Powerful radio galaxies as sources of the highest energy cosmic rays

We summarize the status of the search for the origin of the highest energy cosmic rays. We briefly mention several competing proposals, such as Gamma Ray Bursts also giving rise to energetic protons, high energy neutrinos and cosmological defects, and then concentrate on the possibility that powerful radio galaxies can provide the sources. We describe several tests, some of which have been performed already. First, powerful radio galaxies must be able to accelerate protons to such energies; this entails that there is sufficient space for the Larmor motion. Second, we require at least one candidate radio galaxy with sufficiently strong shock fronts to be the source, at a sufficiently close cosmological distance. Third, the distribution of arrival directions of the highest energy particles on the sky ought to reflect the source distribution as well as the propagation history. The present status can be summarized as inconclusive. Powerful radio galaxies have been tested more than any other candidate source class, but a definitive confirmation is still outstanding. If we were able to confirm this particular theory—or any other—these particles at beyond ${10}^{20}\hspace{0.17em}\mathrm{eV}$ may be turned into tools of high energy physics.