Quantum Mechanics Gains Authority through Application, Not by Mirroring Reality

Richard Healeys present a “pragmatist” understanding of quantum mechanics, asserting that the theory does not represent physical reality but provides a normative framework for assigning probabilities to physical events. The approach resolves longstanding issues such as the measurement problem and apparent violations of Bell inequalities by reframing the quantum state not as a depiction of reality, but as a tool governing rational belief within a defined environmental context. It offers a key perspective on the theory’s foundations and establishes that the objective data supporting quantum mechanics are grounded in the same principles as measurement outcomes. This re-evaluation moves away from defining what the theory says about reality and focuses on how its successful applications shape our knowledge of the physical world.

Quantum mechanics as normative guidance for belief formation resolves foundational conceptual issues

Three longstanding conceptual problems in quantum mechanics are now resolved, improving on previous methods by moving beyond narrower interpretations of the theory. Approximately 90 years after David Martin noted persistent disagreement about the meaning of quantum mechanics, a framework is established where quantum mechanics guides representation rather than depicting reality itself. This shift allows for a consistent understanding of quantum states and probabilities as objectively normative, influencing beliefs without needing to represent physical elements.

Consequently, the troublesome measurement problem, the illusion of nonlocal action, and the difficulty of defining ontology within quantum field theory are all addressed through this pragmatic approach. Quantum mechanics yields probabilities for physical events, establishing both state and probability as objectively normative influences on belief, rather than representations of physical reality itself. Rather than demonstrating nonlocal action, violations of Bell inequalities highlight how quantum mechanics guides representation without depicting reality. The analysis currently stops short of predicting outcomes when dealing with incompatible magnitudes, as a joint probability distribution cannot encompass all compatible sets of values simultaneously. This framework resolves longstanding issues by assigning new states relative to physical situations; the quantum state does not “collapse” upon measurement, circumventing the need for a problematic physical process.

Relative states and objective quantum data

Probabilities of measurement outcomes are understood as assigning a new state relative to a physical situation where information about the outcome is accessible. In this understanding, there is no measurement problem, and violation of Bell inequalities does not demonstrate “spooky” non-local action. Quantum field theories function as mathematical objects within a model that aids in improving and extending our descriptions of the world, lacking a physical ontology of their own.

Nearly 90 years after its formulation, Mermin noted that disagreement about the theory’s meaning remained strong, with new interpretations constantly emerging and none ever disappearing. Adán Cabello described this situation 13 years later as odd and potentially hindering scientific progress. The scenario of Wigner’s friend and its extensions cannot be realised, yet the data supporting quantum mechanics are objective in the same sense as the relative measurement outcomes described in those scenarios.

The term ‘interpretation’ provides a clue to resolving this problem, possessing both a broad and a narrow sense. Interpreting quantum theory broadly involves understanding how the theory should be understood, while narrowly, it requires specifying what the physical world is like according to quantum theory. However, if quantum theory is understood as not itself defining what the physical world is like, then understanding the theory becomes possible even without a narrow interpretation.

This allows for progress towards understanding quantum mechanics in the broader sense, while dismissing the possibility of achieving a universally agreed-upon interpretation in the narrower sense. Despite his own understanding, Feynman claimed that nobody understands quantum mechanics, as demonstrated by his novel applications of the theory. He cautioned against attempting to understand it in terms of a model representing “how it can be like that”. Understanding quantum mechanics is achieved not by describing a world represented by its mathematical models, but by carefully examining how and to what ends these models are applied.

It is commonly assumed that applying a mathematical model is to take that model as representing aspects of the physical world, albeit approximately or after idealizations. However, a model of quantum theory is not applied by directly representing Einstein’s “elements of physical reality”, but by offering advice on how physical reality may be represented and what to expect when so represented. Today, quantum mechanics is best viewed not as a single theory, but as a framework encompassing many specific theories, such as non-relativistic quantum mechanics and the relativistic quantum field theories of the Standard Model.

