Why interference phenomena do not capture the essence of quantum theory (2111.13727v7)

Abstract: Quantum interference phenomena are widely viewed as posing a challenge to the classical worldview. Feynman even went so far as to proclaim that they are the only mystery and the basic peculiarity of quantum mechanics. Many have also argued that basic interference phenomena force us to accept a number of radical interpretational conclusions, including: that a photon is neither a particle nor a wave but rather a Jekyll-and-Hyde sort of entity that toggles between the two possibilities, that reality is observer-dependent, and that systems either do not have properties prior to measurements or else have properties that are subject to nonlocal or backwards-in-time causal influences. In this work, we show that such conclusions are not, in fact, forced on us by basic interference phenomena. We do so by describing an alternative to quantum theory, a statistical theory of a classical discrete field (the `toy field theory') that reproduces the relevant phenomenology of quantum interference while rejecting these radical interpretational claims. It also reproduces a number of related interference experiments that are thought to support these interpretational claims, such as the Elitzur-Vaidman bomb tester, Wheeler's delayed-choice experiment, and the quantum eraser experiment. The systems in the toy field theory are field modes, each of which possesses, at all times, both a particle-like property (a discrete occupation number) and a wave-like property (a discrete phase). Although these two properties are jointly possessed, the theory stipulates that they cannot be jointly known. The phenomenology that is generally cited in favour of nonlocal or backwards-in-time causal influences ends up being explained in terms of inferences about distant or past systems, and all that is observer-dependent is the observer's knowledge of reality, not reality itself.

• The paper demonstrates that interference phenomena do not mandate wave-particle duality by proposing a classical toy field theory.
• It shows that quantum properties like phase and occupation are inherently present, refuting the need for observer-dependent interpretations.
• The study presents a strictly local explanation for interference effects, urging a reexamination of quantum-classical boundaries in foundational physics.

An Analysis of "Why interference phenomena do not capture the essence of quantum theory"

The paper "Why interference phenomena do not capture the essence of quantum theory," authored by Lorenzo Catani, Matthew Leifer, David Schmid, and Robert W. Spekkens, offers a detailed examination of the interpretational landscape surrounding quantum interference phenomena. The authors challenge the prevailing narrative that interference behavior fundamentally distinguishes quantum mechanics from classical physics, a perspective famously endorsed by Richard Feynman, who considered it the "only mystery" of quantum mechanics.

Core Arguments and Methodological Approach

The paper aims to decouple the notion that quantum interference mandates a departure from classical thinking. The authors propose that interference effects, while mysterious and challenging, do not necessitate radical conclusions such as wave-particle duality, observer-dependent reality, or the failure of local causation.

1. Wave-Particle Complementarity: The authors argue against the necessity of wave-particle duality as a fundamental characteristic required by interference phenomena. They propose an alternative description using a classical model dubbed the "toy field theory." This theory incorporates both particle-like and wave-like properties simultaneously without invoking duality, thereby rejecting the standard interpretation that a photon toggles between wave and particle behavior.
2. Observer-Dependent Reality: The authors challenge the notion that reality is contingent on the observer's measurement choices. Within their toy model, the properties attributed to quantum systems—particle-like occupation and wave-like phase—are not observer-dependent but are instead always present in a determinate state, only obscured by the limitations of measurement and knowledge.
3. Local Causation: Importantly, the toy field theory maintains a strictly local explanation for interference effects. By conceptualizing modes as carrying both local and phase information that propagate naturally through local interactions, the theory negates the need for superluminal or retrocausal narratives. It cleverly uses phase relations to account for interference patterns that align with classical expectations of locality.

Theoretical and Practical Implications

The implications of endorsing a classical statistical framework for quantum phenomena extend both theoretically and practically:

• Theoretical Impacts: The model emphasizes the necessity for no-go theorems with well-stated assumptions when claiming interpretational novelty in quantum mechanics. Encouraging mathematical rigor and caution against hasty interpretational claims, the paper echoes the sentiment that what might appear as interpretive license could often stem from an overlooked classical interpretation.
• Practical Consequences: By providing a viable classical statistical account of interference, this research potentially influences how physicists might approach quantum foundations, suggesting alternative pathways for theory development that might be hidden in plain view.

Future Directions

While the paper refocuses the discourse on quantum interference, questions remain about other phenomena in quantum mechanics that might resist classical interpretations, especially in higher-dimensional or multipartite systems. Subsequent research could explore the extent to which classical models can accommodate such phenomena or identify more precise boundaries where classicality must yield to quantum novelties.

In summary, the authors provide a meticulous argument that challenges the orthodox view of quantum interference as inherently nonclassical. By developing a classical framework that accounts for standard interference phenomena, they invite the community to reconsider not only the essence of quantum theory but also the methodologies employed to differentiate between quantum and classical worlds.