Talk:Loop quantum gravity

From Wikipedia, the free encyclopedia
Jump to: navigation, search
WikiProject Physics (Rated B-class, Mid-importance)
WikiProject icon This article is within the scope of WikiProject Physics, a collaborative effort to improve the coverage of Physics on Wikipedia. If you would like to participate, please visit the project page, where you can join the discussion and see a list of open tasks.
B-Class article B  This article has been rated as B-Class on the project's quality scale.
 Mid  This article has been rated as Mid-importance on the project's importance scale.


?? question ??[edit]

am i wrong if i'd suggest to replace "space" by "SPACETIME" in many places, f.e.: Loop quantum gravity (LQG) is a theory that attempts to describe the quantum properties of the universe and gravity. It is also a theory of quantum space and quantum time because, according to general relativity, the geometry of spacetime is a manifestation of gravity. LQG is an attempt to merge and adapt standard quantum mechanics and standard general relativity. The main output of the theory is a physical picture of space where SPACETIME is granular. The granularity is a direct consequence of the quantization. It has the same nature as the granularity of the photons in the quantum theory of electromagnetism or the discrete levels of the energy of the atoms. Here, it is SPACETIME itself that is discrete. In other words, there is a minimum distance resp. TIME-STEP possible to travel through it. (=> each of the four dimensions are granular... ) ... as well as i found in many articles/places "(havy) matter" as the only reason for "bending/curvation of the spacetime" ! Shouldn't it be "matter or energy" at every place as both are equivalent ? — Preceding unsigned comment added by TheuDie (talkcontribs) 12:45, 27 December 2014 (UTC)

12 year old[edit]

Isn't it wikipedia policy for a 12 year old to be able to understand... I'm 12 years of age and my dad (37) couldn't understand a word of it except for the words a, the, it, in, science, gravity and physics! please... can this srticle be simplified!

Then why don't you go to Simple English Wiki kid? That's clearly your and your father's level. —Preceding unsigned comment added by (talk) 04:16, 25 November 2010 (UTC)

The relevant question is -- is the Wikipedia sufficiently self-contained for a patient enough and diligent enough person (even 12) to find answers needed? You don't need to understand something to understand it. You only need to have access to the background needed to understand it. But the background is part of the Wikipedia, too. You can literally start out at 0 (as long as you're able to read, that is) and work your way from there.
I always understood that's how people normally surf the web anyhow (especially younger people for whom this is supposedly their home turf!). When they don't get something they surf in a matter of milliseconds to another link to bone up on what background they don't know, and then go back to what it was whose background they didn't have.
I think the answer is -- the Wikipedia (and the net, as a whole) are completely self-contained ... even to the point (I might add) of making school and college redundant and superfluous. -- Mark, 2007 February 3 —Preceding unsigned comment added by (talk) 23:32, 3 February 2008 (UTC)
That's the dumbest statement I've ever read. I shudder at the thought of the newer generations being educated by the internet. (talk) 19:26, 25 November 2009 (UTC)

Also, this article has gained WWW status as "difficult to understand"

Please sign your comments in the future. The fact of the matter is that LQG is a very esoteric piece of theoretical physics. As such it cannot be explained to a 12 year old unless that 12 year old knows tensor calculus. When I was 12 I was busy learning the basic laws of motion and general analytical skills which helped me learn what I needed in order to understand topic such as these. --Hfarmer 06:28, 13 July 2007 (UTC)
Absolute rubbish. This obviously bright 12 year old is quite right and you are wrong. There is a perfectly comprehensible description of this field in the January 2004 issue of Scientific American that does not use the word 'tensor' once. Like I said, I have a PhD in solid state chemistry and I found the article totally incomprehensible. Don't blame the igonorance of the audience for your lack of expository skills. Deadlyvices 10:16, 14 July 2007 (UTC)
Your education... I really don't see what that has to do with the topic at hand. You learned chenistry and not nearly as much physics as a physicist would. Please respect the fact that solid state chemistry while complex is not physics, that little you know would be applicable. I certinaly would not pretend to know as much about solid state chemistry as you would. Please pay us, your fellow physical scientist, the same respect. Furthermore I have not edited this article's copy in a long time. When I did I gave it the structure of having a plain english introductory lead section, then more technical guts. It still has that basic structure. Does it not say.

