Number 122 (Story #1), April 6, 1993 by Phillip F. Schewe and Ben Stein THE CASIMIR-POLDER FORCE , the force between a neutral atom and a metal plate, has been observed for the first time by Edward Hinds and his co-workers at Yale (Phys. Rev. Lett., 1 Feb. 1993). They measured the force on neutral sodium atoms traveling through a pair of gold plates spaced only microns apart. Predicted as long ago as 1948, the Casimir-Polder force is a consequence of the fact that the presence of a conducting plate (even with no electric field applied) modifies the vacuum around the atom, which in turn affects the quantum behavior of the atom, such as its ability to emit light of a certain wavelength. A force of the Casimir-Polder type is believed to play a major role in binding together the atoms of the recently discovered He2 molecule. In another example of cavity quantum electrodynamics, as the physics of atoms confined by reflecting walls in micron-scale volumes is called, an experiment currently underway in Paris is exploiting the wavelike nature of atoms in an attempt to count precisely the number of photons trapped in a cavity without perturbing the atom-photon system. Such a "quantum nondemolition" technique might lead to the development of more sensitive light sensors. (Scientific American, April 1993; Science News, 13 February 1993; Physics Today, April 1993; Nature, 18 Mar. 1993.)