A Brief Recap of Fundamentals #

Before we dive in, you will need to be familiar with basic particle physics and classical electrodynamics concepts. Here, we prepared a recap of these topics to refresh your memory (or lay out some essential fundamental concepts) before we move on into quantum electrodynamics and Feynman diagrams.

Subatomic Particles and Forces #

The Standard Model #

To explore quantum interactions, you should know have an awareness of our current understanding of the elementary particles that make up our universe. We all know that everything around us is made up of atoms - small blobs constantly in motion: flying around, bumping into one another, etc. - which are supposedly, indivisible building blocks of our world. Modern physics has taught us, however, that is not quite true; we can still cut these atoms up further and further (even protons and neutrons!) into what are termed elementary particles – which are really indivisible (to our knowledge). They are shown in the diagram below, which is called the Standard Model of Particle Physics.

Source: Google Images

You might be curious then: what are protons and neutrons made up of? The answer is quarks. Protons have two up quarks and one down quark (denoted $uud$), while neutrons have one up quark and two down quarks ($udd$). This means that most of our observable universe can be described using just up quarks, down quarks and electrons! (electrons are already elementary particles, so we can’t divide them further)

Source: Google Images

The Four Fundamental Forces #

We also know that all forces we experience are a consequence of four fundamental forces. Usually, we explain how these forces act at a range using the concept of a field. But at a quantum scale, we realise that these fields can be explained using particles too! Of the four, three are known to be mediated by certain bosons. For the last force, gravity, scientists hypothesise that it is also mediated by a boson, called the graviton, but the theory has yet to be supported by empirical observation.

What about the Higgs boson? #

You may have heard about the hype surrounding its confirmation by CERN and Atlas in 2013. While we won’t cover it in depth (you can read more about it here and here), it is an elementary particle that is associated with the mass of every particle (!) and we sometimes see it in Feynman diagrams involving the weak force, or nuclear fusion or nuclear fission where mass is converted into energy, or vice versa. It was theorised to explain what seemed like inconsistencies in the electroweak unification theory (which says that the electromagnetic and weak force can be explained by a single electroweak force), i.e. photons being massless while $Z$ and $W$ bosons having ~$100$ times greater mass than protons.

Classical electrodynamics #

Electrodynamics or Electromagnetism is the study of Electromagnetic Force, which explains the interaction between electrically charged particles.

We know that the electromagnetic force is carried by the electric and magnetic fields. Electric Fields and Magnetic Fields exert a force on a charged particle. For example, the magnetic field of a magnet can arrange iron filings like so:

Magnetic Field

Source: Google Images

Electric Field

This is an electric field. The lines describe the force exerted on the a positively charged particle. We can see that positive charges are attracted to the negative charge, and repelled away from the positive charge.

Source: Google

The idea of electric fields and magnetic fields works well for most day to day interactions. However, for really small objects, like electrons and photons, the classical model of fields begins to break down. Hence, we need another explanation - quantum mechanics. Read more in the QED section.