Complementarity

Complementarity is the principle that quantum systems require mutually exclusive experimental descriptions, each revealing features the others cannot, for a complete account.

Wave–particle duality: The observation that quantum entities exhibit wave-like or particle-like behavior depending on how they are measured. It is the best-known case of complementary descriptions: interference and localized detection require different experimental arrangements.
Niels Bohr: A Danish physicist who developed the atomic model and helped establish quantum theory's conceptual foundations. He formulated complementarity as a central principle of his interpretation of quantum mechanics.
Uncertainty principle: A quantum relation limiting the precision with which certain pairs of observables, such as position and momentum, can jointly be specified. It is mathematically distinct from complementarity, though Bohr connected the two ideas.
Classical limit: The regime in which quantum predictions approximate the behavior described by classical physics. Complementary quantum descriptions must recover familiar classical accounts when quantum effects become negligible.
Double-slit experiment: An experiment in which particles pass through two openings, producing interference when path information is unavailable. Adding a path detector reveals particle-like trajectories while suppressing the interference pattern.
Como lecture: Niels Bohr's 1927 lecture in Como, Italy, presenting ideas that became central to his account of quantum theory. Bohr introduced complementarity in this lecture while addressing the limits of classical description.
Realism: The philosophical view that scientific theories describe a reality existing independently of observation. Complementarity challenges simple expectations that every property has one observer-independent classical description.
Quantum entanglement: A relation between quantum systems whose joint state cannot be represented as independent states of each part. Entanglement supplies correlations that challenge classical intuitions about separable descriptions.
Which-way experiment: An experiment designed to determine which path a quantum particle takes through an interferometer. It makes the tradeoff between path knowledge and interference directly measurable.
Solvay Conference: A series of international physics meetings where leading scientists debated foundational questions in quantum theory. At the 1927 meeting, Bohr argued for complementarity in debates with Einstein.

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