New research suggests that the “swimmers” of the natural world — from ships at sea to microorganisms floating through the bloodstream to ubiquitous quantum particles — exert a predictable influence on each other within seemingly chaotic environments.
Researchers at Yale, Harvard, Oxford, and the Nordic Institute for Theoretical Physics have devised a model for explaining the attraction and repulsion between objects as waves rise and fall around them. The findings, reported August 15 in the Proceedings of the National Academy of Sciences, suggest a unifying theory for how force is generated in nonequilibrium systems.
“What we find is that if the spectrum of waves has a particular structure — sharply peaked — then depending on how far apart floating objects are from each other, sometimes they will attract and sometimes they will repel each other,” said senior author John Wettlaufer, the A.M. Bateman Professor of Geophysics, Mathematics and Physics at Yale.
“An entire range of things can be explained in this manner, from microscopic to boats. If you give us the spectrum of waves or fluctuations we can tell you what the forces will be between objects,” Wettlaufer said.
Publication: Alpha A. Lee, et al., “Fluctuation spectra and force generation in nonequilibrium systems,” PNAS, 2017; doi: 10.1073/pnas.1701739114
Abstract: Many biological systems are appropriately viewed as passive inclusions immersed in an active bath: from proteins on active membranes to microscopic swimmers confined by boundaries. The nonequilibrium forces exerted by the active bath on the inclusions or boundaries often regulate function, and such forces may also be exploited in artificial active materials. Nonetheless, the general phenomenology of these active forces remains elusive. We show that the fluctuation spectrum of the active medium, the partitioning of energy as a function of wavenumber, controls the phenomenology of force generation. We find that, for a narrow, unimodal spectrum, the force exerted by a nonequilibrium system on two embedded walls depends on the width and the position of the peak in the fluctuation spectrum, and oscillates between repulsion and attraction as a function of wall separation. We examine two apparently disparate examples: the Maritime Casimir effect and recent simulations of active Brownian particles. A key implication of our work is that important nonequilibrium interactions are encoded within the fluctuation spectrum. In this sense, the noise becomes the signal.