An experiment to directly test the second postulate of special relativity is described. The speed of photons, resulting from the annihilation of either thermal positrons or in-flight positrons (moving with relativistic velocity), is measured using two complementary variations of the same basic experiment. For both at rest and moving positrons the constancy of the speed of light was confirmed to an accuracy of approximately 1%. This apparatus can be used in an advanced undergraduate laboratory and also used to place limits on alternative theories to special relativity that have transformations other than the Lorentz transformation.

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