Talk:Loop quantum gravity

• /Archive 1 - up to May 2004
• /Archive 2 - created 29 November 2004
• /Archive 3 - As of August 2005

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)</math>

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 ✉ 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.

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 [1], [2], [3] 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 [4] that cites the new preprint [5] 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)

==External Links== I would like to add a link to Abhay Ashtekar's home page. It has some excellent popular articles suitable for beginners about Space, Time, GR, and LQG. http://cgpg.gravity.psu.edu/people/Ashtekar/articles.html Also, I'd like to add a link to the new Scientic American Issue on LQG http://www.sciam.com/special/toc.cfm?issueid=40&sc=rt_nav_list Atoms of Space and Time by Lee Smolin "We perceive space and time to be continuous, but if the amazing theory of loop quantum gravity is correct, they actually come in discrete pieces." I"ll wait a few days. in the absence of adverse comment, I"ll made the additions. Take Care! Will314159--Will314159 19:18, 30 March 2006 (UTC)

Also Loop Quantum Gravity. Lee Smolin. Online at www.edge.org/3rd–culture/smolin03/smolin03–index.html --Will314159 21:33, 30 March 2006 (UTC)

I made the above mentioned external link additions and checked the links. Take Care!--Will314159 13:46, 31 March 2006 (UTC) And the reason these resources are under External links and not the Bibliography is because the Ashtekar home page, the SciAmer Special Issue, and Edge has multiple articles in one setting dealing with LQG and associated ideas. Take Care!--Will314159 14:22, 31 March 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 [6] 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)

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

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)

Lorentz violation

I am under the impression that LQG violates (local) Lorentz invariance by proposing a preferred reference frame for the definition of the Planck scale (the spin foam has a characteristic scale of one Planck length only in a particular frame.) My familiarity with this comes from talk of LGQ tests that probe Lorentz violation in GRBs. I'm pretty sure I'm right, so I'm taking out the unsourced opening statement that it preserves LI. Sdedeo (tips) 05:35, 27 September 2006 (UTC)

That's not necessarily the case, although judging from the literature you've linked to, it does seem to be a common (convenient?) misconception. LQG satisfies Lorentz invariance in local regimes.

To put it more simply than does the matter justice, let's imagine two observers measuring a particular length. One is at rest, the other accelerating. The one at rest measures the length to be minimal, i.e. the Planck length. The idea is that by Lorentz boosting, the other observer might see a reduction in the minimal length (the Plank length) because of Fitzgerald contraction. However, QM might seem to tell us that length perceived will still be the eigenvalue of an ordinary quantum observable (of course!), which means that it doesn't, some say, Fitzgerald contract: observer 2 cannot measure anything smaller than the Planck length, because it won't be an eigenvalue. Hence it can't Fitzgerald contract, and hence it can't be minimal. However, this overlooks a crucial fact. In LQG, the spacetime metric is a quantum field. Measurement is described by a Hermitian operator, but two distinct measurements (as the two observers' measurements are) get two distinct operators. And interestingly, the "length measurement" operators for the two parties involved don't commute: a "length eigenstate" for observer 1 (at rest) is thus not a "length eigenstate" for observer 2 (Lorentz-boosted). But, in quantum mechanics, a property which is not in an eigenstate for a particular observer is described by a probability distribution over a number of states: a superposition. This means that the length observed by observer 2 will have a mean value (over many experiments, say) which is contracted appropriately — weird, isn't it? — but have a minimal nonzero value which is given by the Planck length (because observer 2 can always measure the distance to be zero, i.e. there is no length, the two "points" on either side of the length are actually one point, in his reference frame). Ergo, no violation of Lorentz invariance.

Now, other theories of quantum gravity may have a problem with phenomena of this sort, depending on how they treat the idea of the spacetime metric. But LQG doesn't, precisely because it is a quantisation of GR. Conversely, we could say that an experiment which does detect a violation of local Lorentz invariance would probably falsify LQG!

And in future, please cite properly, instead of linking to search engine results. Not only is this wholly inappropriate by any scholarly standards, but the link you placed in the article is nigh unreadable, and renders it very hard to edit. Thanks. Byrgenwulf 11:42, 27 September 2006 (UTC)

After reading the above, I reverted the recent edits by Sdedeo. I think we need to discuss them here first, before we decide what to do next. 15:39, 27 September 2006 (UTC)

This is an interesting argument, but very limited. What you have established essentially is that a minimal length is compatable with invariance under boosts in the same fashion that the quantization of angular momentum is compatable with invariance under rotations. However, the structure of the "spin foam" is more complicated; theories of LQG that are not simply toy models to demonstrate this statement usually require "doubly special" transformations [7]. Sdedeo (tips) 23:19, 27 September 2006 (UTC)

Recent edits by Sdedeo

A few things here.

