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On the Everettian Epistemic Problem
 Studies in History and Philosophy of Modern Physics
"... Recent work in the Everett interpretation has suggested that the problem of probability can be solved by understanding probability in terms of rationality. However, there are two problems relating to probability in Everett — one practical, the other epistemic — and the rationalitybased program dire ..."
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Recent work in the Everett interpretation has suggested that the problem of probability can be solved by understanding probability in terms of rationality. However, there are two problems relating to probability in Everett — one practical, the other epistemic — and the rationalitybased program directly addresses only the practical problem. One might therefore worry that the problem of probability is only ‘half solved ’ by this approach. This paper aims to dispel that worry: a solution to the epistemic problem follows from the rationalitybased solution to the practical problem.
ManyWorlds and Schrödinger’s First Quantum Theory
, 2009
"... Schrödinger’s first proposal for the interpretation of quantum mechanics was based on a postulate relating the wave function on configuration space to charge density in physical space. Schrödinger apparently later thought that his proposal was empirically wrong. We argue here that this is not the ca ..."
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Schrödinger’s first proposal for the interpretation of quantum mechanics was based on a postulate relating the wave function on configuration space to charge density in physical space. Schrödinger apparently later thought that his proposal was empirically wrong. We argue here that this is not the case, at least for a very similar proposal with charge density replaced by mass density. We argue that when analyzed carefully this theory is seen to be an empirically adequate manyworlds theory and not an empirically inadequate theory describing a single world. Moreover, this formulation—Schrödinger’s first quantum theory—can be regarded as a formulation of the manyworlds view of quantum mechanics that is ontologically clearer than Everett’s. PACS: 03.65.Ta. Key words: Everett’s manyworlds view of quantum theory; quantum theory without observers; primitive ontology; Bohmian mechanics; quantum nonlocality in the manyworlds view; nature of probability in the manyworlds view; typicality.
The Quantum Measurement Problem: State of Play
, 2007
"... 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 physicsinformed philosophers, just where we have got to in the quest for a solu ..."
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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 physicsinformed 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.
The Probability Problem in Everettian Quantum Mechanics Persists. The British Journal for the Philosophy of Science
"... Everettian quantum mechanics (EQM) results in ‘multiple, emergent, branching quasiclassical realities ’ (Wallace [2012]). The possible outcomes of measurement as per ‘orthodox ’ quantum mechanics are, in EQM, all instantiated. Given this metaphysics, Everettians face the ‘probability problem’—how ..."
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Everettian quantum mechanics (EQM) results in ‘multiple, emergent, branching quasiclassical realities ’ (Wallace [2012]). The possible outcomes of measurement as per ‘orthodox ’ quantum mechanics are, in EQM, all instantiated. Given this metaphysics, Everettians face the ‘probability problem’—how to make sense of probabilities and recover the Born Rule. To solve the probability problem, Wallace, following Deutsch ([1999]), has derived a quantum representation theorem. I argue that Wallace’s solution to the probability problem is unsuccessful, as follows. First, I examine one of the axioms of rationality used to derive the theorem, Branching Indifference (BI). I argue that Wallace is not successful in showing that BI is rational. While I think it is correct to put the burden of proof on Wallace to motivate BI as an axiom of rationality, it does not follow from his failing to do so that BI is not rational. Thus, second, I show that there is an alternative strategy for setting one’s credences in the face of branching which is rational and which violates BI. This is Branch Counting (BC). Wallace is aware of BC and has proffered various arguments against it. However, third, I argue that Wallace’s arguments
SelfLocating Uncertainty and the Origin of Probability in Everettian Quantum Mechanics
 In: Struppa, D., & Tollaksen, J. (eds
, 2014
"... A longstanding issue in attempts to understand the Everett (ManyWorlds) approach to quantum mechanics is the origin of the Born rule: why is the probability given by the square of the amplitude? Following Vaidman, we note that observers are in a position of selflocating uncertainty during the peri ..."
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A longstanding issue in attempts to understand the Everett (ManyWorlds) approach to quantum mechanics is the origin of the Born rule: why is the probability given by the square of the amplitude? Following Vaidman, we note that observers are in a position of selflocating uncertainty during the period between the branches of the wave function splitting via decoherence and the observer registering the outcome of the measurement. In this period it is tempting to regard each branch as equiprobable, but we give new reasons why that would be inadvisable. Applying lessons from this analysis, we demonstrate (using arguments similar to those in Zurek’s envariancebased derivation) that the Born rule is the uniquely rational way of apportioning credence in Everettian quantum mechanics. In particular, we rely on a single key principle: changes purely to the environment do not affect the probabilities one ought to assign to measurement outcomes in a local subsystem. We arrive at a method for assigning probabilities in cases that involve both classical and quantum selflocating uncertainty. This method provides unique answers to quantum Sleeping Beauty problems, as well as a welldefined procedure for calculating probabilities in quantum cosmological multiverses with multiple similar observers. CALT 682928 1 ar
Quantum mechanics as classical physics
, 2014
"... Here I explore a novel nocollapse interpretation of quantum mechanics which combines aspects of two familiar and welldeveloped alternatives, Bohmian mechanics and the manyworlds interpretation. Despite reproducing the empirical predictions of quantum mechanics, the theory looks surprisingly class ..."
