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I don't see why all low-energy photon collisions result in lowering the the energy of the particles. If the photons can come from any direction, surely just as many will add energy as drain it? If such a cosmic-ray particle is formed in a super-nova event I would expect it to be boosted by the EM radiation from the SN, after it's creation and initial ejection, until the radiation from the SN fell below background levels.
Einstein's special theory of relativity dictates that any cosmic rays reaching Earth from a source outside our galaxy will have suffered so many energy-shedding collisions that their maximum possible energy is 5 × 1019 electronvolts. This is known as the Greisen-Zatsepin-Kuzmin limit.
Yeah - umm...When a photon is absorbed and re-emitted by a single particle can we be sure that it will be re-emitted back in the direction it came from? When we have a reflective macro-structure we can say yes, but can we say so for a single particle?