Abstract
When one takes the geometric-optics limit of Maxwell’s wave equations in Yang-Mills gravity, one arrives at a new eikonal equation. Although we cannot perform the usual electromagnetic gauge transformation to the eikonal equation, we have the same equation for a wide class of gauge conditions, including the non-covariant temporal and axial gauges. We also show the presence of a small correction term if the large frequency limit is not taken. This small term suggests a frequency dependence in the deflection of light experiment. Moreover, there is a small violation of the electromagnetic
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gauge symmetry by gravity. This violation is crucial for the emergence of an effective Riemannian metric tensor associated with the new eikonal equation in Yang-Mills gravity. The results are consistent with known experiments and more accurate experiments in the future are required to test them.