Hydrogen under the microscope: The experimental set-up for high-precision measurements on the hydrogen atom at the Max Planck Institute of Quantum Optics. (Courtesy: Vitaly Wirthl, MPQ) After more than 15 years of conflicting results, two independent measurements appear to have settled the debate over the charge radius of the proton. The new measurements, which are the most precise to date and are based on protons in normal atoms, suggest that the radius is 0.8406 femtometres (10 -15 m) – very close to the measured value that initiated the controversy back in 2010. Charge radius is a measure of how far the electric charge of a particle extends into space. In protons, researchers have two main ways of measuring it. The first is by scattering electrons from hydrogen atoms, which consist of a single proton bound to an electron. The second is by analysing the Lamb shift, which slightly modifies the gap between energy levels of the hydrogen atom and arises from interactions between the electron and proton.…