Folks might benefit from an explanation of the Casimir effect: Two metal plates separated by a vacuum form an electromagnetic cavity of width W. The cavity al

Folks might benefit from an explanation of the Casimir effect: Two metal plates separated by a vacuum form an electromagnetic cavity of width W. The cavity allows only wavelengths whose mode goes to zero at the edges of the cavity. Normally, you feed a cavity resonator with E&M radiation (for example, your microwave), but the Casimir effect is a recognition of a factoid from quantum field theory: Photon fields have an energy of (N + 1/2) \hbar\omega, where N is the number of photons in the mode, and omega is the frequency of the mode. The strange factoid is that even for N=0 (no photons) there is still energy in the mode. This energy goes by the name "zero-point energy", because there is still energy in the vacuum for zero photons. When you change the width W of the cavity, there is a change in the allowed modes, so there is a change in the total zero-point energy. The Casimir force then makes sense: Since the zero-point energy changes with the distance between the metals, there is a force pushing the system from the higher-energy configuration to a lower-energy configuration. Edit: Since this has gotten attention, my one-sentence summary of the article from below: Phonons cause a microscopic change the distance between the two plates, which by the Casimir force, then allows transmission of phonons across the vacuum gap.