Neutrino Superfluids

Neutrino superfluids aren’t going to be observed any time soon, but the mathematical proof that they could exist helps to augment the catalog of possible physical reality. Superfluids are closely related to superconductors. In both phenomena numerous particles---whether boson particles such as helium-4 atoms or pairs of fermion particles such as electrons or helium-3 atoms---can coalesce into a single, all-encompassing quantum state; examples include supercurrents, superfluids, and Bose-Einstein condensates (BEC).

Joe Kapusta, a physicist at the University of Minnesota, has shown that neutrinos too can become a superfluid. First they must pair up, as electrons do in superconductors. Two electrons with opposite spins can form pairs by the exchange of slight disturbances in the underlying matrix of atoms in the solid sample. Analogously, neutrinos with opposite helicity (for a “left-handed” neutrino, its intrinsic spin is oriented opposite to its direction of motion; for “right-handed” neutrinos it’s the other way around) could pair up by exchanging a disturbance in the all-pervasive sea of Higgs bosons in the universe. (The Higgs boson, in turn, is the much-sought cornerstone of the current standard model of particle physics; it is the particle whose presence confers mass on many of the other known particles.) After pairing up, the nu pairs could then form a superfluid condensate.

Kapusta admits that the chances of observing his superfluid are slim since, first, right-handed neutrinos have never been observed (and might be even more elusive or ghostly than their left-handed partners) and, second, because the superfluid would only occur at temperatures far colder than the 2.7-K average-temperature of the current universe. Kapusta points out that a superfluid of heavy neutrinos would make a great medium for advanced civilizations to send messages over intergalactic distances since the scattering length of pulses (the average distance they go before scattering) moving through the neutrino fluid would be much greater than for electromagnetic pulses. (Kapusta et al.; Physical Review Letters, 17 December 2004; kapusta@physics.umn.edu, 612-624-0506x)