Talk:Loop quantum gravity/Archive 3

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Archive 4

Is it the "main competitor" of string theory?

I am neither a string theorist nor a loop-quantum-gravity theorist, but rather a mathematician with a semi-professional interest in physics. So I have no real stake in saying whether Miguel or Lubos Motl is really right on the physics. But I am convinced that the article as written is not neutral.

The first sentence of the second paragraph says, "As a theory of quantum gravity, LQG is the main competitor of string theory." This sentence introduces one of the main themes of the entire (long) article. But it is not a neutral statement. Most loop quantum gravity theorists might believe it, but very few string theorists do. Most string theorists believe that string theory doesn't have any competitors as a theory of quantum gravity. Rather they only see themselves as trying to finish a vast but incomplete work. Most of the few string theorists who have considered loop quantum gravity only say that it isn't helpful.

Well, that seems to be the rather unfortunate state of affairs. There doesn't seems to be much discussion going on, just invectives. This is NOT supposed to be a popularity contest. In any case, people working in LQG naturally believe it to be a serious candidate for a theory of quantum gravity: otherwise they would be putting their time elsewhere! Similarly, naturally string theorists disagree.

I'll concede that I have some trouble staying neutral myself. To me it mostly sounds like, "we claim competition with string theory because we don't want to learn it." I'll concede that some loop-quantum-gravity theorists understand string theory much better than I do. I'll also concede that when I have asked string theorists what is really wrong with loop quantum gravity, they haven't much enlightened me. I have seen Lubos Motl's criticisms, but they are too partisan to really teach me the intellectual asymmetry that I believe is there.

Despite claims to the contrary on the part of string theorists, I am not aware of a single point of contact with experiment. What will happen if and when CERN's LHC finds no supersymmetry? Will they move the expected supersymmetry breaking energy just above the range of LHC and claim it is around the corner as they have done for the past 20 years? Admittedly, LQG does not make contact with experiment either. That is why I am getting a Ph.D. in math. — Miguel 16:59, 2004 Nov 22 (UTC)

So maybe that's the correct conclusion: The LQG article is too glib, the article criticizing it is too vitriolic, both of them are too long, and neither one solves the problem.

This is NOT supposed to be a popularity contest. Hopefully nobody other than Lubos is criticizing the description of the actual mathematical tools used in loop quantum gravity. — Miguel 16:59, 2004 Nov 22 (UTC)

- Greg Kuperberg, 2 Nov 2004

Too long, yes. After all, one ought to be able to sum up fundamental physics in 10K?! The length issue is going to have to be solved by putting technical development in separate articles, at some point. But for the rest: Most string theorists believe that string theory doesn't have any competitors as a theory of quantum gravity. Nor as a theory of anything else, probably. But would we ask them? I think Greg is perhaps missing what NPOV tries to do, here. It certainly does not consist of saying that string theory has no competitors; the quantum gravity territory is somewhat disputed. Charles Matthews 21:44, 2 Nov 2004 (UTC)

Charles, you are doing an outstanding job of refactoring the article. I have seen you in action lately on other articles and I love it. Must be all that experience from writing Go books ;-)

Anyway, I believe everything up to and including the ingredients of quantum gravity is neutral. If anyone disagrees, I'd like to start addressing specific concerns. I would like to see the article reach a final form that is concise and informative. Maybe there should be a main article link under loop quantization, because that is really where the meat is. The open problems and implications are more iffy and need work in any case (some of the statements are not really correct either). I wouldn't mind if the physical implications were removed from this article, since they are of a more speculative nature, the open problems are important because they would give an idea of what the LQG community thinks it needs to do to succeed. ‐ Miguel 17:18, 2004 Nov 22 (UTC)

I don't think that I am missing the point of NPOV. I will try to say it more clearly: The real dispute is whether not there is a dispute between string theory and loop quantum gravity, and not which theory is better physics. What if I claimed that Velikovskian astronomy ("Worlds in Collision") is "the main competitor" of the Newtonian model of the solar system? Velikovskians would like to say that theirs is the main competitor, but people who understand Newton's laws see it differently. I see nothing neutral about acknowledging a dispute solely because one faction believes that there is one. The page should only say that LQG advocates believe that their theory competes with string theory.

It says that now. — Miguel 16:59, 2004 Nov 22 (UTC)

And yes, you should ask string theorists, and not just Lubos Motl. You could learn from them, as I could.

As for the excessive article length, its main significance is that it filibusters the discussion. That is also typical of people who want to establish an artificial dispute.

- Greg Kuperberg, 3 Nov 2004

On the length, I think plenty of editorial work is required. Patience is a virtue in relation to that, though; in time I think this article should be a summary of main points, and links to more detailed discussion. Yes, to the extent that there are two sides here, we should try to get away from advocacy, and each side trying to bolster their case. It isn't that easy, give the technical level of the points being made.

On the other point, I think Greg meant to write

The real dispute is whether [or] not there is a dispute between string theory and loop quantum gravity,

and I suppose one should address the question of whether what is written is somehow a journalistic blowing-up of something lesser into a schism which is isn't really there. Normal WP stylistic devices for this go like: 'some physicists regard ... ' and on the other hand 'other physicists consider this a sterile debate at present', or whatever matched the main attitudes you could find. One can routinely insert this kind of qualification, and it doesn't matter so much, as long as both views are held. It rather neuters the whole interest of the subsequent discussion, in my view. I find it interesting to have a less qualified answer to 'if string theory isn't right about the ultimate physics, what else is there?'.

Charles Matthews 16:00, 3 Nov 2004 (UTC)

I suppose one should address the question of whether what is written is somehow a journalistic blowing-up of something lesser into a schism which is isn't really there.

Yes, I think that this point is crucial for this particular Wikipedia article. I think that you and I are on the same wavelength on this now.

I find it interesting to have a less qualified answer to 'if string theory isn't right about the ultimate physics, what else is there?'.

Again, I can only address this as an outsider. First off I think that string theorists deserve the first answer to the question and to have it on the string theory page. My understanding is that string theory is somewhere between a hypothesized model and a topic which, in a positive sense, is not falsifiable. E.g., there is no way to argue against combinatorial game theory as a theory of games, because it is just an engulfing desire to analyze games rigorously. Likewise string theory is, to an extent, the general desire to reconcile quantum mechanics with gravity. Some string theorists (possibly not all of them) claim to be willing to engulf any useful mathematical physics towards this aim.

To extend my analogy, although combinatorial game theory cannot be wrong, Berlekamp can be. Even so, if someone says "Berlekamp might have the wrong theory of Go," one good response is, "Well, what would you have him do instead?"

Actually, I've played Elwyn at go, and he should study tesuji. Not meant to be a helpful comment, but I couldn't resist. Charles Matthews

Surely you mean "in addition" rather than "instead" in your friendly aside here. - Greg

Likewise many people like to suppose that string theory could be wrong, but string theorists reply with the same thing, "what would you have us do instead?" The answers to that question have dwindled over 30 years. Obviously loop quantum gravity theorists still say, "some of you should do loop quantum gravity". So the question (which should be addressed on this loop quantum gravity page) is whether their dissent is substantive or just wishful thinking. I strongly suspect it is mostly wishful thinking, but I admit that I personally can't prove it.

- Greg Kuperberg, 3 Nov 2004

One of the difficulties (if one wishes to see all this as a difficulty - I'm mostly just glad we have experts developing WP pages on all this) is that the physics tradition is much more oriented towards survey articles; and they are supposed in a sense to state and overstate positions, so that others can then ignore the original papers where things were less clear. WP likes surveys also, but the constraint (a bit quixotic in some cases) of NPOV and attempted complete coverage means that we can't permanently accept surveys designed to sell one aspect of quantum gravity or anything else. Still, this is an example of an area where the average informed reader can't expect to be able to edit and retain the actual meaning. Charles Matthews 20:57, 3 Nov 2004 (UTC)

Sorry for interrupting this discussion, but if it is a competetion, it only can be a competition of beauty at this stage: Because at present neither string theory not LQP can compete in predictive power and falsifiability - or did I miss something important? --Pjacobi 21:52, 3 Nov 2004 (UTC)

No, I think it is important that the discussion is about theories. I think that Greg is implying that Lubos's style of argument is somehow too engaged (I know it is too polemic for WP, but that is another matter); but as I read it, the state of affairs where Wikipedia simply accepts that the phrase 'quantum gravity' can mean one thing for string theory, another for LQG is also not really satisfactory. If someone reasonably asks, on a talk page, why there are segregated discussions going on - it's a good question. It seems to me quite plausible that there are two traditions going on here. So, I don't see this as an artificial discussion. But then I'm only a mathematician (retired). Charles Matthews 22:53, 3 Nov 2004 (UTC)

How theories develop and compete before you can test them

Peter Jacobi suggests that string theory and LQG can only compete on "beauty" if neither one yet makes any physical prediction. I think that he is indeed missing something important, namely about the way that theory develops in science in general. His comment is like saying, "I see that this is not the Olympics, so it must be a Miss America pageant".

