## Monday, November 01, 2004

### Quantum Gravity

I've put together links for reference on the particular subject titled. If someone has others that they would like to add, please do. I will be placing a permanent link on the sidebar for reference. Hope its useful.

# List of loop quantum gravity researchers

## Quantum Gravity quote

A pessimist might say that combining string theory and loop quantum gravity is like combining epicycles and aether.
(John Baez, TWF281)

What is Quantum Gravity?

Moderator: Stephen Shenker, Panelists: Abhay Ashtekar, Juan Maldacena, Leonard Susskind, Gerard 't Hooft, Cumrun Vafa

• Jan Ambjørn

• Kostas  Anagnostopoulos

• John Baez

• Julian Barbour

• Chris  Isham

• Ted  Jacobson

• Renate  Loll

• Fotini Markopoulou Kalamara at Penn
State University
and Albert
Einstein Institute

• Carlo  Rovelli

• CGPG: Center  for Gravitational Physics and Geometry

• High-Energy Theory  Group at The Niels Bohr Institute

• Imperial College  Theoretical Physics Group

• The Max Planck Institute for Gravitational Physics (Albert  Einstein Institute)

• Penn State Physics Department

• Perimeter Institute for Theoretical Physics

• String theory

• Loop quantum gravity of Smolin and Rovelli

• Noncommutative geometry of Alain Connes

• Twistor theory of Roger Penrose

• Abhay Ashtekar -- Author of Ashtekar variables, he is one of the founders of loop quantum gravity.

• John Baez -- Mathematical physicist.

• Julian Barbour -- Author of The End of Time, Absolute or Relative Motion? and The Discovery of Dynamics.

• Martin Bojowald --

• Louis Crane -- Theorist.

• Rodolfo Gambini -- Author of Loops, Knots, Gauge Theories and Quantum Gravity.

• Brian Greene -- Physicist who is considered one of the world's foremost string theorists.

• Stephen Hawking -- Leading theoretical physicists.

• Peter Higgs -- Proposed the 1960's theory of broken symmetry in electroweak theory,

• Christopher Isham -- Theoretical physicist.

• Michio Kaku -- Theoretical physicist with significant contribution to the string field theory.

• Fotini Markopoulou-Kalamara -- Theoretical physicist interested in foundational mathematics and quantum mechanics

• Roger Penrose -- Mathematical physicist and imade the invention of spin networks.

• Jorge Pullin -- Theoretical physicist.

• Carlo Rovelli -- Obtained, with Lee Smolin, an explicit basis of states of quantum geometry.

• Lee Smolin -- Theoretical physicist who has made major contributions to loop quantum gravity.

• Andrew Strominger -- Theoretical physicist who works on string theory

• Thomas Thiemann -- Researcher.

• Edward Witten -- Mathematical physicist who does research in M-theory

• Centauro event

• String theory

• M-theory

• Loop Quantum Gravity by Carlo Rovelli

• http://gravity.psu.edu/online/Html/Seminars/Fall1999/Amelino-Camelia/Slides/s01.html

• http://www.theory.caltech.edu/people/patricia/test/Einstein3.htmlSolving Relativity in Three Dimensions.

• http://www.tsolkas.gr/english/document1/document1.html

• http://www.pitt.edu/~wbcurry/index.html

• http://www.parcellular.fsnet.co.uk

• http://www.unifiedftheory.com

• http://www.damtp.cam.ac.uk/user/gr/public/qg_home.html

• http://www.maths.nott.ac.uk/personal/jwb/qg.html

• http://www.npac.syr.edu/users/paulc/physics/surfaces.html

• http://www.npac.syr.edu/PROJECTS/PUB/paulc/physics/surfaces.html

• http://www.physics.nyu.edu/faculty/sokal/transgress_v2/transgress_v2.html

• http://graham.main.nc.us/~bhammel/FCCR/qg.html

• http://zebu.uoregon.edu/~js/glossary/quantum_gravity.html

• http://penlee.inje.ac.kr/research/black.htm Quantum Gravity and Black Hole Related Published Articles...

