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...