1. I hope that the message above this section, which labels this article a "crank magnet", is not leading anyone astray here. LQG is not a cranky subject: it is a recognised research program in quantum gravity. LQG papers have been published in journals such as Physical Review: and it does get more respectable than that as far as scientific publication goes. The only cranks here, as far as I am concerned, are those who have an irrational obsession with discrediting LQG beyond what is reasonable (no names mentioned).
2. As far as the "diffeomorphism invariance" section which Sdedeo deleted, I agree that it is not appropriate, as written, for an encyclopaedia...it does give the impression that the writer is, himself, discovering background independence and diffeomorphism invariance as he writes: this does not work. The article needs a section on diffeomorphism invariance, as it is a crucial part of LQG, but I shall write something more appropriate when I get the chance. I have recently completed writing an article, and have been intending to fix the page up for some time, but have not wanted to embark upon the project until I can do it justice. I shall be doing it properly soon.
3. Now, regarding this nonsense of "Lorentz invariance violation". In order to support his idea that LQG violates this principle, Sdedeo links to the blog of someone known for his anti-LQG polemics. He also provides a set of search results of abstracts as a "citation" of supporters of LQG admitting that this is the case. Among these citations that Sdedeo provides, allegedly to "prove" that LQG supporters agree that LQG violates Lorentz invariance, is this paper, which proves, along the lines I discussed above, that this is nonsense: there is no such violation. In fact, it points out that it is a "simple minded" criticism of LQG to insist on this. The paper in question was published in Physical Review D: the premier journal for this sort of research. So, I have removed the paragraph, which contained typographical errors anyway, as nonsense and OR. The sources are not properly cited, and nor do the "cited" sources support the argument made: they contradict it (other than the blog, but citing blogs in quantum gravity articles is not appropriate).
4. My intuition tells me that the unreasonable and spurious attack on this article is motivated, at least in part, by the "crank magnet" message above, and is an offshoot of the "expert rebellion" phenomenon. As such, I shall assume that it is happening in good faith, and I shall not make a fuss about it (yet). Additionally, while I am not so arrogant or presumptuous to term myself an "expert", I do sympathise wholeheartedly with the project being undertaken on that board. Indeed, I have posted an account of one of my own recent "crank encounters" to the relevant subpage. I hope that part of the "expert rebellion" is not deliberately harming the integrity of what should be scholarly articles on scholarly subjects!
5. I know my little rant here will probably make me sound a bit like a crank. I am not. I promise. I am simply an editor who feels very strongly about maintaining Wikipedia's standards, and is more than a little frazzled himself about the endless stream of nonsense that we have to combat. I am disturbed that someone who is apparently very concerned about the same things as I am, can come and deposit a load of nonsense, without proper citation, and which demonstrates a fundamental misunderstanding of the subject, without even discussing their changes on the talk page, and then go on to rant about the events on their user page! This simply does not do. Byrgenwulf 22:30, 27 September 2006 (UTC)

1. Post scriptum. I note that Sdedeo has now put a rant about me on his userpage. He claims to "have been involved in research towards detecting Lorentz violation (sic) effects" predicted by the theory. Searching paper abstracts and linking to search results does not really count as "research", in my book. For all I know, he may be more deeply involved, but then I invite him to discuss the problem properly, here. Personally, I think that someone "involved in research" like that would not have addressed the issue as Sdedeo did, and expressed such uncertainty about his understanding on the talk page. Nor, I believe, would they be wanting to restrict Wikipedia's scientific reporting to the level of popular science, I don't think, as Sdedeo calls for here. I could be wrong, again, I know. I apologise in advance if I cause offence by this. I simply do not appreciate this sort of attack, which is completely uncalled for, and not really very reasonable. Byrgenwulf 22:47, 27 September 2006 (UTC)

As I have reported on my user page, User:Sdedeo#Wikipedia_and_the_problem_of_expert_knowledge, it is a common, mainstream opinion that LQG violates Lorentz invariance. The well-publicised statement that the photon dispersion relation is altered from w=k requires that ordinary Lorentz invariance be broken. A "pseudo-Lorentz invariance", invariance under "doubly special" Lorentz transformations has be invented, although it is impossible to gauge this and recover General Relativity. I'm certaintly willing to acknolege that a minority of workers in the field may disagree. I personally do not find their argument compelling but wikipedia is not the place to debate such claims and I certaintly mentioned this in my edit as a minority viewpoint.

