"If all goes as planned, a laser-cooled clock named PARCS will be installed on the ISS in late 2004 or 2005. Experts expect it to be the most stable clock ever, keeping time within 1 second every 300 million years (1 part in 10¹⁶).

 

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."

 

 

 

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