Everett Interpretation

Wave function realism is an interpretative framework for quantum theories which recommends taking the central ontology of these theories to consist of the quantum wave function, understood as a field on a high-dimensional space. This paper presents and evaluates three standard arguments for wave function realism, and clarifies the sort of ontological framework these arguments support.

We study the conservation of energy, or lack thereof, when measurements are performed in quantum mechanics. The expectation value of the Hamiltonian of a system changes when wave functions collapse in accordance with the standard textbook treatment of quantum measurement, but one might imagine that the change in energy is compensated by the measuring apparatus or environment. We show that this is not true; the change in the energy of a state after measurement can be arbitrarily large, independent of the (...) physical measurement process. In Everettian quantum theory, while the expectation value of the Hamiltonian is conserved for the wave function of the universe, it is not constant within individual worlds. It should therefore be possible to experimentally measure violations of conservation of energy, and we suggest an experimental protocol for doing so. (shrink)

Any viable interpretation of quantum theory needs to account for the Born rule, from which the theory gets its probabilistic empirical predictions. In this paper, we give an overview of possible approaches to this problem in the context of the Many Worlds interpretation. We argue that, for structural reasons, none of them can possibly succeed. More precisely, we argue that the Many Worlds interpretation must obtain the Born rule by proceeding either axiomatically, deductively, or inductively, and that all three of (...) these approaches run into general, fundamental obstructions. (shrink)

This paper provides a detailed historical account of early debates over wave-function realism, the modern term for the view that the wave function of quantum theory is physically real. As this paper will show, the idea of physical waves associated with particles had its roots in work by Einstein and de Broglie, who both originally thought of these waves as propagating in three-dimensional physical space. De Broglie quickly turned this wave-particle duality into an early pilot-wave theory, on which a particle's (...) associated “phase wave” piloted or guided the particle along its trajectory. Schrödinger built on de Broglie's phase-wave hypothesis to provide a comprehensive account of the nascent quantum theory. However, Schrödinger's new “undulatory mechanics” came at the cost of replacing de Broglie's phase waves propagating in physical space with a wave function propagating in a system's abstract configuration space. The present work will argue that this move from three-dimensional physical space to a many-dimensional configuration space was a key reason why the founders of quantum theory uniformly abandoned the physical reality of the wave function. This paper will further clarify that de Broglie introduced two distinct pilot-wave theories, and will then argue that it was Bohm's rediscovery of the second of these two pilot-wave theories over two decades later, as well as Bohm's vociferous defense of wave-function realism, that were responsible for resurrecting the idea of an ontological wave function. This idea ended up playing a central role in Everett's development of the many-worlds interpretation. (shrink)

The original version of the de Broglie-Bohm pilot-wave theory, also called Bohmian mechanics, attempted to treat the wave function or pilot wave as a part of the physical ontology of nature. More recent versions of the de Broglie-Bohm theory appearing in the last few decades have tried to regard the pilot wave instead as an aspect of the theory's nomology, or dynamical laws. This paper argues that neither of these views is correct, and that the de Broglie-Bohm pilot wave is (...) best understood as a collection of latent variables in the sense of a hidden Markov model, a construct that was not available when de Broglie and Bohm originally formulated what became their pilot-wave theory. This paper also discusses several other challenges for the ontological view of the pilot wave. One such challenge is due to Foldy-Wouthuysen gauge transformations, which connect up with the Deotto-Ghirardi ambiguity in the de Broglie-Bohm theory. Another challenge arises from the freedom to carry out canonical transformations in the wave function's own notion of phase space, as defined by Strocchi and Heslot. (shrink)

Superseded Paper Notice This document represents an earlier draft of my work on the multi‑simulation thesis. It has since been fully developed and expanded into the paper: Augusto Bartolomeu, *The Multi‑Simulation Thesis: Quantum Physics as Evidence for Multi‑Simulations* (2025). Readers are strongly encouraged to consult the updated version, which provides systematic arguments, expanded engagement with objections, and a fuller positioning within the scholarship of quantum interpretations and simulation theory. This earlier draft is preserved here as a marker of the project’s (...) intellectual trajectory. This paper advances the multi-simulation thesis, a reinterpretation of quantum mechanics and the simulation hypothesis. Where traditional approaches such as Everett’s Many-Worlds interpretation proliferate infinite universes, this model proposes multiple parallel simulations — sibling runs of a single informational codebase. This reframing aligns with key quantum phenomena: wavefunction collapse as on-demand rendering, entanglement as shared memory access, the Planck scale as pixelation, and quantum computing as cross-simulation querying. Consciousness is treated not as a byproduct of computation but as a portal function within the simulation, a mechanism by which possibility collapses into form. The thesis further extends to cultural domains, arguing that monuments, archives, and collective memory act as civilizational measurement devices — collapsing history much like observation collapses quantum states. By connecting physics, philosophy, and culture, the multi-simulation thesis offers a more parsimonious and coherent framework than multiverse accounts, while also accommodating anomalies such as déjà vu and the Mandela Effect. Though not conventionally falsifiable, its value lies in explanatory scope, coherence, and its ability to unify disparate phenomena under a single informational ontology. © 2025 Augusto Bartolomeu. All rights reserved. (shrink)

