Redefining Reality: The QBist View of Quantum Measurement

Author: Denis Avetisyan

A growing interpretation of quantum mechanics, QBism, challenges traditional notions of objectivity and proposes a participatory reality shaped by the observer’s experience.

The realization shifts from perceiving quantum measurement devices as fixed instruments to understanding them as tools deeply intertwined with an agent’s prior beliefs, ultimately reducing observation to a specification of $P(E\_j|R\_i)$ and raising the question of how personal probabilities manifest within any measurement process.

This review examines the core principles of QBism, emphasizing its normative interpretation of the Born Rule and its implications for understanding quantum indeterminism.

Quantum mechanics continues to challenge our intuitions about reality, prompting ongoing debate over its proper interpretation. In the paper ‘QBism, Polishing Some Points’, we refine the tenets of QBism – a perspective emphasizing the personal, participatory nature of quantum experience – to clarify its distinct position within the broader landscape of quantum foundations. Specifically, this work argues that QBism understands quantum states as expressions of an agent’s beliefs, the Born rule as normative guidance for decision-making, and measurement outcomes as fundamentally personal experiences, thereby offering a unique account of indeterminism and the meaning of quantum phenomena. What further ontological lessons might emerge from embracing this radically subjective view of the quantum world?

Emergent Reality: Introducing QBism

For nearly a century, the elegant mathematical framework of quantum mechanics has existed in a peculiar tension with everyday experience. While remarkably successful at predicting experimental outcomes, conventional interpretations often present a reality drastically different from what humans perceive – a world of superpositions, entanglement, and probabilistic wave functions that seem to defy common sense. Attempts to bridge this gap, such as the Copenhagen interpretation and many-worlds interpretation, grapple with fundamental questions about measurement, objectivity, and the nature of reality itself. These approaches often require accepting concepts that feel counterintuitive, like the collapse of the wave function upon observation, or the existence of an infinite number of parallel universes. This persistent difficulty in reconciling the theory with intuitive understanding has motivated explorations of alternative interpretations, seeking a more natural fit between the quantum world and human experience.

Quantum mechanics, traditionally understood, posits an objective reality described by mathematical equations – a reality existing independently of observation. However, QBism proposes a significant departure from this viewpoint, centering instead on the experiences of an agent – a being that interacts with and learns from the quantum world. This isn’t merely a philosophical shift; it’s a reinterpretation of the quantum formalism itself. Rather than describing objective properties of systems, quantum states are understood as representing an agent’s personal degrees of belief about possible outcomes. Consequently, a quantum state isn’t a thing ‘out there’ but a tool used by the agent to update its expectations based on its interactions. This agent-centric perspective fundamentally alters how quantum mechanics is understood, moving away from a search for observer-independent facts and toward a framework where reality emerges through the ongoing interplay between agents and the world around them.

Rather than describing an objective reality existing independently of observation, QBism posits that quantum states embody an agent’s personal probabilities – their individual degrees of belief about potential measurement outcomes. This challenges the conventional view where $ \Psi $, the wavefunction, is seen as a physical property of a system. Instead, it’s understood as a tool used by an agent to update their expectations based on experience. Consequently, different agents, even when observing the same system, can legitimately assign different quantum states, reflecting their unique histories and informational contexts. This isn’t a flaw in the theory, but a fundamental aspect of how it functions; the quantum state is not ‘out there’ to be discovered, but is intrinsically linked to the knower and their interactions with the world.

QBism proposes a significant realignment in interpreting quantum mechanics by centering the agent – the observer making measurements – as fundamental to the theory’s meaning. Traditional approaches grapple with the perplexing notion of how objective reality, described by quantum states, connects to subjective experience; however, QBism sidesteps this issue by asserting that quantum states don’t represent objective properties of the world itself. Instead, these states are understood as encapsulating an agent’s personal probabilities – their degrees of belief about potential measurement outcomes. This shift resolves longstanding conceptual problems, like the measurement problem, by framing quantum mechanics not as a description of what is, but rather as a framework for an agent to update beliefs based on experience. Consequently, different agents can assign different quantum states to the same physical system, each valid within their own experiential context, thereby removing the need for a single, objective quantum reality.

