# Other Topics in the Philosophy of Quantum Mechanics

### Review of D. Albert, 'A Guess at the Riddle' (2024)

*Mind*, forthcoming.

This is an essay review of Albert's book, which is a linked set of essays on the metaphysics of quantum mechanics. A brief extract:

A Guess at the Riddle doesn’t really pretend to be self-contained. It needs to be understood as a set of moves in a long-standing debate (a debate, it must be said, largely carried out within a fifty-mile radius of New York City). But that debate has been influential in the metaphysics of physics as little else has, and A Guess at the Riddle is a deep and carefully considered set of arguments from which even those not directly invested in the debate can learn much.

### Quantum Systems Other Than the Universe (2024)

To appear in M.E.Cuffaro and S.Hartmann (eds.), *Open Systems: Physics, Metaphysics, and Methodology* (OUP, in preparation).

How should we interpret physical theories, and especially quantum theory, if we drop the assumption that we should treat it as an exact description of the whole Universe? I expound and develop the claim that physics is about the study of autonomous, but not necessarily isolated, dynamical systems, and that when applied to quantum mechanics this entails that in general we should take quantum systems as having mixed states and non-unitary dynamics. I argue that nonetheless unitary dynamics continues to have a special place in physics, via the empirically-well-supported reductionist principles that non-unitarity is to be explained by restriction to a subsystem of a larger unitary system and that microscopic physics is governed by unitary and largely known dynamics. I contrast this position with the `Open Systems View' advocated recently by Michael Cuffaro and Stephan Hartmann.

### Philosophy of Quantum Mechanics (2022)

*Oxford Research Encyclopedia of Physics*, 2024.

This is a general introduction to and review of the philosophy of quantum mechanics, aimed at readers with a physics background and assuming no prior exposure to philosophy, written for the *Oxford Research Encyclopedia of Physics*.

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### Review of A. Ney, 'The World in the Wave Function' (2022)

*Philosophical Review* 131 (2022), 528-532.

This is an essay review of Ney's book, which defends a wavefunction-realist ontology for quantum mechanics. A brief extract:

Wavefunction realism has become a leading contender for the metaphysics of quantum mechanics, particularly in the case of dynamical-collapse and Everettian approaches. The starting point is non-relativistic quantum mechanics expressed in configuration space: mathematically, the subject matter of the theory is a complex function of 3N variables, evolving under the Schrodinger equation (perhaps interrupted by occasional stochastic collapses). Traditionally those 3N variables are interpreted as the 3 coordinates of position of N particles – but in wavefunction realism, they are interpreted as points in a new, high-dimensional space. Quantum mechanics, according to the wavefunction realist, fundamentally concerns the undulations of the wavefunction in this space; three-dimensional goings on are emergent, high-level, derivative. Alyssa Ney has been a leading figure in the development of wavefunction realism for most of its existence. Her papers advocating and developing the position have been highly influential, and the present book can be seen as the culmination of this line of her research and is the definitive statement of wavefunction realism as of 2021.

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### On The Reality of the Global Phase (2022)

Online only.

I respond to a recent argument by David Schroeren (Phil.Sci., forthcoming; philsci-archive.pitt.edu/20419/) that - contrary to a very wide consensus - global phase in quantum mechanics is real. I argue that Schroeren's argument rests on a mistaken assumption about projective representation theory and that, when this is corrected, the argument no longer follows. In doing so I give a brief introduction to projective representation theory.

(Note: after I posted this note, David Schroeren decided to withdraw his paper, and it is no longer available online (philsci-archive, unwisely in my view, permits withdrawal of submitted papers). I have left this paper available partly because it would be hypocritical not to, partly because I think its discussion of projective representation theory is of some interest in its own right.)

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### The Sky is Blue, and Other Reasons Quantum Mechanics is Not Underdetermined by Evidence (2022)

*European Journal for Philosophy of Science* 13 (2023) 54

I criticize the widely-defended view that the quantum measurement problem is an example of underdetermination of theory by evidence: more specifically, the view that the unmodified, unitary quantum formalism (interpreted following Everett) is empirically indistinguishable from Bohmian Mechanics and from dynamical-collapse theories like the GRW or CSL theories. I argue that there as yet no empirically successful generalization of either theory to interacting quantum field theory and so the apparent underdetermination is broken by a very large class of quantum experiments that require field theory somewhere in their description. The class of quantum experiments reproducible by either is much smaller than is commonly recognized and excludes many of the most iconic successes of quantum mechanics, including the quantitative account of Rayleigh scattering that explains the color of the sky. I respond to various arguments to the contrary in the recent literature.

