Scientists Unveil the Mystery of the Arrow of Time

A new study from theoretical physicists at the City University of New York (CUNY) Graduate Center has made progress in identifying how particles and cells give rise to the large-scale dynamics we experience as the passage of time.

The flow of time from the past to the future is a central feature of our experience of the world. But how this phenomenon, known as the arrow of time, arises from microscopic interactions between particles and cells is a mystery that researchers at the CUNY Graduate Center’s Initiative for Theoretical Sciences (ITS) are helping to unravel. unravel with the publication of a new article in the magazine Physical Review Letters. The results could have important implications in various disciplines, such as physics, neuroscience, and biology.

Fundamentally, the arrow of time arises from the second law of thermodynamics: the principle that the microscopic arrangements of physical systems tend to increase in randomness, moving from order to disorder. The more disordered a system becomes, the more difficult it is for it to find its way back to an ordered state, and the stronger the arrow of time. In short, the universe’s tendency to disorder is the fundamental reason why we experience time flowing in one direction.

The two questions our team asked were: if we look at a particular system, will we be able to quantify the strength of its arrow of time, and will we be able to order how it arises from the microscale, where cells and neurons interact, to the complete system?says Christopher Lynn, first author of the article. Our discoveries provide the first step toward understanding how the arrow of time we experience in daily life emerges from these more microscopic details..

To start answering these questions, the researchers explored how the arrow of time could be broken down by looking at specific parts of a system and the interactions between them. The parts, for example, could be the neurons that function within a retina. Observing a single moment, they demonstrated that the arrow of time can be broken down into different pieces: those produced by parts working individually, in pairs, in trios, or in more complicated configurations.

Armed with this way of breaking down the arrow of time, the researchers looked at existing experiments on the response of neurons in a salamander’s retina to different movies. In one of the films, a single object moved randomly across the screen, while in another the full complexity of scenes found in nature was portrayed. In both films, the researchers found that the arrow of time arose from simple interactions between pairs of neurons, not from large, complicated groups. Surprisingly, the team also found that the retina displayed a stronger arrow of time when viewing random motion than a natural scene. Lynn said this latest finding raises questions about how our internal perception of the arrow of time aligns with the external world.

These results may be of special interest to neuroscience researchers.says Lynn. They could, for example, lead to answers about whether the arrow of time works differently in brains that are neuroatypical..

The Chris decomposition of local irreversibility – also known as the arrow of time – is an elegant and general framework that can provide a novel perspective for exploring many high-dimensional, non-equilibrium systems.said David Schwab, professor of physics and biology at the Graduate Center and principal investigator of the study.


The City University of New York | Christopher W. Lynn, Caroline M. Holmes, William Bialek, and David J. Schwab, Decomposing the local arrow of time in interacting systems. Physical Review Letters (2022),