Source-velocity composition as a test framework for the second postulate: three direct tests, a photon speed law, and a 2-enhanced upstream Thomson-scattering test

The invariance of the speed of light with respect to the motion of its source is a foundational assumption of special relativity and one of the most extensively tested principles in physics. However, most existing tests constrain only specific source-velocity-dependent models and leave open the broader question of how potential source-motion effects could be parameterized and experimentally distinguished. Here, we develop a Source Velocity Composition (SVC) framework that continuously interpolates between special relativity and source-dependent emission models through a single dimensionless parameter. The framework yields exact predictions for laboratory and astrophysical observations while preserving a clear experimental interpretation. Reanalysis of classic source-motion tests, including high-energy particle emission measurements, binary-star constraints, and precision Doppler experiments, places strong limits on source-velocity effects. We show that forward-emission configurations possess limited sensitivity, whereas backward-emission geometries exhibit a relativistic enhancement proportional to γ², providing substantially greater discrimination power. Based on this result, we propose an upstream Thomson-scattering experiment capable of probing source-velocity effects far beyond the reach of traditional emission tests. The analysis identifies experimentally accessible observables, derives closed-form timing and frequency-shift relations, and establishes quantitative bounds within a unified framework. Regardless of whether deviations are ultimately observed, the proposed approach provides a systematic and falsifiable methodology for testing one of the central assumptions underlying modern relativistic physics.