Observed threshold anomalies as the first hope of a manifestation of Planck-length physics

Abstract

The observations of photons from the BL Lac object Mk501 with energies above 10 TeV and of cosmic rays with energies above the GZK threshold appear to be inconsistent with conventional theories. Remarkably, among the recent new-physics proposals of solutions of these threshold paradoxes a prominent role has been played by proposals based on quantum properties of space-time. While the experimental evidence (and theory work attempting to interpret it) is much too preliminary to justify any serious hopes that we might have stumbled upon the first manifestation of a "quantum gravity", the fact that for the first time phenomenological models involving quantum-gravity ideas are competing on level ground with other new-physics proposals clearly marks the beginning of a new stage of quantum-gravity research. I emphasize one important aspect of this new phenomenology: combining the determination of the relevant thresholds with data on the time/energy structure of gamma-ray bursts it is possible to distinguish between alternative quantum-gravity scenarios. This point is illustrated focusing on 3 specific scenarios: dispersion-inducing space-time foam, string-theory-motivated non-commutative space-time, and this author's recent proposal of a relativistic theory in which the Planck length has the role of fundamental observer-independent minimum length.