abstract: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, who estimated its energy to be approximately (, about 20 million times more energetic than the highest energy measured in radiation emitted by an extragalactic object);the blazar Markarian 501, measured in 1997 in other words, a subatomic particle with kinetic energy equal to that of 50 Joule, or a baseball traveling at about .