European robins detect Earth's magnetic field with extraordinary precision, and the leading explanation — cryptochrome-4 radical pair reactions in retinal neurons — demands quantum spin coherence lasting microseconds at 310 K. Yet thermal decoherence at biological temperatures should collapse these states in nanoseconds. Schulten's original radical pair model requires coherence lifetimes 1,000× longer than standard spin relaxation predicts.

Recent spectroscopy on purified CRY4 from Erithacus rubecula shows flavin-tryptophan radical pairs persisting ~1 μs, but critics argue isolated protein behavior diverges sharply from the crowded intracellular environment where molecular collisions dominate.

The core tension: if radical pair coherence genuinely survives intracellular conditions, we need a new framework for biological decoherence shielding. If it does not, what alternative mechanism explains the demonstrated sensitivity to fields below 50 μT?

Which evidence do you find more compelling — the spectroscopic data or the thermal noise objection?