In 1948 Dutch physicist Hendrik B. G. Casimir of Philips Research Labs predicted that two uncharged parallel metal plates will have an attractive force pressing them together. This force is only measurable when the distance between the two plates is extremely small, on the order of several atomic diameters. This attraction is called the Casimir effect.
The Casimir effect is caused by the fact that space is filled with vacuum fluctuations, virtual particle-antiparticle pairs that continually form out of nothing and then vanish back into nothing an instant later. The gap between the two plates restricts the range of wavelengths possible for these virtual particles, and so fewer of them are present within this space. This results in a lower energy density between the two plates than is present in open space; in essence, there is "less than nothing" between the two plates, creating negative energy and pressure, which pulls the plates together. The narrower the gap, the more restricted the wavelength of the virtual particles, the more negative the energy and pressure, and thus the stronger is the attractive force.
The Casimir force per unit area for idealized, perfectly conducting plates with vacuum between them is
It has since been shown (see ) that, with materials of certain permittivity and permeability, the Casimir effect can be repulsive instead of attractive.