Cosmic rays are fast-moving particles that constantly bombard the earth from all directions. About 1 million pass through our bodies every night. Most are the nuclei of atoms ranging from protons to uranium. The lower energy particles are bent by magnetic fields in our Galaxy and the Universe. Their directions become so scrambled that we cannot trace where they come from. They have been studied for many years but remarkably little is known about their origins.

Above about 10¹⁴ eV the flux of cosmic rays is so low that it is barely practical to detect them directly even using instruments carried on balloons or spacecraft. Instead one must rely on the extensive air-showers (EAS) that the particles create when they hit the earth’s atmosphere. Above 10¹⁴ eV, where the maximum number of ~ 10⁵ particles is reached at ~ 6 km above sea-level, some particles survive so that remnants of the primary are detectable. The discovery of extensive air-showers is usually credited to Pierre Auger who, in 1938, observed an unexpectedly high rate of coincidences between counters separated by a few metres. Further investigations by his team showed that even when the counters were as far as 300 m apart, the rate of coincidence was significantly in excess of the chance expectation. Speculating that the primaries were photons, and using the newly developed ideas of quantum electrodynamics, Auger demonstrated that the incoming entities had energies as high as 10¹⁵ eV, nearly 6 orders of magnitude above the energy then reached by man-made accelerators.

Cosmic rays remain the most extreme example of the departure of matter from thermal equilibrium. (Additional links to more detailed information can be made, for example, to some of the Auger web sites or to specific papers).

The Pierre Auger Observatory has been designed to study the highest energy cosmic rays – some as energetic as a tennis ball moving at 100 kph. These are expected to travel in straight lines and so reveal their sources. They are studied using the air-shower technique initiated by Pierre Auger. The Observatory must be very large as the rate of events at the very highest energies (~ 10²⁰ eV) is less than 1 per km² per century!

These very rare particles have mysterious origins, so that studying them may open doors to problems in exotic physics and astrophysics.