Observation of Ultra High Energy Cosmic Rays (UHECR) -whose energy exceeds
1020eV- is still a puzzle for modern astrophysics. The transfer of more than
16 Joules to a microscopic particle can hardly be achieved, even in the most
powerful cosmic accelerators such as AGN's, GRB's or FR-II radio galaxy lobes.
Potential sources must also lie within 100 Mpc of the Earth as the interaction
length of protons, nuclei or photons is less than 10Mpc. However no visible
counterpart of those sources has been observed. Calling upon new physics such
as Topological Defect interactions or Super Massive Relic Particle decays is
therefore very tempting, but such objects are yet to be proven to exist. Due to
the very low flux of UHECR only very large dedicated experiments, such as the
Auger observatories, will allow to shed some light on the origin of those
cosmic rays. In this quest neutrinos, if they can be detected, are an
invaluable messengers of the nature of the sources.Comment: Talk Given at the Neutrino 2000 COnference. Sudbury, Toronto June
12-17 2000 7 pages, 8 figure
