
Quantum gravity is the field of theoretical physics attempting to unify the theory of quantum mechanics, which describes three of the fundamental forces of nature, with general relativity, the theory of the fourth fundamental force: gravity. The ultimate goal of some (e.g. string theory) is a unified framework for all fundamental forces—a theory of everything. Much of the difficulty in merging these theories comes from the radically different assumptions that these theories make on how the universe works. Quantum field theory depends on particle fields embedded in the flat space-time of special relativity. General relativity models gravity as a curvature within spacetime that changes as mass moves. The most obvious ways of combining the two (such as treating gravity as simply another particle field) run quickly into what is known as the renormalization problem. Gravity particles would attract each other and adding together all of the interactions results in many infinite values which cannot easily be cancelled out mathematically to yield sensible, finite results. This is in contrast with quantum electrodynamics where the interactions sometimes evaluate to infinite results, but those are few enough in number to be removable via renormalization. Both quantum mechanics and general relativity have been highly successful. Unfortunately, the energies and conditions at which quantum gravity effects are likely to be important are inaccessible to current laboratory experiments. The result is there are no experimental observations which would provide any hints as to how to combine the two. The general approach taken in deriving a theory of quantum gravity is to assume that the underlying theory will be simple and elegant and then to look at current theories for symmetries and hints for how to combine them elegantly into an overarching theory. One problem with this approach is that it is not known if quantum gravity will be a simple and elegant theory. Such a theory is required in order to understand those problems involving the combination of very large mass or energy and very small dimensions of space, such as the behaviour of black holes, and the big bang. == Historical perspective == Historically, there have been two reactions to the apparent inconsistency of quantum theories with the necessary background-independence of general relativity. The first is that the geometric interpretation of general relativity is not fundamental, but just an emergent quality of some background-dependent theory. This is explicitly stated, for example, in Steven Weinberg's classic ''Gravitation and Cosmology'' textbook. The opposing view is that background-independence is fundamental, and quantum mechanics needs to be generalized to settings where there is no a priori specified time. The geometric point of view is expounded in the classic text ''Gravitation'', by Charles Misner, John Archibald Wheeler and Kip Thorne. The two books by giants of theoretical physics expressing completely opposite views of the meaning of gravitation were published almost simultaneously in the early 1970s. The reason was that an impasse had been reached, a situation which led Richard Feynman (who himself had made important attempts at understanding quantum gravity) to write, in desperation, ''"Remind me not to come to any more gravity conferences"'' in a letter to his wife in the early 1960's. Progress has been made on both fronts, leading (as of 2004) to string theory on the one hand and loop quantum gravity on the other. == The incompatibility of quantum mechanics and general relativity == At present, one of the deepest problems in theoretical physics is harmonizing the theory of general relativity, which describes gravitation and applies to large-scale structures (stars, planets, galaxies), with quantum mechanics, which describes the other three fundamental forces acting on the microscopic scale. A fundamental lesson of general relativity is that there is no fixed spacetime background, as found in Newtonian mechanics and special relativity; the spacetime geometry is dynamic. While easy to grasp in principle, this is the hardest idea to understand about general relativity, and its consequences are profound and not fully explored, even at the classical level. To a certain extent, general relativity can be seen to be a relational theory, in which the only physically relevant information is the relationship between different events in space-time. On the other hand, quantum mechanics has depended since its inception on a fixed background (non-dynamical) structure. In the case of quantum mechanics, it is time that is given and not dynamic, just as in Newtonian classical mechanics. In relativistic quantum field theory, just as in classical field theory, Minkowski spacetime is the fixed background of the theory. Finally, string theory started out as a generalization of quantum field theory where instead of point particles, string-like objects propagate in a fixed spacetime background. Although string theory had its origins in the study of quark confinement and not of quantum gravity, it was soon discovered that the string spectrum contains the graviton, and that "condensation" of certain vibration modes of strings is equivalent to a modification of the original background. Quantum field theory on curved (non-Minkowskian) backgrounds, while not a quantum theory of gravity, has shown that some of the core