The Return of Scale: Operational Boundary Activation Beyond Momentum Recoil in Massive Spatial Superpositions

The Return of Scale is a technical response to and extension of Lucas C. Céleri, Diogo O. Soares-Pinto, and Daniel A. Turolla Vanzella's The Apparatus Strikes Back: Momentum Conservation and the Cost of Spatial Superpositions (arXiv:2607.08819). It preserves their exact apparatus-recoil visibility as the immutable baseline prediction and asks whether the finite causal participation of an apparatus supplies an experimentally variable boundary of scale. The cited authors are not affiliated with this response, and no endorsement is implied. The note defines the participating apparatus through its acoustic reach, mechanical correlations, preparation interval, and retained degrees of freedom. It factors the total state space into scale, center-of-mass, internal elastic, and environmental sectors to prevent double-counting. A dimensionless, time-local boundary functional is constructed from the gravitational self-energy contrast of the complete branch-dependent particle-apparatus configurations and a normalized record-participation functional. This activates a provisional inter-scale coupling only after recoil, ordinary environmental decoherence, internal modes, and conservation laws have been accounted for. Coherent amplitude transfer between resolved scale sectors is explicitly distinguished from irreversible visibility loss. An exponential residual is permitted only under a weak-coupling Markov reduction with a suitable destination spectrum. The proposed experiment compares matched apparatuses with similar recoil and environmental baselines but different causal propagation structures. A null result constrains the scale-coupling combination; a controlled residual organized by the preregistered boundary variable would motivate a more complete dynamical derivation. This note operationalizes the missing-boundary hypothesis introduced in The Answers Were There All Along and develops the scale-enclosure program presented across Causality and Attraction and Nothing Is All You Need. It does not claim a measured new effect or a numerical threshold.