Relaxing the vacuum

Ball, Philip

doi:10.1038/nmat2516

News & Views
Published: September 2009

Material witness

Relaxing the vacuum

Philip Ball

Nature Materials volume 8, page 705 (2009)Cite this article

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There are few topics in physics more prone to misrepresentation than the Casimir force. In popular discourse, the term is commonly preceded by 'ghostly', as though there is something barely credible about the manifestation of an attractive interaction between two surfaces separated by a vacuum. Interpretations in terms of virtual particles or suppressed quantum fluctuations of the electromagnetic field only encourage that view. But regarded as the familiar dispersion force resulting from induced dipoles, 'slowed down' by the finite speed of photons, the Casimir force becomes altogether more prosaic.

All the same, proposals to alter its influence — to engineer it — have about them something of the marvellous, as though the inescapable exigencies of nature are somehow being cheated. This possibility, however, was already implicit in Evgeny Lifshitz's recasting of the Casimir force in 1956, when he worked out the theory for real materials with finite dielectric permittivity (that's to say, finite conductivity). It's easy to see from Lifshitz's theory that, for certain choices of plate materials and media separating them, the Casimir force can actually be made repulsive. All the same, it wasn't until earlier this year that the right combination of materials — silica, gold and an organic liquid — was found¹. (It's often overlooked that a classical analogue of this repulsive force, due to density fluctuations of a fluid at its critical point between two surfaces, was seen some time ago in superfluid helium².)

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