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Greisen–Zatsepin–Kuzmin limit
Theoretical upper limit on the energy of cosmic ray protons
Greisen–Zatsepin–Kuzmin limit
The Greisen–Zatsepin–Kuzmin limit (GZK limit or GZK cutoff) is a theoretical upper limit on the energy of cosmic ray protons traveling from other galaxies through the intergalactic medium. The limit is 5×1019 eV, or about 8 joules (the energy of a proton traveling extremely near the speed of light). The limit is set by the slowing effect of interactions of the protons with the microwave background radiation over distances in excess of about 160 million light-years. The limit is the same order of magnitude as the upper limit for energy at which cosmic rays have been detected, although some detected rays appear to have exceeded the limit. For example, one extreme-energy cosmic ray, the Oh-My-God Particle, was measured at a record-breaking 3.12×1020 eV (50 joules) of energy (about the same as the kinetic energy of a 95 km/h baseball). In the past, the apparent violation of the GZK limit inspired cosmologists and theoretical physicists to suggest other ways that circumvent the limit. These...
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