IMAGINE a hedgehog tottering across a road to a grass verge. On reaching the boundary, it relaxes and goes to sleep. There you have it. In figurative, everyday language, that’s a boojum. The reality is a tad more complicated because the hedgehog is really a “single-point topological defect” and the grass is the “A-phase of superfluid helium-3”.
A little background might help. is a “quantum liquid” that can do fantastic things like flow uphill and squeeze through impossibly small holes. The common isotope, helium-4, enters this magical state at 2.2 kelvin, but its rarer and lighter cousin, helium-3, won’t play ball until it is much colder – about…
