Talk:Loop quantum gravity/Archive 4

Part IV Selection of Open Research Problems
FROM Thiemann - Lectures on Loop Quantum Gravity, please use accordingly to improve the article and get an idea of the issues at hand:

Let us summarize the most important open research problems that have come up during the discussion in these lectures.

i) Hamiltonian Constraint and Semiclassical States The unsettled correctness of the quantum dynamics is the major roadblock to completing the quantization programme of QGR. In order to make progress a better understanding of the kinematical semiclassical sector of the theory is necessary.

ii) Physical Inner Product Even if we had the correct Hamiltonian constraint and the complete space of solutions, at the moment there is no really good idea available of how to construct a corresponding physical inner product because the constraint algebra is not a Lie algebra but an open algebra in the BRST sense so that techniques from rigged Hilbert spaces are not available. A framework for such open algebras must be developed so that an inner product can be constructed at least in principle.

iii) Dirac Observables Not even in classical general relativity do we know enough Dirac observables. For QGR they are mandatory for instance in order to select an inner product by adjointness conditions and in order to arrive at an interpretation of the final theory. A framework of how to define Dirac observables, at least in principle, even at the classical level, would be an extremely important contribution.

iv) Covariant Formulation The connection between the Hamiltonian and the Spin Foam formulation is poorly understood. Without such a connection e.g. a proof of covariance of the canonical formulation on the one hand and a proof for the correct classical limit of the spin foam formulation on the other cannot be obtained using the respective other formulation. One should prove a rigorous Feynman Kac like formula that allows to switch between these complementary descriptions.

v) vi) QFT on CST's and Hawking Effect from First Principles The low energy limit of the theory in connection with the the construction of semiclassical states must be better understood. Once this is done, fundamental issues such as whether the Hawking effect is merely an artefact of an invalid description by QFT's on CST's while a quantum theory of gravity should be used or whether it is a robust result can be answered. Similar remarks apply to the information paradoxon associated with black holes etc. Combinatorial Formulation of the Theory The description of a theory in terms of smooth and even analytic structures curves, surfaces etc. at all scales in which the spectra of geometrical operators are discrete at Planck scales is awkward and cannot be the most adequate language. There should be a purely combinatorical formulation in which notions such as topology, differential structure etc. can only have a semiclassical meaning.

vii) Avoidance of Classical and UV Singularities That certain classical singularities are absent in loop quantum cosmology and that certain operators come out finite in the full theory while in the usual perturbative formulation they would suffer from UV singularities are promising results, but they must be better understood. If one could make contact with perturbative formulations and pin point exactly why in QGR 81 the usual perturbative UV singularities are absent then the theory would gain a lot more respect in other communities of high energy physicists. There must be some analog of the tenorrealization group and the running of coupling constants that one usually finds in QFT's and CST's. Similar remarks apply to the generalization of the loop quantum cosmology result to the full theory.

viii) Contact with String (M) Theory If there is any valid perturbative description of quantum gravity then it is almost certainly string theory. It is conceivable that both string theory and loop quantum gravity are comple- mentary descriptions but by themselves incomplete and that only a fusion of both can reach the status of a fundamental theory. To explore these possibilities, Sinolin has launched an ambi- tious programme [82] which to our mind so far did not raise the interest that it deserves 8. The contact arises through Chern Simons theory which is part of both Loop Quantum Gravity and M Theory [83] (when considered as the high energy limit of 11 dimensional Supergravity). Another obvious starting point is the definition of M Theory as the quantum supermembrane in 11 dimensions [84], a theory that could be obtained as the quantization of the classical supermembrane by our non-perturbative methods. Finally, a maybe even more obvious con- nection could be found through the so-called Pohlmeyer String [85] which appears to be a method to quantize the string non-perturbatively, without supersymmetry, anomalies or extra dimensions, by working directly at the level of Dirac observables which are indeed possible to construct explicitly in this case. Slicky 09:06, 21 August 2006 (UTC)

Like many things in physics this is a contentios matter. I have archived the debate thus far because the page was very messy. note that there is a long laundry list of objections to LQG in the third achive which I will link to in the article so please do not add them backin. If you do not "Like" LQG flame it in usenet. This is just an encylopedica article and as such should be brief and aimed at the lay person who just wants to know that the heck it is. This is not a journal please don't take it so serioously. :-)

I just pray this does not reignite the flame/editwar.

