# Oh-My-God particle
The **Oh-My-God particle** was an ultra-high-energy cosmic ray detected on the night of 15 October 1991 by the [Fly's Eye](https://www.edgepedia.org/flys-eye) air-fluorescence detector at [Dugway Proving Ground](https://www.edgepedia.org/dugway-proving-ground), Utah, with a reconstructed energy of (3.2 ± 0.9) × 1020 eV, or 320 ± 90 EeV, equal to about 51 joules of kinetic energy in a single subatomic particle[1](https://www.sciencedirect.com/science/article/abs/pii/S0927650504000593) • [2](https://arxiv.org/abs/astro-ph/9410067). It remains the most energetic cosmic ray ever observed, and no comparable event was recorded until the 244 EeV Amaterasu particle detected by the Telescope Array in May 2021[3](https://www.nature.com/articles/d41586-023-03677-0).
| Key fact | Detail |
|---|---|
| Detection | 15 October 1991, 7:34:16 UT, by the Fly's Eye detector at Dugway, Utah (40°N, 113°W)[1](https://www.sciencedirect.com/science/article/abs/pii/S0927650504000593) |
| Energy | (3.2 ± 0.9) × 1020 eV (320 ± 90 EeV), about 51 joules, substantially greater than any previously reported cosmic ray[1](https://www.sciencedirect.com/science/article/abs/pii/S0927650504000593) • [2](https://arxiv.org/abs/astro-ph/9410067) |
| Everyday scale | Kinetic energy comparable to that of a 60 mph baseball, or a brick dropped on a toe from waist height[4](https://assets.science.nasa.gov/content/dam/science/astro/programs/physics-of-the-cosmos/events/2025/second-early-career-workshop/presentations/8_Austin_Cummings.pdf) • [5](https://science.utah.edu/faculty/faculty-research/cosmic-ray/) |
| Shower maximum | The particle cascade reached maximum size near a depth of 815 g/cm² in the atmosphere, a depth that does not uniquely identify the primary particle type[2](https://arxiv.org/abs/astro-ph/9410067) |
| Arrival direction | Galactic coordinates centered on b = 9.6°, l = 163.4°; zenith angle 43.9°[2](https://arxiv.org/abs/astro-ph/9410067) • [6](https://www.researchgate.net/publication/1800426_Was_the_Highest_Energy_Cosmic_Ray_a_Photon) |
| Primary type | Compatible with any hadron primary between proton and iron nuclei; the photon hypothesis differs from the data at about 1.5σ and is not favored[7](https://arxiv.org/abs/astro-ph/0410739) |
| Source constraint | Propagation limits imply the particle very likely traveled less than about 50 Mpc, yet the source search found no likely source within 50 Mpc in the arrival direction[8](https://arxiv.org/abs/astro-ph/9410069) |
## The Fly's Eye detector
The [University of Utah](https://www.edgepedia.org/university-of-utah) built the Fly's Eye to record ultra-high-energy cosmic rays through atmospheric fluorescence: charged particles in an extensive air shower excite nitrogen in the atmosphere, which emits near-ultraviolet light whose intensity tracks the charged-particle content of the shower and allows the longitudinal shower profile, and hence the primary energy, to be reconstructed[9](https://www.sciencedirect.com/science/article/abs/pii/0168900285906588). The detector was the first successful air-fluorescence detector, operating from 1981 until July 1992[10](https://www.sciencedirect.com/science/article/pii/S0168900200003077). For the record event the instrument comprised a compound eye of 880 photomultiplier tubes at the focal planes of 67 mirrors, each 1.5 m in diameter[1](https://www.sciencedirect.com/science/article/abs/pii/S0927650504000593).
