This Physicist (Also) Says The Wave Function Isn't Real

Quantum physicist Robert Spekkens argues that many seemingly 'quantum' phenomena can be explained classically by assuming a fundamental limit on knowledge, and he advocates for rejecting the conventional 'ontological models framework' in favor of a new approach guided by Leibniz's principle and a reformulated understanding of causation and inference.

• Many quantum phenomena are reproducible in classical models by limiting knowledge.

• Leibniz's principle guides rejecting conventional ontological models in quantum theory.

• Quantum states represent knowledge, not reality, necessitating new causal frameworks.

Quantum physicist Robert Spekkens challenges conventional interpretations of quantum mechanics, asserting that many phenomena often deemed uniquely quantum, such as interference and entanglement, can be reproduced in classical models that incorporate a fundamental limit on knowledge. Spekkens champions a realist philosophy informed by an operationalist methodology, emphasizing Leibniz's principle of the identity of indiscernibles and the necessity of causal explanations for correlations. His research program aims to reformulate the framework for understanding reality, suggesting that the true innovation of quantum theory lies in new notions of causation and inference rather than mystical quantum quirks.

Challenging Quantum Dogma

• 00:00:00 The conventional view that quantum interference is the 'essence of quantum theory' and impossible to reproduce classically is disputed. Robert Spekkens demonstrated in 2004 that a classical world theory, where maximal knowledge is incomplete, can explain phenomena like the no-cloning theorem, teleportation, and even interference effects, thus proving Richard Feynman's assertion wrong. This work suggests a need for radically different ideas in understanding quantum reality.

Leibniz's Principle in Physics

• 00:01:27 Leibniz's 'identity of indiscernibles' principle states that if two distinct scenarios yield identical empirical observations, they are not ontologically distinct. This 'Leibnizian methodological principle' is crucial to Spekkens' research program. Albert Einstein famously applied this principle twice: to eliminate the ether in special relativity and to identify gravity with spacetime curvature in general relativity, demonstrating its effectiveness in guiding fundamental physics.

Empiricism vs. Realism

• 00:06:40 A fundamental dichotomy in quantum theory interpretations exists between empiricism and realism. Empiricism asserts that a theory's sole purpose is to describe observable experimental outcomes, with conceptual building blocks rooted in indisputable facts. Realism, which Spekkens espouses, seeks deeper explanations of predictions through abstract concepts, acknowledging that all observations are theory-laden and interpretation-dependent. Spekkens adopts a 'realism informed by operationalism,' using operational predictions as the goal for realist models.

Causal Explanations for Correlations

• 00:23:51 A critical aspect of realism for quantum theory is the demand for causal explanations of correlations. Spekkens argues that attributing correlations to either direct causation or a common cause is essential for scientific understanding, as illustrated by the drug trial example where correlation does not imply causation. He insists that quantum theory must provide a causal account to truly understand what is happening, even if it requires giving up aspects of conventional classical realism.

No-Go Theorems and Framework Limits

• 00:26:07 The commitment to Leibniz's principle and the need for causal explanations are in tension with the conventional 'ontological models framework' of hidden variable models, leading to contradictions with quantum predictions. No-go theorems like Bell's and Kochen-Specker's, when combined with Leibniz's principle (which implies local causality and non-contextuality), demonstrate this conflict. Spekkens argues that these theorems necessitate abandoning the conventional framework for realism, not the Leibnizian principle, and innovating how we conceptualize causation and inference in quantum theory.

The Toy Theory and Phenomenology

• 00:30:04 Spekkens' 'toy theory,' developed in 2004, demonstrates that many phenomena traditionally considered uniquely quantum—such as non-orthogonal state discrimination, the no-cloning theorem, and teleportation—can be reproduced in classical models where the only innovation is a fundamental restriction on how much knowledge can be acquired about a physical state. This suggests that quantum states represent incomplete knowledge, not a complete description of reality, and highlights that the true distinctiveness of quantum theory lies beyond this common list of phenomena.

Philosophy's Role in Physics

• 01:19:19 Philosophy of physics is crucial for revolutionary progress, despite common dismissals that it has lost its utility. Historically, major revolutions in physics, such as those by Newton and Einstein, were driven by deeply philosophical thinking and often led to changes in the philosophy of science itself. Philosophical training imparts rigor in argumentation, clear definition of terms, and the ability to critically examine foundational principles, skills essential for identifying and addressing the profound conceptual issues that underpin scientific breakthroughs.

The Category Mistake of Quantum States

• 02:18:50 Spekkens posits that the enduring mystery of quantum theory stems from a 'category mistake': viewing the quantum state as describing reality rather than representing knowledge about reality. He draws an analogy to the decipherment of Egyptian hieroglyphs, where initial misinterpretations as ideograms (concepts) hindered progress until their phonetic nature (sounds) was recognized. This suggests that understanding 'knowledge about what' in quantum theory requires discovering a new formalism for causation and inference, akin to Champollion finding the Coptic language for hieroglyphs.