Many modern scientific theories, especially in physics, take a long time to develop, and the process cannot be as simple as running between a blackboard and a lab bench. When a theory cannot be tested, it can still be evaluated on the basis of relevance, consistency, and completeness. It must eventually be tested of course, but a theory's prospects on the eve of experiment can be as different as those of Bush, Nader, or Putin on the eve of an election, depending on its a priori credibility. Beauty does bear on consistency and completeness, but it is not directly the point.

By these measures, many people (not just string theorists) say that string theorists have laid some massive cinder blocks of a plausible, testable theory, even if the final structure will have to be far larger. Very few physicists say the same of loop quantum gravity, nor indeed any other kind of quantum gravity.

- Greg Kuperberg 3 Nov 2004

As a footnote to this discussion: in a shop, the other day, I opened at random a book Magic Universe by Nigel Calder, and it fell open at a page where it was saying that LQG is the main rival to strings ... . On closer examination it was clear that the proponent of this view was [John Baez]. So, this cuts both ways. WP isn't the only 'host' for this argument. But on the other hand perhaps it would be better to identify the view with someone who is putting it forward. Charles Matthews 12:07, 22 Nov 2004 (UTC)

That is a little silly: it is like saying that Lubos Motl is a proponent of the idea that strings have no rival. John Baez works on LQG, which must mean that he thinks it is a serious theory unless you agree with Lubos that Baez is a crackpot or incompetent. I have reworded accordingly. — Miguel 16:15, 2004 Nov 22 (UTC)

Well, you have introduced some unfortunate point of view there now. There are many things on which I do not agree with Lubos, by the way. Charles Matthews 16:33, 22 Nov 2004 (UTC)

I don't understand what you mean (I'm a little dense this morning), but I don't wish to alienate you and I apologize. — Miguel 17:02, 2004 Nov 22 (UTC)

Let's not argue, guys, have not you have a difficult day like me? ;-) Just to be sure, Charles: do you know that Miguel is John Baez's student, by the way? ;-) Lee Smolin, another well-known name in LQG, is giving a colloquium at MIT on Thursday. Unfortunately it overlaps with the Duality Seminar which I organize and cannot skip... In 40 minutes I have dinner with Shahiar Afshar who believes that he has falsified the Copenhagen and other interpretations of quantum mechanics by a simple Thomas Young-like experiment from the 19th century. I hope that I have the energy to continue to be nice today because this may be a tough task, too. ;-) --Lumidek 22:15, 29 Nov 2004 (UTC)

Moved from article

I believe this statement to be incorrect. — Miguel 11:06, 2004 Nov 22 (UTC)

This reflects a philosophical view that gravity is the very geometric fabric of space and time, and that a quantum theory of gravity must be a quantum theory of space and time while all other particles and forces must be separated: LQG predicts that unification of forces can never occur.

Vandalism of Tweet_Tweet

Hi Miguel, hello others,

I don't know whether Tweet-Tweet is another username of yours. At any rate, the user with this nickname keeps on vandalizing the page with the objections, and including rather disorganized replies (Charles Matthews has a lot of work with formatting them). Tweet Tweet is trying to change the page about the problems of LQG into another page promoting the beliefs of LQG.

Lubos, thank you very much for the thinly veiled accusation that I created a sock-puppet account to vandalize your POV rant, as if I cared. — Miguel 18:04, 2004 Nov 29 (UTC)

This is just unacceptable. Either this vandalism stops, or the objections will have to be included in the main page. Best wishes, Lubos --Lumidek 12:28, 29 Nov 2004 (UTC)

Lumidek, why are you smearing me horribly everywhere? Tweet Tweet 04:15, 30 Nov 2004 (UTC)

i think tweet tweet's objections should be reposted.

Article length is itself a bias

At this point, the least neutral aspect of this article is its sheer length. Of course everyone knows about the structural reasons to keep articles short, but I'm talking about something else. I don't think that loop quantum gravity really deserves this many words in Wikipedia. I think that the current presentation is both a pretense and a filibuster, and I think that Lubos Motl has been outsmarted by this filibuster, despite his high reputation in and deep knowledge of physics. He has turned the filibuster into a heated argument, which is understandable, but doesn't solve the problem.

Objections to the theory of loop quantum gravity is a very long POV rant, and Lubos is not even able to address criticism of that article without resorting to ad hominem attacks. — Miguel 17:47, 2004 Nov 29 (UTC)

Whatever the truth of that, that article has its own talk page for such comments. Charles Matthews 18:01, 29 Nov 2004 (UTC)

I was just addressing the suggestion that Lubos has been outsmarted by a this filibuster. Smart as he is, Lubos likes to paint himself into corners. — Miguel 18:06, 2004 Nov 29 (UTC)

There is the policy page Wikipedia:No personal attacks; please do not go further in the direction of personalising the discussion. Charles Matthews 18:41, 29 Nov 2004 (UTC)

On the other hand, Charles Matthews said that shortening this article requires patience, and I don't agree with that either. I think that people should lose patience with this article and cut out about 2/3 of it.

--Greg Kuperberg 13:36, 29 Nov 2004 (UTC)

I can't agree with that. Firstly, no one should take the number of words on a topic in WP to be any sort of metric of its importance (Star Trek trivia, anyone?). Secondly I think the article is moving in the right direction, at this point. It is quite possible that Greg has some substantive grounds here; I wouldn't know. Patience is always in order - this balances the injunction 'be bold' which means that articles are always open to good-faith editing, radical if need be. Charles Matthews 13:54, 29 Nov 2004 (UTC)

Sorry, Greg, but I also disagree with you. It has been a kind of silently accepted agreement that the loop quantum gravity people would have freedom to expand and refine their article, and the problems with the theory would be posted elsewhere. I actually think that the total material about LQG is not too large, and it may even grow. BTW I am not editing this stuff too much - this would be a hopeless task because we would probably disagree about every individual sentence. All these things are rather far-fetched theories where one makes many aasumptions (well, too many in the case of LQG), but one must have the freedom to go on with these assumptions without being interrupted after every word. Of course that in order to make the text meaningful, you would have to interrupt the flow of ideas nearly after every sentence. --Lumidek 15:30, 29 Nov 2004 (UTC)

Look, unless you start pointing out places where the article is *mathematically* incorrect, I think the preceding paragraph also qualifies as an unjustified attack on LQG.

Mathematically, loop quantization is a well-defined quantization procedure in a precise sense, and that's all I really care about. What the classical limit of the quantum theory is is a matter of mathematical investigation. Contact with experiment is so far away that I decided to get a doctorate in math instead.

Then, why is topological quantum field theory respectable when it is carried out perturbatively with formal Feynman integrals, or defined axiomatically a la Wightman, but fringe science when it is carried out using spin networks/spin foams?

Lastly, the state of "experimental" quantum gravity and even of beyond-the-standard-model phenomenology is such that I don't really think much of what has been done in theoretical high-energy physics the last 30 years qualifies as physics. Sure, it's been done by brilliant people, it's clever, it's beautiful, it's intriguing, it suggests lots of cool stuff not even the Star Trek people were able to come up with, but it is not physics. Not even gravitational waves have been detected! The neutrino mass does not require a paradigm shift but the addition of one nonzero coupling constant to the standard model. There is NOT A SHRED of experimental evidence for physics beyond the standard model. We know our current theories are incorrect in that the SM is supposed to violate unitarity and GR predicts singularities, but the difference between the current situation and the period between 1930 and 1973 is that back then experiment was actually driving theoretical physics.

