Locality and Reality in Special Relativity: A Timelike-Boundary Reading

Timelike boundaries provide a natural setting for organizing causal admissibility, reference structure, visible deviation, and local measurement content on the same Lorentzian surface. This manuscript develops a timelike--boundary reading of local measurement using the established language of special relativity and quantum mechanics as the reference framework. The central object is a timelike boundary equipped with a boundary observer field and observer--adapted cuts. Lorentzian geometry supplies the admissible causal domain of a candidate event. The boundary cut supplies the local comparison surface on which selected quantities are read relative to a coarse--grained reference structure. A local record appears when a boundary--relative deviation becomes resolvable on that cut. The analysis deliberately focuses on the shared interface rather than on the full individual dynamics of general relativity or quantum mechanics. The common structure considered here is the cut--level assignment of causal admissibility, reference structure, resolved deviation, and local record formation. The key distinction is between causal admissibility and measurement content. The causal cone constrains which prior data or contextual contributions may be relevant for a candidate event; it does not by itself supply the local record. The reference structure is specified internally by the boundary reading and is accessed in local records only through locally resolved deviations from it. Thus measurement does not expose the reference structure directly, but records a deviation relative to it. The resulting framework identifies the observer--adapted cut on a timelike boundary as a potential interface where Lorentzian causal geometry and quantum--mechanical record language can be organized together without replacing the established content of either theory.