Three Roads to Quantum Gravity

Rating:
Summary: A fair summary of the status of research in quantum gravity
Review: Considering the experimental status of theories of quantum gravity, it is remarkable that research in this area has progressed to the level in which it has in the last few decades. If one examines the history of science it is readily apparent that laboratory and observational data drove the most successful scientific theories. By reading this book and by perusing some of the extensive literature on quantum gravity, it seems justified to view research in quantum gravity as being driven more by internal consistency requirements and reasons of aesthetics. The author gives an interesting overview of this research, and targets the "popular audience" for its readership. The author expresses great optimism that a successful theory of quantum gravity will be attained within the next decade. Considering the current difficulties in this research, this is indeed a refreshing attitude.

All of the approaches to the quantization of gravity rely on mathematical tools that are quite sophisticated, and competence in the use of these tools requires years of study and concentration. Due to the targeted audience, the author does not discuss these in detail, but he does give interesting and intuitive insights into the nature of the mathematical constructions that are used in quantum gravity. One of these, `noncommutative geometry', is quite recent, while the other, `topos theory', has been around for quite some time, albeit in several disguises. If one is to reconcile the two main approaches to quantum gravity, namely the loop approach and string theory, one will need to understand in detail the mathematics behind both of these theories. This will be a formidable undertaking, and it will take disciplined and focused individuals to carry it out. Unfortunately, and the author addresses this in the book, academic and funding pressures discourage such undertakings. This is either an argument for changing the nature of the academy (which will be very difficult) or doing this research outside the academy. But doing research outside the academy runs the risk of it being viewed as low quality, especially by those in the academy, and so this alternative carries high risk also. In either case, research in quantum gravity is difficult not only because of the nature of the subject matter, but also because of the societal and political pressures that make it a very risky endeavor.

The author, and a few others, came to quantum gravity when it was still a relatively young field, and, as he describes in the book, managed to survive in the academic environment. Their zeal is admirable, considering the roller-coast ride of confidence and depression they no doubt felt during their research efforts. There is no doubt now that quantum gravity is considered to be a respectable field of physics, and has attracted some of the best minds that have ever existed on this planet.

The manner in which the author presents the ideas on quantum gravity will no doubt motivate a few bright young people to take up the gauntlet and enter the field. He definitely prefers the loop approach to quantum gravity, being one of the individuals responsible for its development, but he is fair in giving string theory its due. Even professional physicists or mathematicians though who are curious about quantum gravity could gain a lot from a perusal of the book. There are some surprises in store for those who are used to thinking about space and time from a global point of view. This is especially true in the discussion of topos theory and the manner in which it is used in some approaches to quantum gravity. These approaches require that observers always view their place in the world as being one where they must reason using incomplete information. Two or more individuals though who have enough information to decide whether something is true or false will always make the same decision. This `local' view of descriptions, decision-making, and information gathering will be immediately appreciated by the mathematician reader who is acquainted with the concept of a `sheaf'.

The only possible irritation in the book (depending on the reader's theological views) is the discussion on the `weak' and `strong' anthropic principle and its play on very large (and very small) numbers. Those readers (such as this reviewer) who are not troubled by the magnitudes of these numbers will find the discussion somewhat superfluous. Some theologians have been delighted with the ramifications of some of the discussion on the anthropic principle and fine-tuning in recent years, particularly in the use of the "God of The Gap" arguments in cosmology. This will be no doubt continue, due to the need of these theologians to grab at every straw to establish their positions on origins, extremely fragile as they are.

Another one of the virtues of the book is the author's willingness to discuss the social and political context in which research in quantum gravity is done. He describes the string and loop-gravity theorists as effectively being at war with other, but that the degree of cooperation between them has (thankfully) increased in recent years. The contention between these two groups is no doubt partly due to financial pressures from funding agencies and also personal insecurities among the researchers themselves, the latter resulting in sometimes maniacal obsessions for recognition among peers as being the first to arrive at a particular result. Some say this contention is healthy for science, while others say it is a complete waste of time and has no constructive purpose. It is the opinion of this reviewer that the second holds.

Rating:
Summary: What's our universe made of?
Review: 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.

Rating:
Summary: Learn almost nothing
Review: I have a pretty good background in math and physics. But I learned almost nothing here. I think someone with less background would learn even less. The problem is he never really defines anything, never makes clear whether he means a 'loop' is a real thing or just a calculation tool, etc.

