QED

I’ve read very few books twice. Unless a book is so jam-packed with neatness that every read is sure to introduce something new, I figure it’s not worth the time. One book that is worth the time every time, however, is Richard Feynman’s QED. I just finished my second run through this book, and it was absolutely 100% worth it.

QED reduces nearly all physical phenomena to the movement and interactions of electrons and photons. Starting from the most common of phenomena (light reflecting from glass, light bouncing off a mirror, lenses), this book is literally a fascinating journey into the mysterious character of light. Feynman adopts the role of a specialist teaching the layreader the neat, intuitive way to solve physics problems, and describes the bizarre character of light with delightful whimsy.

The first half of the book is an exposition of the path integral formulation of quantum mechanics as it applies to light. The probability that light does a certain thing is represented by the square of the length of a vector that corresponds to the event. Movement of the light and reflection cause turning and shrinking of the “unit arrow” until a final arrow for the event in question is reached. Summing all the arrows for the way an event can happen gives a final arrow for the event. Although there are an infinite number of ways any event can happen, the ways close to the path of minimum time reinforce each other, while those that take longer cancel each other out. This is the principle of least action at work!

After going through several simplified problems, Feynman reduces all events to combination of three simple steps: a photon goes from A to B, an electron goes from A to B, and an electron emits or absorbs a photon. The arrow-length formulas for the first two events are functions of the distance traveled and the time the travel takes (photons don’t have to travel at c, as it turns out). Fascinatingly, the arrow-length formula for emission and absorption is just a constant…the charge of the particle! Everything from optics to atoms can be analyzed using combinations of these three simple events.

I don’t want to spoil the rest of the book, but it is chock full of amazing observations like this. Teaser: an antiparticle is just a particle traveling backwards in time!

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