A Formal Applicability Boundary for Physical Description: Complementarity, Irreversibility, and Thermodynamic Cost

This preprint formalizes the applicability boundary implicit in Minimal Necessary and Sufficient Conditions for a Physical Universe, specifying the logical and thermodynamic conditions under which physical description becomes meaningful at all. Rather than proposing new particles, forces, or modifications to established physical theories, the work identifies the minimal structural requirements that must obtain before physical concepts may be consistently applied. The construction is intentionally pre-physical, concerning definability, distinction, information, and irreversibility independently of any particular ontology. Within the ΔQ: The Irreversible Universe framework, distinction remains the sole necessary and sufficient condition for the existence of a physical universe. The present work neither modifies nor extends that claim. Instead, it clarifies the downstream conditions under which physical quantities, laws, and explanations become well-defined. The framework argues that physical description becomes applicable if and only if distinction is both informationally nontrivial and irreversibly realized. This boundary is expressed formally as: Physical description is applicable ⇔ (I > 0 ∧ ΔQ > 0) where I denotes informational content and ΔQ denotes irreversible thermodynamic cost. The proposal does not elevate thermodynamics to a fundamental ontology, nor does it compete with quantum mechanics, thermodynamics, or relativity. Rather, it identifies the point at which such theories become jointly coherent and physically meaningful. This work constitutes the second paper in ΔQ: The Irreversible Universe, a series examining the relationships among complementarity, irreversibility, thermodynamic cost, locality, and the applicability of physical description.