Reality Is Relational: QBism and Relational Quantum Mechanics

Most attempts to solve the measurement problem try to remove the observer from the story. Two of the most interesting recent interpretations go the other way: they put observers — or, more carefully, perspectives — at the centre, and ask what follows.

QBism: the quantum state as an agent's expectations

QBism (originally "Quantum Bayesianism"), developed by Carlton Caves, Christopher Fuchs and Rüdiger Schack, starts from a radical move: a quantum state is not a property of a system out in the world. It is an agent's degrees of belief about the consequences of their own future actions on that system.

On this view, "collapse" is nothing mysterious. It is simply an agent updating their expectations after experiencing an outcome — the same thing any of us does when new evidence arrives. The Born rule is reinterpreted as a consistency condition: a normative rule that tells an agent how their probabilities ought to hang together, rather than a law describing an agent-independent reality.

QBism does not deny that a world exists. It says quantum theory is a tool each agent uses to navigate that world, and that measurement outcomes are genuinely new events created in the encounter between agent and system.

Relational quantum mechanics: facts are relative

Carlo Rovelli's relational quantum mechanics (1996) takes a different but kindred step. Here there is nothing special about conscious agents — any physical system can play the role of observer. The key claim is that the values of physical quantities are not absolute. They exist only relative to another system with which the first has interacted.

So in Wigner's friend, there is no contradiction: relative to the friend, the outcome is definite; relative to Wigner, who has not yet interacted with the lab, it is not. Just as velocity in relativity only makes sense relative to a frame, a quantum fact only makes sense relative to a system.

Where they agree, and where they part

• Both refuse to treat the wavefunction as a literal, observer-independent object spread through space.
• Both make measurement outcomes relative — to an agent (QBism) or to a physical system (RQM).
• They part on the observer: QBism centres first-person agents and their experiences; RQM spreads "observing" across all physical interactions.

The hard question both leave open

If facts are relational, how do different observers come to agree? Why does the world look shared and stable rather than fragmented into private perspectives? Answering that — how intersubjective, classical reality is stitched together out of many relative facts — is, for me, one of the most important open problems in the foundations of physics. It is also exactly where the physics of decoherence and quantum Darwinism, the subject of the next note, becomes essential.

Further reading: C. Rovelli, "Relational Quantum Mechanics" (1996); C. A. Fuchs, N. D. Mermin & R. Schack, "An Introduction to QBism with an Application to the Locality of Quantum Mechanics" (2014).