ABSTRACT
The difference between normal systems and superfluid systems is perhaps most strikingly manifested in their current flow. Superfluid current flow displays very special features, both in its structure and in its response to external probes. Superfluid flow is irrotational. In an actual superfluid, we shall see that matters are more complicated, in that vortices may appear in the rotating bucket. As a result, one does not encounter the dramatic manifestation of superfluid behavior predicted for a homogeneous superfluid. Superfluid flow is resistance free: when a superfluid flows in a thin capillary, it is not slowed down by the walls. The two properties of irrotational flow and resistance-free flow constitute the key to superfluid behavior. The superfluid flow is thus characterized by a change in some suitably defined "condensate wave-function." Steady superfluid flow involves a velocity field, which is divergence-free. Such "hydrodynamic" flow is beyond the range of perturbation theory.
