tag:blogger.com,1999:blog-8666091.post110701105407629505..comments2021-05-03T21:54:48.969+02:00Comments on The Reference Frame: Troubled loop gravityLuboš Motlhttp://www.blogger.com/profile/17487263983247488359noreply@blogger.comBlogger38125tag:blogger.com,1999:blog-8666091.post-1109514887641163292005-02-27T15:34:00.001+01:002005-02-27T15:34:00.001+01:00Just to say hello from a friendly soul. I thought ...Just to say hello from a friendly soul. I thought that you will find my comments on quantum gravity entertaining - even if it would be surprised if you would agree with them - see my blog <A HREF="opensys.blogsom.com" REL="nofollow">Open System</A>.Legacy Userhttps://www.blogger.com/profile/08455685161107731846noreply@blogger.comtag:blogger.com,1999:blog-8666091.post-1109514887048290312005-02-27T15:34:00.000+01:002005-02-27T15:34:00.000+01:00Just to say hello from a friendly soul. I thought ...Just to say hello from a friendly soul. I thought that you will find my comments on quantum gravity entertaining - even if it would be surprised if you would agree with them - see my blog <A HREF="opensys.blogsom.com" REL="nofollow">Open System</A>.Legacy Userhttps://www.blogger.com/profile/08455685161107731846noreply@blogger.comtag:blogger.com,1999:blog-8666091.post-1107570667033831262005-02-05T03:31:00.000+01:002005-02-05T03:31:00.000+01:00You see that you can be both brief as well as tole...You see that you can be both brief as well as tolerably polite. Your latest posting contains all the meaningful information from your previous multi-kilobytes rants. Everyone can determine for herself whether I am a secret fan of loop quantum gravity. ;-)Luboš Motlhttps://www.blogger.com/profile/17487263983247488359noreply@blogger.comtag:blogger.com,1999:blog-8666091.post-1107569698217273932005-02-05T03:14:00.000+01:002005-02-05T03:14:00.000+01:00Lubos practices censorship.
Admit it, you've got ...Lubos practices censorship.<br /><br />Admit it, you've got a crush on loop quantum gravity and Smolin is your hero!Anonymousnoreply@blogger.comtag:blogger.com,1999:blog-8666091.post-1107564037062643342005-02-05T01:40:00.000+01:002005-02-05T01:40:00.000+01:00This comment has been removed by a blog administrator.Anonymousnoreply@blogger.comtag:blogger.com,1999:blog-8666091.post-1107563432786517302005-02-05T01:30:00.000+01:002005-02-05T01:30:00.000+01:00This comment has been removed by a blog administrator.Anonymousnoreply@blogger.comtag:blogger.com,1999:blog-8666091.post-1107274569515107302005-02-01T17:16:00.000+01:002005-02-01T17:16:00.000+01:00This message occurs at my blog and I think it is r...This message occurs at my blog and I think it is relevant to also post a copy here since now Lubos no longer visit my blog as he promised:<br /><br />http://quantoken.blogspot.com/2005/02/blackhole-entropy-lqg-and-super-string.html<br /><br />Both the super string camp and the LQG camp claimed their derivations of the Bekenstein-Hawking black hole entropy as their biggest success of their theories. In my judgement, claiming the derivation of Bekenstein Hawking entropy, such a trivial feat, as their biggest success, is completely "childish" and only shows the lack of "innate" ability on the part of each camp to comprehend what is the REAL physics behind the blackhole entropy!<br /><br />I am going to show one very trivial derivation of the black hole entropy and how it is proportional to the event horizen surface area divided by Planck area. One that is different from Hawking's but much simpler.<br /><br />But first, one has to realize two things:<br />1.Hawking entropy is not an empirical experimental evidence, but merely the result of a gedanken "experiment", e.g., mind exercise.<br />2. The entropy is a DIMENTIONLESS physical quantity.<br /><br />Since Hawking entropy is just a mind exercise instead of empirical experimental result. Any claim of deriving the same result as Hawking merely shows that your theory does not have any logical inconsistency or conflict against the line of logic that Hawking's gedanken mind exercise. That's all. You still have not made any connection with the physics reality, unless, of course, that the Hawking formula is confirmed by a REAL experiment, not a gedanken one.