Zero-Point Forces in Acoustic Waves
By the acousto-optic effect, an acoustic plane wave produces a 1D index-of-refraction or permittivity wave variation through a medium. But adjacent material planes of alternating permittivity should interact due to the zero-point (ZP) field to produce internal forces, roughly like the Casimir effect in a stack of regularly spaced discrete conducting plates. The ZP force in a smoothly varying 1D permittivity wave is modeled and found to consist mainly of bulk repulsive and double-wavenumber harmonics. It is stronger than the Casimir ZP attractive force in the corresponding discrete alternating-layer stack at all physically meaningful repetition scales, extends to larger scales, falling off universally only as the inverse square of the wavelength, and shows no temperature sensitivity. Thus, at its extremes, a standing acoustic wave exhibits a bulk expansive ZP pressure through the material volume, but as it passes through its null the ZP pressure vanishes, giving a body stress modulated at twice the acoustic wave frequency. But such repeated tensing in a piezo material is a usual energy-harvesting scenario, suggesting that ZP energy transfer may occur naturally with standing acoustic waves in a piezo medium. A voltage effect is predicted for biphonon lattice vibrations in piezo crystals with the possibility of 'crystal power', the extraction of electrical ZP energy across the crystal volume.