A Definitive Test of Light-Speed Invariance Using First-Order Interferometry with Controlled Motion

ABSTRACT Current tests of light-speed invariance have historically measured potential anisotropies due to the unknown motion relative to hypothetical preferred frames. Due to overlapping/indistinguishable predictions, null results can’t rule out light-speed anisotropy but only establish upper limits on this unknown velocity. We propose a fundamentally different experimental approach eliminating that dependency on hypothetical frames with the introduction of a lab-controlled, deterministic motion v , and enabling a discriminating test rather than an upper-limit-only result. An innovative motion-controlled close-loop interferometric setup is designed to detect two-way light speed deviation with first-order sensitivity ( ), to this motion . This first-order sensitivity allows us to omit second-order effects from both the unknown frame velocity and plausible relativistic corrections, which are at the level. Unlike traditional M-M tests, our design achieves theoretical sensitivity to controlled motions as small as 0.1 m/s—approximately 10ˆ4-fold improvement. This precision enables us to set , with as the total experimental uncertainty. The key methodological innovation is that when we set , the predicted outcomes for light-speed invariance versus anisotropy theories become non-overlapping and empirically distinguishable. Hence, it enables a deterministic answer as to which theoretical prediction matches the experimental result. For example, when , a Null result would contradict the prediction of light-speed anisotropy. This design closely aligns with the generalized Sagnac effect and can be extended to test the new prediction of Sagnac effect in inertial frame. This paper takes a deliberately theoretical-agnostic stance by describing an experimental approach that would clearly discriminate between different predictions when . This paper does not discuss which underlying theoretical framework or interpretation is right. Theoretical interpretation of the eventual experimental result is reserved for future work. Regardless of the outcome, implementing this experiment will address a long-standing gap and contribute substantially to fundamental physics.