This year the annual conference in string theory, celebrated at Roma, has not had an internet live TV broadcast as it happened the last year.
Because of that reason I didn't do a post about the topic. I have waited until the slides where out and I could have read some of them. The slides of conferences, if they are detailed enough, are a good thing because they are addressed to non specialists in that particular field, so they can be easily read, and they condense a great amount of information from various papers.
You can get access to the lists of talks, wth the corresponding slides, here.
I have read a few ones already. The first was the one given by Howava. I was greatly interested in reading how he defended his theory against the recent papers with showed the problems of renormalizability it seems to actually has, despite of being power counting renormalizable. Well, I didn't see any mention of it. The slide talks about the "foundational" papers on the subject and explains it's relation to the M2 brane of M-theory, to the CDT (causal dynamics triangulations) result that in the short length the effective dimension of space time is near 2, and that his theory resembles that, and a few other topics. I find specially curious that one of the motivations for his theory is that string theory violates Lorentz symmetry. Well, I am not sure why he says that, but certainly said without further explanation looks weird. It is a pity that there was not live streaming, nor non-live videos, of the talks so one can't see what questions people made him.
About the F-theory GUT's there were three talks. One from Vafa. It's ppt (than not pdf) is very schematic and without some previous knowledge on the subject I am not sure how much information one can get from it. Anyway, if one reads the papers I cited in my post about F-theory for non experts maybe he could get a much better understanding. Vafa makes a decent work explaining the two foundational papers, the paper in cosmology, and the paper in LHC footprints, that I have read. It also talks about some papers I haven't read, as for example the ones in gauge mediation (although I had read some resumes of the results). The conclusions seem to be that there are two clear predictions from their models. One, in cosmology, is that the dark matter candidate is the gravitino. that rules out models on WIMPS and implies that ATIC, PAMELA and similar results that seems to indicate an anomalous ratio of positrons over electrons over certain ranks of energies would have astrophysical origins. Or not exit at all. Recent results from FERMI/GLAST seem to contradcit ATIC and PAMELA (see, for example this post by Jester, in resonances blog) would agree with this prediction.
The other prediction mentioned on the slide is that there will be some charged track on the LHC leaving the detector. It would be due to the NLSP whose lifetime, 10^1-4 secs, is long enough to allow it scape from the detector.
There are two more talks about F-theory. One by Sakura Schafer-Namek. I have read it but from all the part related to spectral covers I coudn't get any useful informrmation. I simply don't know enough form that mathemathical topic. The other paper in F-theory is the one by Jonathan Heckman. It is centred in flavor hierarchies for quarks and leptons. Well, an interesting topic for sure, but not my favourite one. Anyway the slide is good enough to get some general idea of the topic from it.
Another paper I read is the one of Strominger about the KERR/CFT correspondence. About that topic I only had read a paper dated from the last summer. Well, I am not sure if too much progress has been achieved so far neither I have clear whether the whole field is terribly significant, but possibly that is my fault.
Possibly the most awaited paper was the one from Nima-Arkani-Hamed about twistors and the S-Matrix. There are rumorology out there saying that it's not a paper in string theory but an attempt to create some kind of supersymmetric GUT diferent from string theory. I haven't still read the slide and I can't say anything about. But for sure it is a theory that many people will discuses sooner or later, possibly when the actual paper on the subject would be out.
I'll possibly read more slides later, but I am not sure if I will post about them. But everybody can try to rad the linked slides by themselves. There are good choices that anyone with a decent basic on high energy physics could get some amount of info from them.
UPDATE: In a thread in physicis forums someone, seemengly well informed, said that actually Horava recognized the problems recently found in his theory in his talk as strings 2009. Also the same physic forums poster explained that the actual problems where that one couldn't decouplee the gosths from the theory. Curiosulsly that has lead to a posible reinterpretation of that gosths as dark matter. I have not read the relevant papers but at first sight that looks very bizaree. Gosths are negative norm states tht usually appear in the quantizationo of gauge theories as intermediate states that can be shown not to appear in external legs, i.e., are no observabbles. Toclaims thatusually unwanted negative normed states can go in external lines and actually represent viable particles (in the form of dark matter) seems like one could try to do the same thing for any theory and one wouldn't need gauge theories. I suppose that there will be something special in that gosths that make them diferent from the usual ones and permits people doing such conjectures, but, as I said, looks an a priory contravied claim.
P.S. I am looking for an easier way to use LaTeX in this blog that the one I am using (writing the latex code in the url of an image generated by an external LaTeX server). If I don't find a good solution I would seriously consider the option to migrate this blog to wordpress where writing LaTeX is "natively" supported (that's the reason I make an extensive use of it in my other blog).
Showing posts with label quantum gravities. Show all posts
Showing posts with label quantum gravities. Show all posts
Monday, July 13, 2009
Monday, April 13, 2009
Horava´s quantum gravity
I have mentioned many approaches to quantum gravity, other than string theory, in this blog.
Besides LQG none of them has had major success in attracting people to do research in it. Now, at least it seems so, there is a new option, the so called Quantum Gravity at a Lifshitz Point initiated by Peter Horava, a well known string theorist (remember the Horawa-Witten model of heterotic string theory). I had the first new about it in the Lubos blog, but since them a few other papers have appeared. As far as another string theory minirevolution is going on (F-theory GUTs) which is leading to , seemengly, actual predictions testable in the LHC, as well as, maybe, in cosmology, I have had not time to read these articles, beyond an slight overview. I will use this entry mainly to keep track of the actual papers and also to encourage possible readers of this blog to investigate about them.
I will limit, so, to link some of the papers and paste the abstracts. Just to say that the theory will probably be of the liking of the people who likes condensed matter and critical phenomena.
This was the firs paper, Quantum Gravity at a Lifshitz Point. This is the abstract:
We present a candidate quantum field theory of gravity with dynamical critical
exponent equal to z = 3 in the UV. (As in condensed matter systems, z measures the degree
of anisotropy between space and time.) This theory, which at short distances describes
interacting nonrelativistic gravitons, is power-counting renormalizable in 3 + 1 dimensions.
When restricted to satisfy the condition of detailed balance, this theory is intimately related
to topologically massive gravity in three dimensions, and the geometry of the Cotton tensor.
At long distances, this theory flows naturally to the relativistic value z = 1, and could
therefore serve as a possible candidate for a UV completion of Einstein’s general relativity
or an infrared modification thereof. The effective speed of light, the Newton constant and
the cosmological constant all emerge from relevant deformations of the deeply nonrelativistic
z = 3 theory at short distances
This is the second one: Spectral Dimension of the Universe in Quantum Gravity at a Lifshitz Point, and this is the abstract:
We extend the definition of “spectral dimension” (usually defined for fractal and
lattice geometries) to theories on smooth spacetimes with anisotropic scaling. We show that
in quantum gravity dominated by a Lifshitz point with dynamical critical exponent z in D+1
spacetime dimensions, the spectral dimension of spacetime is equal to
ds = 1 + D/z
In the case of gravity in 3 + 1 dimensions presented in arXiv:0901.3775, which is dominated
by z = 3 in the UV and flows to z = 1 in the IR, the spectral dimension of spacetime flows
from ds = 4 at large scales, to ds = 2 at short distances. Remarkably, this is the qualitative
behavior of ds found numerically by Ambjørn, Jurkiewicz and Loll in their causal dynamical
triangulations approach to quantum gravity
The next article is not written by Horova, the authors are Tomohiro Takahashi and Jiro Soda. The paper is this:Chiral Primordial Gravitational Waves from a Lifshitz Point. This is the abstract:
We study primordial gravitational waves produced during inflation in quantum gravity at a Lifshitz
point proposed by Hoˇrava. Assuming power-counting renormalizability, foliation preserving
diffeomorphism invariance, and the condition of detailed balance, we show that primordial gravitational
waves are circularly polarized due to parity violation. The chirality of primordial gravitational
waves is a quite robust prediction of quantum gravity at a Lifshitz point which can be tested through
observations of cosmic microwave background radiation and stochastic gravitational waves.
