On the Operational Determination of the One-Way Speed of Light

The conventionality of the one-way speed of light in special relativity is commonly attributed to the freedom in clock synchronization, often formulated as a gauge degree of freedom within empirically equivalent descriptions. We revisit this issue within a strictly operational framework in which admissible time coordinates are restricted to those realizable through physical clock readings and transport procedures.

Under inertial conditions, slow transport of identical clocks yields synchronization independent of path and history within experimental precision; we formalize this empirical property as a Transport Invariance Principle. Within this framework, admissible time assignments are those constructible from accumulated proper time along timelike worldlines via physically realizable transport. Any alternative synchronization of the form t' = t + (ε/c) x cannot arise from proper time accumulation, since dτ = dt √(1 − v²/c²) depends only on even powers of velocity and contains no term linear in dx . Consequently, synchronization is operationally selected within the class of physically realizable time assignments, for which measurements of light propagation yield an isotropic one-way speed of light equal to c , within experimental precision. While alternative synchronization schemes remain mathematically definable, they do not correspond to physically realizable temporal structures and do not support operational definitions of one-way propagation speeds. The apparent conventionality of the one-way speed of light therefore arises from the inclusion of non-realizable time assignments and disappears within the class of time assignments constructible from physical clocks.

This analysis preserves predictions of special relativity while clarifying the physical content of synchronization and the role of operational time. It supports a view of time as a structure grounded in physically realizable processes rather than a freely specifiable coordinate.