On the significance of Wigner's Friend in contexts beyond quantum foundations (2402.08727v2)

Abstract: There has been a surge of recent interest in the Wigner's Friend paradox, sparking several novel thought experiments and no-go theorems. The main narrative has been that Wigner's Friend highlights a counterintuitive feature that is unique to quantum theory, and which is closely related to the quantum measurement problem. Here, we challenge this view. We argue that the gist of the Wigner's Friend paradox can be reproduced without assuming quantum physics, and that it underlies a much broader class of enigmas in the foundations of physics and philosophy. To show this, we first consider several recently proposed Extended Wigner's Friend scenarios, and demonstrate that some of their implications for the absoluteness of observations can be reproduced by classical thought experiments that involve the duplication of agents. Crucially, some of these classical scenarios are technologically much easier to implement than their quantum counterparts. Then, we argue that the essential structural ingredient of all these scenarios is a feature that we call "Restriction A": essentially, that a physical theory cannot give us a probabilistic description of the observations of all agents. Finally, we argue that this difficulty is at the core of other puzzles in the foundations of physics and philosophy, and demonstrate this explicitly for cosmology's Boltzmann brain problem. Our analysis suggests that Wigner's Friend should be studied in a larger context, addressing a frontier of human knowledge beyond quantum foundations: to obtain reliable predictions for experiments in which these predictions can be privately but not intersubjectively verified.

• The paper demonstrates that the Wigner's Friend paradox can emerge in classical frameworks, challenging its status as a solely quantum phenomenon.
• The paper introduces 'Restriction A' to illustrate the inherent limitation in jointly representing all agents’ observations in both classical and quantum settings.
• The paper's analysis fosters interdisciplinary dialogue by linking thought experiments with philosophical, cosmological, and cognitive considerations of observer identity.

An Examination of "Thinking twice inside the box: is Wigner’s friend really quantum?"

The manuscript explores the Wigner's Friend paradox, a canonical thought experiment that challenges the foundational elements of quantum theory. Traditionally, this paradox highlights the counterintuitive nature of quantum mechanics. However, this paper argues that the paradox’s implications can extend beyond quantum mechanics, venturing into broader philosophical inquiries and classical analogues, challenging the common perception that its enigmas are confined to quantum theory.

Rethinking Wigner's Friend

At the core, the authors contend that while Wigner's Friend is often synonymous with quantum peculiarity—by emphasizing phenomena like superposition or entanglement—it is not inherently restricted to quantum mechanics. Instead, they propose that similar paradoxical insights can arise in classical frameworks through the duplication of agents. This perspective is potent because it simplifies the technological requisites for experimenting with these scenarios, suggesting that the fundamental issues raised by Wigner's Friend are not exclusively quantum.

Restriction A: A Structural Insight

The paper introduces "Restriction A," asserting that no physical theory can offer a joint probabilistic representation of all agents’ observations. This is a foundational assertion, revealing an inherent limitation within both classical and quantum paradigms. This notion is systematically unpacked through classical and quantum scenarios, elucidating its role in the Boltzmann brain problem and self-locating belief challenges seen through the Sleeping Beauty puzzle.

Through the classical lens, the authors use thought experiments involving duplicated agents to demonstrate Restriction A. They suggest that when an agent is reproduced classically, conventional probabilistic descriptions cannot fully encompass their future observations—mirroring what occurs in quantum scenarios like Wigner's Friend.

Implications for Quantum and Classical Domains

The authors propose that Restriction A, while naturally observed in quantum contexts like the Local Friendliness violations, also emerges within speculative classical scenarios, such as cosmological settings involving Boltzmann brains. These evoke many-world like considerations even within a classical field, suggesting broader implications than purely quantum processes.

Theoretical and Practical Outcomes

The identification of Restriction A prompts reevaluation of agent-based measurements in experimental setups. It challenges the deterministic expectations around knowledge sharing and observer correlations in physics and philosophy. The potential overlap with cognitive sciences and theories of mind also suggests interdisciplinary applications of these insights, calling into question the classical and quantum foundations of identity and observation.

Conclusion

Jones and Müller’s work offers pivotal insights that broaden the discourse surrounding the Wigner's Friend paradox. By stripping it of its quintessentially quantum traits, they unravel a philosophical and structural core that transcends traditional interpretations. Their analysis encourages a holistic understanding of thought experiments, intertwining philosophy, quantum mechanics, and classical theories, and setting the stage for further exploration into the nature of reality and observation.