Friday, October 01, 2010

Experimental Search for Quantum Gravity - Workshop Summary

With some delay, here's finally the summary of our summer workshop on Experimental Search for Quantum Gravity. Most of the delay is due to the videos only having been uploaded two weeks ago, but you can now find the link to the recording and slides on the conference website.

The phenomenology of quantum gravity is a still fairly young research field, and it is good to see it is attracting more interest and efforts every year. Experimental test, also in form of constraints, is an important guide on our search for a theory of quantum gravity. The challenge is that gravity is such a weak force compared to the other interactions, which has the consequence that quantum effects of gravity are extremely difficult to detect - they become important only at the Planck scale, at energies 16 orders of magnitude above what the Large Hadron Collider (LHC) will reach. However, during the last decade proposals have been put forward how quantum gravity could be testable nevertheless.

To that end, a number of models have been developed that arguably are at different levels of sophistication and plausibility, not to mention man-hours. As you can guess, this makes the field very lively, with many controversies still waiting to be settled. So far, none of these models have actually been rigorously derived from a candidate theory of quantum gravity. Instead, they are means to capture specific features that the fundamental theory has been argued to have. Such phenomenological models should thus be understood as simplifications, and one would expect them to be incomplete, leaving questions open for the fundamental theory to be answered.

The best place to look for quantum gravitational effects is in regions of strong curvature, that would be towards the center of black holes or towards the first moments of the universe. Since black hole interiors are hidden from our observation by the horizon, this leaves the early universe as the best place to look. It is thus not surprising that the bulk of effort has been invested into cosmology, most notably in form of String Cosmology and Loop Quantum Cosmology. The typical observables to look for are the amplitudes of tensor modes in the cosmic microwave background (CMB) and non-gaussianities.

The other area of quantum gravity phenomenology that has attracted a lot of attention are violations and deformations of Lorentz-invariance. These have been argued to appear in many approaches towards quantum gravity, including Loop Quantum Gravity (LQG), String Theory, Non-commutative geometry and emergent gravity, thus the large interest in the subject. However, the details are subtle. As I mentioned, no actual derivation exists from either LQG nor string theory, so don't jump to conclusions. Violations of Lorentz-invariance, which have a preferred restframe, can be captured in an effective field theory and are testable to extremely high precision with particle physics experiments (both collider and astrophysics) that allows us to tightly constrain them despite the smallness of the Planck scale. Deformations of Lorentz-invariance have no preferred frame and have been argued not be expressible as effective field theories, thus evading the tight constraints on Lorentz-invariance violations. Deformations of Lorentz-invariance generically lead to a modification of the dispersion relation and an energy-dependent speed of light, which may be observable in gamma ray burst events. As you know from my earlier writing, there's some discussion at the moment about the consistency of these models, and Lee Smolin gave a nice talk on that. Giovanni Amelino-Camelia summarized some of the recent work on the field, and added an interesting new proposal.

Besides these areas into which most of the work has been invested, there's a number of interesting models based on ideas about the fundamental structure of space-time. There is, for example, the causal sets approach, which is Lorentz-invariant, yet results in diffusion, aspects of which may be observable in the CMB polarization, which Fay Dowker spoke about at the workshop. Again, note however that the diffusion equation is motivated by, though not yet actually derived from, the causal sets approach. Then there's the quantum graphity models which I personally find very promising. Unfortunately, Fotini Markoupoulo could not make it to our meeting. I am reasonably sure though that we'll hear more about that model and its phenomenological implications in the future. And there's models about space-time foam leading to decoherence and/or CPT violation, models about space-time granularity leading to modifications of Eötvös' experiment (preprint here) - and I won't attempt to make this a complete listing because I'll inevitably forget somebody's pet model.

A class of models that one should discuss separately are those with a lowered Planck scale. It can happen in scenarios with large extra dimensions that quantum gravitational effects are not actually as feeble as we think they are from extrapolating the strength of gravity over 16 orders of magnitude. (For details, see my earlier post on such models.) It might instead be the Planck scale is just around the corner, making it accessible for collider experiments. A lot of work has been done in this area and these models are now up to being tested at the LHC. Thomas Rizzo gave a great talk on these prospects, and Marco Cavaglia spoke about the production of mini black holes in particular.

