[Physics FAQ] -
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Original by Philip Gibbs 24-January-1997

# What is the Casimir Effect?

The Casimir effect is a small attractive force
which 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 there is a small force drawing them together.

The attractive Casimir force between two plates of area *A*
separated by a distance *a* can be calculated to be,

*
pi*^{2} h-bar c
F = ----------- A
240 a^{4}

where *h-bar* is Planck's constant over two pi 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. The Casimir effect comes from a difference
of energies in which the infinities cancel. The
energy of the vacuum is a puzzle in theories of quantum
gravity since it should act gravitationally and produce
a large cosmological constant which would cause space-time
to curl up. The solution to the inconsistency is expected
to be found in a theory of quantum gravity.

### References

H.B.G. Casimir, Proc. Kon. Ned. Akad. Wetensch. B51, 793 (1948)

S. Lamoreaux, Phys Rev Lett, 78, p5 (1996)