Causes of loss of interest in String program

[marcus]

Hi Arivero, Aidyan, Mitchell, MTd2 and others thinking about the causes of observed string decline.
(on that, see post #340 https://www.physicsforums.com/showthread.php?p=3854410#post3854410 )

I'd been favoring the "no-fault" idea that there's nothing wrong that wasn't already known over 10 years ago and the cooling of interest could be attributed to the appearance of non-string alternative approaches to QG. But Matt Visser posted a paper yesterday that I think could represent real substantive trouble.

Visser cites stringy black hole work by people like Strominger, Horowitz,... and calls their results/conjectures into question as unphysical. In other words he finds actual physical fault, not merely failure to be predictive. I'd appreciate any comment on this. Does anyone see reasons to dismiss or minimize Visser's argument?

http://arxiv.org/abs/1204.3138
Quantization of area for event and Cauchy horizons of the Kerr-Newman black hole
Matt Visser (Victoria University of Wellington)
(Submitted on 14 Apr 2012)
Based on various string theoretic constructions, there have been repeated suggestions that the areas of black hole event horizons should be quantized in a quite specific manner, involving linear combinations of square roots of natural numbers. It is important to realize the significant physical limitations of such proposals when one attempts to extend them outside their original framework. Specifically, in their most natural and direct physical interpretations, these specific proposals for horizon areas fail for the ordinary Kerr-Newman black holes in (3+1) dimensions, essentially because the fine structure constant is not an integer. A more baroque interpretation involves asserting the fine structure constant is the square root of a rational number; but such a proposal has its own problems. Insofar as one takes (3+1) general relativity (plus the usual quantization of angular momentum and electric charge) as being paramount, the known explicitly calculable spectra of horizon areas for the physically compelling Kerr-Newman spacetimes do not resemble those of currently available string theoretic constructions.
15 pages

Here are papers Visser cites, which he appears to be shooting down:
[1] G. T. Horowitz and A. Strominger, “Counting states of near extremal black holes”, Phys. Rev. Lett. 77 (1996) 2368 [hep-th/9602051].
[2] E. Keski-Vakkuri and P. Kraus, “Microcanonical D-branes and back reaction”, Nucl. Phys. B 491 (1997) 249 [hep-th/9610045].
[3] G. T. Horowitz, J. M. Maldacena and A. Strominger, “Nonextremal black hole microstates and U duality”, Phys. Lett. B 383 (1996) 151 [hep-th/9603109].
[4] E. Halyo, B. Kol, A. Rajaraman and L. Susskind, “Counting Schwarzschild and charged black holes”, Phys. Lett. B 401 (1997) 15 [hep-th/9609075].
[5] G. T. Horowitz and J. Polchinski, “A Correspondence principle for black holes and strings”, Phys. Rev. D 55 (1997) 6189 [hep-th/9612146].
[6] F. Larsen, “A String model of black hole microstates”, Phys. Rev. D 56 (1997) 1005 [hep-th/9702153].
[7] M. Cvetic and F. Larsen, “General rotating black holes in string theory: Grey body factors and event horizons”, Phys. Rev. D 56 (1997) 4994 [hep-th/9705192].
[8] M. Cvetic and F. Larsen, “Greybody Factors and Charges in Kerr/CFT”, JHEP 0909 (2009) 088 [arXiv:0908.1136 [hep-th]].
[9] M. Cvetic, G. W. Gibbons, and C. N. Pope, “Universal Area Product Formulae for Rotating and Charged Black Holes in Four and Higher Dimensions”, Phys. Rev. Lett. 106 (2011) 121301 [arXiv:1011.0008 [hep-th]].
[10] A. Castro and M. J. Rodriguez, “Universal properties and the first law of black hole inner mechanics”, arXiv:1204.1284 [hep-th].

 

[Physics Monkey]

I believe "shooting down" is way too strong a phrase to use!

Many of the references singled out by marcus concern themselves with supersymmetric situations not encountered in the real world. Supersymmetry can affect, among other things, the running of the coupling constants which happens to be one of Visser's first criticisms.

Let me take pains to emphasize that I am not saying the Visser article is not interesting, I merely suggest that way more parsing of the works involved needs to be done before anyone has any right to claim that Visser is shooting down anyone.

For example, a simple explanation would be that Refs. 1-10 mostly consider susy models (or very nearly susy models) while Visser mostly considers very non-susy models. I'm only half joking when I suggest that "general" to a string theorist could mean "true among susy models".

