Tuesday, October 30, 2012

ESQG 2012 - Conference Summary

Conference Photo: Experimental Search for Quantum Gravity 2012.

The third installment of our conference "Experimental Search for Quantum Gravity" just completed. It was good to see both familiar faces and new ones, sharing a common interest and excitement about this research direction. This time around the event was much more relaxing for me because most of the organizational work was done, masterfully, by Astrid Eichhorn, and the administrative support at Perimeter Institute worked flawlessly. In contrast to 2007 and 2010, this time I also gave a talk myself, albeit a short one, about the paper we discussed here.

All the talks were recorded and can be found on PIRSA. (The conference-collection tag isn't working properly yet, I hope this will be fixed soon. You'll have to go to "advanced search" and search for the dates Oct 22-26 to find the talks.) So if you have a week of time to spare don't hesitate to blow your monthly download limit ;o) In the unlikely event that you don't have that time, let me just tell you what I found most interesting.

For me, the most interesting aspect of this meeting was the recurring question about the universality of effective field theory. Deformed special relativity, you see, has returned in the reincarnation "relative locality" as to boldly abandon locality altogether after the problem could no longer be ignored. It still doesn't have, however, a limit to an effective field theory. A cynic might say "how convenient," considering that 5th order operators in Lorentz-invariance violating extensions of the standard model are so tightly constrained you might as well call them ruled out.

If you're not quite as cynic however, you might take into account the possibility that the effective field theory limit indeed just does not work. That, it was pointed out repeatedly -- among others by David Mattingly, Stefano Liberati and Giovanni Amelino-Camelia -- would actually be more interesting than evidence for some higher order corrections. If we find data that cannot be accommodated within the effective field theory framework, such as for example evidence for delayed photons without evidence for 5th order Lorentz-invariance violating operators, that would give us quite something to think about.

I agree: Clearly one shouldn't stop looking just because one believes to know nothing can be found. I have to add however that the mere absence of an effective field theory limit doesn't convince me there is none. I want to know why such a limit can't be made before I believe in this explanation. For all I know it might be absent just because nobody has made an effort to derive it. After all there isn't much of an incentive to do so. As the German saying goes: Don't saw on the branch you sit on. That having been said, I understand that it would be exciting, but I'm too skeptic myself to share the excitement.

A related development is the tightening of constraints on an energy-dependence of the speed of light. Robert Nemiroff gave a talk about his and his collaborator's recent analysis of the photon propagation time from distant gamma ray bursts (GRB). We discussed this paper here. (After some back and forth it finally got published in PRL.) The bound isn't the strongest in terms of significance, but makes it to 3σ. The relevance of this paper is the proposal of a new method to analyse the GRB data, one that, given enough statistics, will allow for tighter constraints. And, most importantly, it delivers constraints on scenarios in which the speed of highly energetic photons might be slower as well as on the case in which it might be faster than the photons with lower energy. And for an example on how that is supposed to happen, see Laurent Freidel's talk.

A particularly neat talk was delivered by Tobias Fritz who summarized a simple proof that a periodic lattice cannot reproduce isotropy for large velocities, and that without making use of an embedding space. Though his argument works so far for classical particles only, I find it interesting because with some additional work it might become useful to quantify just how well a discretized approach reproduces isotropy or, ideally, Lorentz-invariance, in the long-distance limit.

Another recurring theme at the conference was dimensional reduction at short distances which has recently become quite popular. While there are meanwhile several indications (most notably from Causal Dynamical Triangulation and Asymptotically Safe Gravity) that at short distances space-time might have less than three spatial dimensions, the ties to phenomenology are so far weak. It will be interesting to see though how this develops in the coming years, as clearly the desire to make contact to experiment is present. Dejan Stojkovic spoke on the model of "Evolving Dimensions" that he and his collaborators have worked on and that we previously discussed here. There has however, for all I can tell, not been progress on the fundamental description of space-time necessary to realize these evolving dimensions.

Noteworthy is also that Stephon Alexander, Joao Magueijo and Lee Smolin have for a while now been poking around on the possibility that gravity might be chiral, ie that there is an asymmetry between left- and right-handed gravitons, which might make itself noticeable in the polarization of the cosmic microwave background. I find it difficult to tell how plausible this possibility is, though Stephon, Lee and Joao all delivered their arguments very well. The relevant papers I think are this and this.

