The Structural Inevitability of Quantum Theory in Causal Update Systems Below the Central Regime

This paper establishes the structural inevitability of quantum-theoretical description within the framework of Causal Update Cosmology (CUC I–IX). Rather than deriving quantum-mechanical equations or advocating a specific interpretation of quantum theory, the present work adopts an exclusion-based structural approach. Starting from the completed formulation of CUC and the concept of the central regime, it demonstrates that classical deterministic description is structurally impossible in causal update systems that have not yet reached or cannot maintain the central regime. Below the central regime, causal path competition remains immature, history indistinguishability is unsaturated, and self-averaging does not operate. Under these conditions, states cannot be uniquely defined, deterministic temporal evolution cannot be constructed, and observation cannot be treated as a passive readout. These failures are not due to epistemic limitations or insufficient information, but follow directly from the causal update structure itself. The paper shows that while probabilistic description becomes unavoidable once determinism is excluded, classical probability theory is insufficient, because probability in such systems is intrinsically coupled to the update process and competing causal histories are structurally non-independent. As a result, quantum-theoretical description emerges as the minimal and only consistent remaining framework. Observation is reinterpreted as an irreversible causal update rather than a measurement of a pre-existing state. This provides a structural resolution of the measurement problem and explains why quantum systems consistently appear on the initial side of the central regime upon observation: not because they are fundamentally incapable of reaching the central regime, but because observational updates disrupt the conditions required to maintain it in systems with limited causal redundancy. Classical description, determinism, and decoherence are shown to arise only as derived and conditional features once the central regime is reached and stably maintained. In this sense, classical theory is not fundamental but emergent. The conclusions of this paper are structurally symmetric to those obtained in the analysis of relativistic inevitability within CUC. Just as relativistic spacetime structure is unavoidable in central-regime–admitting systems, quantum theory is unavoidable below the central regime. Both are thus positioned as necessary descriptive frameworks selected by causal structure, rather than as exceptional postulates. This work represents a key step in extending Causal Update Cosmology beyond relativistic structure to the foundations of quantum theory, and sets the stage for a unified structural treatment of gravity and spacetime curvature in future studies. Note on AI Use This work also explores the broader question of whether new physical theories can emerge through collaboration with AI systems. ChatGPT (OpenAI) was used extensively in the generation and structuring of theoretical ideas and manuscript text, beyond simple editing or polishing. All numerical experiments and scientific validation were performed by the human author.