Quantum electrodynamics and the size of the proton

The radius of the proton can best be determined by comparing highly precise measurements on atomic hydrogen with predictions based on the theory of quantum electrodynamics. However, the results obtained with this approach were in conflict with previously published values, which suggested that novel physical effects might be in play. Further work at the Max Planck Institute of Quantum Optics has now demonstrated that the discrepancies can be attributed to errors in the earlier measurements.