The GZK Bound and Strong Neutrino-Nucleon Interactions above 10¹9eV: a Progress Report

Cosmic ray events above 10^19 eV have posed a fundamental problem for more than thirty years. Recent measurements indicate that these events do not show the features predicted by the GZK bound. The events may, in addition, display angular correlations with point sources. If these observations are confirmed for point sources further than 50 - 100 Mpc, then strong interactions for the neutrino are indicated. Recent work on extra spatial dimensions provides a context for massive spin-2 exchanges capable of generating cross sections in the 1 - 100 mb range, as indicated by data. Applications of extra-dimension physics are controversial, and we comment on several contentious issues.