Approaches to Quantum Gravity

22Quantum Gravity phenomenologyG. AMELINO-CAMELIA22.1 The “Quantum Gravity problem”, as seen by a phenomenologistThe “Quantum Gravity problem” has been discussed for more than 70 years [1]assuming that no guidance could be obtained from experiments. But of course ifthere is to be a science of the Quantum Gravity problem it must be treated justlike any other scientific problem, seeking desperately the guidance of experimentalfacts, and letting those facts take the lead in the development of new concepts. Wemust hope this works also for the Quantum Gravity problem, or else abandon it tothe appetites of philosophers.Unfortunately it is not unlikely that experiments might never give us any clearlead toward Quantum Gravity, especially if our intuition concerning the role of thetiny Planck length (∼ 10 −35 m) in setting the magnitude of the characteristic effectsof the new theory turns out to be correct. But even if the new effects were really sosmall we could still try to uncover experimentally some manifestations of QuantumGravity. This is hard, and there is no guarantee of success, but we must try.Our estimate that the Quantum Gravity corrections should be very small in lowenergyexperiments is based on our experience with other similar situations; in fact,we expect that the Planck scale, since it is the energy scale where the current theoriesappear to break down, should also govern the magnitude of Quantum Gravitycorrections to the analysis of processes involving particles with energies smallerthan the Planck scale. For example, in processes involving two particles both withenergy E the magnitude of the new effects should be set by some power of theratio between E and the Planck scale E p (∼ 10 28 eV). Since in all cases accessibleto us experimentally E/E p is extremely small, this is a key challenge for QuantumGravity phenomenology. This is a challenge which, however, can be dealt with byrelying on experience with other analogous situations in physics. 11 As I emphasized elsewhere [2; 3], ongoing studies of proton stability from the grand unification perspectiveand early 1900s studies of Brownian motion could be characterized by a very similar challenge.Approaches to Quantum Gravity: Toward a New Understanding of Space, Time and Matter, ed. Daniele Oriti.Published by Cambridge University Press. c○ Cambridge University Press 2009.