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 $5\times10^{19}$ 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.Comment: Latex2e with ws-p10x7.cls (included), 14 pages, 10 postscript figures. Proceedings of the Lepton-Photon Symposium, Rome, July 200

Multi-Messenger Astronomy: Cosmic Rays, Gamma-Rays, and Neutrinos

Although cosmic rays were discovered a century ago, we do not know where or how they are accelerated. There is a realistic hope that the oldest problem in astronomy will be solved soon by ambitious experimentation: air shower arrays of 10,000 kilometer-square area, arrays of air Cerenkov telescopes and kilometer- scale neutrino observatories. Their predecessors are producing science. We will review the highlights: - Cosmic rays: the highest energy particles and the GZK cutoff, the search for cosmic accelerators and the the Cygnus region, top-down mechanisms: photons versus protons? - TeV-energy gamma rays: blazars, how molecular clouds may have revealed proton beams, first hints of the diffuse infrared background? - Neutrinos: first results and proof of concept for technologies to construct kilometer-scale observatories.Comment: 26 pages, Latex2e with ws-procs9x6.cls (included), 13 postscript illustrations (placed using graphicx.sty). Talk presented at "Texas in Tuscany", 21st Symposium oon Relavitistic Astrophysics, Florence, Italy, Dec. 200