Saturday, February 25, 2012

Gambini and Pullin: A First Course in Loop Quantum Gravity

Briefly after Christmas, I found in my mailbox a copy of Rodolfo Gambini and Jorge Pullin's new book "A First Course in Loop Quantum Gravity" and a note "Sent with compliments of the Editor." It's a lean book with a total of 183 pages, and that includes the index and the references.

A book like this has been long overdue. There are Rovelli's and Thiemann's books of course, and there is Kiefer's book too, but to read one of these you have to make a major commitment, both in terms of time and in terms of finances.

Gambini and Pullin's book in contrast is a masterwork of omission, and as every science blogger knows that's a very under-appreciated art. The book comes at an affordable US$ 45.99 which, for a hardcover textbook really isn't much.

You need to bring only basic knowledge to benefit from the book. It starts very slowly, with special relativity and electromagnetism, goes on to general relativity, Yang Mills theory and quantum field theory. That is about the first half of the book. The rest of the book then deals with the reformulation of general relativity in terms of Ashtekar's variables, the quantization, loop representation, loop quantum cosmology and black holes, and a word or two on spin foams and the problem of time.

I think this book will be very useful for every undergraduate student who considers to work on quantum gravity and wants to know some more about the topic. Needless to say, the brevity comes at a price. That is that many intricate points are covered only very superficially and the reader is referred to the literature. If you really want to know all the details, this is not the right book for you. But if you want to find out if you want to know the details, this is the place to start.

Notably, the book finishes with a chapter on open issues and controversies, stating clearly and honestly that "Loop Quantum Gravity is an incomplete theory. We do not know if the current formulation... really describes nature or not." But, Gambini and Pullin add, "[I]t is refreshing that in loop quantum gravity there are not many things that can be tweaked to make the theory agree with experiment... This is in contrast to string theory, where the theory has evolved considerably over time and is now perceived by some as having too much freedom to be a predictive theory."

You can also read there
"In this modern day and age a lot of the disagreements about loop quantum gravity are aired in blogs and discussion groups on the Internet. In general we do not recommend people learn from the controversy there. Because blogs and Internet discussion groups are unedited opinions of people, some of which have questionable training or knowledge of the details, and are usually written relatively quickly, in many circumstances they contain highly inaccurate statements."

With that note of caution, in this relatively quickly written and unedited blogpost, I recommend Gambini and Pullin's book if you want to know more about Loop Quantum Gravity without getting drowned in details.

Gloria also liked it.


Steven Colyer said...

Five Stars! Thanks, Bee, this is EXACTLY the book I have been dying to see being written!

Any idea when Lee Smolin's new book is coming out? Co-Written with a professional Philosopher, this is another book I've been longing to read. Thanks again! :-)

Giotis said...

"[I]t is refreshing that in loop quantum gravity there are not many things that can be tweaked to make the theory agree with experiment...'

With the large amount of ambgiguity inherited by the Hamiltonian constraint (which determines the physical states too) I can't see why this is true.

Giotis said...

Not to mention that LQG has nothing to say about matter. Practically it is compliant with everything.

Bee said...

Hi Giotis,

The ambiguity in the Hamiltonian constraint is also briefly addressed in the last chapter of the book. They basically acknowledge it and say more work is required. And yes, LQG doesn't tell you anything about matter (not unless you believe all matter is braids that is), but then it wasn't meant to provide unification. Best,


Giotis said...

Bee don't you think is odd that people practicing LQG accuse String theory for lack of predictivity when their theory not only doesn't predict anything at all at the low energy effective world but it can't even make contact with established theories like GR.

They could raise theoretical arguments in favour of LQG but predictivity is not their strong point. They don't even know if they break Lorentz invariance...

Phil Warnell said...

Hi Bee,

All I have to say if its good enough for Gloria then it should be good enough for me!

Uncle Al said...

Quantum gravitation and the standard model both arrive massless - elegant and unphysical. Add artificial curve fittings (SUSY, riiight).

"the reformulation of general relativity in terms of Ashtekar's variables" Ashtekar variables are chiral decomposition of connection one-forms of the local Lorentz group re the Immirzi parameter (arxiv:1104.1800, 1108.0816). Ashetkar torsion is simple, curvature is not (arxiv:1112.1262 ). Quantum gravitation (triangle-like anomalies, arxiv:0811.0181) requires Chern-Simons correction to Einstein-Hilbert action. Boson rules applied to fermionic mass require hierarchies of symmetry breakings. Admit the obvious. The vacuum is fundamentally chiral toward fermionic mass. Test the Equivalence Principle for physical chiral divergence.