I find this one particularly important because it claims that it has a measurable prediction that could falsify (or give credit to) the theory.
The last one is neither written by Horava, the authors are H. L¨u †⋆, Jianwei Mei † and C.N. Pope. The paper is: Solutions to Horava Gravity
And the abstract is:
Recently Horava proposed a non-relativistic renormalisable theory of gravitation, which
reduces to Einstein’s general relativity at large distances, and that may provide a candidate
for a UV completion of Einstein’s theory. In this paper, we derive the full set of equations
of motion, and then we obtain spherically symmetric solutions and discuss their properties.
We also obtain the Friedman-Lemaitre-Robertson-Walker cosmological metric.
I would advise the readers of this blog to read the entries in the other blogs that I have linked in this page because the last month there have been many many interesting things that are worth reading. Maybe I will make a post resuming them.
Besides LQG none of them has had major success in attracting people to do research in it. Now, at least it seems so, there is a new option, the so called Quantum Gravity at a Lifshitz Point initiated by Peter Horava, a well known string theorist (remember the Horawa-Witten model of heterotic string theory). I had the first new about it in the Lubos blog, but since them a few other papers have appeared. As far as another string theory minirevolution is going on (F-theory GUTs) which is leading to , seemengly, actual predictions testable in the LHC, as well as, maybe, in cosmology, I have had not time to read these articles, beyond an slight overview. I will use this entry mainly to keep track of the actual papers and also to encourage possible readers of this blog to investigate about them.
I will limit, so, to link some of the papers and paste the abstracts. Just to say that the theory will probably be of the liking of the people who likes condensed matter and critical phenomena.
This was the firs paper, Quantum Gravity at a Lifshitz Point. This is the abstract:
We present a candidate quantum field theory of gravity with dynamical critical
exponent equal to z = 3 in the UV. (As in condensed matter systems, z measures the degree
of anisotropy between space and time.) This theory, which at short distances describes
interacting nonrelativistic gravitons, is power-counting renormalizable in 3 + 1 dimensions.
When restricted to satisfy the condition of detailed balance, this theory is intimately related
to topologically massive gravity in three dimensions, and the geometry of the Cotton tensor.
At long distances, this theory flows naturally to the relativistic value z = 1, and could
therefore serve as a possible candidate for a UV completion of Einstein’s general relativity
or an infrared modification thereof. The effective speed of light, the Newton constant and
the cosmological constant all emerge from relevant deformations of the deeply nonrelativistic
z = 3 theory at short distances
This is the second one: Spectral Dimension of the Universe in Quantum Gravity at a Lifshitz Point, and this is the abstract:
We extend the definition of “spectral dimension” (usually defined for fractal and
lattice geometries) to theories on smooth spacetimes with anisotropic scaling. We show that
in quantum gravity dominated by a Lifshitz point with dynamical critical exponent z in D+1
spacetime dimensions, the spectral dimension of spacetime is equal to
ds = 1 + D/z
In the case of gravity in 3 + 1 dimensions presented in arXiv:0901.3775, which is dominated
by z = 3 in the UV and flows to z = 1 in the IR, the spectral dimension of spacetime flows
from ds = 4 at large scales, to ds = 2 at short distances. Remarkably, this is the qualitative
behavior of ds found numerically by Ambjørn, Jurkiewicz and Loll in their causal dynamical
triangulations approach to quantum gravity
The next article is not written by Horova, the authors are Tomohiro Takahashi and Jiro Soda. The paper is this:Chiral Primordial Gravitational Waves from a Lifshitz Point. This is the abstract:
We study primordial gravitational waves produced during inflation in quantum gravity at a Lifshitz
point proposed by Hoˇrava. Assuming power-counting renormalizability, foliation preserving
diffeomorphism invariance, and the condition of detailed balance, we show that primordial gravitational
waves are circularly polarized due to parity violation. The chirality of primordial gravitational
waves is a quite robust prediction of quantum gravity at a Lifshitz point which can be tested through
observations of cosmic microwave background radiation and stochastic gravitational waves.
I find this one particularly important because it claims that it has a measurable prediction that could falsify (or give credit to) the theory.
The last one is neither written by Horava, the authors are H. L¨u †⋆, Jianwei Mei † and C.N. Pope. The paper is: Solutions to Horava Gravity
And the abstract is:
Recently Horava proposed a non-relativistic renormalisable theory of gravitation, which
reduces to Einstein’s general relativity at large distances, and that may provide a candidate
for a UV completion of Einstein’s theory. In this paper, we derive the full set of equations
of motion, and then we obtain spherically symmetric solutions and discuss their properties.
We also obtain the Friedman-Lemaitre-Robertson-Walker cosmological metric.
I would advise the readers of this blog to read the entries in the other blogs that I have linked in this page because the last month there have been many many interesting things that are worth reading. Maybe I will make a post resuming them.
Tuesday, November 18, 2008
The fqxi time essay contest
At least some readers of this blog will read some of the links, and also other blogs (and phorums) not linked here.
If so it is very probable that they will already know about the fqxi foundation and it´s content about time nature. The actual webpage of the essay is this .
The content, or at least many of the papers appeared when he id the post, has been judged by Jonh Baez, in a post in the n-cathegory coffeé as "crackpot". I find that crackpot obession somewhat,well, crackpot. For example Jonh Baez itsefl, author of the crackpot indicator belongs (whether if he wants or not) to the LQG comunity which is considered crackpot. I think that this crackpotery issue is becoming totally nonsensic and very boring.
Said this, the truth is that some of the published articles are very philosofical (to classify them in some way) and very non phyisical. And, certainly, up to now no heavy weigth of string theory has sended an essay (and he is not expected). Of course there are good reasons for that which can be guessed easilly by anyone who thinks about that a bit.
That is not necesarilly a bad thing and invites to physicians not belonging to the top notch spheres to participate, simply because they have some reasonable possibilities to win (or at least to get some of the secondary primzs). In fact I am aware that some bloggers such as Mitta pitkannen, Cristinhe Dantas (I must add therem egregium to the links sections some day of these), Carle Brannen and, may be, Kea has sended their essays. Among the best known phyisicians are Carlo Rovelly, Rodolfo Gambini and Clauss Kiefer.
In a diferent category i would point to Hrvoje Nikoli, who has some papers in arxiv about string theory (certainly a bit outside of the mainstream of string theory) and, one of the last essays, by his biographycal note I think that he could be considered as an cosmologist in the orbit of the Randall-Sumdrum brane worlds, that is, in a position related to the mainstream of stringy cosmologists.
Also I will make a separate mention of an Spanish competitor, Venerando Solis Barrado. I must say that for good o bad I am somehat responsible of his participation because it was bye mans of me that he knew of the conent. Al i can say is (almost) good luck ;-).
I have readed some of the papers, and I have intention to read most of them, at least the ones written by known people and that have recibed votes and/or responses.
And now the delicate question. Will I participate?
