Causal Clock Unification — A Design Framework for Unified Timekeeping Across Scales

A boundary-condition design framework for clock-comparison architectures — replacement core. The framework treats a clock as a complete architecture — oscillator, interrogation apparatus, comparison channels, and protocol — and states the causal boundary conditions of timekeeping in three quantities. The causal loading Λ_O = T_req/T_budget of a named operation over its declared protocol event graph, with Λ_O ≤ 1 a necessary feasibility condition by construction: T_req is the infimum over lower-bound schedules, computed by the forward-composed longest path when arrival maps are FIFO. The descriptive delay ratio δ = T_1way/τ_avg, a charting coordinate with no feasibility meaning: δ > 1 is the normal condition of long-baseline one-way records such as pulsar timing. And operational causal closure — realised availability, realised timeliness, and well-definedness at every required decision event — which is distinct from metrological agreement. The central statement of the framework: causality bounds when a comparison result can exist and how fast a loop can respond; it does not bound the admissible averaging window. Supersession (v1.0.0 → v2.0.0) Version 1.0.0 bound the averaging time of a phase comparison by propagation delay; independent external review (August 2026) established this assignment as physically incorrect, and the v1.0.0 parameter η(τ) = L_comparison/(cτ) is retired. Per the framework's own firewall clause — gaps in the core require a new major version — this release replaces the core, following seven rounds of independent external review. Version 1.0.0 remains resolvable at its version DOI and is superseded; nothing was silently edited. Released under stewardship: the steward fixes the release state and takes responsibility for the record without claiming sole authorship of every formulation (see the document's Endorsement Marker). Canonical purpose This record defines a citation-stable conceptual design framework for reasoning about timekeeping architectures across spatial and temporal scales — from co-located optical clocks through fibre and satellite links to pulsar timing — in a single declared vocabulary of latencies, budgets, and protocol event graphs. Firewall The document is intentionally non-operational and non-normative. It proposes no standards, algorithms, timescales, or governance structures, makes no claim of new physics, and asserts no empirical scaling law: the one empirical conjecture is confined to the evolving band and explicitly marked unestablished. Its purpose is a shared design language that can be cited independently of any specific implementation, policy discussion, or institutional process. Lock–key separation Relationship to interpretive commentary. This framework functions as a conceptual reference layer. Narrative interpretation, motivation, and discussion of societal or infrastructural implications are deliberately separated from this record. Commentary or perspective articles published elsewhere may reference the framework as an external design object without reproducing its formal content; such commentary is interpretive context, not a definition or extension of the framework itself. Stability statement and citation Sections §1–4 constitute the frozen, citation-stable core as of version 2.0.0; amendments require a further major version. Marked illustrative-figure blocks and their captions are evolving-band inserts excluded from the frozen text; §5 and the reference-corrections ledger may evolve in future versions without altering the core. The canonical core text is the file ccuf-core-v2.0.0.md (SHA-256 e1acf79a63f24bde373aada667202e4753f7af0896ee69a6c6b230e9d01121c6); the record also ships an illustrated copy (Markdown and PDF) and the figure sources. Cite this version's DOI for the frozen core; the concept DOI resolves to the latest version.