superfluid
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Imagine pouring tea, but the tea never slows down. It races around a loop forever without anything to push it, slips through cracks too narrow for an ordinary liquid, and even climbs up and over the rim of its cup. A superfluid is a liquid that, once cold enough, behaves like this — it has lost all of its internal stickiness.
Ordinary liquids resist flowing because their atoms drag against one another; that drag is called viscosity, and it is what eventually brings any swirl to a stop. Below a special low temperature, certain liquids — most famously helium chilled to a few degrees above absolute zero — drop into a single shared quantum state in which the atoms move in perfect lockstep. With no internal friction, a current set up in a ring just keeps circulating, and the liquid creeps along surfaces in defiance of everyday intuition.
Superfluidity matters because it is one of the few places where the strange rules of the quantum world become visible to the naked eye, on the scale of a whole cup of liquid. A common misconception is that a superfluid has zero friction against everything; in fact it flows without friction only up to a certain speed, and only the frictionless part of the liquid behaves this way while a normal part can still coexist with it.
Set superfluid helium swirling in a sealed ring-shaped channel and it will keep circulating for as long as you keep it cold — researchers have watched such a current persist with no measurable slowing, the liquid equivalent of a wheel that never stops spinning.
A persistent superfluid current: flow with no engine and no brake.
Superfluidity and superconductivity are close cousins — one is friction-free flow of a neutral liquid, the other friction-free flow of electric charge — and both spring from the same idea of countless particles sharing one quantum state.