The reasons for studying the highest energy cosmic rays will be outlined together with a description of the Pierre Auger Observatory, now in full operation. The question posed in the title can now be asked only because of two results obtained using data recorded at the Observatory.
Firstly, it has been established that the flux of the highest energy cosmic rays is suppressed at energies beyond 5 x 1019 eV.
Secondly, above this energy anisotropy in the arrival directions of the particles has been discovered that appears to be associated with sources lying within 75 Mpc.
From these two observations it seems probable that we have observed the long-sought Greisen-Zatsepin-Kuz’min effect, demonstrating that ultra-high energy cosmic rays are of extragalactic origin. It is also probable that these particles are protons, thus offering the possibility of insights into features of particle physics at centre-of-mass energies 30 times greater than will be reached at the LHC.
Preliminary conclusions from studies of detailed features of extensive air showers suggest that extrapolations from Tevatron energies may not be what have been anticipated hitherto. Much further work remains to be done and the next steps will be outlined.