superfluidity

Superfluidity is the strange ability of certain ultracold liquids to flow with zero viscosity — that is, with no internal friction at all. The classic example is liquid helium cooled below about two kelvin, just a couple of degrees above absolute zero. In this state the liquid does things no ordinary fluid can: it creeps in a thin film up and over the walls of its container, escapes through pores too fine to leak any normal liquid, and, once stirred into rotation, keeps swirling indefinitely without slowing.

These antics are the visible signature of a quantum effect grown large. Helium-4 atoms are bosons, and below the critical temperature a large fraction of them condense into one and the same quantum state, described by a single shared wavefunction spread across the whole sample. The liquid then moves as a unified, coherent whole. Friction normally arises from atoms scattering one at a time, but a coherent condensate has no low-energy way to scatter, so it simply glides on without resistance.

Superfluidity is the close cousin of superconductivity: both are macroscopic quantum states with frictionless flow, one of neutral atoms and the other of charged electron pairs. The lighter isotope helium-3, whose atoms are fermions, can also become superfluid, but only at far colder temperatures and only after its atoms pair up, much as electrons do in a superconductor. Superfluidity is one of the most vivid reminders that quantum mechanics is not confined to the microscopic.