  We discuss the hydrodynamic boundary condition for a superfluid moving
tangentially to a rough surface. Specifically, we argue that the scattering of
quantum fluctuations off surface roughness affects the nature of the boundary
condition, and that this has important consequences including a new theorized
critical speed and the presence of normal fluid at any nonzero speed, even if
the boundary is held at zero temperature. This hydrodynamic boundary condition
is relevant not only for superfluid helium experiments but also for experiments
with trapped dilute Bose-Einstein condensates, in particular those involving
atomic waveguides near surfaces.