--Hfarmer 00:49, 23 August 2005 (UTC)

unverified
For one thing, the critics of this theory cite that it does not predict the existence of extra dimensions and does not predict the masses or charges of particles, such as in String theory.

string theory does not predict or allows for post-diction calculation of masses. extradimensions remains unverified as of 2005.

A Long Question
To my understanding, physics is about describing or measuring particular aspects of the nature. This activity is based on the comparison of the subject of discourse such as spatial or chronological distance or the mass with a unit of measure, such as, in a simple case for spatial distance, a yardstick or the measuring unit of length. So the task for example to measure the distance reduces to counting how often, say, a yardstick has to be applied. Derived variables are treated accordingly. For counting numbers are used. “In mathematics, the real numbers are intuitively defined as numbers that are in one-to-one correspondence with the points on an infinite line—the number line. The term "real number" is a retronym coined in response to "imaginary number". …Real numbers may be rational or irrational; algebraic or transcendental; and positive, negative, or zero…Real numbers measure continuous quantities.” More details on real numbers are found in Wikipedia: http://en.wikipedia.org/wiki/Real_numbers). That an uncountable number of real numbers exists is one aspect. The other aspect is, that in practice self-adjoint operators on a Hilbert space (for example, self-adjoint square complex matrices) are used, which “generalize the reals in many respects: they can be ordered (though not totally ordered), they are complete, all their eigenvalues are real and they form a real associative algebra. Positive-definite operators correspond to the positive reals and normal operators correspond to the complex numbers” Wikipedia (http://en.wikipedia.org/wiki/Real_numbers). Why is it necessary to use “CSTAR algebra”? If I understand correctly, the set of the real numbers is a subset of the set of the complex numbers. So: what is wrong with the simple assumption that complex numbers can also be used for the initially mentioned counting purpose? Why is it a problem to use C-Algebra instead of CSTAR-algebra? For instance, one wants to determine the area of a square and the distance between the corner points is artificially counted by using complex numbers instead of a real numbers. “...since if two complex numbers are equal, their real parts must be equal and their complex parts must be equal…We must emphasize, however, that this separation into a real part and an imaginary part is not valid in general, but is valid only for equations which are linear, that is for equations, in which x appears in every term only in the first power or the zeroth power. For instance, if there were in the equation a term λ • x2, then when we substitute xr + i•xi, we would get λ•(xr+ixi)2, but when separated into real and imaginary parts this would yield λ(xr2-xi2) as the real part and 2•i•λ•xr•xi as the imaginary part. So we see that the real part of the equation would not involve just λ•xr2, but also – λ•xi2. In this case …the completely artificial thing we introduced in our analysis, mixed in…” (Feynman, „Lectures on Physics” (1977), 23-2). So, the outcome would be something that could look like a quantization of a surface. If xi is sufficiently small, the experimental falsification or proof would be as difficult as in case of the theory of loop quantum gravity. Friedrich Schmidt, 17 August 2005.

You sound like a matematician
1.) Physicist use real numbers for observable quantities out of convention. All measureing instruments known to man give measurements in real number's.  For example your yardstick has no $$i=\sqrt(-1)$$ on it.  You can draw one in if you want, and put an I infront of all your other measurements  but why?  That's the physicsit reasoning for it.  Some people can give you a much more complicated song and dance but the bottom line facts are that measureing instruments give results in real numbers


 * Where do you get "measureing instruments known to man give measurements in real number's?" I don't see irrational numbers on my ruler.

2.) Why C* Algebra instead of C Algebra? C Algebra probaly leads to needlessly complicated expressions.  Physicist are good at math but we thrive on makeing math as simple as it can be.

3.) The last part of your comment seems to be on string theory and a compariso between LQG and strings based on expreimental testability. I agree with the general idea that theories of gravity will be difficult to test because the effects will always be very very small. --Hfarmer 01:56, 3 November 2005 (UTC)

Why Heyting algebra?
I'm puzzled by the See Also link to Heyting Algebra. There doesn't seem to be any connection between Heyting Algebra the contents of the Loop Quantum Gravity article. If there is, I'd *really* like to see it.

-- hendrik@pooq.com

Stop opinionating!
I agree with Hfarmer. When you state an opinion, don't use the passive voice (see weasel words) and do cite your authority. I have removed the Problems section. It's nothing but unsupported opinion.

