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Assessing relational quantum mechanics

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Abstract

Relational Quantum Mechanics (RQM) is an interpretation of quantum theory based on the idea of abolishing the notion of absolute states of systems, in favor of states of systems relative to other systems. Such a move is claimed to solve the conceptual problems of standard quantum mechanics. Moreover, RQM has been argued to account for all quantum correlations without invoking non-local effects and, in spite of embracing a fully relational stance, to successfully explain how different observers exchange information. In this work, we carry out a thorough assessment of RQM and its purported achievements. We find that it fails to address the conceptual problems of standard quantum mechanics—related to the lack of clarity in its ontology and the rules that govern its behavior—and that it leads to serious conceptual problems of its own. We also uncover as unwarranted the claims that RQM can correctly explain information exchange among observers, and that it accommodates all quantum correlations without invoking non-local influences. We conclude that RQM is unsuccessful in its attempt to provide a satisfactory understanding of the quantum world.

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Notes

  1. As in Rovelli (1996), we refer to O as “he” and to P as “she.”

  2. One could argue that the stance on the conceptual problems of quantum mechanics described in the last pages is only a point of view, as valid as many others. However, a demand of clarity, precision or self-consistency in a realist physical theory is not a negotiable “point of view” that one can simply decide not to adhere to.

  3. In fact, in Rovelli (1996) it is claimed that “the experimental evidence at the basis of quantum mechanics forces us to accept that distinct observers give different descriptions of the same events”; that “it is nature itself that is forcing us to this way of thinking”. But that, of course, is simply not true, as there are quantum formalisms, such as pilot-wave theory (Bohm, 1952) or objective collapse models (Ghirardi et al., 1986; Pearle, 1989; Bassi et al., 2013), that accommodate all the empirical evidence at the basis of quantum mechanics as well than the standard framework, and do so without ever allowing for distinct observers to give different descriptions of the same events. In any case, in a recent private communication, Rovelli informed us that his point of view is now different, namely that “if you take textbook quantum mechanics at its face value, it forces you to this conclusion.”

  4. Of course, all this is a consequence of the more general ambiguity problem that, we noted, affects standard quantum theory when deprived of special roles for measuring devices or observers. If anything, this proposal makes this problem more apparent, by noticing the need for a choice of the operator M, but failing to recognize that such a choice is in no way dictated by the postulates of the theory.

  5. Clearly, the necessity for an infinite tower of observers immediately disappears as soon as one allows for systems to possess absolute states.

  6. Martin-Dussaud et al. (2019) explicitly states that the ‘elements of reality’ must live in space–time.

  7. After posting this manuscript online, some of the issues with RQM described in it have generated a very fruitful discussion (for the preferred-basis problem see (Brukner, 2021; Pienaar, 2021; Stacey, 2021) and for the exchange of information between observers see (Adlam, 2022; Adlam & Rovelli, 2022)).

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Acknowledgements

We would like to thank Quentin Ruyant, Tim Maudlin and Travis Norsen for valuable comments. We acknowledge partial financial support from PAPIIT-DGAPA-UNAM Project IG100120 and CONACyT Project 140630. DS is grateful for the support provided by the Grant FQXI-MGA-1920 from the Foundational Questions Institute and the Fetzer Franklin Fund, a donor advised by the Silicon Valley Community Foundation.

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We acknowledge support from CONACYT Grant 140630.

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  1. Universidad Nacional Autónoma de México, Mexico City, Mexico

    Ricardo Muciño, Elias Okon & Daniel Sudarsky

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  1. Ricardo Muciño
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  2. Elias Okon
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Correspondence to Elias Okon.

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Muciño, R., Okon, E. & Sudarsky, D. Assessing relational quantum mechanics. Synthese 200, 399 (2022). https://doi.org/10.1007/s11229-022-03886-6

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