— Miguel 17:47, 2004 Nov 29 (UTC)

The SM is supposed to violate unitarity??? Now that's something I've never heard before. Tweet Tweet 03:11, 30 Nov 2004 (UTC)

I once took a really comprehensive course on particle physics phenomenology, and at one point the professor claimed that something new must happen around 1 TeV center-of-mass energy because Standard Model calculations run into unitarily problems around that energy. I never got around to finding out exactly what he was referring to, but I guess I should at some point---I might also have misunderstood what he was saying ;-) — Miguel 05:45, 2004 Nov 30 (UTC)

All I can say is that encyclopedias are supposed to explain theories, not advocate or refute them. A three-paragraph summary of LQG -- two for its physics and one for its insecure relationship to string theory -- would be a much better explanation than what Wikipedia has now. I refer not only to this tedious article, but also to the criticism article and the article "quantum gravity". Maybe the ideal 10-megabyte advocacy article and an artful rebuttal could be in Wikisource. --Greg Kuperberg 15:49, 29 Nov 2004 (UTC)

Advocacy articles are already on the arXiv and referenced at the bottom of the current article, thank you very much. If this looks like an advocacy article, then it just means that the article is badly written. That was never the point of the article, or at least not my point. — Miguel 17:52, 2004 Nov 29 (UTC)

The explanations of all other things at Wikipedia involve both advocating as well as refuting the concepts, I think. No one has to read the articles if she's not interested. But if I forget about the actual questions which things are right, it seems to me that no section in these articles is really useless. What do you exactly mean by saying that they're tedious? That you understand all these things? Or you're not interested in them? In the latter case, you should not read them - that's a very simple recipe. There are people who want to know what LQG is, they want to know it deeper, and they go to Wikipedia and read everything. They should be explained why these things are believed to be true, and they should also be explained why these things are believed not to be true. It's like with any other article, open e.g. parapsychology. There is no capacity limitation in the defining rules of Wikipedia. What you want is really censorship, it seems. --Lumidek 16:28, 29 Nov 2004 (UTC)

No, Lubos, NPOV requires a dispassionate exposition, not advocacy and rebuttal. You seem to want to turn wikipedia into your bully pulpit. Isn't that what you created sci.physics.strings for? — Miguel 17:47, 2004 Nov 29 (UTC)

I cut 25% from the article, recently; I would do the same if I understood how, right now. I may do in the future, because it really should not read as a manifesto or grant proposal. Perhaps we can all agree on that.

That being said, I'm not comfortable with assuming this is fringe science or any other such label (Greg's attitude may match the pathological science pejorative label - he can correct me on that). I'd like to point out that WP has managed, with some difficulty, to deal with POV pushers and fringe science articles in the past. That is, where a strong case has been made on the marginality of fringe science pages, something eventually is done. So there is no actual limitation on the community processes.

I'd go with John Horgan's term ironic science. But I honestly believe string theory is also not honest physics yet. It's beautiful (and not-so-beautiful) mathematics, but that's about it. — Miguel 17:47, 2004 Nov 29 (UTC)

Charles Matthews 16:49, 29 Nov 2004 (UTC)

I'm moving this topic list out of the article, since I think it is superfluous given a list of researchers. Charles Matthews 16:53, 29 Nov 2004 (UTC)

I don't see a problem with the article length. Wikipedia is supposed to be as inclusive as possible and if some other theory like Euclidean quantum gravity is underrepresented, the solution is not to restrict the length of string theory and LQG articles but to invite contributors to Euclidean quantum gravity. At any rate, if you look at the total length of all the articles on string theory here, they'd add up to far more than what we have here on LQG. The article might need to be split up, though. Tweet Tweet 04:12, 30 Nov 2004 (UTC)

Ironic science

I mostly agree with Miguel's comments. I can believe that he didn't mean to make the article read like advocacy, and I'm glad that we can agree that it shouldn't. I think that the article's length does contribute to that impression, although thanks to Charles' work it is improving quickly enough today. :-)

There was a time when all the article consisted of was a half-digested version of Smolin's Three Roads.

I also agree that LQG appears to be ironic science, although not exactly in Horgan's sense -- he throws out the baby with the bath water. Witten has referred to at least one LQG paper as "wishful thinking". If I can believe Ed Witten, then his criticism could be the best rational approximation to Horgan's label.

An interested party referring to at least one paper as wishful thinking discredits the whole theory? Holly Molly!

Of course it doesn't really discredit anything; the question is who am I going to believe. The real anecdote is here. I suspect that Witten would say the same of LQG in general, at least as a competitor to string theory. --Greg Kuperberg 21:55, 29 Nov 2004 (UTC)

I also agree that string theory is not complete physics, just like quantum computation is not complete computer science and inflationary cosmology was not complete physics until recently. But it looks like a credible attempt at complete physics, and not ironic science. LQG seems to be a more desperate attempt. In particular, the thesis of competing with string theory makes it look much more desperate than it would otherwise. Some LQG people (like Lee Smolin) may be less committed to the competition thesis, or (like Don Marolf, if you can count him as an LQG person) seem completely uncommitted to it. That makes LQG looks more reasonable. --Greg Kuperberg 20:19, 29 Nov 2004 (UTC)

I think string theory is part of the 30-year-long flight forward from the stupendous success of the standard model. Supersymmetry, GUTs, supergravity, bosonic strings, superstrings, M theory, matrix theory, (mem)brane theory, noncommutative geometry, large extra dimensions...

You are conflating assumptions, results, and speculations. Supersymmetry is an assumption of string theory (except for somewhat ill-looking purely bosonic strings). GUTs, supergravity, membranes, and noncommutative geometry are all answers derived or inferred from calculations. Matrix theory is another theory; it and the five main string theories are conjectured to converge to the same non-perturbative entity, lately called M theory. Large extra dimensions are, I think, a speculation that would make string theory testable. Of course string theory requires extra dimensions regardless, but they may or may not be too small to see in particle accelerators.

I am just giving a partial list of hypes that the high-energy-physics community has gone through over the past 30 years. Each of them promised to be just what was needed to be the theory of everything, and every one of them failed to deliver. At some point you might decide that enough is enough and maybe the flight forward should stop until there is some new experimental input. You might also decide to look at quantum field theory or quantum gravity from a different angle. How about looking at it from general relativity instead of from particle physics? — Miguel 06:07, 2004 Nov 30 (UTC)

...give me a break!

Why? This is what really bothers me. This is what really makes it sound like you want to compete with string theory because you don't want to learn it. It's a terrible reason to compete against a scientific theory.

I wonder where this idea comes from that anyone not working on string theory must just not have spent enough time studying it. This sounds almost like religious poselytism: "if only you tried a little harder you'd see the error of your ways". I will admit that I don't know as much about it as I should or might want to, but I am not totally ignorant of it either, and there is only so much one can learn in a limited amount of time. After you learn about ordinary quantum field theory and particle physics, where do you go? String theory is one of many possible places to go. LQG is another. How about constructive quantum field theory? Or TQFTs? Or discrete models? One should not assume that the choice not to study string theory in more depth is motivated by intellectual laziness, like you (here) and Lubos (in the objections page) seem to do. — Miguel 06:07, 2004 Nov 30 (UTC)

By the way, to keep things in perspective, the reason why 't Hooft and Veltman just got their Nobel prize for renormalizing Yang-Mills is because, until very recently, there were no experimental results that were not reproducible at tree level. One-loop calculations just were not necessary. That is how much of the standard model we still need to test before anyone can honestly start talking about beyond-the-standard-model phenomenology.

Before 't Hooft and Veltman even did their work, people said that the physics Nobel committee is more conservative than Barry Goldwater.

I guess they'd rather be safe than sorry, and I can't blame them. At least their approach keeps theoretical physicists who want a Nobel prize honest.

But I obviously take the opposite view from you of what that conservatism means. They reward seminal contributions to established theories. Apparently it was until very recently that one could argue that for all the elegance of renormalizability as a criterion to distinguish between physical theories, it was not really required by experiment. When 1-loop calculations did become necessary, well, that changed the situation. — Miguel 06:07, 2004 Nov 30 (UTC)

Anyone working in LQG will tell you that they do so because they believe it is a physically reasonable and mathematically consistent attempt at quantizing gravity. Whether that means they believe it is a competitor of strings as a theory of quantum gravity, I don't know.

Well, that is the narrow definition of "anyone working in LQG". The broad definition is anyone who writes a paper on it, which could include skeptics who are still happy to help the believers. Which definition is operative for the people you listed, e.g., Donald Marolf? --Greg Kuperberg 21:55, 29 Nov 2004 (UTC)

Well, if you are willing to help the believers (more bothersome religious language, by the way) you must still think it makes some sort of sense, otherwise you might as well write papers showing it is either physically unreasonable or mathematically inconsistent (like Witten has done recently regarding the Kodama state, for instance).

As for Don Marolf, it seems to me that his papers are mostly about what one could call quantum gravity phenomenology, not necessarily strings or LQG or any other specific fundamental theory. That is probably why (or results from the fact that) he is noncommittal on the issue of what quantum gravity theory is the right one. — Miguel 06:07, 2004 Nov 30 (UTC)

Smolin will say that maybe strings and spin foams are two different approximations to the true quantum gravity. He is not very optimistic about string theory's chances of ultimate success, though.

— Miguel 20:43, 2004 Nov 29 (UTC)

Hype and belief

I'd like to stick to a point that I consider fundamental, and that could be a fair statement for the LQG page even though it is accusatory. I still get the impression that some LQG people want to compete with string theory because they don't want to learn it. Indeed, this is a recurring theme even among non-LQG critics of string theory, going back to the 1980s when string theory was first popular. It is of course a terrible reason to take sides. If you acknowledge a research program as viable, then you ought to understand it very, very well before trying to compete with it. Otherwise you can't win -- if you won you would in effect understand the opposing theory better than its practitioners. You can't even know whether you are winning, if you don't understand what you are trying to refute.