assumptions of quantum field theory cannot be carried over to curved spacetime, let alone to full-blown quantum gravity. In particular, the vacuum, when it exists, is shown to depend on the path of the observer through space-time (see Unruh effect). Also, the field concept is seen to be fundamental over the particle concept (which arises as a convenient way to describe localized interactions). This latter point is not uncontroversial, as it is contrary to the way quantum field theory on Minkowski space is developed by Steven Weinberg's book ''Quantum Field Theory''. Loop quantum gravity is the fruit of the effort to formulate a background-independent quantum theory. Topological quantum field theory provided an example of background-independent quantum theory, but with no local degrees of freedom, and only finitely many degrees of freedom globally. This is inadequate to describe gravity in 3+1 dimensions, which even in vacuum has local degrees of freedom according to general relativity. In 2+1 dimensions, however, gravity is a topological field theory and it has been successfully quantized in several different ways, including spin networks. There are three other points of tension between quantum mechanics and general relativity. First, general relativity predicts its own breakdown at singularities, and quantum mechanics becomes inconsistent with general relativity in a neighborhood of singularities. Second, it is not clear how to determine the gravitational field of a particle, if under the Heisenberg uncertainty principle of quantum mechanics its location and velocity cannot be known with certainty. Finally, there is a tension, but no logical contradiction, between violations of Bell's inequality in quantum mechanics, which imply superluminal influence, and the speed of light as a speed limit in relativity. The resolution of the first two points may come from a better understanding of general relativity [http://arxiv.org/abs/astro-ph/0506506]. == Theories == There are a number of proposed quantum gravity theories: *String theory *Supergravity *AdS/CFT *Loop quantum gravity of Ashtekar, Smolin and Rovelli *Euclidean quantum gravity *Noncommutative geometry of Alain Connes *Twistor of Roger Penrose *Discrete Lorentzian quantum gravity *Sakharov induced gravity *Regge calculus *Process Physics *Chaotic gravitational waves The "direct" way of quantizing gravity comes with many choices. Do we use functional integrals over Wick rotated Riemannian metrics (e.g. by Hawking)? See Euclidean path integral approach. Do we use the covariant Peierls bracket? Do we use BRST/Batalin-Vilkovisky formalism or gauge fixing or gauge factoring? If we pick canonical quantization, do we use the Einstein-Hilbert action with only the metric as dynamical to get the Wheeler-deWitt equation? Or do we treat the metric and the affine connection independently? Or do we have the whole Poincaré group as the gauge group starting with the Einstein-Cartan theory? Or do we use the Cartan method of moving frames with the Palatini action to get second class constraints? Do we eliminate the second class constraints using the Ashtekar variables to get loop quantum gravity or do we do something else? The existence of spinor fields may force us to work with the Cartan formalism or something comparable. Or maybe we should look at representations of the diffeomorphism group just as Wigner looked at representations of the Poincaré group. == Quantum gravity theorists == ''See list of quantum gravity researchers'' == See also == *Centauro event *String theory *M-theory *Semiclassical gravity *Quantum field theory in curved spacetime *Process Physics == External links == The famous spoof of postmodernism by Alan Sokal (see Sokal Affair) was entitled ''Transgressing the Boundaries: Toward a Transformative Hermeneutics of Quantum Gravity''. Protoscience Theoretical physics Quantum gravity
This sentence fragment sounds wrong to me: "The energies and conditions at which quantum gravity are likely to be important are..." I assume it should be something like "quantum gravity effects are...", but unfortunately I don't understand enough about the subject to feel confident in changing it. User:Marsvin 19:54, 2004 Jul 13 (UTC) I heve moved the section "the incompatibility between QM and GR" from LQG. User:Miguel 16:25, 25 Jul 2004 (UTC)Some of the criticisms of LQG from the ST corner are distinctly NPOV. Weeding it a bit. On second thought, the section makes it clear that it's one persons list of criticisms. Rebuttals from the ST side of the coin seem desirable. Perhaps it's even more desirable for most of this to be drastically shortened or summarized, lest the article turns into a soapbox platform for various QG proponents. Nobody yet knows what the right solution looks like, OK? Give it a rest! :-) —User:JRM 11:50, 2004 Sep 14 (UTC)
== Less Emphasis on ST/LQG Debate ==
While the String Theory vs. Loop Quantum Gravity debate is obviously relevent here, the ST objections to LQG take up what, to me, seems an inordiante amount of space on this page--especially with the counter argument confined to a single eternal link.
At least untill the LQG guys get their "merge" worked out, wouldn't it make more sense to link to the Loop_gravity page from here, since that presents a more thourough treatment of the ST criticizms? (And maybe, then, organize the two links under a single "Ongoing Debate" section on this page?)