Everettian quantum mechanics is often presented as ontologically economical because it preserves unitary dynamics and avoids physical collapse. This paper argues that this economy is incomplete unless Everettian realism supplies a principled criterion for counting worlds, branches, histories, or observer-relevant realities. The central worry is not mathematical branching, but ontological overcounting: the move from microphysical or branch-level multiplicity to full world-multiplicity may assign ontological weight to distinctions that make no difference to any observer-constituting record. I introduce an observer-equivalence constraint: two (...) physical or branch-relative realizations are equivalent for an agent A if they agree with respect to all physically accessible, persistent, update-constraining records available to A within its finite integration window. Such realizations may be physically distinct, but they do not define distinct observer-worlds for A. In the conscious case, the relevant observer-constituting record-structure should be understood as temporally integrated over a finite window rather than as an instantaneous observer-slice. This does not refute Everettian quantum mechanics, nor does it deny decoherence or branch structure. It challenges the interpretation of branch multiplicity as unpriced ontological multiplicity. A record-based alternative treats many micro-realizations as equivalent until they generate observer-relevant record differences. The resulting picture is not a new collapse theory, but a constraint on ontological economy: worlds should not be counted more finely than the record-structures that make them difference-making for observers. (shrink)

This paper argues that the unresolved core of the measurement problem is not best understood as a purely dynamical problem. The standard formulation asks how linear quantum evolution yields a single definite outcome. That question is genuine, but it already presupposes that definiteness must be read directly from the global physical state. I argue instead that the deeper residual problem is one of individuation: under what conditions does a physically realized difference count as a fact, an outcome, or a world (...) for a finite record-bearing system? Once the issue is formulated at this level, the role of decoherence looks different. Decoherence and redundant environmental encoding do not select one globally privileged outcome, but they do supply the physical architecture of stable, accessible, and update-constraining records. The remaining task is ontological rather than primarily dynamical. This diagnosis is especially significant for Everettian quantum mechanics. Everett preserves the global physics with unusual clarity, but branching structure alone does not determine when physically realized differences count as distinct observer-worlds for finite agents. A complete account therefore requires an explicit criterion of observer-world individuation. I propose such a criterion in terms of record-constituted definiteness: two physical configurations belong to the same observer-world for an agent when they agree with respect to the agent's accessible, persistent, and update-constraining records. (shrink)

Everettian quantum mechanics replaces physical collapse with unitary branching: after measurement, multiple successor observers are said to exist, each recording a definite outcome. This paper raises a prior metaphysical question about that familiar picture: what licenses the multiplication of conscious subjects, rather than merely the multiplication of physical branch-realizers? I introduce the Informational Identity Constraint (IIC): if conscious subjecthood is individuated by subject-constituting informational or record-based configuration, then there is no subject-multiplicity without subject-constituting informational difference. The point is not that (...) two physical systems cannot be numerically distinct. They can. The point is that physical token-multiplicity does not by itself ground multiplicity at the level of conscious subjectivity unless one supplies an additional criterion of subject-individuation. The paper then adds a necessary refinement. The IIC must not be mistaken for a counting rule. It does not say that every distinct conscious configuration counts once, nor that identical configurations exhaust the ontology of probability. Identity is not measure. A=A provides an individuation constraint; it says what is the same and what is distinct at the level of subject-constituting configuration. Probability or measure, by contrast, belongs to dynamically supported continuations. In schematic form: A=A individuates the node; mu(A -> B) represents the transition-weight of a continuation. This transition-relative notation is not proposed as a new physical law, nor as an alternative to the Born rule. In an Everettian setting, any physically adequate mu would have to be constrained by decoherence, amplitude structure, and the existing debate over Born weights, including decision-theoretic and self-locating approaches. The present claim is conceptual: whatever the correct measure is, it should not be assigned to isolated experiential configurations as such. Teletransportation and fission cases make the distinction vivid. A perfect continuation-copy may inherit the same records and experiences, but the experience itself contains no intrinsic probability label. Boltzmann-brain cases sharpen the point: pseudo-historical records may be experientially indistinguishable from ordinary records, yet lack high-weight generative chains. Thus the mature A=A argument is not a replacement of branch-counting by configuration-counting. It is a pre-probabilistic constraint on observer ontology: before probabilities can be assigned, one must individuate the observer-states and continuations to which those probabilities attach. (shrink)

This paper argues that the philosophical possibility of teleportation creates a neglected pressure point for personal identity in Everettian many-worlds theories. If teleportation can count as survival because it preserves the relevant informational structure of a subject — memories, dispositions, bodily organization, and psychological continuity — then local material continuity cannot be the fundamental criterion of personal identity. But in an Everettian setting, this anti-local implication generates a further question: if a subject may survive informational reconstruction across space, what explains (...) the individuation of future continuations across branches? The paper argues that the two challenges, though structurally distinct — one targeting spatial locality, the other targeting uniqueness of continuation — converge on a common demand for an explicit criterion of continuation. Neither bare material continuity nor bare informational duplication is sufficient. What is needed is a criterion of record-continuity: the structured inheritance of memory, access relations, body-schema, environmental embedding, and world-context through which observer-worlds are individuated. It also distinguishes record-continuity from narrative self-constitution accounts by treating narrative as one record-form within a wider causal-historical structure. This criterion is presented as a record-theoretic specification and extension of broadly Parfitian reductionism, rather than as a rejection of it, and as a friendly clarification of the record-historical dimension implicit in functionalist readings of Everettian observers such as Wallace's. (shrink)

