Quantum Electrodynamics (QED) and Quantum Chromodynamics (QCD) are both quantum field theories that describe the fundamental interactions of particles. However, they differ in several key aspects:
Charge and Color: QED deals with the electromagnetic force, which is mediated by the photon. It describes interactions between electrically charged particles, such as electrons and positrons, and the photon. QCD, on the other hand, deals with the strong nuclear force, which holds quarks together within protons, neutrons, and other hadrons. Unlike electric charge in QED, quarks carry a different type of charge called "color charge," which comes in three varieties: red, green, and blue (as well as their anticolors).
Particle Interactions: In QED, electrically charged particles interact through the exchange of virtual photons. These interactions are characterized by long-range forces and can be either attractive or repulsive, depending on the charges involved. In QCD, quarks interact through the exchange of gluons, which carry color charge. Gluons themselves can also interact with each other, giving rise to the phenomenon known as "gluon self-interaction." Unlike photons, gluons carry color charge themselves, resulting in unique three-gluon and four-gluon vertex interactions.
Asymptotic Freedom and Confinement: One of the most striking features of QCD is asymptotic freedom. At high energies or short distances, the strong force becomes weaker, and quarks and gluons can behave as nearly free particles. This property is in contrast to QED, where the electromagnetic force becomes stronger at short distances. However, at low energies or large distances, the strong force becomes stronger, leading to the phenomenon of confinement. This means that quarks and gluons cannot exist as isolated particles but are always confined within color-neutral bound states, such as protons and neutrons.
Fundamental Particles: In QED, the fundamental particles are electrons, positrons, and photons. Electrons carry a negative electric charge, positrons carry a positive charge, and photons are chargeless. In QCD, the fundamental particles are quarks and gluons. Quarks carry color charge and come in six different flavors (up, down, charm, strange, top, bottom), while gluons carry color charge and mediate the strong interaction between quarks.
These differences in particle properties and interactions make QED and QCD distinct quantum field theories, each describing different fundamental forces and phenomena.