On the fate of Lorentz symmetry in loop quantum gravity and noncommutative spacetimes

I analyze the deformation of Lorentz symmetry that holds in certain noncommutative spacetimes and the way in which Lorentz symmetry is broken in other noncommutative spacetimes. I also observe that discretization of areas does not necessarily require departures from Lorentz symmetry. This is due to the fact that Lorentz symmetry has no implications for exclusive measurement of the area of a surface, but it governs the combined measurements of the area and the velocity of a surface. In a quantum-gravity theory Lorentz symmetry can be consistent with area discretization, but only when the observables ``area of the surface" and "velocity of the surface" enjoy certain special properties. I argue that the status of Lorentz symmetry in the loop-quantum-gravity approach requires careful scrutiny, since areas are discretized within a formalism that, at least presently, does not include an observable "velocity of the surface". In general it may prove to be very difficult to reconcile Lorentz symmetry with area discretization in theories of canonical quantization of gravity, because a proper description of Lorentz symmetry appears to require that the fundamental/primary role be played by the surface's world-sheet, whose "projection" along the space directions of a given observer describes the observable area, whereas the canonical formalism only allows the introduction as primary entities of observables defined at a fixed (common) time, and the observers that can be considered must share that time variable.Comment: 59 pages, LaTe

Exotic Acceleration Processes and Fundamental Physics

Gamma-ray bursts and ultra-high-energy cosmic rays provide an important testing ground for fundamental physics. A simple-minded analysis of some gamma-ray bursts would lead to a huge estimate of the overall energy emitted, and this represents a potential challenge for modelling the bursts. Some cosmic rays have been observed with extremely high energies, and it is not easy to envision mechanisms for the acceleration of particles to such high energies. Surprisingly some other aspects of the analysis of gamma-ray bursts and ultra-high-energy cosmic rays, even before reaching a full understanding of the mechanisms that generate them, can already be used to explore new ideas in fundamental physics, particularly for what concerns the structure of spacetime at short (Planckian) distance scales.Comment: 5 pages, LaTex; Brief overview of the contributions to the section "Exotic Acceleration Processes and Fundamental Physics" of the Huntsville Workshop monograph (proceedings) "Particle Acceleration in Astrophysical Plasmas: Geospace and Beyond

A perspective on Quantum Gravity Phenomenology

I give a brief overview of some Quantum-Gravity-Phenomenology research lines, focusing on studies of cosmic rays and gamma-ray bursts that concern the fate of Lorentz symmetry in quantum spacetime. I also stress that the most valuable phenomenological analyses should not mix too many conjectured new features of quantum spacetime, and from this perspective it appears that it should be difficult to obtain reliable guidance on the quantum-gravity problem from the analysis of synchrotron radiation from the Crab nebula and from the analysis of phase coherence of light from extragalactic sources. Forthcoming observatories of ultra-high-energy neutrinos should provide several opportunities for clean tests of some simple hypothesis for the short-distance structure of spacetime. In particular, these neutrino studies, and some related cosmic-ray studies, should provide access to the regime $E>\sqrt{m E_p}$.Comment: 15 pages, LaTex. These notes provided the basis for the ``summary talk" which I gave as chairman of the QG1 session (``Quantum Gravity Phenomenology") at the "10th Marcel Grossmann Meeting on General Relativity" (Rio de Janeiro, July 20-26, 2003). V2: Additional remarks (especially on synchrotron radiation) and additional reference

Planck-scale structure of spacetime and some implications for astrophysics and cosmology

I briefly review some scenarios for the role of the Planck length in quantum gravity. In particular, I examine the differences between the schemes in which quantum gravity is expected to introduce a maximum acceleration and the schemes in which the Planck length sets the minimum value of wavelengths (maximum value of momentum). I also comment on some pictures for the structure of spacetime at the Planck scale, such as spacetime discretization and spacetime noncommutativity. I stress that some of these proposals can have significant implications in astrophysics and cosmology.Comment: 9 pages, LaTex. Invited talk at ``Thinking, Observing and Mining the Universe", Sorrento, Italy, September 22-27, 2003 (to appear in the proceedings). V2: Paper unchanged, some references adde

Building a case for a Planck-scale-deformed boost action: the Planck-scale particle-localization limit