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### Review of R. Healey, 'The Quantum Revolution in Philosophy' (2019)

*Analysis* 80 (2020) 381-388

This is an essay review of Healey's book, which defends a pragmatist approach to quantum mechanics. The first paragraph:

Richard Healey’s The Quantum Revolution in Philosophy is a terrific book, and yet I disagree with nearly all its main substantive conclusions.1 The purpose of this review is to say why the book is well worth your time if you have any interest in the interpretation of quantum theory or in the general philosophy of science, and yet why in the end I think Healey’s ambitious project fails to achieve its full goals.

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### On the Plurality of Quantum Theories: Quantum theory as a framework, and its implications for the quantum measurement problem (2018)

In S. French and J. Saatsi, *Scientific Realism and the Quantum* (OUP, 2020).

`Quantum theory' is not a single physical theory but a framework in which many different concrete theories fit. As such, a solution to the quantum measurement problem ought to provide a recipe to interpret each such concrete theory, in a mutually consistent way. But with the exception of the Everett interpretation, the mainextant solutions either try to make sense of the abstract framework as if it were concrete, or else interpret one particular quantum theory under the fiction that it is fundamental and exact. In either case, these approaches are unable to help themselves to the very theory-laden, level-relative ways in which quantum theory makes contact with experiment in mainstream physics, and so are committed to major revisionary projects which have not been carried out even in outline. As such, only the Everett interpretation is currently suited to make sense of quantum physics as we find it.

### Quantum Foundations Still Not Cemented (Review of A. Becker, 'What is Real? The Unfinished Quest for the Meaning of Quantum Physics') (2018)

*Physics Today* 71 (2018) 51-52.

This is an review of Becker's book on the foundations of quantum mechanics. A brief extract:

Adam Becker’s remarkable new book ... takes on the puzzles of quantum mechanics with a historical approach that is at once accessible and scholarly. He begins with the founding fathers of quantum theory by revisiting the familiar tale of Niels Bohr versus Albert Einstein. In traditional tellings, Einstein stubbornly failed to understand the wisdom of Bohr’s Copenhagen interpretation; in revisionist versions, Bohr’s mesmerized disciples missed Einstein’s obvious insights. Becker engages deeply with the historical record and provides a convincing and nuanced account that largely vindicates Einstein without simplistically dismissing Bohr.

### Against Wavefunction Realism (2017)

In B. Weslake and S. Dasgupta, *Current Controversies in the Philosophy of Science* (Routledge, 2021).

I argue that wavefunction realism --- the view that quantum mechanics reveals the fundamental ontology of the world to be a field on a high-dimensional spacetime, must be rejected as (a) relying on artefacts of too-simple versions of quantum mechanics, and (b) not conceptually well-motivated even were those too-simple versions exactly correct. I end with some brief comments on the role of spacetime in any satisfactory account of the metaphysics of extant quantum theories.

### Lessons from Realistic Physics for the Metaphysics of Quantum Theory (2016)

*Synthese* 197, 4303-4818 (2020).

Quantum mechanics, and classical mechanics, are framework theories that incorporate many different concrete theories which in general cannot be arranged in a neat hierarchy, but discussion of `the ontology of quantum mechanics' tends to proceed as if quantum mechanics were a single concrete theory, specifically the physics of nonrelativistically moving point particles interacting by long-range forces. I survey the problems this causes and make some suggestions for how a more physically realistic perspective ought to influence the metaphysics of quantum mechanics.

### What is Orthodox Quantum Mechanics? (2016)

In A.Cordero (ed.), *Philosophers Look at Quantum Mechanics* (Springer, 2019).

What is called ``orthodox'' quantum mechanics, as presented in standard foundational discussions, relies on two substantive assumptions --- the projection postulate and the eigenvalue-eigenvector link --- that do not in fact play any part in practical applications of quantum mechanics. I argue for this conclusion on a number of grounds, but primarily on the grounds that the projection postulate fails correctly to account for repeated, continuous and unsharp measurements (all of which are standard in contemporary physics) and that the eigenvalue-eigenvector link implies that virtually all interesting properties are maximally indefinite pretty much always. I present an alternative way of conceptualising quantum mechanics that does a better job of representing quantum mechanics as it is actually used, and in particular that eliminates use of either the projection postulate or the eigenvalue-eigenvector link, and I reformulate the measurement problem within this new presentation of orthodoxy.

### Interpreting the Quantum Mechanics of Cosmology (2016)

To appear in forthcoming OUP volume on Philosophy of Cosmology, Anna Ijjas and Barry Loewer (ed.)

Quantum theory plays an increasingly significant role in contemporary early-universe cosmology, most notably in the inflationary origins of the fluctuation spectrum of the microwave background radiation. I consider the two main strategies for interpreting (as opposed to modifying or supplementing) standard quantum mechanics in the light of cosmology. I argue that the conceptual difficulties of the approaches based around an irreducible role for measurement - already very severe - become intolerable in a cosmological context, whereas the approach based around Everett's original idea of treating quantum systems as closed systems handles cosmological quantum theory satisfactorily. Contemporary cosmology, which indeed applies standard quantum theory without supplementation or modification, is thus committed - tacitly or explictly - to the Everett interpretation.

