Huygens’ Principle and Computation of the Light Trajectory Responsible for the Gravitational Displacement of Star Images

A finite time-slice computational method has been developed to describe the effects of gravity on light rays passing close to the Sun by employing a theoretical approach introduced by L. Schiff in 1960. One of the key assumptions in the latter study is that each local observer finds a given light ray to travel along the same straight line with the same constant speed c. It is concluded that the observation of star image displacements during solar eclipses is primarily a verification of the fact that the speed of light for a stationary non-local observer located in a gravity-free region of space varies with the light ray's lateral distance from the Sun. This circumstance alone is enough to cause wave fronts to be rotated by exactly the amount predicted using Huygens' principle in both Einstein's general relativity and Schiff's simpler theoretical treatment without assuming any curvature in the light path itself. A light refraction experiment is outlined to test this conclusion, and it is pointed out that the resulting procedure would provide a direct method for the measurement of group refractive indices of dispersive media.