Quantum Theory, Groups and Representations: An Introduction

by Peter Woit

PDF available for free download here.

Genres: Physics, Science, Mathematics, Nonfiction, Quantum Mechanics, Textbooks

428 pages, Paperback

First published August 28, 2014

Reviews

[Manny · Rating 4 out of 5]

This book, like nothing else I've read recently, made me feel I was eight years old again. I read the whole thing, and I sort of got it, but I know I didn't really get it. I'm hoping I'll really get it when I'm older. Well, I got some of it. The person who wrote it, Peter Woit, has for most of his life been a huge admirer of the German mathematician Hermann Weyl, who in 1929 published a book on group theory and quantum mechanics. Weyl's book is famous in physics circles, partly because it ended up being very influential, and partly because hardly anyone understands it. It's arguably the most difficult book I know. Woit has written his book to try and explain what Weyl's book is about and make it accessible to ordinary people, by which he apparently means courageous souls with at least advanced graduate knowledge of both math and physics. Woit is definitely much easier to read than Weyl, and maybe he's succeeded; it's a bit difficult to tell, partly because I'm not really in his target group (I don't know enough math or physics), and partly because the book isn't finished yet (it's a preprint which he's been using as lecture notes for the course he teaches at Columbia).

So what is the book about, you want to know? Well, I wish I could give you a proper summary, but that would involve really getting it. The crux, as the title says, is the relationship between quantum mechanics and representations of groups. Groups, if you're not a mathematician, are basically abstractions of symmetries; a representation of a group is basically when you turn it into an actual symmetry. With most groups, there's more than one way to do this. Weyl's great insight, which ended up putting 20th century physics on a new course, was that the mathematical structure of group representations is intimately linked to the mathematical structures of both quantum mechanics and classical mechanics. (For people who know what these words mean: it's not an accident that Lie brackets and Poisson brackets are both commutators). The book is full of very detailed explorations of this general idea. Some of it made good sense; for example, I finally understand what it is that makes the idea of "supersymmetry" so natural and appealing, even though there's no experimental evidence to support it. The mathematics points so clearly in a certain direction that you feel it just has to be there. Other parts made rather less sense; I still don't grasp what it means to say that a particle is essentially an irreducible representation, which disappoints me. I was hoping I'd get that.

Well, my next goal is to read Shankar's Principles of Quantum Mechanics. Woit constantly refers to him, and I can see that I should have read him first. This sort of strategy worked fine when I was eight, and I'm expecting that it'll work now; hopefully I'll be able to reread Woit a year from now and I'll wonder what it was that I couldn't understand. But right now, I'm still unhappy. It's infuriating to see that the big kids get it and I don't.
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[Update, a couple of days later]

When I'd finished, I wasn't sure how much I'd learned. But starting Shankar, I now find that the long first chapter, which gives the mathematical background, is just obvious - I can skim though it, everything makes perfect sense.

Thumbs up, Woit! I'm awarding you a fourth star.

[Wendelle · Rating 5 out of 5]

course textbook.

[Shozab Qasim · Rating 5 out of 5]

I've never been a huge fan of group theory but Woit manages to explain it wonderfully and link it to physics. While I read most sections of it - I'd definitely love to read the whole things once again sometime.