Emory finds non-reciprocal forces in dusty plasma

Emory University physicists published a 2025 PNAS paper showing that machine learning can recover hidden force laws in a laboratory dusty plasma and expose force asymmetries that standard approximations miss. The study analyzed three-dimensional trajectories of charged dust grains suspended in ionized gas, a system used to study many-body physics under controlled conditions. The authors said the inferred interactions were non-reciprocal, meaning the effective force one particle exerts on another need not be matched in equal and opposite form. The work was described by Emory and PNAS as an experimentally validated result, not a social-media claim. ### What did the Emory team actually find? The paper, “Physics-tailored machine learning reveals unexpected physics in dusty plasmas,” said dusty plasma particles interact through plasma-mediated forces that can be nonconservative and nonreciprocal. The authors wrote that their model was trained on 3D particle trajectories from laboratory experiments and learned the effective forces between non-identical particles with “exquisite accuracy,” reported as R² greater than 0.99. (news.emory.edu) Wentao Yu, Eslam Abdelaleem, Ilya Nemenman and Justin C. Burton wrote that the model also inferred particle masses in two independent ways and used that accuracy to measure charge and screening length. The paper said those measurements revealed “large deviations from common theoretical assumptions.” ### Does this mean Newton’s third law was overturned? The study did not say Newton’s third law had been universally overturned. (arxiv.org) The paper described “effective” forces in a nonequilibrium plasma environment, where surrounding ions and electrons mediate the interaction between dust grains. In that setting, the force law between particles can appear asymmetric because each grain is interacting through the plasma, not as an isolated two-body system in vacuum. Emory’s summary of the work used the same framing. Justin Burton, an Emory professor of experimental physics and senior co-author, said the team used AI “to discover new physics,” while Ilya Nemenman, an Emory professor of theoretical physics and co-senior author, said the group could describe the forces with accuracy of more than 99% and show that some common assumptions “are not quite accurate.” (arxiv.org) ### Why use dusty plasma for this? Dusty plasma is common in space and planetary environments, according to the paper, and it offers visible, trackable particles whose motion can be measured directly in the lab. Emory said Burton’s lab developed techniques to track the 3D motion of individual particles, giving the researchers experimental data to test the model’s inferred force laws. (news.emory.edu) The authors argued that many-body systems often lack clean, known interaction laws, which makes them difficult to model from first principles alone. Their approach combined physical constraints with neural-network inference rather than treating the model as a pure black box. ### What was new beyond earlier dusty-plasma work? The PNAS article said the advance was not just identifying non-reciprocal behavior, which plasma physicists already study, but extracting a detailed force law directly from experimental trajectories and quantifying where standard approximations fail. (news.emory.edu) Emory said the result provided its most detailed description yet of the physics of a dusty plasma and yielded precise approximations for the non-reciprocal forces. (arxiv.org) The National Academy of Sciences also recognized the paper with a 2025 Cozzarelli Prize in physical and mathematical sciences, citing the machine-learning model’s ability to discern forces in lab-generated dusty plasmas with high accuracy. ### What about the social-media link to NASA plasma observations? A May 23 social-media post linked the Emory work to NASA observations of plasma double layers in the Van Allen belts, but that connection was not made in the Emory release or the PNAS paper. (pnas.org) The verified record shows Emory’s study focused on laboratory dusty plasma and on force inference from tracked particle motion. Any broader connection to space-plasma observations would be an inference beyond the paper unless separately documented by NASA or the authors. (nasonline.org) The paper is available through PNAS under DOI 10.1073/pnas.2505725122, and the named authors are Yu, Abdelaleem, Nemenman and Burton. Emory said the framework could be applied to other many-body systems, including colloids and living systems, in future work. (pnas.org) (news.emory.edu)