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Quantum-Spacetime Phenomenology Living Rev. Relativity, 16, (2013), 5 ,)6).' 2%6)%73 http://www.livingreviews.org/lrr-2013-5 doi:10.12942/lrr-2013-5 INRELATIVITY Quantum-Spacetime Phenomenology Giovanni Amelino-Camelia Dipart. Fisica Univ. La Sapienza and Sez. Roma1 INFN P.le Moro 2, I-00185 Roma, Italy email: [email protected] http://www.roma1.infn.it/~amelino/ Accepted: 18 May 2013 Published: 12 June 2013 Abstract I review the current status of phenomenological programs inspired by quantum-spacetime research. I stress in particular the significance of results establishing that certain data analyses provide sensitivity to effects introduced genuinely at the Planck scale. My main focus ison phenomenological programs that affect the directions taken by studies of quantum-spacetime theories. Keywords: quantum spacetime, loop quantum gravity, spacetime noncommutativity This review is licensed under a Creative Commons Attribution-Non-Commercial 3.0 Germany License. http://creativecommons.org/licenses/by-nc/3.0/de/ Imprint / Terms of Use Living Reviews in Relativity is a peer reviewed open access journal published by the Max Planck Institute for Gravitational Physics, Am M¨uhlenberg 1, 14476 Potsdam, Germany. ISSN 1433-8351. This review is licensed under a Creative Commons Attribution-Non-Commercial 3.0 Germany License: http://creativecommons.org/licenses/by-nc/3.0/de/. Figures that have been pre- viously published elsewhere may not be reproduced without consent of the original copyright holders. Because a Living Reviews article can evolve over time, we recommend to cite the article as follows: Giovanni Amelino-Camelia, \Quantum-Spacetime Phenomenology", Living Rev. Relativity, 16, (2013), 5. URL (accessed <date>): http://www.livingreviews.org/lrr-2013-5 The date given as <date> then uniquely identifies the version of the article you are referring to. Article Revisions Living Reviews supports two ways of keeping its articles up-to-date: Fast-track revision A fast-track revision provides the author with the opportunity to add short notices of current research results, trends and developments, or important publications to the article. A fast-track revision is refereed by the responsible subject editor. If an article has undergone a fast-track revision, a summary of changes will be listed here. Major update A major update will include substantial changes and additions and is subject to full external refereeing. It is published with a new publication number. For detailed documentation of an article's evolution, please refer to the history document of the article's online version at http://www.livingreviews.org/lrr-2013-5. Contents 1 Introduction and Preliminaries 7 1.1 The \Quantum-Gravity problem" as seen by a phenomenologist ........... 7 1.2 Quantum spacetime vs quantum black hole and graviton exchange . 9 1.2.1 The quantum-black-hole regime ......................... 10 1.2.2 The graviton-exchange regime .......................... 11 1.2.3 The quantum-spacetime regime ......................... 11 1.2.4 Aside on the classical-gravity regime ...................... 12 1.3 20th century quantum-gravity phenomenology ..................... 12 1.4 Genuine Planck-scale sensitivity and the dawn of quantum-spacetime phenomenology 14 1.5 A simple example of genuine Planck-scale sensitivity . 16 1.6 Focusing on a neighborhood of the Planck scale .................... 17 1.7 Characteristics of the experiments ........................... 20 1.8 Paradigm change and test theories of not everything . 20 1.9 Sensitivities rather than limits ............................. 22 1.10 Other limitations on the scope of this review ..................... 23 1.11 Schematic outline of this review ............................. 23 2 Quantum-Gravity Theories, Quantum Spacetime, and Candidate Effects 25 2.1 Quantum-Gravity Theories and Quantum Spacetime . 25 2.1.1 Critical String Theory .............................. 25 2.1.2 Loop Quantum Gravity ............................. 27 2.1.3 Approaches based on spacetime noncommutativity . 28 2.1.4 Other proposals .................................. 29 2.2 Candidate effects ..................................... 30 2.2.1 Planck-scale departures from classical-spacetime symmetries . 31 2.2.2 Planck-scale departures from CPT symmetry . 32 2.2.3 Decoherence and modifications of the Heisenberg principle . 32 2.2.4 Distance fuzziness and spacetime foam ..................... 33 2.2.5 Planck-scale departures from the equivalence principle . 33 3 Quantum-Spacetime Phenomenology of UV Corrections to Lorentz Symmetry 34 3.1 Some relevant concepts .................................. 34 3.1.1 The Minkowski limit ............................... 34 3.1.2 Three perspectives on the fate of Lorentz symmetry at the Planck scale . 