What's really going on in quantum gravity?


by marcus
Tags: gravity, quantum
marcus
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#1
Feb11-04, 08:21 PM
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What do you think is currently going on in quantum gravity?
Are we entering a post-String era in physics theory? Different opinions welcome. What alternative lines of research are showing promise. Here are some straws-in-the-wind----none of them particularly weighty in isolation.

For one thing research in String appears to be stagnating or slumping while that in alternative lines is picking up. I dont have numbers for newer lines like condensed matter modeling. So I'll just give them for LQG. At the arxiv.org "Search Physics Archives" page, I put in [ABS = loop quantum gravity]OR[ABS = spin foam]OR[ABS = loop quantum cosmology] since 2000. The engine found these numbers of papers:
2000                 46
2001                 48
2002                 64
2003                 70
Year-to-date(2/11)   73
These are the preprints at the archive that have somewhere in their ABSTRACTS either the words loop quantum gravity, or the words spin foam, or the words loop quantum cosmology. Year-to-date is for the 12 months up to February 11, 2004.
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Here are the same numbers for
[ABS = string]OR[ABS = brane]OR[ABS = M-theory]

2000               1457
2001               1496
2002               1500
2003               1265
Year-to-date(2/11)  911
This counts those where the abstract summary of the paper has in it somewhere the word string, or the word brane, or the word M-theory.
Year-to-date reflects activity during part of the calendar year 2003 and part of the calendar year 2004.

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I wouldnt say raw research output is particularly significant. Indeed the large number of string papers has not, in the last 3 or 4 years, produced many that turned out to be important in the sense of being highly cited. But whether it means anything or not, there are the stats. For the moment it looks like string research output has peaked.
What else is new?
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The premier physics discussion board "SPR" (sci.physics.research) has had several months of bitter wrangling about the failure of Stringy theories to make testable predictions. The basic issue raised is whether String research should be considered empirical science or a fairyland of mathematical entertainment. Apparently prior to the 1980s it was customary for each new theoretical proposition to face the empirical fire of testing by experiment within a few (like 3 or 4) years of seeing the light. String theory has not undergone this discipline and this worried the posters on "SPR". There were other criticisms as well such as the "anthropic" tendencies of Leonard Susskind and the proliferation of a googleplex of stringy theories with no way of chosing in sight (the "landscape" discussed by Tom Banks and quantified by Michael Douglas).
None of this need concern us in detail but there were on SPR many shrill cat-fights and bloodbaths and bewildering displays of dirty laundry.
Again, this is just a straw in the wind. Would not mean all that much in isolation.
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Another straw in the wind is that I see people switching out of string research and into alternatives. Several times in the past 6 months when I see a new author on the Loop scene I look back in the arxiv to see where they are coming from and they turn out to have been doing String before.

this may be true in other growing areas of quantum gravity. You may be getting people exiting from String and going into these other areas as well.

I dont know about the other areas---I know they do switch from String to Loop because I see it happening.

(that is just anecdotal evidence, dont have statistics on it)
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Another straw in the wind is the rising importance of General Relativity, and cosmology. While by contrast, Loop Gravity quantizes General Relativity, retaining its basic (BI and DI) principles and the fundamental "Gravity = Geometry" idea, quantum field theory on the other hand, and the Stringy theories which developed from it, use a fixed background space--most commonly a flat static non-expanding "minkowski" space.

We seem to be entering an era when the main concepts of physical interest arise from GR-----black holes, dark matter, dark energy, cosmological constant---come out of the curved, dynamic space of GR models. These concepts are foreign to high energy physics, and neither theoretical particle physics nor string theory cope especially well with them.

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Another indicator is the declining importance of accelerators and the rising importance of new astronomical instruments and observations (high energy cosmic rays, gammaray bursts, neutrino astronomy, CMB astronomy, determination of dark energy and dark matter parameters).
I know people involved in the Antarctic neutrino observatory and the Canadian Heavy Water neutrino observatory and I sense there excitement is comparable to the excitement felt back in the Big Accelerator glory days of like Sixties-Eighties.
It's just a feeling---another straw picked up by the wind.
But HEP or theoretical particle physics has always been closely linked to the Big Accelerators. The accelerator boom created the HEP establishment and the faculty positions which String/Brane folk have inherited. But the base has shifted.

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And then there is Thomas Thiemann posting his LQG-string paper, venturing to subsume string into Loop theory. If this checks out it will allow String to dispense with the extra dimensions and solve some other problems. Objections offered here at PF were not especially weighty or convincing. Mainly "we have to wait to see what Abhay Ashtekar and Hermann Nicolai say." And I expect there will be more such papers cropping up, but we will have to wait and see on that one.

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It is a many-faceted picture. I notice that Lubos is using a different sig (at least for now). It quotes Albert Einstein saying

"Only two things are infinite: the Universe and human stupidity. And I am not sure about the former."
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chroot
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#2
Feb11-04, 08:26 PM
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Can we stop with the popularity contests, and just discuss the theories themselves? Thanks.

- Warren


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