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Experimental Evidence of Quantum Gravity?

  1. Mar 3, 2012 #1
    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?
  2. jcsd
  3. Mar 3, 2012 #2


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    Quantum gravity effects would appear under extreme conditions (where ordinary classical GR breaks down and develops "singularities") such as in connection with cosmological big bounce or black holes.

    So most of the research papers studying ways to test are about BB or BH, and most have to do with either the CMB (ancient light from early universe, showing fluctuations in density) or with radiation that might be detected coming from the evaporation of black holes.

    I can only toss out some links to give an idea of the kind of things being studied. I don't know of anybody who has the immediate direct ANSWER of how to test today's QG theories with today's technical means.
    This is just to get the flavor. It is not something to try to understand in depth, all still preliminary and nowhere near ready for mass audience.

    http://physics.republika.pl/PRD7.pdf (Physical Review D 2010)
    Observing the big bounce with tensor modes in the cosmic microwave background:
    Phenomenology and fundamental loop quantum cosmology parameters

    Some slides for a conference talk by the same people--more visual and less texty:

    According to one version of QG theory clouds of microscopic BH would exist and show characteristic radiation. The particular theory could be tested (and perhaps falsified) if one looks for that characteristic radiation (and perhaps does not find it.) Strange rather bold idea...FWIW here is the paper:

    Emission spectra of self-dual black holes
    Sabine Hossenfelder, Leonardo Modesto, Isabeau Prémont-Schwarz
    (Submitted on 2 Feb 2012)
    We calculate the particle spectra of evaporating self-dual black holes that are potential dark matter candidates. ... In this limit, we then derive the number-density of the primary emission particles,... We finally arrive at the expression for the spectrum of secondary particle emission from a dark matter halo constituted of self-dual black holes.
    15 pages, 6 figures

    Here's a more popularized account of some different proposals for testing Loop QG:

    Physicists propose test for loop quantum gravity
    Last edited: Mar 3, 2012
  4. Mar 3, 2012 #3


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    Here's an earlier paper from Physical Review Letters 2009.

    Cosmological footprints of loop quantum gravity
    J. Grain, A. Barrau
    Accepted by Physical Review Letters, 7 pages, 2 figures
    (Submitted on 2 Feb 2009)
    "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."

    This by Grain and Barrau is one of the most highly cited papers of this type. Since 2008 there have appeared are some 40 or 50 papers exploring various ways to test QG theory. I'll get a search link that can dig up some of them.

    Many of the ideas here involve sending up a spacecraft with the right instruments to detect whatever "footprint" of QG they want to look for.

    The technical specialty of figuring out how to test scientific theories is called phenomenology. What we are talking about are "QG Phenomenology" research papers. Designing tests that would show this or that theory is wrong or not wrong---observational tests that would favor or disfavor some QG theory compared with some other.

    It's a small challenging research area that is getting started. Sabine Hossenfelder a QG phenomenologist in Stockholm, has been active in organizing a couple of conferences/workshops on it so far--getting the people who do this kind of work together to compare notes and share ideas.

    Here is a write-up she did on a workshop on the Experimental Search for Quantum Gravity
    It covers a whole range of ideas for testing various QG theories.
    Maybe that link is the best way of answering your question.
    Last edited: Mar 3, 2012
  5. Mar 3, 2012 #4
    I can't say I fully understand the words and equations that these papers present, but thanks for the references and I will read them again, and probably again. It makes sense that we'd look for something so hardly measurable as the quantum of gravity in the CMB.

    I'm guessing that the present LIGO configuration in Louisiana has too many seismic (and especially temperature) unknowns for us to be detecting a gravity wave... does the theory of a gravity wave also imply that gravity is a quantized 'thing'?
  6. Mar 3, 2012 #5


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    There was just recently a video seminar talk by Raphael Porto about LIGO and what it might be able to detect. Years ago Raphael used to post here and I conversed with him. He's very bright and gives a really good slide presentation, fast talking and lots of visual information.
    Google "pirsa porto" and get http://pirsa.org/12030088/

    I can't speculate as to whether or not LIGO type instruments will be able to identify some kind of quantum effect. But you can listen to Rafael, or just glance at his slides PDF and see if anything grabs your interest.

    A lot of people have the basic view that "gravity = geometry" so a gravity wave is a ripple in geometry itself. And quantum gravity effects should appear in the geometry under extreme conditions.
    Fortunately early universe expansion was so extreme that the CMB is like looking at things under a huge microscope. Have to go. Supper.
    Last edited: Mar 3, 2012
  7. Mar 4, 2012 #6


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    Terrific talk. Nothing about quantum gravity, but lots of stuff I'm interested in. I somehow thought LIGO got killed off in funding? Seems not, which is nice.
  8. Mar 4, 2012 #7


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    No. Gravity waves exist in a purely classical (i.e. non-quantum) theory of gravity.
  9. Mar 4, 2012 #8


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    As regards funding trouble, we've been hearing more about LISA
    http://lisa.nasa.gov/mission/ [Broken]
    Laser interferometer SPACE antenna. The idea was an equilateral triangle orbiting the sun (side = 5 million km)

    "Due to NASA budget outlook and to the astro2010 decadal rankings, in April 2011 NASA and ESA ended their ten-year-long LISA partnership. As a consequence:

    ESA is now developing a redesigned European-only gravitational-wave mission, NGO. A new European science team already completed a science performance study, and an industrial study is currently in progress. ESA will decide next year if NGO will go forward as a Cosmic Vision L-class mission, in which case NASA may participate as a minor partner."
    Last edited by a moderator: May 5, 2017
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