Quantum gravity Definition and 470 Threads

Hi, I'm a non-expert on LQG, and have a question about it: What is the status of the correspondence principle applied to LQG? I.e., in which sense can we take the classical (h --> 0) or non-relativistic (c --> oo) limit in order to obtain classical GR or Newtonian gravity out of LQG...

Hey all! So, in my study of general relativity, I've come to understand that gravity is actually what physicists would have classically called a "fictitious force", in that it is a force derived from the fact that the observer is not in an inertial reference frame, like in the case of the...

I am a UG student and want to ask about the current state of quantum gravity research. Is quantum gravity a promising field in physics research right now, as in, is it making much progress in its development? Also, on a related note, what are the mathematics required in this field? Is it...

Hello. I would like to bring to discussion this paper called "Mathematical Foundations of the Relativistic Theory of Quantum Gravity", by Professor Fran de Aquino, from the Maranhao State University, in Brazil. The paper was published in the Pacific Journal of Science and Technology...

Just a crazy thought, does a massive body actually reduce baseline quantum fluctuations nearby (compared with the outer space which has much more baseline fluctuations), thus mimicking Casimir effect on a much larger scale and draws things towards a region having a reduced quantum fluctuations...

I was recently reading a small news article named Experiments Show Gravity Is Not an Emergent Phenomenon. http://www.technologyreview.com/blog/arxiv/27102/?ref=rss. It goes on about gravity not being a traditional force but a emergent phenomenon. But the interseting thing is I've gone...

Do theories of quantum gravity require that space-time is a lattice instead of a continuum? I guess this question has been addressed elsewhere, but I would appreciate hearing different points of view. Please dummy down the responses so a philosopher can understand it.

Einstein's General Realitivity acurately computed the precession of the perihelion of the Mercury, Schwinger's and Feynman's QED was able to calculate the Lamp shift, Bohr's quantum mechanics gave precise explanations of the hydrogen atom. Question: what experimental phenomenon is there that we...

The proportionality constant of big G (like Planck's constant) seems to suggest that gravity is a quantized 'thing'... does anyone know of any experiments (perhaps with a Watt or torsion balance) that show evidence of this?

Sorry, I keep having naive little ideas lately! Humour me anyone if you so wish.. If you took the estimated mass of luminous/energetic bodies estimated in the observable universe (galaxies & black holes etc), disregard dark matter/energy, and make that each galaxy/observed mass can for...

Our major candidates for quantum gravity are as you know String Theory and Loop Quantum Gravity. Loop Quantum Gravity is about the spacetime curvature as a priori. String Theory is about the Flat Spacetime as a priori (although I'm not sure if the 6 compactified dimensions are considered...

How do you understand about Gravitons in Loop Quantum Gravity? All I know about it is from what I heard that "LQG hopes that its predictions for experiments occurring far below the Planck scale will be almost identical to that of gravitons on flat spacetime". So do you consider it a pure...

Hello all, I was wondering if Hawking's approach is still relevant. Found a book on his compilation of papers an amazon and had heard a talk by him suggesting it as a view to continue research. With all the hoo ha on M-theory and etc, would it be possible to buy this collection of papers for...

Can someone give me a rundown on current quantum gravity research namely: Loop Quantum gravity Causal Dynamical Triangulation Group Field Theory I had my attention on them in the past but I lost track of any progress that has been made (the last time I checked I think Rovelli wrote a paper...

Isham wrote: http://arxiv.org/PS_cache/gr-qc/pdf/9310/9310031v1.pdf When we come to a Diff(M)-invariant theory like classical general relativity the role of time is very different. If M is equipped with a Lorentzian metric g, and if its topology is appropriate, it can be foliated in many ways...

Recently, I am very interested in Loop Quantum Gravity. But I hope I can know more about the recent development of Loop Quantum Gravity. I mean the development from 2000 to 2011. Any conceptual or practical or technical development in this realm? Further more, I do not know the relationship...

