Quantum Decoherence
Why macroscopic superpositions die: environment-induced loss of interference.
A pure quantum state $|\psi\rangle = \alpha|0\rangle + \beta|1\rangle$ has off-diagonal density matrix elements $\rho_{01} = \alpha \beta^*$ encoding interference. Coupling to a large environment $|E\rangle$ entangles system + environment:
$$|\psi\rangle |E_0\rangle \to \alpha|0\rangle|E_0\rangle + \beta|1\rangle|E_1\rangle.$$Tracing out the environment:
$$\rho_{\rm sys} = |\alpha|^2|0\rangle\langle 0| + |\beta|^2 |1\rangle\langle 1| + \alpha\beta^* \langle E_1|E_0\rangle |0\rangle\langle 1| + \text{h.c.}$$If $\langle E_1 | E_0\rangle \to 0$ (environment "records" which-state info), off-diagonals vanish — the state becomes a classical statistical mixture. Decoherence is fast for macroscopic objects; the famous Schrödinger's cat is decoherent on timescales ~$10^{-23}$ s in air.
Decoherence time scales with separation: for a free Brownian particle of mass $m$ at temperature $T$ over distance $\Delta x$, $\tau_d / \tau_{\rm relax} \sim (\lambda_{\rm dB}/\Delta x)^2$. "Pointer basis" selected by interaction Hamiltonian.
Decoherence + Born rule reproduces measurement statistics without explicit collapse — but does NOT solve the measurement problem (which outcome occurs).