Loop quantum gravity (LQG), also known as loop gravity and quantum geometry, is a proposed quantum theory of spacetime which attempts to reconcile the seemingly incompatible theories of quantum mechanics and general relativity. This theory is one of a family of theories called canonical quantum gravity. It was developed in parallel with loop quantization, a rigorous framework for nonperturbative quantization of diffeomorphism-invariant gauge theory. In plain English, this is a quantum theory of gravity in which the very space in which all other physics occurs is quantized.

Loop quantum gravity (LQG) is a proposed theory of spacetime which is constructed with the idea of spacetime quantization via the mathematically rigorous theory of loop quantization. It preserves many of the important features of general relativity, while at the same time employing quantization of both space and time at the Planck scale in the tradition of quantum mechanics.

LQG is not the only theory of quantum gravity. The critics of this theory say that LQG is a theory of gravity and nothing more, though some LQG theorists have tried to show that the theory can describe matter as well. There are other theories of quantum gravity, and a list of them can be found on the quantum gravity page.

Just what about that is confusing? where is the word tensor used? There is really no way to make it simpler than the above. Any simpler and the article would be so generic as to be applicable to any theory of quantum gravity. --Hfarmer 13:25, 14 July 2007 (UTC)
Sorry, I should have said I found it almost totally incomprehensible. You don't appear to grasp that the issue is not about education, it's about meeting your audience halfway, and you don't seem to be prepared to budge an inch. Why should I have to be a physicist to understand a Wikipedia article? Kind of makes the writing of it in the first place pretty pointless, wouldn't you say?
I came here looking for a good, clear account of the subject for the intelligent layperson and I found this instead. It's chock-full of material such as

At the core of loop quantum gravity is a framework for nonperturbative quantization of diffeomorphism-invariant gauge theories, which one might call loop quantization. While originally developed in order to quantize vacuum general relativity in 3+1 dimensions, the formalism can accommodate arbitrary spacetime dimensionalities, fermions,[1] an arbitrary gauge group (or even quantum group), and supersymmetry,[2] and results in a quantization of the kinematics of the corresponding diffeomorphism-invariant gauge theory. Much work remains to be done on the dynamics, the classical limit and the correspondence principle, all of which are necessary in one way or another to make contact with experiment.

In a nutshell, loop quantization is the result of applying C*-algebraic quantization to a non-canonical algebra of gauge-invariant classical observables. Non-canonical means that the basic observables quantized are not generalized coordinates and their conjugate momenta. Instead, the algebra generated by spin network observables (built from holonomies) and field strength fluxes is used.

