Talk:Quantum gravity/Archive for 2004

Sentence fragment

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)

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 external 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:

• Loop quantum gravity versus string theory

217.26.0.121 10:03, 10 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)

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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 - 24.59.196.30 16:31, 4 Nov 2004 (UTC)