 

[mitchell porter]

It looks to me like Visser hasn't asked any of the string theorists he cites what they make of his observation - and that's foolish. They could probably set him straight in five minutes. He's acting like one of those people who hope to show that the whole of particle physics is a fallacy because of some single calculation they made. I don't doubt that there are challenges to understanding the entropy of real black holes in stringy terms, but I'm sure this isn't one of them. I'm just a string-theory amateur, but if I figure out what he's overlooking (and whether he nonetheless has a valid point somewhere), I'll post about it.

 

[MTd2]

He's acting like one of those people who hope to show that the whole of particle physics is a fallacy because of some single calculation they made. I

String theory might be a fallacy, so, what s the problem? Developing string theory as a physical theory can indeed be as tricky as programming a very complicated computer game, and that s all. N

 

[mitchell porter]

The specific problem is that Visser frames his paper as a criticism of a string theory formula, but he doesn't understand string theory and didn't consult with anyone who does.

Most of his paper elaborates on non-string formulas found in references 19-22, and I think not in a very sophisticated way. Those formulas relate black hole horizon areas to spin, mass, and charge. What Visser then does is to reexpress the charge in terms of the fine structure constant, also using the fact from particle physics of charge quantization. But he just does this at the level of elementary algebraic substitution. If he's going to talk about the impact of charge quantization on black hole entropy, shouldn't he be using quantum field theory - maybe something like 't Hooft's brick-wall model from 1985?

Anyway, most of his paper is devoted to these slight elaborations of non-string formulas. But in the paper he has released, he starts out by quoting a few string formulas (or conjectures) in which horizon areas are functions of integers. His thesis here is that these formulas can't be valid for the real world, because (so he says) they would imply that the fine structure constant equals 2, not approximately 1/137. And besides, alpha runs, so it has to pass through irrational values.

So here is the most obvious gap in what he writes: he says nothing about how to interpret the significance of a varying alpha for his own new formulas, the ones which he advances as the correct alternative to the string formulas. Is the reader to assume that black hole entropy runs with the energy scale? He's just silent on this issue. If he had bothered to develop his new formulas within a proper QFT framework, he would have an answer. As things stand, apparently the reader has to figure this out for themselves.

Similarly, he doesn't even raise the question of how running coupling constants figure in string theory. In string theory, the tree-level coupling constant depends on the VEV of the dilaton. My ignorance is such that I don't know whether this is already taken into account in the string formulas for horizon area, whether it's an invisible normalization factor, or what. But again, Visser says nothing about this issue, he evinces no awareness of it. And that is precisely why I can say that he's been sloppy in relating his new formulas to string theory - he hasn't bothered to find out how the string formulas work, he's just relying on his own personal guess as to how they work.

 

[MTd2]

I am aware that you are studying string theory for quite some time. You seem to be a smart person, yet, these details, which seem to be not so subtle, still are hard to find and even to study. More over, there is no empiricism involved here.

So, that reinforces my thoughts about giving up on strings because they are too hard with little practical reward. Perhaps, other theories are also hard, but you can cut the loose buds and simplify the teaching/exposition literature, for the reason that you can do experiments.

 

[marcus]

Since we've turned a page, I'll recap the basic input we are trying to explain. There's been a decline in string interest and activity that shows in various ways. One is a downtrend in the rate of first-time faculty hires, starting around 2001. This is visible both in terms of absolute numbers (from average about 9 per year down to around 1 per year) and also in terms of string as a fraction of total Particle Theory. It used to be that around HALF the first-time faculty hires in HEP theory were in string, now it's more like a tenth.

A physicist at the U Toronto (Erich Poppitz) charts first time faculty hires in High Energy Physics Theory (Usa and Canada) by year and keeps track of what fraction of these are in string, and which fractions are in other branches of theory.
http://www.physics.utoronto.ca/~poppitz/Jobs94-08
His chart shows 11 HEP theory hires in 2011 of which one was string.
Here I've smoothed annual rates by averaging over 3 years intervals.

period                 1999-2001    2002-2004   2005-2007    2008-2010    2011
annual HEP theory hires   18             24            23             13            11 
annual string hires           9              8              6               2              1

The source used is http://particle.physics.ucdavis.edu/rumor/doku.php

There's also been a decline in annual citations to recent string research by the theorists themselves.
Number of recent string papers making the top fifty in the annual Spires HEP topcite list

year (some omitted for brev.)   2001    2003    2005    2007    2009    2010
recent work highly cited in year  12         6         2         1         1        0

Here a paper is counted as recent if it appeared in the previous five years. Citations gauge the quality/significance of current work by how much other researchers in the field currently refer to it.

