On the physical reality of the isotropic speed of light

Abstract

It was suggested by Einstein and later greatly elaborated on by others that the methods used to synchronize distant clocks are a matter of convention. The standard method, in which it is assumed that the speed of light is isotropic, obviously yields an isotropic light speed when such clocks are involved in determining the speed of light. Another method, in which clocks travel symmetrically but otherwise arbitrarily in opposite directions, may also be used to synchronize distant clocks. This method establishes whether or not the clocks are synchronized in a physically significant way in the sense that it allows a distinction to be made between a contrived anisotropic light speed and an anisotropic speed that is physically significant or real. Specifically, a contrived anisotropic light speed results in laws of physics that are not symmetric, whereas a true anisotropic light speed does not affect the symmetry of physical laws. Furthermore, when invariance in the speed of light is imposed, the invariant interval may be identified with the lapse of proper time in the case in which anisotropy is contrived. But, in the case of true anisotropy, this identification is not possible. Experiment reveals that, on the basis of symmetry in physical law, any anisotropy in the speed of light is contrived and not physically significant.