Cosmic Rays with 50 Joules of Energy: What, How, and from Where?

The observation of cosmic ray air showers above 10[sup 20] eV poses three serious challenges for particle astrophysics: (i) how the primary particles are accelerated to these fantastic multi-Joule energies, (ii) how the primaries evade energy loss while crossing a Universe filled with radiation, and (iii) why events appear to cluster in direction. The data is briefly reviewed. Then some novel solutions to these problems, generally involving new physics, are explored. Emphasis is placed on the signatures by which cosmic ray experiments will discriminate among proposed solutions, and the role neutrinos may play in resolving puzzles. [ABSTRACT FROM AUTHOR]