'In a nutshell, loop quantization is the result of applying C*-algebraic quantization to a non-canonical algebra of gauge-invariant classical observables. ' Just what the hell does all this mean? The most impenetrable of nutshells, that's what. I may be a chemist but even with my limited knowledge (which extends to Fermi levels, wave vectors, k-space, Brillouin zones and the suchlike) I imagine I could write a far more accessible account which, although lacking this kind of mathematical detail, reaches a much wider audience. Yet when someone points out that the article is largely incomprehensible, you reply 'As such it cannot be explained to a 12 year old unless that 12 year old knows tensor calculus'. You might as well have said 'go away and play with your Newtonian mechanics, little boy/girl'. If you want to be accorded respect, try showing it in the first place, and to everyone, not just 'fellow physical scientists'. I'd suggest that you start by always underestimating your audience's knowledge and never underestimating their intelligence. This article and your subsequent comments seem to do the complete opposite. Deadlyvices 04:14, 16 July 2007 (UTC)
I am sure your education in chemistry is quite comprehensive. I have no doubt that in that field you are totally qualified. I would have to differ with your assertion that you know enough about this topic to write a good article about it. While you know much about quantum levels of atoms and what not (more knowledge of that than the average physicist to be certain). That kind of "quantum physics" has little to nothing to do with the ultra relativistic quantum mechanics that is LQG (Or M theory for that matter). If you had said you knew Quantum Field Theory that would have been a much stronger credential.
However I understand that this is not about credentials. Wikipedia needs to be understandable to everyone who has graduated at least highschool. That is how I had written the introductory paragraph and subsequent editors continued in that spirit.
To make this article comprehensible to a 18 year old the article could only consist of that introductory section.
I suggest this look at the references in the LQG article. Read and study the matterials then please tell me how to break this down without mentioning tensors, or calculus, or the calculus of tensors, or the algebra of operators, etc, etc. It is inherently very complicated. THAT'S NOT MEANT TO INSULT YOU. It's just a statement of facts. LQG is second in complexity only to M-theory. That's just the nature of Quantum Gravity. --Hfarmer 05:16, 20 July 2007 (UTC)
So, are you telling me that there is no middle ground between the article remaining as it is, that is to say, more or less incomprehensible to anybody without a background in relativistic physics, and it being so superficial that it conveys no usful information whatsoever? That, to be of use, it only is of use to professional theoretical physicists? Give me a break!
The article on Heim Theory also deals with a very complicated and dense subject, probably understood by even fewer people than LQG. Yet I was able to understand all of it, not just the introductory paragraph. This is because the person who wrote it is evidently better at communicating difficult subject matter than the people who wrote this article. I'd bet real money that an entry written on LQG by a comparative non-expert would reach a bigger audience than this one and end up doing more ultimate good for the cause. Don't forget; it's the plebs like me who ultimately pay the salaries of the theoreticians who work in this field and we have every right to know why this field is worth bothering about. The 12-year-old you patronised may well end up being your Senator one day.Deadlyvices 16:53, 20 July 2007 (UTC)
Yes. That's what I am saying. At the level of physics a 12 or even 18 year old knows LQG and M-theory would be indistinguishable. For such a person they would probably stop with reading about "quantum gravity". To be a real smart @$$ I will say that a 12 year old Richard Feynmann would understand just fine. :P--Hfarmer 19:55, 16 September 2007 (UTC)
I tend to concur with the other Richard Feynman, the one who said that if a subject wasn't explicable in a freshman lecture, then it hadn't been understood properly. Every other article I've read on the Web about LQG manages to convey much more than this one. I wonder why that is?Deadlyvices 07:33, 23 September 2007 (UTC)
Like most big Wikipedia topics (Star Wars, Pokemon, computational chemistry...) it's written largely by fans and experts in its particular field, and therefore generally written with a certain amount of assumed knowledge about the field, rather than being aimed at a general audience. It's easy to forget that Wikipedia is meant to be an encyclopedia, and not just a collection of all human knowledge. Technical depth can (and arguably should) be dropped for clarity and brevity, so long as proper references and a bibliography are provided. 16:35, 25 October 2007 (UTC)

I agree that the quantum physicists who author these tutorials have little interest in writing down to the layman level since they are playing to their peers, academics who will recognize thier thumbnail descriptions on wiki almost like publications. It is not impossible to explain something this arcane to laymen with some background in math (just read Roger Penrose's article on loop gravity in "Road to Reality") but for a 12 year old without the knowledge of vectors, guage connections, parallel transport, spin networks, etc. it just isn't possible. Piamero (talk) 05:19, 22 September 2008 (UTC)piamero

I think that this piece of the whole article does a better job that this wikipedia article in explaining what loop quantum gravity is. —Preceding unsigned comment added by (talk) 11:38, 9 December 2009 (UTC)

While I have the greatest respect for people who strive to document these complex topics, I wholeheartedly agree with Deadlyvices' objections. Abstract physical concepts that are very difficult to understand have been documented to accommodate readers of nearly all levels, which means this article is unnecessarily complicated. The greatest physicists of all time were able to make earth-shattering discoveries because of their ability to think in physical pictures like speeding trains and such. The most famous example is probably the apple falling on Newton's head. I know that this becomes rather difficult when talking about non-Newtonian physics, but Albert Einstein also thought in 'physical pictures', even though the subjects he studied were far more abstract than gravity. I strongly suggest you take it upon you to represent the intensely abstract LQG with more 'graspable' presentations. Highly respected scientists like Michio Kaku do a great job at publishing literature on advanced scientific concepts that is understandable by pretty much anyone. That said, I'm not telling you to drop the more complex content. I'm merely asking you to at least provide interested readers who do not study physics with a solid explanation of the basics of LQG, significantly increasing the importance and value of this article. — Preceding unsigned comment added by (talk) 18:34, 24 August 2011 (UTC)

Should there be another LQG entry simplified for popular understanding in popular press? For a generally accessible and less technical introduction to the topic, see Introduction to M-theory. — Preceding unsigned comment added by (talk) 03:35, 16 March 2013 (UTC)

I like complicated[edit]

As an "educated layman" i enjoy being challenged and stimulated by wikipedia physics entries and i'd like to suggest that as long as technical terms are linked to their respective articles, allowing users to investigate to whatever depth they feel comfortable with there should be no attempt to simplify to any great extent - some things are just bloody complicated. (talk) 04:50, 21 January 2008 (UTC)

