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Wigner's friend



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

Proper nounEdit

Wigner's friend

  1. (physics, philosophy) 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.
    • 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,
      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 collapse.
    • 2009, Henry Stapp, Wigner's Friend, Daniel Greenberger, Klaus Hentsche, Compendium of Quantum Physics: Concepts, Experiments, History and Philosophy, Springer, page 857,
      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.
    • 2016, John G. Cramer, The Quantum Handshake: Entanglement, Nonlocality and Transactions, Springer, page 82,
      In 1962 Eugene Wigner elaborated on the knowledge issue with his Wigner's Friend paradox, an expansion of the Schrödinger's Cat problem [11]. Wigner replaced the cat with a “friend”, i.e., an intelligent observer and at the same time replaced the hydrocyanic acid mechanism with a less lethal piece of apparatus, e.g., a light bulb that is switched on when a count is recorded.

Usage notesEdit

The thought experiment illustrates the central role the conscious observer has in the quantum mechanical measurement process (and therefore should have in any interpretation of quantum mechanics).


  • 1961, E. P. Wigner, "Remarks on the mind-body question", I. J. Good (editor), The Scientist Speculates, London, Heinemann.
  • 1998, John Gribbin, Q is for Quantum: Particle Physics from A to Z.

Further readingEdit