The Enigma of the Highest Energy Particles in the Universe

Cosmic rays have been observed up to several 10^20 eV through the showers of secondary particles they induce in the atmosphere. This is a macroscopicenergy of up to 50 Joules, presumably in one elementary particle. The existence of such particles and their propagation through the highly structured Universe pose formidable challenges and exciting prospects at the same time:Their origin and  sources have not been identified yet, but they already allow to test physics at  center of mass energies unattained in the laboratory, albeit in a rather indirect way. We will give an overview over possible sources and acceleration mechanisms, propagation in a structured Universe, and how the question of cosmic ray mass composition is connected to the particle physics of hadronic interactions. We also briefly address multi messenger aspects and future prospects related to secondary gamma-rays and neutrinos produced in primary cosmic ray interactions.   Speaker: Günter Sigl (Hamburg University)