LQG people say they compete with strings because strings claim they are the only game in town, and the people in LQG think that they discovered that not to be true around 1990. The string theory people naturally disagree. — Miguel 19:18, 2004 Nov 30 (UTC)

The competition is not as you paint it, though. I don't think LQG people are actively trying to refute string theory, they are just trying to develop what they see as a possible alternative. Some people are interested in investigating possible synthesis. However, since they are constantly challenged for choosing to work on anything other than string theory, they have to give arguments why they choose not to work in string theory. That is, I think, the motivation for most of the criticism of string theory coming from the LQG community. Maybe that's why it appears that people "just don't want to learn it". Also, to learn it at the level of discussing it cogently is a far cry from learning it at the level of making contributions to it. Therefore, if you want to research LQG you are not going to be conversant with strings at the level you would need to do strings research. Most of the most articulate criticism of strings that I am aware of comes from people that it is safe to assume understand it well enough to know what they're saying. — Miguel 19:58, 2004 Nov 30 (UTC)

Miguel asks where I get the idea that non-string theorists haven't spent enough time studying string theory. I'm not a string theorist, I haven't spent much time studying it, and I don't know who has or hasn't spent enough time on it. But some of the comments about string theory in LQG narratives do bespeak of excuses and ignorance, even to me. After acknowledging string theory as the elephant in the quantum gravity room, how can you then say, "there is only so much one can learn in a limited amount of time?" Polchinski looks like a great textbook to me, and it isn't all that fat. I haven't worked through it, but that's because I'm a mid-career mathematician, not a graduate student in mathematical physics. If I wanted to compete with string theory, I would take Polchinski as a fundamental challenge.

You should have heard the dismayed opinions on the wikipedia article Objections to the theory of loop quantum gravity that I heard at a recent conference. To people who seriously work in LGQ, those kinds of arguments also bespeak of excuses and ignorance. An ether theory? Everyone in LQG is just a bad physicist?

You are not saying anything I don't know or agree with. Polchinski has been high on my shopping/reading list for a long time, but I got sidetracked.

At a more technical level, neither supersymmetry nor any other aspect of string theory listed here is "hype". String theorists employ supersymmetry not because they want to, but because they have to. There is a purely bosonic string theory that has no supersymmetry, but it appears to have logical diseases. (There may be other non-supersymmetric string theories that also don't work that I don't know about.) Supersymmetry is considered pretty, but infatuation with it isn't really the point. Again, it sounds like, "I don't want to learn string theory because I don't want to learn supersymmetry". I sense that there is a lot of hype in

Oh, I know about supersymmetry, I just don't find it so compelling a physical idea, nor does it have any experimental justification after decades of looking. The fact that the "predicted" supersymmetry-breaking scale has been sliding just beyond the reach of the current experiments for as long as I remember also bespeaks of excuses. What if the LHC finds no supersymmetric particles? Is the promised supersymmetry breaking scale going to slide again?

I also think that the word "elegant" is misused in LQG commentary as much as the word "hype". String theory can be elegant, but this isn't a beauty contest any more than it is a popularity contest. No one thinks that renormalization is elegant; the founders of quantum field theory thought it was ugly. Maybe the t'Hooft-Veltman idea of dimensional regularization is elegant, in an avante-garde fashion, but that was always a side issue in their work on the Standard Model. The real point is that non-renormalizable quantum field theory isn't predictive; it "isn't even wrong", as the saying goes. Looking at the tree level to side-step renormalizability is shackling a possibly insane patient. That's why I suggested that the Nobel physics committee is too conservative. t'Hooft and Veltman proved that the patient (the Standard Model of particle physics) is sane, but the Nobel committee did not buy it; they wanted to see the patient perform under pressure.

Tree level is part of particle physics phenomenology even if there is no consistent quantum field theory underlying it. In fact, everyone is pretty much convinced that all quantum field theories are, in fact, effective and that the fundamental theory won't be a quantum field theory in the ordinary sense. — Miguel 19:25, 2004 Nov 30 (UTC)

Concerning the word "believers", my main point is that one can play a double game with the posted "list of loop quantum gravity researchers". That title does suggest that they buy into it, i.e., that "they believe it is physically reasonable and mathematically consistent". But that doesn't seem to be true of some of the more famous physicists on the list, e.g., Marolf. I don't mean to insinuate that LQG is pseudoscience in using the word "believers"; rather I was echoing the word "believe" which Miguel introduced into the discussion.

I lifted that list partly from a list of participants in a LQG conference. Maybe the list needs to be reworked. It does seem, on closer examination of his publications, that Marolf is a quantum gravity researcher but not a loop quantum gravity researcher. A similar examination of everyone on the list could be carried out.

Nonetheless, although it is only reasonable to believe the theories that you choose to develop, I sense a strange pessimism among advocates of LQG. It is useful to skeptics if you call your own theory "ironic science", but it also seems defeatist. I don't see that such defeatism is warranted for string theory. --Greg Kuperberg 14:33, 30 Nov 2004 (UTC)

LQG is not my theory. I'm not even sure what I'm going to be doing in the future, but that's beside the point. By the way, I started out in physics and have been drifting more and more into mathematics because I just can't seem to get excited about all these things that people are calling great discoveries. It may sound strange coming from a mathematician, but I think contact with experiment is really important and that has all but been lost since the standard model was formulated. For the record, I believe that most of theoretical high-energy physics is ironic science right now. My biggest fear is that the LHC will confirm the minimal standard model with a 2-component Higgs and a desert after that, and we will be stomped for progress for the foreseeable future.

Some people, like my advisor, feel that we need to make some progress soon on the issues that have been open since the early 1990's. Others like Ashtekar are very optimistic. Smolin is trouble by the state of the entire quantum gravity field, including strings. Many people have the same impression that string theory needs to make some real physical, not mathematical, progress soon: it seems to just gobble up new branches of mathematics in an attempt at finding the definitive breakthrough, and it does seems to be just grasping at straws. Most of the most loudly trumpeted discoveries from string theory are still fairly conjectural and have at bast a tenuous link to experiment. Take AdS/CFT for example: ok, so the large-N limit of conformal SU(N) in (Riemannian) 4 dimensional has the same partition function as some flavour of supergravity on an AdS^5 x S^5 background. This is really cool, but

1. how well-established is the correspondence? Is it just at the level of formal Feynman integral manipulations?;
2. What does the large N limit of conformal Yang-Mills in a Riemannian space have to do with physics (large-N was invented by 't Hooft to study confinement, and you can wick-rotate Riemannian to euclidean, but conformal fields are massless, N is not large...)
3. Is this really the only handle we have on 't hooft's (again him!) "holographic principle"? At least some people in quantum gravity phenomenology are going back to the quantum mechanical and general relativistic draving boards and trying to see if they can: first, understand holography, and second, use it as a fundamental principle to bootstra their way out of the quantum gravity impasse. And yes, I call a theory with 10^300 different low-energy limits an "impasse".
4. People seem to be falling back on using AdS/CFT as a way to study confinement, which might mean string theory is the real thing behind 't Hooft's "planar diagrams" but not a theory of quantum gravity after all.

— Miguel 18:59, 2004 Nov 30 (UTC)

But we're straying away from the point, which is that LQG is a legitimate object of study independently of what one thinks of strings if only because it came as a complete surprise when Ashtekar's variables allowed people to make rigorous sense of the Wheeler-de Witt equation for the first time in 30 years, plus a connection was found to Penrose's ideas about combinatorial quantum geometry. Around 1990 LQG managed to shatter all the conventional wisdom about quantum gravity that people had learnt from perturbative quantum field theory, but some people still call that "wishful thinking". It may still fail to make contact with the classical limit, but the jury is still out on that one. I don't know. It's not a crackpot theory. — Miguel 19:06, 2004 Nov 30 (UTC)

Look, if what needs to happen to this article is that the connection to actual physics needs to be toned down and the focus made more mathematical, so be it. There should be a quantum gravity article about the physics and what people have learnt about the problem since the 1930's, which is a lot and does not all (or even close to that) come from string theory. — Miguel 19:06, 2004 Nov 30 (UTC)

This is way too long for not really discussing the article itself

This discussion is a mammoth of almost 50 Kb. Can someone archive it so we can we move on to some substantive criticism of the article itself that will lead to improvement? — Miguel 20:01, 2004 Nov 30 (UTC)

Moved from article

Research in LQG and related areas

Active research directions

• Spin foam models
  • 2+1 and 3+1 theories
  • Barrett-Crane model
  • relation to the canonical approach
  • the Barbero-Immirzi parameter
  • canonical and spin foam geometries
  • the continuum limit
  • renormalization group flows
• the Hamiltonian constraint
  • 2+1 and 3+1 theories
  • spin-foam and canonical approach
  • quantum cosmology
  • Semi-classical corrections to Einstein equations
  • factor ordering
  • finding solutions and physical inner product
  • Thiemann's phoenix project.
• Semi-classical issues
  • kinematical and dynamical semi-classical states
  • quantum field theory on quantum geometry
  • quantum cosmology
  • Minkowski coherent state and Minkowski spin foam
• Loop quantum phenomenology
  • Lorentz invariance
  • Doubly-special relativity
  • quantum cosmology
  • Kodama state and de Sitter background
• Conceptual issues
  • observables through matter coupling
  • string theory in polymer representation
  • matter couplings on semi-classical states
  • the problem of time
  • spin foam histories
  • quantum groups in LQG

Loop quantum gravity's implications

Space atoms

In LQG, the fabric of spacetime is a foamy network of interacting loops mathematically described by spin networks. These loops are about 10^-35 meters in size, called the Planck scale. The loops knot together forming edges, surfaces, and vertices, much as do soap bubbles joined together. In other words, spacetime itself is quantized. Any attempt to divide a loop would, if successful, cause it to divide into two loops each with the original size. In LQG, spin networks represent the quantum states of the geometry of relative spacetime. Looked at another way, Einstein's theory of general relativity is (as Einstein predicted -- when and where did he do that ???) a classical approximation of a quantized geometry.