• http://wwwgro.unh.edu/compton5/submitted_abstracts/A005_Kaaret.html

• http://www.zebu.uoregon.edu/~js/glossary/quantum_gravity.html

• http://www.fervor.demon.co.uk/qgravity.htm

• http://newton.skku.ac.kr/research-interest/k-research-interest.html

• http://www.nd.edu/~chemlib/journals/cqg.htm

• http://www.maths.nott.ac.uk/personal/jwb/

• http://www.damtp.cam.ac.uk/user/gr/public/bh_hawk.html

• http://suhep.phy.syr.edu/research/randomsurfaces/

• http://www.phys.psu.edu/~beetle/Phys/tqft.html

• http://www.maths.nott.ac.uk/personal/jwb/pg.html

• http://aeneas.ps.uci.edu/aeneas/benchqgmpi.html

• http://www.lib.msu.edu/dbases/item/000362.html

• http://www.deepspace.ucsb.edu/research/grqg.html

• http://www.pitt.edu/~wbcurry/qg.html

• http://physics.syr.edu/research/relativity/rel-fac.html

• http://www.maths.nottingham.ac.uk/personal/jwb/qg.4d.html

• http://www.ps.uci.edu/physics/hamber.html

• http://t8web.lanl.gov/people/emil/Slides/mottola.html

• http://www.kps.or.kr/~kpsjnl/jp/jp1997/jp30-2/jp349/jp349.html

• http://www.physics.ucsb.edu/Research/activities/grqg.phtml

• http://cs-server.aston.ac.uk/~barnesa/publications.html

• http://www.comp.glam.ac.uk/pages/staff/efurse/Maths-is-Scruffy/Penrose-Maths-and-AI/Need-quantum-gravity.html

• http://hepweb.rl.ac.uk/ppUK/_vti_bin/shtml.dll/PressReleases/pr_whipple.html

• http://www.amtp.cam.ac.uk/user/gr/Report96-html/Report96-html.html

• http://digerati.edge.org/3rd_culture/bios/smolin.html

• http://www.phy.syr.edu/research/randomsurfaces/tour.html

• http://www.cs.aston.ac.uk/~barnesa/publications.html

• http://www.physics.ohio-state.edu/~anton/html/Abstract/clifford.html

• http://www.livingreviews.org/Articles/Volume1/1998-1rovelli/contents.html

• http://www.livingreviews.org/Articles/Volume1/1998-1rovelli/node29.html

• http://www.livingreviews.org/Articles/Volume1/1998-1rovelli/node1.html

• http://phy.syr.edu/research/relativity/rel-fac.html

• http://www.livingreviews.org/Articles/Volume1/1998-1rovelli/node8.html

• http://www.livingreviews.org/Articles/Volume1/1998-1rovelli/node7.html

• http://www.damtp.cam.ac.uk/user/gr/public/bh_home.html

• http://www.livingreviews.org/Articles/Volume1/1998-1rovelli/node26.html

• http://www.livingreviews.org/Articles/Volume1/1998-1rovelli/node23.html

• http://www.dallas.net/~matzke/physcomp/physicstoday.html

• http://ds.dial.pipex.com/index/Science-Computing-and-Technology/Science-Computing-and-Technology-Physics-1.html

