QED - The Strange Theory of Light and Matter (Penguin Press Science) [Paperback]

QED The Strange Theory of Light and Matter by Richard Feynman

There are many anecdotes about Richard Feynman: he came from New York and had a strong Brooklyn accent and exuberant character to go with it. He could antagonise his more effete colleagues with his natural humanity. He was once asked what it was like to be genius. His reply was along the line of ` Well, I feel pretty dumb much of the time as I'm stuck on a problem I can't solve.' After winning the Nobel Prize in 1965 he was dubbed `the smartest man in the world', his mother's comment was, `If he's the smartest man, then God help us.' He was also a highly competent safe cracker and made crucial decisive discoveries linking defective `O' rings to the Challenger shuttle disaster

It's quite rare to get close to major figures in science, yet this is the feeling one gets from reading QED.
Richard Feynman gave these lectures as part of the first series of Alix G. Mautner Memorial Lectures. Alix and her husband were close friends of his and it was always Feynman's intention to write a series of lectures, not for his peers or students but for the intelligent, interested layperson that knows little of Theoretical Physics. So this is not a text book for a physics undergraduate but it is for those who strive to have some understanding of the exquisite mysteries and paradoxes which emerge from the subject.

Quantum mechanics, despite its esoteric reputation is one of the most successful theories in science. It explains all the cracks which appeared in classical physics in the early 20th C, such as the photoelectric effect and blackbody radiation and led to enormously powerful discoveries of such as Heisenberg's Uncertainty Principle, wave-particle duality and Planck's famous constant. This is the tiny and irreducible amount of energy known as a quanta and is the quanta in quantum mechanics.
Quantum electo-dynamics is a branch of Quantum Mechanics relating to the interactions solely of photons and electrons. Electrons absorb and emit photons - this is what they do and most importantly both behave as waves and particles. Because of their wave nature it is impossible to measure both the position and the momentum simultaneously with certainty (The Uncertainty Principle) but the probability of finding their location is what Feynman struggles to do in his work and explain in these lectures.

If a beam of light is reflected from a mirror the angle of incidence equals the angle of reflection, we were told at school. `Not so,' says Feynman - the light is reflected from every possible point on the mirror, even if the light is a single photon. It is simply that the probability of it using the direct and shortest path is greatest. Although he does not mention it explicitly, this is the superposition of a photon whereby a single photon can take many different positions at the same time. Feynman goes into fascinating detail of how to calculate these probabilities using little arrows. These have become known as `Feynman arrows', the direction of which relates to the time, and hence position in its wave cycle as it reaches its target. Imagine a bicycle wheel with a ruler attached from the hub to the rim and a direct path taken to a mirror. When the wheel reaches the mirror the ruler will be at some angle to the road. However, if a slightly different and hence longer path were taken then the angle would be different when the wheel reaches the mirror. Photons behave in a similar manner except their frequencies are considerably greater to those of a rotating bicycle wheel. A typical frequency of a photon is in the order of a million billion Hz., with a corresponding wavelength in nanometres, for an electron it is <1nm.
If many of these little arrows are calculated and then put head to tail and a line drawn from the tail of the first one to the head of the last and then this line is measured, then the probability of finding that a photon has taken that route is given by squaring the length of the line This is a basic rule peculiar to Quantum Mechanics and is known as the `Born rule'. In the macroscopic classical world in which we live, to calculate the probability of a number of events you would multiply and individual probabilities, but in the microscopic world of quantum mechanics they are squared - this is one of the many mysteries of the subject.
So, we can say nothing with certainty of the behaviour of a particular photon - whether it went this way or that, only the probability of its behaviour.

He continues his survey with the interaction of electrons with photons. This is altogether more complex. Electrons, like photons have the dual nature of particles and waves, they have rest-mass whereas photons do not. (photons are bosons and electrons are fermions). He uses little diagrams, this time to demonstrate the interaction of photons with electrons. These are space-time graphs showing the particle's path through both space and time and show the possibility of electrons travelling backwards in time to absorb or emit a photon. He creates a picture of the complexity of the multi-dimensional calculations needed to resolve these mammoth probabilities.

The final lecture deals with the interactions of more massive elementary particles such as protons neutrons and their constituents, quarks, muons and neutrinos. He shows how new particles cam be predicted by constructing grid diagrams with known properties but with gaps where new particles may lie.

This is fine book written with passion and conviction by a man utterly absorbed and thoroughly expert in his field. It is not an easy book and hence not a gentle read. He puts highly complex material into very simple language giving the impression that you now have some understanding of these peculiar and sometimes bazaar aspects of Nature which are often paradoxical and contrary to common sense. As a scientific communicator Richard Fenyman is superb.

A.O'Connell
February 2011

Unlike many books relating to quantum mechanics, and the strange universe that exists on the quantum scale, this book is dedicated to a subject that is known and (as far as can be said about anything relating to the quantum scale) understood.

This book does a superb job of explaining to the layman (such as myself) what quantum electrodynamics is, and restricts itself to doing just that job and doing it well.

I know little about physics and upon reading this book i gained a clear understanding of QED and it pushed me into the right direction to find out more about the world of quantum mechanics.
A recommeded read.