Talk:Quantum gravity
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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.
Marsvin 19:54, 2004 Jul 13 (UTC)
I heve moved the section "the incompatibility between QM and GR" from LQG. 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! :-) —JRM 11:50, 2004 Sep 14 (UTC)
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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 66.84.200.34 19:44, 7 Oct 2004 (UTC) (edited stupid link typo 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:
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. 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 - 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. --Lumidek 14:45, 4 Nov 2004 (UTC)
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 - 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 [1] (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. --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. [2] (http://motls.blogspot.com/2004/10/causality-and-entanglement_16.html) --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 - 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.
--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. --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.