The Oh-My-God particle was an ultra-high-energy cosmic ray (most likely an iron nucleus) 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. Its observation was a shock to astrophysicists (hence the name), who estimated its energy to be approximately ×1020 eV ( 3×108 TeV, about 20 million times more energetic than the highest energy measured in radiation emitted by an extragalactic object); 3 in other words, an atomic nucleus with kinetic energy equal to that of 48 Joules, or a 5-ounce (142 g) baseball traveling at about 93.6 kilometers per hour (60 mph).
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 is the particle's energy, and is the mass energy of the proton. For the Oh-My-God particle, this gives ×1014 eV, roughly 50 times the collision energy of the 7.5Large Hadron Collider.
Since the first observation 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. 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.
- the blazar Markarian 501, measured in 1997
- Open Questions in Physics. German Electron-Synchrotron. A Research Centre of the Helmholtz Association. Updated March 2006 by JCB. Original by John Baez.
- "Physicists spot potential source of 'Oh-My-God' particles", sciencemag.org
- "The Particle That Broke a Cosmic Speed Limit | Quanta Magazine". www.quantamagazine.org. Retrieved 2015-05-14.