Well, I wasaware for some years on the problem of canonical gravity (wheler-de Witt or LQG) with the problem of no evolution in time. I had readed their guessed solution by means of the relational time. I had readed (as is reflected by some posts on the topic) about wormholes. Womrmholes can work as time machines and peoplee working in wormhles usually make discusion about the topic, including non womrholmy time machines. I had readed about tachyons outside string theory (I guess I have readed most of the relevants papers) and something about tachyon condensation. I had readed various books by Illya Prigogyne aout the arrow of time. And, still worst, I was demanded by a friend to invent a guessed explanation about the amercian film "the butterfly effects" which covers the topic of time travel. That resulted in a toy theory which I like to call "cronoquantum mechanics".
I think that that means that I have a reasonable background in the topic. Reading some of the already published articles I see that I was not aware of some things, but not of too many. Well, physic of time and specially of time travel is a funny topic for fans of science fiction, and I am one of them. That is mainly the reason I have learned about that topics. For a while I tried to develope a litle bit more the "quantum cronomechanics" but I ended up bored of the topic (and I must add that it is a good source to get a headcache to try to think too much about it).
Even thought now there is this content and they pay a good money to the winner, and also there are some secondary prizes which result interesting. Certainly I can live withouth them, but, hey, this blog is freelance quantum gravity, and my other blog is freelance science. The name of the blogs is inspired in the freelance workers in the infomaic market. The idea is easy to understand. In the informatic buisenes there are people who don´t like to work in fixed position in an enterprise (or governement position) and prefer to work as freelances from their homes. I must say that there is a wide market for that sector and that many people in that sector are formers top notch programmers working on well known enterprises who decided to be iindependents.
The physic market, quite on the countrary, is based on an academic/funtionary paradigm. People want to get positions at universities and research institutes and devote to that purpose a lot of effort, from the very begining of their universiary studies. Of course there are good reasons for that, to begin with, tradition.
But if one analyzes the situation the fact is that the model has some serious drawbacks. The academic positions are got by people who has a reasonable expectation of beeing in the charge for around 40 years. That means that he will be blocking that postion for around 8 promotions of students (assuming a promotions takes a romedia of 5 years). It is important to note that universitary positions are permanent and that new universities (specially ones wich need theoretical physics working in that topic) are not created too ofthem.
That means that there is very few possibilities to get a permanent possition. Once this is realized there are two possibilties. To begin a fierce competition from the very begining triying to get the best possible academic quailifications and triying to convence to your teacher that you are the smartes guy since Newton. It is important to note that getting the best qualifitaions relay a lot in learning `rmarly what your teacher decides that is important about a topic. If you decide that you are interested in other aspects of the topic and study them it results ofthem in a worst qualification.
Other possible way is to organize life in a way that you could have reasonable amounts of time working, when possible, in topics related to physics. You can use that time to learn what you prefer. I certainly have opted always by this way. Of course that doesn´t mean that I reject the possibilty to do a PhD and all that. But it would be in maths, I belive that I know too much physics to do a PhD in physics (yeah, it is contradictory).
Well, all this las diegressions it to congratulate the fqxi foundation for their iniciative. To publish a paper requires to do some very specific work,no always fun. And I like to be payed for my work (at least if it is a good work). I mean, I am sure that now I could publish papers on string theory or LQG. Certainlly they would, very probably, would be not too goood papers (ate least not now). But even that papers would requiere good amounts of time. And I wouldn´t get any rewar for them. So i see not point in triying to publish anything if I am not almost sure that it is not crystal clear that would be a top notch paper (a definitve prrof of the AdS/CFT conjecture, the millenium prize in the yang-mills masss gap or things like that). Certainly I think it is not a good idea to depend on that possibiilities.
On the other sie this more modests fqxi prizes are very interesting because they are a more realistic objective which can serve to animte people to do good papers that he woind´t do otherwise.
Ok, a lot of sociology. Am I going to publish a paper in that content?. Well, today I have had an ocurrence for a very nice title for the paper, It would be a mesh to not use it. Most important, I am really tired of the topic of time physics and I think that the best way to never to have to woory about the topic is to publish the paper. But I still dind´t answer (I know that people who know me personally have realized that point perfectlly), shal I try to send a paper to the content? Well, may be, if I have time... ;-).
Update: The deadline for the submition of the paper is over and I still haven´t it ready. Un unfortunate minor illnes has had me down (for purposes of doing serious work) for around a week and that is bad for someone like me that belongs to the "wait last minute" category. If there is some flexibility byt the fqxi comite maybe still it could be some chance. If not I´ll try to put the paper somewhere (once it is finished) in order people could read it (if any is interested).
If so it is very probable that they will already know about the fqxi foundation and it´s content about time nature. The actual webpage of the essay is this .
The content, or at least many of the papers appeared when he id the post, has been judged by Jonh Baez, in a post in the n-cathegory coffeé as "crackpot". I find that crackpot obession somewhat,well, crackpot. For example Jonh Baez itsefl, author of the crackpot indicator belongs (whether if he wants or not) to the LQG comunity which is considered crackpot. I think that this crackpotery issue is becoming totally nonsensic and very boring.
Said this, the truth is that some of the published articles are very philosofical (to classify them in some way) and very non phyisical. And, certainly, up to now no heavy weigth of string theory has sended an essay (and he is not expected). Of course there are good reasons for that which can be guessed easilly by anyone who thinks about that a bit.
That is not necesarilly a bad thing and invites to physicians not belonging to the top notch spheres to participate, simply because they have some reasonable possibilities to win (or at least to get some of the secondary primzs). In fact I am aware that some bloggers such as Mitta pitkannen, Cristinhe Dantas (I must add therem egregium to the links sections some day of these), Carle Brannen and, may be, Kea has sended their essays. Among the best known phyisicians are Carlo Rovelly, Rodolfo Gambini and Clauss Kiefer.
In a diferent category i would point to Hrvoje Nikoli, who has some papers in arxiv about string theory (certainly a bit outside of the mainstream of string theory) and, one of the last essays, by his biographycal note I think that he could be considered as an cosmologist in the orbit of the Randall-Sumdrum brane worlds, that is, in a position related to the mainstream of stringy cosmologists.
Also I will make a separate mention of an Spanish competitor, Venerando Solis Barrado. I must say that for good o bad I am somehat responsible of his participation because it was bye mans of me that he knew of the conent. Al i can say is (almost) good luck ;-).
I have readed some of the papers, and I have intention to read most of them, at least the ones written by known people and that have recibed votes and/or responses.
And now the delicate question. Will I participate?
Well, I wasaware for some years on the problem of canonical gravity (wheler-de Witt or LQG) with the problem of no evolution in time. I had readed their guessed solution by means of the relational time. I had readed (as is reflected by some posts on the topic) about wormholes. Womrmholes can work as time machines and peoplee working in wormhles usually make discusion about the topic, including non womrholmy time machines. I had readed about tachyons outside string theory (I guess I have readed most of the relevants papers) and something about tachyon condensation. I had readed various books by Illya Prigogyne aout the arrow of time. And, still worst, I was demanded by a friend to invent a guessed explanation about the amercian film "the butterfly effects" which covers the topic of time travel. That resulted in a toy theory which I like to call "cronoquantum mechanics".
I think that that means that I have a reasonable background in the topic. Reading some of the already published articles I see that I was not aware of some things, but not of too many. Well, physic of time and specially of time travel is a funny topic for fans of science fiction, and I am one of them. That is mainly the reason I have learned about that topics. For a while I tried to develope a litle bit more the "quantum cronomechanics" but I ended up bored of the topic (and I must add that it is a good source to get a headcache to try to think too much about it).