In the science fiction novel Quarantine, Greg Egan imagines a universe where interactions with human observers collapse quantum wavefunctions. Aliens, unable to collapse wavefunctions, tire of being slaughtered by these collapses. In response they erect an impenetrable shield around the solar system, protecting the rest of the universe from human interference and locking humanity into a starless Bubble. When confronting scientific realism and the quantum, many philosophers try to do the theoretical counterpart of this fictional practical strategy. Quantum mechanics is (...) beset with many hard-to-resolve interpretational challenges. Philosophers — appealing to decoherence and coarse-graining — try to put these in a bubble and hope that they can go about their philosophizing as before. My chapter aims to burst this Bubble. (shrink)

Two of the most difficult problems in the foundations of physics are (1) what gives rise to the arrow of time and (2) what the ontology of quantum mechanics is. They are difficult because the fundamental dynamical laws of physics do not privilege an arrow of time, and the quantum-mechanical wave function describes a high-dimensional reality that is radically different from our ordinary experiences. In this paper, I characterize and elaborate on the ``Wentaculus” theory, a new approach to time’s arrow (...) in a quantum universe that offers a unified solution to both problems. Central to the Wentaculus are (i) Density Matrix Realism, the idea that the quantum state of the universe is objective but can be impure, and (ii) the Initial Projection Hypothesis, a new law of nature that selects a unique initial quantum state. On the Wentaculus, the quantum state of the universe is sufficiently simple to be a law, and the arrow of time can be traced back to an exact boundary condition. It removes the intrinsic vagueness of the Past Hypothesis, eliminates the Statistical Postulate, provides a higher degree of theoretical unity, and contains a natural realization of “strong determinism." I end by responding to four recent objections. In a companion paper, I elaborate on Density Matrix Realism. (shrink)

This paper lays out a novel proposal about the metaphysical foundations of (non-relativistic) quantum mechanics, which has some elements in common with Everett's “Many Worlds” interpretation and some elements in common with Bohm's ”Pilot Wave” interpretation. The view agrees with the Everettians that the quantum wavefunction can be interpreted be interpreted as a complete description of the world in fundamental terms. But it holds that this truth of this description suffices for the existence of an uncountable plurality of “worlds” of (...) ordinary, non-fundamental objects, where each such “world” corresponds to a mapping of points of time to points of configuration space that obeys that Bohmian “Guidance Equation”. (shrink)

The second law of thermodynamics is traditionally interpreted as a coarse-grained result of classical mechanics. Recently its relation with quantum mechanical processes such as decoherence and measurement has been revealed in literature. In this paper we will formulate the second law and the associated time irreversibility following Everett’s idea: systems entangled with an object getting to know the branch in which they live. Accounting for this self-locating knowledge, we get two forms of entropy: objective entropy measuring the uncertainty of the (...) state of the object alone, and subjective entropy measuring the information carried by the self-locating knowledge. By showing that the summation of the two forms of entropy is a conserved and perspective-free quantity, we interpret the second law as a statement of irreversibility in knowledge acquisition. This essentially derives the thermodynamic arrow of time from the subjective arrow of time, and provides a unified explanation for varieties of the second law, as well as the past hypothesis. (shrink)

We investigate the validity of the field explanation of the wave function by analyzing the mass and charge density distributions of a quantum system. It is argued that a charged quantum system has effective mass and charge density distributing in space, proportional to the square of the absolute value of its wave function. This is also a consequence of protective measurement. If the wave function is a physical field, then the mass and charge density will be distributed in space simultaneously (...) for a charged quantum system, and thus there will exist a remarkable electrostatic self-interaction of its wave function, though the gravitational self-interaction is too weak to be detected presently. This not only violates the superposition principle of quantum mechanics but also contradicts experimental observations. Thus we conclude that the wave function cannot be a description of a physical field. In the second part of this paper, we further analyze the implications of these results for the main realistic interpretations of quantum mechanics, especially for de Broglie-Bohm theory. It has been argued that de Broglie-Bohm theory gives the same predictions as quantum mechanics by means of quantum equilibrium hypothesis. However, this equivalence is based on the premise that the wave function, regarded as a Ψ-field, has no mass and charge density distributions, which turns out to be wrong according to the above results. For a charged quantum system, both Ψ-field and Bohmian particle have charge density distribution. This then results in the existence of an electrostatic self-interaction of the field and an electromagnetic interaction between the field and Bohmian particle, which contradicts both the predictions of quantum mechanics and experimental observations. Therefore, de Broglie-Bohm theory as a realistic interpretation of quantum mechanics is probably wrong. Lastly, we suggest that the wave function is a description of some sort of ergodic motion (e.g. random discontinuous motion) of particles, and we also briefly analyze the implications of this suggestion for other realistic interpretations of quantum mechanics including many-worlds interpretation and dynamical collapse theories. (shrink)

Causal–Continuity Stabilization Theory (CCST) characterizes macroscopic causal persistence as a physical stability property of finite substrates rather than as an ontological feature of the universal wavefunction. In its original formulation, CCST formalized descriptive admissibility: a coarse–grained macroscopic causal history remains well–posed only when thermodynamic dissipation stays below a substrate–dependent bound and when coarse–grained dynamical sensitivity preserves closure. This work develops an extension of CCST that targets a logically distinct stability question in Everettian dynamics: not only whether a macroscopic description remains (...) admissible within a branch, but whether a branch structure itself can remain dynamically coherent as an organized physical entity under sustained environmental coupling. Although unitary quantum mechanics preserves all components of the global quantum state [1, 2], organized branch structure may dynamically disperse into microscopic degrees of freedom under open–system dynamics, thereby losing the correlation patterns required for macroscopic organization [3, 4]. We distinguish structural dispersion (loss of branch–level organizational coherence) from Causal Exit (loss of description–level definability on a finite physical substrate), emphasizing that these correspond to stability failures at distinct physical layers and should not be conflated. We derive a dynamical weighting over coarse–grained path ensembles from open–system effective action and influence functional formalisms [5–7], showing how microscopic dynamics inducesanon–uniformstabilitygeometryinpathspacethroughirreversibledynamicalsuppression rather than probabilistic selection, without introducing collapse, selection rules, or inter–branch transport. Finally, a minimal toy model is introduced to visualize the qualitative sensitivity of branch structural persistence under controlled stability thresholds. The model is strictly illustrative and is accompanied by transparent parameter definitions and a reproducible numerical implementation in the appendix. (shrink)

