Studying gamma radiation arriving from distant gamma-ray bursters, a Boston physicist has concluded that the speed of light is constant to an unprecedented degree of accuracy.
First detected in the 1970s, the flashes of gamma rays last as little as 0.001 s but with energies per photon of up to 1 million times the energy of optical light. Their short duration and great distance travelled make the bursts a precise probe of special relativity, particularly of the postulate that the speed of light is a constant, independent of its source velocity.
If the speed of the rays depended on the velocity of the emitting material, the initial pulse would spread out in time as it propagated towards us. Estimating the speed of the material and the distance to the bursts, the small spread in photon arrival times during a burst implies that the speed of rays is independent of the velocity of their source to more than one part in 1020 - the most stringent test, by a factor of about 1011, of special relativity ever made possible.
Such constancy merits a grander name than "velocity of light", suggest some researchers, proposing the "Einstein constant". AIP