The Oh-My-God particle (as physicists dubbed it) was an ultra-high-energy cosmic ray detected on 15 October 1991 by the Fly's Eye camera in Dugway Proving Ground, Utah, United States. As of 2026, it is the highest-energy cosmic ray ever observed. Its energy was estimated as (3.2±0.9)×1020 eV (320 exaelectronvolt). The particle's energy was unexpected and called into question prevailing theories about the origin and propagation of cosmic rays.
Speed Although most high-energy cosmic rays are protons, it is not known what the ultra-high-energy Oh-My-God particle was. A 2025 study of neutrinos produced by collisions between protons and photons shows that for very high-energy cosmic rays fewer than about 70% are protons, the others probably being heavy ions such as iron. If m p {\displaystyle m_{\mathrm {p} }} is the rest mass of the particle and E K {\displaystyle E_{\mathrm {K} }} is its kinetic energy (energy above the rest mass energy), then its speed was very close to 1 − [ m p c 2 / ( E K + m p c 2 ) ] 2 {\textstyle {\sqrt {1-[m_{\mathrm {p} }c^{2}/(E_{\mathrm {K} }+m_{\mathrm {p} }c^{2})]^{2}}}} times the speed of light. Since E K ≫ m p c 2 {\textstyle E_{\mathrm {K} }\gg m_{\mathrm {p} }c^{2}} , this ratio can be simplified to 1 − 1 2 [ m p c 2 / E K ] 2 {\textstyle 1-{\frac {1}{2}}[m_{\mathrm {p} }c^{2}/E_{\mathrm {K} }]^{2}} . If it was a proton, for which m p c 2 {\displaystyle m_{\mathrm {p} }c^{2}} is 938 MeV, this means it was traveling at 0.9999999999999999999999957 times the speed of light, its Lorentz factor was 3.2×1011 and its rapidity was 27.1. This is 1.3 femtometers per second less than the speed of light, so if a photon were traveling alongside the proton, it would take over 245,000 years for the photon to gain a 1 cm lead, as seen from the Earth's reference frame. Due to special relativity, the relativistic time dilation experienced by a proton traveling at this speed would be extreme. If the proton originated from a distance of 1.5 billion light years, it would take approximately 1.71 days in the reference frame of the proton to travel that distance, as determined by application of the relativistic time dilation equation. By the same formula, if the particle was an iron ion with the same kinetic energy, then its speed would have been "only" 0.999999999999999999987 times the speed of light.
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