• http://newton.skku.ac.kr/research-interest/research-interest.html

• http://www.ma.hw.ac.uk/~des/HCM.html

• http://www.workpage.com/g/16/006g.htm

• http://www.edge.org/3rd_culture/bios/smolin.html

• http://qso.lanl.gov/~carmen/noghost.html

• http://rattler.cameron.edu/EMIS/journals/LRG/Articles/pubNoVol1.html

• http://artemis.phyast.pitt.edu/thesis/index.html

• http://www.weburbia.demon.co.uk/pg/contents.htm

• http://www.npac.syr.edu/techreports/html/0750/abs-0761.html

• http://www.npac.syr.edu/techreports/html/0350/abs-0361.html

• http://www.pitt.edu/~wbcurry/

• http://hepwww.ph.qmw.ac.uk/epp/lectures/lecture27/lecture27.html

• http://www.phy.syr.edu/research/relativity/symposium.html

• http://suhep.phy.syr.edu/research/relativity/symposium.html

• http://www.livingreviews.org/Articles/Volume1/1998-1rovelli/node24.html

• http://www.bbk.ac.uk/Departments/Physics/tpru.html

• http://www.livingreviews.org/Articles/pubNoVol1.html

• http://euler.math.hkbu.edu.hk/conference/genrel.html

• http://www.physics.nus.edu.sg/~phyteoe/research/strings.html

• http://www.cordis.lu/tmr/src/grants/fmbi/950428.htm

• http://t8web.lanl.gov/people/emil/sgrav.html

• http://physics.syr.edu/research/randomsurfaces/lat/node1.html

• http://www.ift.uni.wroc.pl/~poff/symposia/kar99.html

• http://www.physics.brandeis.edu/cosmo.htm

• http://www.harc.edu/96_annualreport/page09.html

• http://phy.syr.edu/research/relativity/symposium.html

• http://physics.syr.edu/research/randomsurfaces/lat/lat.html

• http://www.nr.no/~soleng/papers/strings.html

• http://www.ams.org/new-in-math/sciencewars.html

• http://digerati.edge.org/3rd_culture/bios/davies.html

• http://www.livingreviews.org/Articles/Volume1/1998-1rovelli/node13.html

• http://www.livingreviews.org/Articles/Volume1/1998-13loll/node52.html

• http://www.livingreviews.org/Articles/Volume1/1998-13loll/node17.html

• http://www.livingreviews.org/Articles/Volume1/1998-1rovelli/node5.html 2.3

• http://www.cultsock.ndirect.co.uk/MUHome/cshtml/general/sokal.html

• http://physics.syr.edu/research/randomsurfaces/

• http://www.innerx.net/personal/tsmith/cnfGrHg.html

• http://www.karlin.mff.cuni.cz/asc/knihovna/publik95/publ29.htm

• http://www.nonlocal.com/quantum-d/posts/srh_9-5-96.html

• http://www.ap.univie.ac.at/users/fe/qg/

• http://www-itp.unibe.ch/~whelan/thesis.html

• What is Quantum Gravity

Finally, string theory started out as a generalization of quantum field theory where instead of point particles, string-like objects propagate in a fixed spacetime background. Although string theory had its origins in the study of quark confinement and not of quantum gravity, it was soon discovered that the string spectrum contains the graviton, and that "condensation" of certain vibration modes of strings is equivalent to a modification of the original background.

LQG does not have this feature to describe point particles, where a one dimensional string includes gravity.

According to Wikipedia:

1.loop quantum gravity makes too many assumptions
2. according to the logic of the renormalization group, the Einstein-Hilbert action is just an effective description at long distances

3. loop quantum gravity is not a predictive theory

4. loop quantum gravity has not offered any non-trivial self-consistency checks

5. loop quantum gravity is isolated from particle physics

6. loop quantum gravity does not guarantee that smooth space as we know it will emerge as the correct approximation of the theory at long distances

7. loop quantum gravity violates the rules of special relativity

8. the discrete area spectrum is not a consequence, but an assumption of loop quantum gravity

9. the discrete area spectrum is not testable

10. loop quantum gravity provides us with no tools to calculate the S-matrix

11. loop quantum gravity does not really solve any UV problems

12. loop quantum gravity is not able to calculate the black hole entropy, unlike string theory

13. loop quantum gravity has no tools to answer other important questions of quantum gravity

14. the criticisms of loop quantum gravity regarding other fields of physics are completely misguided

15.loop quantum gravity calls for "background independence" are misguided

16.loop quantum gravity is not science

The numbered points are connected to deeper explanations.

Criticisms of string theory can follow in someone else's post. With the group in favor of LQG they should be able together their heads and come up with lots of things

Current theories of gravity are based on the geometric curvature of space.

Current theories of other fundamental forces in the universe are 'quantum field theories', where particles pass other particles back and forth among themselves to interact.
We know that geometric gravity theories conflict with quantum field theories, and that this conflict means that we don't know what happens under extreme conditions.

A quantum theory of gravity would involve particles passing 'gravitons' back and forth among themselves. This quantum theory would probably be a more accurate description of gravity, and might be accurate enough to describe the extreme conditions found at the center of a black hole.

David Palmer

Quantum Gravity

Quantum gravity is the field of theoretical physics attempting to unify the subjects of Quantum mechanics and General relativity.

Much of the difficulty in merging these theories comes from the radically different assumptions that these theories have on how the universe works. Quantum mechanics depends on particle fields embedded in the flat space-time of either Newtonian mechanics or special relativity. Einstein's theory of general relativity models gravity as a curvature within space-time that changes as mass moves. The most obvious ways of combining the two (such as treating gravity as simply another particle field) run quickly into what is known as the renormalization problem. Gravity particles would attract each other and if you add together all of the interactions you end up with many infinite results which can not easily be cancelled out. This is in contrast with quantum electrodynamics where the interactions do result in some infinite results, but those are few enough in number to be removable via renormalization.

