Physical Society Colloquium

R.E. Bell Lecture

The highest energy cosmic rays:
some past history and recent observations

James Cronin

Nobel Laureate
University of Chicago

Since the discovery by Pierre Auger in 1938 that some cosmic rays have energies in excess of 10¹⁵ eV, much experimental effort has been devoted to the study of these highest energy cosmic rays. We will describe briefly recent experiments which have found cosmic rays with energies in excess of 10²⁰ eV (16 joules) - microscopic particles with macroscopic energy.

The principal part of the talk will be a description of the Pierre Auger Observatory in Argentina. This observatory covers an area of 3000 km² with 1600 particle detectors overlooked by 4 fluorescence telescopes. The observatory even while under construction has been taking reliable data since Jan 1 2004. Preliminary results and the ultimate prospects for the observatory will be described.