# A Stronger Theorem Against Macro-realism

1Department of Computer Science, University of Oxford, Wolfson Building, Parks Road, Oxford, OX1 3QD, United Kingdom
2Faculty of Philosophy, University of Oxford, Radcliffe Humanities, Woodstock Road, Oxford, OX2 6GG, United Kingdom

### Abstract

Macro-realism is the position that certain macroscopic observables must always possess definite values: e.g. the table is in some definite position, even if we do not know what that is precisely. The traditional understanding is that by assuming macro-realism one can derive the Leggett-Garg inequalities, which constrain the possible statistics from certain experiments. Since quantum experiments can violate the Leggett-Garg inequalities, this is taken to rule out the possibility of macro-realism in a quantum universe. However, recent analyses have exposed loopholes in the Leggett-Garg argument, which allow many types of macro-realism to be compatible with quantum theory and hence violation of the Leggett-Garg inequalities. This paper takes a different approach to ruling out macro-realism and the result is a no-go theorem for macro-realism in quantum theory that is stronger than the Leggett-Garg argument. This approach uses the framework of ontological models: an elegant way to reason about foundational issues in quantum theory which has successfully produced many other recent results, such as the PBR theorem.

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### Cited by

[1] George C. Knee, Max Marcus, Luke D. Smith, and Animesh Datta, "Subtleties of witnessing quantum coherence in nonisolated systems", Physical Review A 98 5, 052328 (2018).

[2] Shane Mansfield and Elham Kashefi, "Quantum Advantage from Sequential-Transformation Contextuality", Physical Review Letters 121 23, 230401 (2018).

[3] R. Hermens and O.J.E. Maroney, "Constraints on macroscopic realism without assuming non-invasive measurability", Studies in History and Philosophy of Science Part B: Studies in History and Philosophy of Modern Physics 63, 50 (2018).

[4] J. J. Halliwell, "Comparing conditions for macrorealism: Leggett-Garg inequalities versus no-signaling in time", Physical Review A 96 1, 012121 (2017).

[5] Kunkun Wang, George C. Knee, Xiang Zhan, Zhihao Bian, Jian Li, and Peng Xue, "Optimal experimental demonstration of error-tolerant quantum witnesses", Physical Review A 95 3, 032122 (2017).

[6] J. J. Halliwell, "Necessary and sufficient conditions for macrorealism using two- and three-time Leggett-Garg inequalities", Journal of Physics Conference Series 1275 1, 012008 (2019).

[7] John-Mark A. Allen, "Reality, Causality, and Quantum Theory", arXiv:1901.01618.

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