What is the Casimir Effect?

What is the Casimir Effect?
The Casimir effect is a small attractive force that acts between two close parallel
uncharged conducting plates. It is due to quantum vacuum fluctuations of the
electromagnetic field.
The effect was predicted by the Dutch physicist Hendrick Casimir in 1948. According to the quantum theory, the vacuum contains virtual particles which are in a continuous state of fluctuation (see physics FAQ article on virtual particles). Casimir realised that between two plates, only those virtual photons whose wavelengths fit a whole number of times into the gap should be counted when calculating the vacuum energy. The energy density decreases as the plates are moved closer, which implies that there is a small force drawing them together.
The attractive Casimir force between two plates of area A separated by a distance L can
be calculated to be,

          π h c
F = -------------- A
        480 L^4
where h is Planck's constant and c is the speed of light.
The tiny force was measured in 1996 by Steven Lamoreaux. His results were in
agreement with the theory to within the experimental uncertainty of 5%.
Particles other than the photon also contribute a small effect but only the photon force is
measurable. All bosons such as photons produce an attractive Casimir force while
fermions make a repulsive contribution. If electromagnetism was supersymmetric there
would be fermionic photinos whose contribution would exactly cancel that of the
photons and there would be no Casimir effect. The fact that the Casimir effect exists
shows that if supersymmetry exists in nature it must be a broken symmetry
According to the theory the total zero point energy in the vacuum is infinite when
summed over all the possible photon modes.

References:
H.B.G. Casimir, Proc. Kon. Ned. Akad. Wetensch. B51, 793 (1948)
S. Lamoreaux, Phys. Rev. Lett. 78, 5 (1996)�

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