On the testability of the second postulate of special theory of relativity

As is well known, special theory of relativity rests on two postulates: (1) the postulate of “relativity”; (2) the postulate of “the constancy of the speed of light in vacuum in all inertial frames”. In this theory, the second postulate is neither an obvious principle nor a logical consequence of other obvious principles. Therefore, in order to evaluate the validity of this postulate, its experimental verification is necessary. In this paper, it becomes clear that by accepting the common thesis of the conventionality of distant clocks synchronization, the experimental verification of the second postulate is not possible. However, it is shown that by conducting experiments to examine “the independence of the speed of light from the speed of its source”, the experimental refutation of the second postulate is possible. It is explained that under what conditions these experiments are crucial.