TO MEASURE LOCAL GRAVITY with an uncertainty of 3 parts per billion, Steven Chu uses an atom interferometer, in which cesium atoms are treated like waves, split apart into two wavelets, each of which takes a separate path. When the wavelets are brought back together they produce an interference pattern which depends sensitively on the local force (gravity) tugging on the atoms. Not only is this an improvement (by a factor of a million) in accuracy over previous atom interferometers but represents, according to Chu, "the best confirmation of the equivalence principle between a quantum and macroscopic object." (Peters et al., Nature, 26 August 1999.)