Measuring Length in Relative Motion

This preprint analyzes length measurement in the presence of relative motion using an explicitly specified light-based measurement procedure. An idealized bus is considered in two inertial reference frames: one in which the bus is at rest (IRF) and one moving with constant velocity relative to it (IRF′). The length of the bus is reconstructed exclusively from measured return times of light signals and fixed geometric relations between selected points. The analysis is carried out entirely within classical physics, treating light as a signal propagating in vacuum with speed c. It is shown how the same geometric length can be obtained from measurements performed in both reference frames within the framework of the specified operational procedure, and how the corresponding time differences are related. The results emphasize the central role of operational assumptions and concrete measurement procedures in the determination of length in situations involving relative motion.