Quantum Gravity (Cambridge Monographs on Mathematical Physics)

Rating:
Summary: Wrong philosophy, incorrect math, not very good book
Review: It is guaranteed that many people who have not seen the book - and who have no idea about physics (for example, linux evangelists) - will write a lot of irrelevant "reviews", but I am sure that those who are able to think will know which reviews are serious and which reviews are not.

I am among those who really enjoy searching for errors in physics books. While the good books usually have a couple of hundreds of typos and roughly five errors of a slightly conceptual flavor, it may be easier to enumerate the correct and valuable statements in Rovelli's book rather than the errors. The problems with this book are rather serious, numerous, and fundamental. It's mainly because the chosen point of view on the topic is problematic and perhaps obsolete.

First of all, the title is inappropriate and overly ambitious. The book does not cover quantum gravity (QG) - this term includes, among many other insights, semiclassical gravity, the calculation of particle production and Hawking radiation, the black hole information puzzle and black hole thermodynamics; holography, the AdS/CFT correspondence and other mechanisms of appearance and disappearance of spacetime dimensions; physics at the Planck scale, physics of spacetime singularities and application of quantum theory to cosmology; graviton scattering; quantum corrections to geometry, geometric dualities such as T-duality and mirror symmetry, topology changes and other topics arising in string theory; quantum effects influencing locality, causality, and the arrow of time; the origin of gravity and its interplay with other forces and particles. Rovelli's book does not really explain either of these topics.

The author starts with some terminological issues. For example, he redefines the word "relativistic" in such a way that the special theory of relativity is "nonrelativistic". It's not just a matter of unusual language: Rovelli repeatedly contradicts the fact that a theory XY must locally reduce to special relativity if XY should be called "general relativity" (GR). The models he presents probably do not respect the laws of special relativity - they are not relativistic in the usual meaning of the word - and consequently they're problematic from the viewpoint of experimental validity as well as according to the very purpose of GR: the only reason why Einstein had to look for a new theory of gravity was the required compatibility with his special theory of relativity. Because the adjective "relativistic" has a positive flavor in it, Rovelli decided to redefine it so that his promoted theory can be called "relativistic" even though it is not (according to the usual understanding of the word).

The initial chapter seems as an idiosyncratic account of reasoning about QG before anything sound about QG was known. This chapter also presents another adjective that is very popular in the LQG community: "background-independent". Rovelli correctly points out some complications that quantized geometry adds to the usual concepts of quantum field theory such as the operator product expansions. Many of these problems are simply solved by writing the metric as the sum of a nonzero classical background (a vacuum expectation value) plus a quantum, operator-valued fluctuation. This is the usual approach in particle physics and string theory and the main target of Rovelli's attacks. He does not say that such a decomposition is necessary for the concepts like the S-matrix to make sense. He does not say that such a decomposition does not eliminate the general covariance of the physical results (which really means decoupling of the unphysical modes). He does not say that even Newton's laws can be written in a background-independent fashion. Finally, he does not say that being "background independent" is vacuous unless one can show that the theory predicts many different geometric backgrounds (which is not likely in the case of LQG). He also hides the fact that there are different Hamiltonian LQG theories for different spacetime topologies; unlike string theory, LQG does not allow topology change.

The space limitations are forcing me to truncate the review; see http://schwinger.harvard.edu/~motl/rovelli.html for the full text.

Rating:
Summary: The fifth star is just there to make Motl angry.
Review: It is ironic that in his 30+ paragraph attack on Carlo Rovelli, as in his relentless attacks on the small community of physicists and mathematicians still interested in alternatives to the morass of culture, conjecture and ideology which now goes by the meaningless name "M theory," Lubos Motl has the audacity to dismiss him, and them, as dogmatists!

If the book really was as useless and silly as Motl pretends, then he would not be so afraid of it.

I seriously doubt that anyone will go to the trouble of writing a line-by-line rebuttal of Motl's screed. But I think that open minded readers would be interested to see the very terse, very positive review from Alain Connes, available at the book's CUP webpage.

Rating:
Summary: excellent ! a milestone of contemporary physics ....
Review: These days, a fierce battle is raging between two camps of physicists - strings theory developers and proponents of loop quantum gravity (seems like Amazon is going to be another battlefield :-)) . So far there are no experiments to confirm or reject any of these two theories and it is therefore inevitable to conduct the research in both directions. Rovelli's book seen from this perspective is a major attempt to present a consistent picture of how the LQG fits into the framework of our present knowledge about the world around us. This thoughts provoking book explains the path to the present state of LQG, its current problems and is a great reading not only for active developers but for anyone eager to understand the broader impact of these new physical ideas.

Be warned ! This is not an usual textbook ... You should have a fairly deep understanding of QM, QFT and GR - only then will Rovelli show you how to re-think these ideas from a new point of view.

Still, the rest of us (:-)) can benefit greatly from the crystal clear reasoning and philosofical implications he presents throughout the book.

Note: If you like John Baez books and style - you are going to love this one too ...