At this point, Byrgenwulf finds a single article that holds to the minority viewpoint that standard Lorentz invariance can be recovered. He uses this to decide my edits are "nonsense" and that I have a "fundamental misunderstanding" of the subject. Quite the contrary; the long history of believing in, and searching for, Lorentz violation in LQG can be found pretty easily by someone with an understanding of the field -- see refs in [8].

Sdedeo (tips) 22:57, 27 September 2006 (UTC)

By all means I can't judge the content dispute here, but I humbly ask both of you User:Sdedeo and User:Byrgenwulf to consider the possibilty that some sort of misunderstanding his occured here? I've seen you both on other places as decent guys and would like to see a full scale war only if really, really necessary. --Pjacobi 23:00, 27 September 2006 (UTC)

I have no desire to revert or otherwise edit war with Byrgenwulf and am no longer involved in editing the page. Sdedeo (tips) 23:01, 27 September 2006 (UTC)

My main point here, Sdedeo, is that citing the blog of Lubos Motl, a noted polemic opponent of LQG, does not cut it. Nor does linking to a search page of abstracts in support of a point, one of which abstracts actually contradicts the point made (the Phys Rev paper). And yes, the article I cited was among the list of abstracts which you "graciously provided" ostensibly to prove that the theory does violate Lorentz invariance. See for yourself: click on the second link you provided (not Motl's blog, which is worth less than nothing in this discussion), and you will see the paper I cited.

I also by no means believe that something is correct just because it is published in Phys Rev. But I do believe that by any scholarly standards, a Phys Rev citation is worth more than someone's blog. And I believe that the arguments offered in the paper I cited are far more rigorous and correct than the arguments contra.

Sdedeo says it is a "common, mainstream" opinion that LQG violates Lorentz invariance. This is not really so. It may be a "common, mainstream" misconception that it does, and it is certainly a "common, mainstream" string theorist's strawman, but it is not actually the case. While some (but not all, not by any means all) of LQG's proponents may suggest violation of Lorentz invariance as a possible consequence of the theory, this is not part of the central "core" of the theory. Indeed, I would argue that the idea is a minority one among LQG's more respectable proponents, but one which has seized the imagination of many people, without them stopping to check their facts or think about the arguments a little. I never had any idea that this Lorentz invariance nonsense was quite as widespread as it is (this is not specifically my field, though I do profess a fairly thorough understanding of LQG).

I know that Lee Smolin suggested that it might be a problem, and Sdedeo states on his userpage that he "trumpets" this idea, but this is a bit of an exaggeration. Indeed, Smolin main paper on Lorentz invariance violation [9] (to which it would have been very easy to provide a direct link, by the way, instead of your page of search results), takes the approach that this breaking of Lorentz invariance may or may not be a problem, but if it were, it doesn't matter, because his form of "doubly special relativity" covers it. As it turns out, this doubly special relativity has its own problems, but Smolin never even suggested that the Lorentz invariance violation was a necessary part of LQG, and nor did he stipulate that this new Lorentz group was a necessary "fix" to the theory. He merely said that some physicists have pointed to what they think might be a problem, and he offered a possible solution (he never really analysed the problem).

As far as Sdedeo's analysis goes, I don't accept it. I also found the paper to back me up after I had written my explanation above — although I had read it before, of course. I suppose I am used to thinking of LQG in a certain way...

Yes, the analysis I provided is for the hbar limit. That's the point of the whole argument, really. The hbar limit is a "minimal length", smaller than which there is nothing. But there is no sound reason to insist that Lorentz invariance must only occur in the classical limit, as you seem to be doing, that it is some sort of "emergent phenomenon"...whyever should this be the case? If it is preserved right at the "bottom", then all is well, because this is where the argument is centred. And in the classical limit, obviously, the results will be returned in accordance with expectations, so all is well. The photon dispersion thing, if it is what I am thinking of, is an extremely peripheral issue in LQG, and doesn't need Lorentz invariance violation anyway: slight differences from expected ratios for sufficiently large numbers should be accounted for by gravitational effects, which is ultimately what the theory is trying to do, last time I checked. The same goes for the doubly special relativity idea: these are all very much on the fringes, as it were, of LQG, and are neither a crucial or even a necessary part of the theory — they are hardly what might be called "canonical".

Sdedeo mentions that the "spin foam" model is more complicated. Yes the spin foam model is more complicated in some respects, but strictly speaking the spin foam model is not LQG. While both theories make use of loop quantisation, in LQG proper (the canonical approach) the spin networks are in three dimensions, but in the spin foam model they are in four dimensions. Of course there will be differences, but problems with the spin foam model belong in that model's article, or topological quantum field theory. The header of the LQG article specifically states that the article deals with the canonical approach, not theories of loop quantisation in general. But as has been pointed out, this is not the place for such debating these matters.

