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Physics News Update
The American Institute of Physics Bulletin of Physics News

Number 300 (Story #3), December 20, 1996 by Phillip F. Schewe and Ben Stein

THE CASIMIR FORCE , a 1948 theoretical prediction in which the seemingly desolate "vacuum" creates a tiny force between a pair of conductors, has been precisely measured for the first time. According to quantum mechanics, empty space (the "vacuum") is not truly empty but instead contains fleeting electromagnetic waves and particles that pop into and out of existence. However, when the vacuum is bounded by a pair of conducting surfaces, the only electromagnetic waves that can exist are those with wavelengths shorter than the distance between the surfaces. The exclusion of the longer wavelengths results in a tiny force between the conductors. To measure the Casimir force, Steve Lamoreaux, now at Los Alamos (505-667-5005), employs a torsion pendulum, a twisting horizontal bar suspended by a tungsten wire. The attraction between a gold-plated sphere and a second gold plate causes a small twisting force in the bar. By applying a voltage sufficient to keep the twisting angle of the bar fixed, Lamoreaux determined the force caused by the attraction of the plates. His results agree with theory to a 5% level. (Physical Review Letters, 6 January 1997.) Researchers previously measured the Casimir-Polder force (Update 122), a different but related effect in which the vacuum creates an attraction between a conducting plate and a neutral atom.