Friday, January 04, 2013

Gravitational bar detectors set limits to Planck-scale physics - Really?

Contains 10-31% juice.
Three weeks ago, Nature Physics published, to my surprise, another paper on quantum gravity phenomenology:
The appearance of the word “macroscopic” in the title should be a warning sign.

As we discussed previously, there are recurring attempts in the literature on quantum gravity phenomenology to amplify normally tiny and unobservable effects by using massive systems. This is tempting because in macroscopic terms the Planck mass is 10-5 g and easy to reach. The problem with this attempt is that such a scaling-up of quantum gravitational effects with the total mass of a system isn't only implausible as an amplification, it is known to be wrong. Next two paragraphs contain technical details, you can skip them if you want.

The reason this amplification for massive systems appears in the literature is that such a scaling is what you, naively, get in approaches with non-linear Lorentz-transformations on momentum space that have been motivated by quantum gravity. If Lorentz-transformations act non-linearly the normal, linear, sum of momenta, this linear sum is no longer invariant under Lorentz-transformations and thus does not constitute a suitable total momentum for objects composed of many constituents.

It is possible to introduce a modified sum, and thus total momentum, that is invariant. But this total momentum receives a correction term that grows faster than the leading order term with the number of constituents. The correction term is suppressed by the Planck mass, but if the number of constituents is large enough, the additional term will become larger than the (normal) leading order term. This would mean that momenta of macroscopic objects would not add linearly, in conflict with what we observe. This issue has been called the “soccer ball problem”; accepting it is not an option. Either this model is just wrong, or, as most people working on it believe, multi-particle states are subtle and the correction terms stay small for reasons that are not yet well understood. To get rid of these terms, a common ad-hoc assumption is to also scale the Planck mass with the number of constituents so that the correction terms remain small. Be that as it may, it's not something that makes sense to use for “observable predictions”.

Earlier last year, Nature published a paper in which the questionable scaling was used to make “predictions" for massive quantum oscillators. Since this prediction is not based on a sound model, it is very implausible that anything like this will be observed.

The authors of the new paper now propose to precisely measure the ground state energy of a gravitational wave detector, AURIGA. In theories with a modified commutation relation between position and momentum operators, this energy receives correction terms. Alas, such modified commutation relations either break Lorentz-invariance, in which case they are very tightly constrained already and nothing interesting is to be found there. Or Lorentz-invariance is deformed, which leads to the necessity to modify the addition law and we're back to the soccer-ball problem.

So you might suspect that the new paper by Marin et al suffers from a similar problem as the previous one. And you'd be wrong. It's much better than that.

The authors explicitly acknowledge the necessity to understand multi-particle states in the models that they aim at testing, and present their proposal as a method to resolve a theoretical impasse. And while they talk about very massive objects indeed (the detector bars have a mass of about 105 kg), they do not scale up the effect with the mass (see eq 4). Needless to say, this means that the effect that they get is incredibly tiny, about 33 orders of magnitude away from where you would expect quantum gravitational effects to become relevant. They modestly write “Our upper limit... is still far from forbidding new physics at the Planck scale.”

Here's the amazing thing. For all I can tell, not knowing much about the AURIGA detector, the paper is perfectly plausible and the constraint makes indeed sense. I have nothing to complain about. In fact they even cite my review in which I explained the problem with massive systems.

The only catch is of course that the limit that they obtain really isn't much of a limit. If Nature Physics was consistent in their publication decisions, they should now go on and publish all limits on Planck scale physics that are less than 34 orders of magnitude away from being tested. I am very much looking forward to this. There are literally hundreds of papers that compute corrections due to modified commutation relations for all sorts of quantum mechanics problems. I should know because they're all listed and cited in my review. Expect an exponential growth of papers on the topic. (I am already dreading the day I have to update my review.) Few of them ever bother to put in the numbers and look for constraints because rough estimates show that they're far, far, away from being able to test Planck scale effects.