### Life and Death in the Tails of the GRW Wave Function (2014)

*International Journal of Quantum Foundations 8* (2022), 148-157.

It seems to be widely assumed that the only effect of the Ghirardi-Rimini-Weber ('GRW') dynamical collapse mechanism on the `tails' of the wavefunction (that is, the components of superpositions on which the collapse is *not* centred) is to reduce their weight. In consequence it seems to be generally accepted that the tails behave exactly as do the various branches in the Everett interpretation except for their much lower weight. These assumptions are demonstrably inaccurate: the collapse mechanism has substantial and detectable effects within the tails. The relevance of this misconception for the dynamical-collapse theories is debatable, though.

### Decoherence and its Role in the Modern Measurement Problem (2011)

*Philosophical Transactions of the Royal Society of London A* 370 (2012) 4576-4593.

Decoherence is widely felt to have something to do with the quantum measurement problem, but getting clear on just what is made diffcult by the fact that the "measurement problem", as traditionally presented in foundational and philosophical discussions, has become somewhat discon- nected from the conceptual problems posed by real physics. This, in turn, is because quantum mechanics as discussed in textbooks and in founda- tional discussions has become somewhat removed from scientific practice, especially where the analysis of measurement is concerned.

This paper has two goals: firstly (sections 1-2), to present an account of how quantum measurements are actually dealt with in modern physics (hint: it doesn't involve a collapse of the wavefunction) and to state the measurement problem from the perspective of that account; and secondly (sections 3-4), to clarify what role decoherence plays in modern measurement theory and what effect it has on the various strategies that have been proposed to solve the measurement problem.

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### Interview (2011)

In Elegance and Enigma: The Quantum Interviews, edited by Maximilian Schlosshauer(Springer, 2011).

Answers to a series of written "interview questions" about the foundations of quantum theory. (In the published volume, the answers are grouped by question, not by participant.)

### Quantum Mechanics on Spacetime I: Spacetime State Realism (2009)

(DW and Chris Timpson) *British Journal for the Philosophy of Science* 61 (2010) 697-727

What ontology does realism about the quantum state suggest? The main extant view in contemporary philosophy of physics is wave-function realism. We elaborate the sense in which wave-function realism does provide an ontological picture; and defend it from certain objections that have been raised against it. However, there are good reasons to be dissatisfied with wave-function realism, as we go on to elaborate. This motivates the development of an opposing picture: what we call spacetime state realism; a view which takes the states associated to spacetime regions as fundamental. This approach enjoys a number of beneficial features, although, unlike wave-function realism, it involves non-separability at the level of fundamental ontology. We investigate the pros and cons of this non-separability, arguing that it is a quite acceptable feature; even one which proves fruitful in the context of relativistic covariance. A companion paper [still not extant as of 2021, sorry - DW] discusses the prospects for combining a spacetime-based ontology with separability, along lines suggested by Deutsch and Hayden.

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### The Quantum Measurement Problem: State of Play (2007)

Chapter 1 of D. Rickles (ed), *The Ashgate Companion to the New Philosophy of Physics *(Ashgate, 2008)

This is a preprint version of a chapter in the Ashgate Companion to the New Philosophy of Physics (which appeared under the more straightforward title "Quantum Mechanics"). In it, I aim to review, in a way accessible to foundationally interested physicists as well as physics-informed philosophers, just where we have got to in the quest for a solution to the measurement problem. I don’t advocate any particular approach to the measurement problem (not here, at any rate!) but I do focus on the importance of decoherence theory to modern attempts to solve the measurement problem, and I am fairly sharply critical of some aspects of the "traditional" formulation of the problem.

### Non-locality and gauge freedom in Deutsch and Hayden's formulation of quantum mechanics (2005)

(DW and Chris Timpson) *Foundations of Physics* 37 (2007), 951-955.

Deutsch and Hayden have proposed an alternative formulation of quantum mechanics which is completely local. We argue that their proposal must be understood as having a form of `gauge freedom' according to which mathematically distinct states are physically equivalent. Once this gauge freedom is taken into account, their formulation is no longer local.

(**Note:** So good they published it twice! For some reason, Foundations of Physics also published the paper in the next issue of the journal, one month later.)

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### Solving the measurement problem: de Broglie-Bohm loses out to Everett (2004)

(Harvey Brown and DW) *Foundations of Physics* 35 (2005), 517-540

The quantum theory of de Broglie and Bohm solves the measurement problem, but the hypothetical corpuscles play no role in the argument. The solution finds a more natural home in the Everett interpretation.

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