Even thought now there is this content and they pay a good money to the winner, and also there are some secondary prizes which result interesting. Certainly I can live withouth them, but, hey, this blog is freelance quantum gravity, and my other blog is freelance science. The name of the blogs is inspired in the freelance workers in the infomaic market. The idea is easy to understand. In the informatic buisenes there are people who don´t like to work in fixed position in an enterprise (or governement position) and prefer to work as freelances from their homes. I must say that there is a wide market for that sector and that many people in that sector are formers top notch programmers working on well known enterprises who decided to be iindependents.
The physic market, quite on the countrary, is based on an academic/funtionary paradigm. People want to get positions at universities and research institutes and devote to that purpose a lot of effort, from the very begining of their universiary studies. Of course there are good reasons for that, to begin with, tradition.
But if one analyzes the situation the fact is that the model has some serious drawbacks. The academic positions are got by people who has a reasonable expectation of beeing in the charge for around 40 years. That means that he will be blocking that postion for around 8 promotions of students (assuming a promotions takes a romedia of 5 years). It is important to note that universitary positions are permanent and that new universities (specially ones wich need theoretical physics working in that topic) are not created too ofthem.
That means that there is very few possibilities to get a permanent possition. Once this is realized there are two possibilties. To begin a fierce competition from the very begining triying to get the best possible academic quailifications and triying to convence to your teacher that you are the smartes guy since Newton. It is important to note that getting the best qualifitaions relay a lot in learning `rmarly what your teacher decides that is important about a topic. If you decide that you are interested in other aspects of the topic and study them it results ofthem in a worst qualification.
Other possible way is to organize life in a way that you could have reasonable amounts of time working, when possible, in topics related to physics. You can use that time to learn what you prefer. I certainly have opted always by this way. Of course that doesn´t mean that I reject the possibilty to do a PhD and all that. But it would be in maths, I belive that I know too much physics to do a PhD in physics (yeah, it is contradictory).
Well, all this las diegressions it to congratulate the fqxi foundation for their iniciative. To publish a paper requires to do some very specific work,no always fun. And I like to be payed for my work (at least if it is a good work). I mean, I am sure that now I could publish papers on string theory or LQG. Certainlly they would, very probably, would be not too goood papers (ate least not now). But even that papers would requiere good amounts of time. And I wouldn´t get any rewar for them. So i see not point in triying to publish anything if I am not almost sure that it is not crystal clear that would be a top notch paper (a definitve prrof of the AdS/CFT conjecture, the millenium prize in the yang-mills masss gap or things like that). Certainly I think it is not a good idea to depend on that possibiilities.
On the other sie this more modests fqxi prizes are very interesting because they are a more realistic objective which can serve to animte people to do good papers that he woind´t do otherwise.
Ok, a lot of sociology. Am I going to publish a paper in that content?. Well, today I have had an ocurrence for a very nice title for the paper, It would be a mesh to not use it. Most important, I am really tired of the topic of time physics and I think that the best way to never to have to woory about the topic is to publish the paper. But I still dind´t answer (I know that people who know me personally have realized that point perfectlly), shal I try to send a paper to the content? Well, may be, if I have time... ;-).
Update: The deadline for the submition of the paper is over and I still haven´t it ready. Un unfortunate minor illnes has had me down (for purposes of doing serious work) for around a week and that is bad for someone like me that belongs to the "wait last minute" category. If there is some flexibility byt the fqxi comite maybe still it could be some chance. If not I´ll try to put the paper somewhere (once it is finished) in order people could read it (if any is interested).
Wednesday, October 29, 2008
Non quantum gravity and dark matter
I keep reading Physicsforums, specially the beyond the standard model forum. Recently there was a discussion about a new proposal appeared in arxiv arguing that maybe gravity wouldn’t need to be quantized after all.
The paper in question, authored by Stephen Boughn is this.
It is a very clear paper where the usual assumptions are reviewed. As is commonly known we actually have a quantum theory, the standard model, which describes all known interactions but gravity. The best available description, experimentally supported, of gravity is Einstein theory of gravity, which is a non quantum theory.
In order to approach both theories one can begin by quantizing the standard model in the curved backgrounds of general relativity, instead of doing it in plain Minkowsky space (see my previous post for an easy introduction- in Spanish, sorry for non Spanish people-).
The next step one could try is simply to consider the gravitational field created by the averaged value of the energy momentum tensor and forget the idea of quantizing gravity at all. That is to replace:
1.
by:
2.
This proposal has many well known problems, both theoretical and practical. The author discuss them in the chapter 6 of his paper. Consider a state of matter, with probability !/2 of being in O1 region of space time and a probability ½ of being in a disjoint region O2. If you use equation 2 you get a gravitational field appropriate for matter being distributed in both regions. If later a measurement is made and the state is resolved to one of the Oi then the gravitational field would change in a discontinuous and acausal manner.
The key point of the paper is to keep eqt. 1 as valid and forget about 2. Of course one can’t do it without further assumptions. The author establish that the energy momentum tensor must satisfy the following prerequisite. In the language of decoherence theory, that the system is in a decoherent, mixed quantum state for it is only then that the probability predictions of quantum theory agree with those of classical physics. (You can read about decoherence in, for example, this paper by Lubos Motl.).
This assumption immediately implies another one, that non-localized, coherent quantum systems are not sources of gravity. That sounds as a very hard assumption, but the author argues that It will turn out that for microsopic systems, in which quantum coherence is most commonly observed, the effects of gravity are, in principle, unobservable. For larger macroscopic systems, decoherence is the norm and classical stress-energy is well defined. This leaves open the question of gravitational interactions of mesoscopic, coherent sytsems.
After that he goes through some chapters reviewing the detectability of possible quantum gravity phenomena. He begins, in chapter two, considering the detectability of gravitons. Remember that a graviton should be the quanta that would mediate gravity interactions if one insist in doing quantum gravity in a particle physicist like way. This chapter is very well written, and it relates the gravitons to gravity waves. Note that one of the authors research activities is precisely in the field of experimental detection of gravity waves so he can be considered an authority in that particular.
In chapter 3 he dwells with gravity and quantum interference, that is, double slit like thought experiments. He concludes the existence of a conditions that must be satisfied for a gravitational measurement to be made that will sufficiently localize the incident particle so as to destroy the quantum interference which are stated in terms of the separation of the two slits, r, the acceleration of the test mass at, the velocity of the incoming particle vi. The actual conditions are:
r >¯h r^2/Gm^3 (here ¯h is h bar, i.e. h/2π)
t >¯h^3 / G2m^5
at < G^3m7 /¯h^4
vi < Gm^2 / ¯h
If the conditions are not satisfied, the gravitational interaction is insufficient to detect the incident particle and quantum interference remains intact..He concludes that for quantum coherent systems with masses less than ∼ 10^7mp (mp=Planck mass), there is not a measurable gravitational effect that would compromise their coherence. He does further analysis and get further restrictions. The conclusion of the arguments is that the question of whether or not coherent quantum systems are sources of gravity is unanswerable for systems with masses < 10^10 mp. That leaves unanswered the question of mesoscopic systems, which he analyzes later.
The chapter 4 is a continuation, in a certain sense, of the previous. The most interesting is the chapter 5 where he fully analyzes the central issue of the paper. The key point, if I rightly understand is the following statement:
“Because macroscopic systems
invariably undergo decoherence on very short time scales, they behave as they would
in a classical world, i.e., no quantum interference effects.”
