Wigner's friend

Wigner's friend
Name	Wigner's friend
Known for	Thought experiment in quantum measurement

Wigner's friend is a thought experiment introduced by Eugene Wigner in 1961 that challenges the interpretation of measurement in quantum mechanics by positing a conscious observer inside a sealed laboratory and an external observer describing the laboratory quantum mechanically. It drew attention from figures such as John von Neumann, Niels Bohr, Werner Heisenberg, Erwin Schrödinger, Max Born, and Paul Dirac and has influenced debates among proponents of Copenhagen interpretation, Many-worlds interpretation, and objective collapse theories including Ghirardi–Rimini–Weber and Penrose interpretation. The scenario catalyzed later work by David Deutsch, Chris Fuchs, Rodolfo Gambini, Roland Omnès, and contemporary researchers like Caslav Brukner, Yakir Aharonov, and Antoine Suarez.

Background and motivation

Wigner proposed the thought experiment against the background of foundational puzzles raised by Albert Einstein during the Solvay Conference, and debates over measurement advanced by John Bell and formalized by John von Neumann in his measurement chain and projection postulate. Influences trace to paradoxes such as Schrödinger's cat and the measurement problem discussed by Louis de Broglie and Max Planck, with methodological input from philosophers and physicists including Karl Popper, Hilary Putnam, Thomas Kuhn, and Bas van Fraassen. Wigner aimed to probe the role of consciousness via thinkers like Arthur Eddington and William James while engaging with mathematical structures introduced by Paul Dirac and formal analysis by Eugene Wigner himself.

Original thought experiment

In Wigner's original formulation the setup features an observer inside a sealed room performing a spin measurement on a particle prepared in a superposition described by Erwin Schrödinger's wavefunction, while an external observer, Wigner, assigns a superposed state to the entire room according to unitary evolution articulated by Werner Heisenberg and John von Neumann. The experiment contrasts the collapse postulated by Max Born's rule with the universal unitary dynamics championed by Hugh Everett III in Many-worlds and highlights contradictions discussed by John Bell in his inequalities. Wigner invoked notions of subjective collapse related to work by Henry Stapp and philosophical positions defended by Rudolf Carnap and Ludwig Wittgenstein about observation and meaning.

Interpretations and implications

Responses split among major interpretations: advocates of Copenhagen interpretation (e.g., followers of Niels Bohr and Werner Heisenberg) emphasize classical-quantum cut and observer complementarity discussed by Abram Joffe and Pascual Jordan; proponents of Many-worlds interpretation such as Bryce DeWitt and David Deutsch deny collapse and treat both inside and outside observers as branches; objective collapse models like Ghirardi–Rimini–Weber and proposals by Roger Penrose modify dynamics to restore single outcomes. Relational approaches advanced by Carlo Rovelli and epistemic approaches like Quantum Bayesianism (QBism) defended by Christopher Fuchs, Rüdiger Schack, and Blake C. Stacey recast the paradox in terms of information and agent-centered probabilities, while modal interpretations from Bas van Fraassen and decoherence analyses by Wojciech Zurek connect to environment-induced superselection.

Variants and extensions

Researchers extended the scenario into multi-agent protocols influenced by Renato Renner, Adrian Kent, Francesco Costa, and Felix Leditzky, producing paradoxes related to locality studied in contexts by Antony Valentini and Nicolas Gisin. The extended Wigner-type thought experiments relate to nonlocality and contextuality explored by Simon Kochen and Ernst Specker in the Kochen–Specker theorem, and communicate with studies on macroscopic superpositions pursued by Anthony Leggett and Nicolas Brunner. Connections to decision theory and game-theoretic analyses involve work by David Wallace and Simon Saunders, while operational reconstructions of quantum theory by Lucien Hardy and Giulio Chiribella provide formal frameworks for variant formulations.

Experimental proposals and tests

Proposals to realize Wigner-like tests draw on technologies developed by groups led by Anton Zeilinger, Serge Haroche, John Martinis, Rainer Blatt, Andrew Steane, and Rudolf Blatt for quantum control of atoms, photons, and superconducting qubits. Experimental designs employ interferometry, cavity QED, and optomechanics pioneered by Theodor Hänsch, Immanuel Bloch, Alessandro Romito, and Mikhail Lukin to probe coherence of macrosystems, and leverage quantum tomography techniques advanced by Christopher Monroe and Dirk Bouwmeester. Recent table-top experiments and proposals reference work by Caslav Brukner, Časlav Brukner collaborators, Jacob Taylor, Eugene Demler, and groups at institutions like CERN, MIT, Harvard University, University of Vienna, and University of Oxford exploring witnessable signatures of Wigner-type superpositions.

Criticisms and philosophical debate

Critics such as Tim Maudlin, John S. Bell, Abner Shimony, and Bas van Fraassen challenge the coherence of invoking consciousness as a physical collapse agent, while philosophers including Daniel Dennett, Thomas Nagel, David Chalmers, and Patricia Churchland analyze ramifications for philosophy of mind. Debates engage with issues raised by Frank Jackson's knowledge argument, Gilbert Ryle's category mistakes, and metaphysical concerns addressed by David Lewis and Saul Kripke. Legal and ethical dimensions occasionally surface in discourse involving institutions like UNESCO and commissions on science policy, and pedagogical treatments appear in textbooks by John Preskill, Sean Carroll, Leonard Susskind, and Steven Weinberg.

Category:Quantum measurement