Quantum electrodynamics

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Quantum electrodynamics modifies Maxwell's equations to be consistent with the quantized nature of matter.

And quantum electrodynamics changes in the electromagnetic field are expressed in terms of discrete excitations.

Particles known as photons, which are the quanta of light.

It's a packet of light.

In this case, the symmetry is local phase invariance, and the conserved quantity is electric charge.

At this point, we only need a couple of extra steps to produce the full description of electromagnetism in the quantum world, quantum electrodynamics, or QED.

First, we need to upgrade the Schrödinger equation to the Dirac equation so it works with Einstein's special relativity, and we talked about that in this episode.

Then we need to apply quantum principles to our field, like considering its internal or self-energy and allowing quantized oscillations in the field itself.

Quantum field theory describes a universe filled with different quantum fields in which particles are excitations, quantized vibrations.

We've talked about QFT many times before, starting with the very first quantum field theory, quantum electrodynamics.

QED talks about the electromagnetic field, whose excitations give us the photon.

The calculations of QED describe how this field interacts with charged particles to give us the electromagnetic force, which binds electrons to atoms, atoms to molecules, and therefore, you know, allows you to exist.

QED is a much deeper and more complicated description of electromagnetism than the simple opposite charges attract, like charges repel of classical electrodynamics.

But how do we know it's right? Well, because it makes some predictions that clash with the classical theory, and those predictions are the most precisely tested and thoroughly verified in all of physics.

Today, we're gonna talk about the theory and experiments behind one of these tests, measuring the g factor, or in simple English, measuring the anomalous magnetic dipole moment of the electron.

It doesn't consider the quantum nature of the field.

This, in essence, is because his equation has the charge and mass distributed differently.

QED further tweaks this by showing that charge can smear by, by something called the vacuum polarization.

It's not that the moment should be one, that's the classical theory, which is wrong, and also suggested that electrons should spin faster than light.

QED predicts it should be a bit above two, and that's what we see.

So QED is, if not gospel truth, the most right thing we have for describing electrons.

Okay, nice knowledge bomb there, Gareth Dean.

Epsilon J asks why electrons are thought of as infinitesimal points.

In other words, gravity should produce more gravity, ad infinitum This type of self-interaction or self-energy is seen in other quantum field theories and is hard to deal with even there.

For example, in quantum electrodynamics, the electron has a self-interaction due to its electric charge messing with the surrounding electromagnetic field.

In QED, the mess is fixed with something called perturbation theory.

It's a scheme to calculate a complex interaction, like the buzzing electromagnetic field around an electron, with a series of corrections to a simple, well-understood interaction, which might be the electron in a quiet electromagnetic field.

We talk about this more in our episode on the g factor.