"Space is blue and birds fly through it" (1712.02894v5)

Abstract: Quantum mechanics is not about 'quantum states': it is about values of physical variables. I give a short fresh presentation and update on the $relational$ perspective on the theory, and a comment on its philosophical implications.

Relational Quantum Mechanics: A Critique of Quantum State Ontology

Carlo Rovelli's paper, titled "Space is blue and birds fly through it," presents a critical examination of the commonly held notion that quantum mechanics centers around the concept of quantum states. Instead, Rovelli advocates for a relational interpretation of quantum mechanics, which shifts the focus from quantum states to observable physical variables. This paper offers a fresh perspective on quantum theory by challenging the historical and philosophical underpinnings of quantum states as the foundational pillars of quantum mechanics.

Overview of the Argument

In his work, Rovelli revisits the historical evolution of quantum mechanics, tracing its emergence back to the pivotal contributions of Heisenberg, Born, and Jordan. Notably, Rovelli critiques Schrödinger's introduction of the wave function ψ, which indirectly led to the ontological weight assigned to quantum states. Rovelli argues that this was a conceptual misstep that detracted from the core phenomenological observations that quantum theory ought to be concerned with.

Key Characteristics of Quantum Theory

Rovelli outlines three distinct differences between classical and quantum mechanics that are pivotal to understanding quantum phenomena:

• Discreteness: Quantum mechanics introduces fundamental discreteness into the values that physical variables can assume, characterized by the Planck constant. This represents a deviation from classical physics, where values are assumed to be continuous.
• Probabilistic Predictions: Quantum mechanics allows for predictions of variables' values only probabilistically, deviating from the deterministic approach of classical mechanics.
• Relational Aspect: The values that variables assume are not absolute but inherently relational, contingent upon their interaction with other physical systems.

Relational Quantum Mechanics

Relational Quantum Mechanics (Relational QM), as proposed by Rovelli, challenges conventional interpretations by asserting that all physical quantities derive their meaning from their relational context, rather than as absolute entities. This perspective is consistent with other areas in physics, such as relativistic mechanics, where quantities like velocity and position are understood relative to frames of reference.

Implications and Philosophical Considerations

Rovelli's relational approach to quantum mechanics carries significant philosophical implications. It advocates for a sparse ontology, focusing on the events that occur at interactions between systems rather than assuming a continuous substance. This interpretation requires reconsideration of realism, especially in its strong form, where the expectation of an exhaustive and continuous list of properties is relinquished.

In the historical context of quantum theory, Rovelli's approach confronts traditional expectations and interpretations, including critiques of concepts such as wave-particle duality and the collapse of the wave function. Relational QM reframes these phenomena as interactions that hold meaning only relative to participating systems.

Conclusion

Rovelli's paper presents a compelling critique of the ontological assumptions surrounding quantum states, urging a reconsideration of quantum mechanics through the lens of physical variables and their relational properties. By anchoring quantum theory in observable, relational events rather than abstract quantum states, Rovelli contributes to a broader philosophical discourse on the nature of reality and observation in quantum physics. As this perspective continues to be debated and scrutinized, it holds potential implications for future research in quantum mechanics and quantum gravity, possibly aiding in the development of a more coherent understanding of the quantum world.