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 5×1019 eV, or about 8 joules. The limit is set by slowing-interactions of cosmic ray protons with the microwave background radiation over long distances (~163 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 ultra-high-energy cosmic ray has been detected which appeared to possess a record 50 joules (312 million TeV) of energy (about the same as 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 (3 million TeV) are much closer to the Planck energy (about 2 billion joules, or 1.22×1016 TeV) than any particles that can be made by current particle accelerators (20 TeV, or 3 millionths of a joule). They are thus suitable as a probe into realms where the theory of special relativity breaks down. Physicist Lee Smolin has written that if such cosmic rays which violate the GZK limit can be confirmed, and other possible explanations discounted, it "would be the most momentous discovery of the last hundred years—the first breakdown of the basic theories comprising the twentieth century's scientific revolution."