A New Limit on the Overall Validity of Special Relativity

A new limit on the overall validity of special relativity theory has been established by a group of physicists the University of Konstanz (Germany) quantum optics lab in collaboration with the University of Düsseldorf. In a sense this is the highest accuracy overall test of special relativity, a pillar of modern physics. One of the principles of relativity theory is that the velocity of light, c, will be the same as measured by all observers. Thus, for example, an observer on a train moving very quickly toward a signal lamp will record the same light speed as an observer at rest next to the train tracks; the velocity of the train does not in any make the apparent light speed any greater. In a Michelson-Morley-type experiment (MM), the universality of observed light speed is demonstrated by comparing light beams moving in different directions.

In another class of experiments, called Kennedy-Thorndike (KT) measurements, one tests that c does not depend on the velocity of the laboratory. Since present MM precision is higher than the best KT precision, the Konstanz researchers aimed for a better KT test as a way of confirming, to a new level of accuracy, that c is independent of both the speed and direction of the lab. Basically they keep watch over a set of standing light waves in a chilled cavity over a 190-day period, during which the Earth traces out more than one-half of its orbit around the sun, altering the velocity of the "lab" by an amount equal to 60 km/sec. If c were to vary with lab speed, then the standing waves (constantly compared to a highly stable atomic clock) would fall out of tune with the cavity; the cavity itself, made of sapphire, has very little thermal expansion at a temperature of 4 K, and could be counted upon to keep its shape. In this way the stability of the resonance frequency translated into a three-fold improvement in accuracy over past KT experiments. A 100-fold improvement in the near future is anticipated. (Achim Peters, 49-7531-88-3823, achim.peters@uni-konstanz.de; Holger Mueller, holger.mueller@uni-konstanz.de) (Braxmaier et al., Physical Review Letters, 7 January 2002; also see researchers' webpage on the experiment.)