But what happens in the astonishingly short time before hydrodynamic theory can be used? The collisions produce a plasma-composed of the subatomic particles "quarks" and "gluons"-that emerges very early in the collision and can be described by a hydrodynamic theory-similar to the classical theory used to describe air flow or other moving fluids-well before the plasma reaches local thermal equilibrium. When they are some combination of dense and cold enough, which can vary depending on the context, quantum mechanics-the fundamental theory that describes the properties of nature at the atomic or subatomic scale-is required to describe their dynamics.ĭramatically out-of-equilibrium systems are routinely created in particle accelerators when pairs of heavy ions are collided at speeds near the speed-of-light. These quantum many-body systems are ensembles of particles, like atoms, that are free to move around relative to each other, Weiss explained. In contrast, the behavior of systems that are far from equilibrium has yielded to few such unifying descriptions." "Despite the staggering array of diverse 'many-body' phenomena, like superconductivity, superfluidity, and magnetism, it was found that their behavior near equilibrium is often similar enough that they can be sorted into a small set of universal classes. "Many major advances in physics over the last century have concerned the behavior of quantum systems with many particles," said David Weiss, Distinguished Professor of Physics at Penn State and one of the leaders of the research team.
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