Another difficulty comes from the success of both quantum mechanics and general relativity. Both have been highly successful and there are no
known phenomenon that contradict the two. The energies and conditions at which quantum gravity are likely to be important are inaccessible to laboratory experiments. The result of this is that there are no experimental
observations which would provide any hints as to how to combine the two.

The general approach taken in deriving a theory of quantum gravity is to
assume that the underlying theory will be simple and elegant and then to
look at current theories for symmetries and hints for how to combine them
elegantly into a overarching theory. One problem with this approach is
that it is not known if quantum gravity will be a simple and elegant theory.

Such a theory is required in order to understand those problems involving the combination of very large mass or energy and very small dimensions of space, such as the behaviour of black holes, and the origin of the universe.

There are a number of proposed quantum gravity theories and proto-theories, including (for example) string theory and the loop quantum gravity of Smolin and Rovelli - see http://www.livingreviews.org/Articles/Volume1/1998-1rovelli/
The Noncommutative geometry of Alain Connes, and Twistor theory, of Roger Penrose, are also theories of quantum gravity

The Quantum Gravity Concept Map is a highly experimental work: it's goals are to help the author organize his own understanding of the subject, and to test the hypothesis that html is a natural language for the construction of a concept map.

Quantum gravity is the field of theoretical physics attempting to unify the theory of quantum mechanics, which describes three of the fundamental forces of nature, with general relativity, the theory of the fourth fundamental force: gravity. The ultimate goal is a unified framework for all fundamental forces—a theory of everything

A history of the Planck values provides interesting material for reflections on timely and premature discoveries in the history of science. Today, the Planck values are more a part of physics itself than of its history. They are mentioned in connection with the cosmology of the early universe as well as in connection with particle physics. In considering certain problems associated with a unified theory (including the question of the stability of the proton), theorists discovered a characteristic mass ~ 1016mp (mpis the proton mass). To ground such a great value, one first refers to the still greater mass 1019mp. In the words of Steven Weinberg:

This is known as the Planck mass, after Max Planck, who noted in 1900 that some such mass would appear naturally in any attempt to combine his quantum theory with the theory of gravitation. The Planck mass is roughly the energy at which the gravitational force between particles becomes stronger than the electroweak or the strong forces. In order to avoid an inconsistency between quantum mechanics and general relativity, some new features must enter physics at some energy at or below 1019 proton masses. (Weinberg 1981, p. 71).

The fact that Weinberg takes such liberties with history in this quotation is evidence of the need to describe the real historical circumstances in which the Planck mass arose. As we saw, when Planck introduced the mass (ch/G)1/2 (~ 1019mp) in 1899, he did not intend to combine the theory of gravitation with quantum theory; he did not even suppose that his new constant would result in a new physical theory. The first "attempt to combine the quantum theory with the theory of gravitation," which demonstrated that "in order to avoid an inconsistency between quantum mechanics and general relativity, some new features must enter physics," was made by Bronstein in 1935. That the Planck mass may be regarded as a quantum-gravitational scale was pointed out explicitly by Klein and Wheeler twenty years later. At the same time, Landau also noted that the Planck energy (mass) corresponds to an equality of gravitational and electromagnetic interactions.

Theoretical physicists are now confident that the role of the Planck values in quantum gravity, cosmology, and elementary particle theory will emerge from a unified theory of all fundamental interactions and that the Planck scales characterize the region in which the intensities of all fundamental interactions become comparable. If these expectations come true, the present report might become useful as the historical introduction for the book that it is currently impossible to write, The Small-Scale Structure of Space-Time.

The struggle to free ourselves from background structures began long before Einstein developed general relativity, and is still not complete. The conflict between [B]Ptolemaic and Copernican cosmologies[/B], the dispute between Newton and Leibniz concerning absolute and relative motion, and the modern arguments concerning the problem of time' in quantum gravity -- all are but chapters in the story of this struggle. I do not have room to sketch this story here, nor even to make more precise the all-important notion of geometrical structure'. I can only point the reader towards the literature, starting perhaps with the books by Barbour [9] and Earman [15], various papers by Rovelli [25,26,27], and the many references therein.

String theory has not gone far in this direction. This theory is usually formulated with the help of a metric on spacetime, which is treated as a background structure rather than a local degree of freedom like the rest. Most string theorists recognize that this is an unsatisfactory situation, and by now many are struggling towards a background-free formulation of the theory. However, in the words of two experts [18], it seems that a still more radical departure from conventional ideas about space and time may be required in order to arrive at a truly background independent formulation.