"As of now, there is not a single experiment which verifies or refutes any aspect of LQG." "theory without experiment is just faith" The same can be said for string theory, or black holes, for that matter.
 * black holes have been proven through gravitational lensing. hth. Avriette 01:50, 12 December 2005 (UTC)

"LQG has failed to gain support in the physics community" is an unsupported generalization.

If one wishes to opinionate or preach, they can place their opinions HERE, like this:


 * Problems


 * As of now, there is not a single experiment which verifies or refutes any aspect of LQG. This problem plagues many current theories of quantum gravity. LQG is affected especially, because it applies on a small scale to the weakest forces in nature. There is no work around for this problem, as it is the biggest problem any scientific theory can have; theory without experiment is just faith. The second problem is that a crucial free parameter in the theory known as the Immirzi parameter is a logarithm of a Transcendental number. This has negative implications for the computation of the entropy of a black hole using LQG. To be fair, it must be noted that the transcendental number is the result of a calculation. It does not come from an experiment, which would be the true test of scientific reality. Since Bekenstein and Hawking computed the entropy of a black hole, this computation has become a crucial litmus test for any theory of quantum gravity.


 * Finally, LQG has failed to gain support in the physics community mainly because of its limited scope. Many scientists believe that LQG could be formulated into a theory of quantum gravity just suited for 4 dimensions. However, by using the String theory or M-theory, scientists have come very close to taking everything we know into account and predicting much that we do not know. Hence, the general feeling is that these competing theories are more potent. Loop theorists disagree, because they believe that we need a proper theory of quantum gravity as a prerequisite for any theory of everything. This philosophical problem could be the most fatal problem that LQG faces in the future. Only time and experimentation can decide the matter.


 * Other problems associated with LQG can be found in Talk:Loop Quantum Gravity Archive 3

The last paragraph should begin, "All problems..." Unless you're ready to site authoritative sources, not just other opinion, leave "problems" out of the article.

Also, as Hfarmer said, Wikipedia is to be read by the educated layman. "Nonperturbative quantization of diffeomorphism-invariant gauge theories" indeed!

J M Rice 20:47, 25 November 2005 (UTC)

Criticisms to LQG, Shortening the Article, and issues around.
I personally think it is a pity that I have to search all the history until I find the criticisms of the theory. Many theories around wikipedia have a "Criticism" topic. And also, the shortening of the article on the basis that "wikipedia is for the common man, not the physics" was lame. Wikipedia should be for both, and elsewhere is stated that it is a project to "compile the sum of all knowledge". Any article can have "soft" and "hardcore" parts. If there is such a problem of people trying to bash LQG, there could be a separate page named Criticisms and Notes on Loop Quantum Gravity What do you people think of it? nihil 20:29, 29 November 2005 (UTC)

Agreed. Alienus 14:38, 2 December 2005 (UTC)

I second that. Which I why I am readding the problems section. I am not an advocate of either string theory or loop quantum gravity. In fact I have my own theory that is more like LQG than anything else out there.

Simply ackowledgin the problems of LQG will not discredit it. Active areas of research have problems those problems are what is being researched.

As for the problems section being "un supported opinion" The content of that section is based on my own research and the objections cited a few archives back by the likes of Lubos Motl "lumidek" was his username. He is a well known theoretical physicist. I myself am at least googleable. Search for Hontas Farmer Physics and you will find my credentials. As such people like us can act as "verifiable sources" Afterall this is our very area of reasearch being discussed who knows it better? --Hfarmer 03:49, 7 December 2005 (UTC)

Eh?
The day someone who doesn't already know the theory (aka the only type of person to whom this page would be useful) understands the term "nonperturbative quantization of diffeomorphism-invariant gauge theories," even after following all the links, is the day I eat my hat.


 * As someone how came to this page to try to understand LQG - I must agree with 'Eh?'. This page is entirely useless to the uninitiated.

I agree, the article is unreadable and should be completly rewriten. In addition, the text refers to a discussion archive which is not a part of the encyclopedia at all. --Egg &#9993; 17:53, 30 December 2005 (UTC)

I absolutely agree: I believe that the introduction should ease a reader into learning about the subject, rather than just assume they already understand LQG. What if the reader is looking into it as leisurely research, and not as part of a university course? I do not know more about diffeomorphism than most people, but I am very willing to learn so long as it is explain in terms I understand. Wikipedia is about free knowledge, and I am finding it hard to see that in this article.


