Event-Dependent Physical Records in Open Systems: Accessibility, Reconstruction, and Control

An event occurs and passes. What remains is physical: the surrounding domain evolves differently from how it would have evolved had the event not occurred. That difference—not a symbol or a measurement outcome—is taken here as the primitive object and called an event-dependent physical record. Our central result is that a record can persist exactly while ceasing to be detectable. We exhibit a unitary evolution in which the record's norm is conserved and its spectral modulus remains unchanged at every frequency, yet its projection onto any fixed finite-rank detector falls below every prescribed detection threshold in finite time, with an explicit upper bound on the operational lifetime. Physical persistence and operational accessibility are therefore logically independent properties. Three further results follow. First, persistence is shown to be a consequence of reversibility rather than a postulate. Second, physical distinctness, detectability, and reconstructibility form a strictly nested hierarchy, with counterexamples to both converse implications. Third, for a Gaussian coordinate, the predictive content recoverable from a given observation channel is bounded by the channel's mutual information, independently of the reconstruction algorithm and the control law.