Copenhagenish interpretations of quantum mechanics (2506.00112v1)

Abstract: We define a class of Copenhagenish interpretations encompassing modern interpretations that follow the Copenhagen spirit. These interpretations are characterized by four postulates: Observers Observe, Universality, Anti-$\psi$-ontology, and Completeness. We explain why such interpretations are not equivalent to the textbook (or orthodox) interpretation, nor to the view that one should shut up and calculate, nor to strict operationalism. We then discuss what lessons are implied for Copenhagenish interpretations by the measurement problem, the Wigner's friend thought experiment, and the simple variants of the Wigner's friend thought experiment that we term Wigner's enemy, stalkee, and penpal. In particular, we discuss how Copenhagenish interpretations give multiple distinct descriptions of each experiment, where these descriptions are each individually true, yet cannot be combined into any single description. To make such interpretations consistent, then, one requires epistemological constraints forbidding certain perspectives from being combined. We discuss these constraints, their motivations, and some of the challenges they introduce.

A Review of "Copenhagenish Interpretations of Quantum Mechanics"

This paper by David Schmid, Y. Ying, and Matthew S. Leifer aims at elucidating the principles underlying a class of quantum interpretations they term "Copenhagenish," which draw inspiration from the Copenhagen interpretation but address some of its shortcomings and ambiguities. The authors propose a coherent framework to define and understand Copenhagenish interpretations, emphasizing four main postulates: Observers Observe, Universality, Anti-$\psi$-ontology, and Completeness.

Key Points and Postulates

1. Observers Observe: This postulate asserts that there are entities, particularly observers, who experience definite outcomes during quantum measurements. This challenges the orthodoxy that measurement merely involves abstract state reductions, by insisting on the necessity of definite experiential outcomes at least for human observers. Interpretations may differ on whether these outcomes are objective facts or relative to an observer's perspective.
2. Universality: Quantum theory is fundamentally universal according to this postulate, meaning every system, macroscopic or microscopic, can theoretically be described by it. While this extends to typically classical systems and observers, the postulate maintains that both unitary evolution and non-unitary collapse postulates must be applicable, a balance differentiating it from interpretations like many-worlds or those relying solely on collapse theories.
3. Anti-$\psi$-ontology: Here, the quantum state is not seen as an intrinsic property of a quantum system but rather as a representation of knowledge, information, or beliefs about measurement outcomes. This view permits multiple valid quantum state assignments to the same system, ensuring there is no contradiction in having different state descriptions for different observers.
4. Completeness: This asserts that quantum mechanics provides a complete description within its framework, with no hidden variables or deeper underlying realities. While quantum states do not reveal intrinsic properties of systems, there is no call for additional variables or theories that would complete quantum mechanics in a traditional realist sense.

Dissecting the Orthodox Interpretation

A significant contribution of this paper is its clear distinction between Copenhagenish and orthodox interpretations. The latter assumes a $\psi$-ontic view with the eigenstate-eigenvalue link, positing that quantum states possess intrinsic properties which conflict with the Universality postulate in Copenhagenish views. The authors convincingly argue that, contrary to the orthodoxy, Copenhagenish perspectives avoid ensuing contradictions by embracing anti-$\psi$-ontology.

Addressing Thought Experiments

The authors explore classic thought experiments such as Wigner's Friend, reframing them within a Copenhagenish framework. In these scenarios, they highlight how different observers may assign different quantum states and measurement descriptions without inconsistency, provided they observe epistemological constraints and resist forming a global, objective view of the combined perspectives. This reflects a departure from traditional resolutions offered by realist interpretations, emphasizing observational perspectives rather than attempting to resolve contradictions objectively.

Evaluating the Practical Implications

On the practical side, embracing Copenhagenish interpretations could influence how quantum mechanics is taught, emphasizing the modularity and flexibility of observer-dependent descriptions rather than pushing for a singular objective truth. The paper also posits theoretical implications by questioning long-held assumptions about counterfactual definiteness and the global applicability of quantum characteristics.

Future Directions and Open Questions

The paper prompts further exploration into how these interpretations reconcile with recent advancements in quantum theory and technologies, like quantum computing. Moreover, it challenges researchers to consider how epistemic limitations within a Copenhagenish framework influence experimental designs and philosophical discussions.

In conclusion, the authors propose a compelling re-framing of the Copenhagen interpretation, addressing well-known thought experiments with a blend of anti-realist and complete understandings. Their articulation of how these ideas might develop provides significant insight into ongoing debates and future directions in quantum mechanics interpretations.