Ultracold atoms flow on a chip

Hydrodynamics usefully describes the behavior of many-body systems, from molecules in a cup of tea to cars on a highway. The framework treats a collection of particles as a continuous fluid whose total mass, momentum, and energy are conserved, and it offers the advantage of describing a system in terms of local characteristics rather than individual particle movements. But in some systems that do not reach thermal equilibrium, hydrodynamics fails because many more quantities are conserved. Now Max Schemmer, Isabelle Bouchoule, and their colleagues have provided the first direct evidence in support of new hydrodynamic models that are sufficiently general to describe the nonequilibrium behavior of ultracold atoms in one dimension.