I'd make the change myself but I'm a total Wikipida n00b and don't want to step on anybody's toes... --SMQ
User:66.84.200.34 19:44, 7 Oct 2004 (UTC)
(edited stupid link typo User:66.84.200.34 19:47, 7 Oct 2004 (UTC))
In any case the discussion is too big, so I suggest moving it to another article. I can do it by myself, but let's first decide on title. I suggest:
* Loop quantum gravity versus string theory
User:217.26.0.121 10:03, 10 Oct 2004 (UTC)
== bibliography ==
Hi, can we decide on bibliography? I like some books from loop quantum gravity's bib but I don't want to put them here since in the light on ST/LQG debate this can be considered POV. By the way, as we are far from QG in both approaches this article should IMHO concentrate on principal problems with QM+GR=QG ant not on ST and LQG --- we are moving to it. User:193.124.225.253 16:26, 13 Oct 2004 (UTC)
== The page promotes loop quantum gravity and downplays string theory ==
Neutrality would ideally mean that it's hard to infer the author's opinion, but I could tell immediately that much of this page is written by advocates of loop quantum gravity.
As I said on the other page, I am neither a string theorist nor a loop-quantum-gravity theorist. I respect loop quantum gravity papers as mathematical physics and their authors as mathematical physicists. But I also see some loop quantum gravity people as engaged in a Naderite quest to compete with string theory. This quest is not physics. For better or worse, it should be presented on the LQG page. It does not belong on this second page on quantum gravity in general; this page should only have a reference to it.
: Greg Kuperberg - User:24.59.196.30 14:09, 4 Nov 2004 (UTC)
I agree with Greg, especially if he meant the section about "theories and prototheories". As a string theorist, I must say that this section reminds me Feynman's comments about the cargo cult sciences: the primitive tribes choose a guy with wooden earphones who expects the airplanes to land, much like they did in the Second World War. They don't land. Something is wrong with their science. It's hard to explain them what's exactly wrong - it would be easier to point out how they should change the shape of the wooden earphones.
In a similar way, this paragraph about "theories and prototheories" asks a lot of irrelevant technical questions about the shape of various brackets, constraints, and the redefinitions of the fields, without asking whether the "direct" way of quantizing "pure gravity" is the right approach. Of course that this is not the right approach, according to virtually all particle physicists and string theorists. General relativity, as a quantum theory, is just an effective theory that works at long distances, but breaks down at short distances, and no field redefinition or shaping of the wooden earphones is able to change the fact that quantized GR is "incomplete in the UV". This makes the whole paragraph irrelevant.
A question for Wikipedia is whether the viewpoint - that gravity should still be quantized "directly" - should be given a lot of attention. It is certainly a viewpoint that does not seem to lead anywhere in science. Most scientists in the field "know" that it can't work, even though it is hard to state it as a theorem. Yes, the more you move from the actual scientists via science fans to the laymen, the more they find it plausible that it should be possible to quantize pure gravity directly after all. Well, I still think that Wikipedia should try to prefer the "professional" viewpoint over the misunderstandings of outsiders. If it's true, this paragraph should not assume that "quantizing gravity directly (without any new physics)" is a right approach. --User:Lumidek 14:45, 4 Nov 2004 (UTC)
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Well, Lubos, you may agree with me more than I agree with you. I have no reason to doubt your physics; from the larger context I am inclined to believe it. But at a human level, your explanation is terrible. I said that I respect LQG papers as mathematical physics and their authors as mathematical physicists, but that is not consistent with your narrative at all. Feynman's famous essay refers to sheer crackpots, and not to trained scientists who trip badly in their work. It is not easy to understand string theory, much less to believe it for the proper internal reasons. I believe it largely for external reasons, e.g., that I have never heard of Witten making a big mistake.
In any case you are better at explaining why people don't properly understand string theory than what is really wrong with loop quantum gravity. I have met string theorists who aren't at all adamant that LQG is worthless, much less that it will always be worthless. The most that they will say is that they haven't learned from it. Possibly they are being polite and they really do think that it's worthless. Since popular accounts of string theory are so fashionable, I think that they should do more to counter this apparently Naderite alternative. But it should be more adroit than your severe polemics.
