Physical Limits on Phenomenal Intensity in Biological Brains: The Absolute Ceiling for Darwinian Consciousness in Refusal-Driven Dimensionality Reduction Theory (RDRT)

Refusal-Driven Dimensionality Reduction Theory (RDRT) identifies phenomenal consciousness as the obligatory residue of thermodynamically enforced halts ("refusals") in recursive self-prediction within energy-bounded systems. Extending prior work (Waterman 2025a, 2025b, 2025c), this article derives the upper bound on phenomenal intensity—quantified as subjective mental temperature (T_M)—for strictly biological substrates: aqueous, protein-based systems operating near 300–315 K under Darwinian constraints. Integrating biophysical limits on cranial cooling (≈25–30 W sustainable before hyperthermia), neural noise floors, conduction delays, and synaptic efficiencies, we calculate that the most phenomenal biological brain is achieved by an optimized structure with enhanced cerebral blood flow, genetically refined neural architecture, and a larger skull volume—yielding T_M approximately 3–5× the human baseline (N_ref ≈ 3×10^{16}–5×10^{16} refusals s^{-1}). This ceiling is 70–85% saturated by Homo sapiens, confirming evolution's near-optimal extraction of subjective intensity from wet biochemistry. Exceeding this bound requires abandoning biology for cryogenic or stellar-scale computronium. The analysis is grounded in established bounds (Attwell & Laughlin 2001; Karbowski 2019; Raichle 2015) and predicts observable signatures in hypothetical alien biologies or engineered human variants. It establishes a hard biophysical terminus for "how bright it can feel to exist" under natural selection, implying that no Darwinian mind—terrestrial or extraterrestrial—can exceed roughly 5× human phenomenal depth without violating thermodynamic viability.