Or stated, together with another claims of the chapter, in a more generic way it could be said: “the experimental data available to date only takes account of interactions between matter systems in a decoherent state.”
That raise the question of what would be the behaviour of macroscopic, or at least, mesoscopic, systems which are in coherent states. He talks about the copper pairs in superconductivity, Bose-Einstein condensates and systems like that. Here I would add a few things. A few years ago an condensate-matter physicist, Podkeltnov, made a claim, in a press conference, about some kind of gravity shielding that appeared unexpectectly in experiments which implied some kind of high temperature superconducting devices. He didn’t provide all the details of the experimental device and ulterior attempts to reply the experiment, based on the available data, are until now unsuccessful. Later Podkelnov he improved the experiment and even tried to conjecture an explanation. His argument was related to the suppression of Fourier modes of gravity because of coupling of the Landau-Ginzburg lagrangian which could be used to describe the superconductor to the energy of the cosmological constant. Certainly the “non quantum gravity” proposal could be considered as an alternative explanation if one would try to insist in explaining an effect non firmly established experimentally, of course.
To conclude my review of this proposal I’ll mention a few problems that the own Stephen Boughn recognizes. The main one, in my opinion, is that if a coherent system exchange momentum with a coherent one, and later becomes non coherent his proposal could lead to a violation of momentum conservation. Another one is a legitimate criticism of the conjecture put forward in this paper is its lack of predictive power. Except possibly in the case of the coherent to decoherent transitions in mesoscopic systems, and even in these cases the conjecture makes no specific prediction, the nonquantum conjecture makes no additional predictions that can not already be made by quantum theory and general relativity. There are some more concerns, that the author acknowledge in the final chapter and I´ll not talk here about them.
Let’s go now with the next topic of this post, dark matter. A few weeks after this paper appeared Sean Carrol in his blog, cosmic variance, made this post. Soon there was a reply by Lubos Motl here.
They are very interesting posts in their own. But I bring them here because it is stated there that dark matter, if it interact only by means of gravity with itself, and the rest of the universe, would decohere very solowly. In fact, if the non quantum gravity proposal would be taken to it’s full consequences it could be expected that it wouldn’t decohere at all. But if so, it wouldn’t interact gravitatorilly at all. That is a very bad thing because dark matter is postulated to explain unobserved mass in the universe which accounts the observed rate of cosmological expansion.
In fact, in a very recent paper it is discussed the possibility that dark matter could not exist, or, at least, not be the main responsible of some experimental data. The paper is this. It is discussed in a blog entry by Lubos Motl here. Quickly, the idea is that a field associated to string theory, could take a nonvacuum expected value and that if particles are actually strings, would couple to it resulting in a Lorentz type force which would explain the problem with the way galaxies rotate in an alternative way to the usual explanations of dark matter of MOND (modified newtoninan dynamics). If this non quantum gravity proposal would be taken seriously the stringy paper would gain additional value because dark matter, even if it exists, could not interact gravitationally, or at least not too much.. Of course if we accept the nonquantum gravity proposal string theory would loose one of it’s more important reason to exist, it’s status as a quantum theory of gravity and it would have to be questioned if it’s explanation of galaxies rotations could be still accepted.
In fact I admit that I actually didn’t do the actual calculations of exactly how much dark matter would interactuate gravitationally if the non quantum gravity proposal would be truth. I find surprising that the author, Stephen Boughn, wouldn’t consider it in his paper when he claims that he is actually working on cosmology, but, of course, he could easily not have realized this lack of coherence in dark matter, which is only obvious once one is told about it, but not before.
Anyway, the paper is interesting in it’s own, even if it’s wrong, because of the review of many aspects related to gravity and it has served me to take quote of some issues that have happened in the quantum gravity world in the recent times. Hope the reader would find them interesting.
The paper in question, authored by Stephen Boughn is this.
It is a very clear paper where the usual assumptions are reviewed. As is commonly known we actually have a quantum theory, the standard model, which describes all known interactions but gravity. The best available description, experimentally supported, of gravity is Einstein theory of gravity, which is a non quantum theory.
In order to approach both theories one can begin by quantizing the standard model in the curved backgrounds of general relativity, instead of doing it in plain Minkowsky space (see my previous post for an easy introduction- in Spanish, sorry for non Spanish people-).
The next step one could try is simply to consider the gravitational field created by the averaged value of the energy momentum tensor and forget the idea of quantizing gravity at all. That is to replace:
1.
by:
2.
This proposal has many well known problems, both theoretical and practical. The author discuss them in the chapter 6 of his paper. Consider a state of matter, with probability !/2 of being in O1 region of space time and a probability ½ of being in a disjoint region O2. If you use equation 2 you get a gravitational field appropriate for matter being distributed in both regions. If later a measurement is made and the state is resolved to one of the Oi then the gravitational field would change in a discontinuous and acausal manner.
The key point of the paper is to keep eqt. 1 as valid and forget about 2. Of course one can’t do it without further assumptions. The author establish that the energy momentum tensor must satisfy the following prerequisite. In the language of decoherence theory, that the system is in a decoherent, mixed quantum state for it is only then that the probability predictions of quantum theory agree with those of classical physics. (You can read about decoherence in, for example, this paper by Lubos Motl.).
This assumption immediately implies another one, that non-localized, coherent quantum systems are not sources of gravity. That sounds as a very hard assumption, but the author argues that It will turn out that for microsopic systems, in which quantum coherence is most commonly observed, the effects of gravity are, in principle, unobservable. For larger macroscopic systems, decoherence is the norm and classical stress-energy is well defined. This leaves open the question of gravitational interactions of mesoscopic, coherent sytsems.
After that he goes through some chapters reviewing the detectability of possible quantum gravity phenomena. He begins, in chapter two, considering the detectability of gravitons. Remember that a graviton should be the quanta that would mediate gravity interactions if one insist in doing quantum gravity in a particle physicist like way. This chapter is very well written, and it relates the gravitons to gravity waves. Note that one of the authors research activities is precisely in the field of experimental detection of gravity waves so he can be considered an authority in that particular.
In chapter 3 he dwells with gravity and quantum interference, that is, double slit like thought experiments. He concludes the existence of a conditions that must be satisfied for a gravitational measurement to be made that will sufficiently localize the incident particle so as to destroy the quantum interference which are stated in terms of the separation of the two slits, r, the acceleration of the test mass at, the velocity of the incoming particle vi. The actual conditions are:
r >¯h r^2/Gm^3 (here ¯h is h bar, i.e. h/2π)
t >¯h^3 / G2m^5
at < G^3m7 /¯h^4
vi < Gm^2 / ¯h
If the conditions are not satisfied, the gravitational interaction is insufficient to detect the incident particle and quantum interference remains intact..He concludes that for quantum coherent systems with masses less than ∼ 10^7mp (mp=Planck mass), there is not a measurable gravitational effect that would compromise their coherence. He does further analysis and get further restrictions. The conclusion of the arguments is that the question of whether or not coherent quantum systems are sources of gravity is unanswerable for systems with masses < 10^10 mp. That leaves unanswered the question of mesoscopic systems, which he analyzes later.
The chapter 4 is a continuation, in a certain sense, of the previous. The most interesting is the chapter 5 where he fully analyzes the central issue of the paper. The key point, if I rightly understand is the following statement:
“Because macroscopic systems
invariably undergo decoherence on very short time scales, they behave as they would
in a classical world, i.e., no quantum interference effects.”