The event was recorded at 7:34:16 UT on 15 October 1991 with a reconstructed energy of about (3.2 ± 0.9) × 1020 eV[1](https://www.sciencedirect.com/science/article/abs/pii/S0927650504000593). The detection paper, published in 1994 by D. J. Bird and colleagues, reported it as a 51-joule cosmic ray substantially greater in energy than any previously reported[2](https://arxiv.org/abs/astro-ph/9410067). The nickname came from the reaction of the scientists who first examined the event[11](https://science.utah.edu/faculty/why-scientists-havent-solved-the-mystery-of-the-oh-my-god-particle/).
## Why the energy was surprising
The energy sits far above the Greisen–Zatsepin–Kuzmin (GZK) cutoff, the spectral suppression expected near 6 × 1019 eV, where ultra-high-energy protons lose energy through pion photoproduction on photons of the cosmic microwave background[12](https://www.sciencedirect.com/science/article/abs/pii/S0273117711005874). A [GZK cutoff](https://www.edgepedia.org/greisen-zatsepin-kuzmin-limit) should occur below the Oh-My-God particle's energy unless the particle had traveled less than about 30 Mpc from its source[2](https://arxiv.org/abs/astro-ph/9410067). At 3.2 × 1020 eV the typical range of a proton falls to roughly 20 Mpc, and the range is even smaller for heavy nuclei[13](https://www.sciencedirect.com/science/article/abs/pii/S055032130100565X).
A dedicated source search found no likely source within 50 Mpc in the direction from which the particle arrived; the authors noted that relatively strong magnetic fields deflecting the path through a large angle could allow an origin at a nearby [radio galaxy](https://www.edgepedia.org/radio-galaxy) at an earlier time of strong activity[8](https://arxiv.org/abs/astro-ph/9410069). The absence of an obvious nearby source, together with the energy itself, has kept the event central to the unsolved question of what accelerates and propagates ultrahigh-energy cosmic rays[14](https://www.annualreviews.org/content/journals/10.1146/annurev-astro-081710-102620).
## What kind of particle was it
The depth of shower maximum, 815 g/cm², does not by itself identify the primary[2](https://arxiv.org/abs/astro-ph/9410067). Later analyses comparing the observed longitudinal profile with simulated showers found the event compatible with any hadron primary between a proton and an iron nucleus under both interaction models studied (QGSJET 01 and SIBYLL 2.1), while primary photon profiles differ from the data at about the 1.5σ level; the photon hypothesis is not favored but cannot be rejected for this particular event[7](https://arxiv.org/abs/astro-ph/0410739) • [1](https://www.sciencedirect.com/science/article/abs/pii/S0927650504000593). An earlier study had concluded the event was not initiated by a photon[15](https://www.sciencedirect.com/science/article/pii/0927650594000385).
## Later events and context
The Oh-My-God particle was not an isolated anomaly: subsequent observations of other events in the same energy range confirmed it was a real physical event rather than a miscalculation or instrumentation failure[11](https://science.utah.edu/faculty/why-scientists-havent-solved-the-mystery-of-the-oh-my-god-particle/). Fly's Eye and its successor HiRes, together with the Akeno Giant Air Shower Array, detected more than 70 cosmic rays with energies reliably above the GZK energy of about 4 × 1019 eV[13](https://www.sciencedirect.com/science/article/abs/pii/S055032130100565X). Later experiments, HiRes and the [Pierre Auger Observatory](https://www.edgepedia.org/pierre-auger-observatory), observed a suppression of the flux above about 5 × 1019 eV consistent with the predicted GZK suppression[16](https://www.annualreviews.org/content/journals/10.1146/annurev.nucl.58.110707.171154).
No event has exceeded it. On 27 May 2021 the Telescope Array surface detector in Utah recorded the [Amaterasu particle](https://www.edgepedia.org/amaterasu-particle) at 244 ± 29 (stat.) EeV, the most energetic cosmic ray detected since 1991, and its apparent origin in the Local Void, a region lacking conventional cosmic-ray sources, renewed the source puzzle the Oh-My-God particle had posed three decades earlier[3](https://www.nature.com/articles/d41586-023-03677-0) • [17](https://indico.uchicago.edu/event/427/contributions/1337/attachments/325/676/UHECR_TeVPA2024_ToshihiroFujii.pdf).
