The so-called "GZK cutoff" has been predicted since the 1960s -- cosmic rays of too high energy would be unable to propagate long distances due to various loss effects, mainly (for protons and higher nuclei) collisions with the microwave background radiation (which from the particle's point of view is like incoming gamma rays) blasting them apart. However, this cutoff never seemed to actually happen, leading to mysteries -- what was going on? Why were the ultra-high cosmic rays there despite claims they could not exist? E.g. http://en.wikipedia.org/wiki/Oh-My-God_particle http://en.wikipedia.org/wiki/Greisen–Zatsepin–Kuzmin_limit Now new observations at the South Pole "ice cube" detector indicate the cosmic ray spectrum really does seem cut off starting at 10^18 eV energies: http://icecube.wisc.edu/news/view/141 -- Warren D. Smith http://RangeVoting.org <-- add your endorsement (by clicking "endorse" as 1st step)