Copenhagen interpretation

A family of interpretations of quantum mechanics in which a quantum state predicts probabilities for measurement outcomes, and measurement has a central role in defining those outcomes.

Born rule: A rule that converts quantum amplitudes into probabilities for measurement outcomes. It supplies the probabilities the interpretation assigns to possible measurement results.
Niels Bohr: A Danish physicist whose work on atomic structure and quantum theory shaped twentieth-century physics. Bohr developed complementarity and a central philosophical account associated with Copenhagen.
Many-worlds interpretation: An interpretation of quantum mechanics in which all branches of the universal wave function continue to exist. It avoids collapse by treating every possible outcome as realized in a different branch.
Quantum measurement problem: The conflict between quantum evolution’s linear dynamics and the definite outcomes observed in measurements. It asks how definite results arise if quantum states evolve continuously between measurements.
Wave function collapse: A proposed discontinuous change in a quantum state associated with obtaining a definite measurement outcome. It represents the state update associated with a measurement result.
Werner Heisenberg: A German physicist who formulated matrix mechanics and the uncertainty principle. Heisenberg offered influential accounts of measurement and quantum state reduction.
Bohmian mechanics: A deterministic theory in which particles have definite positions guided by a quantum wave. It offers definite particle trajectories instead of treating the quantum state as only predictive.
Wigner's friend: A thought experiment in which an observer measures a system while another observer treats the combined setup quantum mechanically. It tests whether measurement outcomes can be definite for one observer but indefinite for another.
Quantum measurement: A physical interaction that yields information about a quantum system and is represented by measurement operators. Measurement outcomes are central to how the interpretation connects states with observations.
1927 Solvay Conference: An international physics meeting in Brussels where leading physicists debated quantum theory. The meeting brought Bohr and Einstein’s disagreements over quantum mechanics into sharp focus.

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