  Gamma-ray bursts and ultra-high-energy cosmic rays provide an important
testing ground for fundamental physics. A simple-minded analysis of some
gamma-ray bursts would lead to a huge estimate of the overall energy emitted,
and this represents a potential challenge for modelling the bursts. Some cosmic
rays have been observed with extremely high energies, and it is not easy to
envision mechanisms for the acceleration of particles to such high energies.
Surprisingly some other aspects of the analysis of gamma-ray bursts and
ultra-high-energy cosmic rays, even before reaching a full understanding of the
mechanisms that generate them, can already be used to explore new ideas in
fundamental physics, particularly for what concerns the structure of spacetime
at short (Planckian) distance scales.
