Word.ws English Dictionary

Wigner's friend

Wigner's friend proper noun: the hypothetical observer in a thought experiment in which Wigner, absent when his friend observes the state of a particle (in some versions, Schrödinger's cat) as it collapses from quantum superposition, concludes that both the particle and his friend remain in quantum superposition until Wigner himself learns the result of the observation.

Proper noun

Wigner's friend

  1. The hypothetical observer in a thought experiment in which Wigner, absent when his friend observes the state of a particle (in some versions, Schrödinger's cat) as it collapses from quantum superposition, concludes that both the particle and his friend remain in quantum superposition until Wigner himself learns the result of the observation.
    • There is no collapse until Wigner comes along, yet because different branches of the previous superposition have acted differently to bring about different branches of Wigner's friend, each branch of Wigner's friend is under the illusion of seeing a sharp state, and thus under an illusion of...2001, David Lewis, How Many Lives Has Schrödinger's Cat?, Lecture, republished in 2004, Frank Jackson, Graham Priest (editors), Lewisian Themes: The Philosophy of David K. Lewis, page 13
    • The second step is to treat Wigner's friend as an unobserved inanimate measuring device that has two states: either it registers the photon, χ₁ or it does not χ₂.2009, Henry Stapp, Wigner's Friend, Daniel Greenberger, Klaus Hentsche, Compendium of Quantum Physics: Concepts, Experiments, History and Philosophy, Springer, page 857

Origin of Wigner's friend

Named after Hungarian-American physicist Eugene Wigner, who described the experiment in an article published in 1961.

Questions about Wigner's friend

What does Wigner's friend mean?
Wigner's friend means the hypothetical observer in a thought experiment in which Wigner, absent when his friend observes the state of a particle (in some versions, Schrödinger's cat) as it collapses from quantum superposition, concludes that both the particle and his friend remain in quantum superposition until Wigner himself learns the result of the observation.
What part of speech is Wigner's friend?
Wigner's friend is a proper noun.

Words near Wigner's friend in the dictionary

  1. Wigner crystals
  2. Wigner effect
  3. Wigner effects
  4. Wigner energies
  5. Wigner energy
  6. Wigner's friend
  7. Wigner-Eckart theorem
  8. Wigner-Seitz cell
  9. Wigner-Seitz cells
  10. WIGP
  11. Wigram

Source and license

Adapted from Wiktionary: Wigner's friend (page history and authors). Text available under the Creative Commons Attribution-ShareAlike 4.0 license. Word.ws reformatted the entry and added links. Wiktionary data of 2026-09-02.

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