Giant Air Shower Array Shows Cosmic-Ray Spectrum Violating Greisen Cutoff

For almost nine years, the Akeno Giant Air Shower Array in Japan has been accumulating data on the most energetic cosmic rays. AGASA, with its 111 scintillation detectors deployed over $\text{100\hspace{0.17em}}{\mathrm{km}}^2,$ is by far the world's largest air shower array. (See PHYSICS TODAY, January 1998, page 31.) This collaboration of 14 Japanese institutions is led by Masahiro Teshima (University of Tokyo). The recent publication of its observations through October 1997 appears to confirm a provocative astrophysical paradox: How can it be that the cosmic‐ray energy spectrum is extending beyond ${10}^{20}$ electron volts without any clear sign of a cutoff?