Quantum Chromodynamics (QCD) and Quantum Electrodynamics (QED) are both integral parts of the Standard Model of particle physics, but they are not extensions or rivals of each other. Instead, they describe different fundamental forces and aspects of particle interactions.

QED is the quantum field theory that describes how light and matter interact. It’s all about the electromagnetic force, one of the four fundamental forces, and deals primarily with the interactions between photons (which mediate the electromagnetic force) and charged particles like electrons.

QCD, on the other hand, is the theory that describes the strong nuclear force, another of the four fundamental forces. This force is what holds the protons and neutrons together in an atomic nucleus and involves the interactions between quarks and gluons (gluons are the force carriers for the strong force).

So, while QED and QCD are both quantum field theories and share some mathematical similarities, they apply to different forces and particles. They coexist within the Standard Model, each addressing a unique aspect of particle physics. The real beauty of the Standard Model is how it integrates these different theories into a cohesive understanding of the subatomic world, despite their distinct domains.

Quantum electrodynamics is the theory that quantizes the electromagnetic interaction by introducing the field of photons. This affects particles with charge.

Quantum chromodynamics is the theory that quantizes the strong interaction by introducing gluons. This affects quarks.

These are separate theories because they model interactions that occur on different scales, but they (along with quantum flavorudynamics for the weak interaction) can be unified to form the Standard Model by combining terms in the Lagrangian in a sense.

>Is QCD an extension of QED, or is QCD a “rival” of QED?

Neither, they explain different things. Quantum electrodynamics is the quantum field theory of electromagnetism, while quantum chromodynamics is the quantum field theory of the strong force. Electromagnetism is the force between charged particles mediated by photons (the same particles as the quantisation of light), and is responsible for electrons and nuclei being held together to form atoms, magnets, the repulsion between things when you touch something, etc. The strong force on the other hand is the force that holds together quarks (one of the fundamental particles) to make protons and neutrons, which are what then make up the nuclei of atoms.

The standard model of particle physics is built on both QCD and QED (along with another force, the weak force, but discussion of how that ties in can get complicated). As these forces are different, we need different field theories for them, so they are not extensions or competing theories.

In a very ELI5 way, while they’re describing more unfamiliar things, they’re not extensions or rivals of each other, they’re just different things. Gravity and electromagnetism are two descriptions of different forces in Newtonian mechanics, and they’re also neither rivals nor expansions of each other.

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