Quantum decoherence

Quantum decoherence is the process by which a quantum system loses its coherent superposition and appears to collapse into a classical state due to interaction with its environment. It is not a separate physical law but a consequence of the uncertainty principle applied to composite systems: as the environment's degrees of freedom become entangled with the system, the phase relationships that define quantum interference become delocalized across the combined system-environment state, effectively disappearing from any local observation. Decoherence explains why macroscopic objects behave classically without requiring an ad hoc 'collapse postulate' — the classical world emerges from quantum mechanics through the relentless entanglement of quantum systems with their surroundings. The rate and manner of decoherence depend on the interaction Hamiltonian between system and environment, and the identification of stable, pointer-like states that resist decoherence is central to the quantum measurement problem.