Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

'Quantum grains' smaller than 'Planck length'

  1. Jul 1, 2011 #1
    How is it possible that 'quantum grains' are many orders of magnitude smaller than 'strings' or the 'Planck length'?
  2. jcsd
  3. Jul 1, 2011 #2
    Never heard of a "quantum grain"...unless it's part of a new theory, I don't think anything can yet be described at smaller than Planck length nor at Planck energy...
  4. Jul 2, 2011 #3


    User Avatar
    Gold Member

    Welcome to PF baguyette!

    The Planck length is closely related to the speed of light, Planck's constant, and the gravitational constant, thus getting anything smaller than this is going to cause some 'trouble'... I think...
  5. Jul 2, 2011 #4
    According to the July 1st article (see link below), space is made up of 'quantum grains' which have been shown to be many magnitudes of order smaller than the Planck length. I quote from the article;

    "Some theories suggest that the quantum nature of space should manifest itself at the 'Planck scale': the minuscule 10-35 of a metre, where a millimetre is 10-3 m. However, Integral's observations are about 10 000 times more accurate than any previous and show that any quantum graininess must be at a level of 10-48 m or smaller. This is a very important result in fundamental physics and will rule out some string theories and quantum loop gravity theories,"

  6. Jul 2, 2011 #5


    User Avatar
    Science Advisor

    Yes, I saw the news stories too and wondered what on Earth they were talking about. The real paper is at http://arxiv.org/abs/1106.1068. It was an observation of the Integral satellite, studying polarized light from gamma ray bursts. The effect they looked for (and failed to find) is "birefringence of the vacuum". They excluded it to one part in a hundred trillion. :eek:
  7. Jul 2, 2011 #6


    User Avatar
    Gold Member

    I saw this as well and was going to post a question here about it since it didn't seem to make sense.

    Now, I find that it's incredible all right, but not the results stated, rather the fact that the ScienceDaily story seems to have been made up entirely. It bears no relation to the article that it references other than that it references the same experiment but makes statements that are nowhere found in the original paper.

    I've never seen the ScienceDaily before (but I'm new to all this). Is it normally reliable?
  8. Jul 2, 2011 #7
    The ScienceDaily story was taken, verbatim, from the European Space Agency (ESA) website (http://www.esa.int/esaCP/SEM5B34TBPG_index_0.html). One of the authors of the original paper, Dr. Laurent, is quoted in the ESA article, but I could not tell if he made the statement that "quantum grains must be at a level of 10-48m or smaller". Not sure who made that determination or if it is even an accurate presumption from the facts.
  9. Jul 2, 2011 #8


    User Avatar
    Gold Member

    Seems like they should not have referenced the article they did since it has nothing about that discussion of guantum grains and as Bill_K pointed out is on a different topic entirely.

    Be interesting to see if there's any followup on this since the quantum grains size statement is really radical.
  10. Jul 2, 2011 #9


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    The press release and the Sci Daily make a wildly inaccurate presentation of the actual result of the actual paper.

    Baguette I think you suspected as much.

    What Philippe Laurent was quoted as saying was just hype: The paper is not about grains but about Lorentz Invariant Violation.

    Constraints on Lorentz Invariance Violation using INTEGRAL/IBIS observations of GRB041219A
    P. Laurent, D. Gotz, P. Binetruy, S. Covino, A. Fernandez-Soto
    (Submitted on 6 Jun 2011)

    LQG was mentioned in the hype, but LQG does not predict LIV! As far as we know it is Lorentz covariant so bounds on LIV do not constrain it. So the Laurent et al result is not about LQG.
    And indeed, LQG was not mentioned in the paper, which is at a more professional level than the public hype.

    The LQG theory does says something about the measurement of area and volume, and the quantum states. The area and vol operators have discrete spectra. That is at the level of measurement---geometric observables. It also does not represent space on which geometry lives as consisting of physical grains. Subtle distinction.

    But in any case, no Lorentz violation.

    Some people tried to show LIV a few years back but failed. Recent papers have shown the theory is Lorentz covariant.

    But this is irrelevant. The Laurent et al paper only constrains Lorentz violation. LQG does not violate Lorentz. So the paper has no bearing on LQG---neither the standard version established since around 2007 nor any other versions that I know of.
    Last edited: Jul 2, 2011
  11. Jul 2, 2011 #10


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Education Advisor

    I might just be talking out of my butt here, but I'm guessing Laurent is referring to hypothetical violations of Lorentz invariance at high-energy scales.