I very much enjoyed James Overduin's talk on tests of the equivalence principle, as I agree that this is one of the cases in which pushing the frontiers of parameter space might harbor surprises. He has a very readable paper on the arxiv about this here. And Xavier Calmet is among the brave who haven't given up hope on seeing black holes at the LHC, arguing that the quantum properties of these objects might not be captured by thermal decay at all. I agree with him of course (I pointed this out already in this post 6 years ago), yet I can't say that this lets me expect the LHC will see anything of that sort. More details about Xavier's quantum black holes are in his talk or in this paper.

As I had mentioned previously, the format of the conference this year differed from the previous ones in that we had more discussion sessions. In practice, these discussion sessions turned into marathon sessions with many very brief talks. Part of the reason for this is that we would have preferred the meeting to last 5 days rather than 4 days, but that wasn't doable with the budget we had available. So, in the end, we had the talks of 5 days squeezed into 4 days. There's a merit to short and intense meetings, but I'll admit that I prefer less busy schedules.


  1. "Clearly one shouldn't stop looking just because one believes to know nothing can be found." Den Ast absägen, auf dem man sitzt? Ein Kranke hat gesagt, "Umpolung!" Annotated photograph

    Core physics undone by mere chemistry is consistent with an ironic universe (supernovae!). If you dislike P3(1,2)21 α-quartz (79.64 atoms/nm^3) or P3(1,2) γ-glycine (127.07 atoms/nm^3), try P3(1,2) RbNO_3 (42.42 atoms/nm^3) or P3 (1,2) CsNO_3 (29.66 atoms/nm^3). Alas, their large unit cells are coarse 3-D meshes at scale. Re Tobias Fritz, vacuum models before curve fittings do not tolerate fermionic matter or periodic lattices. Don't conjecture, don't argue, don't curve fit; look.

    ("Asymmetry" not "assymmetry. The latter invites "pun-nishment.")

  2. Hi Uncle,

    Thanks, fixed the typo. For reasons I don't understand almost all of your comments end up in the spam queue. Sorry about that, but for all I know there isn't anything I can do about it. I check it on occasion, so sooner or later they will appear. Best,


  3. Correct atom densities to RbNO_3 21.21 atoms/nm^3 and CsNO_3 18.81 atoms/nm^3. Pesky unit conversions. The exposition is otherwise unchanged.

  4. Perhaps centralized political correctness is threatened by rational dissenting voices. Thanks for your work, Bee, blog and comments. Glassblowing offers a beautiful universe. It is no less beautiful for requiring crocheted hyperbolic outputs,

    crochet hyperbolic 142,000 hits

  5. Tegmark uses the independence of the speed of light on frequency as seen in GRBs as a constraint making the "Big Rip" less likely.

  6. Hi Phillip,

    Thanks, fixed that. I've never heard of that argument from Tegmark, do you have a reference? Best,


  7. It's the Big Snap, not the Big Rip. Reference is http://arxiv.org/abs/1108.3080 (now published in PRD). Max could have titled the paper "Life, the Universe and Everything". :-) It's in section IV.A.3. Max mentions the GRB dispersion (or, rather, lack of it) but doesn't provide a reference.

  8. /*..Experimental Search for Quantum Gravity ..*/
    Quantum Gravity is everything between dimensional scales of quantum mechanics and general relativity, i.e. all objects at the human observer scale. There's no need to search for them experimentally.

    /*..constraints on scenarios in which the speed of highly energetic photons might be slower as well as on the case in which it might be faster than the photons with lower energy..*/
    These constrains actually don't exist at the moment, when the longwavelenght photons will encircle these heavier, slower and short-wavelenght ones inside of gamma burst.

    /*the possibility that gravity might be chiral*/
    As a notoriously known example can serve the black holes with asymmetric jets (which serve as an evidence of magnetic monopoles too)

    /*tests of the equivalence principle*/
    Dark matter violates it sufficiently or not?

    /*who haven't given up hope on seeing black holes at the LHC*/
    During LHC collisions the atom nuclei are routinely formed - they're microscopical black holes in the sense of quantum gravity.

    Apparently, the QG phenomenology isn't developed at the very high level yet...;-) And we all are paying these nonsenses from our taxes...

