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鈥檚 a boojum. The reality is a tad more complicated because the hedgehog is really a 鈥渟ingle-point topological defect鈥� and the grass is the 鈥淎-phase of superfluid helium-3鈥�.
A little background might help. is a 鈥渜uantum liquid鈥� 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鈥檛 play ball until it is much colder 鈥� about 1 millikelvin. That鈥檚 because the atoms in helium-3 must first club together into super-fragile 鈥淐ooper pairs鈥�, which break apart at the merest hint of heat.
The atoms in a spin and they also orbit each other. Physicists visualise the combined spin as an arrow pointing along the spin axis and the orbital motion as another arrow. The two arrows can point in a whole host of different directions relative to each other, leading to a whole host of 鈥減hases鈥� of superfluid helium-3.
Advertisement
In the 鈥淎-phase鈥�, the arrows point in the same direction for all pairs, adding up to make big arrows. However, as the superfluid enters that A-phase 鈥� and this is the key to the boojum 鈥� the big arrows of all the Cooper pairs aligned in one region may point in a different direction to the big arrows in the next region, and so on. This variation is called texture. The liquid will do its utmost to make the texture as smooth as possible, but sometimes it simply cannot maintain smoothness and a bunch of arrows ends up pointing radially outwards from one point. This is a point-like defect 鈥� a hedgehog, or boojum. Also possible are line-like defects, called vortices, and surface-like defects, called domain walls.
The hedgehog hates being out in the open and, if possible, heads for the boundary of the A-phase. There, it releases some of its tense, nervous energy, and rests peacefully for evermore. And herein lies the origin of the term boojum. In his poem, The hunting of the snark, Lewis Carroll described an imaginary creature called a boojum that 鈥渟oftly and suddenly vanished away鈥�.
Boojums crop up in all systems that support topological defects, such as liquid crystals. Even the universe, as it cooled, went through phase changes in which the directions of the 鈥渇orce fields鈥� may have frozen in different directions in different places. Boojums could even turn out to be the universe鈥檚 invisible dark matter, says Shaun Fisher at the University of Lancaster, UK. If so, they may just be the most common objects in the cosmos.
鈥淏oojums could even turn out to be dark matter鈥�
