This blog is the result of one year of studies, discussions with professional scientists and amateurs, and posting of reviews and research papers representing our current understanding of quantum gravity, specially, but not limited to, background independent approaches. This blog was also devoted to cosmology, astrophysics and philosophy of science.
The blog is now shutdown.
The interested visitor can download a partial backup of this blog here (zipped file).
Below I highlight a few posts that are also included in the zipped file: my book reviews, an invited post by Daniele Oriti on his research and new upcoming book, which is a very interesting and important work that offers a view of the current developments of quantum gravity, and finally a basic guide for those interested in learning Loop Quantum Gravity and Spin Foams.
Best wishes to all,
Christine C. Dantas
The Trouble With Physics: The Rise of String Theory, the Fall of a Science, and What Comes Next (Hardcover)
by Lee Smolin
Hardcover: 416 pages
Publisher: Houghton Mifflin (September 19, 2006)
ISBN: 0618551050
Review by Christine C. Dantas
Given the diversity of opinions and thorough accounts already available on Smolin's new book, one cannot disregard that adding some original or unforeseen impressions to what already has been said has become a daunting task.
The book is centered on the theme of telling "the story of a quest to understand nature at its deepest level". The reader will, however, get much more than a simple story. As Smolin calls it, it is something like a "tragedy". Although he prepares the terrain, for some readers it might still have an unantecipated taste, specially in the last part of the book. And it is a well-written tragedy, told with genuine honesty, competence and undeniable lucidity, by a reputable scientist, who has innumerous and well-known contributions to quantum gravity, ranging from highly technical works to popularization articles.
The book is divided into 4 parts:
- The Unfinished Revolution, where he identifies five problems that any quantum theory of gravity must address in its ultimate form, namely: the problem of quantum gravity, the problems in the foundations of quantum mechanics, the problem of unification of particles and forces, the problem of explaining how the values of the free constants of the standard model of particle physics are chosen by nature, and finally the cosmological problems, like dark matter and dark energy. This part is mostly devoted to give a historical and personal account of how these problems are being tackled in theoretical physics.
- A Brief History of String Theory, a very readable review of the history and present status of string theory, and his critical account of the main technical problems of the theory. For fun, should be compared with Greene's The Elegand Universe.
- Beyond String Theory, a bird's eye view on other approaches and research programs to quantum gravity, the new experimental hints that are beginning to emerge from these studies, and a personal view on the "original and impatient mind[s]" behind the scene of these new proposals. Also, a review some recent phenomena unantecipated by string theory is also given. In my opinion, this part could be improved, since one can have the impression that alternative approaches to quantum gravity are somewhat "crazy" ideas from these original and impatient minds, but not in fact coherent and formal independent research lines. Of course this part is used as argument for his thesis on the need to make risks in science (see below), but one might end up confusing some things on this issue.
- Learning from Experience, an exceedingly original and thought-provoking account of what is science, how it works, and what can be done to change the current state of affairs. In my judgement, the best part of the book.
One thing that is important to point out and make it very clear: Smolin is not claiming that string theory is completely wrong:
Another concurrent thesis that Smolin puts brilliantly forth, and which cleared up my comprehension on several personal issues regarding what the scientific activity is all about, can be summarized below:
But Smolin also offers a possible cure for the problem and actually defends it throughout some parts of the book:
As I pointed out, the last part of the book was the one that interested me most, since it is dedicated to a fascinating reasoning over what is science, and I think it could even be subsequently explored in a separate, modern treatise on this important and timely issue.
I would like to end this review with a few words that come from someone highly unconnected with the scientific community in the US and Europe, and whose major area of competence is Astrophysics. It has to do with the fact that Smolin do raise questions of general validity, as I could directly relate to my own experiences.
Before reading Smolin's book, I believed that it would offer me interesting arguments for believing that there was indeed a trouble with string theory. But actually it turned out that it offered me also a vision of science of an unforeseen nature, and hit me personally in an unexpected way.
I do not wish to make public some of my old, deep own feelings about what I think science is and how it should be conducted. There are of course certain points that I often do make public, but there are some others that tormented me for quite a long time now, and are so personal and even of emotive nature that I would rather keep them to myself. But this is the fact per se that should be mentioned here, since this is the contribution that I feel I can give on examining his book: I found out that he was addressing some of my personal views and doubts, of course from his own perspective and wisdom, but it was like talking to an old friend who followed my own career in science and understood what troubled me most for all those years. So this book is for you if you want to be challenged over your own vision of science and how you fit in it.
I am absolutely impressed with Smolin's book and, I should say, alleviated. The reason is simply because Lee Smolin wrote the kind of book that I always thought it was of upmost importance to be written, yet for some reason it seemed that no one was willing to write it; a kind of book I could never write myself. But then, after so many years (at least to me), here it is.
Given my recent interest in background independent approaches to quantum gravity, one might tend to believe that I am simply overexcited by the book. I agree that I may be in shortage of arguments to convince you otherwise. In any case, I hope I have at least convinced you that there are some important elements in Smolin's thesis that should be carefully studied. My personal belief is that most of his arguments sound painfully true.
All the movement around his book is an indication that his work is being seen as a timely one, so I have given myself the final overall task to evaluate whether it would be true to consider that this book had the necessary weight of a historical text. Indeed, in years to come, I have little doubt that it will be recognised as such. Even if someone else does not think that way, or even if the book proves itself in trouble or even wrong in some places, it must be applauded for its timely and wise words. I think this is much needed these days, in any discipline of science.
Smolin's book is now placed on my shelf besides the books that I believe are in the same class: Cosmos, by Carl Sagan, The Selfish Gene, by Dawkings, Gödel, Escher and Bach, by Hofstadter, and The Road to Reality, by Roger Penrose, amongst few others.
Update: A very good review is Sabine's over at Backreaction, including links to other reviews. Another place to look is her Inverse Problem post.
Note: A slightly modified version of this review is available at the amazon site.
Many Worlds in One: The Search for Other Universes
by Alex Vilenkin
Publisher: Hill and Wang (June 27, 2006), a division of
Farrar, Straus and Giroux
235 pages
Review by Christine C. Dantas
Last week I received an email from the Consulting Editor of Farrar, Straus and Giroux, asking whether I would be interested in reviewing Vilenkin's new book Many Worlds in One: The Search for Other Universes. I promptly replied to the editor that I would be happy to review his book. I very much welcome such invitations, specially because they somehow reflect that the Background Independence is being recognized as a serious forum on quantum gravity research.
At the same time, however, some mixed feelings started to agitate my mind while I was waiting for the book to arrive. The problem is that I really do not like the idea of "many universes" to begin with. Also, related to this personal feeling, there is the delicate (infamous?) problem of the string theory anthropic landscape, which certainly the author would have something to say about, and to which I also have my own skeptic disposition.
My approach was then to broadly evaluate the book from two main perpectives. First, I attempted to verify how well Vilenkin was able to transmit the difficult and usually "esotheric" concepts of modern theoretical cosmology into a simple -- but as accurate as possible -- language to the layman, regardless of my own reservations about the subject. In the second perspective, I evaluated whether Vilenkin would offer a balanced account on the current view of the physics community on the multiverse issue. In this case, I did not hesitate to have my skeptic disposition in the back of my mind. But at the end, I very much was willing to learn whether my previous ideas were changed, abandoned, or reinforced due to his book, so I tried to read it enthusiastically, with an open mind.
The book's theme is the origin of (our) universe based on the concepts of quantum theory and inflationary paradigm. The author exposes his own ideas and developments in this area, the so-called "eternal inflation" model. Vilenkin's main general arguments are that "inflation is eternal in practically all models suggested so far" (note 8, page 214). Also, the model leads to the possibility that: (1) our universe came to existence by quantum tunneling from an essentially zero size (page 180), and (2) ours is just one universe amongst an infinitude of other "island universes" eternally coming to existence. As a consequence, he puts forward the idea that there could be many copies of identical universes and histories of events, so that "yes, dear reader, scores of your duplicates are now holding copies of this book" (page 112).
The book is well organized into 4 parts, with more or less five chapters each. The first part develops the main ideas behind the big bang and the inflationary model, and are mostly "standard material". This part is well introduced to the general audience. The subject is written with unusual clarity, originality, and conciseness. However, there are a few minor issues that perhaps would enrich the text, for instance, in page 49 Vilenkin for the first time mentions the word "symmetry", but does not explain in more detail this important notion in physics. He does devote quite further ahead in the book a section to this issue on page 124 ("In search of a deep symmetry"), but it is too short and not very elucidating. At least some reference should have been cited.
The second part aims at explaining the author's "eternal inflation" model. Although the same quality of writing style is found in this part (and in fact maintained throughout to the end of the book). It is where the reader really starts to think on the possible loopholes and alternatives to what is being presented. The author is too much quick (to my taste) to accept the chain of reasonings and conclusions drawn from his theory, whereas to me there are really lots of deep mysteries in between that are largely non-trivial.
The third part deals with the cosmological constant problem, the anthropic principle and superstring theory, and the author attempts to explain that these issues can be addressed or accommodated in his theory. The fluidity of the text starts somewhat to become a little hampered at some points here. Perhaps because one could have the feeling that the logic of the presentation must now be completely devoted to convince the reader that in fact the string landscape of vacua together with eternal inflation can serve as a scientific basis where anthropic arguments can be used to make predictions. According to Vilenkin, "the reluctance of many physicists to embrace the anthropic explanation is easy to understand. The standard accuracy in physics is very high (...). Anthropic predictions are not like that. The best we can hope for is to calculate the statistical bell curve (...). It's little wonder that, given a choice, physicists would not give up their old paradigm in favor of anthropic selection. But nature has already made her choice." (pages 150-151). He also concludes that the "observed value of the cosmological constant gives strong indication that there is indeed a huge muliverse out there". But at the same time adds that "the evidence of the multiverse is, of course, indirect, as it will always be (...). But if, with some luck, we make a few more successful predictions, we may still be able to prove the case beyond a reasonable doubt".
Part four is devoted to the issue whether the universe had a beginning, and I found the most enjoyable passage the issue on whether the physical laws, expressed as mathematical equations, predate the universe. Or "if the medium of mathematics is the mind, does this mean that mind should predate the universe?" (page 205). The book therefore ends mostly in a philosophical mood, but this feeling also pervades the book in several points, specially on the issue of how the universe could have come to existence from nothing. I recall when I realized such an awesome thought at early age, and I could never get free of it. It is inspiring to learn that someone has attempted a physical theory out from such an abyssal idea.
There are no ways to get into a more critical position without going deeper into the original papers. In fact, the book offers a very clarifying set of notes and references, including to the original papers, so it also serves as a good entry to the technical literature.
Overall, the book is a pleasure to read from begining to end, and it is difficult to put it down. The writing style is informal throughout, sprinkled with short narratives of interesting, amusing, incidents or outspoken opinions coming from well-knownscientists. There are several nice cartoons as well. My impressions are that, from the point of view of the layman, Vilenkin does a good job in exposing difficult theoretical issues. So although this is his first attempt on exposing to the general reader, he did it very well. The front cover is outstanding.
The major drawback of the book is the fact that he does not mention at all other alternatives, such as Loop Quantum Gravity, nor Smolin's Cosmological Natural Selection. This is quite devastating in my opinion, specially because I did like Vilenkin's book (in fact it went quite beyond my expectations), since the issues treated there are so difficult and pressing, and must be encouraged.
Finally, I must say that some of my previous ideas were changed after reading this book. Mostly, I do have more sympathy on the idea of eternal inflation than I did before. However, I still do not "believe all this nonsense about our clones" (page 116). Also, I still am not convinced that there could be a way to "prove" that the multiverse exists. Nor I believe that the anthropic reasoning will lead the 22nd century physics. But I am using too much the verb "believe". Whether the reader will "believe" the incredible ideas exposed in the book is unknown, but it is quite certain that he or she will have great hours of good, fascinating reading.