<br /><br />So, if a theory is able to derive Hawking entropy, it's good but really not a big deal. But if it can not, then there is a huge trouble in that it is logically inconsistent with Hawking's reasoning and what Hawing based his reasonings on.<br /><br />Actually, any consistent theory at all will always leads to the Hawking formula, give or take a trivial numerical factor which is of the order of one, a numerical factor that both LQG and super string had struggled a bit to get right.<br /><br />Now back to the dimentionless-ness of entropy. Given some basic physics quantities and known physics constants for a black hole, how would you construct an entropy formula that gives a dimentionless quantity? To construct such a formula, all the units should cancel out. That gives you pretty good clue to almost certainly arrive at the only correct answer.<br /><br />We are given:<br />1.G, the gravity constant that is involved in any thing related to gravity<br />2.hbar, anything that envolved entropy needs to count quantum micro states.<br />3.C, light speed is certainly involved in anything related to spacetime.<br />4.M, the mass of black hole. We certainly need that.<br /><br />There is nothing more we need. How would you construct a dimentionless number out of these 4 quantities? An immediate possibility is similar to the electromagnetic coupling constant, the fine structure constant, we can construct a gravity coupling constant here using:<br /><br />S = G*M^2/(hbar*C) (1)<br /><br />And that is the Hawking entropy formula, give or take a numerical factor!!! Actual it is only bigger than the Hawking entropy by a factor of 4*PI. Actually that is the only simple way to get a dimentionless number out of the 4 quantities!!!<br /><br />How come? We know the radius of a black hole is proportional to its mass:<br />R = 2*G*M/C^2 (2)<br />So:<br />M = R*C^2/(2*G) (3)<br />So the (1) becomes:<br />S = (1/4)*R^2/(G*hbar/C^3) = (1/4) * R^2/lp^2 = (1/(4*PI)) * (1/4) * A/lp^2 (4)<br /><br />So it differ from Hawking formula by 1/(4*PI).<br /><br />Now, let me try to use a totally different but much simpler gedanken experiment to derive an entropy formular similar to Hawkings. It's trivial. Any one can think out a hundred different gedanken experiments, all arrive at the same result, differing by only a numerical factor.<br /><br />Let's start with a black hole of almost zero mass, and gradually increase its size by throwing in photons in appropriate wavelength.<br /><br />We do not want to throw in photons whose wavelength is much smaller than the size of the blackhole, since they will lose a great portion of their energy by gravity red-shift, and we do not know how much the mass of blackhole is increase. We do not want to throw in photons of wavelenth much larger than the blackhole size either, since then the photo will diffract around the blackhole completely, without absorbtion.<br /><br />Let's choose photo wavelength<br />Lambda = diameter of blackhole = 2*R (5)<br /><br />Such photons will be absorbed with its energy largely unchanged, increasing the blackhole mass by the equivalent mass of the photon energy. The increase of mass is:<br />delta M = delta E * C^2 = 2*PI*hbar * C^3/lambda = 2*PI*hbar*C^3/(2*R) (6)<br /><br />We know that each photon carries an entropy of exactly one, regardless of the photon's energy, so the increase of black hole entropy for each photon is one:<br />delta S = 1 (7)<br /><br />Therefore:<br />dM = PI*hbar*C^3/R * dS (8)<br />dS = 1/(PI*hbar*C^3) * R * dM (9)<br /><br />Now, since<br />R = 2*G*M/C^2, which is M = C^2/(2G) * R (10)<br />dM = C^2/(2G) * dR (11)<br /><br />Put it into equ. (9):<br />dS = 1/(PI*hbar*C^3) * R * C^2/(2G) * dR (12)<br /><br />Integrate equ. (12) from zero:<br /><br />S = (1/(4*PI)) * R^2/ (hbar*C^3/G) (13)<br />S = (1/4*PI^2)^2 * (1/4)* A / lp^2 (14)<br /><br />Again we ontained virtually the same Hawking entropy, except for a small numerical factor. I could easily get the factor correct if I am willing to try a little bit numerology like the LQG and super string camp did in their crackpot theories.<br /><br />So you see it is really not a big deal at all to have derived the black hole entropy proportional to horizen area divided by Planck area.