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A perspective on Quantum Gravity Phenomenology1

Giovanni Amelino-Camelia

aDipart. Fisica, Univ. Roma \La Sapienza” and INFN Sez. Roma1

P.le Moro 2, 00185 Roma, Italy

ABSTRACT

I give a brief overview of some Quantum-Gravity-Phenomenology research lines, focusing on studies of cosmic rays and gamma-ray bursts that concern the fate of Lorentz symmetry in quantum spacetime. I also stress that the most valuable phenomenological analyses should not mix too many conjectured new features of quantum spacetime, and from this perspective it appears that it should be dicult to obtain reliable guidance on the quantum-gravity

problem from the analysis of synchrotron radiation from the Crab nebula and from the analysis of phase coherence of light from extragalactic sources. Forthcoming observatories of ultra-high-energy neutrinos should provide several opportunities for clean tests of some simple hypothesis for the short-distance structure of spacetime. In particular, these neutrino studies, and some related cosmic-ray studies, should provide access to the regime E> mEp.

1 Quantum Gravity Phenomenology

Quantum-gravity research used to be completely detached from experiments. The horrifying smallness of the expected quantum-gravity eects, due to the overall suppression by powers

of the ratio of the Planck length (Lp ~ 10-35m) versus the characteristic wavelength of the particles involved in the process, had led to the conviction that experiments could never

possibly help. But recently there has been a sharp change in the attitude of a signicant fraction of the quantum-gravity community. This is reflected also by the tone of recent

quantum-gravity reviews (see, e.g., Refs. [[1,2,3,4])]as compared to the tone of quantum gravity reviews published up to the mid 1990s (see, e.g., Ref. [[5, 6]]).

The fact that the smallness of an eect does not necessarily imply that it cannot be studied experimentally is not actually a new idea. There are several examples in physics, and even

remaining in the context of fundamental physics there is the noteworthy example of studies of the prediction of proton decay within certain grandunied theories of particle physics.

The predicted proton-decay probability is really small, suppressed by the fourth power of the ratio between the mass of the proton and the grandunication scale [mproton=Egut]4 ~ 10-64 ,

but in spite of this truly horrifying suppression, with a simple idea we have managed to acquire an excellent sensitivity to the new eect. The proton lifetime predicted by grandunied theories is of order 1039s and \quite a few” generations of physicists should invest their

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