Causal–Continuity Stabilization Theory (CCST) addresses the persistence of macroscopic causal histories within an Everettian, fully unitary ontology. While environmental decoherence accounts for the emergence of approximately autonomous quasi–classical sectors, it does not by itself specify when a continuous descriptive trajectory remains physically well–posed on a finite physicalsubstrate. CCSTintroducesaneffectivegeometricformulationofthermodynamicstrain Σ and a substrate–dependent maximum dissipation bound Φmax, together with a dynamical stability diagnostic expressed by an effective Lyapunov condition λ≤0. The central claim is that sector measure (Born weight) is conceptually independent (...) of de- scriptive admissibility: a sector may be prevalent in the global state yet fail to support stable frame–to–framecontinuityonagivensubstrateiftherequireddissipationexceedsΦmax oriflocal sensitivity renders the coarse–grained closure ill–posed. To avoid any implication of inter–sector transport or physical relocation, CCST defines the projection ΠS,t0 as a stability–constrained partial readout rule acting on the global wavefunction, yielding a descriptive trajectory ES (t) only on domains where admissibility holds. In this framework, a Causal Exit denotes loss of descriptive definability rather than elimi- nation of any Everettian sector, preserving full compatibility with unitary quantum mechanics while providing an explicit physical stability criterion for macroscopic descriptive persistence. (shrink)

I show in this paper why the universality of quantum mechanics at all scales, which implies the possibility of Schrodinger's Cat and Wigner's Friend thought experiments, cannot be experimentally confirmed, and why macroscopic superpositions in general cannot be observed or measured, even in principle. Through the relativity of quantum superposition and the transitivity of correlation, it is shown that from the perspective of an object that is in quantum superposition relative to a macroscopic measuring device and observer, the observer is (...) already sufficiently well correlated to the measuring device that once the object correlates to the measuring device, there is no time period in which the observer can perform an appropriate interference experiment to show that the measuring device is in a superposition. (shrink)

It is often claimed that the many worlds theory is to be preferred over other realist interpretations of quantum mechanics for its ability to avoid the kind of action at a distance that plagues both hidden variables and collapse models. The aim of this paper is address the question of whether branching should be viewed as a causal process that spreads out from a localized region as some authors (Wallace (2012), Blackshaw, Huggett, and Ladyman (manuscript)) have such suggested, or whether (...) it occurs globally across the total quantum state at an instant as others insist (Sebens and Carroll (2018), McQueen and Vaidman (2019)). An appeal of the local branching model is it that it ensures that branching never involves superluminal influences. After clarifying these two models, the paper will consider the argument in favor of local branching, and then four concerns one might have about that model, that speak in favor of regarding branching as global and instantaneous. Ultimately, although the first three concerns may be addressed by advocates of local branching, the fourth is shown to be more convincing, and speaks in favor of regarding branching as a global, instantaneous change, even if it is a change that is triggered by a local, causal process (decoherence). The last part of paper clarifies the proposed model of branching as global, and explains how it is not in tension with relativity or the many worlds interpretations’ claim to avoid spooky action at a distance. (shrink)

Why does our universe exhibit such a specific and contingent physical architecture? This philosophical unease motivates a rehabilitation of the principle of plenitude—recast here as a thesis within the philosophy of physics rather than an armchair metaphysical postulate. I argue that the bottom layer of physical reality is a version of plenitude: an unbounded space of physically admissible configurations, constrained only by minimal requirements such as probability conservation and dynamical reversibility. Within this framework, I redefine modality philosophically as a measure (...) of the number of mutually exclusive classical configurations simultaneously present in a system's effective description, where higher modality corresponds to greater physical freedom. From the maximally permissive plenitude, our familiar universe arises through modal narrowing, wherein quantum possibilities are progressively curtailed via decoherence, spontaneous symmetry breaking, and the emergence of spacetime structure. This illustrates a modal gradient extending from the high modality quantum domains to the low modality classical domains, which I reason runs inversely to von Neumann entropy. If global modality is conserved, the eternal inflation and Many Worlds multiverses can be reinterpreted as modal consequences of preserving possibility expression by segregating locally incompatible symmetry breaking outcomes and decohering branches. If modality is not globally conserved, the same framework supports a lone-universe reading in which plenitude is confined to the fundamental level alone. (shrink)

This is an introduction to wave function realism for a compendium on the philosophy of quantum mechanics that will be edited and translated into Portuguese by Raoni Arroyo, entitled Compêndio de Filosofia da Física Quântica. This essay presents the history of wave function realism, its various interpretations, the main arguments that are given for the position, and the main objections that have been raised to it.

It is shown that the superposed wave function of a measuring device, in each branch of which there is a definite measurement result, does not correspond to many mutually unobservable but equally real worlds, as the superposed wave function can be observed in our world by protective measurement.

