The Oh-My-God particle was an ultra-high-energy cosmic ray detected on 15 October 1991 by the Fly's Eye camera in Dugway Proving Ground, Utah, U.S. At that time it was the highest-energy cosmic ray that had ever been observed. Although higher energy cosmic rays have been detected since then, this particle's energy was unexpected, and called into question theories of that era about the origin and propagation of cosmic rays.
The Oh-My-God particle's energy was estimated as (3.2±0.9)×1020 eV, or 51±14 J. This is 20 million times more energetic than the highest energy measured in electromagnetic radiation emitted by an extragalactic object. It had 1020 (100 quintillion) times the photon energy of visible light, equivalent to a 140 grams (5 oz) baseball travelling at about 26 m/s (94 km/h; 58 mph). 
Assuming it was a proton, this particle traveled at 99.99999999999999999999951% of the speed of light, its Lorentz factor was 3.2×1011 and its rapidity was 27.1. At this speed, if a photon were travelling with the particle, it would take over 215,000 years for the photon to gain a 1 cm lead as seen from the Earth's reference frame.
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 √, where E is the particle's energy and mc2 is the mass energy of the proton. For the Oh-My-God particle, this gives 7.5×1014 eV, roughly 60 times the collision energy of the Large Hadron Collider.
While the particle's energy was higher than anything achieved in terrestrial accelerators, it was still about 40 million times lower than the Planck energy. Particles of such energy would be required in order to explore the Planck scale. A proton with that much energy would travel 1.665×1015 times closer to the speed of light than the Oh-My-God particle. As viewed from Earth it would take about 3.579×1020 years, or 2.59×1010 times the current age of the universe, for a photon to gain a 1 cm lead over a Planck energy proton as observed in Earth's reference frame.
Later, similar events
Since the first observation, at least seven similar events (energy 5.7×1019 eV or greater) have been recorded, confirming the phenomenon. These ultra-high-energy cosmic ray particles are very rare; the energy of most cosmic ray particles is between 10 MeV and 10 GeV.
- Cosmic ray – High-energy particle, mainly originating outside the Solar system
- Greisen–Zatsepin–Kuzmin limit – A theoretical upper limit on the energy of cosmic ray protons
- Ultra-high-energy cosmic ray – A cosmic-ray particle with a kinetic energy greater than 1018 eV
- Ursa Major Cluster – A spiral-rich galaxy cluster of the Virgo Supercluster
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