Students of particle physics learn early that the proton is made of three quarks: two up and one down. But there must be more to the story than that. The proton’s rest mass is 938 MeV. The three quarks, with masses of just a few MeV each, make up only a tiny fraction of that.
Where does the remaining mass come from? Much of it is the kinetic energy of the quarks—they’re not at rest inside the proton, and special relativity dictates that their effective mass increases with speed. Most of the mass lies in the details of how the strong force holds the proton together.
Gluons, the carriers of the strong force, are themselves massless, but the energy of the gluon field contributes to the total proton mass. And the field can spawn fleeting pairs of quarks and antiquarks, known as the sea, as represented in figure 1....
