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Relational quantum mechanics

An interpretation of quantum mechanics proposed by Rovelli (1996). The quantum state of a system is relative to another physical system. Properties are established through interaction, not possessed independently. There is no observer-independent state.

Core claim

All physical quantities are relational. As velocity is relative in classical mechanics, RQM extends relativity to all properties. A measurement is an interaction between two systems. The result is a fact relative to the system that performed the interaction.

Experimental basis: Bell’s theorem (1964; Aspect 1982; Nobel Prize 2022) established that classical local realism fails. RQM builds on this directly.

Origin (1996)

Rovelli diagnosed the measurement problem as resting on a mistaken assumption: that systems possess observer-independent states. The key analogy was with special relativity — Einstein dissolved the puzzles of the Lorentz transformations by dropping absolute simultaneity. RQM drops absolute quantum states. The 1996 paper was framed in information-theoretic language: a maximum of relevant information extractable from a system, and new information always acquirable.

Evolution

Relational EPR (Smerlak and Rovelli, 2007). Can RQM handle EPR correlations without non-locality? Their argument: apparent non-locality disappears when you stop assuming observer-independent properties. The comparison of results between distant observers is itself a local interaction. Contested — Laudisa (2019) argued the locality claim remains “at best dubious.”

Transcendental reading (Bitbol, 2007, 2010). The first sustained philosophical framework for RQM. Bitbol replaced “physical observers” with functional reference frames and interpreted RQM’s relationalism through transcendental philosophy — expressing the conditions of possibility of quantum experience rather than claims about things-in-themselves. His transcendental structuralism occupies a position between naive realism and pure instrumentalism.

“Rovelli’s World” (van Fraassen, 2010). The central philosophical challenge. If facts are relative, what connects different observers’ accounts? Van Fraassen proposed a postulate linking perspectives — without it, observer accounts float free, threatening intersubjectivity. He concluded consistency could be maintained, but the cross-perspective problem became the defining issue for the next decade.

Wigner’s friend sharpened (Frauchiger and Renner, 2018; Brukner, 2018). Frauchiger-Renner showed that three assumptions — universal quantum mechanics, single outcomes, transferable certainty between agents — cannot all hold. Brukner derived a Bell-type inequality showing that universal QM, locality, freedom of choice, and observer-independent facts are mutually incompatible — stronger than standard Bell. RQM survives both by dropping observer-independent facts. Experimental support: Bong et al. (2020, Nature Physics).

Stable facts (Di Biagio and Rovelli, 2021). The answer to van Fraassen: decoherence spreads correlations across many degrees of freedom, making relative facts effectively shared — stable facts whose relativity can be ignored at macroscopic scales. The classical world is the world of stable facts. Stability is a matter of degree.

Cross-Perspective Link (Adlam and Rovelli, 2023). A new postulate: “the measurement of a system’s pointer accurately reveals the outcome said system has registered in an earlier interaction.” Two readings emerged — a strong version (reintroducing something like observer-independent facts) and a weak version (preserving RQM’s perspectivalism but raising questions about sufficiency). Developed from van Fraassen’s 2010 suggestion. The tension between the two readings is unresolved.

Fact-nets (Martin-Dussaud, Zalamea et al., 2023). A mathematical framework where all facts are binary entities involving two systems symmetrically. Addresses a longstanding gap — RQM had operated as an interpretational layer without its own formalism.

Ongoing debate

Incompatibility claims. Lawrence, Markiewicz, and Żukowski (2023, Quantum) argued that relative facts lead to contradictions with standard quantum predictions using a GHZ-state Wigner’s-friend scenario. Cavalcanti, Di Biagio, and Rovelli (2023) responded: the argument implicitly combines relative facts across perspectives as if they were absolute — the assumption RQM rejects. Lawrence et al. (2025) maintained their position.

Pienaar’s quandaries (2021). Five no-go theorems targeting RQM’s core claims. Central tension: RQM wants quantities to take unique values at interactions while maintaining universality and completeness of quantum theory. Di Biagio and Rovelli (2022) responded: the theorems smuggle in ontic assumptions about the quantum state that RQM denies — the state is a computational tool, not a representation of reality.