The bottom line is that the recent insertion did not even discuss the issue properly at all, as it ought to have: why might Lorentz invariance be violated? how is this dealt with? what are the arguments pro and contra? Linking to a blog and two pages of search results (which do not even properly back up the claims made in the edit!) does not amount to proper citation, I don't think. I am therefore well entitled to remove the paragraph, and insist that it is done properly. And I am well entitled to claim that the insertion is nonsensical, as well.

But I likewise have no lust for edit warring. I think the article needs a complete rewrite, since it manages to say almost nothing. And the whole time, when it does say something, LQG's critics interject, LQG apologises, and the article hobbles on again. This doesn't work. Ideally, a whole article on loop quantisation would be lovely, and we really do need something on background independence/diffeomorphism invariance in the article — since it's the main idea behind LQG. I certainly do not wish to drive you away, Sdedeo (this article needs all the help it can get). It seems that we have merely had a very unfortunate misunderstanding, which is in all likelihood a side effect of the constant befrazzledness that is plaguing many corners of Wikipedia. Byrgenwulf 00:51, 28 September 2006 (UTC)

Friendly Neighbour and Sdedeo

A short explanation. I am also a hero of the recent addition to Sdedeo's Talk page. I'd like to explain that I do not claim expertise in the field of LQG. However, I am a physicist and I am able to understand the papers in the field. Sdedeo made a very botched job in convincing me that he's right. The abstract lists were not a way to support your views with scientific papers. All the papers I looked in seem to confirm that Byrgenwulf is right. Maybe it is because he has the ability to cite one paper at at time, not a whole repository of them.

I do believe the article needs a rewrite. In the case of Lorenz invariance it should be stated that there are different views on whether it is violated in LQG. As I have recently stated on another article discussion page, this is an encyclopedia, not a physics hanbook and all widely reported views should be presented here, even when they are wrong (but they status should be reported as well). Please read the linked edit to see what I mean before you hit me. Friendly Neighbour 06:32, 28 September 2006 (UTC)

Here are the facts:

1. The article has claimed -- for who knows how many months -- that LQG generically preserves ordinary Lorentz invariance. This is incorrect.
2. I corrected this error, explained that the mainstream LQG view, held by both critics and proponents and folks in between, is that LQG violates Lorentz invariance, provided references and noted minority views. (For my choice of defining Lorentz invariance as a "minority" view, see comments below.)
3. My edits were deleted -- not improved upon, not expanded, not refined: deleted -- twice.

Byrgenwulf now claims that he deleted my contribution because it contained a reference to Lubos Motl, a critic of LQG, in addition to pointing the reader to a unedited list of papers on the question of Lorentz Violation. He also has some personal thoughts on LQG, which are irrelevant and seem mainly fixated on the argument he presented above which prove nothing about Loop Quantum Gravity beyond a toy model whose only input is "granularity on the Planck scale."

Friendly Neighbour claims both to be a physicist and yet also unable to browse a list a papers in the field to determine the truth of my claims. His claim that randomly clicking on the paper abstracts convinced him that Byrgenwulf is correct is hard to believe -- the first five papers in the list I linked to are discussions of Lorentz violating effects in Loop Quantum Gravity, and an informal count (plus or minus two, let's say) says that out of the seventeen papers I presented, only two claim unambiguously that standard Lorentz invariance is preserved in LQG (fourteen claim that Lorentz violation is violated in Loop Quantum Gravity and one paper appears to be a "false hit".)

Sdedeo (tips) 14:24, 28 September 2006 (UTC)

I did not say I'n not unable to parse your list. The problem is I do not have time to read 17 papers to check your claims. Even bigger problem is that neither do you! Otherwise you would not include ones that contradict your position. Sorry but if you do not have time to list 2-3 most important papers which confirm your edits (they would be useful as citations, anyway), I'm not going to do your research for you. Friendly Neighbour 14:52, 28 September 2006 (UTC)

Your claim is now not that you read the papers and they contradicted my statement, but that you did not read them at all. In any case, it is immaterial; you are unfamiliar with the research and yet felt happy to delete the claim as "controversial". Sdedeo (tips) 15:12, 28 September 2006 (UTC)

