Reading Feynman’s QED as An Aspiring Physicist

I just finished reading Feynman’s “QED: The Strange Theory of Light And Matter” (henceforth ‘QED’) and, in the context of my post on reading more, I would like to give you a writeup of my thoughts. ‘QED’ is a pop-sci book covering the quantum theory of electromagnetism known as quantum electrodynamics. ‘QED’, however, takes a slightly different approach of presenting the science; it does not dumb things down to give you the illusion of understanding. For me, personally, the book was a breeze to read due to the fact that I am already familiar with most of these concepts from my physics studies. However, let’s look at this book from the perspective of the layperson instead. Do Feynman’s explanations still hold up? I think they do.

Perhaps one of the reasons that I wanted to read this book is because I have recently been trying to find some simple ways of explaining the most important concepts in physics. In fact, I wrote a post on QM which I thought had one of the simplest explanations of quantum mechanics’ basic calculus. Turns out I was wrong. Feynman is ahead of most, if not all, popular-physics authors. His incredible wit keeps you engaged while he delivers the clearest, stupidly simple explanations of QED. All the while, the theory is explained correctly.

More precisely, Feynman gives his intuitions on the so-called ‘path integral formulation’. As someone who has studied it, it is a formalism that is quite hard to grasp due to its mathematical complexity. Feynman avoids the confusion of explaining it from the ground up by telling you what we—physicists—really do when we make calculations, even if we don’t realise it. It’s all just two-dimensional arrows assigned in a particular way to each path that a particle can take. The arrows for each way an event can happen are ‘added up’ and the square length of the final arrow is the probability that event can happen. It’s really that simple and, most importantly, correct.

If you are mathematically inclined, you will understand Feynman’s arrows to be complex numbers \(z \in \mathbb{C}\). If you are physically inclined, you will understand that each arrow is assigned based on the classical action functional, \(S[x(t)]\), according to \(e^{\frac{i}{\hbar} S[x(t)]}\) and the Feynman path integral, \(\int \mathcal{D}{x(t)} \, e^{\frac{i}{\hbar} S[x(t)]}\), on the subspace of functions with start point \(A\) and end point \(B\) gives the propagator for that particle. This does not matter, however; the public doesn’t have to deal with this, but we physicists do. The analogy is that you can know what adding or multiplying is without being able to do it.

Not only does he tell you what it is that we do, but he shows you specific examples of how it is applied. More concretely, he shows how our every day perception of light¹ emerges from this ‘simple’² theory. This is also something that I appreciated. He explains how it turns out that light ‘goes’ in straight lines and how reflections work, among others. Which connects the audience back to a real-life example, showing that this is not just some philosophical mumbo-jumbo.

Finally, in the last chapter, Feynman explains how the other forces fit into the picture of QED. In fact, the forces that come under the quantum field theory fold³ are described in basically the same way as QED with some modifications. The only thing that is missing is gravity⁴. The currently best theory, known as the standard model, is beautiful, to say the least. It fits in one line on a piece of paper if you write it compactly and about a page if you write it all out⁵.

So, as you might imagine, I truly believe that Feynman is an amazing expositor of science. I learned a few things that I would not get from a textbook. Additionally, I have gotten a new perspective on how science should be taught. There is yet much to learn from the master.

References

Feynman, Richard P. (2006). QED: The Strange Theory of Light and Matter, Princeton University Press.