ON THE CONCEPT OF SPONTANEOUSLY BROKEN GAUGE-SYMMETRY IN CONDENSED MATTER PHYSICS

We discuss the concept of spontaneous breaking of gauge symmetry in superconductors and superfluids and, in particular, the circumstances under which the absolute phase of a superfluid can be physically meaningful and experimentally relevant. We argue that the study of this question pushes us toward the frontiers of what we understand about the quantum measurement process, and underline the need for a new theoretical framework that keeps pace with modern technological capabilities.