| The Most Precise Test Yet of Special Relativity by Phil Schewe, James Riordon, and Ben Stein The most precise test yet of special relativity, better than previous tests by a factor of three, has been performed by researchers at the Universities of Konstanz and Düsseldorf in Germany (Holger Mueller, holger.mueller@uni-konstanz.de). Presenting their result at this week's CLEO/QELS meeting in Long Beach, California (Paper QME6), the researchers devised a state-of-the-art version of the 19th-century Michelson-Morley experiment, which first established that the speed of light is isotropic, or the same in every direction. They found that special relativity passes their high-precision test with flying colors: the speed of light does not depend on its direction of propagation to within 1.7 parts in 1015, an accuracy about three times higher compared to the best previous experiment. Recent tests of special relativity are motivated by modern physics developments such as certain versions of string theory, which suggest that special relativity may not hold exactly, and that violations might reveal themselves in tests with a certain level of precision. In their experiment, the Konstanz-Düsseldorf team made use of two devices known as optical cavities. Each cavity basically consists of two mirrors held at a constant distance. How long it takes a light beam to make a round trip between the mirrors is a direct measure of the speed of light perpendicular to the mirror surfaces. Each of the two cavities is oriented in a different direction. Rotating the setup could measure light speed in a variety of directions. However, errors can accrue in this setup through variations in 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. Therefore, they reduced length 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 obtained 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. Studying the isotropy of light is but one of three classes of tests needed to fully verify the special theory of relativity. In another class of experiments, the team had already obtained the most precise result in a Kennedy-Thorndike experiment (Update 571), which showed that the speed of light is constant regardless of the velocity of the laboratory that measures it. As spaceborne experiments, such as OPTIS, SUMO and Space-Time, are also planned, the latest experiment is part of a whole new generation of relativity tests. |