Proving Quantum Gravity: Observable or Formula?

[wolram]

How would one prove quantum gravity, would there be some thing observable or would it just be a mathematical formula.

 

[rootone]

I guess either is possible, but a lot of people would be unhappy with just a mathematical proof, even a very convincing one.
Even a rock solid mathematical solution would be thought suspect by some people unless it predicts something observable.
String theorists lament the fact that their beautiful mathematical description of the universe at its most fundamental cannot be tested with present technology, and maybe that will be the case forever.

 

[wolram]

Surly rootone a theory is not a theory it is only conjecture if there is nothing tangible to it.

 

[rootone]

Yes, the best you can get with only mathematics is a highly plausible hypothesis that doesn't have any sensible known alternative.

 

[wolram]

I for one would be unhappy with just a mathematical proof, i would need some thing tangible, some thing observable with space probes.

 

[rootone]

I for one would be unhappy with just a mathematical proof, i would need some thing tangible, some thing observable with space probes.

... or a souped up LHC, or possibly some kind of microscope with fantastic resolution based on a presently unknown technology.

 

[marcus]

http://arxiv.org/abs/1309.6896
Observational issues in loop quantum cosmology
A. Barrau, T. Cailleteau, J. Grain, J. Mielczarek
(Submitted on 26 Sep 2013 (v1), last revised 8 Jan 2014 (this version, v2))
Quantum gravity is sometimes considered as a kind of metaphysical speculation. In this review, we show that, although still extremely difficult to reach, observational signatures can in fact be expected. The early universe is an invaluable laboratory to probe "Planck scale physics". Focusing on Loop Quantum Gravity as one of the best candidate for a non-perturbative and background-independent quantization of gravity, we detail some expected features.
75 pages, invited topical review for Classical and Quantum Gravity

http://arxiv.org/abs/1410.1714
Loop quantum gravity and observations
A. Barrau, J. Grain
(Submitted on 7 Oct 2014 (v1), last revised 28 Oct 2015 (this version, v2))
Quantum gravity has long been thought to be completely decoupled from experiments or observations. Although it is true that smoking guns are still missing, there are now serious hopes that quantum gravity phenomena might be tested. We review here some possible ways to observe loop quantum gravity effects either in the framework of cosmology or in astroparticle physics.
To be published as a chapter of the book "100 Years of General Relativity" edited by A. Ashtekar and J. Pullin.

http://arxiv.org/abs/1504.07559
Loop quantum cosmology: From pre-inflationary dynamics to observations
Abhay Ashtekar, Aurelien Barrau
(Submitted on 28 Apr 2015 (v1), last revised 30 Sep 2015 (this version, v2))
The Planck collaboration has provided us rich information about the early universe, and a host of new observational missions will soon shed further light on the 'anomalies' that appear to exist on the largest angular scales. From a quantum gravity perspective, it is natural to inquire if one can trace back the origin of such puzzling features to Planck scale physics. Loop quantum cosmology provides a promising avenue to explore this issue because of its natural resolution of the big bang singularity. Thanks to advances over the last decade, the theory has matured sufficiently to allow concrete calculations of the phenomenological consequences of its pre-inflationary dynamics. In this article we summarize the current status of the ensuing two-way dialog between quantum gravity and observations.
Invited review article for the focus issue of Classical and Quantum Gravity : "Planck and the fundamentals of cosmology"

 

[wolram]

I have not read, can not, read all your papers Marcus but from what i can make out is there are a few tentative forecasts that may prove interesting?
It also seems to me that QG may be proven at macro and micro levels with powers of observation we do not have yet or am i reading this all wrong?

 

[marcus]

You can get some idea of current events without reading the papers if you simply read the abstracts. Ivan Agullo is an important figure in Quantum Cosmology (in his own right and frequent co-author with Ashtekar)
http://arxiv.org/abs/1509.05693
Detailed analysis of the predictions of loop quantum cosmology for the primordial power spectra
Ivan Agullo, Noah A. Morris
(Submitted on 18 Sep 2015)
We provide an exhaustive numerical exploration of the predictions of loop quantum cosmology (LQC) with a post-bounce phase of inflation for the primordial power spectrum of scalar and tensor perturbations. We extend previous analysis by characterizing the phenomenologically relevant parameter space and by constraining it using observations. Furthermore, we characterize the shape of LQC-corrections to observable quantities across this parameter space. Our analysis provides a framework to contrast more accurately the theory with forthcoming polarization data, and it also paves the road for the computation of other observables beyond the power spectra, such as non-Gaussianity.
24 pages, 5 figures

The data he's talking about has already been taken (by Planck) but has not yet been released.
Agullo gave the October 13 talk at the ILQGS (International LQG Seminar) and slides and audio are online. The talk covered topics from this earlier paper more definitively---showing a successful postdiction and also making a PREdiction testable based on data soon to be released by the Planck Mission. The tone of this earlier paper is more tentative, less bold:
http://arxiv.org/abs/1507.04703
Loop quantum cosmology, non-Gaussianity, and CMB power asymmetry
Ivan Agullo
(Submitted on 16 Jul 2015)
We argue that the anomalous power asymmetry observed in the cosmic microwave background (CMB) may have originated in a cosmic bounce preceding inflation. In loop quantum cosmology (LQC) the big bang singularity is generically replaced by a bounce due to quantum gravitational effects. We compute the spectrum of inflationary non-Gaussianity and show that strong correlation between observable scales and modes with longer (super-horizon) wavelength arise as a consequence of the evolution of perturbations across the LQC bounce. These correlations are strongly scale dependent and induce a dipole-dominated modulation on large angular scales in the CMB, in agreement with observations.
7 pages, 3 figure. Published in Physical Review D.
http://inspirehep.net/record/1383122?ln=en
Here are slides and audio for the October 13 seminar talk
http://relativity.phys.lsu.edu/ilqgs/agullo101315.pdf
http://relativity.phys.lsu.edu/ilqgs/agullo101315.mp3

 

[ohwilleke]

Sam Cottle, the linked post advances a theory that is almost certainly wrong as is obvious from the very first sentence of the abstract:

Gravity is proportional to mass due to the fact that the number of electrons in an object is is usually proportional to that object’s mass.

1. Gravity is proportional to mass-energy, not mass. Kinetic energy and fields created by other forces (at the quantum level, their carrier bosons), for example, contribute to the stress-energy tensor.

2. Mass is not terribly tightly tied to the number of electrons. Many atoms with the same number of protons and electrons have different masses and hence different responses to gravity due to different numbers of neutrons. Neutrinos, weak force bosons and top quark pairs (which decay prior to hadronization or acquiring electrons) have mass. So do nuclei stripped of electrons. Photons interact with gravity independent of electromagnetic effects. Most of the mass in the universe arises from the energy of gluons that have zero rest mass.

Other criticisms apply to almost all of the other conclusions and statements in the post, but there is no point in piling on criticism once the face that the hypothesis is profoundly and completely flawed is established.

The article also appears to be doing something that people interesting in formulating quantum gravity consistent with the Standard Model generally agree isn't possible or even the kind of thing they're looking for in an answer. Generally speaking, the notion is not that gravity is something derivative of a Standard Model force.

Instead, the notion is that it ought to be possible to formulate gravity in a manner that has a mechanism such as gravitons, discrete space-time, or degree of entanglement or entropy, that can be expressed with equations that are mathematically non-pathological and consistent with the Standard Model while fairly approximating all experimentally tested versions of classical general relativity in the classical limit, but having some phenomenological differences arising from the differences between quantum gravity and classical general relativity that can be experimentally tested.