Detector Bounds or Spacetime Bounds? Operational Limits on Reconstructing Cutoff Fields in Flat and de Sitter Spacetime

Whether asymptotically flat and de Sitter spacetimes are operationally distinguishable depends not only on background geometry, but also on the resources available to an observer. We formulate a conditional model for reconstructing a generic ultraviolet-cutoff field in a finite region. The cutoff gives a finite density of independent field degrees of freedom, while stable reconstruction of a time-dependent field requires a corresponding stream of classical records. If finite reusable memory is employed and the required records are repeatedly reset, their erasure exports entropy. Under explicit assumptions—including a closed operational-system condition in which the detector, its power supply, and its usable entropy sink share an area-scaled entropy budget—we derive a finite bound on the duration of fixed-resolution reconstruction. The result is local: it applies equally to a sufficiently small laboratory in flat and de Sitter spacetime and therefore does not, by itself, establish a de Sitter-specific limitation. A distinction arises only with an additional global premise: de Sitter must preclude an indefinitely enlarging external entropy sink, whereas asymptotically flat spacetime may permit one. The central unresolved issue is thus the physical status of entropy export, rather than finite detector memory alone.