This book is an account of the current (as of 2001) state of quantum gravity theory. Smolin starts by giving us a grounding in the basic rules: there is nothing outside the Universee (i.e. no forces outside the Universe can be tested scientifically, so do not belong in a scientific theory), special care must be taken becasue we are necessarily observers in the quantum system, etc. Perhaps the most important consideration is that any theory must be background-indepented -- there is no preferred frame.

Using these starting points, Smolin gives an overview of what we've learned that can help formulate quantum gravity theories or help disprove those that are wrong. The resulting theories that are not obviously wrong are loop quantum theory and string theory.

Smolin does a fair job explaining them to a scietifically-aware audience (I know a thing or two about physics, so I can't speak to how well he does for a completely general audience). It's not a bad book, though I can't help but wonder if it will horribily outdated in a few years. (Of course, it may take longer than that to develop experiments capable of testing these theories.)

I was happily surprised: I was kind of looking for a book like that (read so far about string theory, in fact I carry at the moment "A first course in string theory" by Barton Zwiebach, a technical book, but very detailed and good to follow for an astrophysicist) but not a whole lot about the other approaches to quantumgravity) and here it was!!
I also was happily surprised about the idea of a traveling book.
I found this one outside Ballroom B on May 31 2005 at the Minneapolis Convention Center at the American Astronomical Society meeting in Minneapolis. I first go read it more carefully, take it with me to the Netherlands and will find a good place to release it again.

CAUGHT IN MINNEAPOLIS MINNISOTA USA