“It must be kept in mind that […] isolated material particles are abstractions, their properties on the quantum theory being definable and observable only through their interaction with other systems” (Bohr 1928, 581).

## Abstract

The Rovelli relational interpretation of quantum mechanics (RQM) is based on the assumption that the notion of observer-independent state of a physical system is to be rejected. In RQM the primary target of the theory is the analysis of the whole network of relations that may be established among quantum subsystems, and the shift to a relational perspective is supposed to address in a satisfactory way the general problem of the interpretation of quantum mechanics. Here I discuss two basic issues, that I take to be serious open problems of the interpretation. First, I wish to show—mainly through an analysis of the so-called *third person problem*—that it is far from clear what a relativization of states to observers exactly achieves and in what sense such an approach really advances our understanding of the peculiar features of quantum phenomena. Second, I argue that the claim, according to which RQM is able to preserve locality, is at best dubious. I conclude that further work needs to be done before RQM may aspire to become a satisfactory interpretational framework for the main foundational issues in quantum mechanics.

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## Notes

Consider for instance the debate that already more than 30 years ago was fueled by the Teller

*relational holism*(Teller 1986, 1989; see Morganti 2009 for a general assessment) or the issue of the priority of the structure of relations over individuals in the debate on the structural realism (Ladyman 2016).The outline of RQM given here is rather sketchy and instrumental to the critical points I wish to discuss in the central sections of the paper. For a wider presentation, sensitive also to the metaphysical background and implications of RQM, see Dorato (2016).

As is well known, what is the story that the Everett interpretation exactly tells is a matter of controversy. As Barrett emphasizes: “There has been considerable disagreement over the precise content of his theory and how it was supposed to work” (Barrett 2018).

On a possibly transcendental view of RQM one can see also Bitbol (2008).

Clearly, the Wigner friend paradox is close in spirit to the Schrödinger cat or the von Neumann chain: essentially, they are all variants of the measurement problem. In more recent times new, more sophisticated versions of the Wigner friend paradox have been presented and discussed: one can see Dieks (2019) for an up-to-date survey.

Dieks suggests that the discussion of the measurement process contained in the above mentioned Chapter 6 of the von Neumann treatise of 1932 has already a relational flavour: “the distinction between collapses and unitary evolution for von Neumann is not a distinction between two competing and potentially conflicting physical interaction mechanisms on the same level of description, but rather concerns what can be said

*in relation to two different points of view*” (Dieks 2019, 2). I find this suggestion unconvincing: what von Neumann wishes to prove in the chapter is the consistency between*statistical*predictions for the same observable in different evolutions—collapses and unitary. This consistency proof does not easily entail, in the von Neumann work, that the tension between a linear and a non-linear dynamics can be solved simply by advocating a relational stance.Eminent physicists have shared this attitude, such as Nobel laureate Sir Antony J. Leggett: “I believe that the results of the present investigation provide quantitative backing for a point of view which I believe is by now certainly well accepted at the qualitative level, namely that the incompatibility of the predictions of objective local theories with those of quantum mechanics has relatively little to do with locality and much to do with objectivity” (Leggett 2003, 1470).

Tipler (2014) argues along similar lines, although in a more explicitly Everettian vein.

The peaceful coexistence thesis, recalled above, is grounded on the fact that standard quantum mechanics—although violating OI—

*does*satisfy PI (Ghirardi et al. 1980). As a consequence, quantum–mechanical non-locality would not be so harmful: the outcomes are somehow non-locally affecting each other and this seems to threaten the prescriptions of special relativity, but such outomes are uncontrollable and thus we cannot exploit them to produce any robust action-at-a-distance. The effectiveness of the PI/OI distinction in carrying the burden of such an ambitious coexistence has been often questioned: see Maudlin (1994, 2011^{3}, 85–90), for a critical analysis.The point seems to a certain extent acknowledged in Martin-Dussaud et al. (2018). There the authors, recalling an argument due to Bell (1976) according to which ordinary quantum mechanics—

*under the assumption of completeness*—is nonlocal (“non-locally causal”, in the somewhat misleading Bell terminology used also by the authors), appear to accept that relational quantum mechanics is nonlocal in that sense, but claim that such kind of nonlocality is “reducible to the existence of a common cause in an indeterministic context”. I find the claim unconvincing, but a detailed discussion of it deserves a different paper. Actually the Bell argument is nothing but a reformulation of the so-called*Einstein boxes argument*, originally developed by Einstein in a letter to Schrödinger of June, 19th, 1935 (Arthur Fine 1986 carefully comments upon this letter in Chapter 5 of his 1986 book), and recalled by De Broglie in his 1964 book on wave mechanics (De Broglie 1964, 24–29). The whole point is re-examined in detail in Norsen (2005).

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Laudisa, F. Open Problems in Relational Quantum Mechanics.
*J Gen Philos Sci* **50**, 215–230 (2019). https://doi.org/10.1007/s10838-019-09450-0

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DOI: https://doi.org/10.1007/s10838-019-09450-0