Hi Christine,
Thanks once more for the invitation to contribute to your blog. And, once more, let me congratulate for its well-deserved success, so: happy blog-birthday and keep up the good work!
So, you suggested I could write about the book I am editing (Approaches to quantum grvity: towards a new understanding of space, time and matter) and that is going to be published by C.U.P.
Ok.
The idea is to present an overview of most of the current approaches to quantum gravity, through a collection of introductory papers and reviews each devoted either to one of them or to one particular aspect of one of them. If you are interested, the exact (but provisional, as for the sectioning) table of content is as follows:
- Foreword; Daniele Oriti
-- Ideas and general formalisms
Oveall I am quite happy with it. I think it is going to fulfill its scope and motivations, which were (at least for me): a) to show how active and diverse is quantum gravity research at present, and that there is a variety of approaches being pursued, and lots of new work, new ideas (radical, speculative and solidly grounded in physics and mathematics at the same time), new directions and results (some partial but suggestive, some well-established, some surprising); b) to provide a comparative perspective on what's going on in the field; to foster not only this comparative perspective, but also, possibly, future collaboration ad 'cross-fertilisation' among different approaches; c) to allow newcomers and students as well as whoever is interested in the subject to be introduced nicely to it. Ultimately, and most importantly, d) to show that this stuff is fun and exciting to work on and to read about!
You may have noticed the Q&A sections at the end of each part. The idea is that authors can ask questions to each other, put forward comments and criticisms, and get/give answers. The aim is twofold: first, to improve the comparative aspect of the book, in that possible difficulties of the various approaches could be pointed out if not already discussed in the papers, and to present additional ideas and points of view that can be relevant for a given approach, but maybe originated or suggested by another; second, to give an example of how research progresses: out of discussions, criticisms, debate, indeed, questions and answers...This part is being prepared, and I really hope it is going to be rich in content and useful.
I enjoyed reading all the papers, and the Q&A, received so far, so I hope other readers will enjoy them to, and find the whole thing useful. Ultimetely, we don't know yet what quantum gravity is, how long it is going to take still to find out (not much, I hope!), and which, if any, of all these approaches will be found to be the closest to the final theory. It is well possible that none of them, as they are currently understood, matches reality as it will look once we have understood more of it; I wouldn't be too surprised. However, I also believe that we can learn a lot from all of them, and that the final theory will involve aspects (formalisms and techniques, ideas, motivations, results, we don't know yet) of many, if not all, of them. If this is the case, this book may be of help.
In my own contribution to the book, I have decided to discuss Group Field Theory, which is an important part of what I am working on at the moment. Most importantly, it is the line of research that I am most excited about! I must confess that this is not due to some incredibly important new result that has just come out, unfortunately. Very little, I think, is known at present about GFTs, at least compared to the width and potential of the whole formalism , that has just begun to be explored. However, the results we have already are suggestive of something big, I think; they point towards the need to take GFTs seriously and start really exploring them as fundamental theories, not just as some sort of auxiliary formalism of purely technical use to do something else. In particular, I believe that GFTs can represent: 1) a general, encompassing, possibly unifying, framework to study and understand better many of the other current approaches to quantum gravity, in particular Loop Quantum Gravity, Quantum Regge calculus, dynamical triangulations, causal set theory, in addition (of course) to spin foam models. Also, I believe that if one takes seriously GFTs as field theories describing the quantum dynamics of the fundamental discrete building blocks of (quantum) space, being these seen as simplices, or spin networks (in GFT these two points of view are equivalent), then they offer an altogether new perspective on the issue of the continuum approximation and on the emergence of a continuum spacetime. What I mean is that this becomes a problem in (a very peculiar indeed) condensed matter, to be tackled (of course) with almost ordinary QFT methods. It is the problem of showing that in certain situations (to be identified) the fundamental 'atoms of space' described by some GFT -condense-, so that a continuum spacetime emerges as a sort of liquid.....ideas of this sort were proposed in the past, of course, but we may have now for the first time the opportunity to realise them explicitly and concretely....
You see! I got excited and carried away! Anyway, I don't now yet if any of the above makes real sense, if it is going to be realised fully or it will turn out not to work. How to say? I am working on it :) We'll see where all this leads....but there is potential, so to say....and of course, this is only my opinion....I am not really sure there is many people out there who agrees with this point of view, actually I doubt it!
At least, however, I am having fun thinking about all this and working on it, which is great!
Anyway, I have taken too much of your space already.....
Thanks again for letting me contribute some of my thoughts to the blogosphere :) and even more, thanks for your interest in them!
Take care.
Ciao
Daniele
This guideline provides a basic background material for studying Loop Quantum Gravity. This is supposed to include the "canonical" LQG as well as more recent "spinfoam" (path integral) approaches to background independent, non-pertubative quantum gravity. I have not included recent research papers nor more specialized applications (e.g., cosmology, phenomenology and numerical simulations), although I invite of course all interested readers to comment on these recent achievements as I eventually find them out and post them in the blog.
Also, this is my personal fragmentary view on how to approach this field, and it also reflects my own "formation history". It is my own guideline, and by no means should be seen as a final, authoritative "curriculum". I do hope, however, it has some usefulness to others.
In order to understand recent LQG research papers, I have been studying the following material in no preferential order, although I believe the most correct and solid approach would be to study all the pre-requisites first and only then the primers and review papers afterwards. This is often difficult to accomplish, mainly due to time constraints, so I have been studying this material in a quite non-linear manner.
The list is divided into 5 parts: Pre-requisites, standard books, primers, introductory texts and reviews, and on-line courses. There are a large number of pre-requisite books/texts which I could have included, but I will only mention those which I am specifically studying now or intend to study opportunely.
Finally, it is a lot of material! I intend to post on a much briefer guide than this one, but for the moment, you make your own choices. Good luck!
