Quantum Electrodynamics (QED) and Quantum Chromodynamics (QCD) are both quantum field theories that describe fundamental forces in particle physics. However, they differ in their focus and the specific interactions they describe:
Quantum Electrodynamics (QED): QED is a quantum field theory that describes the electromagnetic force, which includes interactions between charged particles such as electrons and photons (particles of light). It was developed by combining quantum mechanics with classical electromagnetism. QED successfully explains phenomena like the behavior of electrons in atoms, electromagnetic radiation, and the scattering of particles by electromagnetic fields. It is a U(1) gauge theory, meaning it is based on a symmetry group called U(1) that corresponds to the conservation of electric charge.
Quantum Chromodynamics (QCD): QCD is a quantum field theory that describes the strong nuclear force, which is responsible for interactions between particles called quarks and gluons. Quarks are elementary particles that make up protons, neutrons, and other hadrons. Gluons are the force-carrying particles of the strong force. QCD was developed to understand the behavior of quarks and gluons within the framework of quantum mechanics. Unlike QED, which deals with electric charge, QCD is concerned with a different property called color charge, which is associated with the strong force. QCD is a non-Abelian gauge theory based on the symmetry group SU(3), reflecting the conservation of color charge.
In summary, QED describes the interactions of charged particles through the electromagnetic force, while QCD describes the interactions of quarks and gluons through the strong nuclear force. Both theories are crucial components of the Standard Model of particle physics, which aims to explain the fundamental particles and forces in the universe.