slightest resistance, to—as if by magic—overcome friction. Superfluidity is very much analogous to superconductivity, the ability of a metal to conduct electricity without any loss. Indeed, in a very real sense superconductivity is superfluidity, with the electrical current being carried along through the superconducting metal as a superfluid of electrons. A superfluid is to a regular fluid what a superconductor is to a regular electrical conductor. Until the mid-1990s the only laboratory superfluids known were liquids of both helium isotopes, 4He and the rarer 3He.
In principle, a superfluid gas flowing around a closed loop can keep flowing (or “persist”) forever. At a conceptual level, persistent superflow around a loop can be understood as analogous to a twisted loop of ribbon. Imagine taking a length of ribbon, putting a twist in it, and then bringing the two ends of the ribbon together and permanently gluing them. The twist in the loop of ribbon represents the flow