A fraction of a second after the Big Bang, matter existed as a trillion‑degree quark‑gluon plasma that lasted only a few microseconds before forming protons and neutrons. In large accelerators, scientists recreate this plasma by colliding ions near light speed, producing droplets the size of a nucleus that survive for tens of zeptoseconds. Heavy quarks that traverse the plasma act as probes; their motion encodes information about the plasma’s microscopic structure. Einstein’s 1905 Brownian‑motion theory linked a particle’s drag and random kicks to the fluid’s internal forces, but at relativistic speeds the relationship breaks down because the plasma’s interactions are too strong for the original model. Researchers at MIT and UCSB derived a relativistic‑fluid theory that predicts the probability distribution of momentum changes for a heavy quark moving close to light speed. Starting from a weakly interacting quark‑gluon gas, they calculated the strong‑force kicks and obtained a bell‑shaped curve that tilts at the tails: large decelerating kicks are more common than large accelerating ones, contrary to the symmetric Gaussian expected from classical Brownian motion. The same tilted distribution appears for plasmas with different constituents and even in strongly coupled calculations, suggesting a universal structure. Consequently, the primordial quark‑gluon plasma was likely more strongly interacting—more liquid‑like than a simple gas model implies. The work was funded by the U.S. Department of Energy Office of Nuclear Physics, the National Science Foundation, and the Simons Foundation.\n\nReview: By extending Einstein’s framework with rigorous strong‑interaction calculations, the study provides a concrete method to extract microscopic properties of the early‑universe plasma from heavy‑quark observables, opening a new avenue for experimental validation.