At 5×10¹⁹ eV, this is the theoretical upper limit to the energy a cosmic ray can possess. At energies greater than this, the cosmic ray should interact with the radiation of the cosmic microwave background within a distance of 150 million light years, severely limiting the sources available to produce such high energy particles. These interactions would reduce the energy of the cosmic ray to below the Greisen-Zatsepin-Kuzmin limit (GZK limit), and we should not observe such high-energy particles if they do indeed come sources more distant than that.