Number 242 (Story #1), September 28, 1995 by Phillip F. Schewe and Ben Stein NEW TESTS OF THE MOST SUCCESSFUL THEORY IN PHYSICS show the need for improved information on the proton. Stringently verified for five decades now, quantum electrodynamics (QED) is the modern theory of the atom. QED was initially developed to explain the "Lamb shift" in the hydrogen atom, in which an electron orbiting a proton experiences a slight energy shift due to the interaction between the electron and the teeming virtual particles residing in the surrounding vacuum. Modern experimental measurements have become so precise that theoretical calculations of the Lamb shift can no longer consider the proton as a pointlike charge but must instead view it as a sphere in which positive charge is spread out. In the energy states commonly studied in the Lamb shift, the electron actually has a probability of spending some time inside the proton, whose size, unfortunately, is not known to better than about 8%. Using high-resolution laser spectroscopy, Yale researchers (contact Malcolm Boshier, 203-432-3828) have measured the hydrogen Lamb shift to a new record accuracy of six parts per million. Their value (8,172,827 kHz, expressed in units of frequency) agrees with QED providing they incorporate a relatively large proton radius, such as the one measured in Mainz (Germany), 0.862 x 10**-15 m. However, a significant discrepancy between QED and the new Lamb shift result cannot be ruled out until the proton size is pinpointed to within 1 or 2 percent. According to Boshier, "It is...far from clear that everything is okay with QED," particularly when one considers that experimental measurements of another Lamb shift, that for the He+ ion, violently disagree with the theory. "The motivation for Lamb shift measurements is the search for new physics," he adds. (D.J. Berkeland, E.A. Hinds, and M.G. Boshier, Physical Review Letters, 25 September 1995, see also Physical Review Letters, 18 December 1995.)