Customer Reviews

Three Roads to Quantum Gravity

5 star:		(27)
4 star:		(12)
3 star:		(6)
2 star:		(4)
1 star:		(2)

 

 

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.

Three Roads To Quantum Gravity by Lee Smolin isn't the easiest highway to travel in the universe, but will be rewarding to anyone taking the journey. Smolin, currently a professor at Penn State [my alma mater :)], writes simply, but smartly about the biggest piece of unfinished physics business leftover from the 20th Century, the resolution of the conflicts between our theory of the large, general relativity, and our theory of the small, quantum mechanics. Smolin assumes an interested and educated reader and simplifies our trip by not doing long introductions to relativity and quantum mechanics. After introducing the idea of a theory of quantum gravity, Smolin spends the rest of the book laying out the 'three roads' of the title, spending slightly more time on loop quantum gravity, the 'road' he's spent the most time on. Although not as elegantly written or as detailed as Brian Greene's The Elegant Universe, Three Roads To Quantum Gravity compliments that earlier book by coming at 'theory of everything' territory from a different direction. Since there is no AAA for physics, layfolks like myself should be glad that we have guides like Lee Smolin to direct us through the hidden regions of our universe.

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.

Which roads is Lee Smolin talking about? The first road is superstring theory (or M-theory). The mainstream is convinced that string theory is our leading candidate to describe all the particles and all the interactions and to unify quantum mechanics with general relativity. According to string theory, different particle species are loops of string vibrating in different ways. Brian Greene has described string theory in The Elegant Universe beautifully and the reader will probably learn nothing new from this Smolin's book.

The second road is the so-called Loop Quantum Gravity (LQG). This framework, started by Abhay Ashtekar and his friends, offers a way to reformulate the original equations written down by Einstein using some new variables leading to Wilson loops in the role of the fundamental observables. While most theoretical physicists believe that this approach is marginal and probably inconsistent, Smolin would like to "unify" it with superstring theory although M-theory and LQG contradict each other quite clearly in many aspects. The reader gets disappointed because he or she will not find out what is the relation between the loop of LQG and the string of string theory. There is none. The experts know it but the laymen can become confused.

The third road, according to Smolin, is a fuzzy collection of speculations invented by the philosophers of various kinds who would like to refuse everything that physicists have learned and to base everything on ill-defined "profound" and "deep" philosophical principles. Wolfgang Pauli would certainly say that the third road "is not even wrong". Unfortunately, Smolin likes to pretend that marginal directions of research are essential and that people like Finkelstein, Isham, Penrose, Sorkin (and Smolin himself) are heroes of physics today. Read the book if you liked the readable and anecdotal style of the Smolin's previous book "The Life of the Cosmos" or if you like to hear highly idiosyncratic comments about this exciting subject or if you like to speculate about questions as usually asked by laymen and philosophers. However if you want the author to teach you the right questions and the right answers from physics as understood by real physicists or if you want to learn who are the real heroes of the field and what dramatic discoveries have they done - for example Stephen Hawking or today especially Ed Witten - buy The Elegant Universe instead. Smolin's ideas have always remained outside the mainstream; "Three Roads" are no exception. And the readers should know it.

Since the postulation of theory of relativity (theory of cosmos, which describes the structure of space and time), and quantum mechanics (laws of microcosm, which describes atomic structure, nuclear forces, and nature of basic component of matter); physicists until now have struggled to explain gravity (which is a manifestation of spacetime fabric in presence of matter) in terms of quantum mechanics (quantum gravity). In this book the author attempts to explain three different approaches to quantum gravity; Loop Quantum Gravity (LQG), Superstring - M theory (S. -M), and Blackhole Thermodynamics (BT). While each takes a different starting point, they all agree when viewed on Planck scale, and they also view space and time are not continuous, and space is composed of discrete units. LQG gives us a detailed picture of these units in terms of spin networks, where as S.-M theory proposes continuous space in terms of a continuous string (with compactified extra dimensions) made of string bits, which is governed by uncertainty principle. BT theory states that amount information in any given space is finite and is proportional to the area of the boundary of the region in Planck units. The author is a pioneer in the field of LQG and provides the reader with a good introduction of the theory in a non-mathematical form and then compares with S.-M and BT theories. The book is described in three parts; the first part is a general introduction, which describes historical development of three theories, the second part introduces LQG and then compares with S.-M and BT theories, and the final part attempts to unify the three approaches into a single theory using Holographic Principle.