The best constraints on these types of models is, needless to say, my own, which is a stunning 56 orders of magnitude better than the one published in Nature.

So it seems that for once I have nothing to complain. It's a great paper and it's great it was published in Nature Physics. Now I encourage you all to compute Planck scale corrections to your favorite quantum mechanics problem by adding an additional term to the commutation relation, and submit your results to Nature. How about the g-2, or the Casimir effect? Oh, and don't forget that somebody should think about the soccer-ball problem...


  1. Interesting....

    We consider the behavior of macroscopic bodies within the framework of relative locality [G. Amelino-Camelia, L. Freidel, J. Kowalski-Glikman, and L. Smolin, arXiv:1101.0931]. This is a recent proposal for Planck scale modifications of the relativistic dynamics of particles which are described as arising from deformations in the geometry of momentum space.Relative locality and the soccer ball problem

    Starting from familiar territory we have sailed into strange new waters, but only if we circle back to the physics we know will the journey be complete. John Baez

  2. I have a bias towards the idea of using Webber bars. For good reason.

    Ah here's a story with which I liked to share. You may find it relevant or not.A little unorthodox for sure in this science blog....can I be forgiven?:)

    Einstein being such a figure in going after the basis of foundation equations had to make a living. So as to move forward in his science. It's really not that difficult to understand.

    Here is a dream I had a long time ago. He presented a glass pitcher of ice and water and stirred it for me. He did this so I may hear the sound that he wished to make a point of? Take such a glass rod and fill a pitcher with ice and stir it to hear what I heard?

    So while off the beaten path and filled with subjectivity, I wondered and was attracted to the Webber bars.

    The detector is based on a very low losses ultracryogenic mechanical oscillator: when a burst of gravitational waves hits and excites the oscillator, this will vibrate for a time span much longer than the duration of the burst (typically 1msec), thus allowing the extraction of the signal from the detector noise.Welcome to the AURIGA detector

    Such a basis by way in which analogy is used, one seeks to define this science. So that it can explode to include the questions about how we see the science we use in a "cross modal" way?

    So, there is a conversion process that takes place?

    How would such a geometry be expressed in particle constructs if such conversions are sent to oscillatory frequencies that ring with a basic foundation chart allotted to the elements of the universe, and the back ground noise, a plethora of possibilities of all mass being constructed, as mass is given through that field?



  3. Hi Plato,

    The attempted solution to the soccer-ball problem in the paper you refer to is demonstrably wrong, see here. I wasn't aware it had gotten published, doesn't speak well for PRD. Best,


  4. The trick simply is, the quantum corrections to relativity (quantum gravity effects) are all around us - it's not stuff of some esoteric physics around Planck scale. The title is indeed missleading and it confuses the word "quantum-mechanics" with "Planck-scale physics" (probably because the Planck constant plays the important role in quantum mechanics).

    The quantum effects can be observed even with naked eye (the surface of fluid hellium, for example).

  5. This comment has been removed by the author.

  6. Derivation cannot repair defective postulate(s). Gravitation is geometric, gauge theory; stringy, foamy, quantized; a Wesley Crusher soupe du jour. Observation in existing apparatus can validate prior observation but falsifyg orthodox theory, for a footnote. Theory is a swindle for justifying its failures and a poltroon for denying them.

    "How about the g-2, or the Casimir effect" Observed muon (g-2)/2 is 3.4 sigma away from standard model calculation. One-loop MSSM cheats: a neutralino and a smuon or a chargino and a muon sneutrino. Two-sector vacuum deposition onto a rotating disk's face alternates 70 nm aluminum mirror and 18.4 nm of 60:40 MgF2:LiF (1.628 refractive index) 120 nm quarter wave optical quench etalon. Round and round it goes. Cut out a 2.75 gm/cm^3 square of which 23 wt-% is zero point fluctuation-depleted fluoride alloy. Is Casimatter anomalous? Locate a Foucault pendulum under a total solar eclipse. Is the Allaise effect real? Thermodynamics leaks. Theory that denies observation is wrong.

