Superfluid speed limits

Abstract

Superfluids are extraordinary liquids whose viscosity is zero. This enables them to flow without friction and to pass effortlessly through fine pores or tiny openings (in earthenware pottery for example) that are quite impervious to ordinary fluids such as water. Similarly, an object moving in a superfluid experiences no drag and behaves dynamically as if it were moving in a vacuum. But what will happen if this object is subjected to a constant force? Physical intuition suggests that the object will accelerate uniformly, in the absence of drag, but it is also obvious that this cannot continue indefinitely. Sooner or later, something must surely happen to stop the acceleration (figure 1). The nature of this catastrophic event, in which the superfluidity breaks down at least temporarily, is of considerable intrinsic interest and also sheds much light on the fundamental character of the superfluid state, and the physics of the superfluid under consideration.