All atoms are made up of nuclei orbited by electrons. The nuclei, in turn, are made of neutrons and protons, which are themselves made of particles called quarks. (Related: "'God Particle' May Be Five Distinct Particles, New Evidence Shows.")

For years the accepted value for the radius of a proton has been 0.8768 femtometers, where a femtometer equals one quadrillionth of a meter.

The size of a proton is an essential value in equations that make up the 60-year-old theory of quantum electrodynamics, a cornerstone of the Standard Model of particle physics. The Standard Model describes how all forces, except gravity, affect subatomic particles. (See