Jolly good for you. Most of us, when we start off learning about a new subject, prefer 'simple'. And 'comprehensible'. We also don't like having to engage in wild-goose chases across the Web trying to ascertain the meaning of material that could quite easily have been made easy to understand in the first place. Perhaps you have a lot of free time to do this sort of investigation. I don't, personally, and if Wikipedia wants to be recognised as any kind of authoritative resource, it has to be accessible before sets out to be rigorous.Deadlyvices (talk) 18:57, 29 January 2008 (UTC)
Cool story bro. Fact is, this material has to be abstracted and generalized to the point of distortion in order for it to be 'easy to understand'. The problem is egotistical readers expecting to be able to easily understand whatever they come across, certain that their vast intellects are surely capable of grasping any given topic instantly. If they don't understand something, it's not their fault - it's the material's! Pathetic. Unsigned, because, well, fuck you that's why.
Well, bro, I don't expect anything of the sort. Most people, not me, think that this article is jargonistic and poorly written, and if a professional scientist like me finds it difficult it's not because I'm egotistical to the extent that I expect everything to be handed to me on a plate. Perhaps we have a point. Perhaps its written by a crappy author who is too arrogant to recognise he isn't very good at explaining complex topics. Perhaps it shouldn't be written about at all in an encyclopaedia if it can't be written about comprehendably. And fuck you too, you arrogant, abusive and cowardly little man (that much I'm certain about). Deadlyvices (talk) 22:33, 20 July 2011 (UTC)

Just revisited this page. Up until the section *Constraints and their Poisson bracket algebra* it's pretty good. After that it's totally incomprehensible, over-technical shit. Deadlyvices (talk) 11:53, 7 July 2015 (UTC)

Iyo Iyo Ita[edit]

Why are we citing non-peer reviewed, and seemingly unphysical (multiplication of functionals=0 is just wrong) material? Surely we should only cite journal papers, not just what gets posted to arxiv! —Preceding unsigned comment added by (talk) 01:35, 17 March 2008 (UTC)

The way things are done these days arxiv, is the way allot of physics publishing is done. Some on there don't even bother with paper journals anyway because more people will see an arxiv posting. Another way is to put it on a scientific bloging site, or on your website then a moderated usenet group. From there feedback and peer review is public. By the time something is in a paper journal these days it is old news. --Hfarmer (talk) 02:27, 21 July 2008 (UTC)

Chiral fermion anomalies[edit]

It currently appears that nothing forbids coupling anomalous 
- i.e. quantum mechanically inconsistent - chiral fermions to LQG.
That's not true, LQG is based upon first class constraints and with anomalous chiral fermions, the first class constraints mutate into second class constraints. AnonyScientist (talk) 11:38, 23 August 2008 (UTC)

Block of original research removed[edit]

I've removed the block of original research and synthesis posted in the section previously titled "Diffeomorphism invariance and background independence", and replaced it with what I suppose is an equivalently imperfect summary-style section, presently retitled Loop quantum gravity#General_covariance_and_background_independence. ... Kenosis (talk) 23:34, 8 January 2009 (UTC)

I proposed the following be removed to their own sections/articles

   * 4.4 LQG and the big bang singularity
   * 4.5 LQG and particle physics
   * 4.6 LQG and the Graviton
   * 4.7 The Kodama state
   * 4.8 Spinfoam
   * 4.9 Non commutative geometry and loop gravity
  1. 5 LQG and analogues to condensed matter physics
   * 5.1 LQG and string nets
   * 5.2 LQG and group field theory  —Preceding unsigned comment added by (talk) 18:26, 9 March 2009 (UTC) 

any volunteers to do so? If I do so will someone reverse the edit? —Preceding unsigned comment added by (talk) 18:33, 9 March 2009 (UTC)

is interpretation Fermi results controversial?[edit]

Pra1998 added a reference to a paper about results from Fermi, stating in the article that they "seem to have severe implications for this theory." Is this noncontroversial? There's a recent paper by Amelino-Camelia and Smolin analyzing the results, which does not seem to say "Oh no, we need to give up on LQG." -- (talk) 02:20, 18 August 2009 (UTC)

I think it is at this time premature to rely on a single observation. —Preceding unsigned comment added by (talk) 18:07, 19 August 2009 (UTC)

The good, the bad, and the ugly[edit]

I am a layman who has more than passing interest in this topic. I would like to chime in with my two cents.