Or stated, together with another claims of the chapter, in a more generic way it could be said: “the experimental data available to date only takes account of interactions between matter systems in a decoherent state.”
That raise the question of what would be the behaviour of macroscopic, or at least, mesoscopic, systems which are in coherent states. He talks about the copper pairs in superconductivity, Bose-Einstein condensates and systems like that. Here I would add a few things. A few years ago an condensate-matter physicist, Podkeltnov, made a claim, in a press conference, about some kind of gravity shielding that appeared unexpectectly in experiments which implied some kind of high temperature superconducting devices. He didn’t provide all the details of the experimental device and ulterior attempts to reply the experiment, based on the available data, are until now unsuccessful. Later Podkelnov he improved the experiment and even tried to conjecture an explanation. His argument was related to the suppression of Fourier modes of gravity because of coupling of the Landau-Ginzburg lagrangian which could be used to describe the superconductor to the energy of the cosmological constant. Certainly the “non quantum gravity” proposal could be considered as an alternative explanation if one would try to insist in explaining an effect non firmly established experimentally, of course.
To conclude my review of this proposal I’ll mention a few problems that the own Stephen Boughn recognizes. The main one, in my opinion, is that if a coherent system exchange momentum with a coherent one, and later becomes non coherent his proposal could lead to a violation of momentum conservation. Another one is a legitimate criticism of the conjecture put forward in this paper is its lack of predictive power. Except possibly in the case of the coherent to decoherent transitions in mesoscopic systems, and even in these cases the conjecture makes no specific prediction, the nonquantum conjecture makes no additional predictions that can not already be made by quantum theory and general relativity. There are some more concerns, that the author acknowledge in the final chapter and I´ll not talk here about them.
Let’s go now with the next topic of this post, dark matter. A few weeks after this paper appeared Sean Carrol in his blog, cosmic variance, made this post. Soon there was a reply by Lubos Motl here.
They are very interesting posts in their own. But I bring them here because it is stated there that dark matter, if it interact only by means of gravity with itself, and the rest of the universe, would decohere very solowly. In fact, if the non quantum gravity proposal would be taken to it’s full consequences it could be expected that it wouldn’t decohere at all. But if so, it wouldn’t interact gravitatorilly at all. That is a very bad thing because dark matter is postulated to explain unobserved mass in the universe which accounts the observed rate of cosmological expansion.
In fact, in a very recent paper it is discussed the possibility that dark matter could not exist, or, at least, not be the main responsible of some experimental data. The paper is this. It is discussed in a blog entry by Lubos Motl here. Quickly, the idea is that a field associated to string theory, could take a nonvacuum expected value and that if particles are actually strings, would couple to it resulting in a Lorentz type force which would explain the problem with the way galaxies rotate in an alternative way to the usual explanations of dark matter of MOND (modified newtoninan dynamics). If this non quantum gravity proposal would be taken seriously the stringy paper would gain additional value because dark matter, even if it exists, could not interact gravitationally, or at least not too much.. Of course if we accept the nonquantum gravity proposal string theory would loose one of it’s more important reason to exist, it’s status as a quantum theory of gravity and it would have to be questioned if it’s explanation of galaxies rotations could be still accepted.
In fact I admit that I actually didn’t do the actual calculations of exactly how much dark matter would interactuate gravitationally if the non quantum gravity proposal would be truth. I find surprising that the author, Stephen Boughn, wouldn’t consider it in his paper when he claims that he is actually working on cosmology, but, of course, he could easily not have realized this lack of coherence in dark matter, which is only obvious once one is told about it, but not before.
Anyway, the paper is interesting in it’s own, even if it’s wrong, because of the review of many aspects related to gravity and it has served me to take quote of some issues that have happened in the quantum gravity world in the recent times. Hope the reader would find them interesting.
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.
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.
Wednesday, November 07, 2007
Renormalizable ma non troppo
My intention for this post was to make an exposition of Horava-Witten theory and the phenomenological and cosmological theories related to them (brane universes, ekptyroptic, etc).
But a confluence of factors has decided me to make a brief post about a diferent topic. In a previous entry I talked about conformal gravity. That was a nice theory which could presumably reproduce general relativity in the appropiate limits, and it was renormalizable. I haven´t head nothing new about it since that post, but I neither have had time to search so maybe there is something going on and simply a am not aware of it.
Since them I have had notice of ¡3! diferent claims about new viable paths to quantum gravity. I´ll explain a bit about them.
The first is the effort of Martin Reuter in triying to prove that classical pure (i.e. no matter) general relativity can have an UV fixed point. I am not still very expert in the insights of Wilson and Polchinsky in the renormalization group, but the idea goes as follows. In any QFT loops usually give rise to infinite terms. To get around them you do a cutt-off and you separate the integral in a finte and an infinte part. Them you add a new term to the lagraingian (a counterterm) which generates an aditional diagram whcih cancels the divergent part of the integral. Inthis way ou can deal with any theory, renormalizable or not. But in renormalizable theories the counterterms have the same form as the original terms of the lagrangian so you can cancel the divergences by apropiate redefinitions of parameters of the lagrangian (coupling constants, masses, wavefunctions, etc). If the neccessary counterterms are not of the same form that the orignial lagrangian this resoults into the adition of new parameters of the theory. You can have by this procedure a finite theory,, but with an infinite number of free parameters, i.e., with no predicitive power. General relativity is of this type. But there is a possible scape to this problem. It could be that there are aditional aspects which constraint the values of all that necessary infinte number of parameters so all of them have the same value and the predictive power of the theory is recovered. Theories with such behaviour are said to have an UV (ultra violete) fixed point. (Wilson-Polchinsky renormalization group theory consist in many other aspects, but this is all I need of renormalization group theory to explain what´s going on here).
Martin Reuter is triying to probe that pure general relativy actually has an UV fixed point. To do so he must do some desompositoins which allow to do some numerical analisys. The results of the numerical analisys strongly suggest the existence of the UV fixed point. He also can uses that results to get some results incosmology. I havent readed too mcuh about the details so I´ll not say anything about them. What I must say is that there is a problem with all this. If matter is added to the theory it can be shown that you are adding wht are called relevant operators and that you will go out of the UV fixed point so semmengly all that effort would be poinless. In fact some people in LQG (canonical as well as spin foam versions) are triying to recover matter as some kind of topological deffects, or other, very bizarre, constructons out of pure gravity. If they would be succesfull ther would be no need to add matter and the UV fixed point would be kept. That would be very good for them because the wouold have on one hand a way to do perturbative quantum gravity from conventional Einstein gravity and on the other hand a non perturbative formulation which allows them to prove things about diferent interesting questions (pity that they don´t know how to recover conventional gerneral relativity from that non perturbvative formulation).
Well, very recenly I have had news about a theory made by Krasnov kown as "non metric quantum gravity". You can read about it here.