 * Agreed! Especially since Loop Quantum Gravity is mentioned in the Big Bang Theory (that sitcom, y'know) so more people than ever are likely to come here hoping to get some basic info on what it is. I was extremely frustrated to find I could not get even the most basic of information on what it is. I'm not saying that the more technical stuff should be deleted, but please include a basic layman's overview of what the heck this is.--Sailor Titan (talk) 02:17, 3 January 2009 (UTC) PS: Scientific American has a great article that explains the theory in layman's terms: Big Bang or Big Bounce?: New Theory on the Universe's Birth

It has become clear that this article needs rewriting
I do not have the energy to undertake this matter right this minute but I will tomorrow. What needs to be re imposed is the structure I had imposed a year ago. I will re write a plainly worded introduction to the subject matter. Below that introduction a more technical section which will include a discussion of the short commings of Loop Quantum Gravity. (which as far as I can see are as much the result of the sociology of physics and the politics of funding. String theory people attack LQG with the zeal one would expect from religious fanatics.)  --hfarmer 00:24, 3 January 2006 (UTC)


 * I think the important point there is that this theory, like String Theory, is regarded by many as a form of religion. This is discussed extensively in The Elegant Universe. See also the Sokal Affair. For clarity in this article, I think history of loop quantum gravity offers a little more digestible prose. Avriette 02:37, 3 January 2006 (UTC)


 * Er, whoops. That's Bogdanov Affair. Got my affairs mixed up. Avriette 00:30, 5 January 2006 (UTC)

NPOV back
I've returned the NPOV tag on top of this page because someone has slowly deleted absolutely all concrete criticism that effectively explained why loop quantum gravity is an inconsistent fringe science. I understand that the percentage of LQG fans between those who edit this page exceeds their percentage among the physicists roughly by three orders of magnitude, but that should not be a sufficient justification to allow a Wikipedia page that claim such nonsenses such as that LQG only has the same problems like string theory, which is what this page essentially does. See, , for basic summary why LQG is not a ready candidate for a physics theory from the viewpoint of physicists as opposed to crackpots who push some agenda through Wikipedia. Best wishes, Lubos Motl, Harvard --Lumidek 14:57, 8 January 2006 (UTC)

Yeah, you are right. And I dont think it's fair to mix LQG with Bogdanov theory (which in my view represents a real alternative to major problems non solvable in LQG).

Lubos Motl's comments above are to be read in the context that he is an outspoken string theorist, and not an unbiased/independent authority on the merits of Loop Quantum Gravity - 172.188.245.115 23:21, 18 January 2006 (UTC)

This will shock you Dr. Motl but I agree with that. I myself authored a section on the problems with LQG which some people have sought to remove even that modest section at times. if you do not believe me just look at the revision history. I am currently to busy to clean up this mess. But as soon as I do the Npov will be removed. Because I will write it with a neutral POV. I am not a LQG fan or a string fan. I have my own take on canonical quantum gravity separate and apart from LQG. LQG is by no means perfect. However I do not feel that it's encyclopedia entry should be comprised of a smear campaign.

On second thought I feel that one of the former revisions of this page was just about right. I will revert to the revision as of (http://en.wikipedia.org/w/index.php?title=Loop_quantum_gravity&oldid=32572035). I will do this at 1:00AM CST 1/21/06 if nobody objects to that revision. --Hfarmer 06:30, 20 January 2006 (UTC)

I would also point out two other things to Dr. Motl. you wrote a much longer section full of objections. I placed a link to the talk page where I archived it. I did this from the first rewrite of the page. I though that much of what you had to say was far too technical for the layperson reading the article. I would like you to know that I feel the same way about much of the matterial added going into painful detail on the mathematics of LQG (diffeomorphism invariance indeed!). What use is any of that to the average highschool kid doing a science project who only knows E=mc^2 (and not even F=ma because they have not had a formal physics class)? None. Talk of diffeomorphisms and lack of 11 dimensional super symmetry... will just fly over the heads of 98% of the people served by wikipedia.

Do you understand that I am not your enemy?

