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The origin of cosmic rays

Discovered in 1912, cosmic rays are still proving an enigma. Where exactly do they come from? Although at `low' energies, say below 1011 eV, it is pretty certain that they are generated in sources in our own Galaxy, at higher energies the problems start to multiply. Up to 1016 eV supernova remnants may well be responsible but above that it is almost anyone's guess. At the very highest energies detected so far (~3 × 1020 eV), where a single particle carries as much energy as a mass of 1 kg falling 5 m, the problem of origin is so acute that alarmingly exotic processes have to be invoked. The present status of the search for the origin of cosmic rays will be described here.