Tuesday, April 29, 2008

Gerard ’t Hooft: A diferent string theory

The first oe paper is LOCALLY FINITE MODEL FOR GRAVITY written by Gerard ’t Hooft. If by some casual you dont know who ´t hoof is just to say that he has a nobel prize by proving that the gauge theories, wich are the basic ingredient, of the standard model, are renormalizable. A few physicists I know consider him the last greater phyisicist (Steven Weinberg would be considered slighly earlier in time). ON the other hand be sure that Lubos Motl is not among his fans ;-). Ok, let´s leave sociology and go into physic. The abstract of the paper is this:

Matter interacting classically with gravity in 3+1 dimensions usually gives
rise to a continuum of degrees of freedom, so that, in any attempt to quantize
the theory, ultraviolet divergences are nearly inevitable. Here, we investigate
matter of a form that only displays a finite number of degrees of freedom
in compact sections of space-time. In finite domains, one has only exact,
analytic solutions. This is achieved by limiting ourselves to straight pieces of
string, surrounded by locally flat sections of space-time. Globally, however,
the model is not finite, because solutions tend to generate infinite fractals.
The model is not (yet) quantized, but could serve as an interesting setting
for analytical approaches to classical general relativity, as well as a possible
stepping stone for quantum models. Details of its properties are explained,
but some problems remain unsolved, such as a complete description of the
most violent interactions, which can become quite complex.

The paper begins with some considerations about 2+1 dimensions and the role of pont partile matter as source of curvature, in the form of a wedge in space time. Inspired by that view he considers the extension of this to 3+1 dimensions. The role of the point particles are now strings. Why? Simply because the aditional dimension is perpendicular to the others and so a point becomes an infinite string. In principle it could look a bit arbitrary, and not general. But the idea seems to consider the space-time sourruounding that infinite strings.

He writes the energy momentun tensor for that strings (well known for people aware of cosmic strings). Later he considers moving and interacting strings. In considering this he concerns about holonomy so maybe the reader would consult something about this topic if he does´nt know it previously. The wikipedia entry is specially good about the topic so it could be a quick start guide. The very quick idea of homotopy is to consider a map betwen parallel transport of vectors around a closed curve and the group associated to the bundle (wich defines the very concept of paralell transpoort).

Afther that he considers interactions of strings. The first claim is as a resoult of interactions, connections, he cant´consider infinite strings alone anymore. Diferent types of collisions are analized. He cnsiders vaious posibilities and takes care about some possible issues (for example, rotating strings would create spacetimes with closed timelike curves as is well known since the work of Gott). I am not sure of how much of this work would intersect with the well stablished resoults about networks of cosmic strings (gauge or superstring ones). It would be fine if some reader would know it and could say something about it.

The final conclusion he claims is that he can get all the degrees of freedom of gravity by pieces of straight strings. In this way he could study gravity just from this. Seemengly this is somewhat similar to Regge calculus (a discretized aproach to quantum gravity)using strings instead of points in the nodes. He also says that in this sense is just the opposite to some papers triying to get matter from ure gravity (in a clear reference to Smollin program in the octopy). In the paper a quantizaion of the model is not made, that is announced for a future paper. About that paper he says that he will not follow traditional quantiztion proceduers based in lagrangian mechanisms,partially because the model seems not to admit a Lagrangian formulation). AS an advance a claim is made that the theory will not have ultraviolete divergences but possibly will have problems with infrared regime.

Well, I still have to re-read carefully some pieces of the paper, and a definite juice will only be possible when the paper on quantization would be available. Also it would be interesting to see how it is recived by the mainstream physic comunity. For example Sean carroll has announced that he will speak about the paper (it is how I knew about it´s existence). And, I guess that also Lubos Motl will have something to say, given his aparent animosity against t' hoof (maybe because he has sometimes soped favourably about the LQG comunity). Personally I consider ’t Hooft a very interisting figure and I like to be aware of what he does, even if I don´t necesarilly agree with all his conclusions.

Saturday, April 26, 2008

New, complementary, blog

As announced I have created an aditional blog inwordpress. The actual url is http://freelancescience.wordpress.com/.

The idea is to keep this blog for quantum gravity related stuf and the other for diferent aspects of physics, math, and, ocasionally, another sciences. A separate interest is to see how well it works the LaTeX functionality of wordpress. If it works fine, and blogspot refuses to allow LaTeX, I would consider the posibility to migrate this blog there (afther all they have facilities to do so).

Advise, the other blog will have most of their entries in spanish, because of the expected target audience, if some non spanish speaker is particularly interested in seeing english entries he could let me a message and I would see what it can be done.

Friday, April 04, 2008

Fresh air for string theory

The very recent mounths seem to have brought great news for string theory. I´ll write in this post a brief guide to the relevant papers.

The first, cronologically, is this paper by Beasley, Heckman and Vafa. It is a paper where, for the first time, it is addressed the task of constructing phenomenology from F-theory. A second one is announced where the program started in thiw will be concluded. If 125 pages is more time of what you have available just now you can try to read a sinopsis in the Jackes Distler glog, concretelly here. Lubos also wrote his own comment, try to search for it in his blog ;-).

Maybe the reader has no too much familiarity with compactifications and similars. In that case, or even if he has, it would be recomendable the following paper Les houches lectures on constructing string vacua. The great point of that paper is that it covers in a single paper approach for many types of strings, and many types of vacua (compactifications, fluxes, etc).

Another very interesting topic are advances in M-theory and their interactions. A review explaing the topic can read here. Well, two papers have changed things and some people (yes, you are rgiht, Lubos) has told that it may be the beguining of a third string theory revolution. The papers in question are these:Gauge Symmetry and supersymmetry of Multiple M2-Branes and M2 to D2. You can read a brief description in Lubo´s or Distler´s blogs. I particularly recomend, if you are in a hurry, the one in Lubos blog because it is easier to understand.

Not enought reading? Well, there is a very recent paper in perturbabtive string theory. Concretely some where the four loop amplitude for string theory is explictely calculated. The author of the paper is Samuel Grushevsky and the actual paper is this

For the string heater readers I would recomend reading this paper by Smollin and all. They continuate their "ocutopussy program" where the try to get the standar model from spin networks of pure gravity. To be honnest, I don´t really think that approach is particularly viable, measured by the own standards of the LQG commnuity, but it is up the readers own judice to decide what is interesting or not.

To conclud simply to say that at last I found a great reference for the basics of the renormalization group, as well as in many other topics,the three volumes book of Steven Weinberg in quantum field theory. I had never tried to read that book because of it´s extension (and because, hey, I already had a reasonably good knowledge of the subject) but I must say that despite of ít´s extension it is so well written that one can read it fast. I hope to write soon a post about the subject of renormalization group theory, that could serve as an introduction to those who would want to follow the Martin reuter papers on the subject and the discusions on Distler blog on the particular.