Light deflection in unified gravity and measurable deviation from general relativity

Abstract: Light does not travel in a perfectly straight line when it passes near massive objects. Instead, it follows the curvature of spacetime as predicted by general relativity. In this work, we apply the gauge theory of unified gravity [Rep. Prog. Phys. 88, 057802 (2025)], formulated as an extension of the Standard Model to include gravity. Using dynamical equations, we calculate gravitational deflection of light near astrophysical objects without need to use a curved metric. We do not use a single free parameter, and the ray optics method for the present problem is extremely accurate. The deflection angles obtained from unified gravity and general relativity are equal in the first power of the gravitational constant, which explains previous experiments. However, the second-order terms reveal a measurable relative difference of $1/15\approx6.7\%$. Therefore, experimentally differentiating between the two theories will become possible in the near future.