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A Comparison of Two Presentations of Quantum Mechanics - Everett's Relative-State and Rovelli's Relational Quantum Mechanics

Matthias Ackermann

Published May 7, 2019• 0 citations
OpenAlex

Abstract

Everett’s Relative State Formulation of Quantum Mechanics and Rovelli’s Relational Quantum Mechanics are two eminently interesting and impactful presentations of what the world of quantum mechanics is like. Beginning with a depiction of the theories and their physical implications I conclude with a comparison of their respective picture of reality. I propose here that by letting philosophical deliberations be guided by the very statements from within the theories, one arrives at a structural realist picture of what our world is like.

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Everett’s Relative State Formulation of Quantum Mechanics and Rovelli’s Relational Quantum Mechanics are two eminently interesting and impactful presentations of what the world of quantum mechanics is like. Beginning with a depiction of the theories and their physical implications I conclude with a comparison of their respective picture of reality. I propose here that by letting philosophical deliberations be guided by the very statements from within the theories, one arrives at a structural realist picture of what our world is like.
Rovelli’s RQM is first characterized by contrast with both Everett’s and Bohr’s interpretations of quantum mechanics. Then, it is shown that a basic difficulty arises from the choice of formulating RQM in a naturalistic framework. Even though, according to Rovelli’s interpretation, statements about the world only make sense relative to certain naturalized observers described by means of quantum mechanics, this very meta-statement seems to make sense relative to a sort of super-observer which does not partake of the naturalized status of ordinary observers. The difficulty is solved by substituting functional reference frames for physical (or naturalized) observers throughout. Instead of being relative to physical observers, statements about the state vector of physical systems are here relative to well-defined projects of probabilistic prediction which may be embodied by several physical observers.
Everett's "Relative State Formulation of Quantum Mechanics" (RSQM), which appeared in Reviews of Modern Physics, is based on his thesis "The Theory of the Universal Wavefunction". The most noteworthy property of these works is the claim by other authors that these works are the seminal contribution to Many Worlds theories of branching realities and the claim that practical laboratory experimental tests of RSQM are not possible. This report shows that Everett's two works describe a formulation of quantum mechanics that contradicts an overwhelming body of experimental physics.
In this paper I present and defend Rovelli's relation quantum mechanics from some foreseeable objections, so as to clarify its philosophical implications vis a vis rival interpretations. In particular I ask whether RQM presupposes a hidden recourse to both a duality of evolutions and of ontology (the relationality of quantum world and the intrinsicness of the classical world, which in the limit must be recovered from the former). I then concentrate on the pluralistic, antimonistic metaphysical consequences of the theory, due to the impossibility of assigning a state to the quantum universe. Finally, in the last section I note interesting consequences of RQM with respect to the possibility of defining a local, quantum relativistic becoming (in flat spacetimes).Given the difficulties of having the cosmic form of becoming that would be appropriate for priority monism, RQM seems to present an important advantage with respect to monistic views, at least as far as the possibility of explaining our experience of time is concerned.
Abstract The following paper explores the metaphysical significance of Relational Quantum Mechanics (RQM). For this, I will first outline Rovelli’s philosophical framework, and then offer a reconstruction of Rovelli’s seminal argument in favour of RQM. I will then explore some healthy lines of philosophical work that may be undertaken to strengthen RQM. First, I will frame RQM in the debate between realist and antirealist readings of QM and defend the general claim that RQM is compatible with realism. Then, I discuss the main ontological interpretations of RQM, which I classify as “dispositional”, “transcendental” and “structural”, and argue that all of them remain committed to substantialist assumptions that limit the radicality of Rovelli’s proposal. Finally, I analyse recent developments on RQM’s ontology and suggest that preserving its ontological originality requires an assessment on the concept of “pattern”.
Abstract Accounts of quantum theory that bring to the fore the relationship between the system and the observer, or measuring device, include Everett’s ‘Many Worlds’ interpretation and Rovelli’s ‘Relational Quantum Mechanics’, both of which are considered in Chapter 9. The former understands the state of a system as relative to that of the observer and in Everett’s original formulation, the definite outcomes obtained through measurement were manifested via the observer’s memory sequences. The subjectivist aspect of this interpretation is made even more explicit in the so-called ‘Many Minds’ variant. Relational Quantum Mechanics likewise adopts a relational or perspectival approach to the states of the system under observation. In this chapter both approaches are considered as offering a way of aligning quantum theory with the correlationist feature of phenomenology. Both are then compared with QBism and the pros and cons of all three are considered in terms of possible avenues for developing a phenomenological form of quantum theory.
I begin by examining the question of the quantum limits of knowledge by briefly presenting the constraints of the theory that derive from its mathematical structure (in particular the no-go theorems formulated by von Neumann and Kochen and Specker). I then argue that these theorems reflect on a formal level those practical and experimental settings that are needed to come to know the properties of physical systems. In particular, I discuss some aspects of this relationist and contextualist conception of reality by comparing, in their apparent diversity, Bohr’s holistic and Rovelli’s relationist interpretation of the formalism, that deep down share a unifying metaphysics of dispositions and propensities. Both interpretations are based on the widely shared fact that quantum mechanics does not describe previously definite quantities. In the final part I show that, as a consequence of a relationist and perspectival approach to quantum mechanics, the quantum state of the universe regarded as an isolated system cannot be known in principle, so that the universe must be described “from within” by dividing it into two "parts". This is in fact the only way in which any two systems can exchange information by being physically correlated.