A limit on the energy of cosmic rays imposed by the interaction between the cosmic rays and the cosmic microwave background radiation. This limit, found by Kenneth Greisen, Vadim Kuzmin, and Georgiy Zatsepin in 1966 using the theory of the inverse Compton effect, is about 6×1019 eV. Cosmic rays with higher energies have been detected but it has been established that these come from relatively near sources and so have not had a chance to be slowed down by interaction with the background radiation.