Then there's the possibility that we do already have observational evidence for quantum gravity, we just haven't recognized it for what it is. Stephon Alexander talked about a model that generates the neutrino masses, the cosmological constant, and makes additional predictions. Can you ask for more? (Preprint here.) And Greg Landsberg gave a talk about his recent work, trying out the idea that on short scales space-time is not higher- but lower-dimensional (preprint here). This idea has been around for some years now (even New Scientist noticed), but in my impression it so far lacks a really good phenomenological model.

We had three discussion sessions during the week. One on the question what principles might be violated by quantum gravity, one on experiments and thought experiments, and one on the future of particle physics. Unfortunately the recording of the last one, which was the most lively one, failed, but check out the other two. The discussions went very well, and I think they served their purpose of people getting to know each other and exchanging their opinions about the central questions of the field.

All together, I am very pleased with the workshop. Despite a number of organizational glitches, it went very smoothly. The experimentalists mixed well with the theorists, we covered a fair share of the relevant topics, and it didn't rain on the BBQ. To offer some self-criticism, we did this year have a lack of string phenomenology. Some may want to count Mavromatos as "stringy," but we didn't have anybody speaking on string cosmology for instance. That was not by design, but by chance, since, as usual, some of the people we invited declined or could eventually not make it. One of the lessons that I personally have drawn from this workshop is that there is some degeneracy in the predictions of various models that should be sorted out by combining several predictions. This has been well done in the case of extra dimensional models where a clear distinction between signatures of different scenarios has been invested a lot of effort into. Similar studies are however missing when it comes, for example, to quantum gravity phenomenology in the early universe as predicted by different models.

In any case, I hope that we will have more workshops in this series in the future. I'll keep you posted. And I'm sure, one day the workshop will come when we'll actually have evidence to discuss...


  1. Hi Bee,

    Thanks for posting this. I'm watching "Experiments and Thought Experiments", and around 20 mins someone in the audience states that thought experiments can be used to test the internal consistency of a theory, but the video does not show the audience.

    But then at around 23 mins the video turns to the audience and I guess is that same person again speaking. Who that is please?


  2. I wouldn't feel bad there were no superstring theorists there. At least you tried. One of the things that makes it easy for me to switch from science and engineering to mathematics is the gross immaturity of the string theory wars, with 99% (or better) of the immaturity being on the string theorists. Bunch of crybabies. It's embarrassing that fellow intellectuals behave so.

    That doesn't mean the string theorists are wrong, though. What the heck, maybe God's legal name is Rube Goldberg. Does he/she/it have an impish sense of humor? Possibly.

    Speaking of which, did you take a gander at Ed Witten's latest preprint re Feynman Path Integrals yesterday, here? Does that clear anything up, or just stir the mud more?

  3. Hi Christine,

    That's Daniel Sudarsky. I totally agree with him btw that thought experiments can be used to test the internal consistency of a theory... The reason why I ended up on the panel was that the person who we originally planned to be there, who was supposed to be somewhat provocative, couldn't make it on that evening. So I was trying - pretty unconvincingly I think - to put forward the point of view if I can't measure it, I don't care. Best,


  4. Hi Steven,

    It's not so much that I feel bad about it, but that I'd have liked to hear eg a brief overview on string cosmology or what else is going on in that area. Anyway, next year we'll have a program here on string pheno (there's no details yet), where I hope to catch some interesting talks. Best,


  5. Ah, ok, thanks. Now he goes to the board to elaborate. I'm still watching the video. In any case, I have to think further about the issue.

  6. Sure. I believe I expressed my opinion on this earlier in this post.

  7. Physicists assume that gravitation is "weak" in the subatomic domain, at least until one gets down to the Planck scale.

    But what if what we call the "strong force" is actually the direct physical manifestation of neo-classical gravitation playing a dominant role on scales of 10^-18 cm to 10^-12 cm?

    Can this hypothesis be ruled out scientifically, and by that I mean with empirical evidence from actual physical systems?

    If not, could it be that the initial assumption that gravitation is "weak" in the subatomic domain launches all the standard quantum gravity research programs in the wrong direction from the get-go?

