A Decisive Experiment Comparing Vertical and Horizontal Light Travel Times in Gravitational Fields: Testing the Gravitational-Potential Dependence of Light Speed

It is important to emphasize that, under a coordinate formulation corresponding to the measurement performed by a single ground-based atomic clock, General Relativity (GR) yields the same qualitative prediction: the path-integrated effective propagation speed of light in the vertical direction is slightly greater than that in the horizontal direction. This result is consistent with the prediction of OBWT. The two theories differ only at the level of second-order corrections, which are beyond the resolution of current experimental capabilities. Consequently, this experimental configuration does not constitute a discriminating test between GR and OBWT. We propose a ground-operated decisive experiment to compare the round-trip travel times of laser light along vertical (radial) and horizontal (transverse) paths of equal geometric length, using a single high-precision atomic clock fixed on the ground. The experiment is motivated by a phenomenological hypothesis: in a weak gravitational field, the speed of light increases with gravitational potential as v(r)=c(1−GM/(c2r))v(r)=c(1−GM/(c2r)), while atomic clock rates also depend on the potential, making local measurements of cc always yield cc (explaining the null result of Michelson–Morley type experiments). By keeping the clock on the ground, the vertical path integrates the light speed over varying heights, whereas the horizontal path experiences constant light speed. The model yields a predicted time difference ΔTmodel≈−GMc3(H/R)2ΔTmodel≈−c3GM(H/R)2 (negative, i.e., vertical travel time shorter), while general relativity predicts a positive difference ΔTGR≈+2GMc3⋅H/RΔTGR≈+c32GM⋅H/R. For height H≳2H≳2 km, ∣ΔTmodel∣∼10−18∣ΔTmodel∣∼10−18 s, within the reach of current optical lattice clocks. The experiment can decisively discriminate between the two theories by the sign of the measured time difference. Acknowledgments The author thanks auxiliary AI tools for linguistic revision, typesetting and numerical verification. All original physical ideas, model derivations and conclusions are independently completed by the author. OBWT constructs an independent flat-spacetime gravitational theory named Ontological Basic Wavefield Theory (OBWT), founded on the wave vector distribution axiom: u² + v² = Φc² From this fundamental axiom, a scalar energy formula describing mass-energy flow under gravitational potential is derived: E = γΦv M₀ Φc² The theory predicts that coordinate light speed varies with the gravitational scalar potential Φ. The tangential coordinate speed of light equals √Φ c, while a radial scale compression effect governed by γΦ (analogous to Lorentz contraction for horizontal motion) yields a radial coordinate light speed of Φc. Nevertheless, local observers always measure an apparent light speed of c along both radial and tangential directions. This originates from the synchronous contraction of wave packets and light wavelengths in the vertical direction, together with atomic clock frequencies modulated by the potential-dependent light speed. There are two essential distinctions between OBWT and General Relativity (GR). First, OBWT provides a concrete physical ontology: gravitation is not spacetime curvature, but scalar field modulation of light speed, wave-packet energy, phase evolution rate and radial wave-packet compression within flat spacetime. It offers an explicit physical mechanism, unlike GR’s purely geometric interpretation of gravity. Second, the exact static gravitational potential of OBWT reads Φ = (1−GM/(c²r))² Its weak-field Taylor expansion is 1−2GM/(c²r)+(GM/(c²r))² with a quadratic term coefficient of +1. In contrast, the effective potential of GR in isotropic coordinates carries a quadratic coefficient of +2. This quadratic discrepancy is negligible for weak-field solar system tests but becomes prominent in strong gravity regimes, leading to distinct observational predictio