Beyond Content: Surface Matching, Temporal Divergence, and the Limits of Synthetic Baselines

This preprint examines whether the temporal structure observed in long-form Human-LLM interaction can be reproduced by a stateless synthetic baseline matched on broad surface properties. Building on Beyond Content: Temporal Dynamics of Long-Form Human-LLM Interaction (Baldovino, 2026), the study compares a 25-thread empirical archive against five synthetic runs generated with a surface-matched null designed to preserve broad interactional constraints while excluding persistent identity, biography, long-horizon memory, explicit goals, and user-specific adaptation. The synthetic baseline closely approximated assistant-side bulk behavior, but did not recover the broader user-side variability, amplification structure, or recovery dynamics observed in the empirical archive. Coupled metrics showed the same pattern: rolling amplification remained comparatively flat in the synthetic condition, while the empirical archive displayed stronger positional variation and broader excursions. Lagged coupling analyses provided secondary, directionally supportive evidence of differential local interaction structure. The paper does not make semantic, universal, or agency-based claims. Its contribution is structural and methodological: within this archive and this baseline family, substantial surface matching alone was insufficient to reproduce the full temporal morphology of long-form human-LLM interaction. The results also suggest that constructing a strong synthetic null for long-form interaction is itself a nontrivial methodological problem.