    In the paper, Laurent et al. wrote, "On general grounds, Lorentz violating operators of dimension N = n + 2 modify the standard dispersion relations E2 = p2 + m2 by terms of the order of fnpn/Mn-2Pl where MPl is the reduced Planck scale ... used as a reference scale since LIV is expected to arise in the quantum regime of gravity." They derive a correction to the polarization of order
    [tex]\frac{\xi k}{M_\mathrm{Pl}}[/tex]
    and extract an upper limit of [itex]\xi \lt 1.1\times 10^{-14}[/itex] from their data. LIV will manifest when
    [tex]\frac{\xi k}{M_\mathrm{Pl}} \backsim 1[/tex]
    which corresponds to a length scale of 1/k~10-48 m. If LIV and quantum gravity arise at the same length scale, the results suggest quantum gravity doesn't kick in when most physicists expect it to. Laurent is interpreting this to mean spacetime remains smooth down to at least that length scale. (The paper does mention, though, that some symmetry, like supersymmetry, could imply [itex]\xi = 0[/itex].)
  12. Jul 3, 2011 #11


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    That is right, vela. In my post I assumed that was what we were talking about.

    It turned out to be impossible to derive LIV at very high energy scales from LQG when this was a research goal around 2005-2007.
    So either Laurent was misquoted (mentioning Lqg as one theory that might be effected) or he doesn't know the current state of the field.

    Since you seem interested in the Lorentz covariance of quantum gravity theories you might like to look at this paper published in Physical Review D May 15, 2011, so quite recent!
    Lorentz covariance of loop quantum gravity
    Carlo Rovelli, Simone Speziale
    (Submitted on 8 Dec 2010, last revised 18 Apr 2011)
    The kinematics of loop gravity can be given a manifestly Lorentz-covariant formulation: the conventional SU(2)-spin-network Hilbert space can be mapped to a space K of SL(2,C) functions, where Lorentz covari...
    Last edited: Jul 3, 2011
  13. Jul 3, 2011 #12
    Thanks for that, it wasn't obvious to me but your simple explanation looks right :smile:
  14. Jul 6, 2011 #13

    not seen yet.... (perhaps never........)
    the original article do not states that fact.
    IF exist a quantum foam HAS to be smaller that planck sclae, IF not, then, maybe space is continuous.

    from a previous experiment on high energy photons:

    "Even more importantly, this photon sets limits on a possible linear energy dependence of the propagation speed of photons (Lorentz-invariance violation) requiring for the first time a quantum-gravity mass scale significantly above the Planck mass"

    that is:
    there is not small scale linearity altering the speed of light, According to relativity, the speed of light obeys Lorentz Invariance: it's the same for all observers and all energies of light. but some quantum gravity models suggest that Lorentz Invariance may break down near the Planck length, 1.62 x 10-33cm, causing high-energy photons to travel at different speeds than their low-energy peers.
    quantum theory implies that, at scales trillions of times smaller than an electron, spacetime should be discontinuous. Think of it as frothy or foamy.
    In that case, we would expect that shorter wavelength light (at higher energies) would be slowed compared to light at longer wavelength.

    "A value this close to the Planck length means that quantum gravity models in which there's a linear relationship between photon energy and speed are "highly implausible." That leaves other quantum gravity options open, including those in which the the relationship is non-linear"

    string and loop model predicts a linear relationship.

    Last edited: Jul 6, 2011
  15. Jul 6, 2011 #14


    User Avatar
    Science Advisor

    which models?
  16. Jul 8, 2011 #15
    Last edited: Jul 8, 2011
  17. Jul 9, 2011 #16


    User Avatar
    Science Advisor

    Sorry, I don't understand; the paper says in the abstract "even more importantly, this photon sets limits on a possible linear energy dependence of the propagation speed of photons (Lorentz-invariance violation)"; the paper does not name one single specific model.

    Then you say "string and loop model predicts a linear relationship". Where do you get your statement from? Where and how is this prediction of Lorentz invariance derived? Currently afaik the loop community says that LQG does not predict violation of Lorentz invariance, neither linear nor non-linear.
  18. Jul 9, 2011 #17
  19. Jul 9, 2011 #18
  20. Jul 10, 2011 #19


    User Avatar
    Science Advisor

    afaik the results for LQG do not hold in the covariant approach, therefore something must be wrong with this construction. I guess this is de to the fact that the correct regularized Hamiltonian H is still unknown (and therefore the effective ones are incorrect)and that the semiclassical states used are not in the kernel of H.

    I don't know a paper where these conclusions have been withdrawn, but today nobody in the LQG community believes in violation of Lorentz invariance.

    Perhaps marcus knows more avout the history.
  21. Jul 10, 2011 #20
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook