A good place to begin might be with Enrique Alvarez recent survey "Loops versus Strings" given in July to an audience of HEP people at the Portoroz conference "What comes beyond the standard model?"(adsbygoogle = window.adsbygoogle || []).push({});

http://arxiv.org/gr-qc/0307090 [Broken]

Alvarez is a recognized string theorist who has published some 16 articles since 1997 IIRC. He is at U. Madrid, very likely was the thesis advisor for someone name Marchesano now at Madison doing string postdoc. Dont know but a likely guess. Lethe knows Marchesano. Anyway Alvarez was asked to give a survey on Loops versus Strings and his viewpoint is not that of a relativist (a GR expert) but of a string-brane person.

But he is not necessarily propagandizing, as string-folk sometimes do when they feel threatened and are talking to outsiders. He seems to me to be being fair and objective according to his own lights. This is July 2003.

Page 10:

"4.2 Big Results [of string theory]

Perhaps the main result is that graviton physics in flat space is well defined for the first time, and this is no minor accomplishment...

The other Big Result[ref to Strominger/Vafa] is that one can correctly count states of extremal black holes as a function of charges. This is at the same time astonishing and disappointing. It clearly depends strongly on the objects being BPS states (that is, on supersymmetry), and the result has not been extended to non-supersymmetric configurations. On the other hand, as we have said, itexactlyreproduces the entropy as a function of a sometimes large number of charges, without any adjustable parameter..."

My reaction is that dependence on supersymmetry, which is far from being established, is disconcerting as is the restriction to extremal (and, I understand, near-extremal) holes. These are exotic objects which unlike the black holes one sees evidence of in nature are electrically charged to the max. An extremal hole is as electrically charged as it can possibly be and continue to exist.

It would be more reassuring if there were a stringy result for Schwartzschild holes----the ordinary vanilla electrically neutral hole we are used to thinking about.

However the Loop result which was gotten the same year as the

Strominger/Vafa one (1996) and applies to Schwarzschild holes, has an undetermined parameter called the Immirzi parameter!!!

So the Loop derivation of the same entropy formula is also unsatisfactory. This fly in the ointment has, in turn, generated further theoretical investigation (work by Corichi, Swain etc) and it may be that some new insights will come out of it.

But the most dubious thing about the String-Brane version of the entropy formula is something Alvarez did not even elude to. However another String Theorist, Gary Horowitz of UCSB, did, at the Ninth Marcel Grossman Meeting at Rome July 2000, in his talk

"Quantum Gravity at the Turn of the Millennium".

http://arxiv.org/gr-qc/0011089 [Broken]

[If you catch a string theorist being honest and talking to his peers, he is apt to say similar things to Lee Smolin---but some people choose to discount Smolin as biased! Smolin's words on this particular matter strike me as, if anything, more indulgent and congratulatory to string than those of Horowitz the insider, speaking on the level]

Horowitz on page 12:

"Both string theory and quantum geometry [by this he means LQG: the attempt to quantize spacetime geometry, i.e. GR] have given strong evidence that...They can reproduce the entropy of black holes by counting quantum states. But they do so in very different ways. Quantum geometry is directly counting fluctuations of the event horizon, while string theory extrapolates the black hole to weak coupling and counts states of strings (and branes) in flat spacetime. At the moment, the string calculations give exact results...only for extreme and near extreme charged black holes..."

well, I need to go, but will get back to this later on

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# Black Hole Entropy in Quantum Gravity

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