Measurement problem

The conflict between quantum theory’s continuous evolution of superpositions and the definite outcomes observed in measurements. It asks how, or whether, quantum mechanics accounts for a single observed result.

Schrödinger equation: The equation governing the continuous time evolution of a quantum state. Its unitary evolution preserves superpositions, creating one side of the conflict.
Many-worlds interpretation: An interpretation in which the universal wave function always evolves unitarily and all outcomes occur in branching worlds. It rejects collapse and treats each possible outcome as realized in a different branch.
Niels Bohr: A Danish physicist who developed the Copenhagen interpretation and helped establish quantum theory. His account emphasized classical descriptions of experimental arrangements and outcomes.
Quantum superposition: A quantum state formed from a linear combination of possible states, allowing interference between them. The measurement problem begins when a superposition appears to yield only one observed result.
Wigner's friend: A thought experiment in which one observer measures a system while another treats the observer and system quantum mechanically. It probes whether different observers can assign incompatible accounts of a measurement outcome.
Wave function collapse: A proposed discontinuous change in a quantum state to an outcome associated with a measurement. Collapse supplies definite outcomes but is not part of ordinary Schrödinger evolution.
Bohmian mechanics: A quantum theory in which particles have definite positions guided by a wave function. It adds definite particle configurations to explain observed outcomes without fundamental collapse.
Werner Heisenberg: A German physicist who formulated matrix mechanics and the uncertainty principle. His early interpretation helped shape the debate over quantum states and observation.
Hilbert space: A complete inner-product vector space used to represent quantum states and observables mathematically. Its state vectors provide the formal setting for superpositions and measurement predictions.
Bell's theorem: A theorem showing that no theory satisfying specified locality conditions reproduces all quantum predictions. It rules out some classical explanations but does not alone solve the measurement problem.

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