A new limit on the overall validity of special relativity has been established by a group of physicists in Germany.
The work was done at the University of Konstanz 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 way 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 tests (KT), 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.
(Braxmaier et al., Physical Review Letters, 7 January 2002; text at this URL; see also this URL)
(Editor's Note: This story, with only minor editing, is drawn from PHYSICS NEWS UPDATE, the American Institute of Physics Bulletin of Physics News Number 571, January 2, 2002, by Phillip F. Schewe, Ben Stein, and James Riordon.)
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