Casimir force changes sign.

that the quantum fluctuations of an electromagnetic field, so-called zero-point fluctuations, give rise to an attractive force between objects 1. This force is a particularly striking consequence of the quantum theory of electrodynamics (for a review, see ref. 2). Casimir's calculations were idealized — he considered two perfectly conducting parallel plates at absolute-zero temperature — but there are implications for more realistic objects. In Physical Review Letters, Kenneth et al. 3 have extended these considerations to real-world materials. Their work follows that of Boyer in 1974, who also studied the case of parallel plates but with one plate perfectly conducting and the other having infinite magnetic permeability (permeability is a measure of the material's response to an applied magnetic field). For this special case Boyer found that quantum fluctuations induce a force with the opposite sign, causing the plates to repel each other 4. Kenneth et al. now extend understanding of the Casimir force phenomenon to the more general situation of realistic 'dielectric' materials that are characterized by both their electrical permittivity (a measure of the material's response to an applied electric field) and their magnetic permeability. Their numerical results show that repulsive forces can arise in the general class of materials with high magnetic permeability. Although the Casimir effect is deeply rooted in the quantum theory of electro-dynamics, there are analogous effects in classical physics. A striking example was discussed in 1836, in P. C. Caussée's L'Album du Marin (The Album of the Mariner) 5. Caussée reported a mysteriously strong attractive force that can arise between two ships floating side by side — a force that can lead to disastrous consequences (Fig. 1). A physical explanation for this force was, however , offered only recently, by Boersma 6 , who suggested that it originates in the radiation pressure of water waves acting differently on the opposite sides of the ships. His argument goes as follows: the spectrum of possible wave modes around the two ships forms a continuum (any arbitrary wave-vector is allowed); but between the vessels their opposing sides impose boundary conditions on the wave modes, restricting the allowed values of the component of the wave-vector that is normal to the ships' surfaces. This discreteness created in the spectrum of wave modes results in a local redistribution of modes in the region between the ships, with the consequence that there is a smaller radiation pressure between the ships …