
How big is a proton? The most accurate measurement yet suggests it's smaller than we thought. This could be due to an error – or it might just hint at totally new particle physics.
"The new experiment presents a puzzle with no obvious candidate for an explanation," says Peter Mohr of the international Committee on Data for Science and Technology (CODATA), which calculates values for fundamental constants in physics, who was not involved in the new work.
Like most quantum objects, a proton is fuzzy around the edges. Its size is defined by the extent of its positive charge rather than a crisp physical boundary. This charge radius cannot be measured directly but can be inferred from the hydrogen atom, which consists of a proton and an electron.
The electron can sit in a variety of energy "shells", each with a different distribution in space. One shell's distribution requires the electron to dive in and out of the proton, and another sits entirely outside the proton. The energies of both of these shells can be combined to deduce the proton's radius, using a theory known as quantum electrodynamics (QED).


