A proton is the only stable example of a particle composed of three quarks. But inside the proton, gluons, not quarks, dominate.
One question that every curious child winds up asking at some point or other is, “what are things made of?” Every ingredient, it seems, is made up of other, more fundamental ingredients at a smaller and smaller scale. Humans are made of up organs, which are made of cells, which are made of organelles, which are made of molecules, which are made of atoms. For some time, we thought that atoms were fundamental — after all, the Greek word that they’re named for, ἄτομος, literally means “uncuttable” — since each species (or element) of atom has its own unique physical and chemical properties.
But experiments taught us that atoms weren’t fundamental, but were made of nuclei and electrons. Moreover, although the electron couldn’t be split apart, those nuclei were further divisible: into protons and neutrons. Finally, the advent of modern experimental high-energy physics taught us that even the proton and neutron have smaller particles inside of them: quarks and gluons. You often hear that each nucleon, like a proton or neutron, has three quarks inside of it, and that the quarks exchange gluons, keeping them bound together. But that isn’t…