"Doubly-special relativity" (DSR), the idea of a Planck-scale Minkowski limit that is still a relativistic theory, but with both the Planck scale and the speed-of-light scale as nontrivial relativistic invariants, was proposed (gr-qc/0012051) as a physics intuition for several scenarios which may arise in the study of the quantum-gravity problem, but most DSR studies focused exclusively on the search of formalisms for the description of a specific example of such a Minkowski limit. A novel contribution to the DSR physics intuition came from a recent paper by Smolin (hep-th/0501091) suggesting that the emergence of the Planck scale as a second nontrivial relativistic invariant might be inevitable in quantum gravity, relying only on some rather robust expectations concerning the semiclassical approximation of quantum gravity. I here attempt to strengthen Smolin's argument by observing that an analysis of some independently-proposed Planck-scale particle-localization limits, such as the "Generalized Uncertainty Principle" often attributed to string theory in the literature, also suggests that the emergence of a DSR Minkowski limit might be inevitable. I discuss a possible link between this observation and recent results on logarithmic corrections to the entropy-area black-hole formula, and I observe that both the analysis here reported and Smolin's analysis appear to suggest that the examples of DSR Minkowski limits for which a formalism has been sought in the literature might not be sufficiently general. I also stress that, as we now contemplate the hypothesis of a DSR Minkowski limit, there is an additional challenge for those in the quantum-gravity community attributing to the Planck length the role of "fundamental length scale".Comment: 12 pages, LaTe

Phenomenology of Philosophy of Science: OPERA data

I observe that, as the physics side of the OPERA-anomaly story is apparently unfolding, there can still be motivation for philosophy of science to analyze the six months of madness physicists spent chasing the dream of a new fundamental-physics revolution. I here mainly report data on studies of the OPERA anomaly that could be relevant for analyses from the perspective of phenomenology of philosophy of science. Most of what I report is an insider's perspective on the debate that evolved from the original announcement by the OPERA collaboration of evidence of superluminal neutrinos. I also sketch out, from a broader perspective, some of the objectives I view as achievable for the phenomenology of philosophy of science.Comment: 13 pages, LaTe

Are we at the dawn of quantum-gravity phenomenology?

A handful of recent papers has been devoted to proposals of experiments capable of testing some candidate quantum-gravity phenomena. These lecture notes emphasize those aspects that are most relevant to the questions that come to mind when one is exposed for the first time to these research developments: How come theory and experiments are finally meeting in spite of all the gloomy forecasts that pervade traditional reviews? Is this a case of theorists having put forward more and more speculative ideas until a point was reached at which conventional experiments could rule out the proposed phenomena? Or has there been such a remarkable improvement in experimental techniques and ideas that we are now capable of testing plausible candidate quantum-gravity phenomena? These questions are analysed rather carefully for the recent proposals of interferometry-based tests and tests using observations of gamma rays of astrophysical origin. I also briefly discuss other proposed experiments (including tests of quantum-gravity-induced decoherence using the neutral-kaon system and accelerator tests of models with large extra dimensions). The emerging picture suggests that we are finally starting the exploration of a large class of plausible quantum-gravity effects. However, our chances to obtain positive (discovery) experimental results depend crucially on the magnitude of these effects. In most cases the level of sensitivity that the relevant experiments should achieve within a few years corresponds to effects suppressed only linearly by the Planck length.Comment: 47 pages, Latex. Based on lectures given at the XXXV Karpacz Winter School of Theoretical Physics "From Cosmology to Quantum Gravity", Polanica, Poland, 2-12 February, 1999. To appear in the proceeding

Doubly Special Relativity

I give a short non-technical review of the results obtained in recent work on "Doubly Special Relativity", the relativistic theories in which the rotation/boost transformations between inertial observers are characterized by two observer-independent scales (the familiar velocity scale, $c$, and a new observer-independent length/momentum scale, naturally identified with the Planck length/momentum). I emphasize the aspects relevant for the search of a solution to the cosmic-ray paradox.Comment: Invited piece for the N&V section of Nature. 5 pages, LaTe

Status of Relativity with observer-independent length and velocity scales

I have recently shown that it is possible to formulate the Relativity postulates in a way that does not lead to inconsistencies in the case of space-times whose structure is governed by observer-independent scales of both velocity and length. Here I give an update on the status of this proposal, including a brief review of some very recent developments. I also emphasize the role that one of the kappa-Poincare' Hopf algebras could play in the realization of a particular example of the new type of postulates. I show that the new ideas on Relativity require us to extend the set of tools provided by kappa-Poincare' and to revise our understanding of certain already available tools, such as the energy-momentum coproduct.Comment: 24 pages, Latex (to appear in proceedings of 37th Karpacz Winter School