Investigators at the Max-Planck-Institute for Nuclear Physics in Heidelberg, Germany, are measuring the relativistic Doppler shift of light, a consequence of time dilation. Approaching objects shine more blue light, which has a higher frequency, and receding ones shine more red, which has lower energy. The effect is analagous to the whine of a passing fire engine that rises as it approaches and then falls as it speeds by and recedes in the distance. The group is currently beaming laser light onto lithium ions spinning at seven percent of the speed of light in the heavy ion storage ring TSR (actually shaped like a rectangle). If it is coming from behind the atoms, the light has to be at a higher-than-normal frequency to make the ions fluoresce; if it is coming head on, a lower frequency. The physicists can compare the exact frequency shift with what Einstein's equations predict. The current measurement should be nearly 10 times more accurate than previous tests, says team member Gerald Gwinner. Once finished, the group might move to a different storage ring, whose ions fly by at 40 percent of the speed of light, he adds.

In the "extension" to the standard model of particle physics, formulated by Alan Kostelecky of Indiana University and colleagues, light could perform an entirely new trick. Its polarization might change as it shines through empty space, dependent on its frequency and the distance traveled. Overall, Kostelecky says, this is the best test of special relativity violations in any system to date, and improved imaging of distant galaxies will only sharpen it.

The standard model extension suggests places for high-energy particle-physics experiments to look as well. Measurements of the muon's magnetic moment at Brookhaven National Laboratory could spot an effect, as could experiments at the Fermi National Accelerator Laboratory on kaons and D mesons--two exotic, short-lived particles. Fermilab researchers are currently poring over data obtained from the Kaons at the Tevatron (KTeV) and the FOCUS (FOtoproduction of Charm: Upgraded Spectrometer) experiments on kaon and D-meson decay. A violation could show up as a variation in certain decay parameters on a 24-hour cycle, says Bruce Winstein of the University of Chicago. Winstein reports that researchers have gone through half the KTeV data so far, with another year and a half of number crunching to go, and no anomalies have cropped up yet. "It's probably not there at all," he says. But "it's important to look for things you don't expect to see."

An array of atomic clocks set to board the International Space Station within the decade or fly off into space on satellites will offer additional high-accuracy tests. Their measurements won't supersede those of Earth-based tests, says Kostelecky. Rather, he explains, because the clocks will fly and rotate faster, and in a different plane, than they would on Earth, they will give experimenters access to a wider spectrum of potential violations. "It's like a spacetime roller coaster ride," he remarks. --JR Minkel

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