What's really going on in quantum gravity?

In summary, there is currently a lot of debate and discussion surrounding the field of quantum gravity. One major topic of discussion is the idea of a post-String era in physics theory, with some researchers arguing that alternative lines of research are showing more promise. This is reflected in the number of papers being produced in these different areas, with Loop Quantum Gravity (LQG) showing a steady increase while String theory has stagnated. Another concern is the lack of testable predictions in String theory, leading to debates about its place as empirical science. There also seems to be a shift in focus towards General Relativity and cosmology, with concepts like black holes and dark matter becoming more important. This shift is accompanied by a decline in the importance of
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marcus
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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 don't 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:
Code:
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]

Code:
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 wouldn't 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?
------------------------------

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.
------------------------------------

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 don't know about the other areas---I know they do switch from String to Loop because I see it happening.

(that is just anecdotal evidence, don't have statistics on it)
------------------------------------------

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.

---------------

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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Can we stop with the popularity contests, and just discuss the theories themselves? Thanks.

- Warren
 
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That is hardly an advertisement for String Theory. I'm not sure what the message is but it's not a ringing endorsement of String.

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In my opinion, the current state of quantum gravity research is a mix of stagnation and progress. On one hand, as the content mentioned, there seems to be a decline in the research output and excitement surrounding string theory. This could be due to the lack of testable predictions and the proliferation of multiple versions of the theory, leading to a sense of confusion and dissatisfaction among physicists. On the other hand, there is a growing interest in alternative lines of research, such as loop quantum gravity and condensed matter modeling. This could be a sign of a shifting focus towards more empirical and experimentally testable theories.

As for the question of whether we are entering a post-string era, I think it is too early to tell. While there is evidence of a decline in the prominence of string theory, it is still a widely studied and influential theory in physics. It has also made significant contributions to other areas of physics, such as black hole physics and holography. It is possible that string theory will continue to evolve and adapt in response to criticisms and challenges, much like how it has incorporated ideas from loop quantum gravity.

In terms of alternative lines of research showing promise, I believe that loop quantum gravity and condensed matter modeling are two promising areas. The former offers a different approach to quantizing gravity, while the latter has shown potential for connecting quantum mechanics to gravity through the study of emergent phenomena. Other areas, such as causal set theory and twistor theory, also hold promise but may require more development and experimental validation.

Overall, it is an exciting time in quantum gravity research as we continue to explore different approaches and theories. While string theory may have dominated the field for some time, it is important to keep an open mind and continue to pursue alternative lines of research in order to gain a deeper understanding of the elusive nature of gravity.
 

1. What is quantum gravity?

Quantum gravity is a theoretical framework that aims to reconcile the theories of general relativity and quantum mechanics. It attempts to explain how gravity, which is described by general relativity, can be reconciled with the principles of quantum mechanics, which govern the behavior of particles at a subatomic level.

2. Why is quantum gravity important?

Quantum gravity is important because it seeks to provide a more complete understanding of the fundamental laws of nature. It could potentially explain phenomena such as the behavior of matter at the center of a black hole and the origin of the universe. Additionally, a successful theory of quantum gravity could have practical applications, such as improving our understanding of gravity and enabling the development of advanced technologies.

3. What are the current theories of quantum gravity?

Some of the current theories of quantum gravity include string theory, loop quantum gravity, and asymptotic safety theory. These theories all attempt to reconcile general relativity with quantum mechanics in different ways, and they are all actively being researched and developed by scientists.

4. How do scientists study quantum gravity?

Scientists study quantum gravity through a combination of theoretical and experimental methods. Theoretical physicists use mathematical models and calculations to develop theories and make predictions about the behavior of quantum gravity. Experimental physicists conduct experiments and analyze data to test these theories and provide evidence for their validity.

5. What are the challenges in understanding quantum gravity?

One of the main challenges in understanding quantum gravity is that it requires the development of a theory that can unify two seemingly incompatible theories – general relativity and quantum mechanics. Additionally, the extreme conditions where gravity is strongest, such as inside a black hole, are difficult to study and require advanced technologies and techniques. There is also a lack of experimental evidence for quantum gravity, making it a highly speculative field of study.

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