An experiment that has been on physicists’ wish-lists for many decades has delivered an unexpected result: the proton seems to be about 4% smaller than has been thought. As reported in Nature (doi: 10.1038/nature09250), the result calls into question either the value of the most accurately known fundamental constant or the validity of a remarkably successful physical theory.

The theory of quantum electrodynamics, which describes how light and matter interact, has provided many successful and highly precise predictions of atomic properties. The precision of both the theory and atomic spectroscopy have advanced to the point where accurate knowledge of the size of the proton (specifically, its “charge radius”) is the limiting factor for comparing experiment with theory. The currently accepted value for the proton radius, based mostly on spectroscopy of the hydrogen atom, is known to an accuracy of only 1%.

Randolf Pohl and colleagues have improved this accuracy by a factor of ten, by performing a technically challenging experiment that has only recently become feasible. Replacing the electron in hydrogen by its heavier counterpart, the muon, increases the effect of the proton radius on the measured atomic spectrum. The resulting, more accurate, value for the radius differs from the previously accepted value by an amount that cannot be explained. As the authors discuss, the result seems to require either a change in the previously well accepted value of the Rydberg constant (which plays an important role in the hydrogen spectrum), or a problem with quantum electrodynamics itself. More information about the project is available at: https://muhy.web.psi.ch/wiki/.

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