The good[edit]

This is a remarkably well-balanced article in a field in which every researcher thinks he/she has THE answer. Theory is not confused with fact. Limitations are admitted. Contrary opinions are acknowledged. All objectively presented. Kudos to the contributors. I would like to see this objectivity maintained in future editions.

The bad[edit]

The article is almost completely impenetrable. It's not the concepts; it's the vocabulary. My favorite sentence is:

"While originally developed in order to quantize vacuum general relativity in 3+1 dimensions, the formalism can accommodate arbitrary spacetime dimensionalities, fermions, an arbitrary gauge group (or even quantum group), and supersymmetry, and results in a quantization of the kinematics of the corresponding diffeomorphism-invariant gauge theory."

This is a cutting-edge subject--some level of knowledge must be assumed. The article cannot go all the way back to explaining photons, electron, protons, and neutrons--that would require an entire encyclopedia. However a better balance needs to be achieved between precision and comprehension. Some suggestions: shorter sentences; fewer "BIG" words per sentence; more analogies; more examples. I don't think a professional physicist is needed--just writing each sentence with conscious attention to the target audience.

The ugly[edit]

There is no ugly!

The para you quoted can be understood by anyone with even a basic understanding of basic physics, . Dimension10 (talk) 13:53, 23 July 2013 (UTC)

Citation needed?[edit]

I was a bit surprised by the following in the article, " of now there is no experimental observation for which loop quantum gravity makes a prediction not made by the Standard Model or general relativity (a problem that plagues all current theories of quantum gravity)." Could someone provide a citation for this as i thought LQG did make a prediction which was supposed to be tested by GLAST. —Preceding unsigned comment added by (talk) 21:57, 15 May 2011 (UTC)

You are quite right, looking for it as I write.[edit]

It was the question if a photons 'speed' could vary with its 'energy' that GLAST was supposed to look for, amongst other things. In loop quantum it seems as if this could be a effect of the theory, not invalidating Einstein, but modifying it slightly. Although, be aware of that we are discussing 'energy', as defined from interactions, not 'rest mass'. So those of you wanting to give a photon a 'mass' have to search elsewhere. .. Can't seem to find anything recent though? discuss it. And here is GLAST .. The best I could find is from 2008 .. but? I'm surprised that I can't find anything better on it. It should be resolved by now, one way or another..

Ahh a.. Hmm :) But, I do like Smolin, he's what all scientists should be, open to change, keeping his cool. Theories are only as good as the experiments proving or disproving them. And Smolin is constantly at the forefront, no matter how controversial he might be deemed to be. Still, as I expect light to be a 'clock' of sorts myself, I'm happy that we haven't found it with several 'beats', not that it's impossible, though. Here is the link to the wiki (deformed special relativity) that seems to discuss that idea in more depth, hopefully :)

Yoron (talk) 17:51, 25 June 2011 (UTC)


What's the central formula of this theory? I mean, there has to be something likem a Lagrange density or something similar... -- (talk) 18:08, 20 July 2011 (UTC)

Yup, the Einstein - Hilbert - Palatini - Ashtekar (EHPA) or Einstein - Hilbert - Ashtekar (EHA) action (Source: LQG for (and by) the bewildered) .. Dimension10 (talk) 13:56, 23 July 2013 (UTC) . m.
Would there be something like a (quantum) physical system, with a Hilbert space of states or whatever is appropriate for LQG, and observables and dynamics? You would be looking for the formula for dynamics of the system? --Telecomtom (talk) 01:33, 3 April 2012 (UTC)
From the article on spin networks: "In loop quantum gravity (LQG), a spin network represents a "quantum state" of the gravitational field on a 3-dimensional hypersurface. The set of all possible spin networks (or, more accurately, "s-knots" - that is, equivalence classes of spin networks under diffeomorphisms) is countable; it constitutes a basis of LQG Hilbert space." Maybe someone will write the formula for the dynamics ... --Telecomtom (talk) 02:08, 3 April 2012 (UTC)

Overlooked aspects and gaining information[edit]

