B What kind of experiment is needed to quantize gravity?

[Ken Gallock]

Hi everyone.
I heard that there are several theories or hypotheses to quantize gravity such as superstring theory, loop quantum gravity, etc., and I believe none of them are conclusive (that's what I heard).
So, here is my question.
1) Is it because there are no experimental results which prove of disprove those theories?
2) If so, what kind of experimental results could lead us to an ideal theory?

 

[bhobba]

We already have a quantum theory of gravity valid to energies we can currently probe:
http://arxiv.org/pdf/1209.3511v1.pdf

To go beyond that we need to experimentally probe what we currently cant, or figure out some consequence that can be checked. Its being worked on.

Thanks
Bill

 

[marcus]

there was a paper about this on the fourth quarter MIP poll
https://www.physicsforums.com/threads/poll-fourth-quarter-2015-mip-most-important-qg-papers.851080/

here it is:
http://arxiv.org/abs/1512.02083
Tests of Quantum Gravity induced non-locality via opto-mechanical quantum oscillators
Alessio Belenchia, Dionigi M. T. Benincasa, Stefano Liberati, Francesco Marin, Francesco Marino, Antonello Ortolan
(Submitted on 7 Dec 2015)
Several quantum gravity scenarios lead to physics below the Planck scale characterised by nonlocal, Lorentz invariant equations of motion. We show that such non-local effective field theories lead to a modified Schrödinger evolution in the nonrelativistic limit. In particular, the nonlocal evolution of opto-mechanical quantum oscillators is characterised by a spontaneous periodic squeezing that cannot be generated by environmental effects. We discuss constraints on the nonlocality obtained by past experiments, and show how future experiments (already under construction) will either see such effects or otherwise cast severe bounds on the non-locality scale (well beyond the current limits set by the Large Hadron Collider). This paves the way for table top, high precision experiments on massive quantum objects as a promising new avenue for testing some quantum gravity phenomenology.
5 pages, 1 figure

one single experiment would not be expected to answer all questions. do-able experiments would constrain the possible qg theory bit by bit.
There may be some relevant idea on how to constrain QG theories here as well:
https://www.physicsforums.com/threads/poll-first-quarter-2016-mip-most-important-qg-papers.864637/

 

[marcus]

a successful qg theory may need to correctly predict the observed value of the cosmological constant (i.e. longterm expansion rate)
this may arise from the quantum microstructure underlying geometry. so here is one qg theory that makes a prediction

http://arxiv.org/abs/1603.08658
The Atoms Of Space, Gravity and the Cosmological Constant
T. Padmanabhan
(Submitted on 29 Mar 2016)
I describe an approach which connects classical gravity with the quantum microstructure of spacetime. The field equations arise from maximizing the density of states of matter plus geometry. The former is identified using the thermodynamics of null surfaces while the latter arises due to the existence of a zero-point length in the spacetime. The resulting field equations remain invariant when a constant is added to the matter Lagrangian, which is a symmetry of the matter sector. Therefore, the cosmological constant arises as an integration constant. A non-zero value Λ of the cosmological constant renders the amount of cosmic information (Ic) accessible to an eternal observer finite and hence is directly related to it. This relation allows us to determine the numerical value of Λ from the quantum structure of spacetime.
Invited Review; 32 pages; 3 figures

it's another way proposed theory can be constrained by observation

 

[ohwilleke]

Most quantum gravity theories with gravitons would both have a coupling constant that runs with energy scale, and would also subtly influence the beta functions of the running of the Standard Model constants with energy scale. The energy scales at which this would be possible to detect probably exceed our engineering capabilities, but I could imagine astronomy observations of "natural experiments" involving very extreme circumstances that might be able to catch phenomena like this (e.g. some way to infer SM coupling constants with precision in high energy environments near supermassive black holes).