Wigner’s Friend Thought Experiment Challenges Absolute Event Descriptions in Mechanics

The nature of reality and how observation defines it remains a central question in physics, and a particularly challenging thought experiment, Wigner’s Friend, continues to fuel debate. Hervé Zwirn from the Centre Borelli and IHPST, along with colleagues, revisits this classic puzzle to explore its implications for our understanding of absolute events. The research presents a comprehensive analysis, beginning with the original proposal and extending to recent theoretical developments and ingenious variations of the experiment. The team’s work suggests that the very notion of events occurring independently of observation may be flawed, potentially reshaping how physicists conceptualise the fundamental structure of reality and challenging long-held assumptions about objectivity.

This article document presents a detailed exploration of Wigner’s Friend thought experiment and its modern extensions, investigating the philosophical and physical implications for our understanding of quantum mechanics and reality itself. It provides a comprehensive summary, broken down into key areas of inquiry.

Wigner’s Friend introduces a scenario where one observer, conventionally termed how does the wave function collapse from a superposition of states into a definite outcome when a measurement occurs? What precisely defines a measurement, and what role does the observer play in this process? The standard formulation of quantum mechanics, described by the Schrödinger equation, predicts unitary evolution, meaning superpositions evolve predictably, yet measurement seems to violate this principle, introducing an apparent discontinuity.

The initial Wigner’s Friend scenario has undergone significant evolution, with modern extensions involving multiple layers of observers observing each other, creating increasingly complex and nested scenarios. These multi-observer chains amplify the paradox, as each observer’s perception of reality diverges from the others. Recent theoretical research, notably the work of Dziewiński and others, has produced ‘no-go’ theorems demonstrating inherent contradictions if one simultaneously assumes both the universality of quantum mechanics —that quantum laws apply to all systems, including observers —and the existence of observer-independent facts. These theorems rigorously demonstrate that it is logically impossible to maintain both principles, strengthening the argument that reality is, in some sense, observer-dependent. The implications are profound; if facts do not exist independently of observation, then the very notion of an objective reality is called into question. Experiments are currently being conducted, utilising entangled photons and sophisticated measurement techniques, to test the predictions of these extended scenarios. These experiments aim to determine whether the correlations predicted by quantum mechanics hold even in these complex multi-observer settings, pushing the boundaries of our understanding and potentially revealing the limits of quantum theory itself.

The paradoxes raised by Wigner’s Friend compel us to confront the interpretational challenges inherent in quantum mechanics. The Copenhagen Interpretation, the traditional view, struggles to provide a satisfactory explanation of what constitutes a measurement, often relying on an undefined distinction between the quantum system and the classical observer. The Many-Worlds Interpretation (Everett), proposes that every quantum measurement causes the universe to split into multiple branches, each representing a different possible outcome. This avoids wave function collapse but introduces the concept of parallel universes, a notion that many find counterintuitive. Relational Quantum Mechanics (Rovelli) focuses on the relationships between observers and systems, arguing that quantum states are not absolute properties but are defined relative to an observer. This interpretation suggests that there is no privileged frame of reference and that reality is fundamentally relational. QBism (Quantum Bayesianism) treats quantum states as subjective degrees of belief, emphasizing the role of the observer’s information and personal experience. Convivial Solipsism (Zwirn) proposes that each observer experiences their unique reality, and there is no objective reality independent of observation, presenting a maximally perspectival interpretation. Objective Collapse Theories (Ghirardi-Rimini-Weber) propose modifications to quantum mechanics that introduce spontaneous wave function collapse, independent of observation, attempting to resolve the measurement problem by altering the fundamental laws of physics. Each interpretation offers a different perspective on the nature of reality and the role of the observer, and each faces its own challenges and criticisms.

Wigner’s Friend thought experiment and its extensions profoundly challenge the notion of objective realism, the idea that there is a single, observer-independent reality. The no-go theorems suggest that facts may not exist independently of observation, implying that reality is not a pre-existing entity waiting to be discovered, but rather is, in some sense, created by the act of observation. This raises fundamental questions about the nature of truth and knowledge. If reality is observer-dependent, then what does it mean to say that something is true? Is truth relative to the observer, or is there some deeper, underlying reality that transcends individual perspectives? The thought experiment highlights the pivotal role of the observer in shaping reality, prompting us to reevaluate our assumptions about the relationship between consciousness and the physical world. It raises questions about how we acquire knowledge and whether universal truths are possible in a quantum universe, challenging the foundations of epistemology and metaphysics.

The article supports the idea that reality is, in some sense, observer-dependent, and that the pursuit of an entirely objective understanding of quantum mechanics may be fundamentally impossible. Convivial Solipsism is presented as a particularly radical, yet logically consistent, perspective on this issue, suggesting that each observer inhabits their unique quantum reality.