What principle, formulated by Bohr, states that quantum mechanics must reduce to classical physics?

The correspondence principle states that quantum mechanics must reproduce classical physics in the appropriate limit, especially for large quantum numbers or macroscopic systems.

Niels Bohr developed the idea while refining his early quantum theory of the atom. He coined the term “correspondence principle” in 1920, building on the observation that quantum predictions should connect smoothly with established classical results. In the Bohr model, for example, the frequencies and intensities of spectral lines approach classical expectations as the quantum number becomes large.

The principle helped guide early quantum theory, although modern physics treats the classical limit more carefully. Quantum mechanics does not simply become classical in every situation; classical behavior usually emerges under particular conditions, such as large actions, large quantum numbers, environmental decoherence, or suitable approximations.

It is often confused with Bohr’s complementarity principle, which concerns the mutually exclusive wave and particle descriptions of quantum objects. The uncertainty principle, associated with Heisenberg, concerns limits on simultaneously knowing pairs such as position and momentum.