ABSTRACT
This chapter discusses the theory of hydrodynamic fluctuations in a superfluid. It presents a derivation of the "two-fluid model" of superfluidity. This derivation, based on the memory function formalism, seems to be new in detail although in substance it is closely related to the discussion given by Hohenberg and Martin. The chapter focuses on those macroscopic features of superfluids which can be understood with little detailed calculation. A more useful paradigmatic characterization of superfluidity is by what the name implies, the existence of persistent macroscopic flow — superflow. The symmetry which is broken is not one of the conventional ones, translations or rotations under which superfluid Helium is certainly invariant. The chapter shows that the persistent macroscopic flow which characterizes superfluidity is an aspect of long-ranged momentum correlations. It also focuses on more microscopic features of He II, such as the spectrum and nature of the elementary excitations.