Some large aspects of this theory have been seriously overlooked, this article only explains the very basics of loop quantum gravity and a lot more information is needed to give the reader a sufficient understanding of this theory. I'm a physicist, and I'm new to wikipedia, but an expert is not the the only option, for example i can't really help because i specialize in string theory and occasionally other similar fields. But the truth is that anyone, given they put in a sufficient amount of time and research can construct an understanding of anything, well enough to give a brief outline of it. A large population of the planet have vast amount of information at their disposal, everyone reading this has access to the internet, this allows us to research near enough any topic we desire, including quantum loop gravity. It may be hard to find but i can assure you, it is there. All we need is a team of people, maybe even just one person to research this topic, their understanding to begin with is irrelevant, as long as they put in enough effort to begin with and gain an interest in the desired topic. This person, or people, could then update this page with their findings, sure it would help if there was a specialist who could explain it without research, but the notification that one has been needed has been up there for 2-3 years, so we will have to do the next best thing. Someone with a respectable amount of knowledge of physics could do this brilliantly, they could grasp the concepts of this theory easily, and identify, false or misleading data. I have a 'quite' good understanding of this theory and if no one steps up and does this research I will do it myself, if i have the time. Dr J. Hill — Preceding unsigned comment added by Joehill11 (talkcontribs) 20:59, 6 February 2012 (UTC)

Well, hurry the frick up, Doc - and when you get going, leave me a note on my Talk Page so I can follow along behind you as your assistant, tweaking your grammar! :-)

Welcome to Wikipedia! We can use you! And BTW, if you know any lung cancer experts, send them to me - I NEED THE HELP!

Very best regards: Cliff (a/k/a "Uploadvirus") (talk) 11:47, 8 February 2012 (UTC)

black hole entropy contradictions[edit]

the below 2 passages contradict one another, Carlo Rovelli's edit is based on more recent research which does away with the Immirzi parameter and Carlo Rovelli is a LQG researcher who has written papers and textbooks.

I propose deleting the older bottom paragraph as no longer current.

Another application of LQG concerns understanding the thermal behavior of black holes. A recent success of the theory in this direction is the computation of the entropy of all non singular black holes [16]. The result is the expected formula S=A/4, where S is the entropy and A the area of the black hole, derived by Bekenstein and Hawking on heuristic grounds. This is the only known derivation of this formula from a fundamental theory, for the case of generic non singular black holes.

Another problem is that a crucial free parameter in the theory, known as the Immirzi parameter, can only be computed by demanding agreement with Bekenstein and Hawking's calculation of the black hole entropy. Loop quantum gravity predicts that the entropy of a black hole is proportional to the area of the event horizon, but does not obtain the Bekenstein-Hawking formula S = A/4 unless the Immirzi parameter is chosen to give this value. A prediction directly from theory would be preferable.


Main article: History of loop quantum gravity

In 1986, "someone" reformulated Einstein's general relativity in a language closer to that of the rest of

changed that to twitch

(cur | prev) 02:18, 14 May 2013‎ AnomieBOT (talk | contribs)‎ . . (112,172 bytes) (+53)‎ . . (Rescuing orphaned refs ("ReferenceA" from rev 554904443)) (undo) (cur | prev) 16:08, 13 May 2013‎ (talk)‎ . . (112,119 bytes) (+4)‎ . . (→‎History) (undo) (cur | prev) 16:01, 13 May 2013‎ (talk)‎ . . (112,115 bytes) (-9)‎ . . (→‎History) (undo) (cur | prev) 15:56, 13 May 2013‎ (talk)‎ . . (112,124 bytes) (-9,375)‎ . . (→‎An overview) (undo) (Tag: section blanking) (cur | prev) 15:54, 13 May 2013‎ (talk)‎ . . (121,499 bytes) (+7)‎ . . (→‎An overview) (undo)

i think someone should undo vandals and then lock the article only ibyan and rovelli may add content ban

Time in general relativity[edit]

Under "The problem of time in quantum gravity", the introduction states:

"Roughly speaking the problem of time is that there is none in general relativity. This is because in general relativity the Hamiltonian is a constraint that must vanish. However, in any canonical theory, the Hamiltonian generates time translations. Therefore we arrive at the conclusion that "nothing moves" ("there is no time") in general relativity."

This is probably a reflection of Rovelli's thinking ("Forget time", "Time does not exist"). The claim that there is no time in general relativity as such is inappropriate, however, as it is at odds with generally accepted descriptions of general relativity. — Preceding unsigned comment added by (talk) 22:54, 24 March 2014 (UTC)

The Problem of Time section belongs on a separate page, and should be removed from this page. I submitted a request for a "Problem of Time" page ( ) and added a link to that proposed page as a "main article" link. — Preceding unsigned comment added by (talk) 12:34, 27 April 2014 (UTC)