First off, I need to explain why I wanted to read this book. I thought the 3 roads of the title would be string theory, loop quantum gravity (LQG) and twistor theory. I have studied both string theory and twistor quite extensively, so was looking forward to a recap of those two with a nice easy introduction into LQG. This is not the case though. The book begins by trying to take the issue of quantum gravity in as broad a scope as possible, before looking at LQG and string theory. After reading it, I was still none the wiser as to what he thought the third road was.
I have to say I was quite disappointed with the start of this book. In chapter 3, Smolin makes the very correct observation that “If one is not careful, [the superposition principle] can lead to a kind of mysticism in which its meaning is over-interpreted far past what the evidence calls for.” However, he fails to take note the irony that the first two chapters contain conclusions which over-step the boundary set by evidence, and so the foundation of the book is based on some unjustifiable assumptions. Along with that, on page 22, there is possibly one of the least helpful diagrams I have ever seen in any scientific literature. Though he acknowledges that he is not the most eloquent of writers, he unfortunately seems to want to emphasise this point with some very hand-wavey descriptions of general relativity (GR). If you have not studied GR at university or even read other popular science literature on the subject, then the introduction will likely leave you completely baffled and clueless. If that sounds like you, I'd recommend going for A Brief History of Time first to get a clearer picture of GR.
From here, Smolin goes on to talk about quantum cosmology and the restrictions it can impose on our worldview. This was quite interesting to read, though probably not for the reasons intended. The book is (at the time of writing this review) 10 years old and it is quite fascinating to see how scientific opinion has shifted in even this short space of time. Smolin dismisses the many-worlds hypothesis as an interpretation of quantum mechanics (QM) though he doesn't really explain QM particularly well. What we are left with is a chapter that tries very hard to explain things in a lively, straightforward way, but which fails in that aim and is quite garbled and confusing, which is a terrible shame.
However, it's not all doom and gloom. The book picks up significantly in terms of quality and clarity when Smolin goes on to give the background to his own speciality: loop quantum gravity. He makes a good case for pinning it to sound and well evidenced basis, even if no direct evidence has yet been found to confirm it. He is also keen to stress that LQG is not necessarily a candidate for a theory of everything, and treats his own subject with a level of humility and healthy scepticism that is very welcome in a science text. There is also a hint at the end of the book of the introduction to his later book, The Trouble With Physics, which details certain sociological problems that surround and inhibit some aspects of research into quantum gravity.
There is a helpful critique of string theory given, though possibly not enough time is spent explaining it properly, and readers interested in that could do a lot worse that Brian Greene's The Elegant Universe.
Overall, it is worth reading but prepared to quite frustrated, particularly early on.