The author gives us several interesting accounts of physicists working in these fields are in a climate of mutual ignorance and complacency with the belief that their theory is correct and others are wrong. There are instances when one group can't solve certain problems, and they seek the help from the other camp. The author also briefly explains other theories such as Twister theory, and Non-Commutative Geometry. This is one of the few books I have read which describes LQG in some detail, although there are several books in literature, which describes S.-M theory. The author is very honest in comparing the three approaches to offer the best explanation for quantum gravity. Anyone who wants to understand LQG must have this book.

I am very impressed with this book. This is going to be one of the seminal science book for lay persons, right up there with Stephen Hawking's "A Brief History of Time".

Lee Smolin is an active researcher in Loop Quantum Gravity. He laid out in a clear manner the research progress of Quantum Gravity. Quantum Gravity, once completed, should able to reveal the basic fabric of our universe, and say a lot about what is the stuff that space and time are made of. There are no mind boggling math in the book, but there are lots of mind boggling ideas that once Smolin explains them, I am left with a sense indeed this must be how our universe works.

Smolin thinks understanding how Black Hole works is key to progress in Quantum Gravity. He gives a modern update of what we know about Black Holes, siting a few fresh research results. This is very valuable, because the typical Black Hole books for the lay person are very dated indeed. A modern treatment is exactly what we need.

This book is a must for the fans following the going-ons in the newsgroup sci.physics.research.

For me, the most useful reviews are those that tell me something about the book, along with some of the reviewers' opinions. I hope that the reader finds this one to be in that category. Some of the reviews of this book were less than completely positive and so is this present one. For the reasons listed below, I am giving this book a positive review, but a strongly qualified one.

First a little about the book and its good features:

1. Quantum gravity is the merger of general relativity and quantum mechanics. It is being applied to describe nature at the smallest of size scales, the behavior of black holes and the origin of the universe.

2. More than gravity per se, this book is about the quantum nature of space and time. It addresses the question as to whether space and time are continuous and can be divided into infinitely smaller pieces, or whether they are discreet, with minimum possible sizes (the Planck distance and Planck time). The book supports this discreet view of space and time.

3. The quantum of space is discussed in terms of quantum loops and atomic structure is described in terms of the spin networks, developed by Roger Penrose, that are made up of these quantum loops.

4. This book discusses the entropy of black holes and how this relates to quantum gravity.

5. There is a brief explanation of what string theory is about and how it relates to quantum gravity. It is postulated that these strings may themselves be made from spin networks.

6. There is a brief section of supersymmetry and its impact on string theory and quantum gravity.

7. There is a brief chapter discussing theological implications of this work.

8. This book focuses on the questions raised by modern physics. If you read it you will definitely get an idea about these questions and about quantum gravity, string theory and the entropy of black holes. These are all good reasons to read this book, but before you invest the time and effort to do so you should consider the reservations listed below.

Now why the above features were not enough for me to give this book 5 stars:

1) You definitely need some physics background to get much from this book. You need to know about entropy, quantum mechanics, nuclear physics and general relativity. This is not good a book for high school students or those who have no background in the required prerequisites. This would be a good book for people in the field, for interested scientists and for laymen who are well read in the required prerequisites. To be fair, the author does mention the need for some previous knowledge, but this is not stated on the book cover or where it might warn prospective readers. I therefore feel the need to do so here.

2) There is no attempt to define some of the important things being discussed. For instance, conical figures depicting photons in space-time are shown with little or no discussion. World lines are shown in one figure, but not defined. (Under W in the index are Warhol, Andy and Wittgenstein, Ludwig, but no world line.)

3) The features mentioned above are given in only general terms. Without more specificity and more background information the text is rather murky in places. After about 50 pages I was ready to stop and go on to other things, but I decided to read a bit further. Fortunately, I enjoyed some of the subsequent chapters a bit more and finished the book. No math is included, which is probably a very good thing since the math is daunting even for theoretical physicists.

