: Astronomy: Greisen-Zatsepin-Kuzmin cut off

high-energy astrophysics > particle astrophysics > cosmic ray > Greisen-Zatsepin-Kuzmin cut off

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Greisen-Zatsepin-Kuzmin cut off

The Greisen–Zatsepin–Kuzmin limit (GZK limit) is a theoretical upper limit on the energy of cosmic ray protons traveling from other galaxies through the intergalactic medium to our galaxy. The limit is 5×10¹⁹eV (50 EeV), or about 8 joules (the energy of a proton travelling at ≈ 99.99999999999999999998% the speed of light). The limit is set by the slowing effect of interactions of the protons with the microwave background radiation over long distances (≈ 160 million light-years). The limit is at the same order of magnitude as the upper limit for energy at which cosmic rays have experimentally been detected, although indeed some detections appear to have exceeded the limit, as noted below. For example, one extreme-energy cosmic ray, the Oh-My-God Particle, which has been found to possess a record-breaking 3.12×10²⁰eV (50 joules) of energy (about the same as the kinetic energy of a 95 km/h baseball). (Wikipedia, The Free Encyclopedia, https://en.wikipedia.org/wiki/Greisen%E2%80%93Zatsepin%E2%80%93Kuzmin_limit)

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