  8. Hi Robert,

    As I wrote in my post, there are models in which the Planck scale is stronger on short distances than GR suggests. Subatomic though doesn't work, we've tested particle interactions to high precision down below the radius of a proton already. The strong interaction has an SU(3) gauge group, which has testable and tested predictions, gravity hasn't, so they can't just be the same. I vaguely recall people have been playing around with the idea of gravity being a risidual forge. Or maybe it was just something I thought about at some point, wondering if people had played around with it. In any way, it doesn't work either. There are SU(3) risidual forces, but they're not long-ranged. Best,


  9. Err, I meant, gravity is stronger or the Planck scale is lower, not the Planck scale is stronger. Need coffee.

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  11. Hi Bee,

    So kind of you to write such a thorough synopsis of the Quantum Gravity workshop which you helped host at Nordita this past summer. There is certainly lots of information to review of which in the end if lucky I will at best only partially understand. It’s interesting you make note of Fotini Markoupoulo’s work in this area as I’ve always held the impression she being one of those brilliant people who’s contributions fly just under the radar in terms of general recognition. However I must also admit that the take she has on all this I have trouble in even being able to follow. It is experiences such as this that has me wonder if a self consistent and experimentally convincing theory of quantum gravity ever is developed if only the very few will be able to understand it. Here’s then hoping that Markoupoulo’s grandmother is a much sharper person than me:-)



    P.S. By the way I did notice that bulge you are sporting in the conference photo yet never mentioned it as I thought you might take it that I was suggesting you were approaching middle age. This goes to show better to leave ones speculations to the side until having more evidence to support them. One thing for certain I would bet you’ve become more empathetic to the emergent take on things as of late:-)



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    The Strong Force and Gravitation couldn't be more unalike.

    The strong force gets stronger the farther the quarks fly apart (NPP to Politzer, Wilczek and Gross for discovering the QFT equations that describe it, it's called Asymptotic freedom), gravitation gets weaker with distance.

    The strong force appears discrete, but gravity appears continuous.

    However, I openly question whether or not gravity should be considered a "force." Semantics trouble time, I guess. I always thought Einstein's Gen Rev theory made it plain that gravity is a geometrical consequence of spacetime.

    But regarding gravity's small (infinitesimal, negligible, but not zero) effects in the submicroscopic realm, wasn't Penrose looking into that circa 2004? What became of that? Thanks for reminding me Robert.

  14. Here are some comments to consider.

    (1) General Relativity is a very complex, subtle and physically rich theory. It is extremely doubtful that we have plumbed the full depths of this remarkable theory.

    (2) Kerr, Kerr-Newman and Reissner-Nordstron ultracompact objects are capable of interacting (merging, scattering,...) in a variety of different, sophisticated and as yet incompletely understood ways. The physics of these "exotic" objects might involve some major surprises.

    (3) I am far less sure than others that empirical knowledge of particles like protons and nuclei precludes very strong analogies to gravitational phenomena. What exactly and physically is it about SU(3) gauge theory that is incompatible with strong and discretely scaled gravitation?

    (4) Many of the objections to strong gravitation in the atomic domain are based on the assumptions that gravitation has an absolute coupling constant, and that there are no cutoffs. The model I work on (a global discrete self-similar paradigm for the interaction between 4d S-T geometry and mass/energy) says that both of these assumptions might be wrong. In a discrete fractal paradigm, G scales in a discrete manner and there are strong cutoffs intimately related to the different discrete scales.
    I seriously doubt that this paradigm conflicts with any well-observed empirical knowledge. But I am willing to calmly discuss these issues with those willing to approach them scientifically.

    (5) My mentors taught me to question authority and to question assumptions. Not in a negative or counterproductive way, but in the manner of healthy skepticism that is a sine qua non of science.

  15. Robert,

    Please question authority elsewhere. As our comment rules state, this is not the place to discuss your pet theory. There's about 1 million experiments on subatomic and subnuclear scales that are successfully, to extremely high accuracy described by QCD, including some that test explicity for the gauge group. I believe I told you that previously, including the relevant references, and I'm currently not in the mood to dig them out again. You find everything you need to know in the Particle Data Book. Or, make that, you find everything you need to explain without QCD in the PDB. Gravity is not QCD, it doesn't have SU(3) gauge bosons, it doesn't come with quarks, it isn't asymptotically free, just to mention the core issues. If you want to attempt to explain 3+ decades worth of data without SU(3), go ahead, write a paper and good luck. You can bring up the topic again when your paper got published. More on that topic is not welcome here. Best,


  16. Contemporary gravitation theories derive from maximum symmetries. Nothing so constrains the universe. False vacuum decay powered cosmic inflation. Where is the antimatter? The Weak interaction is strictly left-handed. Left- and right-handed antineutrinos and neutrinos empirically oscillate flavors and so must be massed, then are Majorana particles: neutrinos must be their own antiparticles. (See-saw? Hee haw.) Biological homochirality. Each and all are chiral anomalies.