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Here I explore a novel nocollapse interpretation of quantum mechanics which combines aspects of two familiar and welldeveloped alternatives, Bohmian mechanics and the manyworlds interpretation. Despite reproducing the empirical predictions of quantum mechanics, the theory looks surprisingly classical. All there is at the fundamental level are particles interacting via Newtonian forces. There is no wave function. However, there are many worlds. On the face of it, quantum physics is nothing like classical physics. Despite its oddity, work in the foundations of quantum theory has provided some palatable ways of understanding this strange quantum realm. Most of our best theories take that story to include the existence of a very nonclassical entity: the wave function. Here I offer an alternative which combines elements of Bohmian mechanics and the manyworlds interpretation to form a theory in which there is no wave function. According to this theory, all there is at the fundamental level are particles interacting via Newtonian forces. In this sense, the theory is classical. However, it is still undeniably strange as it posits the existence of a large but finite collection of worlds, each completely and utterly real. When an experiment is conducted, every result with appreciable Born Rule probability does actually occur in one of these worlds. Unlike the many worlds of the manyworlds interpretation, these worlds are fundamental, not emergent; they are interacting, not causally isolated; and they never branch. In each of these worlds, particles follow welldefined trajectories and move as if they were being guided by a wave function in the familiar Bohmian way. In fact, their trajectories are determined by a combination of intra and interworld forces. In this paper I will not attempt to argue that this theory is unequivocally superior to its competitors. Instead, I would like to establish it as a surprisingly successful alternative which deserves attention and development, hopefully one day meriting inclusion among the list of promising realist responses to the measurement problem. 1 ar
Against the Empirical Viability of the DeutschWallaceEverett Approach to Quantum Mechanics
"... The subjective Everettian approach to quantum mechanics presented by Deutsch and Wallace fails to constitute an empirically viable theory of quantum phenomena. The decision theoretic implementation of the Born rule realized in this approach provides no basis for rejecting Everettian quantum mechani ..."
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The subjective Everettian approach to quantum mechanics presented by Deutsch and Wallace fails to constitute an empirically viable theory of quantum phenomena. The decision theoretic implementation of the Born rule realized in this approach provides no basis for rejecting Everettian quantum mechanics in the face of empirical data that contradicts the Born rule. The approach of Greaves and Myrvold, which provides a subjective implementation of the Born rule as well but derives it from empirical data rather than decision theoretic arguments, avoids the problem faced by Deutsch and Wallace and is empirically viable. However, there is good reason to cast doubts on its scientific value.
To appear in the British Journal for the Philosophy of Science. Branching and Uncertainty
"... Abstract: Following Lewis, it is widely held that branching worlds di¤er 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 worl ..."
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Abstract: Following Lewis, it is widely held that branching worlds di¤er 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 utterances typically made about quantum mechanical contingencies, including statements of uncertainty, if the Everett interpretation of quantum mechanics is true. The ‘incoherence problem’of the Everett interpretation, that it can give no meaning to the notion of uncertainty, is thereby solved. 1
Everett and the Born Rule
, 810
"... During the last ten years or so, derivations of the Born rule based on decision theory have been proposed and developed, and it is claimed that these are valid in the context of the Everett interpretation. This claim is critically assessed and it is shown that one of its key assumptions is a natural ..."
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During the last ten years or so, derivations of the Born rule based on decision theory have been proposed and developed, and it is claimed that these are valid in the context of the Everett interpretation. This claim is critically assessed and it is shown that one of its key assumptions is a natural consequence of the principles underlying the Copenhagen interpretation, but constitutes a major additional postulate in an Everettian context. It is further argued that the Born rule, in common with any interpretation that relates outcome likelihood to the expansion coefficients connecting the wavefunction with the eigenfunctions of the measurement operator, is incompatible with the purely unitary evolution assumed in the Everett interpretation. Key words:
Formalism and Interpretation in Quantum Theory1
"... QuantumMechanics can be viewed as a linear dynamical theory having a familiar mathematical framework but a mysterious probabilistic interpretation, or as a probabilistic theory having a familiar interpretation but a mysterious formal framework. These points of view are usually taken to be somewhat ..."
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QuantumMechanics can be viewed as a linear dynamical theory having a familiar mathematical framework but a mysterious probabilistic interpretation, or as a probabilistic theory having a familiar interpretation but a mysterious formal framework. These points of view are usually taken to be somewhat in tension with one another. The first has generated a vast literature aiming at a “realistic ” and “collapsefree ” interpretation of quantum mechanics that will account for its statistical predictions. The second has generated an at least equally large literature aiming to derive, or at any rate motivate, the formal structure of quantum theory in probabilistically intelligible terms. In this paper I explore, in a preliminary way, the possibility that these two programmes have something to offer one another. In particular, I show that a version of the measurement problem occurs in essentially any nonclassical probabilistic theory, and ask to what extent various interpretations of quantum mechanics continue