<br /><br />QuantokenQuantokenhttps://www.blogger.com/profile/08057876770160255308noreply@blogger.comtag:blogger.com,1999:blog-8666091.post-1107243513685947052005-02-01T08:38:00.000+01:002005-02-01T08:38:00.000+01:00"I also think that one can show that a theory in w..."I also think that one can show that a theory in which only "nice" configurations are summed over in its Feynman's formulation may be proved to lead to a non-unitary theory in general."<br /><br />Certainly most of the time, I would agree with that. But not in general! There are textbook cases of 1+1 gravity where doing such a prescription yields perfectly sensible and unitary results. Bizarre, but true.<br /><br />In general im very skeptical of the path integral, especially in the case where d > 4. You need quartic terms or higher in the lagrangian to be bounded from below (a stable theory) and its quite straightforward to see that you end up with strong coupling at the cutoff point, hence any UV limit cannot be treated by perturbation series. Such an enterprise is speculative by nature, lest you can find some nonperturbative examples and match answers.Anonymousnoreply@blogger.comtag:blogger.com,1999:blog-8666091.post-1107239341528602252005-02-01T07:29:00.000+01:002005-02-01T07:29:00.000+01:00This comment has been removed by a blog administrator.Anonymousnoreply@blogger.comtag:blogger.com,1999:blog-8666091.post-1107238899145648852005-02-01T07:21:00.000+01:002005-02-01T07:21:00.000+01:00This comment has been removed by a blog administrator.Anonymousnoreply@blogger.comtag:blogger.com,1999:blog-8666091.post-1107238318875395482005-02-01T07:11:00.000+01:002005-02-01T07:11:00.000+01:00Dear lumo,
thanks for your guidance across the str...Dear lumo,<br />thanks for your guidance across the string landscape; I guess I will be busy for a while! ;-) "Review" was Smolins words.<br /><br />And darn it, that was supposed to be 'Captains blog' and 'the lifes of'! Why is it that my keyboard always seems to be a few keys short? :-)torbjornhttps://www.blogger.com/profile/03072037567907266083noreply@blogger.comtag:blogger.com,1999:blog-8666091.post-1107211594529018352005-01-31T23:46:00.000+01:002005-01-31T23:46:00.000+01:00Dear torbjorn,
you don't need to call it a "revie...Dear torbjorn,<br /><br />you don't need to call it a "review". We have textbooks of String theory, and Joe Polchinski's book is now the most famous one.<br /><br />About 200 different reviews of string theory and various aspects of it are summarized in<br /><br /><A HREF="http://www.blogger.com/r?http%3A%2F%2Farxiv.org%2Fabs%2Fhep-th%2F0311044">http://arxiv.org/abs/hep-th/0311044</A>Best<br />LubosLuboš Motlhttps://www.blogger.com/profile/17487263983247488359noreply@blogger.comtag:blogger.com,1999:blog-8666091.post-1107211151073489392005-01-31T23:39:00.000+01:002005-01-31T23:39:00.000+01:00While L.M. with more or less logic refutes most of...While L.M. with more or less logic refutes most of Smolins letter, he can apparently not find a general string rewiew - surely AdS-CFT isn't all of the subject. This is unfortunate since it would be interesting if it exists.<br /><br />The discussion illustrates why it is so difficult for an interested outsider to find the right track. The 'stringulation' of sci.phys.strings and the venom makes it hard to feel the fangs,... uh, facts, behind. It throw me in a loop for 3 months before I got stringed out.<br /><br />Often you get the feeling of watching the following episode of the String Trek series:<br /><br />"Captain's log, string date 20+ (?):<br /><br />Captain Quirk (Motl): <br />- That Loopy Qlingon Group is still out there! And now it seems the Higgs field is leaking in the space-time manifold!<br /><br />Chief Engineer (Wissen):<br />- Aye! I can fix that with the gauge couplings. But it will take the life of 3 undergraduates and a massive release of papers.<br /><br />Science Officer (Schrieber):<br />- But remember Jim, Qlingons are people too!