I have these blog somewhat stopped while I am updating my knoledege in string theory.

My previous background in the field came mainly from three books. The Lüst- Theisen one: "lectures on String theory" from the series "lectures notes in physis of Springer Verlag, "Quantumf field theory of point particles and strings" by Brian Hatfield" and "Strings theory and M-Theory" from Micho Kaku. For the necesary supersymmetry background i had relaied mainly in the book "Cosmology and particles physics" and the chapters devoted in it to that topic. Also i have readed some chapters in dedicated books in the topic for puntual questions.Appart of it I had readed various articles on some random topics. Of all these articles the most interesting ones where the ones by Lisssa Randall in the topic of wrapped dimensions. Also I was aware of the advances, at least in a semidiviulgative way, throught the forum (now inactive) of the web www.superstingtheory.com

Those who knows these books will realize that they are mainly about the "old" (pre-branes) string theory. The kakus book somewhat covers them, but in a a somewhat tangential way.

With that base, and a reasonably good background in advanced QFT, general relativity, and of course modern mathemathics I got knowledege of loop quantum gravity. I found their modest premises and their stated results interestings so I studied it. First, and mainly, the canonical part and some of it´s developments, specially y the ones abot singularities in cosmology and in black holes. I also have studied the spin-foam, "path integral" aproach, but I don´t like it as mcuh as the canonical one. too begin with there are too many spin-foam models (corresponding to too many diferent lagrangians), but well, anyway i recognize that if someone intends to know LQG and mostly their recent developments it is necessary to know spim-foams. The other thing I really misslike of LQG is the invasion of category theory in the field. Fortunatellly someone told me that it is mainly a personal iniciative of Jonh Baez and that not too much people are interested in folllowing it seriously. It is not that I don´t respect Jonh Baez, of course, but for me t looks as an unnecesary try to introduce his favourite mathematical tool without a very real reason.

Once I had learned LQG i became aware of the current status of the "string wars". I have readed the technical objections of string theorists to loop quantum gravity. I recognize that they point questions wich must get serious attention but, in my humble opinion, don´t invalidate LQG as a research field. Most ugly seems to me the calificative "crackpots" used by them to the LQG comunity. And evn less I find reasonable to include in that group to someone like Gerard t´hoof who has made possilby the last theorethic development in high-energy physicis wich is refrended by experiments. I mean, his proof that gauge theories are renormalizable, and the fact that every tested particle physic theory is a gauge theory.

But the question ifs that now string theory people claim that the actual formal development of the theory is good enoguht to state definitive questions about what a quantum gravity can be and what it can´t. It lloks me very unlikely that these could be truth. But I don´t like to criticis from a layman viewpoint. That´s way I am nowadays studiying the recent advances instring theory. I begined with some review papers wich would give me a broad idea. For example I begined with the onescited in www.superstringtheory.com (just now i have been unable to find the exact url where they are listed) about d-branes (you can read also their technical "divulgative" section here http://www.superstringtheory.com/basics/basic7a.html

Appart i have readed a very good review article, wich appeared in a recopilative book, writne by Thomas mohaupt, it can be easily found on arxiv also.

But have decided to go an step beyond review papers and I have gone into books. I beguined reading the one by Tomás Ortin "gravity and strings". I have already finished the part

of "gravity" (wich include to chapters in supersymmetry). I have found it very interesting in his extense development of the idea of "gravity from gravitons". But the chapteres about some specific solutions on Einstein equations seems more a catalogue of "what an string theoriest needs to know about general relativity" that a serious aproach to general relativity. I also readed the first three chapters in string theory. Afther a review of basic (and maybe not some basic) string theory he beguins obaianing effective attions for strings in target spaces. I find that it also shares the "catalogue" aproach that he uses in the last chapters about gravity.

So I decided to pause reading that book and have gone into a diferent one. The V. Clifford book "D-Branes". I had beguined reading it some time ago, but i had finished opting by going into the review articles I commented before. Now that i am more aware of the results thanks to that articles (and the explanations about string theory in many blogs) I am finding it lecture more productive. I had another posiibles choices, for example the polchinsky books, but at last i Have preferred these.

While I am studying string theory LQG people don´t stop and have appeared some interesting papers. For example it has seem light the article of frienklin about feynman diagrams from spin foams in 4-d. I have not readed the article. Only the comments about it in the apropiate phorum (you can see i in the links sections of the blog). It looks interesting. Also it has appeared a series about kodama state wich seems to share some light about the question of the classical limit of LQG wich are very wellcome news.

But still i have opted by keep learning string theory. Appart of the point of the need to kknow something ito properly cricitc it i am interested in understanding in deep their works about black holes. And also i have the viewpoint that quantum gravity rise some questions wich are independent of the viewpoint you use to study it. SO i like to see them somtimes form a string theorist viewpoint and other ones from an lQG viewpoint and see the similtudes and difrences.

Also there has been at least to papers stating that "old" perturbative quantum gravity on one sied and supergravities in the other could be renormlizable afther all (despite the "theorems" wich stated the contrary. If these probes to be firmly stated I think it can be learned a lot from them and how the results of these "naive" viewpoint compares to most sophisticated aproachs.

And even from non full aproachs to quantum gravity, that is quantization in curved backgrounds seems to be a few things to be learned. I just can understand how "pure" string people can feel safe if they only learn string theory. Well, do they only learn it? I don´t pretend to know what other pople knows of course.

## Monday, November 20, 2006

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