Wigner's friend

Wigner's friend. This thought experiment, introduced by the physicist Eugene Wigner in 1961, is a provocative extension of the famous Schrödinger's cat paradox. It challenges the foundations of quantum mechanics by questioning the nature of objective reality and the role of consciousness in the wave function collapse. The scenario places an observer, "the friend," inside a sealed laboratory, creating a chain of observation that leads to profound questions about quantum measurement and the interpretation of physical states.

Thought experiment

The scenario involves two observers: Wigner outside a sealed laboratory and his friend inside it. The friend performs a quantum measurement on a microscopic system, such as a photon whose polarization is in a superposition of horizontal and vertical states. According to standard Copenhagen quantum mechanics, the friend's interaction causes the system's wave function to collapse into a definite state, which the friend then records. However, from Wigner's external perspective, who treats the entire laboratory—including his friend and all apparatus—as a single quantum system, no collapse has occurred. For Wigner, the friend, the measuring device, and the original photon remain in a vast, entangled superposition of all possible outcomes. This creates a stark contradiction: the friend has a definite, subjective experience of a single result, while Wigner's quantum description insists on an indefinite, objective reality.

Interpretations of quantum mechanics

Different interpretations of quantum mechanics resolve this paradox in radically different ways. The Copenhagen interpretation, associated with Niels Bohr, typically holds that measurement by a conscious observer is fundamental, but it struggles to define when such consciousness applies, leading to an ambiguous chain. The Many-worlds interpretation, proposed by Hugh Everett III, eliminates collapse entirely; both possible measurement outcomes occur in branching, non-communicating universes, so Wigner and his friend simply inhabit different branches of a multiversal wave function. The de Broglie–Bohm theory posits an objective, deterministic reality guided by a pilot wave, giving definite positions to particles at all times, thus avoiding the paradox of indefinite states. In contrast, the Quantum Bayesianism or QBist approach, developed by thinkers like Christopher Fuchs, treats quantum states as subjective degrees of belief, so each agent (Wigner and his friend) simply has different information, with no single objective reality.

Experimental tests and developments

Recent advances in quantum optics and quantum information theory have transformed Wigner's friend from a purely philosophical puzzle into a subject for experimental inquiry. In 2019, a team led by researchers at Heriot-Watt University and the University of Vienna performed a laboratory test using entangled photon pairs and human-level observers simulated by automated measurement devices. Their work, published in journals like *Science*, demonstrated a violation of inequalities derived by Daniela Frauchiger and Renato Renner, suggesting that quantum theory cannot consistently describe the use of itself by multiple agents. These developments sit at the intersection of foundations of physics and quantum computing, pushing the boundaries of what constitutes a valid observation.

Implications for measurement

The thought experiment forces a re-examination of the very concept of a quantum measurement. It challenges the assumption that there is a clear, physical threshold at which a quantum system transitions from a superposition to a classical, definite state—a problem known as the Heisenberg cut. If the friend's consciousness is not a sufficient cause for collapse, then the chain could extend indefinitely, perhaps requiring the consciousness of the entire universe. This has spurred discussions in the philosophy of science about scientific realism and the role of the observer, influencing fields like quantum cosmology and studies of the early universe.

Related thought experiments

Wigner's friend is part of a rich lineage of conceptual challenges in quantum foundations. It directly extends Schrödinger's cat, which entangled a microscopic event with a macroscopic outcome. The Einstein–Podolsky–Rosen paradox, formulated by Albert Einstein, Boris Podolsky, and Nathan Rosen, questioned the completeness of quantum mechanics and led to the concept of quantum entanglement. John Bell's theorem and the subsequent Bell test experiments provided a way to test local realism against quantum mechanics. More recently, the Frauchiger–Renner theorem has formalized the contradictions inherent in applying quantum theory to complex, self-referential agents, creating a modern extension of Wigner's original dilemma.

Category:Thought experiments Category:Quantum mechanics Category:Philosophy of physics