Three Roads to Quantum Gravity

It's more difficult still given some of the contradictions and inconsistencies that obtain between quantum theory, which "was invented to explain why atoms are stable and do not instantly fall apart" but has little to say about space and time, and general relatively theory, which has everything to say about the big picture but tends to collapse when describing the behavior of atoms and their even smaller constituents. Whence the hero of Smolin's tale, the as-yet-incomplete quantum theory of gravity, which seeks to unify relativity and quantum theory--and, in the bargain, to move toward a "grand theory of everything." Smolin ably explains concepts that underlie quantum gravity, such as background independence, the superposition principle, and the notion of causal structure, and he traces the development of allied theories that have shaped modern physics and led to this new view of the universe.

Although he allows that "it has not been possible to test any of our new theories of quantum gravity experimentally," Smolin predicts that a solid framework will be established by 2015 at the outside. If he's correct, the years in between promise to be an exciting time for students of the physical sciences, and Smolin's book makes an engaging introduction to some of the big questions they'll be asking. --Gregory McNamee

Product Description
From one of the world's most distinguished scientists, the first popular book to present the controversial and exciting ideas behind quantum gravity.

The Holy Grail of modern physics is a theory of the universe that unites two seemingly opposing pillars of modern science: Einstein's theory of general relativity, which deals with large-scale phenomena (planets, solar systems and galaxies), and quantum theory, which deals with the world of the very small (molecules, atoms, electrons).

In Three Roads to Quantum Gravity, Lee Smolin provides the first concise and accessible overview of current attempts to reconcile these two theories in a final "theory of everything." This is the closest anyone has ever come to devising a completely new theory of space, time and the universe to replace the Newtonian ideas that were the foundation of all science until the beginning of the twentieth century.

Lee Smolin, who has spent his career at the forefront of these new discoveries, presents for the first time the main ideas behind the new developments that have brought a quantum theory of gravity in sight. He explains in simple terms what scientists are talking about when they say the world is made from exotic entities such as loops, strings, and black holes. As he does so, he tells the fascinating stories behind these discoveries: the rivalries, epiphanies, and intrigues he witnessed firsthand.

Science Masters Series

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.


a layperson's best shot at QG   November 9, 2001
Edward A.Cohen, MD (Chicago, IL United States)
10 out of 11 found this review helpful

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 book got me started on a 2 year study of QFT   November 10, 2004
Douglas Mckenzie (Denville, NJ USA)
6 out of 6 found this review helpful

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.


A fair summary of the status of research in quantum gravity   February 7, 2005
Dr. Lee D. Carlson (Saint Louis, Missouri USA)
10 out of 12 found this review helpful

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.