An extended analysis is given of the program, originally suggested by Deutsch, of solving the probability problem in the Everett interpretation by means of decision theory. Deutsch's own proof is discussed, and alternatives are presented which are based upon different decision theories and upon Gleason's Theorem. It is argued that decision theory gives Everettians most or all of what they need from `probability'. Contact is made with Lewis's Principal Principle linking subjective credence with objective chance: an Everettian Principal Principle is (...) formulated, and shown to be at least as defensible as the usual Principle. Some consequences of (Everettian) quantum mechanics for decision theory itself are also discussed. -/- [NB: this this long (70 pages) and occasionally rambling online-only paper has been almost entirely superseded by material in subsequent; if something is not included in them it usually means that I have had second thoughts. I include it for completeness only. (shrink)

Problems in moral philosophy and philosophy of religion can take on new forms in light of contemporary physical theories. Here we discuss how the problem of evil is transformed by the Everettian "Many-Worlds" theory of quantum mechanics. We first present an Everettian version of the problem and contrast it to the problem in single-universe physical theories such as Newtonian mechanics and Bohmian mechanics. We argue that, pace Turner (2016) and Zimmerman (2017), the Everettian problem of evil is no more extreme (...) than the Bohmian one. The existence and multiplicity of (morally) terrible branches in the Everettian multiverse in contrast to the mere possibility of them in the Bohmian universe does not entail there is "more evil" in the former than in the latter. Low probability in the Bohmian case and low branch weight in the Everettian case should modulate how we respond to them in exactly the same way. We suggest that the same applies to the divine decision of creating an Everettian multiverse. For an empirically adequate Everettian quantum mechanics that justifies the Born rule, there is no special problem of evil. In order for there to be a special Everettian problem of evil, the Everettian interpretation must already have been exposed to decisive refutation. In the process, we hope to show how attention to the details of physical and metaphysical theories can and should impact the way we think about problems in moral philosophy and philosophy of religion. (shrink)

Electromagnetism is the paradigm case of a theory that satisfies relativistic locality. This can be proven by demonstrating that, once the theory’s laws are imposed, what is happening within a region fixes what will happen in the contracting light-cone with that region as its base. The Klein-Gordon and Dirac equations meet the same standard. We show that this standard can also be applied to quantum field theory (without collapse), examining two different ways of assigning reduced density matrix states to regions (...) of space. Our preferred method begins from field wave functionals and judges quantum field theory to be local. Another method begins from particle wave functions (states in Fock space) and leads to either non-locality or an inability to assign states to regions, depending on the choice of creation operators. We take this analysis of quantum field theory (without collapse) to show that the many-worlds interpretation of quantum physics is local at the fundamental level. Debates about measurement and the branching of worlds do not challenge this fundamental locality. (shrink)

Interpretation is not the only way to explain a theory's success, form and features, and nor is it the only way to solve problems we see with a theory. This can also be done by giving a reductive explanation of the theory, by reference to a newer, more accurate, and/or more fundamental theory. We are seeking a theory of quantum gravity, a more fundamental theory than both quantum mechanics and general relativity, yet, while this theory is supposed to explain general (...) relativity, it's not typically been thought to be necessary, or able, to explain quantum mechanics---a task instead assigned to interpretation. Here, I question why this is. I also present a new way of assessing the various interpretations of quantum mechanics, in terms of their heuristic and unificatory potential in helping us find a more fundamental theory. (shrink)

This book consists in seventeen chapters devoted to physical, metaphysical, and methodological questions concerning open systems. The chapters in the volume address questions such as: Are (theories of) open systems more fundamental than (theories of) closed systems? How have concepts of open and closed systems have been used throughout the history of physics, and how should we understand their use in contemporary physical theories? Must the universe be a closed system? Must there be a such thing as the universe at (...) all? -/- Contributors: Emily Adlam, Luis C. Barbado, Caslav Brukner, Eddy Keming Chen, Michael E. Cuffaro, Gemma De les Coves, George Ellis, Doreen Fraser, Henrique Gomes, Sean Gryb, William L. Harper, Stephan Hartmann, Molly Kao, Adam Koberinski, James Ladyman, Simon Langenscheidt, Olimpia Lombardi, Jorn Klovfjell Mjelva, Wayne C. Myrvold, Daniele Oriti, Josh Quirke, David Sloan, Karim P. Y. Thebault, Lev Vaidman, David Wallace, and Alastair Wilson. (shrink)

I argue here that progress in understanding the lessons of quantum physics has been hindered by the tendency to cast Niels Bohr as a villain. Building on the work of Favrholdt, Faye, and Howard, I present a more accurate view of Bohr's proposal for the "epistemological lesson" of quantum physics. I then argue that several interpretive programs -- often presented as alternatives to Copenhagen -- are, after substantial conceptual work, arriving at a view that is notably similar to Bohr's. -/- (...) Using Maudlin’s formulation of the measurement problem as a foil, I argue that it relies on a notion of wave-function completeness that is inappropriate for a genuinely indeterministic theory, and one that Bohr consistently rejected. On Bohr's view, quantum theory permits concrete agents, situated within specific experimental contexts, to make objective descriptions, while refusing the demand for a single, context-free "description from nowhere". (shrink)

