Feynman diagram

We'll get back to those in another episode.

This is a good time to introduce our first Feynman diagram.

The brilliant Richard Feynman developed these pictorial tools to organize the painful mathematics of quantum field theory.

But they also serve to give a general idea of what these interactions look like.

In a Feynman diagram, one direction is for time, in this case, up.

The other axis represents space, although the actual distances aren't relevant.

Here we see two electrons entering in the beginning and moving towards each other.

They exchange a virtual photon, this squiggly line here.

Time direction is irrelevant for the photon.

Throw these on a plot of space versus time, and we have a Feynman diagram.

a useless one.

None of these particles are doing anything worth calculating.

It looks like this.

This vertex alone represents six very different seeming interactions.

And it can be used to construct infinite Feynman diagrams.

Let's look at the possibilities.

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First matter is extremely useful in simplifying quantum field theory calculations because it massively cuts down the number of Feynman diagrams you need.

For example, this one diagram for electron and photon scattering represents both the double deflection of an electron or of a photon producing an electron-positron pair before the positron annihilates with the first electron.

That virtual particle in the middle may be an electron traveling forwards or backwards in time.

Now that we completely understand the fundamental nature of antimatter, let's go back and look at the solution to the Feynman diagram challenge question.

I asked you to draw all of the two vertex and four vertex diagrams for the interaction where electrons and positrons scatter off each other.

Using the simple rules I laid out for building Feynman diagrams, well, here are the two vertex diagrams.

These are really interesting because the processes seem so different.

To find the value of m, physicist Richard Feynman developed rules of scattering.

He also developed a system of diagrams, creatively named Feynman diagrams, that help to represent particle interactions and calculate relevant values for these interactions.

These are powerful tools for calculation, and they will be discussed later in the series.

We now understand why particles decay, and how to use physical quantities like decay rate and cross section to investigate and predict the decays and interactions between different particles in the standard model.