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Shown is an artist rendition of the quantized vortices in a bucket of supefluid helium. Not indicated in the drawing is the fact that the vortex cores consist of ordinary fluid. An important point to make here is that while the superfluid condensate is whirling around the vortex cores, the cores themselves are rotating rigidly around the axis of rotation. The reason for this that the vortex cores must be in equilibrium with the rigidly-rotating excitations or ordinary fluid in the bucket. (The fluid is never completely condensed into the superfluid state at finite temperatures. The ordinary component of the fluid has to rotate rigidly, of course, or it suffers viscous dissipation.) In other words, if the vortices did not rotate rigidly around the axis of rotation, the vortex cores would scatter off the rigidly-rotating ordinary fluid component of the system---producing dissipation known as mutual friction. Much more will be said about mutual friction a few overheads after this one.

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