QED (Quantum Electrodynamics)

by Richard Feynman


I stumbled across this book by accident in a Waterstones. When I saw Feynman’s name, I bought it instantly without checking what QED even was.

Notes on the book

Quantum Electrodynamics

Essentially about the way light behaves as a photon. This book is based on lectures given by him, explaining the theory to non-science people in 1983. He says that many people dumb things like this down, such that they are not explaining it at all, but something different. In contrast he promises not to distort the picture, and tell it how it is. This ability is very much Feynman’s gift.

Light is a particle

Although light can be modelled as a wave, it’s a photon, a particle.

Behaviour can’t be predicted

The behaviour of an individual photon is not predictable. We cannot say ahead of time whether it will reflect off of a pane of glass or pass through it.

We can only apply probabilities

Although a single photon cannot be predicted, the general behaviour of photons does conform to probabilities, so we can determine the likelihood of something happening.

The probability of photons doing one thing or another is the core of the lectures.

Everything we observe as the behaviour is light, is actually just the most likely outcome

It is our belief that light travels in straight lines, but in fact this is only the most probable behaviour of photons.

Refraction, defraction, mirages, and lenses are all explained through the probabilites of photon behaiour.

Representing Probabilities

Probabilities of a given event happening are represented by an arrow.

The direction of the arrow relates to the length of time, from the start of the event to the end, according to a mystical stop watch which turns. When the event ends, the stopwatch stops. The direction of the hand is the direction of the arrow. The stopwatch, as far as I understand, is only relevant in that it means that events which take roughly the same amount of time will have roughly the same direction.

The length of the arrow is the square root of the probability of the event. e.g. probability of 4%, length of 0.02.

Combining Probabilities

To determine whether one of two events will occur (an OR operation), you add the two arrows, by sticking the tail of one to the arrow of the other, just like adding vectors. The resulting vector is the overall probability. Notice how, if the two arrows point in the same direction, they create a large overall probability than if they oppose one another.

To determine whether two events both occur (an AND operation), you multiply or “twist and shrink” the arrows. You multiply the lengths, and then twist it as many degrees as the other, from the unit arrow, pointing directly up.

The Mystery of Partial Reflection

When you aim monochromatic light at a pane of glass, it has a 4% likelihood of reflecting off.

What would you expect if you added another pane of glass behind the first? The probability should go up another 4%? Well, it depends on the gap between them. At certain gap sizes, it is 8%, then as you move it further away it gradually increases up to 16%. Then as you go still further, it starts to drop again. Keep going and it will go down, past 8%, past 4%, to 0% total reflection from the two panes.

Think about that: by placing one piece of glass behind another, at a certain distance, you can stop light bouncing off of the first. Mind-blowing.

This might be something to do with the delayed-choice quantum eraser but I’m not sure.

Occasionally I send out an idea & ask for your thoughts.