<br /><br />Unfortunately the real McCoy (experimental evidence) is still absent from the plot."torbjornhttps://www.blogger.com/profile/03072037567907266083noreply@blogger.comtag:blogger.com,1999:blog-8666091.post-1107149732579995392005-01-31T06:35:00.000+01:002005-01-31T06:35:00.000+01:00I love a good thrashing of LQG as much as anyone, ...I love a good thrashing of LQG as much as anyone, but jeez Lubos, get over yourself.Anonymousnoreply@blogger.comtag:blogger.com,1999:blog-8666091.post-1107143464266010522005-01-31T04:51:00.000+01:002005-01-31T04:51:00.000+01:00Thank you Lumo for orienteering and your Martian p...Thank you Lumo for <A HREF="http://www.blogger.com/r?http%3A%2F%2Fcosmicweb.uchicago.edu%2Fimages%2Fx.gif">orienteering</A> and your Martian patience.<br /> <br /><B>String Theory, Universal Mind, and the Paranormal</B> <br /><br /><I>The point in regard to mathematical thinking, which motivates our model, is the following. Consider first of all what the brain does in visual perception. Here the primary information from the visual receptors goes through various levels of processing until it ends up as a high-level representation of the content of the visual field. It is not unreasonable to identify mathematics as a similar process, except that higher levels of abstraction are involved in this case. With the visual case, the mechanics are straightforward: the visual field typically contains for example edges, for which abstraction a dedicated neural system has evolved, related to our ability to perceive edges. It is hard to see why we should have such ready access to higher mathematical abstractions having little connection with experience (Penrose 1994). <B>One resolution of the problem would be for mathematical concepts to be in some way ‘in the physics’</B>, rather than being emergent properties of brains. In case it is felt that such a drastic solution is not necessary to explain our ready access to mathematical ideas, and that neural networks can provide an adequate explanation, a stronger argument for the existence of some kind of Platonic realm can be made on the basis of the aesthetic aspect of music (Josephson and Carpenter 1996)</I>Yes I recognized <A HREF="http://www.blogger.com/r?http%3A%2F%2Feskesthai.blogspot.com%2F2004%2F11%2Fnon-euclidean-geometry-and-universe.html">high energy</A>, and the return to the planck epoch.<br /><br /><I>Since Reinmannian geometry is the mathematical core of general relativity, this means that it too must be modified in order to reflect faithfully the new short distance physics of string theory. Whereas general relativity asserts that the curved properties of the universe are described by Reinmannian geometry, string theory asserts this is true only if we examine the fabric of the universe on large enough scales. On scales as small as planck length a new kind of geometry must emerge, one that aligns with the new physics of string theory. This new geometry is called, quantum geometry." Brian Greene</I> <br /><br />Issues with Glast, were headed in this direction from what I understood when looking at the gamma ray burst?<br /><br /> Being restricted, with regards to energy determinations, cosmological views become interesting applications looking towards the planck era. I do <B>see</B> Glast determinations falling short of planck epoch being fully discriptive by those same glast determinations. Would Smolin agree with this? <br /><br />Although, Smolin's approach is leading through scientific procedures, when moving towards planck views, a new geometry must emerge?<br /><br />Ideally, it was one more view of the harmonical spectrum being understood as far I as I could see.:)Legacy Userhttps://www.blogger.com/profile/01554579710459395706noreply@blogger.comtag:blogger.com,1999:blog-8666091.post-1107132225646878012005-01-31T01:43:00.000+01:002005-01-31T01:43:00.000+01:00This comment has been removed by a blog administrator.Anonymousnoreply@blogger.comtag:blogger.com,1999:blog-8666091.post-1107128390050583612005-01-31T00:39:00.000+01:002005-01-31T00:39:00.000+01:00At sufficiently short distances (in the particle p...At sufficiently short distances (in the particle physics language: high energies), the Poincare subgroup of the diffeomorphism group is always locally restored. This is called the equivalence principle. Physics in freely falling frames locally follows the rules of special relativity without gravity. A theory that does not have this feature, at least approximately with a very small epsilon, is falsified.Luboš Motlhttps://www.blogger.com/profile/17487263983247488359noreply@blogger.comtag:blogger.com,1999:blog-8666091.post-1107126906081730422005-01-31T00:15:00.000+01:002005-01-31T00:15:00.000+01:00I accept the status of crackpot being the laymen t...I accept the status of crackpot being the laymen that I am and it is hard for such junior members to penetrate the language barrier, but I am sincerely trying.:)<br /><br />Thank you for the correction on symmetry breaking.