LONG BEACH, Calif., May 22 -- Searching for possible violations of Einstein's special theory of relativity, researchers from Germany have performed the most precise experimental test to date of one of special relativity's central principles -- that the speed of light is isotropic, or the same in every direction. In a new version of the 19th-century Michelson-Morley experiment, which first established this principle, researchers at the Universities of Konstanz and Düsseldorf found that the theory passes with flying colors: The speed of light does not depend on its direction of propagation to within 1.7 parts in 10^15, an accuracy about three times higher compared to the best previous experiment. Holger Mueller of the University of Konstanz presented these findings at a press conference during CLEO/QELS 2002 Tuesday. Mueller said recent tests of special relativity are motivated by modern physics developments such as string theory, which suggests that special relativity may not hold exactly, and that violations might reveal themselves in tests with a certain level of precision. The Konstanz-Düsseldorf team made use of two devices known as "optical cavities" pointing in different directions. Such cavities basically consist of two mirrors held at a constant distance. The round-trip time of a light beam between the mirrors is a direct measure of the speed of light perpendicular to the mirror surfaces. Any dependence of this speed on the direction of space would show up when the setup is rotated. However, errors creep in through variations of the cavity length, caused by temperature effects and material aging processes. Thus, the researchers employed cavities made from an ultra-pure sapphire crystal, which is virtually impervious to aging effects, and operated it at the temperature of liquid helium, near absolute zero. Length changes are thus reduced to a level significantly lower than what can be achieved at room temperature. Using advanced laser techniques for reading out the cavity round-trip time, the team could obtain a new limit on possible violations of light propagation isotropy. With the experiment still under way, the researchers hope to obtain another threefold improvement after taking sufficient data. As spaceborne experiments are also planned, the latest experiment is part of a whole new generation of relativity tests, Mueller said.