Simultaneity in the Foundations of Special Relativity

Einstein’s synchronization condition is commonly introduced as a conventional
procedure for defining simultaneity using light signals, from which the relativity of simultaneity is subsequently derived. This paper examines the logical role played by this condition at the foundations of special relativity, independently of later coordinate constructions. It is shown that the synchronization criterion implicitly presupposes a physically meaningful symmetry in light propagation in order for simultaneity to be well-defined at all, rather than merely operationally stipulated. This symmetry is
later treated as a consequence of the theory, creating a dependency inversion
between premises and results. The analysis identifies an equivocation between
operational and physical notions of simultaneity that remains unresolved within the standard formulation. Implications for the interpretation of light-speed invariance and frame equivalence are discussed, without proposing experimental
modification of special relativity.