Three accounts of effective realism (ER) have been advanced to solve three problems for scientific realism: Fraser and Vickers (forthcoming) develop a version of ER about non-relativistic quantum mechanics that they argue is compatible with all the main realist versions (‘interpretations’) of quantum mechanics avoiding the problem of underdetermination among them; Williams (2019) and Fraser (2020b) propose ER about quantum field theory as a response to the problems facing realist interpretations; Robertson and Wilson (forthcoming) propose ER to deal with the (...) dubious ontological status of the entities belonging to superseded theories. This paper argues for the unification of these proposals based on realism about modal structure and the idea of scale relativity of ontology developed by ontic structural realists. This solves problems some or all the accounts of ER face, especially that of making explicit in what way they are realist. Furthermore, we respond to a recent critique that has been raised against the ontic structural realist account of quantum mechanics that we employ. (shrink)

Can multiverse hypotheses ever receive empirical support? Critics argue that multiverse scenarios posit unobservable entities, face severe underdetermination, or fall outside the bounds of science. This chapter challenges that view by offering a naturalistic metaphysical counterpoint to Bayesian approaches, distinguishing fragmented from holistic multiverses. Scientific proposals are almost always holistic: they embed universes within a unifying physical or metaphysical structure that can, in principle, leave empirical signatures inside the universes. I develop a typology of such signatures and show how it (...) applies to leading scenarios from quantum theory, cosmology, and string theory. This framework clarifies why objections such as the `this universe' objection and a newly articulated generalization, the epistemic isolation objection, fail against scientifically motivated multiverses. The upshot is a qualified defence: while fragmented multiverses remain empirically inaccessible, certain holistic multiverses could, in principle, be supported by the same epistemic standards used elsewhere in physics. (shrink)

Why are quantum correlations so puzzling? A standard answer is that they seem to require either nonlocal influences or conspiratorial coincidences. This suggests that by embracing nonlocal influences we can avoid conspiratorial fine-tuning. But that’s not entirely true. Recent work, leveraging the framework of graphical causal models, shows that even with nonlocal influences, a kind of fine-tuning is needed to recover quantum correlations. This fine-tuning arises because the world has to be just so as to disable the use of nonlocal (...) influences to signal, as required by the no-signaling theorem. This places an extra burden on theories that posit nonlocal influences, such as Bohmian mechanics, of explaining why such influences are inaccessible to causal control. I argue that Everettian Quantum Mechanics suffers no such burden. Not only does it not posit nonlocal influences, it operates outside the causal models framework that was presupposed in raising the fine-tuning worry. Specifically, it represents subsystems with density matrices instead of random variables. This allows it to sidestep all the results (including EPR and Bell) that put quantum correlations in tension with causal models. However, this doesn’t mean one must abandon causal reasoning altogether in a quantum world. When decoherence is rampant and there’s no controlled entanglement, Everettian Quantum Mechanics licenses our continued use of standard causal models. When controlled entanglement is present---such as in Bell-type experiments---we can employ recently-proposed quantum causal models that are consistent with Everettian Quantum Mechanics. We never need invoke any kind of non-local influence or any kind of fine-tuning. (shrink)

Local Quantum Mechanics explores the idea that, if one adopts the many worlds interpretation, then one can avoid what Einstein called “spooky action at a distance,” the non- locality that is supposed to be a consequence of quantum entanglement according to other approaches. The essays in this volume aim first to articulate a clear and defensible formulation of the argument from locality for preferring the many worlds interpretation of quantum mechanics over its rivals, and then evaluate it. Contributors from both (...) philosophy and physics question in what sense those in quantum foundations should care about locality. In particular, Local Quantum Mechanics explores whether and how an interpretation of quantum mechanics should avoid action at a distance to maintain consistency with special relativity. Finally, these essays assess whether the many worlds interpretation needs a particular metaphysical or mathematical interpretation to achieve the aim of providing a local interpretation of quantum mechanics. (shrink)

Everettian quantum mechanics tells us that the fundamental dynamics of the universe are deterministic. So what are the `probabilities' that the Born rule describes? One popular answer has been to treat these probabilities as rational credences. A recent alternative, Isaac Wilhelm's centered Everett Interpretation (CEI), takes the Born probabilities to be centered chances: the objective chances that some centered propositions are true. Thus, the CEI challenges the `orthodox assumption’ that fundamental physical laws concern only uncentered facts. I provide three arguments (...) against the centered Everett Interpretation. First, I argue that the CEI is in apparent tension with a significant motivation for adopting Everettian quantum mechanics: rejecting the attribution of special significance to observers or agents in fundamental physics. I suggest the CEI can avoid this tension, but only at the cost of sacrificing its central claim that there are objective chances in an Everettian multiverse. My second argument concerns the CEI’s claim that the centered Born rule is a fundamental physical law. I provide two plausible notions of fundamentality for physical laws, and I argue that the centered Born rule satisfies neither. My final argument is that the CEI’s branch-relative laws cannot explain or constrain an agent’s rational credences in the way that the CEI claims. (shrink)

Abstract: This paper develops and advocates a rule for assigning self-locating credences in quantum branching scenarios, called Indexed Branch-Counting. It is argued that Indexed Branch-Counting can be justified on both accuracy-theoretic grounds and on the grounds that it satisfies a requirement of exchangeability for probability assignments. Since Indexed Branch-Counting diverges from the Born Rule, this poses trouble for Everettian approaches to probability. The paper also addresses a common argument against branch-counting, namely that the rule is incoherent in light of putative (...) vagueness in the number of branches. Finally, the paper addresses a recent proposal from Simon Saunders that aims to reconcile branch-counting with the Born Rule, arguing that the proposal faces challenges. (shrink)

