Time Mystery Fire

Physicists are unraveling the mysteries of time’s arrow

Scientists are unraveling the mystery of the arrow of time with implications for physics, neuroscience and biology.

A new study by theoretical physicists has made progress in determining how particles and cells generate the large-scale dynamics we experience over time.

A central feature of how we experience the world is the flow of time from the past to the future. But exactly how this phenomenon, known as the arrow of time, arises from microscopic interactions between particles and cells is a mystery.Researchers at the CUNY Graduate Center for Theoretical Sciences (ITS) program are helping to unravel the mystery, publishing a new paper in the journal Physical Review Letters. These findings could have important implications for 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 the microscopic arrangement of physical systems tends to increase in randomness, from order to disorder. The more disordered a system is, the harder it is to get back into order, and the more powerful the arrow of time is. In short, the universe’s propensity for disorder is the fundamental reason why we experience time flowing in one direction.

“Two questions for our team are, if we look at a particular system, can we quantify the strength of its arrow of time, and whether we can sort out how it emerges from the microscopic scale, where cells and neurons relate to the system as a whole interaction?” said Christopher Lynn, a postdoc in the ITS program and lead author of the paper. “Our findings provide the first step in understanding how the arrow of time that we experience in our daily lives emerges from these more microscopic details.”

To begin answering these questions, physicists explored how to break down the arrow of time by looking at specific parts of the system and how they interact. For example, these parts may be neurons that function within the retina. Looking at a moment, they show that the arrow of time can be broken down into different parts: parts that result from parts that work individually, in pairs, in groups of three, or in more complex configurations.

With this method of breaking down the arrows of time, the scientists analyzed existing experiments on how salamander retinal neurons responded to different movies. In one film, an object moves randomly across the screen, while another depicts the full complexity of a scene in nature. In both films, the team found that the arrows of time came from simple interactions between pairs of neurons, rather than large, complex groups. Surprisingly, the researchers also observed that when viewing random motion, the retina showed a stronger arrow of time than in natural scenes. The latter finding raises the question of how our internal perception of the arrow of time aligns with the external world, Lynn said.

“These results may be of particular interest to neuroscience researchers,” Lynn said. “For example, they could derive answers about whether the arrow of time functions differently in atypical neuronal brains.”

“Chris’s decomposition of local irreversibility — also known as the arrow of time — is an elegant general framework that can shed new light on the exploration of many high-dimensional, non-equilibrium systems,” said the study’s principal investigator and professor at the University of Wei Schwab said the Graduate Center’s Department of Physics and Biology.

Reference: “Decomposing Local Time Arrows in Interactive Systems”, Christopher W. Lynn, Caroline M. Holmes, William Bialek, and David J. Schwab, accepted, Physical Review Letters.

Authors in order: Christopher W. Lynn, Ph.D., Postdoc, CUNY Graduate Center; Caroline M. Holmes, Princeton Ph.D. student; William Bialek, Ph.D., Professor of Physics, CUNY Graduate Center; CUNY Graduate Center, Physics and Biology Dr. David J. Schwab

Funding: National Science Foundation, National Institutes of Health, James S McDonnell Foundation, Simons Foundation, and Alfred P Sloan Foundation.

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