1) Pre-requisites:
- General Relativity (GR)
In particular: the Hamiltonian formulation of GR:
[1.1] Wald R M, 1984, General Relativity (Chicago: Chicago University Press)
- Quantum Mechanics and Quantum Field Theory (QFT)
[1.2] Zee A 2003 Quantum Field Theory in a Nutshell, Princeton University Press
In particular: QFT in flat and curved space-time:
[1.3] Wald R M, 1994, Quantum field theory in curved space-time and black hole thermodynamics (Chicago: Chicago University Press)
- Gauge Field Theory
[1.4] Baez J and Muniain J P, 1994, Gauge fields, knots and gravity, Singapore: World Scientific -- wait for the upcoming second edition!
[1.5] George Svetlichny, Preparation for Gauge Theory [math-ph/9902027]
- Systems with constraints:
[1.6] Sanjeev S. Seahra, The Classical and Quantum Mechanics of Systems with Constraints
[1.7] Andreas W. Wipf, Hamilton’s Formalism for Systems with Constraints [hep-th/9312078]
[1.8] Paul A. M. Dirac. Lectures on Quantum Mechanics. Dover, Mineola, New York,
1964.
- Mathematical Methods:
[1.9] Frankel T 2003 The Geometry of Physics, Cambridge University Press
[1.10] Bishop R L and Goldberg 1968 Tensor Analysis on Manifolds, Dover
[1.11] Byron F W and Fuller R W 1970 Mathematics of Classical and Quantum Physics, Dover
[1.12] Henle M 1979 A Combinatorial Introduction to Topology, Dover
[1.13] Tung W 1985 Group Theory in Physics, World Scientific
[1.14] Nakahara M 1990 Geometry, Topology and Physics, Institute of Physics Publishing Bristol and Philadelphia
2) Standard Books (I find them quite advanced, though):
[2.1] Rovelli C 2004 Quantum gravity (Cambridge, UK: Cambridge University Press)
[2.2] Thiemann T 2001 Introduction to modern canonical quantum general relativity [gr-qc/0110034]
3) Primers:
[3.1] Pullin J, Knot theory and quantum gravity in loop Space: A Primer [hep-th/9301028]
[3.2] Rovelli C and Upadhya P, Loop quantum gravity and quanta of space: A primer [gr-qc/9806079]
[3.3] Major S A, A spin network primer [gr-qc/9905020]
[3.4] Alejandro Corichi, Loop Quantum Geometry: A primer [gr-qc/0507038]
4) Introductory texts and reviews:
[4.1] Ashtekar A, Gravity and the quantum [gr-qc/0410054]
[4.2] Smolin L, An invitation to loop quantum gravity [hep- th/0408048]
[4.3] Ashtekar A and Lewandowski J, Background independent quantum gravity: A status report [gr-qc/0404018]
[4.4] Perez A, Introduction to loop quantum gravity and spin foams [gr-qc/0409061]
[4.5] Thiemann T, Lectures on loop quantum gravity [gr-qc/0210094]
[4.6] Nicolai H, Peeters K and Zamaklar M, Loop quantum gravity: An outside view [hep-th/0501114] (warning: outside view)
[4.7] Rovelli C, The century of the incomplete revolution: Searching for general relativistic quantum field theory [hep-th/9910131]
[4.8] Rovelli C 1998 Loop quantum gravity Living Rev. Rel. 1 1 [gr-qc/9710008]
[4.9] Pullin J Canonical quantization of general relativity: The last 18 years in a nutshell [gr-qc/0209008]
[4.10] Rovelli C, Strings, loops and others: A critical survey of the present approaches to quantum gravity [gr-qc/9803024]
[4.11] Rovelli C, Notes for a brief history of quantum gravity [gr-qc/0006061]
[4.12] Rovelli C, A dialog on quantum gravity [hep-th/0310077]
[4.13] Smolin S, How far are we from the quantum theory of gravity? [hep-th/0303185]
[4.14] Muxin Han, Weiming Huang, and Yongge Ma, Fundamental Structure of Loop Quantum Gravity [gr-qc/0509064]
[4.15] Lee Smolin, The case for background independence [hep-th/0507235]
[4.16] Alejandro Perez, The spin-foam-representation of LQG [gr-qc/0601095]
[4.17] Hermann Nicolai and Kasper Peeters, Loop and Spin Foam Quantum Gravity: A Brief Guide for Beginners [hep-th/0601129] (warning: outside view)
5) On-line courses:
- Baez J Quantum Gravity Seminar
- Smolin L, Introduction to Quantum Gravity -- just look at the sidebar of this blog.
[Photo credit here]
The blog is now shutdown.
The interested visitor can download a partial backup of this blog here (zipped file).
Below I highlight a few posts that are also included in the zipped file: my book reviews, an invited post by Daniele Oriti on his research and new upcoming book, which is a very interesting and important work that offers a view of the current developments of quantum gravity, and finally a basic guide for those interested in learning Loop Quantum Gravity and Spin Foams.
Best wishes to all,
Christine C. Dantas
The Trouble With Physics: The Rise of String Theory, the Fall of a Science, and What Comes Next (Hardcover)
by Lee Smolin
Hardcover: 416 pages
Publisher: Houghton Mifflin (September 19, 2006)
ISBN: 0618551050
Review by Christine C. Dantas
Given the diversity of opinions and thorough accounts already available on Smolin's new book, one cannot disregard that adding some original or unforeseen impressions to what already has been said has become a daunting task.
The book is centered on the theme of telling "the story of a quest to understand nature at its deepest level". The reader will, however, get much more than a simple story. As Smolin calls it, it is something like a "tragedy". Although he prepares the terrain, for some readers it might still have an unantecipated taste, specially in the last part of the book. And it is a well-written tragedy, told with genuine honesty, competence and undeniable lucidity, by a reputable scientist, who has innumerous and well-known contributions to quantum gravity, ranging from highly technical works to popularization articles.
The book is divided into 4 parts:
- The Unfinished Revolution, where he identifies five problems that any quantum theory of gravity must address in its ultimate form, namely: the problem of quantum gravity, the problems in the foundations of quantum mechanics, the problem of unification of particles and forces, the problem of explaining how the values of the free constants of the standard model of particle physics are chosen by nature, and finally the cosmological problems, like dark matter and dark energy. This part is mostly devoted to give a historical and personal account of how these problems are being tackled in theoretical physics.
