Greisen–Zatsepin–Kuzmin limit

The Greisen–Zatsepin–Kuzmin limit (GZK limit) is a theoretical upper limit on the energy of cosmic rays (high energy charged particles from space) coming from "distant" sources. The limit is 7000801088243500000♠5×10¹⁹ eV, or about 8 joules. The limit is set by slowing-interactions of cosmic ray photons 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. For example, one extreme-energy cosmic ray has been detected which appeared to possess a record 7001499879063944000♠3.12×10²⁰ eV (50 joules) of energy (about the same as the kinetic energy of a 60 mph baseball).

Cosmologists and theoretical physicists have regarded such observations as key in the search for explorations of physics in the energy realms which would require new theories of quantum gravity and other theories which predict events at the Planck scale. This is because protons at these extreme energies (6999480652946100000♠3×10¹⁸ eV are much closer to the Planck energy (about 7009195465531414000♠1.22×10²⁸ eV, or 2 thousand million joules) than any particles that can be made by current particle accelerators (6994224304708180000♠1.4×10¹³ eV, or 2.2 millionths of a joule). They are thus suitable as a probe into realms of novel physics.

The limit was independently computed in 1966 by Kenneth Greisen,Vadim Kuzmin, and Georgiy Zatsepin, based on interactions between cosmic rays and the photons of the cosmic microwave background radiation (CMB). They predicted that cosmic rays with energies over the threshold energy of 7000801088243500000♠5×10¹⁹ eV would interact with cosmic microwave background photons ${\displaystyle \gamma _{\rm {CMB}}}$, relatively blueshifted by the speed of the cosmic rays, to produce pions via the ${\displaystyle \Delta }$ resonance,

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