the gluon
/ GLOO-on /
If quarks are permanently imprisoned, the gluon is both the jailer and the bars of the cell. It is the particle that carries the strong force, the glue that binds quarks into protons and neutrons and binds those, in turn, into atomic nuclei. But the gluon is a stranger kind of glue than the photon: where light carries no charge and lets electromagnetism reach across a room, the gluon carries the very charge it responds to, and that single fact makes the strong force behave like nothing else in nature.
A gluon is the massless, spin-1 gauge boson of quantum chromodynamics, the gauge theory of the SU(3) color symmetry. Quarks carry one of three colors; a gluon carries a color and an anticolor, and the algebra of SU(3) yields exactly eight independent color combinations, so there are eight gluons. Because gluons themselves carry color, they couple directly to one another, three-gluon and four-gluon vertices exist, unlike photons, which are electrically neutral and do not. This self-coupling makes the strong coupling constant run in the opposite direction from electromagnetism: it weakens at short distances (asymptotic freedom) and grows at long distances (confinement).
The consequence is dramatic. Try to pull two quarks apart and the gluon field does not spread out and fade like an electric field; it collapses into a taut tube, or string, of nearly constant tension around one GeV per femtometer, storing more and more energy until it snaps by creating a fresh quark-antiquark pair. This is why you never see a free gluon or a free quark, only color-neutral hadrons, and why gluons, though massless, mediate a force that is effectively short-ranged. Gluon self-interaction is also why lattice QCD, not pencil-and-paper perturbation theory, is needed to compute most low-energy strong-interaction quantities.
About half of a fast-moving proton's momentum is carried not by its quarks but by gluons, as deep inelastic scattering revealed; the glue is as substantial as the things it glues.
Gluons are not a passive background: they carry a large share of a proton's energy and momentum.
It is tempting to picture a single kind of gluon like a single kind of photon, but there are eight color states, and it is precisely this color-charged multiplicity and self-interaction that distinguishes the strong force from electromagnetism.