- A Brief History of String Theory, a very readable review of the history and present status of string theory, and his critical account of the main technical problems of the theory. For fun, should be compared with Greene's The Elegand Universe.
- Beyond String Theory, a bird's eye view on other approaches and research programs to quantum gravity, the new experimental hints that are beginning to emerge from these studies, and a personal view on the "original and impatient mind[s]" behind the scene of these new proposals. Also, a review some recent phenomena unantecipated by string theory is also given. In my opinion, this part could be improved, since one can have the impression that alternative approaches to quantum gravity are somewhat "crazy" ideas from these original and impatient minds, but not in fact coherent and formal independent research lines. Of course this part is used as argument for his thesis on the need to make risks in science (see below), but one might end up confusing some things on this issue.
- Learning from Experience, an exceedingly original and thought-provoking account of what is science, how it works, and what can be done to change the current state of affairs. In my judgement, the best part of the book.
One thing that is important to point out and make it very clear: Smolin is not claiming that string theory is completely wrong:
"There are good reasons to take string theory seriously as a hypothesis about nature, but this is not the same as declaring its truth. String theory rests on several key conjectures, for which there is some evidence but no proof. What we have, in fact, is not a theory at all but a large collection of approximate calculations, together with a web of conjectures that, if true, points to the existence of a theory."And:
"Disagreement and controversy are necessary for science to progress, but there is always supposed to be a way to resolve a dispute by means of experiment or mathematics. In the case of string theory, however, this mechanism seems to have broken down."Those passages summarize one of the main thesis of his book: string theory, as it is currently developed, can be seen as a canonical example of a style of doing science that is wrong. But it is not the case that he thinks it should be put to an end. I think he is clear enough on this matter throughout the book.
Another concurrent thesis that Smolin puts brilliantly forth, and which cleared up my comprehension on several personal issues regarding what the scientific activity is all about, can be summarized below:
"Theoretical physics is hard, not because a certain amount of math is involved but because it involves great risks."A good strategy to do science right, therefore, involves diversity and the willing to take risks: without them the scientific activity gets corrupted and dominated by sociological phenomena. Lee Smolin offers plenty of reasonable evidences along his book that this is the case for string theory.
But Smolin also offers a possible cure for the problem and actually defends it throughout some parts of the book:
"To keep science healthy, young scientists should be hired and promoted based only on their ability, creativity, and independence, without regard to whether they contribute to string theory or any other established research program."There are obvious things that, although obvious, sometimes hide unexplored reasoning inside them:
"The truth lies in a direction that requires a radical rethinking of our basic ideas about space, time, and the quantum world."Clean enough. But to get in the right direction, things will necessarily have to change. The book is an intelligent discourse over simple and undeniable true statements such as this, and makes it very compelling from the consequences that can be draw from them.
As I pointed out, the last part of the book was the one that interested me most, since it is dedicated to a fascinating reasoning over what is science, and I think it could even be subsequently explored in a separate, modern treatise on this important and timely issue.
I would like to end this review with a few words that come from someone highly unconnected with the scientific community in the US and Europe, and whose major area of competence is Astrophysics. It has to do with the fact that Smolin do raise questions of general validity, as I could directly relate to my own experiences.
Before reading Smolin's book, I believed that it would offer me interesting arguments for believing that there was indeed a trouble with string theory. But actually it turned out that it offered me also a vision of science of an unforeseen nature, and hit me personally in an unexpected way.
I do not wish to make public some of my old, deep own feelings about what I think science is and how it should be conducted. There are of course certain points that I often do make public, but there are some others that tormented me for quite a long time now, and are so personal and even of emotive nature that I would rather keep them to myself. But this is the fact per se that should be mentioned here, since this is the contribution that I feel I can give on examining his book: I found out that he was addressing some of my personal views and doubts, of course from his own perspective and wisdom, but it was like talking to an old friend who followed my own career in science and understood what troubled me most for all those years. So this book is for you if you want to be challenged over your own vision of science and how you fit in it.
I am absolutely impressed with Smolin's book and, I should say, alleviated. The reason is simply because Lee Smolin wrote the kind of book that I always thought it was of upmost importance to be written, yet for some reason it seemed that no one was willing to write it; a kind of book I could never write myself. But then, after so many years (at least to me), here it is.
Given my recent interest in background independent approaches to quantum gravity, one might tend to believe that I am simply overexcited by the book. I agree that I may be in shortage of arguments to convince you otherwise. In any case, I hope I have at least convinced you that there are some important elements in Smolin's thesis that should be carefully studied. My personal belief is that most of his arguments sound painfully true.
All the movement around his book is an indication that his work is being seen as a timely one, so I have given myself the final overall task to evaluate whether it would be true to consider that this book had the necessary weight of a historical text. Indeed, in years to come, I have little doubt that it will be recognised as such. Even if someone else does not think that way, or even if the book proves itself in trouble or even wrong in some places, it must be applauded for its timely and wise words. I think this is much needed these days, in any discipline of science.
Smolin's book is now placed on my shelf besides the books that I believe are in the same class: Cosmos, by Carl Sagan, The Selfish Gene, by Dawkings, Gödel, Escher and Bach, by Hofstadter, and The Road to Reality, by Roger Penrose, amongst few others.
Update: A very good review is Sabine's over at Backreaction, including links to other reviews. Another place to look is her Inverse Problem post.
Note: A slightly modified version of this review is available at the amazon site.
Many Worlds in One: The Search for Other Universes
by Alex Vilenkin
Publisher: Hill and Wang (June 27, 2006), a division of
Farrar, Straus and Giroux
235 pages
Review by Christine C. Dantas
Last week I received an email from the Consulting Editor of Farrar, Straus and Giroux, asking whether I would be interested in reviewing Vilenkin's new book Many Worlds in One: The Search for Other Universes. I promptly replied to the editor that I would be happy to review his book. I very much welcome such invitations, specially because they somehow reflect that the Background Independence is being recognized as a serious forum on quantum gravity research.
