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Topic: Loop quantum gravity

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  Dialogos of Eide: Quantum Gravity
Quantum gravity is the field of theoretical physics attempting to unify the subjects of Quantum mechanics and General relativity.
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.
eskesthai.blogspot.com /2004/11/quantum-gravity_01.html   (1779 words)

 Wikinfo | Loop quantum gravity   (Site not responding. Last check: 2007-10-03)
Quantum field theory on curved (non-Minkowskian) backgrounds, while not a quantum theory of gravity, has shown that some of the core assumptions of quantum field theory cannot be carried over to curved spacetime, let alone to full-blown quantum gravity.
LQG inherits this geometric interpretation of gravity, and posits that a quantum theory of gravity is fundamentally a quantum theory of spacetime.
The quantization of gravity in the Ashtekar formulation was based on Wilson loops, a technique developed in the 1970's to study the strongly-interacting regime of quantum chromodynamics.
wikinfo.org /wiki.php?title=Loop_quantum_gravity   (3571 words)

Quantum mechanics is weird science; quantum gravity is both weird and mysterious.
Quantum gravity might be the link between general relativity and quantum mechanics.
Loop quantum gravity is a very attractive theory in the sense that, it very well handles the non perturbative quantum field theory.
dotworlds.free.fr /anglais/gravity.htm   (1024 words)

 Quantum Gravity (Stanford Encyclopedia of Philosophy)
Quantum Gravity: A physical theory describing the gravitational interactions of matter and energy in which matter and energy are described by quantum theory.
Loop quantum gravity is less plagued by a lack of predictions, and indeed it is often claimed that the discreteness of area and volume are concrete predictions of the theory.
Indeed, it is a major challenge of loop quantum gravity to show that it has general relativity as a low-energy limit, and a major challenge of string theory to show that it has the standard model of particle physics plus general relativity as a low-energy limit.
plato.stanford.edu /entries/quantum-gravity   (6049 words)

 Loop quantum gravity - Wikipedia, the free encyclopedia
Loop quantum gravity (LQG) is a proposed theory of spacetime which is constructed with the idea of spacetime quantization via the mathematically rigorous theory of loop quantization.
Loop quantum gravity is an effort to formulate a background-independent quantum theory.
At the core of loop quantum gravity is a framework for nonperturbative quantization of diffeomorphism-invariant gauge theories, which one might call loop quantization.
en.wikipedia.org /wiki/Loop_quantum_gravity   (2125 words)

 The Reference Frame: Objections to loop quantum gravity
In theoretical physics, loop quantum gravity is one speculative approach to quantum gravity, sometimes cited as a competitor theory to string theory.
Loop quantum gravity is not a unifying theory.
Loop quantum gravity, on the other hand, says that geometry is a good variable at all distance scales, and the areas etc. have discrete spectrum, which contradicts Lorentz invariance in any theory with local excitations.
motls.blogspot.com /2004/10/objections-to-loop-quantum-gravity.html   (5350 words)

 Quantum Foam and Loop Quantum Gravity
Loop quantum gravity has opened up a new window to investigate deep cosmological questions such as the origin of the universe.
Recent loop quantum gravity calculations indicate that the big bang is actually a big bounce; before the bounce the universe was rapidly contracting.
Loop quantum gravity is completely unperturbative and is also background-independent (geometry of spacetime is not fixed), and appears to lead to a pregeometry in which space and time are derived concepts (instead of being a pre-defined entity).
universe-review.ca /R01-07-quantumfoam.htm   (1110 words)

 Physics News Update
Loop Quantum Gravity (LQG), rival of string theory in the quest to unite quantum mechanics with general relativity, does not suffer from certain mathematical "infinities" (corresponding to ephemeral, but numerous, alternatives in the way that interactions take place in spacetime), a new study shows.
In a theory that would combine quantum and gravity features, spacetime would then have to be both deformable and fuzzy, and this has been difficult to do.
In loop theory, the merger is attempted by imagining that space itself consists of moveable tiny loops.
www.aip.org /enews/physnews/2001/split/562-1.html   (279 words)

Quantum gravity is the name we give to the theory that unifies all of physics.
Loop quantum gravity differs from other approaches to quantum gravity, such as string theory, in that apart from using Ashtekar's formalism we made no modifications to the principles of relativity and quantum theory.
Loop quantum gravity is such a one; string theory is not.
www.edge.org /3rd_culture/smolin03/smolin03_print.html   (6853 words)

