The limit was independently computed in 1966 by Kenneth Greisen, Georgy Zatsepin, and Vadim Kuzmin 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 5 × 10 eV would interact with cosmic microwave background photons γ γ C M B , relatively blueshifted by the speed of the cosmic rays, to produce pions through the Δ Δ resonance,