I would like to continue discussing SF (i.e. PI) models of LQG based on chapter 3 from http://arxiv.org/abs/1009.4475 Critical Overview of Loops and Foams Authors: Sergei Alexandrov, Philippe Roche (Submitted on 22 Sep 2010) Abstract: This is a review of the present status of loop and...

I would like to continue discussing canonical LQG based on chapter 2 from http://arxiv.org/abs/1009.4475 Critical Overview of Loops and Foams Authors: Sergei Alexandrov, Philippe Roche (Submitted on 22 Sep 2010) Abstract: This is a review of the present status of loop and spin foam...

There are still unsettled questions in loop quantum gravity, especially regarding uniqueness of the Hamiltonian constraint, constraint algebra, on-shell vs. off-shell closure, operator norm and convergence, ultra-locality, possibly quantization anomalies. These questions have been asked in...

Can loop quantum gravity tell us something about the recent phenomena of the neutrinos?

http://arxiv.org/abs/1109.1290 [B]Linking covariant and canonical LQG: new solutions to the Euclidean Scalar Constraint[/B Authors: Emanuele Alesci, Thomas Thiemann, Antonia Zipfel (Submitted on 6 Sep 2011) Abstract: It is often emphasized that spin-foam models could realize a projection on...

Is there any good arguments or reasons for why it seems the general consensus is that the linear Schroedinger equations is fundamental? I know that if it turns out either of these are nonlinear it would falsify certain readings of QM so it would be interesting if there is indeed no real good...

Is anybody here familiar with Asselmeyer's work on exotic smoothness and quantum gravity?

If spacetime is composed of tiny quantum "grains," the gamma-ray photons' polarization should change from random polarization (at the GRB source) to biased toward a certain polarization when received by the Integral spacecraft . The Integral polarization results depend on spacetime being...

Could however the quantum gravity works itself be the solution to the measurement problem? I'm not talking about the whole theoretic approach to QG, but just the weird quantum effects being directly caused by the quantum gravity?

Hi everybody, before I begin this exercise for myself I want to make sure I have a few things right. Would the Lagrangian for low energy quantum gravity be \mathcal{L}=\frac{1}{2}\partial^\mu \bar{\psi}\partial_\mu \psi...

I'm a novice in Loop Quantum Gravity, so please bear with me if this is a trivial question. As far as I know, LQG is usually formulated non-perturbatively, but is there also a perturbative approach in the sense that we use a series expansion in order to calculate vacuum expectation values? Are...

A mass in 3d space yet, sorry if this so pleb to be ignored.

http://arxiv.org/abs/1105.6359 Is string theory a theory of quantum gravity? Steven B. Giddings (Submitted on 31 May 2011) Some problems in finding a complete quantum theory incorporating gravity are discussed. One is that of giving a consistent unitary description of high-energy scattering...

Does anybody else besides Fra believe that quantum mechanics' Measurement Problem, Quantum Gravity, and Unification Program of all Forces are somehow related? Why and why not (pls. state or elaborate why you think or don't think so)?

General relativity is all about acceleration (equivalence principle). Quantum theory never mentions acceleration. New object speeds and directions only arise from discrete interaction events. Is this the fundamental reason why the two theories are so hard to reconcile?

In this new paper today http://arxiv.org/abs/1105.4184, is it considered whether quantum gravity actually has fermionic fields rather than just bosonic fields: Abstract It is generally assumed that the gravitational field is bosonic. Here we show that a simple propagating torsional theory...

I don't quite understand the properties of loop quantum gravity. I have searched around and have not come up with anything very helpful. I'm pretty good when it comes to understanding quantum mechanics and string theory, but please, NO MATH! I'm only in high school! thanks

What theories of quantum gravity are currently UV complete?

In the thread https://www.physicsforums.com/showthread.php?t=485800&page=17 posz #257 fzero mentions that all known approaches to use gauge theory to describe canonical gravity fail nonperturbatively (Witten in http://arxiv.org/abs/0706.3359 "Three-Dimensional Gravity Revisited"): - gauge...

