= Superfluidity
{wiki}
<Superfluidity> is a quantum-fluid response with <phase stiffness> and flow without viscous dissipation in an appropriate low-speed regime. A <superfluid> supports a phase-gradient current; defects and excitations can limit that flow. <Bose-Einstein condensation> concerns macroscopic one-particle occupation and is a distinct criterion: the Gaussian <phase stiffness> theory can have divergent absolute-phase fluctuations while retaining a cost for <gradients>.
Why does cooling a <liquid> make it flow more freely through a tiny gap? <Helium-4> supplies an experimental entrance to this question: the <helium slit-flow experiment> compares the motion of liquid levels on opposite sides of the transition into <superfluidity>. A flow measurement probes resistance to motion; a measurement of <Bose-Einstein condensation> probes occupation of a <quantum state>. The <two-fluid model of superfluidity> and the <Landau criterion> connect these questions to <entropy> transport and to the <energy> required to create an excitation.
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