: Greg Kuperberg - User:24.59.196.30 16:31, 4 Nov 2004 (UTC)
== A new answer to Greg from Lubos ==
Hey Greg, obviously, I am not gonna agree with anything you wrote above. ;-) Which string theorists are you exactly talking about? Be sure that string theorists such as Witten, Strominger, Vafa, Gross, Polchinski, Susskind, Banks, etc., but also particle physicists like Nima Arkani-Hamed, ... I could continue for a long time, all of them are convinced that loop quantum gravity is rubbish - and they will tell you about it, even though (sometimes) with a more diplomatic language (but sometimes tougher language). Sure, you can find a string theorist - especially if she or he is a young one - who will tell you that (s)he is open-minded about LQG. But sorry, this just proves a lack of experience with the subject.
Concerning Feynman, he had DEFINITELY said a lot of these explicit comments about the "general relativity" community. (Well, he was slightly critical of string theory, too, but it was not as emotional as the LQG-like people.) If you read his books, you would know that Feynman was really angry about these relativists at the conferences, and he asked his wife to remind him that he should never visit another conference about general relativity - exactly because they often like to discuss the religious rubbish about "background independence" and "special role of gravity" WITHOUT TRYING TO MAKE A QUANTITATIVE CONTACT WITH DOABLE EXPERIMENTS, which is something that Feynman could not stand. Feynman definitely thought that these people were lousy scientists, and he was never hiding it.
There are thousands of pages, even on the web, where you can learn about it. For example, open [http://www.bbc.co.uk/science/space/spaceguide/hawking/transcript1.shtml]
Feynman has given an amusing account of attending the conference on general relativity and gravitation, in Warsaw in 1962. In a letter to his wife, he said.
I am not getting anything out of the meeting. I am learning nothing. Because there are no experiments, this field is not an active one, so few of the best men are doing work in it. The result is that there are hosts of dopes here (126) and it is not good for my blood pressure. Remind me not to come to any more gravity conferences!
Once again, your statement that Feynman did not say that these people "admiring the exceptional beauty and role of GR" were stupid is not true, and can easily be shown incorrect. And if you had doubts that the way of thinking of the dopes on the 1962 conference was essentially identical to the LQG community today (and there are no real new discoveries either), I can give you references for it, too. --User:Lumidek 15:23, 7 Nov 2004 (UTC)
-----
A brief comment:
"Both have been highly successful and there are no known phenomena that contradict the two" is incorrect. Quantum entanglement contradicts quantum mechanics in that information is supposedly not able to travel faster than the speed of light, yet this is exactly what happens.
: Whoever wrote this nonsense, has not signed his or her contribution, so I hope that others will know that it is irrelevant. Quantum entanglement is a successfully verified prediction of quantum mechanics, and saying that they "contradict each other" is simply a stupidity. No real information is propagating in these experiments. The outcomes are correlated, but it does not require any propagation of real information superluminally. [http://motls.blogspot.com/2004/10/causality-and-entanglement_16.html] --User:Lumidek 15:37, 7 Nov 2004 (UTC)
Although I don't really understand string theory, I do study quantum information theory and I can comment on this. If you attempt to stick to the non-quantum model of information, it is not only that it can travel faster than light in quantum mechanics. Rather, quantum mechanics overthrows the old notion of information entirely. So you have to properly redefine information in quantum mechanics. Once you do that, the plain answer is that quantum entanglement does not send any quantum information faster than light. It does allow classically impossible things (like quantum secrecy and quantum computation), but faster-than-light communication is not one of those things.
- Greg Kuperberg - User:24.59.196.30 22:13, 8 Nov 2004 (UTC)
----
As a young scientist there is one thing I know for certain about theoretical physics. I know that theories that are not falsifiable are not science. As far as I know String theory and loop gravity are very weak in that regard.
The above is an example of a neutral statement. Study it.
--User:HFarmer 03:21, 11 Jan 2005 (UTC)
: If memory serves, there were a few experiments that placed bounds on variations of string theory/m theory and LQG that _could_ be correct. For string theory, there were searches for dimensions that had curled to a size large enough to be detected (as opposed to planck-scale), and for LQG someone had searched for the effects of space quantization on the propagation of photons from very distant sources. Both searches turned up empty, but the point is that at least a _few_ experiments were done.