The Born Rule, according to QBism, doesn't define probabilities in isolation but rather establishes a connection between the probabilities generated by a reference device when it is both on-producing multiple probabilities-and off-producing a single probability. — The Born Rule, according to QBism, doesn’t define probabilities in isolation but rather establishes a connection between the probabilities generated by a reference device when it is both on-producing multiple probabilities-and off-producing a single probability.

From Calculation to Action: The QBist Approach

QBism fundamentally reinterprets the role of quantum mechanics, shifting its focus from ontological claims about an external reality to its utility as a framework for informed action. Instead of describing what is, quantum mechanics, within QBism, provides a set of rules for agents to assess and update their subjective probabilities based on experience. This perspective posits that quantum states represent an agent’s degrees of belief regarding the outcomes of future measurements, not inherent properties of a physical system. Consequently, the predictive power of quantum mechanics stems not from mirroring an objective reality, but from facilitating rational decision-making under conditions of uncertainty, allowing agents to consistently improve their expectations and actions.

Within the QBist interpretation of quantum mechanics, the Born Rule is not considered a fundamental law governing the external world, but rather a prescription for coherent belief updating. Specifically, it dictates how an agent’s subjective probabilities – representing their degrees of belief about possible measurement outcomes – should be revised in light of new experience. The rule, expressed mathematically as $P(x|y) = | \langle x | y \rangle |^2$, is thus not describing a physical relationship, but providing a rational norm for assigning probabilities to future events based on past observations. This normative interpretation avoids the ontological commitments inherent in standard interpretations, framing the Born Rule as a tool for making optimal decisions rather than a statement about objective reality.

QBism leverages the foundations of Bayesianism, specifically the use of prior beliefs and likelihoods to calculate posterior probabilities-represented mathematically as $P(A|B) = \frac{P(B|A)P(A)}{P(B)}$-but significantly departs from objective Bayesian approaches. While objective Bayesianism seeks probabilities that represent objective states of the world, independent of any observer, QBism explicitly centers the agent’s subjective experience. This means probabilities are assigned based on the agent’s beliefs about the world and their expectations for measurement outcomes, rather than reflecting an inherent property of the system itself. Consequently, differing agents, with differing prior beliefs, may legitimately assign different probabilities to the same quantum event, and both assignments can be rational within the QBist framework, a departure from the unique posterior distribution sought in objective Bayesian analysis.

QBism’s adoption of a personalist probability framework addresses foundational issues in quantum measurement by defining probability as a degree of belief held by an agent. This contrasts with interpretations assigning objective probabilities to quantum events. Within this view, measurement outcomes are not revealed properties of a pre-existing system, but rather are experiences an agent has, influenced by their prior beliefs and actions. The consistency of this approach stems from its reliance on the principles of Bayesian updating; agents revise their beliefs based on observed evidence, maintaining coherence. Pragmatically, this interpretation avoids the need to postulate hidden variables or wavefunction collapse, offering a self-consistent framework where quantum mechanics provides a guide for making rational decisions in the face of uncertainty, rather than a description of objective reality.

Experience as Measurement: The Agent’s Perspective

Within the QBist interpretation of quantum mechanics, a measurement is not a physical interaction that disturbs a quantum system. Instead, it is defined as an action performed by an agent – an observer with beliefs and intentions – that yields a definite outcome experienced by that agent. This experience is not a discovery of a pre-existing value, but rather a personal update of the agent’s beliefs based on the measurement interaction. The outcome is therefore subjective, representing what the agent believes given the action they took, rather than an objective property revealed through the measurement process. This contrasts with interpretations positing an external, objective measurement process independent of any conscious observer.

Within the QBist interpretation of quantum mechanics, a measurement outcome is directly related to an agent’s prior beliefs and the actions those beliefs motivate. The agent assigns probabilities to potential outcomes based on their subjective state of knowledge, and the act of measurement updates these probabilities through Bayesian inference. Crucially, the measurement doesn’t reveal a pre-existing value of a property; rather, it is the agent’s experience of the outcome that defines the value in that specific context. Subsequent actions are then based on this updated belief, forming a closed loop between belief, measurement, and action, and emphasizing the personal nature of quantum information.