OK, I was not precise. Mea culpa. What I meant was that I looked only into the papers of links provided by Byrgenwulf because he bothered to link actual papers, not search result lists and explain why they are relevant (it saves a lot of time as one needs not read the whole paper to see what it's supposed to prove). However, I did ask you three times (including once on your Talk page) for actual paper links. The time you wasted rebuking me could be used to find some papers which could actually make the article better balanced and better referenced. Friendly Neighbour 15:19, 28 September 2006 (UTC)

I had dinner on Monday with some friends of mine: a computer scientist and a mathematician. The subject of wikipedia came up and the mathematician said that she found the entire project totally ridiculous. Every time she googles a term and comes to the wikipedia page she finds massive errors. Being a bit of an idealist, I said, yes, wikipedia has problems, but you know, it's a collaborative effort, why not edit the page and fix it -- you have all this valuable knowledge, why not make it available, it's a wonderful thing, etc., etc.. Little did I know that the average lifetime for such a contribution would be measured in hours.

You deleted my content before Byrgenwulf came along, so one assumes that this revert was made in a complete (as opposed to near-complete) state of ignorance. You are certaintly correct that time has been wasted. However, I did not waste my time -- you did. Further, your unwillingness to acknoledge the incomplete state of your knowledge, your belief that reading papers to correct it is unnecessary (even when a list is provided to you), and the fact that there are appear to be others like you forces me to agree with my friends. Sdedeo (tips) 15:42, 28 September 2006 (UTC)

You're wrong. I reverted you almost four hours after Byrgenwulf gave his reply on the talk page. This is my revert:

• 15:34, 27 September 2006 Friendly Neighbour (Talk | contribs) (rv the contested claims by Sdedeo to the last version by Byrgenwulf at least until a consensus emerges)

and these are the relevant changes on the Talk page:

• 15:39, 27 September 2006 Friendly Neighbour (Talk | contribs) (›Lorentz violation - Why I reverted Sdedeo)
• 11:42, 27 September 2006 Byrgenwulf (Talk | contribs) (›Lorentz violation - Nonsense - I'll change the article later)

I see you are not willing providing any article links. You seem to believe that thrashing my reputation is a better way to improve the article. Which is funny because I was open to discussion and willing to change my mind if only you deigned to provide me the information I needed. So be it. Friendly Neighbour 16:30, 28 September 2006 (UTC)

I have provided article links -- and discussed their content over and over on this page. Your first contribution was to delete these links. Your later contributions were to explain why you did not, and would not, read them. Yes, the timing does suggest that you read Byrgenwulf's (incorrect) statements before reverting. Sdedeo (tips) 16:34, 28 September 2006 (UTC)

My later contributions can be summed up in one sentence: "Please help me support you". But you refused. OK, I'm done with the controversy. I'll not comment here at least until you decide to help yourself with some actual references (not a search result list - I can create them myself). Friendly Neighbour 16:46, 28 September 2006 (UTC)

As I have said, I am no longer involved with editing the page, and I no longer have faith that wikipedia can create and maintain articles on technical subjects beyond a popular level. I will be curious to see if this particular issue is ever corrected given the way editors behave on physics-related articles. You and Byrgenwulf have decided to delete the content I provided; fine -- I will not engage in the usual wikipoliticking to reinstate it. I have provided you and Byrgenwulf with information about an important defect in the article, and, if you did not have them already, the tools to verify this defect and correct it. The rest is up to you. Sdedeo (tips) 17:03, 28 September 2006 (UTC)

Oh, I see I'm now featured in a name of a section. What a honor! - as Dr. Zoidberg would say.

Here is the contested (it's a fact) section in last version of Sdedeo's edit:

Unlike General Relativity, LQG is acknoledged by both critics [10] and supporters (see, e.g., [11]) not to satisfy local Lorentz invariance -- although controversy exists on this point (see, e.g., [12]). Tests of LQG-induced local Lorentz violation through modification of the photon dispersion relation have been suggested through searches for energy-dependent photon arrival from distant gamma ray bursts [13].

I did make my search. The Lorenz invariance (not only of LQG but also of string theories) was indeed postulated by many authors. However most recent papers do not see any need for it. The strongest quotation I found comes from the very beginning of Fay Dowker, Joe Henson and Rafael D. Sorkin "About Lorentz invariance in a discrete quantum setting", 2004:

"Contrary to what is often stated, a fundamental spacetime discreteness need not contradict Lorentz invariance."

