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"Quantum-Spacetime Phenomenology"

Giovanni Amelino-Camelia

	Abstract
1	Introduction and Preliminaries
	1.1	The “Quantum-Gravity problem” as seen by a phenomenologist
	1.2	Quantum spacetime vs quantum black hole and graviton exchange
	1.3	20th century quantum-gravity phenomenology
	1.4	Genuine Planck-scale sensitivity and the dawn of quantum-spacetime phenomenology
	1.5	A simple example of genuine Planck-scale sensitivity
	1.6	Focusing on a neighborhood of the Planck scale
	1.7	Characteristics of the experiments
	1.8	Paradigm change and test theories of not everything
	1.9	Sensitivities rather than limits
	1.10	Other limitations on the scope of this review
	1.11	Schematic outline of this review
2	Quantum-Gravity Theories, Quantum Spacetime, and Candidate Effects
	2.1	Quantum-Gravity Theories and Quantum Spacetime
	2.2	Candidate effects
3	Quantum-Spacetime Phenomenology of UV Corrections to Lorentz Symmetry
	3.1	Some relevant concepts
	3.2	Preliminaries on test theories with modified dispersion relation
	3.3	Photon stability
	3.4	Pair-production threshold anomalies and gamma-ray observations
	3.5	Photopion production threshold anomalies and the cosmic-ray spectrum
	3.6	Pion non-decay threshold and cosmic-ray showers
	3.7	Vacuum Cerenkov and other anomalous processes
	3.8	In-vacuo dispersion for photons
	3.9	Quadratic anomalous in-vacuo dispersion for neutrinos
	3.10	Implications for neutrino oscillations
	3.11	Synchrotron radiation and the Crab Nebula
	3.12	Birefringence and observations of polarized radio galaxies
	3.13	Testing modified dispersion relations in the lab
	3.14	On test theories without energy-dependent modifications of dispersion relations
4	Other Areas of UV Quantum-Spacetime Phenomenology
	4.1	Preliminary remarks on fuzziness
	4.2	Spacetime foam, distance fuzziness and interferometric noise
	4.3	Fuzziness for waves propagating over cosmological distances
	4.4	Planck-scale modifications of CPT symmetry and neutral-meson studies
	4.5	Decoherence studies with kaons and atoms
	4.6	Decoherence and neutrino oscillations
	4.7	Planck-scale violations of the Pauli Exclusion Principle
	4.8	Phenomenology inspired by causal sets
	4.9	Tests of the equivalence principle
5	Infrared Quantum-Spacetime Phenomenology
	5.1	IR quantum-spacetime effects and UV/IR mixing
	5.2	A simple model with soft UV/IR mixing and precision Lamb-shift measurements
	5.3	Soft UV/IR mixing and atom-recoil experiments
	5.4	Opportunities for Bose–Einstein condensates
	5.5	Soft UV/IR mixing and the end point of tritium beta decay
	5.6	Non-Keplerian rotation curves from quantum-gravity effects
	5.7	An aside on gravitational quantum wells
6	Quantum-Spacetime Cosmology
	6.1	Probing the trans-Planckian problem with modified dispersion relations
	6.2	Randomly-fluctuating metrics and the cosmic microwave background
	6.3	Loop quantum cosmology
	6.4	Cosmology with running spectral dimensions
	6.5	Some other quantum-gravity-cosmology proposals
7	Quantum-Spacetime Phenomenology Beyond the Standard Setup
	7.1	A totally different setup with large extra dimensions
	7.2	The example of hard UV/IR mixing
	7.3	The possible challenge of not-so-subleading higher-order terms
8	Closing Remarks
	References
	Footnotes
	Figures

It used to be natural to expect [111 ] that indeed the highest energy cosmic rays are protons. However, this is changing rather rapidly in light of recent dedicated studies using Auger data [7 , 242

, 509

], which favor a significant contribution from heavy nuclei. The implications for the Lorentz symmetry analysis of the differences between protons and heavy nuclei, while significant in the detail (see, e.g., Ref. [488 ]), are not as large as one might naively expect. This is due to the fact that it just happens to be the case that the photodisintegration threshold is reached when the energy of typical heavy nuclei, such as Fe, is $∼ 5⋅1019 eV$ , i.e., just about the value of the photopion-production threshold, expected for cosmic-ray protons.