Quantum Gravity Phenomenology

I give a brief non-technical review of " Quantum Gravity Phenomenology " and in particular I describe some studies which should soon allow to establish valuable data-based constraints on the short-distance structure of spacetime. The " quantum-gravity problem " has been studied for more than 70 years, but we still do not have a single experimental result whose interpretation requires us to advocate a quantum theory of gravity. The search of a first case in which data provide some evidence of a quantum property of spacetime is at this point understandably characterized by a certain anxiety. Even the status of this research as a truly scientific endeavor could be questioned if the present lack of confrontation with experiments persists. And it does not help that one can still encounter in the literature (although it is fortunately becoming increasingly rare) some descriptions of the " quantum-gravity problem " that are not of the type that one expects in introducing a truly scientific problem. For example , as motivation for reseach in quantum gravity it is sometimes mentioned that it is unsatisfactory to have on one side our present description of electromagnetic , weak and strong forces, unified within the Standard Model of particle physics, which is a quantum field theory, and on the other side gravity described by General Relativity, which is governed by the very different rules of classical mechanics. This type of " human discomfort " , this type of urgency to correct the lack of elegance of our present worldview, does not of course define a scientific problem. But there is a well-defined scientific problem which can be naturally called " quantum-gravity problem ". This is the problem of obtaining quantitative predictions for processes in which both gravity effects and Standard-Model effects cannot be neglected. The (quantum) Standard Model of particle physics has been hugely successful in describing the microscopic phenomena involving fundamental particles, where gravity can be ignored, and (classical) General Relativity has been equally successful in describing the motions of macroscopic bodies, whose quantum and Standard-Model properties can be safely neglected. We have no available data on situations in which neither can be neglected, but we already know that we would not be able to describe those data with our current theories, because some logical and mathematical inconsistecies are encountered even before getting to the point of obtaining a numerical prediction for these processes. Like two …