In Everettian quantum mechanics, justifications for the Born rule appeal to self-locating uncertainty or decision theory. Such justifications have focused exclusively on a pure-state Everettian multiverse, represented by a wave function. Recent works in quantum foundations suggest that it is viable to consider a mixed-state Everettian multiverse, represented by a (mixed-state) density matrix. Here, we develop the conceptual foundations for decoherence and branching in a mixed-state multiverse, and extend arguments for the Born rule to this setting. This extended framework provides (...) a unification of 'classical' and 'quantum' probabilities, and additional theoretical benefits, for the Everettian picture. (shrink)

Our experience of the world is temporally asymmetric: coffee spontaneously cools, glasses smash, people age and die, while the reverse processes never occur. However, our best theories of fundamental physics are temporally symmetric, making no distinction between past and future directions. This creates a puzzle: how can our asymmetric experiences emerge from a symmetric foundation? The usual solution is to posit a special initial condition for the universe. This solution, however, amounts to putting in the asymmetry by hand, and one (...) may be left looking for a further explanation of this asymmetric boundary condition. In this paper I explore another possible solution, namely that the asymmetry of experience can be explained anthropically, via an observer selection effect. In particular, I use the formalism of decoherent histories to argue that such an explanation is available in the Everettian, or “many worlds”, interpretation of quantum mechanics. Along the way, we will discover that Everettian quantum mechanics implies the existence of many more worlds than are traditionally recognised. (shrink)

The question of whether chemical structure is reducible to Everettian Quantum Mechanics (EQM) should be of interest to philosophers of chemistry and philosophers of physics alike. Among the three realist interpretations of quantum mechanics, EQM resolves the measurement problem by claiming that measurements (now interpreted as instances of decoherence) have indeterminate outcomes absolutely speaking, but determinate outcomes relative to emergent worlds (Maudlin, 1995). Philosophers who wish to be sensitive to the practice of quantum chemistry (e.g. Scerri, 2016) should be interested (...) in EQM because Franklin and Seifert (2020) claim that resolving the measurement problem also resolves the reducibility of chemical structure, and EQM is the interpretation which involves no mathematical structure beyond that used by practicing scientists. Philosophers interested in the quantum interpretation debate should be interested in the reducibility of chemistry because chemical structure is precisely the kind of determinate three-dimensional fact which EQM should be able to ground if it is to be empirically coherent (see Allori, 2023). The prospects for reduction of chemical structure are poor if it cannot succeed in EQM; the prospects for EQM as a guide to ontology are poor if it cannot reduce chemical structure. -/- Unfortunately for proponents of chemical reduction and EQM, there are three serious barriers to the reduction of chemistry to EQM. The first concern is that quantum treatments of chemical structure rely on the Born-Oppenheimer approximation, which holds nuclear locations fixed while minimizing the energy of the electronic configuration (Hendry, 2022), but this approximation is not licensed by EQM. The Born-Oppenheimer approximation relies on nuclei and molecular orbitals being simultaneously present, but in the three-dimensional ontology following from the Everett interpretation these only emerge at different energy scales and are not simultaneously present (Miller, 2023). The second concern is that the emergent worlds of EQM are supposed to be decoherent at the macro-scale (A. Wilson, 2020), but the recent development of superchemistry suggests that chemical reactions can occur in coherent states (Zhang et al., 2023). The third concern is that emergent worlds are only pragmatic pseudo-processes (Wallace, 2012b), but this means EQM trades realist physics for mere instrumentalism about chemistry. Absent a commitment to chemical realism, reduction is an empty promise. The prospects for reduction of chemical structure to EQM are therefore poor. (shrink)

One motivation for preferring the many worlds interpretation of quantum mechanics over realist rivals, such as collapse and hidden variables theories, is that the interpretation is able to preserve locality (in the sense of no action at a distance) in a way these other theories cannot. The primary goal of this paper is to make this argument for the many worlds interpretation precise, in a way that does not rely on controversial assumptions about the metaphysics of many worlds.

Recent accounts of probability in the many worlds interpretation of quantum mechanics are vulnerable due to their dependence on probability theory per se. For this reason, the many worlds interpretation continues to suffer from the incoherence and quantitative problems. After discussing various theories of probability, I discuss the incoherence problem and argue that self-locating probabilities centered in time-extended worlds can solve it. I then discuss and refute various solutions to the quantitative problem. I argue that the only tenable way to (...) ground these self-locating probabilities is to identify the mathematical form of the Born rule as a generic pattern in a time-extended wavefunction, and to distribute degrees of belief over the region of wavefunction occupied by this pattern. I then outline a time-symmetric version of quantum mechanics - the Fixed Point Formulation - which, interpreted within a time-symmetric Everettian framework, can provide the foundation for a theory of quantum probability. (shrink)

Perspectivist positions have been proposed in physics, notably in order to address the interpretive difficulties of quantum mechanics. Recently, some versions of perspectivism have also been proposed in general philosophy of science to account for the plurality of scientific practice. Both kinds of views share the rejection of what they metaphorically call the “view from nowhere”. However, beyond this superficial similarity, they are very different: while quantum perspectivism entertains a concrete notion of perspective associated with individual agents or systems or (...) concrete contexts, perspectival realism adopts a more abstract notion associated with explanatory aims or conceptual schemes. The aim of this paper is to clarify what is at stake with perspectivism _in general_. The general notion of a perspective, as well as the various attitudes one can entertained towards them, are characterised using the concepts of harmless contradiction and cross-perspectival accessibility. A taxonomy of positions ranging from absolutism to relativism is proposed on this basis. Then the framework is applied to quantum perspectivism and perspectival realism to show its fruitfulness. Finally, I argue that abstract versions of perspectivism are bound to be metaphysically weaker than concrete versions. (shrink)

