The major goal of the present book is to outline the field-theoretic analysis of the dynamical behaviour of a Bose-condensed fluid that has developed since the late 1950's. While we often use the weakly interacting dilute Bose gas (WIDBG) for illustrative purposes, the emphasis is on the dynamical properties of a specific Bose-condensed liquid, superfluid 4He. We attempt to develop a coherent picture of the excitations in liquid 4He which is consistent with, and rooted in, an underlying Bose broken symmetry. Recent high-resolution neutron-scattering studies in conjunction with new theoretical studies have led to considerable progress and it seems appropriate to summarize the current situation. The only other systematic account of superfluid 4He as a Bose-condensed liquid is the classic monograph by Nozières and Pines (1964, 1990).

The phenomenon of Bose condensation plays a central role in many different areas of modern condensed matter physics (Anderson, 1984). Historically it was first studied in an attempt to understand the unusual properties of superfluid 4He (London, 1938a). In a generalized sense, however, it also underlies much of the physics involved in superconductivity in metals and the superfluidity of liquid 4He, in which Cooper pairs play the role of the Bosons (see, for example, Leggett, 1975 and Nozières, 1983). In recent years, there has been increased research on the possibility of creating a Bose-condensed gas, involving such exotic composite Bosons as excitons in optically excited semiconductors, spin-polarized atomic Hydrogen, and positronium atoms.