Relative Information Defines Relative Facts, Dissolving Previous Problems in the Relational Interpretation of Mechanics

The nature of reality and how we perceive it remains a central question in physics, and a new approach to the relational interpretation of mechanics offers a compelling perspective. Andrea Di Biagio from the Institute for Quantum Optics and Quantum Information (IQOQI) Vienna, Austrian Academy of Sciences, and Carlo Rovelli from Aix-Marseille University, explore this concept by grounding it in quantifiable notions of relative information. This work establishes a clear definition of a relative fact, crucially without introducing any new assumptions beyond established physics or assigning a privileged status to observers. By linking perspectives to specific measurable properties rather than entire systems, the researchers dissolve longstanding problems within the relational interpretation and, notably, demonstrate that a measurement is best understood as a continuous process, rather than an instantaneous event.

This approach defines a relative fact without altering established quantum theory or assigning a special role to observers. By linking perspectives to commutative observables, rather than entire quantum systems, the team resolves several longstanding problems within the interpretation. As a result, they demonstrate that a quantum measurement, when properly described, is a continuous process, rather than an instantaneous event.

Relational Quantum Mechanics and Observer Dependence

This extensive collection of papers and notes centers around Relational Quantum Mechanics (RQM), its implications for the foundations of physics, and the challenges it presents to our understanding of reality, objectivity, and scientific practice. The core idea of RQM is a rejection of absolute, observer-independent properties of quantum systems. Instead, a quantum state is relative to an observer; a system doesn’t have a property, it has a property relative to a specific observer. This interpretation emphasizes that quantum states represent information an observer has about a system, and this information is fundamentally physical, representing correlations between the observer and the observed.

The focus shifts from states describing reality to facts, statements about correlations confirmed through interactions. Unlike many interpretations, RQM doesn’t require a universal wavefunction describing the entire universe; reality is constructed through local interactions and information exchanged between observers. The papers analyze the Wigner’s Friend paradox and its extensions as crucial tests for RQM, highlighting the problem of defining objective reality when multiple observers measure the same system. RQM attempts to resolve this by asserting that each observer’s measurement creates a new, relative fact.

A major concern is ensuring the consistency of relative facts when different observers have conflicting information, and the papers explore conditions under which these facts can be reconciled or remain logically independent. The combination problem, how to combine the perspectives of multiple observers, is a central challenge for any relational interpretation. If all states are relative, how can science claim to discover objective truths? The papers argue that objectivity isn’t about finding observer-independent properties, but about establishing intersubjective agreement, consistent correlations confirmed by multiple observers.

Science, in this view, is a process of building a shared web of information. RQM can lead to non-classical causal structures where the order of events is observer-dependent, challenging our intuitive notions of cause and effect. The papers also touch upon the physical resources required to perform quantum measurements, highlighting the limitations of ideal measurements, and utilize information theory as a fundamental tool for understanding quantum mechanics and the limits of knowledge. This collection of papers presents a radical rethinking of quantum mechanics and its implications for our understanding of reality, objectivity, and the nature of scientific knowledge, proposing a relational view where reality is not a pre-existing entity but is constructed through interactions and the exchange of information between observers.

Observer-Relative Facts in Quantum Mechanics

This work presents a novel perspective on relational quantum mechanics, redefining how facts are understood within the quantum realm. Researchers demonstrate that a relative fact can be defined without adding to established quantum theory or assigning a special role to observers. By associating perspectives with commutative observables, rather than entire quantum systems, the team resolves several longstanding problems within the interpretation. This approach fundamentally reframes the nature of measurement, demonstrating it is properly described as a continuous process, rather than an instantaneous event.

The study establishes that before a measurement is performed, the outcome of an experiment is a fact relative to the observer performing it. However, when another observer chooses to perform a different experiment, they relinquish the possibility of learning the original outcome before their own measurement. This does not imply a single, pre-existing value for the observed property, but rather a relational truth dependent on the perspective of the observer. The team mathematically shows that while probabilities can be computed at any moment during the experiment, these probabilities do not satisfy standard equations when observers differ in their experimental choices.

Furthermore, the research confirms that an observer’s outcome is not a stable fact relative to another observer who has complete control over the system. This failure of conventional probability theory is presented as a symptom of the inherent relationality of facts within quantum mechanics. The work builds upon established principles of information theory, including Shannon’s mathematical theory of communication, to solidify this relational framework and explore its implications for understanding the foundations of quantum physics. This breakthrough delivers a new lens through which to view quantum reality, emphasizing the observer-dependent nature of facts and offering a pathway toward resolving long-standing interpretive challenges.

Relational Quantum Mechanics, Mathematically Defined

This work presents a refined formulation of relational quantum mechanics, grounding its central ideas in quantifiable definitions compatible with standard quantum theory and probability. Researchers demonstrate that a relative fact can be defined without adding to established quantum theory or assigning a special role to observers. By associating perspectives with commutative algebras of observables, rather than entire quantum systems, the team addresses previous criticisms concerning the vagueness of the interpretation and provides a mathematically precise framework. This approach allows for the definition of relative facts based on relative information, demonstrating how the relational interpretation emerges directly from the mathematical structure of quantum mechanics itself.

The team demonstrates that this reformulation requires no modifications to quantum theory, nor does it introduce additional postulates regarding measurement collapse or the role of consciousness, remaining firmly within a naturalistic worldview. Quantum theory, within this framework, describes views from anywhere, rather than a view from nowhere, with facts understood as relative to these perspectives. While acknowledging this approach moves beyond straightforward realism by relinquishing questions lacking empirical content, it offers a coherent and mathematically sound account of quantum phenomena, avoiding both mere instrumentalism and unnecessary ontological assumptions. The authors note that the information measures used are symmetric only when considering the combined information between variables, not the conditional information from one variable to another. They also point out that the maximum information shared between variables is limited by the minimum of their individual information content. Future work, they suggest, may explore the implications of these findings for understanding the emergence of objectivity from the agreement between perspectives, shaped by the interactions between variables.