THE SOFT AND THE HARD WAY TO LOOK FOR THE OLD QUANTUM THEORY OF GRAVITY

摘要

Toward the end of the 1990s it was fashionable to argue that within 10 or even 5 years there would have been a fully satisfactory solution of the quantum-gravity problem, but it is still not in sight. I here take as working assumption that the path to Quantum Gravity will not consist of a single huge conceptual step, but it will rather be a path analogous to the one that took us to Quantum Mechanics only after a long and disorienting multi-step process, the first three decades of the 20th century which we now label as the phase of the "Old Quantum Theory". In order for us to gain access to such a phase of "Old Quantum-Gravity Theory" phenomenology should necessarily take the lead, and I here comment on a few quantum-gravity-phenomenology research programs that at least should be viewed as illustrative examples of what we can realistically try. And I stress that, in addition to the better known phenomenology inspired by "ultraviolet aspects" of the quantum-gravity problem, the possibility of "ultraviolet/infrared mixing" should invite us to also consider the opportunities provided by the fundamental-physics "infrared" studies performed at some particle decelerators.