When Noise Becomes a Signal: A Study of Random Motion

On the open ocean, the number of waves generated between the boats is much smaller than the number generated in the ocean waters surrounding the boats, thus causing an attractive force that brings the boats together once the boats are within a certain radius of one another.

Two ships are travelling past each other on a choppy sea. Both are without captain or crew—listless objects floating in an ocean, buffeted by wind and water. However, as soon as one ship glides within a certain distance of the other, the two ships begin to move towards each other, as if guided by some miraculous attractive force.

Chaotic ocean waters, particles drifting in coffee, pollen jerking back and forth in water—these systems are considered by physicists to be “non-equilibrium systems,” or systems that are influenced by external forces. John Wettlaufer, the A.M. Bateman Professor of Physics and Geophysics at Yale University, has been studying quantum electrodynamics (QED), the theory behind such mysterious attractive forces for 20 years.

Here, Wettlaufer and team examined how noise becomes signals in a variety of different non-equilibrium systems. Image courtesy of John Wettlaufer.

In a non-equilibrium system, random forces constantly bombard small particles. The term “small” here is relative: a ship is small compared to the expanse of the ocean. But the implications of research on these non-equilibrium forces are endless. For example, billions of small particles crucial to a cell’s function are at the whims of forces generated in the non-equilibrium system of the cytosol. Similarly, studying the Casimir effect prevents collision on rough seas. Consequently, if scientists could understand how to model these non-equilibrium systems, then scientists could better understand the rules that govern both objects as large as cruise ships and molecules smaller than a cell.

Wettlaufer’s paper, published in 2015 in the Proceedings of the National Academy of Sciences of the United States of America (PNAS), delineates how Wettlaufer and team describe force generation in non-equilibrium systems. Wettlaufer studied two seemingly unrelated phenomena: the maritime Casimir effect, the strange attractive force described previously, and Brownian motion, which describes the random movement of microscopic particles suspended in fluid, in order to discover a mathematical model that could apply generally to non-equilibrium systems.

In order to find this model, Wettlaufer first studied the noise, or the external forces that cause non-equilibrium, in each phenomenon. In contrast, the signal of the system refers to the internal forces at play when the system is in equilibrium. Wettlaufer found that if the noise is large enough, the noise can overpower the signal and become the signal that drives force generation. “Imagine driving a car along a road. The signal would be the forward movement of the car. External forces, like a strong gust of wind, can force you off track. This wind would be the noise,” Wettlaufer said. Noise can be graphed on a sharply peaked fluctuation spectrum, in which each peak describes a force generated in the system. This spectrum can then be analyzed to understand how non-equilibrium systems operate.

Through analysis of these spectra, scientists like Wettlaufer may help us become better equipped to predict the results of supposedly random movement. The universe may not be as unpredictable as it seems.