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Idea for experimental test of LQG

  1. Feb 21, 2012 #1
    Hello! Long time peeker, first time poster. I recently made a GR joke/brainteaser on a pop sci blog I frequent, and the tirade of nutty reactions it provoked drove me to an agitated contemplation of the matter, which gave rise to a few ideas.

    I was thinking that maybe a Gallileo type orange/grape experiment in a decent sized vacuum chamber, shielded with a high powered electromagnetic armour (maybe with testing at different altitudes) might give results that could differ enough from the GR warped space-time model to support a quantum gravity based theory?

    Does anyone know if such an experiment has been done already, and if not does anyone want to lend me a blowtorch, a few shipping containers, some jet engines, a power station, and a mountain? And an orange?
  2. jcsd
  3. Feb 21, 2012 #2


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    I guess you are suggesting a test of the equivalence principle? I don't think that is related to LQG specifically. A related proposal might be:

    Breaking Diffeomorphism Invariance and Tests for the Emergence of Gravity
    Mohamed M. Anber, Ufuk Aydemir, John F. Donoghue

    Apparently, a failure of Lorentz invariance would lead to a failure of the equivalence principle, so one could also test for a failure of Lorentz invariance:

    Modern tests of Lorentz invariance
    David Mattingly
    http://arxiv.org/abs/grqc/0502097 (section 2.5)

    I think the Eot-Wash group has done a test very similar to what you propose.

    Test of the Equivalence Principle Using a Rotating Torsion Balance
    S. Schlamminger, K.-Y. Choi, T. A. Wagner, J. H. Gundlach, and E. G. Adelberger
    http://www.npl.washington.edu/eotwash/publications/pdf/schlamminger08.pdf [Broken]
    Last edited by a moderator: May 5, 2017
  4. Feb 21, 2012 #3
    Hi atyy,

    Yeah, I suppose that is the kind of thing I was trying to propose (in a more break than make capacity!). What I envisaged was that if a differential could be obtained in the proposed "subatomic foam" that could form a mass-entropic conception of quantum gravity, via a high energy filter (such as an electromagnetic shield, or maybe a vacuum surrounded by a zero energy state BEC like a superconductor or superfluid?) then maybe you could filter out enough 'foam' to provide a result detectably different from one expected by equivalence/GR spacetime.

    Thanks for the links (strange dreams for me tonight..), sorry if I'm just typing out loud!
  5. Feb 21, 2012 #4


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    There are feasible observational tests being proposed, a physicist at Stockholm (S. Hossenfelder) has organized a couple of conferences/workshops on *Experimental Search for Quantum Gravity*. Here is a writeup on the one in Stockholm in 2010:

    Hossenfelder and co-workers recently proposed a new trace of Loop qg to look for in cosmic ray spectrum. From Loop-type BH. Google "hossenfelder emission spectra" and get http://arxiv.org/abs/1202.0412

    Loop leads to a simple explanation of what makes up dark matter, which if correct, would result in a signature in high energy radiation such as cosmic ray. This is new (2012) and not in the 2010 workshop report I mentioned.

    Some other papers propose a LQG "footprint" to look for in the microwave background (left over light from early universe where LQG predicts a particular start to expansion).

    Google "grain barrau footprint loop" and get http://arxiv.org/abs/0902.0145
    Cosmological footprints of loop quantum gravity
    J. Grain, A. Barrau
    (Submitted on 2 Feb 2009 (v1), last revised 4 Mar 2009 (this version, v2))
    The primordial spectrum of cosmological tensor perturbations is considered as a possible probe of quantum gravity effects. Together with string theory, loop quantum gravity is one of the most promising frameworks to study quantum effects in the early universe. We show that the associated holonomy correction should modify the potential seen by gravitational waves during the inflationary amplification. The resulting power spectrum should exhibit a characteristic tilt. This opens a new window for cosmological tests of quantum gravity.
    Comments: Accepted by Phys. Rev. Lett., 7 pages, 2 figures

    Most tests would require a new generation astrophysical instruments, or data that has not yet been reported from the Planck spacecraft, but this one by Wen Zhao uses already collected data:
    Google "zhao constraints early universe models" and get http://arxiv.org/abs/1007.2396

    Wen Zhao is already working with available CMB data from the WMAP spacecraft to limit the range of parameters for Loop and other models of early universe. He participated in Hossenfelder's 2010 workshop and the work is included in the report I mentioned.

    More papers related to observational testing of LQG, these are all 2009 or later. The link is slow, Spires system will be replaced soon, but efficient. Some 49 papers including some I mentioned.
    http://www-library.desy.de/cgi-bin/spiface/find/hep/www?rawcmd=FIND+%28DK+LOOP+SPACE+AND+%28QUANTUM+GRAVITY+OR+QUANTUM+COSMOLOGY%29+%29+AND+%28GRAVITATIONAL+RADIATION+OR+PRIMORDIAL+OR+inflation+or+POWER+SPECTRUM+OR+COSMIC+BACKGROUND+RADIATION%29+AND+DATE%3E2008&FORMAT=www&SEQUENCE=citecount%28d%29 [Broken]

    Here's a less efficient search by the new tool which will replace Spires, but still pretty good:
    Finds 48 papers from the period 2008-2011, mostly the same papers.
    Last edited by a moderator: May 5, 2017
  6. Feb 28, 2012 #5
    Good stuff. I'm just an interested observer, but I do find the whole quantum gravity concept a really beautiful, emergent way that could explain a whole raft of phenomena!

    Had another idea today (maybe more of an engineering quandry than a theoretical issue?);

    If several satellites of varying mass require orbit adjustment from time to time, assuming unperturbed trajectories & excluding lunar interference etc, is there a point (some kind of LEO lagrangian point) before which adjustment of trajectories is consistently away from from the gravity well, and beyond consistently towards it, to maintain stable orbits?
    IE in warped spacetime, any idealised stable geostationary orbit will inevitably eventually decay into the gravity well, while with quantum gravity an idealised stable geostationary orbit has a quantum uncertainty, and will eventually decay either towards or away from the gravity well (or not exist?!). Gakk. Brain off.. :/
    Last edited: Feb 28, 2012
  7. Jul 18, 2012 #6


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    We should check those links again, to see if more LQG and LQC phenomenology papers show up.
    The DESY search that I linked in post#4 used to get 49 papers back in February. Now it finds 58.
    It is slow but still works!
    http://www-library.desy.de/cgi-bin/spiface/find/hep/www?rawcmd=FIND+%28DK+LOOP+SPACE+AND+%28QUANTUM+GRAVITY+OR+QUANTUM+COSMOLOGY%29+%29+AND+%28GRAVITATIONAL+RADIATION+OR+PRIMORDIAL+OR+inflation+or+POWER+SPECTRUM+OR+COSMIC+BACKGROUND+RADIATION%29+AND+DATE%3E2008&FORMAT=www&SEQUENCE=citecount%28d%29 [Broken]
    Last edited by a moderator: May 6, 2017
  8. Jul 27, 2012 #7
    I heard some rumours along the lines of "proton dissapearance at super low temperatures" & some purported support for astronomical observations with an inverse lambda value for the cosmological constant (which superficially sound like they may be conducive to analysis by some of the more leftfield physical theorems) a while ago, but I haven't seen any papers directly citing such. I suppose I've been to busy contemplating the immense philosophical, metaphysical & industrial ramifications of the discovery of the higgs bourbon to look into it right now.
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