What is the origin and nature of ultra-high-energy cosmic rays?
Cosmic rays are high-energy particles, mainly protons and alpha particles, which come from outer space and hit the Earth’s atmosphere producing a shower of other particles. Most of these are believed to have picked up their energy by interacting with shock waves in the interstellar medium. But the highest-energy ones remain mysterious–nobody knows how they could have acquired such high energies. The record is a 1994 event detected by the Fly’s Eye in Utah, which recorded a shower of particles produced by a cosmic ray of about 300 EeV. A similar event has been detected by the Japanese scintillation array AGASA. An EeV is an “exa-electron-volt”, which is the energy an electron picks up going through a potential of 1018 volts. 300 EeV is about 50 joules–the energy of a one-kilogram mass moving at 10 meters/second, presumably all packed into one particle! Nobody knows how such high energies are attained–perhaps as a side-effect of the shock made by a supernova or gamma-ray burster? The
