Hydrodynamics deals primarily with the flow of water and other fluids. A branch of fluid dynamics, it describes the behavior of many-body systems, from molecules in a cup of tea to cars on a highway. The framework, which assumes that each point in a fluid-like collection of particles is locally at equilibrium, characterizes the local quantities that are conserved for an entire system. For a classical fluid like water, those local quantities are mass density, momentum density, and energy density. Models encode the evolution of local quantities in the form of continuity equations based on mass, momentum, and energy conservation.
In the quantum world, many-body systems are notoriously difficult to model because the computational memory needed to retain their characteristics grows exponentially with the number of particles. Instead of trying to describe the complex movement of each individual particle, quantum theorists seek ways to explain many-body systems in terms of...
