Wigner’s friend

A thought experiment in which an observer inside a laboratory measures a quantum system while an outside observer treats the laboratory quantum mechanically. It probes whether measurement outcomes can be definite for one observer while remaining in superposition for another.

Quantum superposition: A quantum state that combines multiple possible outcomes until an interaction or measurement yields a particular result. The outside observer can describe the friend and measured system as a superposition of outcome branches.
Eugene Wigner: A Hungarian-American physicist whose work helped establish symmetry principles in quantum mechanics and particle physics. Wigner introduced the friend scenario to examine the role of consciousness in measurement.
Many-worlds interpretation: An interpretation of quantum mechanics in which all components of the quantum state continue to evolve without collapse. It treats the friend’s distinct outcomes as branches rather than selecting one observer-independent result.
Extended Wigner’s friend scenario: A multi-observer thought experiment in which several laboratories and agents make quantum measurements. It generalizes the original setup to compare the consistency of several observers’ accounts.
Quantum measurement: An interaction that correlates a quantum system with an apparatus or observer and yields a recorded outcome. The friend’s measurement creates the outcome whose status is disputed between observers.
Measurement problem: The problem of reconciling quantum evolution into superpositions with the definite outcomes observed in experiments. The thought experiment gives this foundational problem an explicit observer-inside-observer structure.
Objective collapse theory: A class of theories in which quantum superpositions spontaneously collapse through physical processes. It predicts a physical limit to extending coherent quantum descriptions to a macroscopic laboratory.
Frauchiger–Renner paradox: A proposed contradiction arising when agents apply quantum theory to other agents’ measurements under specific assumptions. It sharpens the friend scenario into an argument that universal quantum reasoning, single outcomes, and consistency cannot all hold.
Schrödinger equation: The equation governing the continuous time evolution of a quantum state in the absence of measurement. Wigner can apply this unitary evolution to the entire laboratory, including the friend.
Von Neumann–Wigner interpretation: An interpretation proposing that consciousness plays a role in the transition from quantum possibilities to definite outcomes. Wigner’s original discussion connected the friend scenario to a consciousness-based account of measurement.

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