It could be that the decline in String research is not primarily due to any flaw or inadequacy in stringy physics that was not evident already, say, by 2001. It could simply be that newer approaches to QG and explaining the SM have arisen, and that researchers have a natural tendency to spread out seeking fresh ideas and new areas to work on. The String program is some 40 years old and there has been plenty of time for new ideas to germinate and new research directions to develop.

On the other hand we can try to identify inherent shortcomings in the physics which might be responsible. Arivero made the point that we should consider String separately as a source of particle models (where it seems to have limited utility) and of candidate theories of the quantum geometry of the universe.
In the latter role, does it offer promising ways to resolve the cosmological singularity and model conditions leading up to the start of expansion? Are these testable by astronomical observation? Such seem among the main things one wants from a QG theory. In short, it's still debatable whether there are valid physics reasons for the decline.

 

[marcus]

The other topic that just came up (on the previous page) is this recent paper by Matt Visser.
Visser call some stringy black hole work into question as unphysical.
http://arxiv.org/abs/1204.3138
Quantization of area for event and Cauchy horizons of the Kerr-Newman black hole
Matt Visser (Victoria University of Wellington)
(Submitted on 14 Apr 2012)
Based on various string theoretic constructions, there have been repeated suggestions that the areas of black hole event horizons should be quantized in a quite specific manner, involving linear combinations of square roots of natural numbers. It is important to realize the significant physical limitations of such proposals when one attempts to extend them outside their original framework. Specifically, in their most natural and direct physical interpretations, these specific proposals for horizon areas fail for the ordinary Kerr-Newman black holes in (3+1) dimensions, essentially because the fine structure constant is not an integer. A more baroque interpretation involves asserting the fine structure constant is the square root of a rational number; but such a proposal has its own problems. Insofar as one takes (3+1) general relativity (plus the usual quantization of angular momentum and electric charge) as being paramount, the known explicitly calculable spectra of horizon areas for the physically compelling Kerr-Newman spacetimes do not resemble those of currently available string theoretic constructions.
15 pages

==quote from the first paragraph of the introduction==
Various string theoretic constructions have led to the suggestion that black hole event horizon areas might follow the quantization rule [1–10]
...
...
These specific string-inspired proposals are conjectured to have universal validity, far beyond the realm in which they were originally conjectured. It is this conjecture of universal validity which will be addressed in this current article.
==endquote==

Here are papers [1–10] which Visser begins by citing in his lead paragraph.
[1] G. T. Horowitz and A. Strominger, “Counting states of near extremal black holes”, Phys. Rev. Lett. 77 (1996) 2368 [hep-th/9602051].
[2] E. Keski-Vakkuri and P. Kraus, “Microcanonical D-branes and back reaction”, Nucl. Phys. B 491 (1997) 249 [hep-th/9610045].
[3] G. T. Horowitz, J. M. Maldacena and A. Strominger, “Nonextremal black hole microstates and U duality”, Phys. Lett. B 383 (1996) 151 [hep-th/9603109].
[4] E. Halyo, B. Kol, A. Rajaraman and L. Susskind, “Counting Schwarzschild and charged black holes”, Phys. Lett. B 401 (1997) 15 [hep-th/9609075].
[5] G. T. Horowitz and J. Polchinski, “A Correspondence principle for black holes and strings”, Phys. Rev. D 55 (1997) 6189 [hep-th/9612146].
[6] F. Larsen, “A String model of black hole microstates”, Phys. Rev. D 56 (1997) 1005 [hep-th/9702153].
[7] M. Cvetic and F. Larsen, “General rotating black holes in string theory: Grey body factors and event horizons”, Phys. Rev. D 56 (1997) 4994 [hep-th/9705192].
[8] M. Cvetic and F. Larsen, “Greybody Factors and Charges in Kerr/CFT”, JHEP 0909 (2009) 088 [arXiv:0908.1136 [hep-th]].
[9] M. Cvetic, G. W. Gibbons, and C. N. Pope, “Universal Area Product Formulae for Rotating and Charged Black Holes in Four and Higher Dimensions”, Phys. Rev. Lett. 106 (2011) 121301 [arXiv:1011.0008 [hep-th]].
[10] A. Castro and M. J. Rodriguez, “Universal properties and the first law of black hole inner mechanics”, arXiv:1204.1284 [hep-th].

It may be that what he is finding fault with, or opposes, is what he considers an unwarranted claim of generality--an overextension of some results beyond where they were originally derived. So, for example, see Alejandra Castro's http://arxiv.org/abs/1204.1284. But this overextension might not apply to some of the earlier work. As I recall Castro is one of Alex Maloney's associates (postdoc?) at McGill. Finn Larsen (see [7-8]) was her PhD advisor. References [8-10] are all to recent work, since 2009.