According to Smolin, there are three "roads" currently leading to a theory of quantum gravity: the first road begins from quantum theory and adds relativity (string theory), the second begins from general relativity and adds quantum theory (loop quantum gravity), and the third rejects both and tries to consider the question from first principles. (This third road is basically not discussed, and later in the book the third road becomes thermodynamics of black holes and the "holographic principle".) There are a few mentions of other approaches such as Alain Connes' non-commutative geometry and Roger Penrose's twistor theory, but none is actually explained. Smolin himself worked for a time on string theory, but today is identified with the loop quantum gravity approach. As of the time this book was written (i.e. about fifteen years ago; I haven't gotten to anything more recent yet, since I'm reading the library's astronomy and related physics books chronologically) about ninety percent of those working on quantum gravity were committed more or less strongly to string theory as the sole viable approach, so Smolin's views are in a minority, and much of his book is arguing that minority approaches should be encouraged (and funded). Looking on Amazon at his later books and the reviews of them, I get the impression that he has continued to move further from the mainstream.Late last year, I read Smolin's Life of the Cosmos, and was very impressed by his approach and his use of philosophy, and the clearness of his explanations of the basic theories such as general relativity and quantum theory, the Anthropic Principle, and the questions which they raise, and though I was not convinced by his speculative theory of cosmological natural selection, I was intrigued by the idea. Of course, I'm very much a layman when it comes to physics and astronomy, so my opinions don't count for much except for my own view of the world. Much of my popular science reading in the past few months has favored string theory (e.g. the awful Hyperspace by Michio Kaku, and the excellent The Elegant Universe by Brian Greene) so I was looking forward to reading this account and learning about the alternative(s). Unfortunately, this is a much less well-written book than Life of the Cosmos. It does cover many aspects of modern physics and cosmogony that I was unfamiliar with, and it begins well. In the first section, called "Points of Departure", Smolin emphasizes the relational aspects of space-time as constituted not by absolute space and time (he argues that many relativists ignore the basic breakthrough of relativity, using it to derive local geometries of space-time which are then treated as absolute backgrounds in a non-relativistic manner) nor even by relations among things (either particles or strings) but by relations among events or processes. (Surprisingly, he mentions Hegel and Heidegger but not Whitehead in this connection, but this is a much less philosophical book than the earlier one, at least in terms of discussing writers who would be called "philosophers".) The second section, "What We Have Learned", discusses the history of loop quantum gravity theory (partly from a personal perspective) and has chapters on the entropy of black holes and a chapter on string theory as well. The problem is that I got no real understanding of what LQG is actually saying. He does tell us (over and over, the book is very repetitive) that space (and time? he doesn't discuss this) is discrete rather than continuous, and that the loops are also discrete. But he never really explained what they are, except that they are "like" the discrete lines of magnetic field in a superconductor. The "glossary" at the end of the book defines "loop" as "a circle drawn in space"! That's the whole definition of the most important concept in the book! The problem may be that he is so involved in the theory that he doesn't realize the lay reader (for whom the book is supposedly written; this is explicitly a popularization and not a book for physicists) has no idea of what LQG is, and probably doesn't know a lot about superconductors either, so he assumes that the analogy will make everything clear. It doesn't, at least to me. He claims that he has been very evenhanded in discussing the three "roads", and particularly that he is fairer to string theory than string theorists are to other approaches (this at least is true, since I've never read a book on string theory that mentioned any alternative except leaving the standard model of quantum theory as is), but the chapter on string theory basically is just about what's wrong with it (not background independent, not testable -- though neither is LCG, not unique) and I would not have understood it without having read other books on the subject. The chapters on black holes and their entropy were the most interesting part of the book, and the ones I got the most out of, but here again he alternates between beating the horse with simplistic analogies and just assuming advanced ideas without explanation. There is no math in the book, and this may be part of the problem, since these ideas make little sense in purely descriptive language.The last part of the book explains the "holographic principle", whines a bit about how non-string-theorists aren't taken seriously enough (or paid well enough) and then gives his conclusions on what the final theory of quantum gravity will look like: LQG (no surprise) will be the basic framework; it will involve discrete space; it will make extensive use of the holographic principle, and generally will be based on what observers in different places can actually observe; it will borrow ideas from other approaches such as non-commutative geometry and twistors; string theories will turn out to be approximations to the true theory on the assumption of various fixed space-times. This will all be developed (and possibly empirically confirmed) by about 2010, or 2015 at the latest. (While I haven't read the latest developments, I am fairly sure we're not that far along -- which may be why his latest books all have phrases like "the crisis of physics" in the title.)To sum up: an interesting book about an interesting subject, but not well written and not a good introduction to LQG, which is the main reason for reading it; I hope one of the later books on my list will give me a better idea of alternatives to string theory, as well as where we actually are now. (I know at least Penrose's book on twistor theory will cover that approach, IF I can understand the math -- he claims the book teaches all the math needed, but I'm skeptical, even though I was a math minor in college.)