    Physics denies emergent phenomenon chirality is fundamental. "Opposite shoes" in vacuum free fall can falsify all achiral theory without contradicting prior observation. Look. Unlike hectares of theory, an inflated Big Bang chiral vacuum remnant is testable in existing apparatus using commercial materials within 90 days,
    The worst it can do is succeed.

    Theory predicts what observation tells it to predict. Observe.

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  18. Hi Bee,

    here is a link to something, I find spectacular

    The researcher claims that Quantum Gravity effects are possible to measure.

    Best, Kay.

  19. As get I gather, QG is in a challenging state, basically from two (maybe more) empirical finds that don't fit in with certain theoretical expectations:
    1. The dark energy, which has a value about 10^-120 of the expectation from unit-simplicity (Planck density.) Why? (Also, yet another "cosmic coincidence" if the time derived from DE value in a/r (L/T^2/L = T^-2 ---> get a time) is on the order of age of the universe.)
    2. The lack of scattering of high-energy photons/particles off any proposed "space-time foam" at the Planck scale. As I gather, that intrinsically just "should be there" so it's odd to be missing. (I got an old lava lamp at yard sale yesterday: the shifting blobs remind me of what that foam was supposed to be like.) [Heh, at first I wrote "shifting blogs ..."]

    There may be some more issues, like Pioneer anomaly and residual galactic oddities, that need explaining but are those two the main "thorns" that have to be resolved?

    Kay, the URL got truncated. Try using HTML coding.

  20. Hi Neil,

    1) That the CC has a small but nonzero value is a challenge, alright, but the solution to that puzzle may or may not be in Quantum Gravity. It might equally well be something entirely different, and there's dozens of proposed explanations that you'll easily find on the arxiv (some are mentioned in my post on the CC).

    2) Photons may or may not scatter off space-time foam which may or may not be there. And if they do so, this might or might not be observable with presently possible measurement precision. I personally believe the effect to be there, but it's way too feeble to be observable. The scenarios that people propose for phenomenology, you see, have a tendency of being overly optimistic, otherwise there's no phenomenology.


  21. Hi Kay,

    Yes, I had seen the paper on the arxiv. Don't let yourself be confused by what this means. It's a non-gravitational system that mimics a gravitational system. It's not "really" gravity, just an analogy that, alright, tests a similar calculation. There's other examples of that sort in condensed matter systems. Also, note that Hawking radiation is quantum field theory in a curved background and not quantum gravity. It's a very interesting experiment, but one has to be clear about what it means and doesn't mean. Best,


  22. Here's a better link Kay to the article you post.

    Of course such analogies are useful, as if one might of considered waterfalls for such conceptual extensions?

    Swimming "up river" while recognizing a point of "no return?"

    Then, even this is in question once considering the "thought experiments of Susskind and further extensions of Seth Lloyd's?"


  23. Here is a very important distinction that should be made scrupulously when we judge potential new ideas, or old ones, in quantum gravity research.

    If the idea conflicts with current consensus within the physics community, does it:

    (a) conflict with discovered, and well-tested, physical knowledge,

    (b) conflict with invented, and incompletely-tested, abstract models,

    (c) run afoul of both (a) and (b)?

    If (a) or (c) applies then the new idea would appear to be in big trouble.

    But if only (b) applies, then the new idea should not be dismissed summarily, for obvious reasons, given the history of science and its philosophy. In this case, one should objectively weigh the merits, potential advantages and weaknesses of the new model/theory/idea against the merits and potential advantages and weaknesses of the currently accepted invented models/theories/ideas. They are all "free creations of the human mind". Nature alone decides which are correct, and which are not.

  24. I don't know whether this would be considered out of the mainstream, (as far as quantum-gravity conjectures go) or if it is being given thought within the physicist community. The idea is this: if the Planck scale changes, and there is abundant evidence in the CMB temperature being reduced as the universe expands that there is, what other phenomenological evidence might there be?

    There is much interest right now in using black hole phenomenology as a construct similar to fundamental particle formation in the early high temperature universe. This would be similar to energy compactification into a larger volume corresponding to a now larger Newtonian Constant. This would presumably entail finding gluonic and quark similarities in the central giant black holes in the middle of 90% of the known galaxies.

