quantum chromodynamics (QCD)
/ QCD = cue-see-dee /
Quantum chromodynamics is the rulebook for the strongest of nature's forces — the force that binds quarks into protons and neutrons, and ultimately holds the heart of every atom together. "Chromo" means color, after the color charge that quarks carry. QCD is, quite simply, the theory of how color charges interact by exchanging gluons.
Its ingredients are spare. Quarks carry color charge in three varieties; the strong force is carried by gluons; and unlike the photons of electromagnetism, the gluons carry color themselves, so they interact with one another. From just these rules — written down as a beautiful symmetry called SU(3) — flows everything: that quarks attract via gluons, that they can never be isolated (confinement), and that the force paradoxically gets weaker, not stronger, at very short distances (asymptotic freedom). QCD is part of the Standard Model, sitting alongside the theories of electromagnetism and the weak force.
What makes QCD hard is also what makes it powerful. Because the force is so strong at ordinary distances, the usual trick of adding up small corrections (perturbation theory) breaks down, and physicists must turn to massive computer simulations on a grid of spacetime — lattice QCD — to calculate, for instance, the mass of a proton from first principles. That such calculations now match experiment to a few percent is one of the great triumphs of modern physics, and confirms that this strange three-color theory really is how the strong force works.
QCD was built in the early 1970s. Gross, Wilczek, and Politzer showed in 1973 that it predicts asymptotic freedom — the force fading at short range — and won the 2004 Nobel Prize for it.
QCD is the quantum field theory of color charge and gluons, completed in the 1970s.
QCD is the theory of the strong force as it acts on quarks. The force that holds protons and neutrons together inside a nucleus is a residual, leftover effect of QCD — related, but a step removed, like the weak stickiness between neutral molecules left over from the electric forces inside them.