- For the public
- Teachers and pupils
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 macroscopic energy 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, issues related to cosmic ray mass composition, and other open questions and future prospects, including the role of secondary gamma-rays and neutrinos produced in primary cosmic ray interactions.