Supervisor: Hans Kjeldsen

Every second the Earth is hit by thousands of very energetic particles per square meter – either gamma rays or charged particles. In 1991 a cosmic ray called the Oh-my-god particle was detected with an estimated energy of 51J - nearly the same as the kinetic energy of a tennis ball fired from a tennis machine (115km/h) – and millions of times the highest energy of an proton from a particle accelerator. In fact, before we had modern particle accelerators at our disposal this natural radiation source was the only window into the particle zoo beyond the atom.

In this colloquium I will explain what happens when such energetic particles hit the atmosphere of Earth, how showers of secondary particles are produced, the phenomena that makes it possible to detect these events on the ground, and some of the detectors used to measure them . I will round up the colloquium by discussing what reasons for concern we might have about these cosmic rays.