Review From User :
Goodreads friends who know about physics told me that this book really does provide a comprehensible explanation of loop quantum gravity. I was a little sceptical, having already seen a couple of unsuccessful attempts, but now that I've read it I'm convinced. It does what it says on the box.
If loop quantum gravity (LQG) hasn't been on your radar, let me give you some background and explain why this is a big deal. Physics has for the last century rested on two basic theories, quantum mechanics and general relativity. Each of these is very successful at what it sets out to do (roughly, quantum mechanics explains atoms and relativity explains galaxies), but, embarrassingly, the two theories are incompatible. This hardly ever matters, which is why physicists have been able to get away with it. But when you're dealing with extreme situations - black holes, and the beginning of the universe - it matters a great deal. As you probably gathered if you watched Interstellar, physicists don't really understand what goes on inside black holes. They are even less certain about what happened right at the beginning of the universe.
The current situation is, to say the least, unsatisfactory, and physicists have been trying to find a way to stick the two theories together for nearly a century: that's what "quantum gravity" is. The leading contender over the last few decades has been string theory, which has achieved a high enough profile that almost everyone at least recognises the name. For a good while, string theorists tried to make out that they were the only approach worth mentioning. When Lee Smolin wrote Three Roads to Quantum Gravity at the end of the last century and gave equal billing to the alternative approach of LQG, there was angry muttering from some string theorists that he was being unrealistically partial to his pet idea.
Since then, however, LQG has made steady progress while string theory has run into problems. Most worryingly for the string theorists, none of the supersymmetric particles essential to the theory have been found by the LHC, our friendly local particle accelerator. That doesn't necessarily mean they aren't there; maybe they don't quite have the predicted properties and are harder to find than expected. But the old Feynman crack about string theorists making excuses rather than predictions is starting to sound unpleasantly apposite. Now LQG has taken another step forward in making itself respectable. This book does a great job of describing the ideas in simple, straightforward language and explaining their appeal. Non-experts - including, one assumes, some who control funding - will soon think that there's a real choice available. If I were a string theorist, I'd be feeling that someone had better start writing a reply.
I won't try and explain here how LQG works. As noted, the book does that very well, and is also fun to read. If you know a little quantum mechanics, my guess is that you'll find it surprisingly simple and intuitive: basically, it's just the way you'd expect to be able to combine quantum mechanics and gravity. I'm sure that there's a huge amount of complexity not visible under the surface, and if I'm suspicious it's because it seems just a bit too good to be true. But deep learning, a subject I know more about, was like this. The idea is super-simple, but it took ages to fix some annoying technical problems that held up progress; once they were solved, the whole field took off. It sounds like LQG could be headed in the same good direction.
Okay, I can't deny it: I'm interested. Where do I find a level 2 treatment with a few more equations, so I can look under the hood and kick the tires I'm seriously thinking of buying.
(I also have a frivolous review here).
Update: the level 2 treatment is Covariant Loop Quantum Gravity: An Elementary Introduction to Quantum Gravity and Spinfoam Theory . It requires nontrivial knowledge of quantum mechanics and general relativity - but if you wanted more detail about the footnotes in Reality Is Not What It Seems, there's no doubt that it'll deliver. Available for free download here.
Media Size : 3 MB