•  585
    Time-dependent symmetries: the link between gauge symmetries and indeterminism
    In Katherine Brading & Elena Castellani (eds.), Symmetries in Physics: Philosophical Reflections, Cambridge University Press. pp. 163--173. 2002.
    Mathematically, gauge theories are extraordinarily rich --- so rich, in fact, that it can become all too easy to lose track of the connections between results, and become lost in a mass of beautiful theorems and properties: indeterminism, constraints, Noether identities, local and global symmetries, and so on. One purpose of this short article is to provide some sort of a guide through the mathematics, to the conceptual core of what is actually going on. Its focus is on the Lagrangian, variatio…Read more
  •  528
    QFT, antimatter, and symmetry
    Studies in History and Philosophy of Science Part B: Studies in History and Philosophy of Modern Physics 40 (3): 209-222. 2009.
    A systematic analysis is made of the relations between the symmetries of a classical field and the symmetries of the one-particle quantum system that results from quantizing that field in regimes where interactions are weak. The results are applied to gain a greater insight into the phenomenon of antimatter.
  •  373
    Gravity, Entropy, and Cosmology: in Search of Clarity
    British Journal for the Philosophy of Science 61 (3): 513-540. 2010.
    I discuss the statistical mechanics of gravitating systems and in particular its cosmological implications, and argue that many conventional views on this subject in the foundations of statistical mechanics embody significant confusion; I attempt to provide a clearer and more accurate account. In particular, I observe that (i) the role of gravity in entropy calculations must be distinguished from the entropy of gravity, that (ii) although gravitational collapse is entropy-increasing, this is not…Read more
  •  306
    Saunders and Wallace reply
    British Journal for the Philosophy of Science 59 (3): 315-317. 2008.
    A reply to a comment by Paul Tappenden (BJPS 59 (2008) pp. 307-314) on S. Saunders and D. Wallace, "Branching and Uncertainty" (BJPS 59 (2008) pp. 298-306)
  •  249
    Quantum probability from subjective likelihood: Improving on Deutsch's proof of the probability rule
    Studies in History and Philosophy of Science Part B: Studies in History and Philosophy of Modern Physics 38 (2): 311-332. 2007.
    I present a proof of the quantum probability rule from decision-theoretic assumptions, in the context of the Everett interpretation. The basic ideas behind the proof are those presented in Deutsch's recent proof of the probability rule, but the proof is simpler and proceeds from weaker decision-theoretic assumptions. This makes it easier to discuss the conceptual ideas involved in the proof, and to show that they are defensible.
  •  246
    Taking particle physics seriously: A critique of the algebraic approach to quantum field theory
    Studies in History and Philosophy of Science Part B: Studies in History and Philosophy of Modern Physics 42 (2): 116-125. 2010.
    I argue against the currently prevalent view that algebraic quantum field theory (AQFT) is the correct framework for philosophy of quantum field theory and that “conventional” quantum field theory (CQFT), of the sort used in mainstream particle physics, is not suitable for foundational study. In doing so, I defend that position that AQFT and CQFT should be understood as rival programs to resolve the mathematical and physical pathologies of renormalization theory, and that CQFT has succeeded in t…Read more
  •  242
    Epistemology quantized: Circumstances in which we should come to believe in the Everett interpretation
    British Journal for the Philosophy of Science 57 (4): 655-689. 2006.
    I consider exactly what is involved in a solution to the probability problem of the Everett interpretation, in the light of recent work on applying considerations from decision theory to that problem. I suggest an overall framework for understanding probability in a physical theory, and conclude that this framework, when applied to the Everett interpretation, yields the result that that interpretation satisfactorily solves the measurement problem. Introduction What is probability? 2.1 Objective …Read more
  •  237
    Everettian rationality: defending Deutsch's approach to probability in the Everett interpretation
    Studies in History and Philosophy of Science Part B: Studies in History and Philosophy of Modern Physics 34 (3): 415-439. 2003.
