Wigner's friend

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

Proper noun

 * 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, concludes that both the particle and his friend remain in quantum superposition until Wigner himself learns the result of the observation.
 * 2) * 2001,, 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.
 * 1) * 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.

Translations

 * German: