We describe a simple experiment to validate the principle of light isotropy. The method is based on measurement of the ratio between refractive indices of two different optical media by using a collimated beam. The method exploits the speed-of-light dependence of light propagating at an angle across optical interfaces. The experiment provides a means to test for light anisotropy with respect to a preferred reference frame, for example, determined from measurements of the Cosmic Microwave Background anisotropy. Presently, the operational management of the GPS system applies corrections indicating the existence of a universal clock. Other researchers have identified evidence of altitude dependence for the speed-of-light and other speed variation effects. These phenomena do not fully comply with the definition of the inertial frame according to Special Relativity. Previous tests of the speed of light may be categorized into one-way or two-way dependency tests. Two-way tests, such as Michelson and Morley's original experiment average a round trip velocity and, consequently, can only provide limited bounds for some anisotropic effects. One-way tests, such as the experiment described here, measuring the speed of light in a single direction may be designed with significantly increased sensitivity to time-dependent variations in light propagation. They may also be designed to be resilient to clock and wavelength variation errors. Our preliminary results indicate a time-dependent variation of the speed of light that is not correlated with CMB anisotropy but is consistent with anisotropy reported by other investigators. The identification of an absolute or preferred reference frame would provide new experimental evidence that may constrain theories that seek to unify gravity with the other fundamental forces or improve the standard model.
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