    Just from my own clumsy heuristic thinking, (that's all I do), wouldn't it make sense to look at that 10% of galaxies that do not contain a central BH. Not being an astrophysicist I don't know if there are any examples of those galaxies that are near enough to make good observations, nor if the gravitational haloes on the peripheries of those galaxies can be observed. But if the gravitational haloes, and corresponding changes in angular momentum of bodies on the periphery can be observed it seems to one should look and see if there are significant differences in those haloes in galaxies absent a central black hole. Right now we assume all galaxies have ideosyncratic angular momentum on the periphery of galaxies. Is that still true in these special galaxies?

    Eric Habegger

  25. I'm not sure if it was clear what I was alluding to here. The normal orbital motion of bodies in galaxies (not near the periphery)is attributable to the total mass of other massive bodies being clustered towards the center of the galaxy, this includes the central black hole. The mass of that central black hole is already included, (please correct me if I'm wrong), in the "normal" calculated angular momentum of distributed bodies in the galaxie.

    However the angular momentum of bodies at the periphery of a galaxy has more angular momentum than can be attributable to the mass of gravity from the rest of the galaxy. In fact, if one compares the activity of those bodies on the outskirts of a galaxy they are acting in a way suspiciously like asymtotic freedom does in the quark-gluon interaction. The farther those bodies get from the center of mass of the galaxy the more rigidly they are confined.

    So if galaxies without central black holes still had that same "asymtotic freedom" it would mean that the central black hole existance or non-existance had nothing to do with it and the galaxie as a whole was the valid fundamental particle on the larger G scale. That wouldn't suit very many people but it seems like it might be testable. I always tend to disdain bandwagon approaches, which is what black hole analogies to fundamental particles seems to be. There is a certain "cool" factor that may not hold up under further cosmological evidence. My understanding is that if the change in the gravitation constant DOES correlate to the change in the CMB radiation temperature with expansion of the universe the cross section length of these new particles in present day would be galaxy size and not central black hole size.

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  27. Eric, I am quite surprised to hear there's evidence (I presume from you saying "...if the Planck scale changes, and there is abundant evidence in the CMB temperature being reduced as the universe expands that there is, ...) that the Planck scale has changed. We could ask: relative to what, since not dimensionless. I'd heard some intimations that alpha (which is dimensionless, the famous ratio and inexplicably equal to about 1/137 - a value helpful to our existence but "logically ugly" ...) may have changed a little but not about L-P etc.

    Also, I have heard of residual questions about gravity (Pioneer, galaxy rotation as you mention) that some don't think that even the DE/CC can resolve (e.g., they continue to explore MOND etc.) Of course, dark matter continues to complicate things but we rather well know it must exist because of extra lensing effects and motions - it can be mapped and clings around galaxies and clusters: it is more than a homogeneous correction like the CC. So you think DM can't explain a residue of galactic motion?

  28. Hi Eric, Neil,

    There's direct constraints on the time-variation of Planck's constant from baryogenesis. Thing is that the processes depend on the mean free path of particle reactions, which in turn depends on the expansion rate, which in turn depends via Friedmann's equations on energy density times Planck's constant. Whether or not the processes are compatible with evidence thus depends on the value of Planck's constant. I believe the constraint is something like 10% or so. In any case, please note that Brans-Dicke theory realizes exactly the idea of a spatial and/or temporal variation of G. Not a new idea and one that's tightly constrained, see reviews on modifications of General Relativity. Best,


  29. Bee,
    Yes, I agree that it is not a new idea. Brans-Dicke, and I think Jordan before them, put the whole CMB radiation in context by introducing a changing G. But it seems to me that without that contextual change that they introduced about 50 years ago we still wouldn't know anything about the CMB. That part of history of the understanding of CMB is like the crazy uncle you hide in the attic that the whole family is embarrassed about. Would we really have a firm foundation to base the CMB temperature change without that? I'm not sure.

  30. Neil,
    "So you think DM can't explain a residue of galactic motion?"

    Like so many things its more a matter of both semantics and perspective. I don't think it explains it in the usual sense. Did you ever see the movie the "Usual Suspects" where towards the end one of the detectives is talking about his messy office. Paraphrasing, he says, "up close it looks like a completely disorganized mess, but if you stand back and take it in it all in then it makes sense."

    The galaxy IS the dark matter. Their existance at the later stage of the evolution of the universe corresponds to baryogenesis at higher temperatures. The gravitational force overwhelms and hides the asymptotic freedom of bodies closer in to the center of mass of the galaxies. As the gravity weakens farther out the asymptotic freedom confinement becomes stronger and is allowed to become evident.

    Just my humble opinion (and I'm sure many will complain that it's not as humble as it should be).:)



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