Doubts about relativity have been caused partly by a mystery regarding cosmic rays, the high-energy particles that rain down on Earth from outer space.
According to relativity, there should be a limit -- known as the G.Z.K. limit -- after Kenneth Greisen, Georgi Zatsepin and Vadim Kuzmin, three theorists who independently came up with it in 1968 -- to the energies of cosmic rays that can hit Earth from distant realms in the cosmos.
Cosmic rays carrying about 10 20 electron volts of energy would bang into the haze of microwaves left behind by the Big Bang fireball and break up before traveling more than a few hundred million light-years.
But in the last decade cosmic ray detectors have recorded a sprinkling of cosmic rays with energies greater than the G.Z.K. limit, which is a hundred million times the energy produced in the Fermi National Accelerator Laboratory's giant Tevatron particle accelerator. The result has been a flood of papers proposing explanations as diverse as unknown particles and the breakdown of relativity.
In the late 1990's Dr. Luis Gonzalez-Mestres of the National Center for Scientific Research in France, and, independently, the Harvard theorists Dr. Sheldon Glashow and Dr. Sidney Coleman proposed that a small violation of relativity would allow high-energy cosmic rays to evade the G.Z.K. energy limit on travel.
''Is the sky sending us a signal?'' Dr. João Magueijo, a theorist at Imperial College London, asked rhetorically last spring.
No one can say for sure. A new experiment in Utah, the High Resolution Fly's Eye, hasn't seen as many high-energy events as it should have based on the earlier data. This fall Dr. John Bahcall, a theorist at the Institute for Advanced Study in Princeton, and Dr. Eli Waxman, from the Weizmann Institute in Israel, combined the data from Fly's Eye and two other detectors, in Yakutsk, Russia, and Halverah Park in Britain and said they found nothing inconsistent with the G.Z.K. cutoff after all.
That would mean that relativity would be off the hook. But the analysis of Dr. Bahcall and Dr. Waxman has been criticized because they left out the Akeno Giant Air Shower Array in Japan, which has continued to record the high-energy rays.
The answer, all agree, will come from the Pierre Auger Observatory, a giant array, named after a pioneer cosmic ray scientist, operated by a multinational collaboration in Argentina. A tiny fraction of the array began operating this year.
Even a negative result from Auger would not relieve physics of a deep mystery, however, said Dr. James Cronin, a University of Chicago physicist and Nobelist who is the American spokesman for the project. Tinkering with relativity, he pointed out, would explain only how such high-energy cosmic rays get here, but not how they are produced in the first place. ''How does a source produce cosmic rays of 1020 electron volts?'' wondered Dr. Cronin. ''It's such a fascinating problem,'' he said, ''one for the twilight of my career.''
