A vacuum chamber used to measure electron transitions in atomic hydrogen, from which the proton’s size was inferred Axel Beyer/MPQ At long last, we have pinned down the size of a proton. More than 15 years after an experiment unexpectedly shook the world of particle physics, researchers are regaining their grip on one of this fundamental particle’s most basic properties. Look around you, and everything you see will be filled with protons. The proton is a fundamental building block of our world – and until 2010, we thought we understood it fairly well. We knew its composition – it is made from three quarks – and we knew its size. Then, a measurement based on an exotic hydrogen atom showed that the proton may actually be about 4 per cent smaller than expected. Physicists scrambled, exploring sources of experimental error as well as theories about new physics phenomena that could resolve this “ proton radius puzzle ”.…