Traditional interpretations of quantum mechanics posit that physical properties of a quantum system exist independently of observation. Measurement, in this framework, is understood as a process by which an observer discovers these pre-existing values. The observer is considered passive, and the act of measurement does not fundamentally alter the system itself, but simply reveals information about its objective state. This contrasts with QBism, which asserts that quantum measurements are personal experiences assigned by an agent and do not uncover pre-existing properties but rather bring them into being through the agent’s interaction with the system.

Within the QBist interpretation of quantum mechanics, the traditional “measurement problem”-which arises from the apparent need for an objective wave function collapse-is resolved by redefining measurement itself. QBism posits that measurement is a personal experience assigned by an agent, rather than a physical interaction causing an objective change in the quantum system. Consequently, there is no requirement for an external mechanism to enforce collapse; the wave function is understood as representing an agent’s subjective degrees of belief, which are updated with each new experience. This eliminates the need to explain how an objective collapse occurs, as it does not occur as an objective physical process, thereby avoiding the associated paradoxes and inconsistencies.

Beyond Local Realism: QBism’s Impact

The enduring puzzles stemming from Bell’s Theorem, particularly those concerning seemingly instantaneous connections between distant particles, find a distinctive resolution within the QBist interpretation of quantum mechanics. Traditional interpretations often grapple with non-locality – the idea that events can influence each other faster than light – and frequently attempt explanations through hidden variables, properties of particles not accounted for in standard quantum theory. However, QBism sidesteps this challenge by dismissing the EPR criterion – the assumption that physical properties exist independently of observation. Instead, it posits that quantum states represent an agent’s personal degrees of belief about possible measurement outcomes, not objective features of reality itself. This shift in perspective dissolves the paradox; the correlations observed in Bell tests aren’t evidence of spooky action at a distance, but rather reflections of an agent updating their beliefs based on experience, effectively negating the need for non-local connections or hidden variables to explain the observed correlations.

QBism fundamentally reconsiders the implications of Bell’s Theorem by shifting focus from the properties of quantum systems themselves to the experiences of agents interacting with those systems. Traditional interpretations often posit non-locality – the seemingly instantaneous correlation of distant particles – as evidence of a deeper, interconnected reality or the existence of hidden variables determining particle behavior. However, QBism proposes that quantum states aren’t objective features of reality, but rather represent an agent’s personal degrees of belief about possible measurement outcomes. Consequently, correlations aren’t evidence of spooky action at a distance, but rather reflect the updating of an agent’s beliefs based on their individual experiences and measurements. By rejecting the necessity of hidden variables and prioritizing the agent’s perspective, QBism dissolves the perceived paradox of non-locality, suggesting that quantum mechanics describes how agents learn and act within their own experiential world rather than revealing an objective, nonlocal reality.

The QBist interpretation of quantum mechanics proposes a radical departure from traditional notions of reality by positing not a single, objective universe, but a ‘pluriverse’ of coexisting possibilities. This framework suggests that an agent’s experiences are fundamental in shaping their personal reality; each measurement doesn’t reveal a pre-existing state, but rather brings forth a specific outcome for that agent within their unique experiential context. Consequently, different agents, even when presented with the same physical system, may legitimately experience divergent realities, each internally consistent and valid. This isn’t simply a matter of subjective interpretation superimposed on an objective truth, but a core tenet of the theory: reality isn’t ‘out there’ to be discovered, but is dynamically brought into being through the interactions of agents and the quantum world, generating a multitude of equally real, yet potentially distinct, universes.

The prevailing image of quantum mechanics as a bizarre realm dictating the behavior of reality often obscures its practical potential. However, a growing perspective, championed by QBism, reframes quantum theory as a fundamentally rational tool for making informed decisions. This isn’t about discovering what is, but about refining what an agent believes and using those beliefs to navigate uncertainty. By shifting the focus from objective reality to subjective experience – specifically, the updating of probabilities based on an agent’s actions and observations – quantum mechanics becomes a powerful extension of classical decision theory. This pragmatic view suggests a future where quantum principles aren’t confined to physics labs, but integrated into diverse fields like artificial intelligence, economics, and even everyday strategic planning, allowing for more effective responses to complex and unpredictable situations.

