Ramona Vogt

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.

When a heavy atomic nucleus splits, it doesn't break into even halves. First, two fragments form that look like pears pressed together at their narrow ends. Then the junction begins to smooth out, like two droplets merging and the surface quickly leveling. In that brief moment, the surface moves inward, and particles inside the nucleus can bounce off it, like a ball off an oncoming wall.