    An analysis is made of Deutsch's recent claim to have derived the Born rule from decision-theoretic assumptions. It is argued that Deutsch's proof must be understood in the explicit context of the Everett interpretation, and that in this context, it essentially succeeds. Some comments are made about the criticism of Deutsch's proof by Barnum, Caves, Finkelstein, Fuchs, and Schack; it is argued that the flaw which they point out in the proof does not apply if the Everett interpretation is assumed…Read more
  •  219
    I attempt to get as clear as possible on the chain of reasoning by which irreversible macrodynamics is derivable from time-reversible microphysics, and in particular to clarify just what kinds of assumptions about the initial state of the universe, and about the nature of the microdynamics, are needed in these derivations. I conclude that while a “Past Hypothesis” about the early Universe does seem necessary to carry out such derivations, that Hypothesis is not correctly understood as a constrai…Read more
  •  215
    A formal proof of the born rule from decision-theoretic assumptions [aka: How to Prove the Born Rule]
    In Simon Saunders, Jon Barrett, Adrian Kent & David Wallace (eds.), Many Worlds?: Everett, Quantum Theory & Reality, Oxford University Press. 2009.
    I develop the decision-theoretic approach to quantum probability, originally proposed by David Deutsch, into a mathematically rigorous proof of the Born rule in (Everett-interpreted) quantum mechanics. I sketch the argument informally, then prove it formally, and lastly consider a number of proposed ``counter-examples'' to show exactly which premises of the argument they violate. (This is a preliminary version of a chapter to appear --- under the title ``How to prove the Born Rule'' --- in Saund…Read more
  •  213
    Branching and Uncertainty
    British Journal for the Philosophy of Science 59 (3): 293-305. 2008.
    Following Lewis, it is widely held that branching worlds differ in important ways from diverging worlds. There is, however, a simple and natural semantics under which ordinary sentences uttered in branching worlds have much the same truth values as they conventionally have in diverging worlds. Under this semantics, whether branching or diverging, speakers cannot say in advance which branch or world is theirs. They are uncertain as to the outcome. This same semantics ensures the truth of utteranc…Read more
  •  206
    An investigation is made into how the foundations of statistical mechanics are affected once we treat classical mechanics as an approximation to quantum mechanics in certain domains rather than as a theory in its own right; this is necessary if we are to understand statistical-mechanical systems in our own world. Relevant structural and dynamical differences are identified between classical and quantum mechanics (partly through analysis of technical work on quantum chaos by other authors). These…Read more
  •  205
    Quantum Mechanics on Spacetime I: Spacetime State Realism
    with Christopher Gordon Timpson
    British Journal for the Philosophy of Science 61 (4): 697-727. 2010.
    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 re…Read more
  •  199
    Review of Laura Ruetsche's "Interpreting quantum theories" (review)
    British Journal for the Philosophy of Science 65 (2): 425-428. 2014.
  •  189
    This is a preliminary version of an article to appear in the forthcoming Ashgate Companion to the New Philosophy of Physics.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 t…Read more
  •  186
    Solving the measurement problem: De broglie-Bohm loses out to Everett (review)
    Foundations of Physics 35 (4): 517-540. 2004.
    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.
  •  183
    Non-locality and Gauge Freedom in Deutsch and Hayden’s Formulation of Quantum Mechanics
    with Christopher G. Timpson
    Foundations of Physics 37 (6): 951-955. 2007.
    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
  •  183
    I analyse the conceptual and mathematical foundations of Lagrangian quantum field theory (QFT) (that is, the ‘naive’ (QFT) used in mainstream physics, as opposed to algebraic quantum field theory). The objective is to see whether Lagrangian (QFT) has a sufficiently firm conceptual and mathematical basis to be a legitimate object of foundational study, or whether it is too ill-defined. The analysis covers renormalisation and infinities, inequivalent representations, and the concept of localised s…Read more
  •  175
    The Quantum Revolution in Philosophy
    Analysis 80 (2): 381-388. 2020.
    Richard Healey’s The Quantum Revolution in Philosophy is a terrific book, and yet I disagree with nearly all its main substantive conclusions.1 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.
  •  162
    Everett and structure
    Studies in History and Philosophy of Science Part B: Studies in History and Philosophy of Modern Physics 34 (1): 87-105. 2003.