35 3.1.3 Aside on broken versus deformed spacetime symmetries . 36 3.2 Preliminaries on test theories with modified dispersion relation . 38 3.2.1 With or without standard quantum field theory? . 39 3.2.2 Other key features of test theories with modified dispersion relation . 41 3.2.3 A test theory for pure kinematics ........................ 41 3.2.4 A test theory based on low-energy effective field theory . 42 3.2.5 More on \pure-kinematics" and “field-theory-based” phenomenology . 44 3.3 Photon stability ...................................... 45 3.3.1 Photon stability and modified dispersion relations . 45 3.3.2 Photon stability and modified energy-momentum conservation . 46 3.4 Pair-production threshold anomalies and gamma-ray observations . 47 3.4.1 Modified dispersion relations and 훾훾 ! e+e− . 47 3.4.2 Threshold anomalies and modified energy-momentum conservation . 50 3.5 Photopion production threshold anomalies and the cosmic-ray spectrum . 50 3.6 Pion non-decay threshold and cosmic-ray showers ................... 52 3.7 Vacuum Cerenkov and other anomalous processes ................... 53 3.8 In-vacuo dispersion for photons ............................. 55 3.9 Quadratic anomalous in-vacuo dispersion for neutrinos . 59 3.10 Implications for neutrino oscillations .......................... 59 3.11 Synchrotron radiation and the Crab Nebula ...................... 60 3.12 Birefringence and observations of polarized radio galaxies . 61 3.13 Testing modified dispersion relations in the lab .................... 62 3.14 On test theories without energy-dependent modifications of dispersion relations . 63 4 Other Areas of UV Quantum-Spacetime Phenomenology 66 4.1 Preliminary remarks on fuzziness ............................ 66 4.2 Spacetime foam, distance fuzziness and interferometric noise . 67 4.2.1 Spacetime foam as interferometric noise .................... 68 4.2.2 A crude estimate for laser-light interferometers . 68 4.2.3 A simple-minded mechanism for noise in laser-light interferometers . 70 4.2.4 Insight already gained and ways to go beyond it . 72 4.2.5 Distance fuzzyness for atom interferometers . 74 4.3 Fuzziness for waves propagating over cosmological distances . 74 4.3.1 Time spreading of signals ............................ 74 4.3.2 Fuzziness from nonsystematic symmetry-modification effects . 75 4.3.3 Blurring images of distant sources ....................... 77 4.4 Planck-scale modifications of CPT symmetry and neutral-meson studies . 79 4.4.1 Broken-CPT effects from Liouville strings ................... 80 4.4.2 Departures from classical CPT symmetry from spacetime noncommutativity at the Planck scale ................................ 81 4.5 Decoherence studies with kaons and atoms ....................... 81 4.5.1 Spacetime foam as decoherence effects and the \훼, 훽, 훾 test theory" . 81 4.5.2 Other descriptions of foam-induced decoherence for matter interferometry . 83 4.6 Decoherence and neutrino oscillations ......................... 84 4.7 Planck-scale violations of the Pauli Exclusion Principle . 85 4.8 Phenomenology inspired by causal sets ......................... 85 4.9 Tests of the equivalence principle ............................ 87 4.9.1 Aside on tests of the equivalence principle in the semiclassical-gravity limit 87 4.9.2 On the equivalence principle in quantum spacetime . 87 5 Infrared Quantum-Spacetime Phenomenology 89 5.1 IR quantum-spacetime effects and UV/IR mixing ................... 89 5.2 A simple model with soft UV/IR mixing and precision Lamb-shift measurements . 91 5.3 Soft UV/IR mixing and atom-recoil experiments ................... 91 5.4 Opportunities for Bose{Einstein condensates ..................... 93 5.5 Soft UV/IR mixing and the end point of tritium beta decay . 93 5.6 Non-Keplerian rotation curves from quantum-gravity effects . 94 5.7 An aside on gravitational quantum wells ........................ 95 6 Quantum-Spacetime Cosmology 96 6.1 Probing the trans-Planckian problem with modified dispersion relations . 96 6.2 Randomly-fluctuating metrics and the cosmic microwave background . 97 6.3 Loop quantum cosmology ................................ 98 6.4 Cosmology with running spectral dimensions ..................... 98 6.5 Some other quantum-gravity-cosmology proposals ................... 99 6.5.1 Quantum-gravity-induced vector fields ..................... 99 6.5.2 A semiclassical Wheeler{DeWitt-based description of the early Universe . 99 6.5.3 No-singularity cosmology from string theory . 100 7 Quantum-Spacetime Phenomenology Beyond the Standard Setup 101 7.1 A totally different setup with large extra dimensions . 101 7.2 The example of hard UV/IR mixing . 102 7.3 The possible challenge of not-so-subleading higher-order terms . 102 8 Closing Remarks 104 References 105 Quantum-Spacetime Phenomenology
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