At the same time, however, some mixed feelings started to agitate my mind while I was waiting for the book to arrive. The problem is that I really do not like the idea of "many universes" to begin with. Also, related to this personal feeling, there is the delicate (infamous?) problem of the string theory anthropic landscape, which certainly the author would have something to say about, and to which I also have my own skeptic disposition.
My approach was then to broadly evaluate the book from two main perpectives. First, I attempted to verify how well Vilenkin was able to transmit the difficult and usually "esotheric" concepts of modern theoretical cosmology into a simple -- but as accurate as possible -- language to the layman, regardless of my own reservations about the subject. In the second perspective, I evaluated whether Vilenkin would offer a balanced account on the current view of the physics community on the multiverse issue. In this case, I did not hesitate to have my skeptic disposition in the back of my mind. But at the end, I very much was willing to learn whether my previous ideas were changed, abandoned, or reinforced due to his book, so I tried to read it enthusiastically, with an open mind.
The book's theme is the origin of (our) universe based on the concepts of quantum theory and inflationary paradigm. The author exposes his own ideas and developments in this area, the so-called "eternal inflation" model. Vilenkin's main general arguments are that "inflation is eternal in practically all models suggested so far" (note 8, page 214). Also, the model leads to the possibility that: (1) our universe came to existence by quantum tunneling from an essentially zero size (page 180), and (2) ours is just one universe amongst an infinitude of other "island universes" eternally coming to existence. As a consequence, he puts forward the idea that there could be many copies of identical universes and histories of events, so that "yes, dear reader, scores of your duplicates are now holding copies of this book" (page 112).
The book is well organized into 4 parts, with more or less five chapters each. The first part develops the main ideas behind the big bang and the inflationary model, and are mostly "standard material". This part is well introduced to the general audience. The subject is written with unusual clarity, originality, and conciseness. However, there are a few minor issues that perhaps would enrich the text, for instance, in page 49 Vilenkin for the first time mentions the word "symmetry", but does not explain in more detail this important notion in physics. He does devote quite further ahead in the book a section to this issue on page 124 ("In search of a deep symmetry"), but it is too short and not very elucidating. At least some reference should have been cited.
The second part aims at explaining the author's "eternal inflation" model. Although the same quality of writing style is found in this part (and in fact maintained throughout to the end of the book). It is where the reader really starts to think on the possible loopholes and alternatives to what is being presented. The author is too much quick (to my taste) to accept the chain of reasonings and conclusions drawn from his theory, whereas to me there are really lots of deep mysteries in between that are largely non-trivial.
The third part deals with the cosmological constant problem, the anthropic principle and superstring theory, and the author attempts to explain that these issues can be addressed or accommodated in his theory. The fluidity of the text starts somewhat to become a little hampered at some points here. Perhaps because one could have the feeling that the logic of the presentation must now be completely devoted to convince the reader that in fact the string landscape of vacua together with eternal inflation can serve as a scientific basis where anthropic arguments can be used to make predictions. According to Vilenkin, "the reluctance of many physicists to embrace the anthropic explanation is easy to understand. The standard accuracy in physics is very high (...). Anthropic predictions are not like that. The best we can hope for is to calculate the statistical bell curve (...). It's little wonder that, given a choice, physicists would not give up their old paradigm in favor of anthropic selection. But nature has already made her choice." (pages 150-151). He also concludes that the "observed value of the cosmological constant gives strong indication that there is indeed a huge muliverse out there". But at the same time adds that "the evidence of the multiverse is, of course, indirect, as it will always be (...). But if, with some luck, we make a few more successful predictions, we may still be able to prove the case beyond a reasonable doubt".
Part four is devoted to the issue whether the universe had a beginning, and I found the most enjoyable passage the issue on whether the physical laws, expressed as mathematical equations, predate the universe. Or "if the medium of mathematics is the mind, does this mean that mind should predate the universe?" (page 205). The book therefore ends mostly in a philosophical mood, but this feeling also pervades the book in several points, specially on the issue of how the universe could have come to existence from nothing. I recall when I realized such an awesome thought at early age, and I could never get free of it. It is inspiring to learn that someone has attempted a physical theory out from such an abyssal idea.
There are no ways to get into a more critical position without going deeper into the original papers. In fact, the book offers a very clarifying set of notes and references, including to the original papers, so it also serves as a good entry to the technical literature.
Overall, the book is a pleasure to read from begining to end, and it is difficult to put it down. The writing style is informal throughout, sprinkled with short narratives of interesting, amusing, incidents or outspoken opinions coming from well-knownscientists. There are several nice cartoons as well. My impressions are that, from the point of view of the layman, Vilenkin does a good job in exposing difficult theoretical issues. So although this is his first attempt on exposing to the general reader, he did it very well. The front cover is outstanding.
The major drawback of the book is the fact that he does not mention at all other alternatives, such as Loop Quantum Gravity, nor Smolin's Cosmological Natural Selection. This is quite devastating in my opinion, specially because I did like Vilenkin's book (in fact it went quite beyond my expectations), since the issues treated there are so difficult and pressing, and must be encouraged.
Finally, I must say that some of my previous ideas were changed after reading this book. Mostly, I do have more sympathy on the idea of eternal inflation than I did before. However, I still do not "believe all this nonsense about our clones" (page 116). Also, I still am not convinced that there could be a way to "prove" that the multiverse exists. Nor I believe that the anthropic reasoning will lead the 22nd century physics. But I am using too much the verb "believe". Whether the reader will "believe" the incredible ideas exposed in the book is unknown, but it is quite certain that he or she will have great hours of good, fascinating reading.
Invited contribution: Daniele Oriti
Hi Christine,
Thanks once more for the invitation to contribute to your blog. And, once more, let me congratulate for its well-deserved success, so: happy blog-birthday and keep up the good work!
So, you suggested I could write about the book I am editing (Approaches to quantum grvity: towards a new understanding of space, time and matter) and that is going to be published by C.U.P.
Ok.