  9. Zephir: It is really difficult to solve a problem if you haven't understood what the problem is.

  10. Or really easy, depending on your perspective.

  11. This comment has been removed by the author.

  12. Well, WHY the quantum gravitists excluded the region between dimensional mass/energy density scale of quantum mechanics and general relativity?

    Isn't the QG supposed just to find common solution of these two theories - i.e. to INTERPOLATE between them?

    And plz, try to argue with my question - not with my person. My person, POV or qualification are solely irrelevant to the above question.

  13. Zephir: I have no clue whatsoever what you think which region of what parameter somebody has excluded, or by which means they have done so. That you evidently believe black hole jets are gravitons and LHC collisions form atom nuclei that are black holes, as well as plenty of previous comments that are little more than a meaningless assembly of words you probably picked up elsewhere, lets me doubt that you actually know what you're talking about.

  14. Bee,
    are all talks posted?
    According to http://www.perimeterinstitute.ca/Events/Experimental_Search_for_QG/Schedule/
    I don't see the talks on Oct 23rd afternoon posted.
    (for eg starting from John Kelley)

    Also you can see he conference collection if you click on the same conference title from 2007 (and in the end)

  15. /* you evidently believe black hole jets are gravitons */

    Nope, I don't believe it. Why the atom nuclei prepared at LHC COULDN'T be the micro-black holes stabilized with extradimensions?

  16. Zephir: To begin with atomic nuclei are vastly too large to be black holes of the same mass, they also have substructure as you might have heard. Micro black holes at the LHC have been looked for - and not been found. I also believe I've told you that several times already. Best,


  17. Hi Shantanu,

    Sorry, I don't know what's up with the missing talks. If they're not up by next week I'll inquire. Best,


  18. /* atomic nuclei are vastly too large to be black holes of the same mass*/
    Yes, general relativity predicts, all massive objects would collide into singularities. But the quantum mechanics predicts instead, all objects are formed with quantum wave packets, which do expand into infinite volume in accordance to Schrodinger equation of free particles. So that the apparent puffiness of atom nuclei in the role of microblack holes is essentially the consequence of quantum mechanics and whole the atom nuclei are quantum gravity artifacts.

    /*they also have substructure as you might have heard.*/

    Yep, this is how the extradimensions usually do manifest itself...

  19. Zephir: First, General Relativity does not predict that "all massive objects collide into singularities." Second, quantum mechanics does not predict that "all objects are formed with quantum wave-packets", whatever that is supposed to mean. Also, there are different dispersion relations. Third, we understand the structure of atomic nuclei very well, there's no evidence for additional spatial dimensions at the scale of atomic nuclei, and if you want to replace QCD with the "manifestation of extra dimensions" then I encourage you to write a paper about it, but please refrain from elaborating on your "theories" here. Best,


  20. /*..General Relativity does not predict that "all massive objects collide into singularities." */

    Wheeler's geon concept was abandoned just with respect to finding, all objects formed with space-time curvature only are predestined to end with pin-point singularity with no mercy, when only general relativity was taken into account.

    /* quantum mechanics does not predict that "all objects are formed with quantum wave-packets" */

    It describes all objects with Schrodinger equation, which belongs into six QM postulates. The time evolution of objects in form of quantum wave packets therefore belongs into predictions of QM.

  21. /* please refrain from elaborating on your "theories" here..*/

    My theory is solely independent to quantum mechanics and general relativity. I wanted to illustrate the contemporary situation with interpretation MAINSTREAM physics theories instead.

    So, have a nice weekend, you've enough of private problems already, as I can read...;-)

  22. Zephir:

    I think you're confusing superpositions with wavepackets, not the same thing. Also, you might have noticed there are other forces in nature besides general relativity which prevent that everything which collides forms black holes, which is why we're here and which is why atomic nuclei don't just collapse to a point.

    "I wanted to illustrate the contemporary situation with interpretation MAINSTREAM physics theories instead."

    Well, your illustration might have been well intended but was not particularly accurate. Best,


  23. /* ..you might have noticed there are other forces in nature besides general relativity which prevent that everything which collides forms black holes..*/

    Just these forces are manifestation of extradimensions, as they're violating the inverse square law. They should be predicted with quantum gravity theory.

  24. A lost generation? M. Shifman provides an "impressionistic portrait" of the current state of particle theory. I wouldn't call the contemporary physicists a "lost generation" - rather the generation which completely misunderstood the practical meaning of its own insights.


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