In 1991, the most energetic particle known was detected, with an energy of 3.2 × 10^ {20} electron volts, or equivalently 320 billion times the rest-mass energy of a proton. This particle was appropriately named the “Oh-My-God particle” because it was rare and unique. However, in 2023, another ultra-high-energy cosmic ray with 75% of this record energy was also reported.
A cosmic ray is composed of high-energy elementary particles — often protons — that move near the speed of light. Ultrahigh-energy cosmic rays have energy levels that are more than a million times larger than those reached by the most powerful human-made particle accelerators. Cosmic rays with energies of hundreds of billions of times the proton rest-mass energy are rarely found — fewer than one particle per square kilometer per century. They might flag new physics if they are produced by some unknown physical processes that might enable them to travel over much vaster distances than previously thought.
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