The Oh-My-God particle was an ultra-high-energy cosmic ray detected on the evening of 15 October 1991 over Dugway Proving Ground, Utah, by the University of Utah‘s Fly’s Eye Cosmic Ray Detector.[1][2] Its observation was a shock to astrophysicists (hence the name), who estimated its energy to be approximately 3×1020 eV or 3×108 TeV. This is 20,000,000 times more energetic than the highest energy measured in electromagnetic radiation emitted by an extragalactic object[3] and 1020 (100 quintillion) times the energy of visible light. Therefore, the particle was an atomic nucleus with a kinetic energy of 48 joules, equivalent to a 142 g (5 oz) baseball travelling at about 26 m/s (94 km/h; 58 mph).[4]

This particle had so much kinetic energy it was travelling at ~ 99.999,999,999,999,999,999,999,510% of the speed of light. As a result, its Lorentz factor was ~ 3.2×1011. This is so near the speed of light that if a photon were travelling with the particle, it would take over 215,000 years for the photon to gain a 1-centimeter lead.

The energy of this particle is some 40,000,000 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

${displaystyle textstyle {sqrt {2Emc^{2}}}}$

, 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.[5]

While the particle’s energy was higher than anything achieved in terrestrial accelerators, it was still about 40,000,000 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 less than 0.000,000,000,000,000,000,000,49% of c faster 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-centimeter lead over a Planck energy proton.

Since the first observation, at least fifteen similar events 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. More recent studies using the Telescope Array have suggested a source for the particles within a 20-degree “warm spot” in the direction of the constellation Ursa Major.[2][6]