Opposite shoes on a vacuum left foot locally pursue divergent minimum action vacuum free fall trajectories. Chemically and macroscopically identical, enantiomorphic atomic mass distributions violate the EP. Single crystals of alpha-quartz in space groups P3(1)21 versus P3(2)21 violate the EP. Single crystals of gamma-glycine in space groups P3(1) versus P3(2) violate the EP. 90 days in an Eotvos balance versus 40 years of empirically sterile theory. Somebody should look.

Bee said...

Hi Giotis,

Do I find it odd that every side points at the other's faults while praising their own virtues? No, I can't say I'm surprised. Best,


Bee said...

Hi Phil,

The cover is also pretty actually. It has a Pollockian spin-network on it (if you look closely, you can see it on the picture). I'm not a Pollock-fan (too messy) but it works nicely for the purpose at hand. Best,


Giotis said...

Bee I see that you point to the second edition of kiefer's book. Note that there is a third enchanced edition coming on July. So your readers if they are patient enough should wait until July to purchase the new version. It would be a pity to pay that amount of money for the old edition when the new one is just around the corner.

Phil Warnell said...

Hi Bee,

Not a Pollock fan eh, then I would advise you keep the lids on your paints tight as from that look in Gloria’s eyes she seems inspired to express herself abstractly. That is taking into account Gloria’s nature and that of LQG could be a serious thing; that is especially if she comes to realize her work not to be background dependent. Then again I suppose if all else fails you could always ask Sundance if he has any braids which might restrain her from expressing herself on the carpet at least :-)



Juan F. said...

OMG, BEEEE!!! How could you give such a big book to your child?LOL Do you remember what J did to a girl with a QM book? I am wondering what J would do to your precious child!

Anonymous Snowboarder said...

Bee - I think your exchange with Glotis speaks to what the authors were getting at with their caution of using the web as an introduction to the subject, especially as string theory is the "in" theory and certainly has a lot more vested interest in terms of time and careers

Bee said...

Hi Juan,

Who is "J"? I didn't actually "give" the book to Gloria. She pulled it out among the pillows and blankets. (I have the unfortunate habit to sleep with books. Though the books aren't the main problem; the main problem are the pens.) However, she was to my surprise very very careful with the book, she opened and closed it a few times and looked very interested at the (few) illustrations of graphs. My biggest concern wasn't the book but that she falls off the bed, because she hasn't really learned yet that "head first" isn't always a good idea. Best,


Arun said...

Whatever interest Gloria's mom will pique her interest too! She's no doubt observed that books are great friends with her mom. :)

Shawn Halayka said...

I very much look forward to counting the number of times that the names Delaunay and Voronoi are mentioned.

Chanda @ Disordered Cosmos said...

I just wanted to second this review. I got a copy of the book a few months ago, and I think Sabine says exactly what I would say about it :-)

Arun said...

The rational head knows that a baby cannot meaningfully endorse a book. But just that smile makes me want to read the book.

Bee said...

I've been joking Gloria looks like right out of a cookie advertisement: blond, blue-eyed, beaming at you if you just hand her something, may that be a plush toy, a cookie, or an empty bottle ;o)

PS: There's something very wrong with the comment feature this morning.

N said...

Hi Bee,

Some time ago I read Lee Smolin's "Three Roads to...", very readable, but I admit I did not understand, well, almost anything.

This one, is it closer to, well, not exactly a layman (I am a - retired - EE Engineer)?

all the best,


Bee said...

Hi N,

I am afraid, if the "three roads" were too technical, then this book will not solve the problem. Unless that is the problem was lack of equations. Martin Bojowald has written a book, maybe this is useful. (I have it, but got stuck in chapter 2. He covers extensively all sorts of tests of General Relativity which isn't so terribly interesting if you did the same thing in your master's thesis. So unfortunately I can't tell you much about it.) Best,


rabraha3 said...

I really liked this book. I think it'd be nice if there were a couple additional problems at the end of each chapter, but it was well written and thought out. Good to see that people with more expertise think it is a worthwhile book.

I'm trying to get my advisor to read it, but what use does an observer have for it (who doesn't do cosmology)?