--Hfarmer 06:50, 20 January 2006 (UTC)


 * I do like that revision more than the present one. I'd be happy to do a copyedit on it if you revert it. Also, before you do any reverting, please make sure that nothing is left out from the current version. No need to lose data. Avriette 07:43, 20 January 2006 (UTC)


 * True we do not want to loose data. However we do not want to make the article too long or too technical.  As a matter of fact look at the string theory article.  It is through, dignified and ackowledges that string theory also has it's problems.  It seems pretty neutral.  Look at the artilcles about other theories of Quantum gravity.  The are neutral brief and to the point.  --Hfarmer 20:02, 20 January 2006 (UTC)


 * Dear Hfarmer, I apologize because I had to be really sloppy in judging your viewpoint. Incidentally, the list of meaningful papers reviewing LQG rationally has just expanded by one - see that cites the new preprint  and explains its points, including the critical ones. I am not sure whether it is physically possible to sustain a realistic page about LQG here. Concerning the technical level of these articles, it seems to me that Wikipedia in general does try to explain topics at the technical level, so I don't understand why it should avoid things like diffeomorphisms that are critical for anything about gravity, among other examples of technical terms. In the case of string theory, there have been many quite deeply technical points explained on Wikipedia - see lists of string theory topics etc. --Lumidek 02:14, 21 January 2006 (UTC)


 * Think nothing of it Dr. Motl. I have had much worse things said over less important topics.  What I was shooting for when I rewrote this about a year ago was an article with two major sections.  An introduction in plain language.  Laypeople need read no further.  Then the technical nuts and bolts of the theory which would include discussion of it's problems and pitfalls.  I have learned to be satisfied when a wikipedia page does not contain hidden swearwords and links to pornography.  You get what you pay for. --Hfarmer 04:03, 21 January 2006 (UTC)

Dr.Motl. I have read the paper you referenced as well as your interpretation of it. Both the paper and your blog seem to attempt to explain this in the a way that a non theoretical physicist could understand. This seems to be the technical level appropriate for the wikipedia.

What would it take to make this article neutral enough that you would remove the npov tag? The way it is as of 1:43 CST is npov as I see it. Perhaps if you added another paragraph to the problems section that better explained how string theorist look at LQG? Just a suggestion. --Hfarmer 07:45, 21 January 2006 (UTC)

Balance Restored
As Dr. Motl has not responded to my last entry here. I will take it for balance being restored. Whomever alters this article in the future should realize that this is supposed to be an encyclopedia. Simple description of LQG its claims failures and sucesses will do. There is no need for a commercial for or against. The people who read this encyclopedia know that what is written in it would not be gospel truth. --Hfarmer 07:35, 27 January 2006 (UTC)

Experimental verification
It is important and should be mentioned that LQG makes a testable prediction: speed of light is not constant but depends on the energy of the photon. cf Smolin sez that GLAST satellite will carry out the experiment. According to NASA, GLAST will go up in 2007.

Theory of everything and Heim theory
I came over here to the LQG talk page to make people aware of some recent changes to the Theory of everything article that seemed relevant to those with an interest in LQG. It seems, however, that some of my concerns have already found their way into the LQG article itself, which is probably a more pressing concern.

First, there have been numerous people over time who have tried to describe LQG as a would-be "Theory of Everything" in that article. I have done my best to explain that this is false, and that LQG researchers generally view its focus on quantum gravity only as a strength, not a weakness. (Judging LQG as a TOE rather than on its own terms would be fundamentally unfair.) However, some people have then accused me of trying to suppress and censor rival theories (I happen to be a string theorist, though I'm not of the LQG-hating variety). Getting some support on this point from people who are not string theorists would be appreciated.

And second, there has been a recent effort to elevate the ideas of Burkhard Heim so that they are treated as mainstream physics. "Heim theory", as it's called, claims to have predicted a bunch of particle masses and coupling strengths, and a paper about faster than light travel based on Heim's work was recently given a prize by a group of aerospace engineers. As far as I can tell, Heim theory is simply bad physics that hides its fundamental flaws behind complicated-looking mathematics; I seem to recall that only one paper by Heim has been published in a peer reviewed journal (and none on the topic by his current followers). One of the people in the pro-Heim effort at the TOE article (Slicky) also added a reference to Heim to the introduction here.