Kinematics

Kinematics in loop quantum gravity is the physics of space and time at the Planck scale. It is expressed in terms of area and volume operators, and spin foam formalism.

Area and volume operators

One of the key results of loop quantum gravity is quantization of areas: according to several related derivations based on loop quantum gravity, the operator of the area $A$ of a two-dimensional surface $\Sigma$ should have discrete spectrum. Every spin network is an eigenstate of each such operator, and the area eigenvalue equals

$A_{\Sigma }=8\pi G_{\mathrm {Newton} }\gamma \sum _{i}{\sqrt {j_{i}(j_{i}+1)}}$

where the sum goes over all intersections $i$ of $\Sigma$ with the spin network. In this formula, $G_{\mathrm {Newton} }$ is the gravitational constant, $\gamma$ is the Immirzi parameter and $j_{i}=0,0.5,1,1.5,\dots$ is the spin associated with the link $i$ of the spin network. The two-dimensional area is therefore "concentrated" in the intersections with the spin network.

Similar quantization applies to the volume operators but the mathematics behind these derivations is less convincing.

Spin foams

Quantum cosmology

An important principle in quantum cosmology that LQG adheres to is that there are no observers outside the universe. All observers must be a part of the universe they are observing. However, because light cones limit the information that is available to any observer, the Platonic idea of absolute truths does not exist in a LQG universe. Instead, there exists a consistency of truths in that every observer will report consistent (not necessarily the same) results if truthful.

Another important principle is the issue of the "cosmological constant", which is the energy density inherent in a vacuum. Cosmologists working on the assumption of zero cosmological constant predicted that gravity would slow the rate at which the universe is expanding following the big bang. However, astronomical observations of the magnitude and cosmological redshift of Type I supernovae in remote galaxies implies that the rate at which the universe is expanding is actually increasing. General relativity has a constant, Lambda, to account for this, and the observations, recently supported by independent data on the cosmic microwave background, appear to require a positive cosmological constant. In string theory, there are many vacua with broken supersymmetry which have positive cosmological constant, but generically their value of Lambda is much larger than the observed value. In LQG, there have been proposals to include a positive cosmological constant, involving a state referred to as the Kodama state after Hideo Kodama, a state described by a Chern-Simons wave function. Some physicists, for example Edward Witten, have argued by analogy with other theories that this state is unphysical. This issue is considered unresolved by other physicists.

Standard quantum field theory and supersymmetric string theories make a prediction based on calculation of the vacuum energy density that differs from what has actually been observed by 120 orders of magnitude. To date, this remains an unsolved mystery that a successful quantum theory of gravity would hopefully avoid

Black hole thermodynamics

While experimental tests for LQG may be years in the future, one conceptual test any candidate for QG must pass is that it must derive the correct formula Hawking derived for the black hole entropy.

With the proper Immirzi parameter, LQG can calculate and reproduce the Hawking formula for all black holes. While string/M-theory does not need the Immirzi parameter, it can as yet only derive the Hawking formula for extremal black holes and near-extremal black holes -- black holes with a net electric charge, which differ from the nearly neutral black holes formed from the collapse of electrically neutral matter such as neutron stars. To date, the Immirzi parameter cannot be derived from more fundamental principles, and is an unavoidable artefact of quantization of general relativity's field equations.

LQG's interpretation of black hole entropy is that the spacetime fabric that makes up the black hole horizon is quantized per Planck area, and the Bekenstein-Hawking entropy represents the degrees of freedom present in each Planck quantum. LQG does not offer an explanation why the interior of the black hole carries no volume-extensive entropy. Instead, it assumes that the interior does not contribute. The spacetime is truncated at the event horizon, and consistency requires to add Chern-Simons theory at the event horizon. A calculation within Chern-Simons theory leads to the desired result for the entropy, proportional to the horizon area.

Additionally, the spectrum of radiation of particles emanating from the event horizon of a black hole has been calculated from LQG's theoretical framework and precisely predicted. This prediction disagrees with Hawking's semiclassical calculation, but the use of a semiclassical calculation that is so far unconfirmed by experiment as a benchmark for an exact nonperturbative fully quantum calculation may be problematic. Modulo the Immirzi parameter, which is the only free parameter of LQG, it matches it on average, and additionally predicts a fine structure to it, which is experimentally testable and potentially an improvement.

The big bang

Several LQG physicists have shown that LQG can, at least formally, get rid of the infinities and singularities present when general relativity is applied to the big bang. While standard physics tools break down, LQG have provided internally self-consistent models of a big bounce in the time preceding the big bang.

Theory of everything: unification of the four forces

Grand unification theory refers to a theory in particle physics that unifies the strong interaction and electroweak interactions. A so-called theory of everything (TOE) is a putative theory that unifies the four fundamental forces of nature: gravity, the strong nuclear force, the weak nuclear force, and electromagnetism. Since the strong and electroweak interactions are described by quantum field theory, such a theory would require gravity also to be quantized, bringing with it the inconsistencies noted above.

One candidate for a consistent quantum gravity is string theory, which in addition to gravity contains gauge vector bosons and matter particles reminiscent of those experimentally observed. This has led to attempts (so far unsuccessful) to construct TOE's within its framework. In contrast, LQG is just a theory of one part of the Universe, namely quantum gravity.

Unification in field theory or string theory is difficult or impossible to test directly, due to the extremely large energy (greater than 10¹⁶ GeV) at which unification is manifest. However, indirect tests exist, such as proton decay and the convergence of the coupling constants when extrapolated to high energy through the renormalization group. The simplest unified models (without supersymmetry) have failed such tests, but many models are still viable. Incorporating the correct strength of gravity in string unification is particularly challenging. While unified theories have greater explanatory and predictive power, it may be that nature does not favour them.

Supersymmetry and extra dimensions

See supersymmetry for detailed discussion

LQG in its current formulation predicts no additional spatial dimensions, nor anything else about particle physics. Lee Smolin, one of the originators of LQG, has proposed that loop quantum gravity incorporating either supersymmetry or extra dimensions, or both, be called loop quantum gravity II, in light of experimental evidence.

Chaos theory and classical physics

Sensitivity on initial conditions, in the light of chaos theory means that two nonlinear systems with however small a difference in their initial state eventually will end up with a finite difference between their states. Loop quantum gravity suggests that the Planck scale represents the physical cut-off allowed for such sensitivity.

Differences between LQG and string/M-theory

Main article: objections to the theory of loop quantum gravity

String theory and LQG are the products of different communities within theoretical physics. It is not generally agreed whether they are in any sense compatible, and their differences have sometimes been represented as different ways of doing physics. This is a sharp debate, or at times presented as such: in other words matters are currently subject to dialectic rather than experimental test.

String theory emerged from the particle physics community and was originally formulated as a theory that depends on a background spacetime, flat or curved, which obeys Einstein's equations. This is now known to be just an approximation to a mysterious and not well-formulated underlying theory which may or may not be background independent.

In contrast, LQG was formulated with background independence in mind. However, it has been difficult to show that classical gravity can be recovered from the theory. Thus, LQG and string theory seem somewhat complementary.

String theory easily recovers classical gravity, but so far it lacks a universal, perhaps background independent, description. LQG is a background independent theory of something, but the classical limit has yet not proven tractable. This has led some people to conjecture that LQG and string theory may both be aspects of some new theory, or that, perhaps there is some synthesis of the techniques of each that will lead to a complete theory of quantum gravity. For now, this is mostly a fond hope with little evidence.

Experimental tests of LQG in the near future

Observation may affect the future theoretical development in quantum gravity in the areas of dark matter and dark energy. The year 2007 will see the launch of GLAST (space-based gamma-ray spectrometry experiments), and perhaps the completion and operation of LHC.