    I address the problem of indefiniteness in quantum mechanics: the problem that the theory, without changes to its formalism, seems to predict that macroscopic quantities have no definite values. The Everett interpretation is often criticised along these lines, and I shall argue that much of this criticism rests on a false dichotomy: that the macroworld must either be written directly into the formalism or be regarded as somehow illusory. By means of analogy with other areas of physics, I develop…Read more
  •  143
    Worlds in the Everett interpretation
    Studies in History and Philosophy of Science Part B: Studies in History and Philosophy of Modern Physics 33 (4): 637-661. 2002.
    This is a discussion of how we can understand the world-view given to us by the Everett interpretation of quantum mechanics, and in particular the role played by the concept of 'world'. The view presented is that we are entitled to use 'many-worlds' terminology even if the theory does not specify the worlds in the formalism; this is defended by means of an extensive analogy with the concept of an 'instant' or moment of time in relativity, with the lack of a preferred foliation of spacetime being…Read more
  •  132
    Protecting cognitive science from quantum theory
    Behavioral and Brain Sciences 27 (5): 636-637. 2004.
    The relation between micro-objects and macro-objects advocated by Kim is even more problematic than Ross & Spurrett (R&S) argue, for reasons rooted in physics. R&S's own ontological proposals are much more satisfactory from a physicist's viewpoint but may still be problematic. A satisfactory theory of macroscopic ontology must be as independent as possible of the details of microscopic physics.
  •  123
    Decoherence and Ontology (or: How I learned to stop worrying and love FAPP)
    In Simon Saunders, Jon Barrett, Adrian Kent & David Wallace (eds.), Many Worlds?: Everett, Quantum Theory & Reality, Oxford University Press. pp. 53--72. 2010.
    NGC 1300 (shown in figure 1) is a spiral galaxy 65 million light years from Earth.1 We have never been there, and (although I would love to be wrong about this) we will never go there; all we will ever know about NGC 1300 is what we can see of it from sixty-five million light years away, and what we can infer from our best physics. Fortunately, “what we can infer from our best physics” is actually quite a lot. To take a particular example: our best theory of galaxies tells us that that hazy glow…Read more
  •  120
    Who's afraid of coordinate systems? An essay on representation of spacetime structure
    Studies in History and Philosophy of Science Part B: Studies in History and Philosophy of Modern Physics 67 125-136. 2019.
    Coordinate-based approaches to physical theories remain standard in mainstream physics but are largely eschewed in foundational discussion in favour of coordinate-free differential-geometric approaches. I defend the conceptual and mathematical legitimacy of the coordinate-based approach for foundational work. In doing so, I provide an account of the Kleinian conception of geometry as a theory of invariance under symmetry groups; I argue that this conception continues to play a very substantial r…Read more
  •  115
    I criticise the view that the relativity and equivalence principles are consequences of the small-scale structure of the metric in general relativity, by arguing that these principles also apply to systems with non-trivial self-gravitation and hence non-trivial spacetime curvature (such as black holes). I provide an alternative account, incorporating aspects of the criticised view, which allows both principles to apply to systems with self-gravity.
  •  110
    Diachronic Rationality and Prediction-Based Games
    Proceedings of the Aristotelian Society 110 (3pt3): 243-266. 2010.
    I explore the debate about causal versus evidential decision theory, and its recent developments in the work of Andy Egan, through the method of some simple games based on agents' predictions of each other's actions. My main focus is on the requirement for rational agents to act in a way which is consistent over time and its implications for such games and their more realistic cousins
  •  90
    An examination is made of the way in which particles emerge from linear, bosonic, massive quantum field theories. Two different constructions of the one-particle subspace of such theories are given, both illustrating the importance of the interplay between the quantum-mechanical linear structure and the classical one. Some comments are made on the Newton-Wigner representation of one-particle states, and on the relationship between the approach of this paper and those of Segal, and of Haag and Ru…Read more
  •  89
    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…Read more
  •  87
    David Wallace argues that we should take quantum theory seriously as an account of what the world is like--which means accepting the idea that the universe is constantly branching into new universes. He presents an accessible but rigorous account of the 'Everett interpretation', the best way to make coherent sense of quantum physics