A new study by theoretical physicists has made strides in identifying how particles and cells give rise to large-scale dynamics that we experience as the passage of time.
A central feature of how we experience the world is the flow of time from the past to the future. But it’s a mystery exactly how this phenomenon, known as the arrow of time, arises from the microscopic interactions between particles and cells. Researchers at the CUNY Graduate Center Initiative for the Theoretical Sciences (ITS) help unravel this conundrum with the publication of a new paper in the journal Physical Assessment Letters. The findings could have important implications across a wide range of disciplines, including physics, neuroscience and biology.
Fundamentally, the arrow of time stems from the second law of thermodynamics. This is the principle that microscopic arrangements of physical systems tend to increase in randomness, from order to disorder. The more disorderly a system becomes, the harder 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 that our team had were: if we looked at a particular system, would we be able to quantify the strength of the arrow of time, and would we be able to figure out how it works? emerge from the microscale, where cells and neurons interact, with the whole system?” said Christopher Lynn, a postdoctoral fellow with the ITS program and the paper’s lead author. “Our findings provide the first step to understanding how the arrow of time we experience in everyday life emerges from these more microscopic details.”
To answer these questions, the physicists examined how to decompose the arrow of time by observing specific parts of a system and the interactions between them. For example, the parts can be the neurons that function in a retina. By looking at a single moment, they showed that the arrow of time can be broken down into different pieces: pieces produced by parts working individually, in pairs, in triplets or in more complicated configurations.
Armed with this method of decomposing the arrow of time, the scientists analyzed existing experiments on the response of neurons in a salamander’s retina to various films. In one film, a single object moved randomly across the screen, while another portrayed the full complexity of scenes in nature. In both movies, the team found that the arrow of time arose from the simple interactions between pairs of neurons — not large, complicated groups. Surprisingly, the researchers also saw that the retina showed a stronger time arrow when looking at random movements 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 particular interest to neuroscience researchers,” Lynn said. “For example, they may lead to answers as to whether the arrow of time functions differently in neuroatypical brains.”
“Chris’s decomposition of local irreversibility — also known as the arrow of time — is an elegant, general-purpose framework that could provide a new perspective for exploring many high-dimensional, non-equilibrium systems,” said David Schwab, principal investigator of the study and a professor of physics and biology at the Graduate Center.
Reference: “Decomposing the local arrow of time in interacting systems” by Christopher W. Lynn, Caroline M. Holmes, William Bialek and David J. Schwab, Accepted, Physical Assessment Letters.
Authors in Order: Christopher W. Lynn, Ph.D, Postdoctoral Fellow, CUNY Graduate Center; Caroline M. Holmes, PhD student, Princeton; William Bialek, Ph.D, professor of physics, CUNY Graduate Center; and David J. Schwab, Ph.D., professor of physics and biology, CUNY Graduate Center
Funding Sources: National Science Foundation, National Institutes of Health, James S McDonnell Foundation, Simons Foundation, and Alfred P Sloan Foundation.