LQG predicts that more energetic photons should travel ever so slightly faster than less energetic photons; this effect would be too small to observe within our galaxy. Giovanni Amelino-Camelia points out that photons which have traveled from distant galaxies may reveal the structure of spacetime.

If GLAST detects violations of Lorentz invariance in the form of energy-dependent photon velocity, in agreement with theoretical calculations, such observations would support LQG. However, string theory would not necessarily be disfavoured.

Let's port everything over to Wikinfo

I have an idea. Why not just port everything relating to quantum gravity to Wikinfo: and add interwiki links to the quantum gravity articles here? Tweet Tweet 04:28, 30 Nov 2004 (UTC)

No, quantum gravity is a subject with a long and tortuous history and it should have its own articles in Wikipedia. You can go and add whatever material to Wikinfo, too. — Miguel 05:39, 2004 Nov 30 (UTC)

Back to editing

Since Miguel reasonably suggested a discussion about editing, here we go. The article says, "a proof by example that it is not necessary to have a theory of everything in order to have a candidate for a quantum theory of gravity." I have no idea what this convoluted clause means, given that it is not established that LQG is a viable quantum theory of gravity. (It would have to have gravity as its macroscopic limit, but this is listed later as an open problem, and many people think that it is a false conjecture.) So I'm going to delete this. --Greg Kuperberg 01:24, 2 Dec 2004 (UTC)

Thanks. — Miguel 05:08, 2004 Dec 2 (UTC)

I didn't like Charles' edit about string theorist who "have not informed themselves about the debate", besides that the grammar was convoluted. The "debate" is not (or not yet) at the level of science. There is a procedural debate as to whether or not there should be any scientific debate. So it inaccurate to say that string theorists have not informed themselves. --Greg Kuperberg 19:06, 5 Dec 2004 (UTC)

Objections taking more of the page

Ok when I found this page there were more objections to loop quantum gravity than there was explaination of what loop quantum gravity says! I thought it would be a good idea for the objections to have their own page. Lo and behold they already had a page of thier own. So the lenthly objections section has been mostly relocated to it's own page. --HFarmer 04:24, 17 Jan 2005 (UTC)

It is against Wikipedia policy to have separate articles for different POVs of a subject. For this reason, editiht had started to merge the objections back into the main page. For this reason, I've reverted your edit. --Pjacobi 07:24, 2005 Jan 17 (UTC)

I understand compleatly where you are comming from. Since Wikipedia prefer's to have one page per topic then logically all of the matterials on LQG should be in one place. --HFarmer 15:58, 17 Jan 2005 (UTC)

On a related note, More LQG is up for deletion; the page and vote may interest editors of this page. —Ben Brockert (42) UE News 01:17, Jan 21, 2005 (UTC)

@Tweet Tweet

Merging two articles just doesn't mean cut'n'paste from one article to the other. And side-by-side of different POVs is not NPOV. --Pjacobi 01:25, 2005 Jan 23 (UTC)

Have you ever heard of the old "equal time" law that the FCC used to enforce on network news & commentary programs? Well before the 80's they did enforce such a rule wich said that a TV/radio station needed to sell equal ammounts of ad time to both parties. Or on a new debate show they would have to have a conservative and a liberal like "Crossfire" or "HANNITY and Colmes". --HFarmer 04:09, 23 Jan 2005 (UTC)

'Equal time', 'balance' etc are not WP policies. The policy is that writing should be NPOV - ideally one should get no information from it with which to reconstruct the writer's POV, just a summing up. Charles Matthews 11:12, 23 Jan 2005 (UTC)

What is POV? What is a scientific opinion based on facts?

I understand Wikipedia's policy. However, scientific theories are all debateable and all present a scientific POV. This is different from normal POV in that scientific POV is based on the available information. Where a regular POV is just an opinion. A theory could be defined as a point of view that is backed up by facts and predicts what will happen under certain circumstances.

This may not be the most diplomatic way of putting it but this is the truth. An example would be the opinion of expert witnesses at a murder trial. The lawyers spend much time trying to belittle the opposing sides expert's education or methods. Then they bring on thier own experts who's scientific opinion is more friendly to them.

have you all looked at the talk pages for other scientific theories. Basic bedrock science is even controversial. I'm talking Special relativity and Evolution. Even they are a source of controversey.--HFarmer 21:14, 23 Jan 2005 (UTC)

Here's an idea

Related to the idea of a scientific POV (SPOV)is the fact that we all have different SPOV's based on our backgrounds.

The controversial tag should be put on this page and any other page related to quantum gravity. I will bet any ammount of money that many of the people who criticize this article just do not agree with LQG. They have their SPOV it is a valid opinion and it is addressed in the "objections" section. That said their ought to be an objections section in each and every article on quantum gravity. The fact that theri is no accepted theory of quantum gravity says that there are arguments for and against all of them. --HFarmer 03:28, 24 Jan 2005 (UTC)

from VfD

A section of this article has been the subject of controversy. An attempt was made to move the disputed section to a separate article. As the result of a Votes for Deletion discussion, that page has been redirected back to this article. The page history has been preserved in case it is useful in the continuing resolution of the dispute. Rossami (talk) 01:22, 3 Feb 2005 (UTC)

What is loop quantum gravity?

As someone who is neither a physicist or mathematician (I have an interest in this area), I really don't understand what loop quantum gravity is. Why? Nothing in the article actually says what it is. First it is shown as an alternative to string theory, then it talks about various aspects and problems of it, but never once does the article actually state what the theory is proposing as a solution to the problem of quantum gravity and what brought about the theory in the first place.

It does say so. But the subject of quantum gravity is just so complex that it is not as simple as saying. This theory of quantum gravity says..blah.. What I would tell a non physicist of loop quantum gravity is just what I would say of any theory of quantum gravity to such a person. Theories of quantum gravity are attempts to understand gravity as it exist (if it exist) at the quantum level. On a higher level I would say that Loop quantum gravity is one canonical theory of quantum gravity whereby space-time is quantized "directly". Note that statements like this leave out so much information that the question what is LQG may not be satisfied by them.

Look up this paper for a wealth of technical information on the subject. arXiv:gr-qc/03113V2 I hope this helps--Hfarmer 06:28, 7 Mar 2005 (UTC)

No, I agree with the original poster in this topic. It would be really nice to have a small summary of what it is before string theory is even mentioned. There is kinda one now, but it's very technical and doesn't contain much information as far as I can tell. I don't know enough about LQG to write a correct one off the top of my head, but it sounds like it might go something like this?

Loop quantum gravity (LQG) is a proposed theory of spacetime which is built from the ground up with the idea of spacetime quantization via the mathematically rigorous theory of loop quantization. It preserves many of the important features of general relativity, such as local Lorentz invariance [local Minkowskian geometry? are they the same thing? I don't know enough to be sure], while at the same time employing quantization of both space and time at the Planck scale in the tradition of quantum mechanics. In this sense, both general relativity and quantum mechanics can be thought of as approximations to LQG in their respective domains; thus, LQG is one of the several competing theories that attempts to combine the two into a Theory of Everything. However, both the mathematics and physics behind LQG are controversial and it is not clear whether LQG truly unifies the two theories, of if this unification is more "forced" than would be hoped.

Anyone want to comment on that? It seems more informative than what we have now, which is more technical and less summary-like. But then again, this is coming out of the keyboard of a 17-year-old whose most advanced physics comes from Penrose's The Emperor's New Mind.

Thank you. I am the original poster of this section and that paragraph says more to clear it up for the layman (which I assume is the purpose of Wikipedia) than anything I have found about it yet. Unless you object, I would like to move this into the main article. --207.111.236.2 15:37, 14 Jun 2005 (UTC)

What will LQG mean for Mass in special relativity ?

I have a question i couldn't find an answer on this topic so far, maybe somebody here is able to explain: Mass in special relativity is described as based on one's frame of reference, and LQG is based on a relativistic time-space room ... Does this room in LQM include a relativic view of masses as well ? To clearify what i mean: Earth is moving with some speed through space, does this make any difference for an oberserver looking at masses of particle near light-speed if he is in oberserving it in LQG than described so far ? To point out the diffence: I would expect masses in LQM to grow with speed, but an observer with high speed itself should detect that he has gained relativistic mass and not the other system as the intertial system predicts. Thank you. --Yoxx 22:09, 24 May 2006 (UTC)

Gravitons

The gravitons section states that gravtions need to be able to occour "in unlimited numbers", to account for field density near heavy stars. But surely - it might possibly be the case that there is an upper limit, which we get at black hole event horizons.

Pmurray bigpond.com 04:50, 19 May 2005 (UTC)

clean this up

someone please make this article shorter why is string theory so short and lqg so long???

edit: for the person above me, time is quantized in lqg but so is space? wow, um, dunno what to say

"time is quantized in lqg but so is space?" yes Space-time is one unified entity in relativistic physical theory. As relativistic gravity is a theory of space-time and geometry it makes sense to many physicist to try and quantize some aspect of space time (length, area, curvature, etc).

