A liquid that won't spin like water
Stir a mug of tea and the whole liquid turns smoothly, fastest at the rim or fastest at the centre depending on how you stir — but in any case it rotates as a connected body, every part free to move at its own pace. A superfluid flatly refuses to do this. Because it is one shared quantum wave, it cannot rotate as a smooth whole at all; the wave's strict internal bookkeeping simply forbids it.
So what happens if you do put a bucket of superfluid helium on a turntable and spin it? The liquid finds a clever compromise. Most of it stays perfectly still, refusing to rotate. The spin is forced instead into a scattering of tiny, isolated whirlpools — needle-thin tornadoes threading through the otherwise motionless fluid. Between them, the helium does not turn at all.
Why the whirlpools come in fixed sizes
Here is the truly strange part. Every one of these whirlpools carries exactly the same amount of swirl — the same fixed "strength" of circulation. You cannot make one a little stronger or a little weaker. If you spin the bucket faster, the liquid does not spin its existing whirlpools harder; instead it simply makes *more* of them, each identical to the last, arranged in a neat lattice like apples in a crate.
These are called [[quantized-vortex|quantized vortices]] — "quantized" meaning they come only in whole, indivisible units, never in fractions, the way money comes in whole coins. Each is a single coin's worth of rotation. The reason traces straight back to the [[macroscopic-quantum-coherence|macroscopic quantum coherence]] of the last guide: the one shared wave must reconnect smoothly with itself all the way around any loop, and that requirement permits only whole numbers of swirl-units. Half a vortex would tear the wave; nature does not allow it.
The fountain that runs on heat
Now for a second wonder, and one of the prettiest demonstrations in all of physics. Take a glass tube packed at the bottom with fine powder, dip it into a bath of superfluid helium, and gently warm the liquid inside the tube — say, by shining a little light on it. The helium responds by squirting up out of the top of the tube in a fountain that can leap tens of centimetres into the air.
Warming a liquid to make it shoot upward sounds backwards — heat usually just makes things expand or boil, not fountain. The [[two-fluid-model|two-fluid model]] explains it beautifully. Remember that the superfluid part carries no heat, while the normal part carries it all. The fine powder lets the frictionless superfluid part slip through easily but blocks the sticky normal part. Warming the tube converts some superfluid into normal fluid, which lowers the superfluid concentration inside the tube. The superfluid outside then rushes in through the powder to even things out — and with nowhere else to go, it overflows the top as a fountain.
This is the [[superfluid-fountain-effect|superfluid fountain effect]]. Its lesson is that in a superfluid, heat and flow are linked in ways that have no parallel in everyday liquids: a temperature difference all by itself can pump the fluid, no mechanical pump required.
A second flavour of superfluid
Everything so far has been about ordinary helium, [[superfluid-helium-4|helium-4]], whose atoms are bosons and so condense in the most direct way. But helium has a rarer, lighter isotope, helium-3, whose atoms are fermions — the loners that refuse to share a state. By the rules of the last few guides, those atoms ought to be unable to condense at all.
And yet, cooled a thousand times colder still — to a few thousandths of a degree above absolute zero — helium-3 *also* becomes a superfluid. The trick is that its atoms pair up two at a time, and a pair of fermions, taken together, behaves like a single boson that can then condense. This is [[superfluid-helium-3|superfluid helium-3]], and it is even richer than its cousin: the pairs can align in several different patterns, giving it multiple superfluid phases with distinct personalities. It is exactly the same trick by which electrons pair up to make a superconductor — another reminder that all these frictionless states are deeply related.