The Highest Energy Cosmic Rays, Gamma Rays and Neutrinos: Facts, Fancy and Resolution

Although cosmic rays were discovered 90 years ago, we do not know how and where they are accelerated. There is compelling evidence that the highest energy cosmic rays are extra-galactic – they cannot be contained by our galaxy’s magnetic field anyway because their gyroradius exceeds its dimensions. Elementary elementary-particle physics dictates a universal upper limit on their energy of $5\times {10}^{19}$ eV, the so-called Greisen-Kuzmin-Zatsepin cutoff; however, particles in excess of this energy have been observed, adding one more puzzle to the cosmic ray mystery. Mystery is nonetheless fertile ground for progress: we will review the facts and mention some very speculative interpretations. There is indeed a realistic hope that the oldest problem in astronomy will be resolved soon by ambitious experimentation: air shower arrays of ${10}^4$ km${}^2$ area, arrays of air Cerenkov detectors and kilometer-scale neutrino observatories.