The idea is to present an overview of most of the current approaches to quantum gravity, through a collection of introductory papers and reviews each devoted either to one of them or to one particular aspect of one of them. If you are interested, the exact (but provisional, as for the sectioning) table of content is as follows:
- Foreword; Daniele Oriti
-- Ideas and general formalisms
- Unfinished revolution; Carlo Rovelli
- The fundamental nature of space and time; Gerardus 't Hooft
- Choice of variables and initial value problems in classical General
- Relativity: prolegomena to any future Quantum Gravity; John Stachel
- Non-locality in Quantum Gravity; Rafael Sorkin
- Spacetime symmetries in histories canonical gravity; Ntina Savvidou
- Categorical geometry and the mathematical foundations of quantum gravity; Louis Crane
- Holography: a keystone of any quantum gravity theory? Rafael Bousso
- Questions and Answers
- Gauge/Gravity duality; Gary Horowitz and Joe Polchinski
- String theory, holography and quantum gravity; Tom Banks
- String field theory; Washington Taylor
- Mirror symmetry and quantum gravity; Brian Greene
- Questions and Answers
- Loop Quantum Gravity; Thomas Thiemann
- Towards a covariant loop quantum gravity; Etera Livine
- Questions and Answers
- The spin foam representation of loop quantum gravity; Alejandro Perez
- 3-dimensional spin foam quantum gravity; Laurent Freidel
- The group field theory approach to Quantum Gravity; Daniele Oriti
- Questions and Answers
- Quantum Gravity, or The Art of Building Spacetime; Renate Loll, J. Jurkiewicz and Jan Ambjorn
- Quantum Regge calculus; Ruth Williams
- Consistent discretizations as a road to quantum gravity; Rodolfo Gambini and Jorge Pullin
- Questions and Answers
- The causal set approach to Quantum Gravity; Joe Henson
- Towards gravity from the quantum; Fotini Markopoulou
- Questions and Answers
- Quantum gravity and precision tests; Cliff Burgess
- Asymptotic safety; Roberto Percacci
- Emergent General Relativity; Olaf Dreyer
- Questions and Answers
- Quantum Gravity phenomenology; Giovanni Amelino-Camelia
- Algebraic approach to quantum gravity II: noncommutative spacetime; Shahn Majid
- Effective non-commutative models of quantum flat spacetime; Florian Girelli
- Doubly special relativity; Jurek Kowalski-Glikman
- Lorentz invariance violation and its role in quantum gravity phenomenology; Daniel Sudarsky, John Collins and Alejandro Perez
- Generic predictions of quantum theories of gravity; Lee Smolin
- Questions and Answers
Oveall I am quite happy with it. I think it is going to fulfill its scope and motivations, which were (at least for me): a) to show how active and diverse is quantum gravity research at present, and that there is a variety of approaches being pursued, and lots of new work, new ideas (radical, speculative and solidly grounded in physics and mathematics at the same time), new directions and results (some partial but suggestive, some well-established, some surprising); b) to provide a comparative perspective on what's going on in the field; to foster not only this comparative perspective, but also, possibly, future collaboration ad 'cross-fertilisation' among different approaches; c) to allow newcomers and students as well as whoever is interested in the subject to be introduced nicely to it. Ultimately, and most importantly, d) to show that this stuff is fun and exciting to work on and to read about!
You may have noticed the Q&A sections at the end of each part. The idea is that authors can ask questions to each other, put forward comments and criticisms, and get/give answers. The aim is twofold: first, to improve the comparative aspect of the book, in that possible difficulties of the various approaches could be pointed out if not already discussed in the papers, and to present additional ideas and points of view that can be relevant for a given approach, but maybe originated or suggested by another; second, to give an example of how research progresses: out of discussions, criticisms, debate, indeed, questions and answers...This part is being prepared, and I really hope it is going to be rich in content and useful.
I enjoyed reading all the papers, and the Q&A, received so far, so I hope other readers will enjoy them to, and find the whole thing useful. Ultimetely, we don't know yet what quantum gravity is, how long it is going to take still to find out (not much, I hope!), and which, if any, of all these approaches will be found to be the closest to the final theory. It is well possible that none of them, as they are currently understood, matches reality as it will look once we have understood more of it; I wouldn't be too surprised. However, I also believe that we can learn a lot from all of them, and that the final theory will involve aspects (formalisms and techniques, ideas, motivations, results, we don't know yet) of many, if not all, of them. If this is the case, this book may be of help.
In my own contribution to the book, I have decided to discuss Group Field Theory, which is an important part of what I am working on at the moment. Most importantly, it is the line of research that I am most excited about! I must confess that this is not due to some incredibly important new result that has just come out, unfortunately. Very little, I think, is known at present about GFTs, at least compared to the width and potential of the whole formalism , that has just begun to be explored. However, the results we have already are suggestive of something big, I think; they point towards the need to take GFTs seriously and start really exploring them as fundamental theories, not just as some sort of auxiliary formalism of purely technical use to do something else. In particular, I believe that GFTs can represent: 1) a general, encompassing, possibly unifying, framework to study and understand better many of the other current approaches to quantum gravity, in particular Loop Quantum Gravity, Quantum Regge calculus, dynamical triangulations, causal set theory, in addition (of course) to spin foam models. Also, I believe that if one takes seriously GFTs as field theories describing the quantum dynamics of the fundamental discrete building blocks of (quantum) space, being these seen as simplices, or spin networks (in GFT these two points of view are equivalent), then they offer an altogether new perspective on the issue of the continuum approximation and on the emergence of a continuum spacetime. What I mean is that this becomes a problem in (a very peculiar indeed) condensed matter, to be tackled (of course) with almost ordinary QFT methods. It is the problem of showing that in certain situations (to be identified) the fundamental 'atoms of space' described by some GFT -condense-, so that a continuum spacetime emerges as a sort of liquid.....ideas of this sort were proposed in the past, of course, but we may have now for the first time the opportunity to realise them explicitly and concretely....
You see! I got excited and carried away! Anyway, I don't now yet if any of the above makes real sense, if it is going to be realised fully or it will turn out not to work. How to say? I am working on it :) We'll see where all this leads....but there is potential, so to say....and of course, this is only my opinion....I am not really sure there is many people out there who agrees with this point of view, actually I doubt it!
At least, however, I am having fun thinking about all this and working on it, which is great!