<br /><br />To make the leap of <A HREF="http://www.blogger.com/r?http%3A%2F%2Fgraphics.nytimes.com%2Flibrary%2Fnational%2Fscience%2F040400sci-universal-theory.A.jpg">photons held to the brane</A> it was from the "other end" that being in the <A HREF="http://www.blogger.com/r?http%3A%2F%2Feskesthai.blogspot.com%2F2004%2F12%2Fgravity-for-instance-varies-with-time.html">bulk graviton concentration</A> would have also helped to identify the boundry of this brane<B>?</B>Legacy Userhttps://www.blogger.com/profile/01554579710459395706noreply@blogger.comtag:blogger.com,1999:blog-8666091.post-1107123256971599282005-01-30T23:14:00.000+01:002005-01-30T23:14:00.000+01:00In general relativity, the relevant symmetries are...In general relativity, the relevant symmetries are not translations and Lorentz transformations but diffeomorphisms and of course diffeomorphisms are spontaneously broken by ANY nonzero metric tensor a.k.a. vierbein or densitized dreibein as the case may be.Anonymousnoreply@blogger.comtag:blogger.com,1999:blog-8666091.post-1107119767116875922005-01-30T22:16:00.000+01:002005-01-30T22:16:00.000+01:00Dear Anonymous,
what you write is much more correc...Dear Anonymous,<br />what you write is much more correct than you think. <br /><br />You example is not just an example - it is an exact description of the situation here. The fact that one can't define the generators of translations (momenta) and the Lorentz transformations is indeed exactly because the states related by translations belong to different superselection sectors. This really means that all continuous geometric operations with the spacetime are spontaneously broken.<br /><br />In LQG, there is a huge continuous number of superselection sectors, which is related to its non-separability.<br /><br />This clearly violates observations and very consistency with reality. Locally, the translations are definitely unbroken in anything that at least remotely resembles our Universe.<br /><br />Best<br />LubosLuboš Motlhttps://www.blogger.com/profile/17487263983247488359noreply@blogger.comtag:blogger.com,1999:blog-8666091.post-1107116943137555062005-01-30T21:29:00.000+01:002005-01-30T21:29:00.000+01:00In a theory with spontaneous symmetry breaking, th...In a theory with spontaneous symmetry breaking, the generators of the internal symmetry cannot be defined within a superselection sector. This is because the "full" Hilbert space is nonseparable. Therefore, spontaneous symmetry breaking is inconsistent and there is definitely something wrong with the math.<br /><br />Let's attack the spontaneous symmetry breaking crackpots, shall we?Anonymousnoreply@blogger.comtag:blogger.com,1999:blog-8666091.post-1107113803920552892005-01-30T20:36:00.000+01:002005-01-30T20:36:00.000+01:00From a laymen perspective and having spent time fo...From a laymen perspective and having spent time following the dvelopement of these two different approaches I was quite interested to see that the SRian approach of LQG and the GRian approach of Strings would not have found relatiosnhips based on the Srian approach?<br /><br />String Amplitudes(continous feynman path) and LQG (discrete images of monte carlo methods relegated to the varying changes) relationship of quantum gravity perspectives seem so close to me, but of course my knowledge seems far from the frames of references of both yourself Lubos Motl and Lee Smolin.