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Ultra high energy cosmic ray sources & experimental results

Here we discuss the latest developments in the debate, where the ultrahigh energy cosmic ray particles come from. In this brief review, we emphasize the predictions that necessarily follow from the various concepts proposed. We discuss both sources and propagation, and spend some space on the by now most conservative model, giant radio galaxies and their hot spots and jets, because it allows more steps in the reasoning to be checked, and is more tightly constrained than any other model. It has survived about forty years of debate already. We summarize the task ahead of us at the end, including the development of new technologies to observe ultra high energy cosmic rays, e.g., with the radio emission of air showers, which may lead to an alternative to the fluorescence detectors with their small duty cycle. Due to HiRes, AUGER, and the plans for EUSO, the future is bright, and the ultra high energy cosmic ray particles may yet allow us to explore new physics.