The impasse has livened up conversations at recent cosmic-ray conferences and underlines the need for a bigger experiment that can detect many more UHECR events.
The unresolved disparity between HiRes and AGASA has added a new urgency to the UHECR problem, and the tools to resolve it are now under construction.
But, ironically, UHECRs are subject to a moderating force that saps their energy before they can travel that far.
Thanks to the GZK cutoff, UHECRs cannot be the result of quasars, gamma-ray bursts, or anything else at cosmological distances.
It's a situation that scientists working on UHECRs want to change.
In the decade since evidence for UHECRs was first announced, theorists have raced ahead of their experimentalist colleagues, hoping to predict where and how such powerful particles are made.
In contrast, UHECRs are moving too fast to be deflected by the Milky Way's magnetic field.
With no obvious connection between UHECRs and particular astronomical objects, researchers have begun to look for more subtle clues to the particles' origins.
Bottom-up" theories postulate that UHECRs start off as low-energy protons and then are powered up by a natural accelerator.
Bottom-up descriptions of UHECRs focus on settings where protons may be energized in a loosely analogous fashion --but to much greater degrees--by various combinations of jets, shocks, and magnetic fields.
In contrast to these astrophysical "bottom-up" possibilities, even more esoteric "top-down" theories depend on hyperenergetic microscopic entities that release UHECRs as byproducts.
While these ideas are more radical, they help explain why UHECRs are attracting so much attention from theorists.