 Wikinfo | Quantum gravity
Quantum mechanics depends on particle fields embedded in the flat space-time of either Newtonian mechanics or special relativity.
Gravity particles would attract each other and adding together all of the interactions results in many infinite values which can not easily be cancelled out.
This is in contrast with quantum electrodynamics where the interactions sometimes evaluate to infinite results, but those are few enough in number to be removable via renormalization.
wikinfo.org /wiki.php?title=Quantum_gravity   (431 words)

 Spin networks, spin foams and loop quantum gravity
Loop quantum gravity is a theory that results from the canonical quantization of general relativity.
Quantum gravity is one of the focus areas at the newly founded Perimeter Institute in Waterloo, Ontario, Canada, which has recruited Lee Smolin as well as several other influential researchers in quantum gravity.
Loop Quantum Cosmology, Boundary Proposals, and Inflation, 2003.
jdc.math.uwo.ca /spin-foams   (2360 words)

 Loop quantum gravity
According to Einstein, gravity is not a force - it is a property of space-time itself.
Loop quantum gravity is an attempt to develop a quantum theory of gravity based directly on EinsteinĀ“s geometrical formulation.
Part of the problem is an as yet unanswered question: This approach leads to a quantum formulation for gravity, but researchers are still busy trying to work out how, from such a quantum foundation, a universe like ours can arise, which obeys the classical (i.e.
www.einstein-online.info /en/elementary/quantum/loops/index.html   (337 words)

The turning point was the invention of not one but two approaches: loop quantum gravity and string theory.
This is important, because we're in the uncomfortable situation of having two well-developed candidates for the quantum theory of gravity.
LEE SMOLIN, a theoretical physicist, is concerned with quantum gravity, "the name we give to the theory that unifies all the physics now under construction." More specifically, he is a co-inventor of an approach called loop quantum gravity.
www.edge.org /3rd_culture/smolin03/smolin03_index.html   (628 words)

Apart from the first sentence, all this is a perfectly factual, lucid account of the state of loop quantum gravity at the turn of the third millennium.
Loop quantum gravity has a much easier time with the former than the latter, basically because it's a form of "canonical quantization" - more jargon, which means that at the very start of the day one chops spacetime into space and time.
Riemannian quantum gravity is a toy model where the gauge group is the 4d rotation group rather than the Lorentz group.
math.ucr.edu /home/baez/week177.html   (2657 words)

 Loop Quantum Gravity vs. M-Theory II.
And this problem [to derive semiclassical quantum gravity from a consistent theory] is also the physical motivation to study both string theory and LQG, I think.
One of the nice things of LQG is the quantization of areas etc. but of course it is not too surprising that operators such as J^2 (angular momentum) in a SU(2) spin-network theory of some sort have quantized eigenvalues of some sort - in fact physicists put this information in.
To include gravity consistently to a quantum theory, we were forced to go to 10 dimensions, to discover supersymmetry, to allow topology change etc. Some people criticize string theory that it predicts nothing new, some people complain the string theory involves/predicts too much new stuff (excited strings, higher dimensions, SUSY etc.).
www.lns.cornell.edu /spr/2001-06/msg0033389.html   (1536 words)

 Quantum Gravity
Loop quantum gravity has too many problems and isn't considered a serious candidate by anyone (eg, it is artificial), not even its proponents.
Loop Quantum Gravity is an honest attempt to unify Quantum Mechanics and General Relativity.
Loop Quantum Gravity is a background independent non-perturbative quantization of spacetime.
c2.com /cgi/wiki?QuantumGravity   (1461 words)

 Quantum Gravity - Cambridge University Press
Quantum gravity is perhaps the most important open problem in fundamental physics.
It is the problem of merging quantum mechanics and general relativity, the two great conceptual revolutions in the physics of the twentieth century.
Appendices include a detailed history of the subject of quantum gravity, hard-to-find mathematical material, and a discussion of some philosophical issues raised by the subject.
www.cambridge.org /uk/catalogue/catalogue.asp?isbn=0521837332   (314 words)

 Loop Quantum Gravity Forum - Advanced Physics Forums
In Loop Quantum Gravity, also referred to as LQG, the attempt is made to introduce the concept of quantum gravity.
Loop Quantum Gravity is the mathematical description of quantum gravity in terms of loops on a manifold.
These loops are quantum excitations of the Faraday-lines that live in the field and who represent the gravitational force.
www.advancedphysics.org /forum/showthread.php?t=1372   (2293 words)

