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The Casimir effect refers to the tiny attractive force that appears between two uncharged plates in a vacuum. This Casimir force is only measurable when the plates are extremely close together (several atomic diameters). This force was predicted in 1948 by Hendrik Casimir, a Dutch theoretical physicist. It was experimentally verified in 1958 by Marcus Spaarnay, again at Philips in Eindhoven while he was studying the properties of colloidal solutions. The recognized cause of the Casimir effect is the quantum vacuum fluctuations (zero-point fluctuations) of the electromagnetic field between the plates.
The attractive force occurs because, as quantum theory indicates, even a so-called vacuum contains a multitude of virtual electromagnetic particles and anti-particles in a continuous state of fluctuation. This is known as the vacuum energy. Because the gap between the plates constrains the possible wavelengths of the virtual particle pairs, there are fewer virtual particles within the space between the plates relative to the space outside them. This means the energy density between the plates is less than that of the energy density of the surrounding space, creating a negative pressure which pulls the plates together ever so slightly.
The closer the plates become, the smaller the vacuum energy density. It was not until 1997 that the precise magnitude of the Casimir force was measured by Steve K. Lamoreaux of the Los Alamos National Laboratory along with Umar Mohideen and Anushree Roy from UC Riverside. Because using two parallel plates would require impractically high standards for precise alignment, a plate and a nearly perfect sphere were used. Within a margin of error of 5%, the intensity was found to be just that predicted by quantum theory; defined as the zero-point energy of the Fourier modes of the electromagnetic field between the plates.
With certain materials and in certain configurations, it has been shown that the Casimir effect can be repulsive as well as attractive. It seems that the Casimir force is too small to be applied usefully to any of our present-day technology, though knowledge of its existence may be essential for those designing micromechanical or nanomechanical robotic devices in the present and in the decades to come. One day it may be possible to exploit the Casimir effect for the generation of energy, though this day is very far off and it is likely that more efficient energy sources will be discovered before this even becomes possible.