The presentation sounds very promising. Using Plebansky action (a very familar for LQG people alternative formulation of general relativity equivalent to it, under addecquate constraints, in the classical limit). They quantize it in somethnig resembling the well known background field method . This method was introduced in the fifties by de Witt to obtain for the first time a (quasi)covariant quantization of general relativity which gave a more rigurous tratement of the previous work of Feynman who had obtained Feynman graphs (obvious xD) for general relativity. The theory is based in separating the field to be quantized in two parts, one, the background, is not quantized and only the fuctuations about it are quantized. Ideally the nonquantizd part would be a limit in which the quantum fluctuations of the quantized part are irrelevant. It is interesting thatthis formalism later impulsed the technology wich would allow to quantize yang mills theories (in fact later better ways were found to quantize them and in the QFT textbooks usually there is no reference to the background field method). In fact there is nothing terribly special about the background field method, it is justa diferent way to do perturbative quantization. So one could ask how this would render Plebansky theory renormalizable. The asnwer is ¡it doesn´t !. If you read the article you find that contrary to the claims the theory is not renormalizable. But beofre wondering if you would make a demand to the authros about fake publicity continuate reading -if you have arrived untill here a litle more effort doesn´t matter ;)-. The trick is that they claim that they can prove that the theory has an UV fixed point (yeah, again). The advantage that I see of this aproach over the Reuter´s one is that there are not numerical calculus and artificial algoritmic constructions involved. But still the same drawback applies. They are theories of pure gravity and the UV would dissapear if matter is added (well, I guess it would be so, I cant say for sure).
There are aditional papers exploring the properties of the theory. Seemengly it can explain dark matter, quasars anomalous red shifts and such that. Pitty it can´t explain ordinary, vissible, matter.
If I must decide among these thre theories, conformal gravity, reuters approach, and kranosv non metric gravity I clearly prefer conformal gravity. It is a renormalizbel theory, without need to use fixed points, and it allows the introductoin of matter. I have readed in Jackes Distler blog some claism which maybe would be a problem for this theory, but I am not totally sure about it.
And to end I´ll say a few words about the new baby quantum gravity, or I would better say a TOE (theory of everything). It´s author is Garret Lissi. You can find his paper here. Sabine Hossfander makes a good blog post about it here so I invite to read that entry. I just will quote a phrase of Sabbine:
He neither can say anything about the quantization of gravity, renormalizability, nor about the hierarchy problem
Well, the requirements for a TOE seem to have drop a bit since the last time I watched ;).
Of courseI again reiterate that I don´t consider myself nothin remotely similar to an authority in this questions and that I invite to the reader to form his own conclusions.
But a confluence of factors has decided me to make a brief post about a diferent topic. In a previous entry I talked about conformal gravity. That was a nice theory which could presumably reproduce general relativity in the appropiate limits, and it was renormalizable. I haven´t head nothing new about it since that post, but I neither have had time to search so maybe there is something going on and simply a am not aware of it.
Since them I have had notice of ¡3! diferent claims about new viable paths to quantum gravity. I´ll explain a bit about them.
The first is the effort of Martin Reuter in triying to prove that classical pure (i.e. no matter) general relativity can have an UV fixed point. I am not still very expert in the insights of Wilson and Polchinsky in the renormalization group, but the idea goes as follows. In any QFT loops usually give rise to infinite terms. To get around them you do a cutt-off and you separate the integral in a finte and an infinte part. Them you add a new term to the lagraingian (a counterterm) which generates an aditional diagram whcih cancels the divergent part of the integral. Inthis way ou can deal with any theory, renormalizable or not. But in renormalizable theories the counterterms have the same form as the original terms of the lagrangian so you can cancel the divergences by apropiate redefinitions of parameters of the lagrangian (coupling constants, masses, wavefunctions, etc). If the neccessary counterterms are not of the same form that the orignial lagrangian this resoults into the adition of new parameters of the theory. You can have by this procedure a finite theory,, but with an infinite number of free parameters, i.e., with no predicitive power. General relativity is of this type. But there is a possible scape to this problem. It could be that there are aditional aspects which constraint the values of all that necessary infinte number of parameters so all of them have the same value and the predictive power of the theory is recovered. Theories with such behaviour are said to have an UV (ultra violete) fixed point. (Wilson-Polchinsky renormalization group theory consist in many other aspects, but this is all I need of renormalization group theory to explain what´s going on here).
Martin Reuter is triying to probe that pure general relativy actually has an UV fixed point. To do so he must do some desompositoins which allow to do some numerical analisys. The results of the numerical analisys strongly suggest the existence of the UV fixed point. He also can uses that results to get some results incosmology. I havent readed too mcuh about the details so I´ll not say anything about them. What I must say is that there is a problem with all this. If matter is added to the theory it can be shown that you are adding wht are called relevant operators and that you will go out of the UV fixed point so semmengly all that effort would be poinless. In fact some people in LQG (canonical as well as spin foam versions) are triying to recover matter as some kind of topological deffects, or other, very bizarre, constructons out of pure gravity. If they would be succesfull ther would be no need to add matter and the UV fixed point would be kept. That would be very good for them because the wouold have on one hand a way to do perturbative quantum gravity from conventional Einstein gravity and on the other hand a non perturbative formulation which allows them to prove things about diferent interesting questions (pity that they don´t know how to recover conventional gerneral relativity from that non perturbvative formulation).
Well, very recenly I have had news about a theory made by Krasnov kown as "non metric quantum gravity". You can read about it here.
The presentation sounds very promising. Using Plebansky action (a very familar for LQG people alternative formulation of general relativity equivalent to it, under addecquate constraints, in the classical limit). They quantize it in somethnig resembling the well known background field method . This method was introduced in the fifties by de Witt to obtain for the first time a (quasi)covariant quantization of general relativity which gave a more rigurous tratement of the previous work of Feynman who had obtained Feynman graphs (obvious xD) for general relativity. The theory is based in separating the field to be quantized in two parts, one, the background, is not quantized and only the fuctuations about it are quantized. Ideally the nonquantizd part would be a limit in which the quantum fluctuations of the quantized part are irrelevant. It is interesting thatthis formalism later impulsed the technology wich would allow to quantize yang mills theories (in fact later better ways were found to quantize them and in the QFT textbooks usually there is no reference to the background field method). In fact there is nothing terribly special about the background field method, it is justa diferent way to do perturbative quantization. So one could ask how this would render Plebansky theory renormalizable. The asnwer is ¡it doesn´t !. If you read the article you find that contrary to the claims the theory is not renormalizable. But beofre wondering if you would make a demand to the authros about fake publicity continuate reading -if you have arrived untill here a litle more effort doesn´t matter ;)-. The trick is that they claim that they can prove that the theory has an UV fixed point (yeah, again). The advantage that I see of this aproach over the Reuter´s one is that there are not numerical calculus and artificial algoritmic constructions involved. But still the same drawback applies. They are theories of pure gravity and the UV would dissapear if matter is added (well, I guess it would be so, I cant say for sure).
There are aditional papers exploring the properties of the theory. Seemengly it can explain dark matter, quasars anomalous red shifts and such that. Pitty it can´t explain ordinary, vissible, matter.
If I must decide among these thre theories, conformal gravity, reuters approach, and kranosv non metric gravity I clearly prefer conformal gravity. It is a renormalizbel theory, without need to use fixed points, and it allows the introductoin of matter. I have readed in Jackes Distler blog some claism which maybe would be a problem for this theory, but I am not totally sure about it.
And to end I´ll say a few words about the new baby quantum gravity, or I would better say a TOE (theory of everything). It´s author is Garret Lissi. You can find his paper here. Sabine Hossfander makes a good blog post about it here so I invite to read that entry. I just will quote a phrase of Sabbine:
He neither can say anything about the quantization of gravity, renormalizability, nor about the hierarchy problem
Well, the requirements for a TOE seem to have drop a bit since the last time I watched ;).
Of courseI again reiterate that I don´t consider myself nothin remotely similar to an authority in this questions and that I invite to the reader to form his own conclusions.
Tuesday, July 17, 2007
Conformal gravity, a new theory of quantum gravity?
I have just seen in physics forums the following paper:
Conformal Gravity Challenges String Theory
I had no previous knoledge of these theory and now I have no time to search in google references for it so I just expose it without any claim about how good or flawed it could be.
