The Oh-My-God particle was an ultra-high-energy cosmic ray (most likely a proton) detected 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 (3×108 TeV, about 20 million times more energetic than the highest energy measured in radiation emitted by an extragalactic object); in other words, a subatomic particle with kinetic energy equal to that of 50 Joules, or a 5-ounce (142 g) baseball traveling at about 100 kilometers per hour (60 mph).
The particle was traveling very close to the speed of light — assuming the particle was a proton, its speed was only about 1.5 femtometers (quadrillionths of a meter) per second less than the speed of light, translating to a speed of approximately 0.9999999999999999999999951c. At that speed, in a year-long race between a photon and the particle, the particle would fall behind only 46 nanometers, or 0.15 femtoseconds (1.5×10−16 s); or one centimeter every 220 000 years. Its energy is around 25 billionths of the Planck energy, the fundamental unit of energy in quantum gravity and the highest conceivable energy scale in all but the most speculative theories of particle physics.
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.