Classicality

Classicality is the property of a system, theory, or description that conforms to classical physics, especially Newtonian mechanics, determinism in phase space, and definite properties in measurement. In physics, many macroscopic phenomena appear classical because quantum effects become negligible when actions are large compared with Planck's constant and because environmental interactions rapidly suppress coherence.

Emergence from quantum mechanics: The quantum-to-classical transition is investigated through decoherence, where coupling to an environment destroys interference between components of a superposition. This process selects a preferred basis, or pointer states, that behave approximately classically. Coarse-graining, dissipation, and the thermodynamic limit further align the dynamics with classical probability distributions.

Classical limits and approximations: In semiclassical methods, such as the WKB approximation or the use of coherent states, quantum dynamics follows approximate Newtonian trajectories. For many-particle systems, statistical descriptions (Liouville or Boltzmann equations) arise as effective theories when microstate information is limited.

Realism and observables: Classicality is often linked to approximately commuting observables that can possess definite values, and to the emergence of a single outcome in measurements. This is a central topic in discussions of macrorealism and the quantum-to-classical boundary, where violations of classical intuition (e.g., entanglement, nonlocal correlations) mark nonclassical behavior.

In summary, classicality denotes the regime or description in which classical physics provides accurate, practical predictions and quantum features are effectively suppressed.