The author, Philip D. Manhein, semmengly a refuted cosmologist, reviews the genesis of general relativity and isolates two parts, kinematics, background independence, and dynamics, Einstein equations. The point is to search an alternative dynamics. The ultimate reason for Einstein equations is a fenomenológical law, the Newton potential V=1/r. These can be shown to be a solution to the Poisson equation. But if we want to allow small variations compatible with actual observations we could go to V=b/r + c.r with a very small c. A solution of these kind can be shown to be compatible with a fourth order derivative Poisson equation:
In background independent terms we can make a theroy based in the Weyl (conformal) tensor Cuvnk form which we can derivate equations of motion of the form:
4αWuv=Tuv.
These equations have Schwarschild type solutions and some other aspects coincident with Einstein theory. Even thought the most important concern, of course, are de diferences with Einstein which are basically 3:
1)At galactic scales the mass distribution deduced from the apropiate equations fits the observed distribution without requiring dark matter (here I would point that recently seemengly there have been indirect observations of dark matter so maybe these could be a problem afther all).
2) At cosmologic scales an equation equivalent to the Friedman-Robertson-Walker (with an apropiate energy moment tensor) can be formulated and we obtain a solution whic describes an aceleratedly expanding universe withouth a cosmological constant (which is forbiden in that theory because of conformal invariance).
3) It is a power counting renormalizable theory. That means that if we construct a perturbative quantum gravity from it it could be renormalizable, i.e. fully consistent. And it would be a 4 dimensional theory, no need for extra dimensions.
Until now all seems very correct.But beeing a relativelly easy theory (as compared to string theory for example) one could think that there is some sublety involved and in fact that was the case. If one calculates the propagator for the gravity sector one finds a term which from knowledge of quantization of gauge theories seems to be associated to a ghost state which without some apropiate way to handle it would remove the unitarity of the theory. Well, the author in these paper, and these is the important development here, claims to have resolved that problem, which seemed to be a generic problem for theories with fourth order derivatives.
I´ll try to read the article more carefullly, and of course also wait for posible reactions in the physic comunity. All it sounds very interesting but suposedly one would be carefull with "fundamental" theories developed by a cosmologist. Well, the paper was brief so in the worst of the cases it didn´t mean an excesive mess of time.
P.S. String theorists claim that there are two factors which seem to indicate that stringn theory must be the "only game in town". One of them is that a quantum gravity would be a fixed point in the renormalization flow defined by the beta funtions of the theory if it going to advoid the need of an innite number of parameters. You can read a carefull exposition of the argument in Jackes Distler blog, concretelly here. Well, conformal theories have ultraviolet fixed points and these is a conformal theory so it would fit the requirement (in that link Distler claims that string theory escapes the problem by a diferent method, even though string theory seen as a conformal theory on the world sheet fits the requirement of ultraviolete fixed point, I am not sure if I am mising some point with these two appearences of UV fixed point from two slightly diferents viewpoints)
I am also aware that something called Poisson deformations (or something similar) seems to indicate some uniquiniess of string theory. I dón´t know how the result is obtained and the secenaries it covers so I can´t judge it´s relevance for the present theory.
Update Afther a bit of search I have found that there articles which have the details of the calculations:
http://arxiv.org/abs/astro-ph/0505266 (classical part)
http://arxiv.org/abs/0706.0207 (quantum part)
You can read another opinion about the article here:
http://www.scienceagogo.com/forum/ubbthreads.php?ubb=showflat&Number=21054&page=2
The theory was presented in the recent Pascos 2007 conferences and seemengly it had a good aceptation and now people is studying the details of the math in search of possible faliures. If not found seems it definitivelly looks like a promising theory.
Conformal Gravity Challenges String Theory
I had no previous knoledge of these theory and now I have no time to search in google references for it so I just expose it without any claim about how good or flawed it could be.
The author, Philip D. Manhein, semmengly a refuted cosmologist, reviews the genesis of general relativity and isolates two parts, kinematics, background independence, and dynamics, Einstein equations. The point is to search an alternative dynamics. The ultimate reason for Einstein equations is a fenomenológical law, the Newton potential V=1/r. These can be shown to be a solution to the Poisson equation. But if we want to allow small variations compatible with actual observations we could go to V=b/r + c.r with a very small c. A solution of these kind can be shown to be compatible with a fourth order derivative Poisson equation:
In background independent terms we can make a theroy based in the Weyl (conformal) tensor Cuvnk form which we can derivate equations of motion of the form:
4αWuv=Tuv.
These equations have Schwarschild type solutions and some other aspects coincident with Einstein theory. Even thought the most important concern, of course, are de diferences with Einstein which are basically 3:
1)At galactic scales the mass distribution deduced from the apropiate equations fits the observed distribution without requiring dark matter (here I would point that recently seemengly there have been indirect observations of dark matter so maybe these could be a problem afther all).
2) At cosmologic scales an equation equivalent to the Friedman-Robertson-Walker (with an apropiate energy moment tensor) can be formulated and we obtain a solution whic describes an aceleratedly expanding universe withouth a cosmological constant (which is forbiden in that theory because of conformal invariance).
3) It is a power counting renormalizable theory. That means that if we construct a perturbative quantum gravity from it it could be renormalizable, i.e. fully consistent. And it would be a 4 dimensional theory, no need for extra dimensions.
Until now all seems very correct.But beeing a relativelly easy theory (as compared to string theory for example) one could think that there is some sublety involved and in fact that was the case. If one calculates the propagator for the gravity sector one finds a term which from knowledge of quantization of gauge theories seems to be associated to a ghost state which without some apropiate way to handle it would remove the unitarity of the theory. Well, the author in these paper, and these is the important development here, claims to have resolved that problem, which seemed to be a generic problem for theories with fourth order derivatives.
I´ll try to read the article more carefullly, and of course also wait for posible reactions in the physic comunity. All it sounds very interesting but suposedly one would be carefull with "fundamental" theories developed by a cosmologist. Well, the paper was brief so in the worst of the cases it didn´t mean an excesive mess of time.
P.S. String theorists claim that there are two factors which seem to indicate that stringn theory must be the "only game in town". One of them is that a quantum gravity would be a fixed point in the renormalization flow defined by the beta funtions of the theory if it going to advoid the need of an innite number of parameters. You can read a carefull exposition of the argument in Jackes Distler blog, concretelly here. Well, conformal theories have ultraviolet fixed points and these is a conformal theory so it would fit the requirement (in that link Distler claims that string theory escapes the problem by a diferent method, even though string theory seen as a conformal theory on the world sheet fits the requirement of ultraviolete fixed point, I am not sure if I am mising some point with these two appearences of UV fixed point from two slightly diferents viewpoints)
I am also aware that something called Poisson deformations (or something similar) seems to indicate some uniquiniess of string theory. I dón´t know how the result is obtained and the secenaries it covers so I can´t judge it´s relevance for the present theory.
Update Afther a bit of search I have found that there articles which have the details of the calculations:
http://arxiv.org/abs/astro-ph/0505266 (classical part)
http://arxiv.org/abs/0706.0207 (quantum part)
You can read another opinion about the article here:
http://www.scienceagogo.com/forum/ubbthreads.php?ubb=showflat&Number=21054&page=2
The theory was presented in the recent Pascos 2007 conferences and seemengly it had a good aceptation and now people is studying the details of the math in search of possible faliures. If not found seems it definitivelly looks like a promising theory.
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