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The Turn in Science

In SPLectrum’s philosophy, language is the medium through which a subject — whatever the subject — experiences reality, and shared reality arises from the convergence of subjects. This piece reads across physics, from classical mechanics to relational quantum mechanics (RQM), where observations of a like shape turn up in physics’ own vocabulary: properties manifest only through interaction, shared facts arise through decoherence, and the observer is any physical system. What follows traces how physics got there.

The trajectory

Leibniz (1646–1716) rejects absolute space entirely — space is not a container, it is the ordering of things that coexist. No space without things. A relational view, but without a mathematical theory it remains scientifically inert. Physics keeps Newton’s absolute space for the next two centuries.

Maxwell (1865) unifies electricity, magnetism, and light into a single framework of fields and waves. The source of the classical triumph — and its undoing. His equations predict a fixed speed of light — and, combined with classical statistical mechanics, the ultraviolet catastrophe.

Planck (1900) fits the radiation spectrum by letting energy come in discrete packets — the move that removes the catastrophe. Einstein extends it: light itself comes in quanta, and builds relativity — space and time coupled, gravity as geometry. De Broglie turns it around: particles behave as waves.

Heisenberg (1927) — position and momentum cannot both be definite. Not a limit of instruments, a feature of nature. Born — the wave function gives probabilities, not certainties. Classical mechanics tells you what is there. Quantum mechanics tells you what you will find if you look.

The impasse. Bohr offers complementarity. Bohm restores determinism through hidden variables at the cost of non-locality. Everett proposes all outcomes happen in branching worlds. No consensus.

Bell (1964) shows quantum correlations violate any classical, locally deterministic picture. Aspect (1982) and others confirm it experimentally — loopholes closed by 2015, Nobel Prize 2022. Nature does not behave classically. Separability fails.

Relational quantum mechanics

Rovelli (1996) takes what the physics says and drops the artificial observer status. The observer is any physical system. Properties are facts established through interaction. In Rovelli’s account, dropping observer-independent facts dissolves the paradoxes.

The connection to loop quantum gravity — where spacetime itself is quantised and relational, with no fixed background — is natural. If spacetime is relational, quantum states should be too.

Growth. A small community: Laudisa, Smerlak, Bitbol. The SEP entry (2002) gave institutional recognition. In 2010, van Fraassen published “Rovelli’s World” — identifying the central challenge: if facts are relative, what connects different observers’ accounts?

Decoherence as mechanism. Early RQM stated the principle: facts are relative. Maturation came through decoherence — the physical process producing stable, shared facts from relational ones. Di Biagio and Rovelli (2021) introduced stable facts — facts whose relativity can be ignored. The classical world is the world of stable facts. But decoherence is itself relational.

Frauchiger-Renner (2018) sharpened everything. Three assumptions cannot all hold: QM applies universally, measurements produce single outcomes, reasoning transfers between agents. RQM keeps the physics universal and outcomes single — but facts transfer only where decoherence anchors them into a shared context.

The affinity

The scientific arc runs parallel to the philosophical one: the absolute giving way to the relational — a view Leibniz held for space — carried through Einstein, quantum mechanics, to RQM. Each step extends the turn — Einstein couples space and time, quantum mechanics puts observer-independent properties in question, RQM frames facts themselves as relational. A collective movement, not a point of arrival.