Wave function collapse

Wave function collapse is the update from a quantum state containing multiple possible measurement outcomes to a state associated with one definite outcome. In standard quantum mechanics, it complements continuous evolution under the Schrödinger equation.

Quantum measurement: A physical interaction that produces information about a quantum system and yields an observed outcome. Collapse is the state update associated with a measurement outcome.
Many-worlds interpretation: An interpretation in which quantum evolution never collapses and measurement outcomes occur in branching components of the state. It retains the full superposition rather than selecting a single outcome by collapse.
Niels Bohr: A Danish physicist whose work helped establish quantum theory and the Copenhagen interpretation. His account of measurement helped shape the framework in which collapse is a state update.
Schrödinger's cat: A thought experiment placing a cat in a superposition correlated with an unpredictable quantum event. It dramatizes the question of when a superposition becomes one definite outcome.
Stern–Gerlach experiment: An experiment that separates particles by spin components using a nonuniform magnetic field. Its discrete detector spots exemplify definite outcomes from quantum alternatives.
Born rule: The rule assigning probabilities to measurement outcomes from a quantum state's amplitudes. It determines the probabilities of the outcomes that collapse can select.
Objective-collapse theory: A family of theories in which quantum states undergo spontaneous, physical collapses governed by modified dynamics. It treats collapse as a real process, not merely a measurement rule.
Werner Heisenberg: A German physicist who formulated matrix mechanics and the uncertainty principle. His early quantum theory helped establish the measurement problem surrounding definite outcomes.
Wigner's friend: A thought experiment in which an observer measures a system while another observer treats the combined setup quantum mechanically. It exposes disagreement over whether collapse depends on an observer's perspective.
Double-slit experiment: An experiment in which particles produce interference after passing through two openings when path information is unavailable. Obtaining path information changes the observed pattern, raising questions about measurement and collapse.

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