Experimental Proposal: A Synchronization-Free Differential Test of One-Way Light Speed Constancy Under Macroscopic Relative Motion

We propose two complementary laboratory experiments to test whether theone-way speed of light depends on the relative velocity between source and receiver—adirect test of Special Relativity’s second postulate. Prior experiments share three unresolveddeficiencies. Michelson-Morley involved no relative motion between source and receiverand tested only the round-trip speed. Alv¨ ager and Sadeh tested source motion only, usingsubatomic particles whose gamma rays traversed bulk scintillator material before detection, risking a light-speed reset via the Ewald-Oseen extinction theorem before the timing signalwas ever registered. Beckmann & Mandics were rendered inconclusive by mirror and slitre-emission artifacts. Crucially, no prior experiment has tested the receiver-motion half ofthe postulate—whether a moving receiver facing a stationary source measures the samelight speed. This proposal is, to our knowledge, the first to test the second postulate using a macroscopicclassical source in continuous windowless vacuum with no mirrors or slits, under controlledsource–receiver relative motion, for both a moving source and—for the first time—a moving receiver with a stationary source. Configuration A uses an accelerating macroscopiclaser in UHV (10−9 Torr) with two stationary detectors. Configuration B inverts the roles: a stationary laser with accelerating detectors, directly probing receiver-motion dependencefor the first time. A further obstacle is the clock synchronization problem. Measuring an absolute one-waytransit time requires synchronized clocks at two separated locations. Any synchronizationprocedure must assume a value for the one-way speed of light—the very quantity undertest. The two measurements are therefore logically circular and fundamentally irresolvable. Our proposal dissolves this problem entirely. We do not measure absolute transit times. Instead, we measure only how the difference in arrival times ∆t between two co-stationarydetectors changes as the source–receiver relative velocity v is varied. Any synchronization offset τsync between the detectors is a fixed constant. It cancels exactly when we formδ (∆t) = ∆t (+v) −∆t (−v) across approach and recession runs, regardless of the value ofτsync or the synchronization convention used. Under Special Relativity, δ (∆t) = 0 for allv. Otherwise, δ (∆t) ≈ 2vL/c2, a femtosecond-scale signature well within reach of moderntime-correlated single-photon counting (TCSPC) hardware. At the detector surfaces, sinceboth APDs are identical, any surface interaction is the same at D1 and D2 and likewise cancels in δ (∆t). The full experiment is implementable for under 300k within 6–12 monthsusing off-the-shelf components