Wigner's friend

Meaning

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.

Origin

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