Gerald A. Miller

The proton is not a solid ball. Its charge, mass, and other properties are distributed differently in space, and each property has its own radius. Physicists have long been able to measure where the proton's charge is concentrated and where its mass lies.

To uncover the proton's true size, physicists swapped the electron in a hydrogen atom for its heavy copy — the muon. Acting like a magnifying glass with thousandfold power, the muon gets right up against the nucleus and 'probes' the proton with unprecedented precision.

If you try to break a proton apart and pull out a quark, you're in for a surprise. Ordinary forces, like the electric force, weaken as particles fly apart. Here, it's the opposite. The farther a quark gets from its neighbors, the harder it's pulled back.