THE SPEED OF LIGHT IS INDEPENDENT of the speed of the light source to within one part in 1020. Kenneth Brecher of Boston University (617-353-3423, reaches this conclusion by studying gamma radiation arriving from distant gamma ray bursters (GRBs). Consider, he says, the gamma production at the GRB: radiation will come to us from the near side of a presumed expanding object, and from the receding far side. Because of expected explosive nature of the GRB engine, its near and far sides might, at least in some cases, be moving apart at a fair fraction of the speed of light. Any differential in the speed of light arising from these two gamma-emitting locations would then result (after a very long extragalactic journey) in a stretched-out gamma pulse upon arriving at Earth. In addition, the emitted gamma rays would scatter off of energetic thermal electrons on leaving the burst sources, further broadening the pulses. From the observed sharpness of the arriving pulses, one can deduce the independence of c from the source speed to be less than a part in 1020, an improvement by a factor of 100 billion, says Brecher, over previous tests of this tenet of relativity theory. At last week's APS meeting in Long Beach, CA, Brecher argued that the speed of light is even more fundamental a concept than light itself since it is related to the intimate relation between space and time. Therefore he urged that c be referred to as "Einstein's constant," in analogy to Planck's constant, which sets the scale of quantum measurements.
