Physicists at MIT and the University of California at Santa Barbara reported on Nov. 5, 2026, a theoretical prediction for how a heavy quark moving at close to the speed of light would be pushed and pulled by a fluid. Published in Physical Review Letters, the work extends Einstein's 1905 description of Brownian motion, which the researchers say breaks down for particles at relativistic speeds. The authors are Jean Du Plessis of MIT's Center for Theoretical Physics and Bruno Scheihing-Hitschfeld of UCSB's Kavli Institute for Theoretical Physics.

The team calculated strong-force interactions for a heavy quark passing through a gas of weakly interacting quarks and gluons. The resulting probability distribution of momentum changes was bell-shaped for small kicks but tilted at the tails for relativistic quarks: large kicks that slow the quark more than drag would predict were found to be more likely than equally large kicks that speed it up, and such rare kicks were more common than a bell curve predicts. The researchers reported the same tilted structure across plasmas of different composition, including in an earlier strongly coupled-plasma calculation.

Du Plessis said the findings suggest primordial quark-gluon plasma is likely more strongly interacting than Brownian modeling implies, more like a liquid than a gas. The work is theoretical; the authors envision comparing such predictions with accelerator experiments to characterize the plasma's microscopic structure. Funding included the U.S. Department of Energy's Office of Nuclear Physics, the National Science Foundation and the Simons Foundation.