Access Constraints and Irreversibility in Quantum Measurement

We present an operational account of quantum measurement in which outcome definiteness and irreversibility arise from causal structure, decoherence, and computational constraints rather than from fundamental collapse. We show that in any globally unitary quantum system with environmental coupling and nontrivial causal partitioning, no physically realizable observer can access all information pathways simultaneously. As a result, observer-dependent descriptions—definite outcomes for embedded observers and coherent superpositions for external observers—are mutually consistent and cannot lead to operational contradiction. This framework resolves Wigner-type measurement paradoxes and clarifies the origin of irreversibility without modifying quantum mechanics.