A Pragmatic Quantum Future

QBism, a compelling interpretation of quantum mechanics, departs from traditional approaches by grounding its principles in pragmatism. Rather than seeking to define an objective reality independent of the observer, QBism posits that quantum states represent an agent’s personal degrees of belief about possible measurement outcomes. This perspective resolves long-standing paradoxes by emphasizing that quantum mechanics isn’t a description of the world itself, but a framework for making informed decisions. The core proposal centers on the idea that assigning probabilities to events is fundamentally a matter of personal experience and action, transforming quantum states into tools for rational behavior. Consequently, QBism provides a consistent and actionable pathway for utilizing the predictive power of quantum theory without getting entangled in metaphysical debates about the nature of reality, offering a distinctly practical lens through which to understand and apply quantum principles.

QBism fundamentally shifts the focus of quantum mechanics away from questions of objective reality and toward the subjective experience of the agent – the observer making the measurement. Rather than attempting to define what quantum states actually are, or whether they represent a physical reality independent of observation, QBism treats these states as personal probabilities reflecting an agent’s beliefs about future measurement outcomes. This pragmatic approach sidesteps longstanding ontological debates – arguments about the nature of being – by asserting that quantum mechanics isn’t describing an external world, but rather providing a framework for rational decision-making in the face of uncertainty. By centering the agent’s experience, QBism transforms quantum states from objective properties into tools for informed action, effectively dissolving the paradoxes that arise from interpreting them as representations of an independently existing reality.

The principles of QBism offer a novel framework with promising implications for fields beyond fundamental physics, particularly in artificial intelligence and decision science. Traditional approaches to AI often struggle with uncertainty and subjective judgment; however, QBism’s emphasis on personal experience and belief as central to quantum mechanics provides a way to model these aspects mathematically. By framing quantum states as degrees of belief, rather than objective properties of reality, researchers can leverage quantum algorithms to enhance decision-making processes under conditions of incomplete information. This allows for the development of AI systems that not only process data but also quantify and manage the agent’s own uncertainty, leading to more robust and adaptable intelligent systems. Furthermore, the QBist perspective could inform the design of algorithms for Bayesian inference and risk assessment, offering a fundamentally different approach to modeling subjective probability and rational action.

The exploration of QBism, as detailed within, suggests a universe fundamentally shaped by the interactions of agents and the continuous updating of beliefs. This resonates with the notion that reality isn’t a pre-existing structure ‘out there,’ but emerges from the ongoing processes of measurement and experience. As Louis de Broglie stated, “Every man’s mental image of the world is blurry, incomplete, and inaccurate.” This inherent fuzziness, acknowledged by de Broglie, mirrors QBism’s core tenet: the Born rule doesn’t describe an objective probability distribution, but rather prescribes how a rational agent should update their beliefs given a quantum event. The article demonstrates how each measurement isn’t revealing a pre-existing state, but actively creating information, further solidifying the participatory nature of reality.

What’s Next?

The insistence on agency within QBism, while a productive departure from static interpretations, invites further scrutiny of what constitutes that agency. The paper rightly shifts focus from what is to what an agent can do, but the nature of ‘doing’ remains largely unaddressed. It is not enough to say measurement is a personal evaluation; the mechanisms by which such evaluations reliably give rise to the observed statistical regularities require deeper investigation. Robustness emerges, it’s never engineered, and understanding the constraints shaping those emergent patterns will be crucial.

A significant challenge lies in scaling this participatory reality beyond isolated measurements. The article highlights ‘inter-participatory’ events, but a complete account must demonstrate how these localized interactions coalesce into the macroscopic world-a world seemingly governed by laws independent of any single agent. Small interactions create monumental shifts, but tracing those shifts requires moving beyond descriptive accounts of individual evaluations to understanding the dynamics of collective belief.

Ultimately, the true test of QBism isn’t whether it feels right, but whether it offers genuinely novel predictive power. The emphasis on normative rules, rather than ontological pronouncements, is a refreshing change. The field should now prioritize developing formalisms that allow for testable predictions arising from this fundamentally different worldview – a worldview where order doesn’t need architects, it simply emerges.

Original article: https://arxiv.org/pdf/2512.14122.pdf

Contact the author: https://www.linkedin.com/in/avetisyan/