Approaches to Quantum Gravity

428 G. Amelino-CameliaFor this phenomenology the key concern for a long time has not been the oneof development but rather the one of articulating a basic claim of existence. Theresults of this effort allow me to provide, in this section, robust evidence that wecan really do Quantum Gravity phenomenology.Then Section 22.2 presents a (incomplete but representative) list of effects thatshould be considered as candidate Quantum Gravity effects, and in Section 22.3 Ibriefly describe the experiments and/or observations which are being analyzed asopportunities to provide related insight.The rest of this chapter focuses on the most studied area of Quantum Gravityphenomenology, the one that concerns the possibility of Planck-scale departuresfrom Poincaré (Lorentz) symmetry. It starts with a small aside (Section 22.4) ondoubly special relativity, which I describe (as originally proposed) as a scenario forPlanck-scale physics, rather than one or another choice of formalism. And I showthat the doubly special relativity idea can be falsified, a rare example of a falsifiableproposal for Planck-scale physics.Section 22.5 may be used as a compact point of entry to the literature on the phenomenologyof Planck-scale departures from Poincaré (Lorentz) symmetry. I donot give detailed accounts but I try to touch on a representative subset of the ideasthe community is pursuing, and in doing so I try to show as clearly as possible howimportant it is to rely on some carefully tailored, commonly adopted, test theoriesin mapping the progress of this phenomenology.Some closing remarks are offered in Section 22.6.22.1.1 Quantum Gravity phenomenology existsTask number one for any phenomenology (usually an easy task but a challengingone here) is to show that effects of the type that could be expected from the relevantclass of theories could be seen. The key source of pride for Quantum Gravity phenomenologistscomes from the fact that, over the past few years, and over a timethat indeed spanned only a handful of years, we managed to change the perceptionof Quantum Gravity research from the traditional “no help from experiments possible”to the present intuition, shared by most workers in the field, that these effectscould be seen. We might need some luck actually to see them, but clearly it is notimplausible. There is a legitimate phenomenology to be developed here.Once task one is accomplished it is important to show that the type of observationsthat are doable not only provide opportunities to luckily stumble upon amanifestation of the new theory, but actually the data could be used to falsify candidatetheories. This task two clearly requires much more of task one, both at thelevel of our understanding of the theories and for what concerns the quality of thedata and their phenomenological analysis.