This is one of the small list of books that has forever changed my paradigm. Unlike the other ones, I was aware of the paradigm change as it was happening and I could not put this book down."...the first principle of cosmology must be 'There is nothing outside the universe' . . . This first principle means that we take the universe to be, by definition, a closed system. It means that the explanation for anything in the universe can involve only other things that also exist in the universe . . . If something has a position, that position can be defined only with respect to the other things in the universe. If it has a motion, that motion can be discerned only by looking for changes in its position with respect to other things in the universe."Space is, therefore, a network of relationships.Secondly, any observer within the closed system is observing a system of which she is a part. The act of observation changes the closed system, which is the founding principle of quantum mechanics. This is the most simple and profound explanation I've ever heard of this phenomenon.Further, space is a set of processes, not things. These processes are "transfer[s] of information." Smolin believes that space is discrete, i.e. that it is made up of elementary parts, which he calls "elementary events." The Planck scale is the measure of these events. Space being discrete means that this region of 'space' represents a finite number of events and that a finite amount of information is contained in it. By information, he means that a finite number of yes/no questions may be answered about that elementary unit.Further, the presence of matter, changes the PRIORITY of causes in this event-driven model of space. I've typically heard this referred to as the shape of space-time, but Smolin, having redefined space as a network of relationships (and I think "changing, contiguous relationships" is implied, due to the nature of life), shows that matter simply rearranges the relationships.I love this view of space and it fits everything I have studied about everything. In particular, it reminds me of Bertrand Russell's epistemology. At first Russell seemed to be saying that we could only *know* relationships, but he then radically redefines "me" as a set of changing relationships. Smolin takes this one step further, redefining all that exists as a network of relationships affected by the observer.A couple more unrelated but cool observations:"It is commonly assumed that anything that is observer-dependent is subjective, meaning that it is not quite real. But . . . in a universe defined by [the theory of relativity], something may be both objectively tru and at the same time knowable only by some observers and not others.""So, in the end, the most improbable and hence the most puzzling aspect of space is its very existence. The simple fact that we live in an apparently smooth and regular three-dimensional world represents one of the greatest challenges to the developing quantum theory of gravity. If you look around at the world seeking mystery, you may reflect that one of the biggest mysteries is that we live in a world in which it is possible to look around, and see as far as we like. . . the greatest gift the quantum theory of gravity could give the world would be a renewed appreciation of the miracle that the world exists at all, together with a renewed faith that at least some small aspect of this mystery may be comprehended."
Do You like book Three Roads To Quantum Gravity (2002)?
To start this review off, I must say that I was impressed with how well this book was written. Being a high school student with no prior knowledge of quantum gravity or quantum mechanics, I was pleased that this book was written in a way that I could actually understand the content and learn from it. Also, Lee Smolin's "Three Roads to Quantum Gravity" not only taught me about today's groundbreaking physics, but has also sparked my interest in pursuing further knowledge in the fields of theoretical physics. This book discusses how scientists are working to create a "theory of everything" by combining Einstein's theory of general relativity with quantum theory. No prior knowledge of these subjects is required to understand his explanations and conjectures, because Smolin does an excellent job defining everything for the reader. Smolin also describes three approaches to reaching an understanding of the structure of space and time on the Planck scale, being string theory, loop quantum gravity, and black hole thermodynamics. All the approaches have their own issues, such as how string theory is background-dependent while space itself is background-independent. However, all three approaches yield the same result: that space is finite. Though nothing can be determined for sure due to the current inability to prove these theories incorrect, all of the theories proposed in this book provide great insight into the structure of our universe.
—Michelle
It's been 10 years since I read Three Roads to Quantum Gravity, so what I have to say about it is minimal. Lee Smolin is probably the best of popular physics writers today, with a very firm grasp of his field of expertise, and an even-handed, detailed treatment of 'competing' models of unifying theories. He's also a refreshingly honest doubter of the superstring hype that pollutes much of pop-physics these days. This book isn't so much a declaration of his preferred "road" to quantum gravity, but a summary of the three prime paths (at the time of publication), as well as a clear and well-written work illustrating the problems associated with each. The book is most valuable as a glimpse at what may come next in particle physics, though I'm happy to say my own field of research will never overlap with this tireless search for unification.
—Bukk
Probably one of the most readable books about the extremes of the universe I've come across. I was grabbed by the opening questions "Is space infinitely divisible?" and half a dozen more. he goes on to explore the very small to the very large and to look at the consequences of what we can see, how we can see it and where we are and might be in the universe.I passed it along to some Christian friends who enjoy advanced science - you might think that a book of superb science, founded in relativity, particle physics, quantum mechanics and cosmology would trouble them - no way, well no way unless you think that the world and the universe are really 6000 years old!
—Steve Alker