The reason for the clutter and extra length is that LQG is subject to more criticism than String Theory wich is it's main competition. It was my idea to move the Objections section to a different article thus cleaning up this article. That was voted down a while ago. I propose that we try it again, move the objections section to it's own article with a prominent link on this page. --Hfarmer 00:26, 13 Jun 2005 (UTC)

The actual article is supposed to be encyclopedic

NOT a dicussion forum use the talk page for that. I am changeing the article and refering to the objections which have been moved below. Keep in mind Wikipedia is not a source hard core physicist turn to this is just for the general interseted public. We shoudl write this in such a way that it will appeal to them. There is no need to spam this article with a bunch of objections. Most people coming here just want to know what the heck loop quantum gravity is. The new eddition should do just that.

Objections to the theory

{{disputed}}{{SectNPOV}}{{merge|Objections to the theory of loop quantum gravity}}

If anything Objections to the theory.... ought to beit's own article.

As a physical theory, loop quantum gravity has been subject to some heavy criticisms. Some objections to the ideas of loop quantum gravity are given here.

Too many assumptions

OBJECTION Loop quantum gravity makes too many assumptions about the behavior of geometry at very short distances. It assumes that the metric tensor is a good variable at all distance scales, and it is the only relevant variable. It even assumes that Einstein's equations are more or less exact in the Planckian regime.

The spacetime dimensionality (four) is another assumption that is not questioned, much like the field content. Each of these assumptions is challenged in a general enough theory of quantum gravity, for example all the models that emerge from string theory.

These assumptions have neither theoretical nor experimental justification. Particular examples will be listed in a separate entry.

The most basic, underlying assumption is that the existence of a meaningful classical theory, of general relativity, implies that there must exist a "quantization" of this theory. This is commonly challenged. Many reasons are known why some classical theories do not have a quantum counterpart. Gauge anomalies are a prominent example. General relativity is usually taken to be another example, because its quantum version is not renormalizable.

It is known, therefore, that a classical theory is not always a good starting point for a quantum theory. Theorists of loop quantum gravity work with the assumption that "quantization" can be done, and continue to study it even if their picture seems inconsistent.

Commentary from the renormalization group aspect

OBJECTION According to the logic of the renormalization group, the Einstein-Hilbert action is just an effective description at long distances; and it is guaranteed that it receives corrections at shorter distances. String theory even allows us to calculate these corrections in many cases.

There can be additional spatial dimensions; they have emerged in string theory and they are also naturally used in many other modern models of particle physics such as the Randall-Sundrum models. An infinite amount of new fields and variables associated with various objects (strings and branes) can appear, and indeed does appear according to string theory. Geometry underlying physics may become noncommutative, fuzzy, non-local, and so on. Loop quantum gravity ignores all these 20th and 21st century possibilities, and it insists on a 19th century image of the world which has become naive after the 20th century breakthroughs.

As a predictive theory

OBJECTION Loop quantum gravity is not a predictive theory. It does not offer any possibility to predict new particles, forces and phenomena at shorter distances: all these objects must be added to the theory by hand. Loop quantum gravity therefore also makes it impossible to explain any relations between the known physical objects and laws.

Loop quantum gravity is not a unifying theory. This is not just an aesthetic imperfection: it is impossible to find a regime in real physics of this Universe in which non-gravitational forces can be completely neglected, except for classical physics of neutral stars and galaxies that also ignores quantum mechanics. For example, the electromagnetic and strong force are rather strong even at the Planck scale, and the character of the black hole evaporation would change dramatically had the Nature omitted the other forces and particles. Also, the loop quantum gravity advocates often claim that the framework of loop quantum gravity regularizes all possible UV divergences of gravity as well as other fields coupled to it. That would be a real catastrophe because any quantum field theory - including all non-renormalizable theories with any fields and any interactions - could be coupled to loop quantum gravity and the results of the calculations could be equal to anything in the world. The predictive power would be exactly equal to zero, much like in the case of a generic non-renormalizable theory. There is absolutely no uniqueness found in the realistic models based on loop quantum gravity. The only universal predictions - such as the Lorentz symmetry breaking discussed below - seem to be more or less ruled out on experimental grounds.

Self-consistency

OBJECTION Unlike string theory, loop quantum gravity has not offered any non-trivial self-consistency checks of its statements and it has had no impact on the world of mathematics. It seems that the people are constructing it, instead of discovering it. There are no nice surprises in loop quantum gravity - the amount of consistency in the results never exceeds the amount of assumptions and input. For example, no answer has ever been calculated in two different ways so that the results would match. Whenever a really interesting question is asked - even if it is apparently a universal question, for example: "Can topology of space change?" - one can propose two versions of loop quantum gravity which lead to different answers.

There are many reasons to think that loop quantum gravity is internally inconsistent, or at least that it is inconsistent with the desired long-distance limit (which should be smooth space). Too many physical wisdoms seem to be violated. Unfortunately the loop quantum gravity advocates usually choose to ignore the problems. For example, the spin foam (path-integral) version of loop quantum gravity is believed to break unitarity. The usual reaction of the loop quantum gravity practitioners is the statement that unitarity follows from time-translation symmetry, and because this symmetry is broken (by a generic background) in GR, we do not have to require unitarity anymore. But this is a serious misunderstanding of the meaning and origin of unitarity. Unitarity is the requirement that the total probability of all alternatives (the squared length of a vector in the Hilbert space) must be conserved (well, it must always be 100%), and this requirement - or an equally-strong generalization of it - must hold under any circumstances, in any physically meaningful theory, including the case of the curved, time-dependent spacetime. Incidentally, the time-translation symmetry is related, via Noether's theorem, to a time-independent, conserved Hamiltonian, which is a completely different thing than unitarity.

A similar type of "anything goes" approach seems to be applied to other no-go theorems in physics.

Gap to high-energy physics

OBJECTION Loop quantum gravity is isolated from particle physics. While extra fields must be added by hand, even this ad hoc procedure seems to be impossible in some cases. Scalar fields can't really work well within loop quantum gravity, and therefore this theory potentially contradicts the observed electroweak symmetry breaking, the violation of the CP symmetry, and other well-known and tested properties of particle physics.

Loop quantum gravity also may deny the importance of many methods and tools of particle physics - e.g. the perturbative techniques; the S-matrix, and so on. Loop quantum gravity therefore potentially disagrees with 99% of physics as we know it. Unfortunately, the isolation from particle physics follows from the basic opinions of loop quantum gravity practitioners and it seems very hard to imagine that a deeper theory can be created if the successful older theories, insights, and methods (and exciting newer ones) in the same or closely related fields are ignored.

Smooth space as limiting case

OBJECTION Loop quantum gravity does not guarantee that smooth space as we know it will emerge as the correct approximation of the theory at long distances; there are in fact many reasons to be almost certain that the smooth space cannot emerge, and these problems of loop quantum gravity are analogous to other attempts to discretize gravity (e.g. putting gravity on lattice).

While string theory confirms general relativity or its extensions at long distances - where GR is tested - and modifies it at the shorter ones, loop quantum gravity does just the opposite. It claims that GR is formally exact at the Planck scale, but implies nothing about the correct behavior at long distances. It is reasonable to assume that the usual ultraviolet problems in quantum gravity are simply transmuted into infrared problems, except that the UV problems seem to be present in loop quantum gravity, too.

Clash with special relativity

OBJECTION Loop quantum gravity violates the rules of special relativity that must be valid for all local physical observations. Spin networks represent a new reincarnation of the 19th century idea of the luminiferous aether - environment whose entropy density is probably Planckian and that picks a privileged reference frame. In other words, the very concept of a minimal distance (or area) is not compatible with the Lorentz contractions. The Lorentz invariance was the only real reason why Einstein had to find a new theory of gravity - Newton's gravitational laws were not compatible with his special relativity.

Despite claims about the background independence, loop quantum gravity does not respect even the special 1905 rules of Einstein; it is a non-relativistic theory. It conceptually belongs to the pre-1905 era and even if we imagine that loop quantum gravity has a realistic long-distance limit, loop quantum gravity has even fewer symmetries and nice properties than Newton's gravitational laws (which have an extra Galilean symmetry, and can also be written in a "background independent" way - and moreover, they allow us to calculate most of the observed gravitational effects well, unlike loop quantum gravity). It is a well-known fact that general relativity is called "general" because it has the same form for all observers including those undergoing a general accelerated motion - it is symmetric under all coordinate transformations - while "special" relativity is only symmetric under a subset of special (Lorentz and Poincaré) transformations that interchange inertial observers. The symmetry under any coordinate transformation is only broken spontaneously in general relativity, by the vacuum expectation value of the metric tensor, not explicitly (by the physical laws), and the local physics of all backgrounds is invariant under the Lorentz transformations.

