Atomic Clocks

According to Einstein's theory of gravity and space-time - called "general relativity" - clocks in strong gravity tick slower than clocks in weak gravity. Because gravity is weaker on the ISS than at Earth's surface, PARCS should accumulate an extra second every 10,000 years compared to clocks ticking on the planet below. PARCS won't be there that long, but the clock is so stable that it will reveal this effect in less than one year. (Strayer notes that clocks on GPS satellites experience this relativistic phenomenon, too, and that onboard systems must correct for it.)

"Putting atomic clocks in orbit is a good way to test general relativity," says Maleki. "General relativity has passed every test so far, but no theory is perfect - not even Einstein's. Eventually, as we extend the precision of our experiments, we expect to find flaws in it, and that will dramatically change what we know about the nature of the Universe."

The stretching of time by relativity has been felt and measured by other orbiting clocks - GPS, for example - but PARCS will measure the effect with errors one hundred times smaller than its predecessors did. Furthermore, PARCS will test technologies to be used in a next-generation clock named RACE slated for installation on the ISS in 2006. Stable within 1 part in 10¹⁷, RACE will keep time so well that if it ran for three billion years it would lose less than 1 second.

Clocks like RACE will test physics like never before. They will improve telecommunications on Earth - "in ways we can't imagine yet" says Maleki - and do wondrous things for navigation. Indeed, with RACE on board, a mariner could navigate not only by the stars, but between them as well.

Perhaps if Masefield were alive today, he would craft his poem differently: "I must RACE down to the launch pad, to my craft so sleek and true; All I need is a stable clock and a star to steer her to...."