It’s a subatomic mystery with big implications. Six years after physicists announced a bafflingly too small measurement of the size of the proton, we’re still not sure what’s going on. With the release of new data today, the mystery has, if anything, got deeper. Protons are particles found inside the nucleus of atoms. For years, the proton’s radius seemed pinned down at about 0.877 femtometres, or less than a quadrillionth of a metre. But in 2010, Randolf Pohl at the Max Planck Institute of Quantum Optics in Garching, Germany, got a worryingly different answer using a new measurement technique. Pohl’s team altered the one proton, one electron composition of a hydrogen atom by switching the electron
Although tiny, a proton takes up a finite amount of space, enough to fit three quarks, a host of virtual particles, and their associated gluons. The size of a proton's radius is determined by these particles and their interactions, and so is fundamentally tied in to theories like the Standard Model and quantum chromodynamics. We can measure the radius because the proton's charge is spread across it, which influences the orbit of any electrons that might be circling it. Measurements with electrons produce a value that's easily in agreement with existing theories. But a few years back, researchers put a heavier version of the electron, called a muon, in orbit around a proton. This formed an exotic,