## Other Topics in the Philosophy of Quantum Mechanics (July 2014)

### Life and Death in the Tails of the GRW Wave Function

#### In submission.

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 (November 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 measure- ment theory and what eect it has on the various strategies that have been proposed to solve the measurement problem.

### Interview (July 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 (May 2009)

#### (DW and Chris Timpson)

*British Journal for the Philosophy of Science* 61 (2010) pp. 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 2011, sorry - DW] discusses the prospects for combining a spacetime-based ontology with separability, along lines suggested by Deutsch and Hayden.

- PDF of published version (requires subscription)
- preprint PDF

### The Quantum Measurement Problem: State of Play (December 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 (March 2005)

#### (DW and Chris Timpson)

*Foundations of Physics* 37 (2007), pp. 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.)

- PDF of published version (requires subscription)
- Preprint PDF
- Preprint PostScript

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

#### (Harvey Brown and DW)

*Foundations of Physics* 35 (2005), pp. 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.

- PDF of published version (requires subscription)
- Preprint PDF
- Preprint PostScript