Is pure quantum gravity known to exist? I had thought it exists in 3D, but Strominger writes http://arxiv.org/abs/0906.1313 "Determining Z for pure 3D quantum Einstein gravity - if it exists - is an important open problem" Eg. Does the Turaev-Viro model not describe 3D QG?

I'd always assumed it was, since LQG spin foam models are based on BF theory. And it is typically said that in 3D it is BF, and in 4D constrained BF. The 4D quantum case is not known to be gravity, but I had thought the 3D case was. Yet in http://math.ucr.edu/home/baez/week254.html Baez...

It is generally belived that there exists fundamental length scale given by the Planck length at which GR and in particular local Lorentz invariance breaks. But accoring to me we can define a resonable length scale only if we assume beforehand that Lorentz invariance is broken. Otherwise how...

Hi everyone, I will soon finish my B.Sc. in mathematics & physics, and I'm currently reviewing options for PhD. One of the subjects that interest me the most is LQG, but most places I've checked (MIT, Caltech, Harvard etc.) don't seem to acknowledge its existence... Please, recommend places...

Meaning of "low energy" in loop quantum gravity In the recent Immirzi parameter thread we accumulated some evidence that to resolve and/or clarify the issues raised there, it would be necessary to have an understanding of the flow to low energy or the flow from micro to macro in loopy models of...

Is the need for Quantum Gravity only related to understand what goes on in the Planck Scale between quantum fields and curvature of spacetime where they collide. Or is it a more general solution to how quantum object is connected to spacetime (or quantum spacetime)? Let's take the example of a...

Given that in GR there's no gauge invariant choice of "equal-time slices", how is unitarity formulated in quantum gravity? I guess the problem may be absent if spacetime is asymptotically flat. But what happens in other cases? In AdS/CFT, the notion of unitarity comes from the CFT side. Is this...

Which of the following is the most promising road to quantum gravity? 1. quantising General Relativity 2. quantising a different classical theory, while still having general relativity emerge as a low-energy (large-distance) limit. 3. having general relativity emerge as a...

I have read that MWI predicts that gravity is quantised. Is this prediction unique to MWI? If for example, someone discovered a quantum theory of gravity that was correct, that MWI would be declared the only valid interpretation of QM and that interpretations such the Copenhagen interpretation...

http://arxiv.org/abs/1102.0270 "The first hazard is well known in Euclidean quantum gravity. It is called “minbus” or “baby universe” [6]. ... The approach is called “causal dynamical triangulation” (CDT) and has been shown numerically to provide “birth control” [9]"

http://arxiv.org/abs/1101.5113 Foundations of a theory of quantum gravity Johan Noldus (Submitted on 26 Jan 2011) After a long technical and consequently philosophical disgression about the necessity of the construction presented in this book, a logically consistent and precise theory of...

What is k-Minkowski spacetime, and... In http://www.physorg.com/news157203574.html, k-Minkowski spacetime, which decreases down towards zero at small scales, is mentioned as a possible tool in a theory of quantum gravity and spacetime. But it is very, very vague. First, how is k-Minkowski...

What Does "Quantum Gravity Scattering Amplitudes" Mean? I've seen it referenced in a few papers but I can't seem to find a definition. I know what "scattering amplitudes" means but don't understand the relationship with quantum gravity. What precisely is doing the scattering in quantum gravity?

What is the Precise Meaning of "Canonical" in Quantum Gravity Context I keep seeing "canonical" in the context of the expression "canonical quantum gravity" and it is clear the meaning has a precise defintion. But I haven't seen it articulated anywhere and a search doesn't bring up anything...

In another forum that I frequent, I have been having a discussion where the state of quantum gravity research came up. Another poster claimed that one of the first thing that any gravity scientist checks for the theory is the theory's prediction for the precession of Mercury's perihelion. Now...