It is all just so mushy.

Plus he talks a lot about theories that have no experimental tests, and none presently envisioned. This seems like metaphysics to me.

You want a real popular science book that explains real science to a layman? Read QED by Feynman. Read Relativity Visualized by Epstein.

Rating:
Summary: In a dynamic discrete space-time 'is' is an illusion
Review: Prof. Smolin is a brilliant teacher.
His first book 'The Life of the Cosmos' contained already the simplest and brightest explanations of the basic elements, processes and principles of modern physics. Here he does it again with the latest progress in the search for a theory of quantum gravity: loop quantum gravity, string theory and black hole thermodynamics.
He dissects the strenghts and weaknesses of the different approaches and shows how they culminate into the holographic principle, where one set of events receives information about other parts of the world.

Lee Smolin explains in a very comprehensible vocabulary that space and time are not continuous, but discrete; that the world is made of processes, not things; and that the world is nothing more than an evolving network of relationships, of which causality is the most important.
These characteristics have important philosophical implications; e.g. they refute the belief that observer dependence rules out objectivity.

This book contains some mind-boggling propositions. One from Lee Smolin himself, where he admits that he doesn't belief in the uncertainty principle, although he continues to work with it. Another one is the use of the whole universe as an instrument.

In the end, the author is very optimistic and predicts that a theory of quantum gravity will be found in the first decades of the 21st century.

This book is a fascinating tale about the real nature of the universe. A must read.

Rating:
Summary: Mathematical physicists at work.
Review: Professor Lee Smolin's little compact book delivers great load of information about puzzles, anomalies, dilemmas and inconsistencies residing within cutting edge of theoretical physics.
This was probably most difficult but enjoyable reading for me, since author writes about particles and Universe from the depth of the tiny Planck's space.
I have studied several popular books about spacetime, yet this one still managed to show many new ways that one can use looking at intricacies of modern cosmological science.
I recommend "Three Roads to Quantum Gravity" to advanced and committed learner who wants to find how quantum gravity may possibly become an experimental science.
Advice: "Black Holes and Time Warps" by Kip Thorne is a very good place to start education about gravity.

Rating:
Summary: An Introduction to Loop Quantum Gravity
Review: 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.

Rating:
Summary: This book got me started on a 2 year study of QFT
Review: Smolin's presentation of the three theories was so thought provoking for me that I realized that I wanted to learn more. I had majored in Physics 35 years ago so I had QM and SR under my belt but nothing more modern. One thing that I particualry liked about Smolin's presentation of the three theories currently being worked on - superstring theory, quantum loop theory and blackhole thermodynamics - was the balanced way he kept saying how all three undoubtedly would contribute to the ongoing progress rather than trying to say one has to be right to the exclusion of the other two - it's simply too early to tell. I've spent 2 years (with a tutor) studying QFT in order, hopefully, to be able to at least approach the mathematics behind these theories. I've just ordered a "First Course in String Theory" and realized that I had never written a review of Smolen's book to so-to-speak thank him so I now fill that gap.

Rating:
Summary: Separate Conquests
Review: Sorry about the review title, just musing on the phrase "Divide and Conquer". As every reader probably knows, general relativity and quantum theory are both extremely successful, predictive theories, but have yet to be brought together in a single, complete framework that seems to clearly apply to our universe. Smolin did an excellent job in this book, already several years old, of discussing approaches to the problem, with emphasis on Loop Quantum Gravity. As usual while reading it I wished that I could plumb the depths of the underlying math, which of course the author did not attempt to present.

Since the predictions of relativity seem to be holding up so well, I can't help wondering whether the current vogue is correct in assuming that gravity is much the same as other forces and is mediated by "gravitons". Might there be some deeper path to unification?

Rating:
Summary: Oooh Look, Quantum Black Holes
Review: This is an overview of current quantum gravity research -- in particular, string, canonical quantum gravity and loop quantum gravity. However, it is much more. The book starts out with a very fundumental philosophical question: Is there an external observer, or are all observers within the universe. By within the universe means, that is, the whole universe, no matter how many sub- or multi- universes there may be. Next, the book covers the history of physics that has been retained in our current understanding. This means there is no history-lesson-feeling to this book. After a discussion of the current thinking, the conclusions speculate on what will happen next in physical theories. Finally, the most important part of the book are the on-line refernces to supplement a book that is really too short.

Rating:
Summary: Provoking speculations and misleading physics
Review: 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.