In the early 1960s, Arno Penzias and Robert Wilson discovered that low-energy microwaves permeate the universe. Kenneth Greisen, Vadem Kuzmin and Georgi Zatsepin pointed out that high-energy cosmic rays would interact with the microwave background. The interaction would reduce their energy, so that particles traveling long intergalactic distances could not have energies greater than 5x10¹⁹ eV.