On the Feigel Effect: Extraction of Momentum from Vacuum?

The Green-function formalism for the electromagnetic field in a magnetoelectric (ME) medium is constructed, as a generalization of conventional Casimir theory. Zero temperature is assumed. It is shown how the formalism predicts electromagnetic momentum to be extracted from the vacuum field, just analogous to how energy is extracted in the Casimir case. The possibility of extracting momentum from vacuum was discussed recently by Feigel [Phys. Rev. Lett. 92, 020404 (2004)]. By contrast to Feigel's approach, we assume that the ME coupling occurs naturally, and is not produced by external strong fields. We also find the same effect qualitatively via another route, by considering one single electromagnetic mode.