4) There is a lot of personal anecdotal information in this book, which in my opinion only adds to its length, but does not provide any insights into the physics being discussed. According to the author (see item #9 below) this was included to show "science in the making", but I personally do see how this is done by recounting how the author had two flat tires while driving a colleague to the airport, or that he was rear ended by a Maserati. This information may great for those who know the author, but I felt that it just slowed down the flow of the text. I would have preferred replacing this anecdotal information with a better discussion of quantum gravity. Most of the book is about quantum loops and in my opinion there is insufficient discussion of how this relates to gravity, quantum or otherwise.

5) One of my main concerns is that at present there is no way to experimentally determine the validity of any of the theories presented in the book and much of what is being presented will likely be shown to be wrong. Without experimental verification, physics tends to degenerate into metaphysics. While it is a somewhat invidious comparison, at times reading this book reminded me of the medieval preoccupation with trying to count the number of angels that could dance on the head of a pin.

6) To use the author's own analogy, this book is like reading about the work of Galileo and Kepler, without the unifying principles developed by Newton. Perhaps, one should wait until someone makes sense of all of this and then spend the time to read a book that discusses this synthesis. (See #9, below, for Smolin's reason for publishing this book before all the evidence was in.)

7) Quantum gravity is clearly in competition with string theory for graduate students and funding dollars, and Smolin clearly favors QG. He feels that it is more fundamental than String Theory since he views strings as perhaps ultimately being made from spin networks of quantum loops. I would have liked to have seen more of the String Theory side of this debate. Compared to String Theory, QG is a minority view of physics on the smallest size and time scales. I am sure that there are many string theorists who could have contributed some balance to Smolin's views. As it stands, the picture that is presented is very one sided. (I am not a String Theorist or even a physicist, just a retired scientist from another field who has the time to write lengthy reviews.)

8) In the epilogue, Smolin predicts, "We shall have the basic framework of the quantum theory of gravity by 2010, 2015 at the outside.". This view is supported by the postscript to the 2001 paperback edition, in which Smolin cites recent experimental work that may provide the experimental evidence to validate one or more of the theories that he presents. All the more reason, in my opinion, to wait as little as 3 years (8 years at the outside) until some of the theories have been winnowed down by experimental evidence and Smolin can write a new book, or update this one, that gives a more definitive view of quantum gravity.

9) At the very end of the book, Smolin questions the wisdom of presenting science "in the making" to a general audience, but decided that it was best for the public to see how science goes about seeking the truth. I have no trouble with this, so long as this is clearly stated on the book cover and in the literature provided by the publisher. I for one would have preferred to wait until all this was sorted out before I expended the energy required to follow the discussion.

In summary, this book is OK if you want to know about some of the important questions being raised in modern physics and if you want to get a glimpse of science "in the making", but this book (and the science being described) is short on verified answers. (Not that anyone else has them to give.)

Reviewers more familiar with Quantum Gravity may find the book 'idiosyncratic' or 'subjective'. But I'm thankful for its articulation of concepts so profound as to elude conventional language. And this is Smolin's intent: not to be thorough (the book is 1 inch thick) but to give the layperson with little or no vocabulary, a chance to resonate with ideas whose very contemplation--let alone solution--has challenged thinkers from Aristotle to Witten (ok, Zeno).

He's expert in this deep-under-deep subject. Equally important in a book aimed at the general audience, he posesses Wigner's "unreasonable effectiveness" in using English to explicate the interface between new mathematics and new physics. And does it gracefully within the multitasked, competitive environment which scientists now inhabit.

The concepts discussed stretch beyond Feynman's use of 'weird' . Though many are now aware of the ubiquity of Black Holes, it still comes as a surprise to learn that---in the few years since Hawking's seminal contributions---theorists have significantly extended Hawking. Black Holes now can be addressed with quantum theory to yield a principle called the Beckenstein bound. And that allows them to formulate a quantum physics of information (or an informational underpinning to physics---Wheeler's "It to Bit"), thus closing in on a complete theory which reconciles relativity and quantum theory

Smolin has the gift of making the reader feel he's understanding these ideas. [Does the reader 'understand' in any meaningful sense? That's a subject for philosophers to debate]. He succinctly describes his own loop theory in comparison to 'conventional' string theories and their successor M-theory (as Brian Greene did more extensively in his book The Elegant Universe). He feels these contenders for the Ultimate are looking at the same elephant; and presents the competition's ideas with (well, almost) the conviction he has for his own.

