gluon
/ GLOO-on /
Forces in nature are carried by messenger particles passed back and forth, the way two people on ice skates affect each other by tossing a ball. Electromagnetism is carried by the photon. The strong force — the one that holds quarks together inside protons and neutrons — is carried by a particle called the gluon, named because it acts like glue. It is the most powerful force carrier known.
A gluon, like the photon, is massless and travels at the speed of light, but in one crucial way it is stranger. The photon carries no electric charge, so light beams pass straight through each other. The gluon, by contrast, carries color charge itself — a color and an anticolor at once. So gluons not only bind quarks; they also bind to each other. When a quark emits or absorbs a gluon, the quark's color can change, and the gluon ferries that color away, keeping the books balanced. There are eight distinct gluon types in all.
Because gluons act on each other, the strong force behaves in a way no other force does: it does not weaken with distance the way gravity or electromagnetism do. Picture the gluons between two quarks bundling into a taut tube or string; pull the quarks apart and the string just stores more and more energy, never letting go. This self-stickiness of gluons is the root cause of confinement, of why most of a proton's mass is sheer strong-force energy, and of the spray of particles called jets seen in high-energy collisions.
A red quark can turn blue by emitting a red-antiblue gluon; the gluon carries away the color change so the total color is conserved.
Gluons carry color, so they change a quark's color as they shuttle the strong force between quarks.
Naively three colors and three anticolors suggest nine gluons, but one colorless combination drops out, leaving eight. That count is not a guess — it follows from the SU(3) symmetry of QCD and is confirmed by how often gluons are produced.