Realism Instrumentalism Ontology

Ontology is a theory’s account of what physical entities or events exist. Realism treats at least part of the theory as describing an observer-independent world; instrumentalism emphasizes its role in organizing predictions and interventions. These are broad orientations. Neither label uniquely specifies a quantum model, and Bell’s theorem does not contain a premise simply called realism that settles every version at once.

Describe the commitments before naming the position

A quantum account can posit particles with positions, a universal wavefunction, localized stochastic events, relational facts or an operational interface of preparations and outcomes. To assess it, ask how its proposed entities relate to the mathematical state, which laws govern them, and how apparatus records emerge.

A realist commitment to physical laws need not imply pre-existing values for every observable. Conversely, accepting operational outcomes as data need not commit an instrumentalist to a hidden classical ontology behind them. The precise property assignments matter more than the label.

One probability law can support different accounts

Consider a two-path interference experiment with phase $\phi$ and ideal output probability

$$p_0(\phi)=\frac{1+\cos\phi}{2}.$$

An operational account can treat this as a calibrated rule connecting preparation, phase setting and detection. A wavefunction-realist account may regard the interfering amplitudes as part of physical reality. A pilot-wave account can add an actual configuration guided by a wavefunction and reproduce the same probability under its equilibrium assumptions.

Agreement on this experiment does not show that the ontologies are identical. It also does not provide experimental evidence selecting one of them without a further prediction that differs. Explanatory comparison and empirical discrimination are related but distinct tasks.

Completeness, economy and explanatory cost

Additional variables can answer questions left open by a state-only account, but they require dynamics and an explanation of their relation to accessible data. A minimal operational account can be predictive and coherent while deliberately leaving some ontological questions unanswered. A universal wavefunction account must explain the relation between its mathematical structure and the records agents use.

These are tradeoffs to articulate, not scores that a page should assign by preference. The interpretation comparison uses common criteria, and How to Evaluate an Interpretation distinguishes mathematical coherence, empirical adequacy and explanatory aims.

Why no-go slogans are unreliable

Kochen–Specker constrains a global noncontextual value rule. Bell constrains a local correlation model with setting independence. PBR constrains overlap of ontic preparation measures under preparation independence. A philosophical position is affected only insofar as it makes the corresponding precise commitments.

Saying “the state is real” does not specify whether it is complete, whether its dynamics is unitary or whether other variables exist. Saying “the state is information” does not specify the referent of the information or whether the account fits an ontological-model framework. Those further questions belong in an explicit theory statement.

Exercise: an incomplete label

A proposed interpretation says only, “The wavefunction is a real physical object.” List three questions still needed to assess its treatment of a measurement.

Solution

One must know its dynamics, whether additional physical variables exist, and how definite records and their probabilities are represented. It also matters which space or structure the wavefunction is taken to describe. The quoted sentence alone selects neither collapse nor Everett nor a wavefunction-plus-configuration theory.

References

• Bell, John S., Speakable and Unspeakable in Quantum Mechanics, 2nd edition, Cambridge University Press (2004).
• Harrigan, Nicholas, and Robert W. Spekkens, “Einstein, Incompleteness, and the Epistemic View of Quantum States,” Foundations of Physics 40, 125–157 (2010), doi:10.1007/s10701-009-9347-0.
• Maudlin, Tim, Philosophy of Physics: Quantum Theory, Princeton University Press (2019) — explicit physical commitments in interpretive comparison.