In so doing, he bridges both an explanatory and sociologic gap . And he does so with welcome doses of speculation and humility. This original thinker deserves better reviews than those preceding mine; I'd suggest these folks levy their criticism in a speciality journal.

This is one of the most interesting books I've read in a while, about a subject that is both difficult and strange. The concept of quantum gravity rests on the notion that space is quantized. Quantization has been around for a long time. The Greeks established the idea of quantized atoms of matter - the concept that matter could be divided only a finite number of times. Most of us are accustomed, at some level or another, to the idea that you can divide elements down to atoms, and no further (they lose their elemental properties if you do).

But how about space? Is it continuous? Can it be subdivided to any arbitrary level? Is there a minimum increment in space? And how about time? Is there a smallest increment of time? According to Smolin (and quite a few other scientists) the answer is yes to both questions, and such quantization is fundamental to the concept of quantum gravity. To me, it seems at once both strange and common sense.

When I took physics in High School, Mr. Lewis once asked (philosophically); "what is time?" I replied that it's what defines motion. The answer came across, I'm afraid, somewhat flippantly, but I've considered it more seriously over the years and wondered what a universe without motion would really look like. Would there be time in such a universe? Would it even make sense to talk about time if there was no motion - no events - taking place? I've wondered if, at some level, we can understand the motion of particles in their zero state as a requirement that time cannot stop. Of course, this motion is also understood as a consequence of the Heisenberg uncertainty principle. Is there a relationship?

Smolin takes the point of view that time and space are sequences. Processes, in his words. Linkages. He introduces the concepts found in general relativity and quantum theory that there is no "space" that is independent of the observer, but only observers that are an integral part of space. Thus, we cannot talk about a separate space and time, but only of a universe in which we are part of the process. Consider the following, from his book:

"time and causality are synonymous. There is no meaning to the past of an event except the set of events it will influence."

"One cannot talk meaningfully about the existence of any object or the truth of any statement without completely specifying the questions that are to be asked."

".... Our world cannot be understood as a collection of independent entities living in a fixed, static background of space and time. Instead, it is a network of relationships the properties of every part of which are determined by its relationships to the other parts."

"if we look finely enough at our world the continuity of space and time will dissolve as surely as the smoothness of materials gives way to the discrete world of molecules and atoms."

As you can see, the book takes a high-level, qualitative, and philosophical approach. This makes it a bit difficult to make the mental bridge to understanding specific issues with the different theories Smolin discusses. Part of the problem is an unspoken understanding among Physicists that a theory is essentially a mathematical construct, and that even though you might write the equations of the theory, you may have no idea what they say. This is a concept that's probably new for many non-physicists, and I think it would be a good idea in a book such as Smolin's to discuss the situation a little bit. This might involve writing a few equations, like the Lagrangian for a four-body system. I know publishers don't like to use equations (and I disagree strongly with them over this point) but the theories of quantum gravity are mathematical, and the problems with understanding them cannot really be approached without explaining some of these characteristics. Otherwise, the author runs the hazard of saying things like "we don't really understand what the theory is telling us" and someone from the life sciences wondering why on earth a scientist does not understand her own theory.

The book's high-level discussion includes M-theory (string theory), loop quantum gravity, and the holographic principle, complete with Smolin's conjectures (at the end of the book) about how the story will unfold.

Smolin made me sit up and think, and I like that in a book. But the book is also written at the 50,000-foot level, and that made it somewhat frustrating. I'd like to understand Smolin's concepts at a more quantitative level, but don't really want to invest the time required to comprehend the full mathematical theory. I kept wishing there was some middle ground, but never found it.

Those issues aside, Smolin's book is worth reading. I'm glad I did.