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 (hence the name), 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 0.999 999 999 999 999 999 999 9951c. 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.
The speed of the particle, if it was a proton, is so high that it would experience relativistic time dilation by a factor of about 320 billion. At that rate, the particle could have traveled for the entire duration of the universe's existence while experiencing less than sixteen days subjective time.
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 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. More recent studies using the Telescope Array have suggested a source for the particles within a 20-degree "warm spot".
- 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.
- Bird, D. J. (March 1995). "Detection of a cosmic ray with measured energy well beyond the expected spectral cutoff due to cosmic microwave radiation". Astrophysical Journal, Part 1 (ISSN 0004-637X), vol. 441, no. 1, p. 144-150. Retrieved February 14, 2014.
- J. Walker (January 4, 1994). "The Oh-My-God Particle". Fourmilab.
- "Physicists spot potential source of 'Oh-My-God' particles", sciencemag.org