: That having been said, I'm staying away from touching any of the articles until it starts looking less like the plasma cosmology flame war. --User:Christopher Thomas 00:49, 28 Feb 2005 (UTC)
:: Yes, let us think about this: ''Science'' and ''flame war.'' Both sides' objectives are the same--explaining how the world works. As far as I can tell as an aerospace engineering outsider (particle physics make my brain hurt), my suggestion is that we just have one section each on LQG and ST ideas on this and leave the rebuttals to a minimum. I very much doubt the average Wiki person wanting to learn more about quantum gravity -really- wants his head blown off by petty factions. That being said, I've removed the apparently aggrivating suggestion of "proto-theories" and completely removed the latter section of the LQG/ST debate, as that was already mentioned in the history. --The Centipede, 12 Apr 2005
== Quantum Gravity. ==
Some time ago, I watched a television documentary in which scientists claimed the universe was created at the instant of the big bang by a collision between two pre-existing universes.
With this idea in mind, I have wondered if the reason why a theory of quantum gravity cannot be found is because our universe is a hybrid universe, represented by two mathematical theories, wholly alien to each other, which can never be unified.
This is just a thought from a layman.
Derek R Crawford.
Theoretical physicsQuantum field theoryGeneral relativityGravity
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Quantum gravity
Quantum gravity is the field of theoretical physics attempting to unify the theory of quantum mechanics, which describes three of the fundamental forces of nature, with general relativity, the theory of the fourth fundamental force: gravity. The ultimate goal of some (e.g. string theory) is a unified framework for all fundamental forces—a theory of everything. Much of the difficulty in merging these theories comes from the radically different assumptions that these theories make on how the universe works. Quantum field theory depends on particle fields embedded in the flat space-time of special relativity. General relativity models gravity as a curvature within spacetime that changes as mass moves. The most obvious ways of combining the two (such as treating gravity as simply another particle field) run quickly into what is known as the renormalization problem. Gravity particles would attract each other and adding together all of the interactions results in many infinite values which cannot easily be cancelled out mathematically to yield sensible, finite results. This is in contrast with quantum electrodynamics where the interactions sometimes evaluate to infinite results, but those are few enough in number to be removable via renormalization. Both quantum mechanics and general relativity have been highly successful. Unfortunately, the energies and conditions at which quantum gravity effects are likely to be important are inaccessible to current laboratory experiments. The result is there are no experimental observations which would provide any hints as to how to combine the two. The general approach taken in deriving a theory of quantum gravity is to assume that the underlying theory will be simple and elegant and then to look at current theories for symmetries and hints for how to combine them elegantly into an overarching theory. One problem with this approach is that it is not known if quantum gravity will be a simple and elegant theory. Such a theory is required in order to understand those problems involving the combination of very large mass or energy and very small dimensions of space, such as the behaviour of black holes, and the big bang. == Historical perspective == Historically, there have been two reactions to the apparent inconsistency of quantum theories with the necessary background-independence of general relativity. The first is that the geometric interpretation of general relativity is not fundamental, but just an emergent quality of some background-dependent theory. This is explicitly stated, for example, in Steven Weinberg's classic ''Gravitation and Cosmology'' textbook. The opposing view is that background-independence is fundamental, and quantum mechanics needs to be generalized to settings where there is no a priori specified time. The geometric point of view is expounded in the classic text ''Gravitation'', by Charles Misner, John Archibald Wheeler and Kip Thorne. The two books by giants of theoretical physics expressing completely opposite views of the meaning of gravitation were published almost simultaneously in the early 1970s. The reason was that an impasse had been reached, a situation which led Richard Feynman (who himself had made important attempts at understanding quantum gravity) to write, in desperation, ''"Remind me not to come to any more gravity conferences"'' in a letter to his wife in the early 1960's. Progress has been made on both fronts, leading (as of 2004) to string theory on the one hand and loop quantum gravity on the other. == The incompatibility of quantum mechanics and general relativity == At present, one of the deepest problems in theoretical physics is harmonizing the theory of general relativity, which describes gravitation and applies to large-scale structures (stars, planets, galaxies), with quantum mechanics, which describes the other three fundamental forces acting on the microscopic scale. A fundamental lesson of general relativity is that there is no fixed spacetime background, as found in Newtonian mechanics and special relativity; the spacetime geometry is dynamic. While easy to grasp in principle, this is the hardest idea to understand about general relativity, and