Loop quantum gravity proponents often and explicitly state that they think that general relativity does not have to respect the Lorentz symmetry in any way - which displays a misunderstanding of the symmetry structure of special and general relativity (the symmetries in general relativity extend those in special relativity), as well as of the overwhelming experimental support for the postulates of special relativity. Loop quantum gravity also depends on the background in a lot of other ways - for example, the Hamiltonian version of loop quantum gravity requires us to choose a pre-determined spacetime topology which cannot change.

One can imagine that the Lorentz invariance is restored by fine-tuning of an infinite number of parameters, but nothing is known about the question whether it is possible, how such a fine-tuning should be done, and what it would mean. Also, it has been speculated that special relativity in loop quantum gravity may be superseded by the so-called doubly special relativity, but doubly special relativity is even more problematic than loop quantum gravity itself. For example, its new Lorentz transformations are non-local (two observers will not agree whether the lion is caught inside the cage) and their action on an object depends on whether the object is described as elementary or composite.

Global justification of variables

OBJECTION The discrete area spectrum is not a consequence, but a questionable assumption of loop quantum gravity. The redefinition of the variables - the formulae to express the metric in terms of the Ashtekar variables (a gauge field) - is legitimate locally on the configuration space, but it is not justified globally because it imposes new periodicities and quantization laws that do not follow from the metric itself. The area quantization does not represent physics of quantum gravity but rather specific properties of this not-quite-legitimate field redefinition. One can construct infinitely many similar field redefinitions (siblings of loop quantum gravity) that would lead to other quantization rules for other quantities. It is probably not consistent to require any of these new quantization rules - for instance, one can see that these choices inevitably break the Lorentz invariance which is clearly a bad thing.

Testability of the discrete area spectrum

OBJECTION The discrete area spectrum is not testable, not even in principle. Loop quantum gravity does not provide us with any "sticks" that could measure distances and areas with a sub-Planckian precision, and therefore a prediction about the exact sub-Planckian pattern of the spectrum is not verifiable. One would have to convert this spectrum into a statement about the scattering amplitudes.

The S-matrix

OBJECTION Loop quantum gravity provides us with no tools to calculate the S-matrix, scattering cross sections, or any other truly physical observable. It is not surprising; if loop quantum gravity cannot predict the existence of space itself, it is even more difficult to decide whether it predicts the existence of gravitons and their interactions. The S-matrix is believed to be essentially the only gauge-invariant observable in quantum gravity, and any meaningful theory of quantum gravity should allow us to calculate it, at least in principle.

Ultraviolet divergences

OBJECTION Loop quantum gravity does not really solve any UV problems. Quantized eigenvalues of geometry are not enough, and one can see UV singular and ambiguous terms in the volume operators and most other operators, especially the Hamiltonian constraint. Because the Hamiltonian defines all of dynamics, which contains most of the information about a physical theory, it is a serious object. The whole dynamics of loop quantum gravity is therefore at least as singular as it is in the usual perturbative treatment based on semiclassical physics.

We simply do have enough evidence that a pure theory of gravity, without any new degrees of freedom or new physics at the Planck scale, cannot be consistent at the quantum level, and loop quantum gravity advocates need to believe that the mathematical calculations leading to the infinite and inconsistent results (for example, the two-loop non-renormalizable terms in the effective action) must be incorrect, but they cannot say what is technically incorrect about them and how exactly is loop quantum gravity supposed to fix them. Moreover, the loop quantum gravity proponents seem to believe that the naive notion of "atoms of space" is the only way to fix the UV problems. String theory, which allows us to make real quantitative computations, proves that it is not the case and there are more natural ways to "smear out" the UV problems. In fact, a legitimate viewpoint implies that the discrete, sharp character of the metric tensor and other fields at very short distances makes the UV behavior worse, not better.

Moreover, as explained above, the "universal solution of the UV problems by discreteness of space" implies at least as serious loss of predictive power as in a generic non-renormalizable theory. Even if loop quantum gravity solved all the UV problems, it would mean that infinitely many coupling constants are undetermined - a situation analogous to a non-renormalizable theory.

Black hole entropy

OBJECTION Despite various claims, loop quantum gravity is not able to calculate the black hole entropy, unlike string theory. The fact that the entropy is proportional to the area does not follow from loop quantum gravity. It is rather an assumption of the calculation. The calculation assumes that the black hole interior can be neglected and the entropy comes from a new kind of dynamics attached to the surface area - there is no justification of this assumption. Not surprisingly, one is led to an area/entropy proportionality law. The only non-trivial check could be the coefficient, but it comes out incorrectly (see the Immirzi discrepancy).

The Immirzi discrepancy was believed to be proportional to the logarithm of two or three, and a speculative explanation in terms of quasinormal modes was proposed. However it only worked for one type of the black hole - a clear example of a numerical coincidence - and moreover it was realized in July 2004 that the original calculation of the Immirzi parameter was incorrect, and the correct value (described by Meissner) is not proportional to the logarithm of an integer. The value of the Immirzi parameter - even according to the optimists - remains unexplained. Another description of the situation goes as follows: Because the Immirzi parameter represents the renormalization of Newton's constant and there is no renormalization in a finite theory - and loop quantum gravity claims to be one - the Immirzi parameter should be equal to one which leads to a wrong value of the black hole entropy.

Nonseparable Hilbert space

OBJECTION While all useful quantum theories in physics are based on a separable Hilbert space; i.e. a Hilbert space with a countable basis, loop quantum gravity naturally leads to a non-separable Hilbert space, even after the states related by diffeomorphisms are identified. This space can be interpreted as a very large, uncountable set of superselection sectors that do not talk to each other and prevent physical observables from being changed continuously. All known procedures to derive a different, separable Hilbert space are physically unjustified.

Foundational lacks

OBJECTION Loop quantum gravity has no tools and no solid foundations to answer other important questions of quantum gravity - the details of Hawking radiation; the information loss paradox; the existence of naked singularities in the full theory; the origin of holography and the AdS/CFT correspondence; mechanisms of appearance and disappearance of spacetime dimensions; the topology changing transitions (which are most likely forbidden in loop quantum gravity); the behavior of scattering at the Planck energy; physics of spacetime singularities; quantum corrections to geometry and Einstein's equations; the effect of the fluctuating metric tensor on locality, causality, CPT-symmetry, and the arrow of time; interpretation of quantum mechanics in non-geometric contexts including questions from quantum cosmology; the replacement for the S-matrix in de Sitter space and other causally subtle backgrounds; the interplay of gravity and other forces; the issues about T-duality and mirror symmetry.

Loop quantum gravity is criticised as a philosophical framework that wants us to believe that these questions should not be asked. As if general relativity is virtually a complete theory of everything (even though it apparently can't be) and all ideas in physics after 1915 can be ignored.

Prejudices claimed

OBJECTION The criticisms of loop quantum gravity regarding other fields of physics are misguided. They often dislike perturbative expansions. While it is a great advantage to look for a framework that allows us to calculate more than the perturbative expansions, it should never be less powerful. In other words, any meaningful theory should be able to allow us to perform (at least) approximative, perturbative calculations (e.g. around a well-defined classical solution, such as flat space). Loop quantum gravity cannot do this, definitely a huge disadvantage, not an advantage as some have claimed. A good quantum theory of gravity should also allow us to calculate the S-matrix.

Background independence

OBJECTION Loop quantum gravity's calls for "background independence" are misled. A first constraint for a correct physical theory is that it allows the (nearly) smooth space[time] — or the background — which we know to be necessary for all known physical phenomena in this Universe. If a theory does not admit such a smooth space, it can be called "background independent" or "background free", but it may be a useless theory and a physically incorrect theory.

It is a very different question whether a theory treats all possible shapes of spacetime on completely equal footing or whether all these solutions follow from a more fundamental starting point. However, it is not a priori clear on physical grounds whether it must be so (It can be just an aesthetic feature of a particular formulation of a theory, not the theory itself.), and moreover, for a theory that does not predict many well-behaved backgrounds the question is meaningless altogether. Physics of string theory certainly does respect the basic rules of general relativity exactly - general covariance is seen as the decoupling of unphysical (pure gauge) modes of the graviton. This exact decoupling can be proved in string theory quite easily. It can also be seen in perturbative string theory that a condensation of gravitons is equivalent to a change of the background; therefore physics is independent of the background we start with, even if it is hard to see for the loop quantum gravity advocates.

Claims on non-principled approach

OBJECTION Loop quantum gravity is not science because every time a new calculation shows that some quantitative conjectures were incorrect, the loop quantum gravity advocates invent a non-quantitative, ad hoc explanation why it does not matter. Some borrow concepts from unrelated fields, including noiseless information theory and philosophy, and some explanations why previous incorrect results should be kept are not easily credible.