## References
1. [Primary particle type of the most energetic Fly's Eye air shower (Astroparticle Physics, 2004)](https://www.sciencedirect.com/science/article/abs/pii/S0927650504000593)
2. [D. J. Bird et al. (1994). Detection of a Cosmic Ray with Measured Energy Well Beyond the Expected Spectral Cutoff due to Cosmic Microwave Radiation. arXiv.](https://arxiv.org/abs/astro-ph/9410067)
3. [Gemma Conroy (23 November 2023). The most powerful cosmic ray since the Oh-My-God particle puzzles scientists. Nature news.](https://www.nature.com/articles/d41586-023-03677-0)
4. [Austin Cummings. Extreme Cosmic Rays, NASA Physics of the Cosmos early-career workshop presentation](https://assets.science.nasa.gov/content/dam/science/astro/programs/physics-of-the-cosmos/events/2025/second-early-career-workshop/presentations/8_Austin_Cummings.pdf)
5. [Second highest-energy cosmic ray ever, University of Utah College of Science](https://science.utah.edu/faculty/faculty-research/cosmic-ray/)
6. [Was the Highest Energy Cosmic Ray a Photon? (Astroparticle Physics, 2004)](https://www.researchgate.net/publication/1800426_Was_the_Highest_Energy_Cosmic_Ray_a_Photon)
7. [On the primary particle type of the most energetic Fly's Eye event. arXiv.](https://arxiv.org/abs/astro-ph/0410739)
8. [In Search of a Source for the 320 EeV Fly's Eye Cosmic Ray. arXiv.](https://arxiv.org/abs/astro-ph/9410069)
9. [The Utah Fly's Eye detector (Nuclear Instruments and Methods A, 1985)](https://www.sciencedirect.com/science/article/abs/pii/0168900285906588)
10. [The prototype high-resolution Fly's Eye cosmic ray detector (Nuclear Instruments and Methods A, 2000)](https://www.sciencedirect.com/science/article/pii/S0168900200003077)
11. [Why Scientists Haven't Solved the Mystery of the OMG Particle, University of Utah College of Science](https://science.utah.edu/faculty/why-scientists-havent-solved-the-mystery-of-the-oh-my-god-particle/)
12. [On the origin of ultrahigh energy cosmic rays (Advances in Space Research, 2012)](https://www.sciencedirect.com/science/article/abs/pii/S0273117711005874)
13. [The high energy cosmic ray spectrum from relic particle decay (Nuclear Physics B, 2001)](https://www.sciencedirect.com/science/article/abs/pii/S055032130100565X)
14. [The Astrophysics of Ultrahigh-Energy Cosmic Rays, Annual Review of Astronomy and Astrophysics (2011)](https://www.annualreviews.org/content/journals/10.1146/annurev-astro-081710-102620)
15. [The highest energy cosmic ray (Astroparticle Physics, 1995)](https://www.sciencedirect.com/science/article/pii/0927650594000385)
16. [The Highest-Energy Cosmic Rays, Annual Review of Nuclear and Particle Science (2008)](https://www.annualreviews.org/content/journals/10.1146/annurev.nucl.58.110707.171154)
17. [Toshihiro Fujii (2024). UHECR presentation, TeVPA 2024](https://indico.uchicago.edu/event/427/contributions/1337/attachments/325/676/UHECR_TeVPA2024_ToshihiroFujii.pdf)
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*Topic: Encyclopedia › Physical world and mathematics › Physics › Particles and nuclei › Astroparticle physics › Cosmic rays › Ultra-high-energy cosmic rays › Energy spectrum and flux suppression*
*Initially written Sep 17, 2026 · Reviewed: Sep 30, 2026; Oct 15, 2026 · Edited: Sep 30, 2026; Oct 15, 2026 · Last review: Oct 15, 2026*
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License: Edgepedia Community License 1.0, https://www.edgepedia.org/edgepedia/license