Cosmic rays: the most energetic particles in the universe

Cosmic rays are an ever present aspect of nature. The birth of the field of elementary-particle physics can be traced to studies of cosmic rays. Now advances in technology and new instrumentation are changing the nature of cosmic-ray research. New forms of astronomy are being created. Ground-based instruments, spawned by cosmic-ray techniques, permit the observation of astrophysical objects emitting radiation in very-high-energy gamma rays, $(>~100\mathrm{GeV}),$ high-energy neutrinos (\ensuremath{\geqslant}1 TeV), and the most energetic particles found in the cosmic radiation $(>~5\ifmmode\times\else\texttimes\fi{}{10}^{19}\mathrm{eV}).$ At these energies the galactic and intergalactic magnetic fields deflect the cosmic-ray protons by only a few degrees. The interaction of these cosmic rays with the cosmic background radiation limits the possible sources to redshifts far less than unity. The origin of these highest-energy cosmic rays is not understood. The present status of knowledge of these cosmic rays and the prospects for solving the mystery concerning their origin are the subjects of this brief article.