Physics opportunities above the Greisen-Zatsepin-Kuzmin cutoff: Lorentz symmetry violation at the Planck scale

Special relativity has been tested at low energy with great accuracy, but these results cannot be extrapolated to very high-energy phenomena: this new domain of physics may actually provide the key to the, yet unsettled, question of the ether and the absolute rest frame. Introducing a critical distance scale, a, below $10−25 cm$ (the wavelength scale of the highest-energy observed cosmic rays) allows to consider models, compatible with standard tests of special relativity, where a small violation of Lorentz symmetry (a can, for instance, be the Planck length) leads to a deformed relativistic kinematics (DRK) producing dramatic effects on the properties of very high-energy cosmic rays. For instance, the Greisen-Zatsepin-Kuzmin (GZK) cutoff does no longer apply and particles which are unstable at low energy (neutron, some hadronic resonances like the $Δ++,$ possibly several nuclei…) become stable at very high energy. In these models, an absolute local rest frame exists (the vacuum rest frame, VRF) and special relativity is a low-momentum limit. We discuss the possible effects of Lorentz symmetry violation (LSV) on kinematics and dynamics, as well as the cosmic-ray energy range (well below the energy scale associated to the fundamental length) and experiments (on earth and from space) where they could be detected.