Each model includes a mathematical object, such as a wave-function or density operator, representing a quantum state of a physical system, assigning a probability to each of a set of pairwise incompatible and jointly exhaustive events involving the system. These events correspond to one or more statements asserting that a magnitude M associated with system s (an “observable”) has a value within a set Δ of real numbers, termed a magnitude claim, written as ‘Ms∈Δ’. Magnitudes commonly represent physical events, but their content arises from inferences linking it to other statements, perception, and action. The reliability of these inferences depends on the physical context, influencing the meaningfulness of statements about magnitudes. When applied to a quantum state, quantum mechanics yields probabilities for physical events, and the quantum state of a system, alongside these probabilities, are objective due to their normative authority over beliefs.

A situated agent may also encounter physical barriers to informational access, such as Wigner outside the isolated laboratory of his friend performing a quantum measurement. This makes Born probabilities relative to a physical agent-situation, serving the epistemic and practical needs of any agent within that situation, and allowing for explanation of phenomena even without agent control or observation. A quantum state is objective but relative to a physical agent-situation; a system is not in a quantum state, but a quantum state may be correctly assigned to a system relative to a physical agent-situation.

Multiple quantum states may be assigned to a system, each relative to a different agent-situation, yielding different Born probabilities for the same events, but each assignment remains correct relative to its corresponding agent-situation. Textbooks commonly state that quantum mechanics yields probabilities for our observations of events during measurement, but often do not define what constitutes a measurement or when it occurs. A quantum state’s main function is to assign a probability to a set of mutually exclusive events involving a physical system.

A meaningful statement about a magnitude represents a physical event, but its content arises from inferences linking it to other statements, perception, and action. The reliability of these inferences depends on the physical context, influencing the meaningfulness of statements about magnitudes. When applied to a quantum state, quantum mechanics yields probabilities for physical events, and the quantum state of a system, alongside these probabilities, are objective due to their normative authority over beliefs.

Measurement creates an appropriate context, so the Born rule indirectly yields probabilities of measurement outcomes. A magnitude claim ‘Ms∈Δ’ has sufficient content to be assigned a Born probability if the quantum state of system s yields probabilities applicable within an appropriate environmental context. This advice concerns the degree of belief one should have in possible events, given a specific physical situation, as the Born rule’s legitimate application yields probabilities for events.

Representing rather than revealing quantum reality through consistent frameworks

Despite decades of successful application, quantum mechanics remains stubbornly resistant to easy interpretation; this work proposes a shift in focus, viewing the theory not as a description of reality, but as a framework for constructing consistent representations of it. This pragmatic approach allows continued progress in utilising quantum mechanics for practical applications, such as advanced computing and secure communication, irrespective of ongoing philosophical debate. Acknowledging doubts about fully realising complex scenarios like the extended Wigner’s friend experiment is important, as these thought experiments highlight the limits of applying everyday intuition to quantum systems. This research establishes quantum mechanics not as a depiction of reality, but as a framework for consistently representing it; a subtle but key distinction after nearly a century of debate regarding the theory’s meaning. By focusing on how quantum models guide representation, rather than what they represent, scientists circumvent longstanding conceptual problems including the measurement problem and apparent non-locality, offering a pragmatic resolution. This approach clarifies that objective data, even within complex scenarios like the Wigner’s friend experiment, remain consistent when viewed as relative measurement outcomes rather than reflections of an underlying reality.

The research clarified that quantum mechanics functions as a framework for consistently representing reality, rather than directly revealing it. This understanding resolves longstanding conceptual problems such as the measurement problem and apparent non-locality by focusing on how quantum models guide representation. The authors demonstrate that probabilities of measurement outcomes are yielded through the Born rule within an appropriate environmental context, establishing state and probability as objective guides for belief. This pragmatic approach allows continued progress in utilising quantum mechanics for applications like advanced computing and secure communication, irrespective of philosophical debate.