Therefore it's certainly not true that LQG needs to break the invariance. Although it's true that a lot of work has really went in proving it must. Therefore I feel that Byrgenwulf (and the third article you cited) say it's a common misconception. I feel some mention of the controversy is indeed needed in the article but probably not a whole section. That were my $0.02. Friendly Neighbour 17:49, 28 September 2006 (UTC)

As I explained above, a discrete spacetime alone is comptable with Lorentz invariance in the same fashion that quantization of angular momentum is compatable with rotational invariance. LQG postulates much, much more than just a fundamental spacetime discreteness and leads to the Lorentz violation effects that are discussed in at least fifteen papers I have given you and which you still refuse to read or remark upon. You have apparently still not read them or understood them in any way.

If you had done more than read the first sentence of the abstract of the paper you cite, you would discover that after presenting a toy model, the authors proceed to discuss realistic LQG models. As the authors themselves state:

We now would like to also give a Lorentz invariant toy model with a fixed discrete length spectrum. Unfortunately, it can not be directly inspired from 3d Lorentzian loop quantum gravity since this theory predicts a continuous spectrum.

When moving to a discussion of realistic LQG models, they use DSR:

Deformed Special Relativity (DSR) [14] was especially introduced to address the issue of constructing a relativistic theory, i.e. one in which inertial observers see equivalent physics, with the equivalence being given by a group of transformations with both a invariant speed c and a invariant length lP . A feature of such a theory is now that the speed of light depends on the energy E of the beam and that c is only the speed of light as E → 0 [14]. The theory is still Lorentz invariant, even though the action of the Lorentz transformations becomes non-linear, and the structure of the translations is modified.

It should be emphasised that when they declare the theory to be "Lorentz invariant" here, they do not mean Lorentz invariant under the usual Lorentz transformations, but rather a different set -- as the authors say later, the Lorentz invariance is "hidden" (I believe what they mean here is that there is a different representation of SO(3,1) in play because the spacetime coordinates now no longer commute, although this is unclear to me.) See Eq. 64 of [14] where the new Lorentz contraction formula is presented for DSR.

The violation of Lorentz invariance in the usual sense can be seen trivially by the alteration of the photon dispersion relation. The number of times I must restate this point, and list articles that refer to this, is rapidly approaching infinity. Sdedeo (tips) 18:28, 28 September 2006 (UTC)

Do you humbly aim at aleph zero or ambitiously for continnum? And what exactly is your point if you do not want to edit the article? Friendly Neighbour 18:39, 28 September 2006 (UTC)

First you delete my contribution. Then you refuse to read the relevant articles. Then, when I do attempt an (incomplete) explanation of an article of your choice, you make it clear you're not interested. This argument arose when you objected to my characterization of your and Byrgenwulf's deletions as ill-informed. Sdedeo (tips) 18:50, 28 September 2006 (UTC)

The more papers on the subject I read (you are wrong that I refuse to read them - but I have more work as you refuse to cooperate) the more I see that:

1. Lorenz violation (LV) in LQG is an option, not a necessity. I already cited a paper on this. You yourself cited another one.
2. It was introduced to solve problems (ultraviolet divergences of quantum field theory among others) and not because it was inherent in LQD. In fact it was proposed in string theory (V.A. Kostelecky, S. Samuel, Phys. Rev. D 39 (1989) 683) much earlier than in LQG (and string theories have actually more ways of LV than LQG). Good history of Lorenz violation research is given in Ted Jacobson, Stefano Liberati, David Mattingly, 2004, "Quantum Gravity Phenomenology and Lorentz Violation"
3. The modified photon dispersion relation is simply a consequence of the modified Lagrangian introduced to create the LV. You were already presented with two papers stating that LV is not necessary in LQG. So the fact that LV "can be seen trivially by the alteration of the photon dispersion relation" (your words) is true but in no way proves that LV is necessary in LQG.

Therefore I believe more strongly than before that your edit I cited above is in part ("is acknoledged (sic!) by both critics and supporters not to satisfy local Lorentz invariance") plainly wrong and in part (where it comapres LQG to GRT instead of other quantum gravity theories which all tried to use LV as a tool) simply misleading. Friendly Neighbour 20:13, 28 September 2006 (UTC)

I am familiar with Kostelecky and Jacobson's work. In fact, I have written a paper on it. One thing you'll learn from them is that a modification of the dispersion relation is a signature of Lorentz violation. The mainstream of LQG predicts modified dispersion relations. String Theories can only violate Lorentz invariance in a "spontaneous" fashion -- i.e., unlike the forms of LV in LQG where Lorentz violation is a fundamental aspect of the theory. Kostelecky and Samuel's foundational 1989 paper discuss LV only in this spontaneous context.

The mainstream opinion is that LQG predicts modified dispersion relations for the photon. In the edit you deleted, I discuss this and also mention the fact that some dispute the inherency of LV in LQG.

