New Evidence Confirms Old Predictions of Retarded Forces

Some of the more subtle predictions of quantum mechanics have had to wait decades before experiments were capable of validating them. One of these is the so‐called retarded force between charges that are so far apart that it takes a relatively long time for a photon to travel between them. (See the article by Larry Spruch in PHYSICS TODAY, November 1986, page 37.) In the 1940s Hendrik B. G. Casimir and D. Polder (then at the scientific laboratory of N. V. Philips's Gloeilampenfabriken in Eindhoven, the Netherlands) analyzed interactions affected by the finite speed of light. They found that the van der Waals potential between two atoms, which goes as $r−6,$ becomes a potential that varies as $r−7$ when the atoms are separated by distances greater than a few hundred Bohr radii. A related prediction is that the interaction between a neutral atom and a conducting wall changes from an $r−3$ to an $r−4$ potential when the atom is far enough from the wall. Casimir subsequently formulated an expression for the force between two conducting plates which is now known as the Casimir force