Thursday, October 04, 2007

Why Loop quantum gravity?

I had a few entries in this blog wondering about some unnaturallness that i find in string theory.

On the other side I am aware about what some people in the string theory comunity think about LQG, so I have decided to make a post about why someone could worry about LQG nowaday (or why not).

One important thing is your academic enviroment. One of my teachers (probably the best in the field of string theory, in my facoulty, when I was studiying) played atention to LQG in his general reviews about quantum gravity. By LQG I mean a broad conception which included to explain the wheeler-de Witt equation and the Astekhar connection in paers previous to the actual form of canonical LQG. In newer papers he stills includes some information about LQG (altought his main interest is in strig theory). I never asked him personally about his opinions so maybe he was just beeing polite, but from his papers one would think that it is not crazy to study LQG. So judging from direct influence of your teachers it was a free way to study LQG.

For a brief (unhappilly too brief) time I beguined to study for a tessis in geometrical quantization (in the math faculty). When later I saw that geometrical quantization played a role in LQG I really enjoyed it because it somewhat meaned that the time I studied geometrical quantization was not lost time.

As I stated before in this blog for a time my knowledge of string theory came mainly from the two books of Michio Kaku and the Lüst-Theisen (and secondary in theh Hatfield book "quantum field theouy of point particles and strings"). While they cover fine the "first revolution" string theory I must say that the second KAku book (string theory and M-theory) althought readed a posteriory when you allready understand the topics, is correct I feel that it doesn´t makes a good job presenting the intuitive ideas of the second string revolution. It would need more space to trate the topics.

Another aspect is that in my faculty library always there has been a democratic representation of string and LQG. You get all the new books in string theory (nowadys you can get the Michel Dine and Becker-Becker Schwartz ones) as well as books by LGG people (for example the book o Baez "lopps, knots and gauge fields). Also you can have books in euclidean quantum gravity and other topics (A hurray for my faculty library).

Also I must say that previously to going hard into string theory I wanted to have an as strong as possible basic in general relativity and quantum field theory. The quantum field theory books usually contained an intro to string theory (kaku´s book on QFT). Pariticularly usefull I found the book "particle physics and cosmology" to understand some more advanced topics in QFT and physicis beyond the standard model. But more important than QFT was for me general relativity. My main source where the book by wald and a really extense book about black holes writen by Frolov & Novikov. Also I found very usefull the online website "living reviews on relativity". I think that I don´t need to say that a general relativity formation favours to apreciate LQG over string theory.

Well, all these means that there is no a priory pressure agianst LQG. This leads me to a famous paper in 2003 (or around that date) in which it is presented the new (for that time) canonical LQG, with the very interesting promise of a soon to come experiment (the one related to the MAGIC experiment that I bloged in the last entry). As a complement living reviews had a somewhat complementary introduction to canonical LQG. You can doubt about the goodness of he physics behind, but as an introduction/presentation both articles (specially the one by Thieman) are ver well writen.

With all these background to study LQG and not string theoy seemed a very good idea. Also an a priory netural forum, physics forums, was very pro LQG (by that time the forum in had been hacked).

Well, what came next? I guest that LQG has somewhat killed itself. Canonical quantum gravity seemed a very elegant and usefull reformulation of the old idea behind wheeler de witt equation (reached from canonical gravity, not euclidean quantum gravity) But very soon the atention was deviated to spin-foams, arguably because of "the problem of time". But spin foams are not so obviously related to gravity as canonical LQG. I mean, there one starts not from the general relativty lagrangian, that is what one, at least naively would expect but from lagrangians which hopfully, with some constraints, reproduce classical general relativiy. Worse still, it is not clear which of all them is favoured so people study many of them (ok, the Crane one´s is the most favoured, seemingly).

But the history doesn´t end there. It is adviced to study causal triangulations, and quantum groups. Not to say there is not a clear relation betwen all these aporaches (besides a like ofr discretized space-time). I must say that I studied spin foams, mainlly thanks to the review articles by Alejandro Perez, which I find very well writen, and which, IMHO present the physics ideas a lot better than the reviews by Jon Baez. What I couldn´t find the interest enoguht to read are the recomended literature about causal triangulations and the like (maybe because I never liked too much lattice QCD, but I guess that it was not that the ultimate cause).

Later I found Lubos Motl blog and it´s comments about some famous papers in LQG (as for example the one in the calculation of the graviton propagator), and well, I beguined to be aware abouth that fraticide strings wars and the "not even wrong" blog (whcih at first I thought it was a blog pro string theory, why else would someone would devote so atention to string theory afhter all? :P).

Most interesting that his attacks to LQG I found interesting in Lubos Motl the presentation of some intuitive ideas of string theory that were somewhat ausent in the literature I had readed.

Well, nowadays I don´t really follow too much LQG. I am aware about it´s developments while I am devoting time to update my string theory knowledges (I feel I am ending that task at last, at least in the main fields, and some not so mianstream aspects also). Would I give any advise against LQG?

Well, LQG people, at least someones, have a like for "Filosofing". I find entertaining reading the prose of their papers (I specially recomend one containing a discusion about the entropy of black holes and if some would count or not the "inner" degrees of freedom. I would recomend people to read also the introductory papers that I have cited, and maybe also some paper of Router about his claim that conventinal quantum gravity has a renormalization group fixed point. Later I would recomend that people to read some of the frequent blog entries on Lubos Motl blog critizazing LQG approachs. At that point it would be an individual decision of every one to go further on LQG or not. But I think that not devoting the relativelly few time that it requires to read the papers I mention(if somone is interested I could give him the exact references if he is not abble to find them himself) is not a good idea.

Well, I have been almost a mounth without any post. In this time I have been reading about some topics, that I hope that my eventual readers could find interesting.

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