While I believe that LQG predicts Lorentz violation, and further that many workers in the field have made similar predictions, I never claimed that LQG required Lorentz violation. Further, in Kostelecky and Jacobson's work they introduce LV through a LV term in the Lagrangian, but this is not the case in LQG, where Lorentz violation arises from, for example, the introduction of DSR. Again, take a look at the DSR paper where the modified Lorentz transformations are presented.

I am happy to see passive aggressive use of the word "sic" is alive and well. Your belief that what I wrote ("LV is acknoledged by critics and supporters not to satisfy local Lorentz invariance, although controversy exists on this point") is adaquately represented by half of what I wrote is incorrect. Sdedeo (tips) 20:37, 28 September 2006 (UTC)

further remarks

From my talk page: "You should have started with an explanation on the article Talk page. The way you edited the article with no explanation made YOU look like the non-scientist party. That's why I reverted you. I could have been wrong but you seemed to send all the well-known "wiki-kookism" signals. However, I made it clear that I mean the revert as temporary. And you, instead of providing the links to the relevant papers in a human readable way, started a revert war."

Your claim is now that you reinstated the incorrect statement that LQG generically satisfies Lorentz invariance because it looked like I was a "wiki-kook". Perhaps your version of wikipedia has special ultraviolet coloring that tells you if an edit is by a "kook" -- if it does, you now know that it this coloring can not be trusted and that, in fact, there are better methods to determining the truth of a claim. There is also a strange contention that this page is not "human readable". Sdedeo (tips) 16:02, 28 September 2006 (UTC)

---

I shall try to be brief here. My objections to Sdedeo's edit have not been properly portrayed by him. The way I see it is as follows:

1. Linking to a polemic blog is not an acceptable citation for an encyclopaedia article (cite a peer-reviewed paper by Motl saying the same thing, and I'll be fine with that. But not his blog.)
2. Linking to a page of search results in support of a point, when some items among those results contradict the point being made, is not an acceptable citation for an encyclopaedia article
3. Sdedeo posted a note on the talk page here, expressing uncertainty over his understanding of the problem. I posted my understanding of the problem, which I deliberately kept quite simple, and an admonition to cite sources properly. But initially, I left the paragraph intact, saying I would fix the article later, rather than remove it altogether. As far as I am concerned, it does not do to just say that "some people think this, and others think that". The article already explains so little, that no-one can immediately see why LQG might require violation of Lorentz invariance.
4. Friendly Neighbour, seeing this, and the fact that Sdedeo had not responded on the talk page, I guess, reverted Sdedeo's changes.
5. Sdedeo replaced his edit, despite the concerns voiced on the talk page, without discussing it.
6. Seeing that, I removed Sdedeo's paragraph, similarly without discussion, as he was still not citing it properly.
7. Sdedeo then took umbrage at all the above, and this situation has deteriorated since.

The relevant facts are much simpler: I corrected an error in the page, and my contribution was deleted.

Now, as far as the actual situation goes. The "paradox" is actually very simple. The solution I provided is similarly simple, although underlying it is the proof that the two operators (for each observer) do not commute, which is where the details come in. Now, Sdedeo, judging from his previous comment, seems to have the opinion that LQG (in its canonical, three-dimensional spin network form) is a "toy model" which is superseded by the 4d spin-network spin foam idea. That is not the case...3D spin-network models are pursued as a viable solution to the quantum gravity problem: that what LQG is. And since the article is about canonical LQG, it doesn't matter what the literature on spin foam models says, or how any other theory making use of loop quantisation handles the problem.

Sorry, I mispoke when I used the phrase "spin foam" above. As I have noted repeatedly (and will note again below), your explanation of the satisfaction of Lorentz invariance applies only to a toy model and not to the full LQG.

In fact, I don't think Sdedeo has been reading and understanding the papers to which he links. He explains to Friendly Neighbour above, after a bit of incivility, that in this paper, the authors use DSR to analyse LQG "more realistically" than with the toy model they present earlier in the paper. Only, they don't. They treat DSR theories an entirely different group of theories to LQG, which, by rights, they are. They then compare the results they got from LQG with the results from DSR, and while the results are comparable, the details of the analysis differ. That the whole point to their paper!

What I described as the essential aspects of this paper are entirely correct. Byrgenwulf's statements here and elsewhere that the mainstream belief holds DSR totally ancillary to the LQG discussion are incorrect.

