Three Roads to Quantum Gravity [Anglais] [Relié]

The completion of a quantum theory of gravity (quantum gravity for short) is one of the most challenging problems in science in the twenty-first century. This theory aims at unifying Einstein's theory of general relativity for large-scale phenomena with the quantum theory for the micro-world, to get understanding of everything from space and time to matter and the universe. Lee Smolin, Professor of Physics at Pennsylvania State University, tells the story of recent and future research pursuing this theory for the intelligent layperson.

The author writes earlier chapters very understandably. The reader who knew nothing about the quantum gravity learns easily the following interesting things: There are three approaches to quantum gravity, i.e., the route from quantum theory (string theory), the road from the theory of general relativity (loop quantum gravity), and the path from fundamental principles. To do cosmology the classical logic demanding that every statement be either true or false is inadequate. A theory of quantum gravity has to answer about the nature of the information tapped in a quantum black hole. The search for the meaning of the temperature and entropy of a black hole is now leading to the discovery of the atomic structure of space and time. Etc.

In the middle of the book the author states that the style of these chapters will be more narrative than others because he can describe from personal experience some of the episodes in the development of loop quantum gravity. Lessons told are, for example, as follows: Science progresses quickly when people with different backgrounds and educations join forces. Einstein's example teach us that trying to invent new laws of physics requires not only intelligence and hard work but also insight, stubbornness, patience and character. Of course, these are also quite understandable.

In the last three chapters some or most of readers might find it difficult to follow the author's explanation. After reading the whole book, however, all the readers would feel that they have gotten at least a vague picture about the difficult problem of proceeding to quantum gravity. This is an exciting book for those who want to catch a glimpse of theoretical physics at its forefront.

There are some typos. Among them the followings are especially unfortunate, because the meanings of one of the laws of thermodynamics and the Heisenberg uncertainty principle are completely reversed to lead laypersons astray: In chapter 7, "The second law of thermodynamics requires only that the total entropy of the world never increase" should read "The second law . . . never decreases." In two inequalities in chapter 11, the symbol of "less than" should be that of "greater than or equal to."

I approached this book with great enthusiasm, hoping for a pedestrian treatment of loop quantum gravity (LQG). To be fair, most of this book is pretty good. Smolin writes pretty well, especially about relational quantum mechanics and how it relates to quantum gravity and cosmology. In addition, Smolin clearly points out why many relativists have issue with string/m-theory's lack of background independence.

I was, however, mildly disappointed in his discussion of the physical meaning of spin networks and loops and in his exposition of a possible synthesis of M-theory and LQG. Perhaps I overlooked it, but this book doesn't directly point out how you go from spin networks and spin foams to spacetime. But, you can figure it out... if you know enough general relativity and quantum field theory.

The appendix of this book is excellent! It provides many useful references to the literature.

All things considered though, this book is worth a read, especially to learn about the connection between spacetime, gravity, and quantum mechanics.

I originally rated this three stars. I recently reread the book and now want to give it four stars.

As my title states, this book is very good, but with the significant caveat that no particular audience is ideally served by it.

If you are truly a layman, you may initially be pleased to find that the book has essentially no formal mathematics and the technical vocabulary isn't too extensive. Smolin himself says that the book is aimed at the "intelligent layman" and that "the reader who has not read anything previously on these subjects will be able to follow this book." However, the book tries to convey a meaningful understanding of some rather advanced (some would say speculative) physics in the areas of thermodynamics of black holes, loop quantum gravity, and string theory, and it does this in a manner which is philosophically sophisticated, with many fundamental questions being raised about the nature of space, time, and scientific theories in general. Moreover, despite Smolin's claim to the contrary, one can't adequately appreciate what the book is about without a basic (at least popular-level) background in quantum mechanics and relativity, which the book doesn't provide.

As a result, I anticipate that the true layman would find this book to be difficult going. Instead, a more realistic audience would be the "advanced layman" who has some prior familiarity with this subject matter, particularly the basics of quantum theory, relativity, the standard model, and cosmology. This audience (which includes me) would probably find the book to be quite stimulating and interesting, and would get a sense of what this advanced physics is all about. However, because the presentation lacks the mathematics and technical details needed for anything resembling a rigorous understanding, they will probably also be left wishing the book had 50 more pages of "meat" and some key equations (explained carefully, but without derivations). For the same reason, the advanced reader would probably also wish the book was at least one notch more advanced, but even these readers may still find the book to be a fun and interesting overview of the subject.

Finally, I'd like to suggest that potential readers should be wary of reviewers who strongly favor the book because they agree with it's thesis, or condemn the book because they disagree. We are dealing with subject matter about which there is no consensus even among eminent physicists, so these reviews are not the place for readers to throw in their two cents about which theories are right and wrong. Instead, this sort of book should be judged mainly on criteria such as whether good questions are asked, whether the context for these questions is well formed, whether the alternative answers proposed for these questions are explained clearly and fairly, and whether the text is well-written. On all of these counts, my judgement is that Smolin does a good job.