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

Abstract

Several years ago Schwinger proposed a physical mechanism for sonoluminescence in terms of changes in the properties of the quantum-electrodynamic (QED) 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 wavenumber, 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 a companion paper we discuss the technical complications due to finite-size effects, but for reasons of clarity in this paper we confine attention to bulk effects.

Geometrodynamics of variable-speed-of-light cosmologies

“…Particle creation from a time-varying refractive index is a well-known effect [51][52][53][54] § and shares many of the features calculated for its inflationary counterpart (e.g., the particles are also produced as squeezed couples). We point out at this stage that these mechanisms are not identical.…”

Cosmological Particle Creation and Dynamical Casimir Effect

“…A new application of the dynamical Casimir effect has recently appeared in connection with the suggestion by Schwinger [17] that the photon production associated with changes in the quantum electrodynamic vacuum state arising from a collapsing dielectric bubble could be relevant for sonoluminescence (the phenomenon of light emission by a sound-driven gas bubble in a fluid [18]). For the further developments and discussions this quantum-vacuum approach see [21,19,20,22,23] and references therein. The possibility of particle production due to space-time curvature has been discussed by Schrodinger [24], while other early work is due to DeWitt [25], and Imamura [26].…”

Sonoluminescence as a QED vacuum effect. II. Finite volume effects

“…Using these basis states and the sudden approximation we can calculate the Bogolubov coefficients relating "in" and "out" vacuum states. In the infinite volume, the discussion of the companion paper [2] is recovered, while for physically realistic finite volumes we see significant but not overwhelming modifications. The uncertainties in our knowledge of the refractive index as a function of frequency are also significant.…”

“…To achieve this, we adjust basic aspects of the model: We will move away from the original Schwinger suggestion, in that it is no longer the collapse from R max to R min that is important. Instead we postulate a rapid (femtosecond) change in refractive index of the gas bubble when it hits the van der Waals hard core [2].…”

“…We discussed Schwinger's original version of the model [3][4][5][6][7][8][9], the Eberlein variant [10][11][12][13], and proposed a variant of our own [2]. (See also [14][15][16][17]. )…”