I'm not enough of an expert on LQG to feel comfortable changing this article myself (and I don't want to be accused of waging a lone crusade against this stuff), but if anyone who has been active here agrees with my impressions of "Heim theory" then they might want to revert that recent edit. And, of course, any support that you could give over at the TOE article would be appreciated as well. (For discussions of the merits of Heim theory or the lack thereof, take a look at the lengthy (archived) discussions at Talk:Burkhard Heim, Talk:Heim theory, and even Talk:Theory of everything.)--Steuard 16:20, 18 April 2006 (UTC)

Obviously, Mr. Steuard, does not like Heim Theory Take Care. --Will314159 10:42, 7 May 2006 (UTC)Edited--Will314159 16:46, 8 May 2006 (UTC)

Yes, he does not like Heim so much. In fact, Heim theory does have a lot in common with LQG. It is background independent, has a quantum of area of planck length squared arrayed in a lattice. Only difference is that Heim's lattice spans 6 dimensions, 3 of which are timelike (including time itself). 2007 should see a review paper on this in the mainstream literature. Also, the artificial gravity results of Martin Tajmar et al. stand to be confirmed at Berkeley and elsewhere in 2007, which would spell a revolution in physics! Walter Dröscher and Jochem Hauser used Heim theory predictions from the 1980s to derive the magnitude and direction of this effect. Did LQG or String have such a prediction? Dream on! --hughey 12:52, 17 January 2007 (UTC)

Heim theory has nothing at all to do with this theory. Trust me. -Hfarmer

is there saomone informed what the name of the account is which has as a result the speed of ligt that results when the light has a very short frequency??? —The preceding unsigned comment was added by 87.78.126.191 (talk • contribs).
 * Could you rephrase (repeat in diffeent words) your question? In present form it's impossible to understand what you are asking about. At least I have no idea what you are trying to say. Friendly Neighbour 15:28, 14 December 2006 (UTC)

I rephrase it: Does anybody know the formula which you use to calculate the deviation that occurs when the wave length of light is near the planck length and hope this one is now understandable —The preceding unsigned comment was added by 87.78.95.6 (talk • contribs).


 * Yes, now it is understandable. I believe several version of the light speed dispersions are possible. For a good example see Equation 2 in Amelino-Camelia G., Testable scenario for Relativity with minimum-length, Phys. Lett. B510 (2001) 255-263, hep-th/0012238. Another version is proposed in Equation 8 of Kowalski-Glikman J., Observer-independent quanta of mass and length, Phys. Lett. A286 (2001) 391-394, hep-th/0102098. However, the subject has nothing to do with Heim theory which this section of the Talk page is about. Friendly Neighbour 15:05, 16 December 2006 (UTC)

Crank magnet articles
I am looking for information from experienced WP editors on the problem of keeping good editors on Wiki. See the page here User:Dbuckner/Expert rebellion

This is no more than a list of people who have left Wikipedia, or thinking of leaving, or generally cheesed off, for the reason (1) what I will unpolitely call 'cranks', i.e. people engaged in a persistenta and determined campaign to portray their highly idiosyncratic (and dubious) personal opinion as well-established mainstream scientific or historical fact, or 'crank subculture' i.e. fairly sizeable subcultures which adhere strongly to various anti-scientific conspiracy theories (e.g. Free energy suppression) or anti-scientific political movements (e.g. Intelligent design) masquerading as "scholarship". (2) the problem of edit creep, i.e. the tendency of piecemeal editing to make articles worse over time, rather than better.

If you are in this category, leave a link to your user page there. If you can, put something on your user page that indicates reason for discontent. I particularly like war stories, so let me have any of those (links please, not on the page).

There is a more general discussion of this issue on Lina Mishima's page. User:LinaMishima/Experts Problem Note I am not in agreement with her title as it is not in my view a problem about experts, but more of adherence to scholarly standards, ability to put polished and balanced articles together. But her idea is good.

I don’t know much about this subject except that it's a possible crank magnet. If you know of any other, let me know, or even better, cut and paste this message on those pages. I'm going round the obvious places like intelligent design, Goedel, Cantor and so forth, but there must be many such. Dbuckner 15:03, 31 August 2006 (UTC)

Earlier version is a lot better
Upon review, I have to agree that the version in http://en.wikipedia.org/w/index.php?title=Loop_quantum_gravity&oldid=32572035 is substantially better than the current version.

