Shrinking protons

If the electron is the currency of chemistry, then the proton is perhaps its bank deposit box and like banking in the real world there is news that can rock their foundations. Randolf Pohl and his colleagues at the Max-Planck-Gesellschaft have been harboring just such news for several years and recently confirmed their findings regarding the size of the proton. Using muonic (as opposed to electronic) hydrogen, the team measured the charge radius of the proton with an accuracy of better than one thousandth of a femtometer. Their measurements show that the hydrogen nucleus measures 0.8418 fm. This result is beyond the margin of error on previous measurements by a factor of five. Imagine the state of international finances if banks were to see accounting errors on a similar scale. The discovery has implications for spectroscopy and the Rydberg constant, the rock on which other fundamental constants are built.