  7. Hi Bee and a happy new year!

    A little bit provocative but what is the actual content of QG phenomenology if, as it seems to be the case, QG does not have any characteristic imprints on our low energy world?

    QG phenomenology might be useful for debugging or constrain theories/models but with the large freedom one has to adjust a theory or model of QG, you really can't point to the right direction that a theory/model of QG should follow.

    So what's the use?

  8. Physics has long been in pursuit of an increasingly fine-grained description of the physical world.
    I wonder what would be revealed by the most coarse-grained view, the one with fewest necessary elementals.
    Does the one dictate the other? Don’t know if that even a meaningful question.

  9. Hi Giotis,

    Well, what you say is true for the model and the experiment being discussed here, thus my sarcasm. But what you say isn't generally true. The whole rule of the game is that you are trying to find a way in which you are sensitive to QG effects in the natural range of parameters. The best known example are violations of Lorentz-invariance that are meanwhile constrained many orders of magnitude beyond the Planck scale. Another example that I discussed here are effects of decoherence by space-time fluctuations. Some simple models of space-time fluctuations are already ruled out in the natural parameter range. And then there's the dispersion in deformed special relativity which I think is nonsense for theoretical reasons, but is also on the edge of being experimentally ruled out (again, in the natural range of parameters). Yet another area to look is the early universe or today's relics that tell us about it, this is the domain of models like string cosmology or loop quantum cosmology, etc. In each case, whether or not you find the signal you are looking for, you'll learn something about the underlying theory.



  10. Exotic gravitation has femtometer scale anomalies. Drop a negative muon into a fully ionized heavy atom, U-238 or Cm-248. Five compactified dimensions would emerge at seven times uranium nuclear diameter, gravitation varying as r^(-7) [5+3 spatial dimensions]. Newtonian 4.4×10^(-7) m/s^2 gravitational acceleration at the Cm-248 nuclear surface instead would be 1.5×10^64 m/s^2, 10^63 gees. Exotic gravitation is not even wrong.

  11. Latest from AAS 221 just wrapped up in LA.

    Nemiroff extends Gamma-ray search for subquantum foam.

    "Bolstered by the evidence garnered from the three photons, Nemiroff's analysis supports earlier indications but takes them clearly below the Planck length: "If foaminess exists at all, we think it must be at a scale far smaller than the Planck length, indicating that other physics might be involved," he says."

    Einstein - yes!
    Conventional QG assumptions - No!

    Discrete Scale Relativity

  12. Robert: Nothing "new" about this. It's the same three photons we discussed here, just took some time to get published.


  13. The presentation was at AAS 221 which was held this week.

    Even if the data were not new the presentation was another opportunity to free ourselves from the legend that space-time is foamy at the conventional Planck scale.

    If one wants to observe subquantum particles, clusters and related phenomena, then one is going to have to do high resolution experiments with condensed matter.

    Fluctuations will show up at the 10^-26 cm to 10^-30 cm range.

    In fact they already have:
    Observational hints of such Subquantum Scale gravitational signatures in the 10-31 cm to 10-26 cm range have been seen in experiments at HERA (2.57 +/- 0.71 x 10-26 cm) and at SLC (3.50 +/- 0.04 x 10-28 cm), as reported by V. Gharibyan, Physical Review Lett., 109, 141103, 2012 [ ].

    Of course, if one never wants to seriously question one's fixed assumptions, then one must settle for the status quo, which is becoming increasingly untenable.


  14. Robert:

    And we had the same presentation at the ESQG 2012 which I mentioned here. I'm just saying there's nothing really new about this result. Besides, it's got nothing to do with space-time foam. People just sometimes seem to be using this expression as a general placeholder for "Planck scale effects", which can be terribly misleading.


  15. I share your stated dislike for semantic arguments, especially when they deflect attention from more substantive issues.




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