Summary of “On perspectivism and relationalism of Relational Quantum Mechanics”
Joanna Luc and Tomasz Placek examine whether Relational Quantum Mechanics (RQM) is best understood as a form of perspectivism, relationalism, or both. They distinguish between:
- Old RQM: Rovelli’s original formulation from 1996.
- New RQM: the formulation developed by Adlam and Rovelli in 2023.
Their central conclusion is that the two versions differ significantly in how they handle observer agreement and perspectival facts.
1. Perspectivism and relationalism
The authors distinguish:
- Ontic perspectivism: reality or facts are relative to a perspective, with no perspective being privileged.
- Relationalism: entities such as properties, events, or facts are constituted by relations between systems.
RQM clearly treats physical facts as relational: a system acquires a value only through interaction with another system. However, the authors argue that RQM’s relation to perspectivism is more complicated, because it is unclear whether facts are merely relative to an observer or are partly constituted by the observing system.
2. Old RQM
In old RQM:
- Physical systems and relational events are part of the ontology.
- Quantum states are not real physical entities; they are bookkeeping and prediction devices.
- A fact has the form, roughly:
[
R(O,X,A,v),
]
meaning that observable (O) of system (X) has value (v) relative to system (A).
The authors identify (A) both as the system involved in the interaction and as the perspective relative to which the fact obtains.
3. Four kinds of agreement
The article distinguishes four senses in which observers might agree:
- Strong agreement
Two observers measuring the same observable on the same system obtain the same value. - Weak agreement
One observer’s result agrees with another observer’s observation of that result—for example, Wigner sees that the Friend saw (v). - Very weak agreement
Within Wigner’s own perspective, Wigner’s measurement of the system agrees with Wigner’s measurement of the Friend’s record. - Perspectival agreement
A third observer finds the records of the two observers correlated within that third observer’s perspective.
The authors argue that old RQM guarantees only very weak and perspectival agreement, not strong or weak agreement. In a Wigner’s Friend-type scenario, the Friend may obtain (+\frac12), while Wigner can later obtain (-\frac12), without contradicting the formalism of old RQM.
4. The temporal problem
A further difficulty concerns the fact that measurements occur at different times. If time is part of the identity of an event, then the Friend and Wigner do not literally measure the same event, even when they measure the same observable on the same system.
Consequently, it is unclear why their results should agree merely because they measured “the same thing.” The authors argue that old RQM requires additional principles explaining how outcomes at different times are connected.
5. Other problems with old RQM
The article also discusses:
- Transition amplitudes: RQM restricts probabilities to events relative to the same system, but probabilities involving events relative to different systems require a more elaborate probability framework.
- Free-floating bare particulars: If a system has properties only when it interacts, it is unclear what individuates or characterizes systems when they are not interacting.
- Underspecified dynamics: Treating the wave function as merely a bookkeeping device does not by itself fully specify how relational facts and probabilities evolve.
The authors therefore conclude that RQM may require more than a purely conceptual reinterpretation of standard quantum mechanics. Some modifications or additional postulates appear necessary.
6. New RQM
Adlam and Rovelli’s new RQM is presented as an attempt to address the agreement and testimony problems. According to the authors, new RQM improves the situation by guaranteeing weak agreement in addition to very weak and perspectival agreement.
However, the authors maintain that problems concerning probabilities and the specification of dynamics remain unresolved in both versions.
Overall conclusion
The article’s main thesis is:
- Old RQM is relational and strongly perspectival, but permits failures of strong and weak inter-observer agreement.
- New RQM preserves the relational and perspectival character of RQM while improving its account of agreement.
- Neither version, as currently formulated, completely resolves the problems of cross-perspective probabilities and underspecified dynamics.
- Contrary to claims that RQM changes only the interpretation of quantum mechanics, the authors argue that it may need additional formal or structural postulates.
