Experiment #10 · Thought experiment

Wigner's Friend

Whose measurement counts?

Eugene Wigner · 1961 · Quantum foundations

First published: E. P. Wigner, "Remarks on the Mind-Body Question", in *The Scientist Speculates*, ed. I. J. Good (1961), 284–302.

A friend performs a measurement inside a sealed lab. To them the outcome is definite; to Wigner outside, the friend is in superposition.

Wigner extends Schrödinger's cat by replacing the cat with a conscious experimenter inside a closed laboratory. The friend measures a quantum system and records a definite outcome. From outside, with no information leaked, Wigner is still required by linear quantum mechanics to describe the friend-plus-system as in superposition. Two observers, two ostensibly correct descriptions. Recent "extended" versions (Frauchiger–Renner 2018) sharpen the case into a contradiction between observer reports if standard quantum mechanics, locality, and observers-apply-to-themselves are all assumed.

Formulation

Friend in isolated lab measures |↑⟩+|↓⟩, records outcome. Outside observer assigns superposition state to (friend ⊗ system). Each observer's account is correct in its own frame; they cannot both be right under naive realism about quantum states. Frauchiger–Renner extends to show contradictions in iterated nested-observer settings.

Compare this experiment

Dimensions Engaged

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Observer

Strikes directly at Observer · Number and Observer · Metaphysical Agency: can the same physical event be at once "measured" (for one observer) and "in superposition" (for another), and if so, what does "outcome" mean?

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Information

Bears on Information · Ontological Status: if quantum states encode an observer's information rather than the world's state, the paradox dissolves — but at the cost of a substantival reading of information.

Responses — How Schools Engage

Affirms / takes the bait 2

Relationalism

Relational QM (Rovelli) takes Wigner's friend as the argument: properties exist only relative to systems that interact with them. There is no "view from nowhere" — the friend's outcome and Wigner's state are both correct, *for different observers*.

Idealism

Some idealist readings welcome the asymmetry: the friend's conscious observation collapses the wave function for them, but Wigner has performed no collapse. Consciousness is the ontologically relevant variable.

Denies / rejects the premise 1

Logical Positivism

No empirical disagreement is possible until Wigner opens the lab; talk of "the friend's superposition" is meaningful only as a calculational device. The paradox is a confusion of state-talk with fact-talk.

Reframes the question 2

Multiverse Theory

Everettian: the friend exists on multiple branches simultaneously; Wigner, before correlation, has not yet entered any branch and so describes the joint state correctly. No contradiction, only nested decoherence.

Analytic Metaphysics / Logical Atomism

Frauchiger–Renner shows that at most three of {standard QM, single outcomes, observer-independence, locality} can be retained. The metaphysical work is choosing which to drop.

Holds it inconclusive 1

Quantum Realism

The case is genuinely live: realism about ψ is in tension with conscious observers being themselves physical systems. QBism and relational accounts handle it; collapse-realist accounts struggle.