Anyway, I have taken too much of your space already.....
Thanks again for letting me contribute some of my thoughts to the blogosphere :) and even more, thanks for your interest in them!
Take care.
Ciao
Daniele
A basic curriculum for Quantum Gravity
This guideline provides a basic background material for studying Loop Quantum Gravity. This is supposed to include the "canonical" LQG as well as more recent "spinfoam" (path integral) approaches to background independent, non-pertubative quantum gravity. I have not included recent research papers nor more specialized applications (e.g., cosmology, phenomenology and numerical simulations), although I invite of course all interested readers to comment on these recent achievements as I eventually find them out and post them in the blog.
Also, this is my personal fragmentary view on how to approach this field, and it also reflects my own "formation history". It is my own guideline, and by no means should be seen as a final, authoritative "curriculum". I do hope, however, it has some usefulness to others.
In order to understand recent LQG research papers, I have been studying the following material in no preferential order, although I believe the most correct and solid approach would be to study all the pre-requisites first and only then the primers and review papers afterwards. This is often difficult to accomplish, mainly due to time constraints, so I have been studying this material in a quite non-linear manner.
The list is divided into 5 parts: Pre-requisites, standard books, primers, introductory texts and reviews, and on-line courses. There are a large number of pre-requisite books/texts which I could have included, but I will only mention those which I am specifically studying now or intend to study opportunely.
Finally, it is a lot of material! I intend to post on a much briefer guide than this one, but for the moment, you make your own choices. Good luck!
1) Pre-requisites:
- General Relativity (GR)
In particular: the Hamiltonian formulation of GR:
[1.1] Wald R M, 1984, General Relativity (Chicago: Chicago University Press)
- Quantum Mechanics and Quantum Field Theory (QFT)
[1.2] Zee A 2003 Quantum Field Theory in a Nutshell, Princeton University Press
In particular: QFT in flat and curved space-time:
[1.3] Wald R M, 1994, Quantum field theory in curved space-time and black hole thermodynamics (Chicago: Chicago University Press)
- Gauge Field Theory
[1.4] Baez J and Muniain J P, 1994, Gauge fields, knots and gravity, Singapore: World Scientific -- wait for the upcoming second edition!
[1.5] George Svetlichny, Preparation for Gauge Theory [math-ph/9902027]
- Systems with constraints:
[1.6] Sanjeev S. Seahra, The Classical and Quantum Mechanics of Systems with Constraints
[1.7] Andreas W. Wipf, Hamilton’s Formalism for Systems with Constraints [hep-th/9312078]
[1.8] Paul A. M. Dirac. Lectures on Quantum Mechanics. Dover, Mineola, New York,
1964.
- Mathematical Methods:
[1.9] Frankel T 2003 The Geometry of Physics, Cambridge University Press
[1.10] Bishop R L and Goldberg 1968 Tensor Analysis on Manifolds, Dover
[1.11] Byron F W and Fuller R W 1970 Mathematics of Classical and Quantum Physics, Dover
[1.12] Henle M 1979 A Combinatorial Introduction to Topology, Dover
[1.13] Tung W 1985 Group Theory in Physics, World Scientific
[1.14] Nakahara M 1990 Geometry, Topology and Physics, Institute of Physics Publishing Bristol and Philadelphia
2) Standard Books (I find them quite advanced, though):
[2.1] Rovelli C 2004 Quantum gravity (Cambridge, UK: Cambridge University Press)
[2.2] Thiemann T 2001 Introduction to modern canonical quantum general relativity [gr-qc/0110034]
3) Primers:
[3.1] Pullin J, Knot theory and quantum gravity in loop Space: A Primer [hep-th/9301028]
[3.2] Rovelli C and Upadhya P, Loop quantum gravity and quanta of space: A primer [gr-qc/9806079]
[3.3] Major S A, A spin network primer [gr-qc/9905020]
[3.4] Alejandro Corichi, Loop Quantum Geometry: A primer [gr-qc/0507038]
4) Introductory texts and reviews:
[4.1] Ashtekar A, Gravity and the quantum [gr-qc/0410054]
[4.2] Smolin L, An invitation to loop quantum gravity [hep- th/0408048]
[4.3] Ashtekar A and Lewandowski J, Background independent quantum gravity: A status report [gr-qc/0404018]
[4.4] Perez A, Introduction to loop quantum gravity and spin foams [gr-qc/0409061]
[4.5] Thiemann T, Lectures on loop quantum gravity [gr-qc/0210094]
[4.6] Nicolai H, Peeters K and Zamaklar M, Loop quantum gravity: An outside view [hep-th/0501114] (warning: outside view)
[4.7] Rovelli C, The century of the incomplete revolution: Searching for general relativistic quantum field theory [hep-th/9910131]
[4.8] Rovelli C 1998 Loop quantum gravity Living Rev. Rel. 1 1 [gr-qc/9710008]
[4.9] Pullin J Canonical quantization of general relativity: The last 18 years in a nutshell [gr-qc/0209008]
[4.10] Rovelli C, Strings, loops and others: A critical survey of the present approaches to quantum gravity [gr-qc/9803024]
[4.11] Rovelli C, Notes for a brief history of quantum gravity [gr-qc/0006061]
[4.12] Rovelli C, A dialog on quantum gravity [hep-th/0310077]
[4.13] Smolin S, How far are we from the quantum theory of gravity? [hep-th/0303185]
[4.14] Muxin Han, Weiming Huang, and Yongge Ma, Fundamental Structure of Loop Quantum Gravity [gr-qc/0509064]
[4.15] Lee Smolin, The case for background independence [hep-th/0507235]
[4.16] Alejandro Perez, The spin-foam-representation of LQG [gr-qc/0601095]
[4.17] Hermann Nicolai and Kasper Peeters, Loop and Spin Foam Quantum Gravity: A Brief Guide for Beginners [hep-th/0601129] (warning: outside view)
5) On-line courses:
- Baez J Quantum Gravity Seminar
- Smolin L, Introduction to Quantum Gravity -- just look at the sidebar of this blog.
[Photo credit here]
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