What is an Ultra-High Energy Cosmic Ray?
Cosmic rays are subatomic particles flying through space at great speed in all directions. About 90% of them are protons, 9% are helium nuclei, and 1% are electrons. Contrary to their naming, cosmic rays are energetic particles rather than rays per se. On Earth, we can detect cosmic rays when they collide with the upper atmosphere and release a shower of charged particles. By observing how intense the particle shower is, we can infer the energy and direction of the cosmic ray. An ultra-high energy cosmic ray (UHECR) is a type of cosmic ray with an anomalously high energy. There is no official cutoff limit defining an ultra-high energy cosmic ray, but the term generally refers to cosmic rays approaching or exceeding the Greisen-Zatsepin-Kuzmin (GZK) limit on cosmic ray energy. The GZK limit corresponds to a threshold energy of 6 x 1019 electronvolts, about ten million times more energetic than the particles in the most powerful particle accelerators. Above this limit, it is thought that
