Sonoluminescence as a QED vacuum effect. I. The physical scenario

Several years ago Schwinger proposed a physical mechanism for sonoluminescence in terms of changes in the properties of the quantum-electrodynamic vacuum state during collapse of the bubble. This mechanism is most often phrased in terms of changes in the Casimir energy (i.e., changes in the distribution of zero-point energies) and has recently been the subject of considerable controversy. The present paper further develops this quantum-vacuum approach to sonoluminescence: We calculate Bogolubov coefficients relating the QED vacuum states in the presence of a homogeneous medium of changing dielectric constant. In this way we derive an estimate for the spectrum, number of photons, and total energy emitted. We emphasize the importance of rapid spatio-temporal changes in refractive indices and the delicate sensitivity of the emitted radiation to the precise dependence of the refractive index as a function of wave number, pressure, temperature, and noble gas admixture. Although the basic physics of the dynamical Casimir effect is a universal phenomenon of QED, specific and particular experimental features are encoded in the condensed matter physics controlling the details of the refractive index. This calculation places rather tight constraints on the possibility of using the dynamical Casimir effect as an explanation for sonoluminescence, and we are hopeful that this scenario will soon be amenable to direct experimental probes. In the following paper we discuss the technical complications due to finite-size effects, but for reasons of clarity in this paper we confine our attention to bulk effects.