Number 253 (Story #2), January 4, 1996 by Phillip F. Schewe and Ben Stein A FEYNMAN THOUGHT EXPERIMENT HAS BEEN DEMONSTRATED IN AN ATOM INTERFEROMETER . In his lectures, Richard Feynman imagined an experiment that explored what would happen when you shine light at an object passing through an interferometer, a device that can split the object into a pair of wavelets which are later recombined to produce an interference pattern. This experiment has been unrealizable in electron and neutron interferometers because neither type of particle interacts strongly with light. Now, using their atom interferometer, an MIT team has shone single photons on atoms inside the interferometer. As Feynman correctly described, scattering a photon from an object inside the interferometer can destroy its wave properties. If one can in principle determine a pathway for the atom by detecting the position of the scattered photon, then the object acts as a particle. On the other hand, if the two atom-wave paths are separated by less than about one-half the wavelength of the light, a scattered photon can no longer provide information on which path the atom traversed, so the wave properties are not destroyed. In a variant of the experiment, the group detected only those atoms that scattered photons in a narrow range of directions. As it turns out, the once-washed-out interference patterns reappear at the sacrifice of information about the paths the atoms took. (Michael Chapman et al, Phys. Rev. Lett., 20 November 1995.)