Over the last 15 years there has been considerable interest in the possibility of quantum-gravityinduced in-vacuo dispersion, the possibility that spacetime itself might behave essentially like a dispersive medium for particle propagation. Two very recent studies have exposed what might be in-vacuo dispersion features for GRB (gamma-ray-burst) neutrinos of energy in the range of 100 TeV and for GRB photons with energy in the range of 10 GeV. We here show that these two features are roughly compatible with a description such that the same effects apply over 4 orders of magnitude in energy. We also characterize quantitatively how rare it would be for such features to arise accidentally, as a result of (still unknown) aspects of the mechanisms producing photons at GRBs or as a result of background neutrinos accidentally fitting the profile of a GRB neutrino affected by in-vacuo dispersion.

We report a general analysis of worldlines for theories with deformed relativistic symmetries and momentum dependence of the speed of photons. Our formalization is faithful to Einstein's program, with spacetime points viewed as an abstraction of physical events. The emerging picture imposes the renunciation of the idealization of absolutely coincident events, but is free from some pathologies which had been previously conjectured.

Two recent publications have reported intriguing analyses, tentatively suggesting that some aspects of IceCube data might be manifestations of quantum-gravity-modified laws of propagation for neutrinos. We here propose a strategy of data analysis which has the advantage of being applicable to several alternative possibilities for the laws of propagation of neutrinos in a quantum spacetime. In all scenarios here of interest one should find a correlation between the energy of an observed neutrino and the difference between the time of observation of that neutrino and the trigger time of a GRB. We select accordingly some GRB-neutrino candidates among IceCube events, and our data analysis finds a rather strong such correlation. This sort of studies naturally lends itself to the introduction of a "false alarm probability", which for our analysis we estimate conservatively to be of 1%. We therefore argue that our findings should motivate a vigorous program of investigation following the strategy here advocated.Comment: In V2 the possible helicity dependence of the effects (only mentioned in a footnote of V1) is discussed in the main text, and we give an estimate of possible values the parameters could take in order to produce the feature found in the dat

We investigate a connection between recent results in 3D quantum gravity, providing an effective noncommutative-spacetime description, and some earlier heuristic descriptions of a quantum-gravity contribution to the fuzziness of the worldlines of particles. We show that 3D-gravity-inspired spacetime noncommutativity reflects some of the features suggested by previous heuristic arguments. Most notably, gravity-induced worldline fuzziness, while irrelevantly small on terrestrial scales, could be observably large for propagation of particles over cosmological distances.Gravitational phenomena weigh on our daily lives very noticeably, but are the phenomena whose description is most unknown at subatomic scales. A fair assessment of the present situation is that we have access to non-gravitational phenomena down to distance scales of the order of 10 −20 m (LHC scales) whereas we have so far gained access to gravitational phenomena only at scales no smaller than 10 −6 m. The challenge of quantum-gravity research is accordingly overwhelming: we have apparently solid indirect evidence (see, e.g., Refs. [1, 2]) of the necessity of a new quantum theory of both gravitational and non-gravitational phenomena with onset at a scale of the order of the minute Planck length ℓ P (∼ 10 −35 m), but any experimental guidance we could seek for attempting to describe this new realm of physics only concerns much larger distance scales.Over the last decade there has been a determined effort [2,3] attempting to improve this state of affairs by using the whole Universe as a laboratory. We focus here on an intriguing example of how this might work out, in investigations of the "spacetime-foam" scenario first discussed by John Wheeler in the 1960s [4] (also see Refs. [5-8]). In some recent studies, such as those in Refs. [9][10][11][12][13][14], the spacetimefoam intuition has guided efforts aimed at characterizing gravity-induced contributions to the "fuzziness" of the worldlines of particles. One attempts to describe the dynamics of matter particles as effectively occurring in an "environment" of short-distance quantum-gravitational degrees of freedom. And it is expected that for propagating particles with wavelength much larger than the Planck length, when it may be appropriate to integrate out these quantum-gravitational degrees of freedom, the main residual effect of short-distance gravity would indeed be an additional contribution to the fuzziness of worldlines. The idea that this might lead to testable predictions originates from heuristic arguments [9][10][11][12][15][16][17][18] suggesting that these quantum-gravity effects should grow with propagation distance. In particular this could produce an observably-large contribution to the blurring of the images of distant astrophysical sources, such as quasars [17,18].We here do not review the relevant heuristic arguments. Actually our starting point is the realization that heuristics was surely valuable for inspiring this phenomenological program, but has run out of steam as a resource ...