Earth is constantly bombarded by high energy particles from space, cosmic rays. Most are protons, the positively-charged particles that normally live in the nuclei of atoms. Many come from the sun, but the highest energy cosmic rays are an enigma. Some have scarcely believable energies, way beyond what could be produced by any conceivable particle accelerator here on Earth. We do not know where they came from, how they could get here, and we have no idea what process could give them such stupendous energies. Now a new observatory has allowed a collaboration of over 300 scientists to answer the first of these questions.
With an array of 1600 detectors that stretch over 3000 km2 of Argentina, the Pierre Auger Observatory detects the showers of particles generated when high energy cosmic rays whack into air molecules. The researchers have discovered that the origin of the highest-energy cosmic rays are quasars. All galaxies have black holes at their centre. In some galaxies gas and stars are falling into the central black hole and prodigious amounts of energy are radiated out as a result, turning the nucleus of the galaxy into a brilliant quasar. How quasars could generate such high energy cosmic rays is a total mystery.