its consequences are profound and not fully explored, even at the classical level. To a certain extent, general relativity can be seen to be a relational theory, in which the only physically relevant information is the relationship between different events in space-time. On the other hand, quantum mechanics has depended since its inception on a fixed background (non-dynamical) structure. In the case of quantum mechanics, it is time that is given and not dynamic, just as in Newtonian classical mechanics. In relativistic quantum field theory, just as in classical field theory, Minkowski spacetime is the fixed background of the theory. Finally, string theory started out as a generalization of quantum field theory where instead of point particles, string-like objects propagate in a fixed spacetime background. Although string theory had its origins in the study of quark confinement and not of quantum gravity, it was soon discovered that the string spectrum contains the graviton, and that "condensation" of certain vibration modes of strings is equivalent to a modification of the original background. Quantum field theory on curved (non-Minkowskian) backgrounds, while not a quantum theory of gravity, has shown that some of the core assumptions of quantum field theory cannot be carried over to curved spacetime, let alone to full-blown quantum gravity. In particular, the vacuum, when it exists, is shown to depend on the path of the observer through space-time (see Unruh effect). Also, the field concept is seen to be fundamental over the particle concept (which arises as a convenient way to describe localized interactions). This latter point is not uncontroversial, as it is contrary to the way quantum field theory on Minkowski space is developed by Steven Weinberg's book ''Quantum Field Theory''. Loop quantum gravity is the fruit of the effort to formulate a background-independent quantum theory. Topological quantum field theory provided an example of background-independent quantum theory, but with no local degrees of freedom, and only finitely many degrees of freedom globally. This is inadequate to describe gravity in 3+1 dimensions, which even in vacuum has local degrees of freedom according to general relativity. In 2+1 dimensions, however, gravity is a topological field theory and it has been successfully quantized in several different ways, including spin networks. There are three other points of tension between quantum mechanics and general relativity. First, general relativity predicts its own breakdown at singularities, and quantum mechanics becomes inconsistent with general relativity in a neighborhood of singularities. Second, it is not clear how to determine the gravitational field of a particle, if under the Heisenberg uncertainty principle of quantum mechanics its location and velocity cannot be known with certainty. Finally, there is a tension, but no logical contradiction, between violations of Bell's inequality in quantum mechanics, which imply superluminal influence, and the speed of light as a speed limit in relativity. The resolution of the first two points may come from a better understanding of general relativity [http://arxiv.org/abs/astro-ph/0506506]. == Theories == There are a number of proposed quantum gravity theories: *String theory *Supergravity *AdS/CFT *Loop quantum gravity of Ashtekar, Smolin and Rovelli *Euclidean quantum gravity *Noncommutative geometry of Alain Connes *Twistor of Roger Penrose *Discrete Lorentzian quantum gravity *Sakharov induced gravity *Regge calculus *Process Physics *Chaotic gravitational waves The "direct" way of quantizing gravity comes with many choices. Do we use functional integrals over Wick rotated Riemannian metrics (e.g. by Hawking)? See Euclidean path integral approach. Do we use the covariant Peierls bracket? Do we use BRST/Batalin-Vilkovisky formalism or gauge fixing or gauge factoring? If we pick canonical quantization, do we use the Einstein-Hilbert action with only the metric as dynamical to get the Wheeler-deWitt equation? Or do we treat the metric and the affine connection independently? Or do we have the whole Poincaré group as the gauge group starting with the Einstein-Cartan theory? Or do we use the Cartan method of moving frames with the Palatini action to get second class constraints? Do we eliminate the second class constraints using the Ashtekar variables to get loop quantum gravity or do we do something else? The existence of spinor fields may force us to work with the Cartan formalism or something comparable. Or maybe we should look at representations of the diffeomorphism group just as Wigner looked at representations of the Poincaré group. == Quantum gravity theorists == ''See list of quantum gravity researchers'' == See also == *Centauro event *String theory *M-theory *Semiclassical gravity *Quantum field theory in curved spacetime *Process Physics == External links == The famous spoof of postmodernism by Alan Sokal (see Sokal Affair) was entitled ''Transgressing the Boundaries: Toward a Transformative Hermeneutics of Quantum Gravity''. Protoscience Theoretical physics Quantum gravity
Quantum gravity
This sentence fragment sounds wrong to me: "The energies and conditions at which quantum gravity are likely to be important are..." I assume it should be something like "quantum gravity effects are...", but unfortunately I don't understand enough about the subject to feel confident in changing it. User:Marsvin 19:54, 2004 Jul 13 (UTC) I heve moved the section "the incompatibility between QM and GR" from LQG. User:Miguel 16:25, 25 Jul 2004 (UTC)
Quantum gravity
Theoretical physicsQuantum field theoryGeneral relativityGravity
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