The iteration problem (Riedel, 2024). Are facts about relative facts themselves relative? The Unrestricted Iteration Principle says yes — leading to infinite regress. Riedel argued RQM may need to commit to perspectivalism over simple relationalism.

The interaction problem (Faglia, 2025). RQM claims events occur at interactions but never adequately defines “interaction.” Plausible specifications either fail or require new postulates.

Ontological proposals. Dorato and Morganti (2022) argued neither priority monism nor ontic structural realism fits RQM — proposed metaphysical coherentism with an event-based ontology. Bitbol (2007, 2010) offered a neo-Kantian reading: transcendental structuralism, metaphysically agnostic. Fano and Sanchioni (2025) constructed an ontology grounded in relational properties.

Philosophical affinities

Constructive empiricism (van Fraassen), ontic structural realism, and Nagarjuna’s relational ontology — see Rovelli for detail. QBism (Fuchs) shares the information-theoretic approach but privileges agents where RQM naturalises the observer. RQM and Rovelli’s loop quantum gravity share the same commitment — no fixed background, no absolute structure — and were developed together: background independence in quantum gravity naturally suggests that quantum states should be relational too.

Current status

RQM is at a crossroads (Calosi and Riedel, 2024 — Foundations of Physics special issue). The community is small but intellectually active. Core tenets are settled; fundamental challenges remain — cross-perspective consistency, the interaction criterion, the iteration problem.

References

Rovelli, C. “Relational Quantum Mechanics,” Int. J. Theor. Phys. 35, 1637–1678 (1996)
Smerlak, M. and Rovelli, C. “Relational EPR,” Found. Phys. 37, 427–445 (2007)
Bitbol, M. De l’interieur du monde (Flammarion, 2010)
Van Fraassen, B. “Rovelli’s World,” Found. Phys. 40(4), 390–417 (2010)
Brukner, C. “A No-Go Theorem for Observer-Independent Facts,” Entropy 20(5), 350 (2018)
Frauchiger, D. and Renner, R. “Quantum theory cannot consistently describe the use of itself,” Nat. Commun. 9, 3711 (2018)
Laudisa, F. “Open Problems in Relational Quantum Mechanics,” J. Gen. Phil. Sci. 50, 215–230 (2019)
Bong, K.-W. et al. “A strong no-go theorem on the Wigner’s friend paradox,” Nat. Phys. 16, 1199–1205 (2020)
Di Biagio, A. and Rovelli, C. “Stable Facts, Relative Facts,” Found. Phys. 51, 30 (2021)
Pienaar, J. “A Quintet of Quandaries,” Found. Phys. 51, 97 (2021)
Di Biagio, A. and Rovelli, C. “Reply to Pienaar and Brukner,” Found. Phys. 52, 62 (2022)
Dorato, M. and Morganti, M. “What Ontology for RQM?” Found. Phys. 52, 63 (2022)
Brukner, C. “Wigner’s Friend and Relational Objectivity,” Nat. Rev. Phys. 4, 628–630 (2022)
Adlam, E. and Rovelli, C. “Information is Physical: Cross-Perspective Links,” Phil. Phys. 1(1), 4 (2023)
Lawrence, J. et al. “Relative Facts are Incompatible with Quantum Mechanics,” Quantum 7, 1015 (2023)
Cavalcanti, E., Di Biagio, A. and Rovelli, C. “On the Consistency of Relative Facts,” Eur. J. Phil. Sci. 13, 55 (2023)
Martin-Dussaud, P. et al. “Fact-nets,” Found. Phys. 53, 26 (2023)
Calosi, C. and Riedel, F. “RQM at the Crossroads,” Found. Phys. 54, 74 (2024)
Riedel, F. “Iteration of Relativity,” Stud. Hist. Phil. Sci. (2024)
Faglia, S. “RQM Does Not Resolve the Problem of Measurement,” Phil. Phys. (2025)
Fano, V. and Sanchioni, M. “Relational Properties and RQM,” Found. Phys. 55 (2025)
Laudisa, F. and Rovelli, C. “Relational Quantum Mechanics,” Stanford Encyclopedia of Philosophy (2002, revised 2021)

Persons

Rovelli