 [No title]
Abstract: In quantum field theory, knowing all transition amplitudes or the S-matrix is usually considered sufficient to specify the physical content of a theory.
We are often accustomed to adopt a similar attitude when developing quantum gravity, concentrating on technical advances.
Based on the Einstein-Hilbert approximation of asymptotically safe quantum gravity we discuss a consistent renormalization group based framework which allows for the inclusion of such quantum gravitational effects into the cosmological field equations and applies to all stages of the cosmological evolution.
relativity.phys.lsu.edu /ilqgs   (650 words)

 Quantum gravity for real (February 2006) - News - PhysicsWeb
If successful, the test -- which is based on "loop quantum gravity" -- would be the first ever experimental evidence for quantum gravity.
Indeed, physicists expect that quantum gravity would modify Einstein's theory near the singularity so that it is not formed at all.
By applying the techniques of loop quantum gravity -- a leading candidate for a quantum theory of gravity -- Joshi and co-workers calculated that a dying star does not form a naked singularity but has all its mass thrown away in a flash instead.
physicsweb.org /articles/news/10/2/2/1   (539 words)

 Forums - Loop Quantum Gravity   (Site not responding. Last check: 2007-10-03)
THEN they spring quantum mechanics on us and teach us that a lot of the principles of quantum physics doesn't agree with relativity -- or that they cannot make a connection between the two.
From this magic, hugh Everett III in 1957 said that, "hey, since on the quantum scale anything is possible, perhaps every nanosecond a somewhat less than infinite -- maybe half an infinite -- number of universes is spreading out in a tree-like fashion.
Relativity and quantum mechanics are accurate to MANY significant decimal places and are highly applicable when applied appropriately.
www.elitetrader.com /vb/showthread.php?threadid=25977   (1355 words)

 The Loop Quantum Gravity Group of NUS
LQG unlike String Theory is notoriously difficult to be understood by beginners, because of various different and confusing notations used, deep and involved mathematical skills needed, very sparce novice-friendly resources and skipping of derivation steps in all review papers.
Lee Smolin's Lectures on Quantum Gravity (search for the link "Introduction to quantum gravity" on the left panel).
The Loop Quantum Gravity Group of NUS is supported by Department of Physics, National University of Singapore.
sps.nus.edu.sg /~wongjian/lqg.html   (550 words)

 Introduction To Loop Quantum Gravity
Thus he says he views lqg basically as a laboratory for investigating certain fundamental issues in quantum gravity.
Be this as it may, jim bjorken in the forward of carlo's book states quite plainly that effective field theory has taught us that GR must be viewed as just an effective field theory, and it's difficult to believe that carlo would've allowed such a statement if it fundamentally contradicted his position.
The time-like dimension is shown to appear to be unique to a moving observer, and preserves the appearance of freedom of choice as one perspective in a structure which can also be viewed from other perspectives as competely deterministic.
www.physicsforums.com /showthread.php?p=385578   (4635 words)

 Loop Quantum Gravity   (Site not responding. Last check: 2007-10-03)
Loop quantum gravity is an approach to this challenge that incorporates quantum theory into our description of spacetime from the very start.
Quantum states of the geometry of space are described by "spin networks" - graphs with certain labellings of their edges and vertices.
John Baez, An Introduction to Spin Foam Models of BF Theory and Quantum Gravity.
math.ucr.edu /home/baez/acm   (296 words)

 String Theory Acquires Rival In Loop Quantum Gravity
String Theory Acquires Rival In Loop Quantum Gravity
Loop quantum gravity (LQG), rival of string theory in the quest to unite quantum mechanics with general relativity, does not suffer from certain mathematical "infinities," a new study shows.
Carlo Rovelli (Center for Theoretical Physics, Marseilles, also University of Pittsburgh) argues that loop theory does not have to import the extra commodities (additional dimensions and particles) needed by string theory.
unisci.com /stories/20014/1031014.htm   (390 words)

 Home page of Carlo Rovelli
Here is an image of a three-dimensional model giving an intuitive picture of the "loopy" structure of space predicted by loop quantum gravity at very short scale.
The center of my research interest is in quantum gravity.
A comprehensive review article on loop quantum gravity has appeared on the electronic journal " Living Reviews in Relativity".
www.cpt.univ-mrs.fr /~rovelli/rovelli.html   (337 words)

 Notes Loop Quantum Gravity   (Site not responding. Last check: 2007-10-03)
Loop Quantum Gravity is a background independent, diffeomorphism invariant quantum gauge field theory of gravity.
[1] Lee Smolin, An invitation to Loop Quantum Gravity
[3] Ashtekar, Lewandowski, Background independent quantum gravity: a status report
mcelrath.org /Notes/LoopQuantumGravity   (104 words)

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