Ultra-high-energy Cosmic Ray - Observational History

Observational History

The first observation of a cosmic ray particle with an energy exceeding 1.0×1020 eV (16 J) was made by Dr John D Linsley and Livio Scarsi at the Volcano Ranch experiment in New Mexico in 1962.

Cosmic ray particles with even higher energies have since been observed. Among them was the Oh-My-God particle observed on the evening of 15 October 1991 over Dugway Proving Ground, Utah. Its observation was a shock to astrophysicists, who estimated its energy to be approximately 3×1020 eV (50 J)—in other words, a subatomic particle with kinetic energy equal to that of a baseball (5 ounces or 142 grams) traveling at about 100 kilometers per hour (60 mph). It was most likely a proton traveling very close to the speed of light, slower by only about 1.5 femtometers (quadrillionths of a meter) per second, or about 0.9999999999999999999999951c, based on its observed energy. At that speed, in a year-long race between light and the particle, the particle would fall behind only 46 nanometers, or 0.15 femtoseconds (1.5×10−16 s).

The energy of this particle is some 40 million times that of the highest energy protons that have been produced in any terrestrial particle accelerator. However, only a small fraction of this energy would be available for an interaction with a proton or neutron on Earth, with most of the energy remaining in the form of kinetic energy of the products of the interaction. The effective energy available for such a collision is the square root of double the product of the particle's energy and the mass energy of the proton, which for this particle gives 7.5×1014 eV, roughly 50 times the collision energy of the Large Hadron Collider.

Since the first observation, by the University of Utah's Fly's Eye Cosmic Ray Detector, at least fifteen similar events have been recorded, confirming the phenomenon. These very high energy cosmic ray particles are very rare; the energy of most cosmic ray particles is between 10 MeV and 10 GeV.