I made a few edits to delete some of the worst material in the current version. The "Criticisms" were simply unscientific, as they amounted to complaints that the theory made assumptions -- which, uh, all scientific theories make; that's not actually a criticism, it's a way of missing the point. I reintroduced the emphasis on the problem of lack of experimental verification, which is a genuine scientific criticism. However, I still think the prior version was better; perhaps someone could revert and lock. 24.59.105.242 03:29, 20 November 2006 (UTC)


 * I suggest instead of reverting to a much older article, you do the work of reintegrating what you think should not have been deleted with the current version. Reverting the article I think would be disrespectful to the large number of people who have worked on it since I think it was September 2006 (!). Sdedeo (tips) 19:23, 24 January 2007 (UTC)


 * After all, it was Sdedeo who deleted half of the article, being disrespectful to the large number of people who have worked on it. That's the reason why I reverted it. 124.54.118.8 07:39, 30 January 2007 (UTC)

kodama state seperate article?
should kodama state, and lqg and standard model be seperate articles?

background independence
Rather worried here that the article pushes a rather controversial view -- that LQG is "background independent" while String Theory is not. The formulation of much of string theory is peturbative, but this does not mean that String Theory is itself background dependent. I think Witten (? -- not sure) made this point recently in a review of Lee's book. In any case, I'll try to dig up this reference, but I think it is possibly not good to describe LQG as background independent in a fashion that ST is not. Sdedeo (tips) 19:28, 24 January 2007 (UTC)

Some restraint in adding preprints
New, interesting, preprints are popping up every day on arXiv and are discussed on the relevant blogs. But this here aims at being an encyclopedic article, doucumenting established knowledge. So can we pls wait adding new stuff until it has evolved to a review article written by a respected author, or something similar? --Pjacobi 18:32, 21 April 2007 (UTC)

Artifacts?
"Many string theorists believe that it is impossible to quantize gravity in 3+1 dimensions without creating matter and energy artifacts."

What does this sentence mean, in plain English? What is a matter artifact? --Deadlyvices 20:48, 4 May 2007 (UTC)


 * Things that break fundamental laws of physics. Like breaking conservation of matter/energy. I have a different problem with this sentence. I do not think 'any string theorists believe it is possible to quantize gravity in 3+1 dimensions. I never heard of such proposition (at least since Einstein's times). --Friendly Neighbour 22:03, 22 June 2007 (UTC)

Quantization??
I think the article should explain how the space time is quantizied, i mean how do you get a 'Quantum' of Volume or Surface.

Also there is a contradicting method, when referring to 3-D geommetry are you supposing your metric is of the form:

$$ g_{ab}=N(t)dt^{2}+g_{ij}dx^{i}dx^{j} $$

Einstein criterion applied to i and j —The preceding unsigned comment was added by 85.85.100.144 (talk • contribs).

Still too technical
My earlier comment about 'artifacts' can be generalised to a criticism of the overall content of this article. It is far too technical and assumes far too much on the part of the reader. I happen to have a Ph.D. in solid state chemistry (which encompasses a lot of physics) but virtually all of this went straight over my head. There is not much point in an expert in LQG writing an encyclopaedia entry intended for other experts in LQG. Deadlyvices 17:38, 28 June 2007 (UTC)

Martin Bojowald paper in Nature Physics expected August 2007
In light of the "Some restraint in adding preprints" plea above, I won't add this to the article itself, but I wanted to make people aware of What happened before the Big Bang? which is a news release about a paper by Martin Bojowald and others to be published in the August 2007 issue of Nature Physics. I don't know whether a paper in Nature Physics is likely to lead to the same kind of media attention that a paper in Nature would. I'm not knowledgable enough to say what the impact (if any) for this article, and/or loop quantum cosmology, is, but my first reaction was that it would go in the list of the papers (currently 3) cited after the sentence "These topics are an active research in loop quantum cosmology." Kingdon 03:11, 2 July 2007 (UTC)

Non-Encyclopedic Tone
There shouldn't be any use of "I" and there should be less use of "!". Crasshopper 05:37, 5 July 2007 (UTC)
 * It should also be aimed more at the lay reader. At the moment large sections are essentially incomprehensible for anyone who doesn't already know the subject intimately, which amusingly gives it something in common with our articles on Pokemon.137.195.68.169 16:31, 25 October 2007 (UTC)

Proper Time and Active Diffeomorphisms
A maximum proper time difference between two given events does indeed exist in general relativity. I suspect a bit of obscurantism in the current article on this point. So, let's be clear that active diffeomorphisms imply an imagined change in real curvature as part of the transformation where ever this transformation is different from a mere transformation of coordinate systems.

67.102.139.25 07:03, 6 July 2007 (UTC)

'So, let's be clear', he says... Some strange new use of the word 'clear' to which I have previously not been exposed. Deadlyvices 21:36, 7 July 2007 (UTC)