence of the irreversibility of pairwise dynamics paves the way for future research aimed at determining whether and how the physical connections between neurons combine to produce a collective arrow of time, and how, the researchers conclude.

Source: C. Lynn et al., arXiv

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