Physicists in Germany have carried out the most accurate measurement to date of the width of the proton. By examining a previously unexplored energy-level transition in the hydrogen atom, Lothar Maisenbacher and colleagues at the Max Planck Institute of Quantum Optics have shown that the Standard Model continues to hold up under extraordinarily tight scrutiny, leaving even less room than before for rival theories that contradict our best understanding of how the universe behaves. The research has been published in Nature.