The initial results puzzled the world three years ago: the size
of the proton (to be precise, its charge radius), measured in
exotic hydrogen, in which the electron orbiting the nucleus is
replaced by a negatively charged muon, yielded a value
significantly smaller than the one from previous investigations
of regular hydrogen or electron-proton-scattering. A new
measurement by the same team confirms the value of the electric
charge radius and makes it possible for the first time to
determine the magnetic radius of the proton via laser
spectroscopy of muonic hydrogen (Science, January 25, 2013). The
experiments were carried out at the Paul Scherrer Institut
(PSI), Villigen, Switzerland, which is the only research
institute in the world providing the necessary amount of muons.
The international collaboration included the Max Planck
Institute of Quantum Optics (MPQ) in Garching near Munich, the
Swiss Federal Institute of Technology ETH Zurich, the University
of Fribourg, the Institut für Strahlwerkzeuge (IFSW) o