From Academic Kids
The Greisen-Zatsepin-Kuzmin limit (GZK limit) is a theoretical upper limit on the energy of cosmic rays from distant sources.
This limit was computed in 1966 by Kenneth Greisen, Vadem Kuzmin and Georgi Zatsepin, based on interactions predicted between the cosmic ray and the photons of the cosmic microwave background radiation. They predicted that cosmic rays with energies over the threshold energy of 5×1019 eV would interact with CMB photons to produce pions. This would continue until their energy fell below the pion production threshold. Therefore, extragalactic cosmic rays with energies greater than this threshold energy should never be observed on Earth. Template:Unsolved
A number of observations have been made by the AGASA experiment that appear to show cosmic rays from distant sources with energies above this limit (whimsically dubbed Oh-My-God particles). The observed existence of these particles is the so-called GZK paradox or cosmic ray paradox.
These observations appear to contradict the predictions of special relativity and particle physics as they are presently understood. However, there are a number of possible explanations for these observations that may resolve this inconsistency. Firstly, the observations could be due to an instrument error or an incorrect interpretation of the experiment. Secondly, the cosmic rays could have local sources (although it is unclear what these sources could be). Another suggestion involves ultra-high energy weakly interacting particles (for instance neutrinos) which might be created at great distances and later react locally to give rise to the particles observed.
A number of exotic theories have been advanced to explain these observations, of which the most notable is the theory of doubly-special relativity.