Difference Between QED and QCD

The key difference between QED and QCD is that QED describes the interactions of charged particles with the electromagnetic field, whereas QCD describes the interactions between quarks and gluons.

QED is quantum electrodynamics whereas QCD is quantum chromodynamics. Both these terms explain the behaviour of small scale particles such as subatomic particles.

1. Overview and Key Difference
2. What is QED
3. What is QCD
4. Side by Side Comparison – QED vs QCD in Tabular Form
5. Summary

What is QED?

QED is quantum electrodynamics. It is a theory that describes the interactions of charged particles with electromagnetic fields. For example, it can describe the interactions between light and matter (which has charged particles). Moreover, it describes the interactions between charged particles as well. So, it is a relativistic theory. Besides, this theory has been considered as a successful physical theory since the magnetic moment of particles, such as muons, agree with this theory to nine digits.

Basically, the exchange of photons acts as the force of the interaction because particles can change their speed and direction of movement when releasing or absorbing photons. Moreover, photons may be emitted as free photons which appear as light (or another form of EMR – Electromagnetic radiation).

Figure 01: QED Elementary Rules

The interactions between charged particles occur in a series of steps with increasing complexity. That means; first, there is only one virtual (unseen and undetectable) photon, and then in a second-order process, there are two photons that involve in the interaction and so on. Here, the interactions occur via exchange of photons.

What QCD?

QCD is quantum chromodynamics. It is a theory that describes the strong force (a natural, fundamental interaction that occurs between subatomic particles). The theory was developed as an analogy for QED. According to QED, electromagnetic interactions of charged particles occur via absorption or emission of photons, but with uncharged particles, it is not possible. According to QCD, the force carrier particles are “gluons”, which can transmit a strong force between particles of matter called quarks. Primarily, QCD describes the interactions between quarks and gluons. We assign both quarks and gluons with a quantum number called “colour”.

In QCD, we use three types of “colours” to explain the behaviour of quarks: red, green and blue. There are two types of colour-neutral particles as baryons and mesons. Baryons include three subatomic particles such as protons and neutrons. These three quarks have different colours and a neutral particle forms as a result of a mixture of these three colours. On the other hand, mesons contain pairs of quarks and antiquarks. The colour of antiquarks can neutralize the colour of quark.

The quark particles can interact via the strong force (by exchanging gluons). Gluons also carry colours; thus, there must be 8 gluons per interaction in order to allow the possible interactions between the three colours of quark. Since gluons carry colours, they can interact with each other (in contrast, photons in QED cannot interact with each other). Thus, it describes the apparent confinement of quarks (quarks are found only in bound composites in baryons and mesons). Thus, this is the theory behind QCD.

What is the Difference Between QED and QCD?

QED stands for quantum electrodynamics where QCD stands for quantum chromodynamics. The key difference between QED and QCD is that QED describes the interactions of charged particles with the electromagnetic field, whereas QCD describes the interactions between quarks and gluons.

The following infographic presents more comparisons regarding the difference between QED and QCD in more details.

Summary – QED vs QCD

QED is quantum electrodynamics where QCD is quantum chromodynamics. The key difference between QED and QCD is that QED describes the interactions of charged particles with the electromagnetic field, whereas QCD describes the interactions between quarks and gluons.

Reference:

1. “Quantum Electrodynamics.” Encyclopædia Britannica, Encyclopædia Britannica, Inc., 23 May 2018, Available here.
2. “String Theory and Quantum Chromodynamics.” Dummies, Available here.

Image Courtesy:

1. “Qed elementary rules” By Pra1998 – Own work (Public Domain) via Commons Wikimedia
2. “QCD – Quantum Chromodynamics” By Nikk (CC BY 2.0) via Flickr