If you take a glass of water and tap on the side of it, you’ll generate waves on the water’s surface. The form of the waves depends on surface tension and gravity, and viscosity governs how quickly the waves fade away. In a recent experiment, researchers performed an equivalent tap for a container of ultra-cold atoms, and the results they found were odd indeed.
The researchers used lithium-6 atoms chilled so close to absolute zero that they could form a superfluid. The “glass” they were contained in consisted of intersecting laser beams, and the “tap” came from toggling the intensity of one of the lasers. This created rippling waves through the atoms that the group could observe.
Measuring at various temperatures, the group found that the waves in the atoms always decayed the way one expects for a classical fluid like water. Even when the atoms transitioned into a superfluid, the wave decay did not change. Since superfluids are considered to have zero viscosity, you’d expect their waves to decay more slowly, but it turns out, that’s not the case! (Image credit: F. Mittermeier; research credit: M. Zwierlein et al., see also; via Physics; submitted by Kam-Yung Soh)