Quantum chromodynamics

In QCD, quarks are the fundamental fermions that make up hadrons such as protons and neutrons, while gluons are the force-carrying bosons that mediate the strong interaction between quarks. Unlike photons in electromagnetism, gluons themselves carry the color charge they mediate, which is why the strong force becomes stronger as quarks are pulled apart.

This phenomenon, known as color confinement, arises because the strong force between quarks does not diminish with distance, unlike gravity or electromagnetism. As you try to separate two quarks, the energy in the gluon field between them increases until it becomes energetically favorable to create new quark-antiquark pairs, meaning you always observe quarks bound in color-neutral combinations.

Asymptotic freedom is the property that quarks interact weakly at very short distances or high energies, effectively behaving as free particles within hadrons. This was discovered by David Gross, David Politzer, and Frank Wilczek, who shared the 2004 Nobel Prize in Physics for this work, which explained why the strong force behaves oppositely to other fundamental forces.

While QED describes the electromagnetic force mediated by photons between electrically charged particles, QCD describes the strong force mediated by gluons between color-charged quarks. The key difference is that gluons themselves carry color charge—there are eight types of gluons—whereas photons are electrically neutral, leading to fundamentally different behaviors like confinement and asymptotic freedom.