How classicality emerges — pointer states survive environment monitoring
S(S:fE) → S_class (plateau)
Einselection: only pointer states ∣p⟩ with [H_S,H_int]=0 survive decoherence. Quantum Darwinism: classical information is recorded redundantly in many environment fragments — mutual information I(S:fE) plateaus at classical value.
Quantum Darwinism (Zurek 2003): Classical reality emerges because pointer states imprint redundant copies of themselves into the environment. An observer intercepts only a fragment (fraction f) of the environment yet recovers all classical information — the mutual information I(S:fE) rises sharply then plateaus. Non-pointer superpositions decohere: their off-diagonal density matrix elements decay as ρ_{ij} → ρ_{ij} e^{-γt|i-j|²}. The redundancy number R_δ measures how many independent observers can learn the system's state — it scales with system size, explaining the robustness of macroscopic classicality.