<br /><br />It will be interesting to see this dialogos of eide developed, form <A HREF="http://www.blogger.com/r?http%3A%2F%2Feskesthai.blogspot.com%2F2005%2F01%2Ftopology-and-early-history.html">this harmonic convergence</A>?Legacy Userhttps://www.blogger.com/profile/01554579710459395706noreply@blogger.comtag:blogger.com,1999:blog-8666091.post-1107100352704835692005-01-30T16:52:00.000+01:002005-01-30T16:52:00.000+01:00It's not often I'm mistaken for a Nobel Laureate (...It's not often I'm mistaken for a Nobel Laureate (or Paul Ginsparg who I don't know).<br />Thanks for the chuckle.<br /><br />However, you research leading to a wrong conclusion (two misses now) does illustrate what can happen when theory is not subject to experimental verification.<br /><br />To continue with the analogy, the space of all possible people who could have written these comments is very large (as is the space of possible theories), but the probability of you naming the right person (finding the right theory) is vanishingly small without experimental guidance.<br /><br />I'm a complete dilletante in the string field theory, <br />but I do have a simple question.<br /><br />Q: In standard QFT the perturbative calculations are done in the context of a flat background metric. In string theory, which is said to unify QFT and GR, does this unification now enable one to do the analogous perturbative string calculations in a background-free context, ie, do the spacetime variable naturally become part of the dynamical system as in GR?Anonymousnoreply@blogger.comtag:blogger.com,1999:blog-8666091.post-1107059046679117822005-01-30T05:24:00.000+01:002005-01-30T05:24:00.000+01:00Dear Jesse,
whether one can prove it *rigorously...Dear Jesse, <br /><br />whether one can prove it *rigorously* for any version of LQG? Hardly. Certainly not me. One would first have to define rigorously what "any version of LQG" means. Of course, if the concepts of LQG are generalized in such a way that it includes string theory, for example, you may also get a Lorentz-invariant LQG. ;-)<br /><br />The assumptions of these proofs are a bit vague, and therefore the proof is also vague. But yes, I seem to think that a summation over discrete histories filling the spacetime can be proved as rigorously as your definition of the theory to be either singular, or topological without local degrees of freedom, or Lorentz-violating.<br /><br />In all regular path integrals we've ever seen, the typical configuration contributing to the path integral is non-differentiable anyway, and the localization to the smooth configurations is just a method to calculate. I also think that one can show that a theory in which only "nice" configurations are summed over in its Feynman's formulation may be proved to lead to a non-unitary theory in general.<br /><br />Best<br />LubošAnonymousnoreply@blogger.comtag:blogger.com,1999:blog-8666091.post-1107057602791032382005-01-30T05:00:00.000+01:002005-01-30T05:00:00.000+01:00Lee's repsonse has a very different effect on grad...Lee's repsonse has a very different effect on grad students, who do not belong to the LEE club. His comments on Nicolai's very thoughtful paper is that the issues raised in this paper are "non issues". According to Lee, 10 years ago, a "few" people addressed the issues and "moved away" from the Hamiltonian formalism. Of course as lesser mortals we and "nicolai" did not know anything about it!!<br /><br />Hey Hamiltonian constraint is not an issue people, since it can be so easily swept under the carpet and now we make progress with spin foams. So the basis upshot is what? Nicolai's paper is 10 years too late? <br /><br />I agree with the Prof. Glashow, if that is who it is!! that this debate is fierce and rabid!!<br /><br />But, I think it should be. If 10 years ago, this issues was resolved, what import does bojowald's work Have (amongst the very few LQG works that I liked only because it actually tied to solve a problem)? and by the way why did someone not write an article saying "forget Hamiltonian: onwards with spin foam" so that we could be better informed. That's why this debate needs to rabid!!! communication has been very very poor. At least Nicolai's paper elicits a response, but for the entire community, this response should be on arxiv.org, and not confined to this blog, so that young researchers can form an opinion from this debate.Anonymousnoreply@blogger.com