Similarly, this latest paper to which Sdedeo has linked about DSR has nothing to do with loop quantum gravity, specifically. Indeed, they treat DSR as a 5d theory and fix an appropriate gauge to deal with a 4d spacetime. This is very, very different to the approach taken in Loop Quantum Gravity. Fundamentally so. It is also ironic that Sdedeo dismisses two particle LQG systems as "toy models" when this paper, which gloriously presents his new Lorentz transformation equation, deals only with a single particle system. What Sdedeo is doing is a little like citing papers dealing with tachyons in bosonic string theory to try to argue that heterotic string theory has a problem with them.

I presented that paper to explain how invariance under DSR differs from invariance under the usual Lorentz transformations and why invariance under DSR is not ordinary Lorentz invariance. There is a subtlety here in that "Lorentz invariance" in Livine & Oriti is being used in a special way: preservation of the group SO(3,1), and not the preservation of the ordinary representation of the group. Since you seem to think that invariance under DSR is invariance under the ordinary Lorentz transformations, you might read this article, or at least look at the equation I point you to.

The bottom line, Sdedeo, is that this Lorentz invariance issue is not a big deal for LQG, despite what Motl might say on his blog. Some people have taken it to heart, evidently, and developed all these doubly special thingies but they are not a part of LQG.

Your opinions about the "true nature" of LQG are outside of currently accepted developments of the field. Indeed, in the very paper that you claim establishes ordinary Lorentz invariance above, the use of DSR -- the theory that Byrgenwulf dismisses -- is crucial.

Had you written something comprehensive on the matter and cited it properly, I wouldn't have had to waste so much time chasing down all these references, and we could no doubt have dealt with this more amicably. But as things stand, I can only conclude that you don't really have your ducks in a row on this one, as your initial post seems to indicate. And since the paragraph was not appropriately cited, it is best left out. I would also ask you again (I already did on your talk page) to remove your remarks about me from your userpage. Otherwise, I shall do it myself. Thank you. Byrgenwulf 19:38, 28 September 2006 (UTC)

Your claim that I have provided no references is ridiculous, given my presentation of seventeen articles and detailed discussion of their results. You have not "chased anything down." You have only repeated, continuously, that the belief that Lorentz invariance is a possible prediction of LQG is "nonsense" and betrays a "fundamental misunderstanding" of LQG. You should drop Carlo Rovelli a line. Simp – Ah! Nice. So, does loop gravity predicts Lorentz violation or not? Sal – I am not sure. I think so far it is like large extra dimensions for strings. Could be. Could not. [15]

You ask on my talk page "Is there any chance that you adjust your userpage so that it does not give the appearance that I am an irrational buffoon who randomly reverts positive contributions to scientific articles?" My userpage does not say that. The impression it hopes to give is that you are someone who reverts positive contributions to scientific articles. I do not believe you are an irrational buffoon. Sdedeo (tips) 20:05, 28 September 2006 (UTC)

this fight has absolutely ruined my day

I'm sorry, this fight has absolutely ruined my day. I have never had such a bad time talking about physics in my life. I am completely depressed and am almost in tears. I bear much of the responsibility for this given my uncollegial tone. I would never talk to colleagues like this. It just got out of hand, and I am sorry for it.

While I really don't want to continue as part of the wikipedia project anymore, you guys -- for all that I think your beliefs about Lorentz violation and loop quantum gravity are incorrect -- seem like nice people. I have presented, as far as I am able, arguments to support my edit. Both you guys (and I'm going to guess we're all guys here) have read them, as I have read what you have written.

I would like to suggest we let this matter drop. In addition, I would like to suggest that we delete the entire discussion we have had to this point. Nobody other than us is going to read through it, and interspersed with actual physics talk is just a huge amount of bile, most of which is due to me and which is really upsetting me now. I have edited my User page to remove all discussion of this. Please let me know if you agree with this by signing your name. The third person to sign should go ahead and delete (or archive, if you really must.)

Sdedeo (tips) 21:05, 28 September 2006 (UTC)

I'm sorry for your day. I also spent a lot of time on this. But at least I learned a lot about LQG. Anyway, I've checked Wikipedia:Talk page guidelines. It says "Archive — don't delete" nut only in the context of articles being too long. It has nothing on a situation like this one.However, the spirit of the guideline is certainly against a deletion. Personally, I don't mind if you delete the whole discussion but I will not do it myself. Maybe archiving is the best solution, after all? Regards, Friendly Neighbour 21:17, 28 September 2006 (UTC)

I think the precident WP:IAR and WP:PI is adaquate for deleting this. We're not trying to conceal anything, we're all in agreement, nobody else has participated (except to tell us to chill out), and the whole thing is in good faith. I understand if you don't want to. Let's wait to hear from B to see if he is OK with deleting and then I will go ahead. Sdedeo (tips) 21:20, 28 September 2006 (UTC)