For scientific realists, quantum mechanics is unsatisfactory because it suffers from the measurement problem. However, there are at least three promising solutions: the pilot-wave theory, the many-worlds theory, and the theory of spontaneous collapse. In this paper I argue that the measurement problem is a false problem for the realist: it was proposed as the last resort to convince the positivists that the theory is not empirically adequate. Instead realists should focus on preserving the reductive explanatory schema that had worked (...) so well in physics before, which requires a theory to have a three-dimensional ontology. Incompleteness argument to this effect have been proposed in the 1920s, but effectively ignored due to the positivistic climate, other unscientific reasons, and theorems which claimed that this project was impossible. When realists re-examined quantum mechanics in the 1950s and later, they happened to focus on the measurement problem. In this paper I speculate on what would have happened if realists instead focused on finding a three-dimensional ontology to complete quantum theory. I show that most paradoxes, puzzles and mysteries connected with quantum mechanics would have never emerge, and that many of what are now considered possible ontological interpretations of the theory would have hardly been taken as viable options. (shrink)

A strongly deterministic theory of physics is one that permits exactly one possible history of the universe. In the words of Penrose (1989), "it is not just a matter of the future being determined by the past; the entire history of the universe is fixed, according to some precise mathematical scheme, for all time.” Such an extraordinary feature may appear unattainable in a world like ours. In this paper, I show that it can be achieved in a simple way and (...) discuss its implications for metaphysics and philosophy of science, including natural properties, free will, explanation, and modality. First, I propose a precise definition of strong determinism. Next, I discuss its philosophical ramifications and a toy example. Finally, I provide a realistic example of a strongly deterministic (and simple) physical theory---the Everettian Wentaculus. A surprising consequence is that whether or not our world is strongly deterministic may be empirically underdetermined. (shrink)

There is a deeply entrenched view in philosophy and physics, the closed systems view, according to which isolated systems are conceived of as fundamental. On this view, when a system is under the influence of its environment this is described in terms of a coupling between it and a separate system which taken together are isolated. We argue against this view, and in favor of the alternative open systems view, for which systems interacting with their environment are conceived of as (...) fundamental, and the environment's influence is represented via the dynamical equations that govern the system's evolution. Taking quantum theories of closed and open systems as our case study, and considering three alternative notions of fundamentality: (i) ontic fundamentality, (ii) epistemic fundamentality, and (iii) explanatory fundamentality, we argue that the open systems view is fundamental, and that this has important implications for the philosophy of physics, the philosophy of science, and for metaphysics. (shrink)

The question of whether Everettian quantum mechanics (EQM) justifies the existence of metaphysical indeterminacy has recently come to the fore. Metaphysical indeterminacy has been argued to emerge from three sources: coherent superpositions, the indefinite number of branches in the quantum multiverse and the nature of these branches. This paper reviews the evidence and concludes that those arguments don’t rely on EQM alone and rest on metaphysical auxiliary assumptions that transcend the physics of EQM. We show how EQM can be ontologically (...) interpreted without positing metaphysical indeterminacy by adopting a deflationary attitude towards branches. Two ways of developing the deflationary view are then proposed: one where branches are eliminated, and another where they are reduced to the universal quantum state. (shrink)

The deterministic nature of EQM (the Everett Interpretation of Quantum Mechanics) seems to be inconsistent with the use of probability in EQM, giving rise to what is known as the “incoherence problem”. In this paper, I explore approaches to solve the incoherence problem of EQM via pre-measurement uncertainty. Previous discussions on the validity of pre-measurement uncertainty have leaned heavily on intricate aspects of the theory of semantics and reference, the embrace of either four-dimensionalism or three-dimensionalism of personhood, or the ontology (...) of EQM. In this paper, I argue that, regardless of the adoption of three-dimensionalism or four-dimensionalism of personhood, the overlapping view or the divergence view of the ontology of EQM, the pre-measurement uncertainty approach to the incoherence problem of EQM can only achive success while contradicting fundamental principles of physicalism. I also use the divergence view of EQM as an example to illustrate my analyses. (shrink)

I defend the Deutsch-Wallace (DW) theorem against a dilemma presented by Dawid and Thébault (2014), and endorsed in part by Read (2018), and Brown and Porath (2020), according to which the theorem is either redundant or in conflict with general frequency-to-chance inferences. I argue that neither horn of the dilemma is well-posed. On the one hand, the DW theorem is not in conflict with general frequency-to-chance inferences on the most natural way of stating the theorem. On the other hand, the (...) DW theorem is crucial for establishing the Born rule as a prediction of Everettian quantum mechanics (EQM), and so cannot be redundant within the theory. (shrink)

In recent years, there has been a growing interest in the possibility of temporal nonlocality, mirroring the spatial nonlocality supposedly evidenced by the Bell correlations. In this context, Glick (2019) has argued that timelike entanglement and temporal nonlocality is demonstrated in delayed-choice entanglement swapping (DCES) experiments, like that of Ma et al. (2012), Megidish et al. (2013) and Hensen et al. (2015). I will argue that a careful analysis of these experiments shows that they in fact display nothing more than (...) “ordinary” spacelike entanglement, and that any purported timelike entanglement is an artefact of selection bias. Regardless any other reason one may have for challenging the assumption of temporal locality, timelike entanglement as evidenced by these experiments is not among them. I conclude by discussing what lessons on the nature of entanglement might be drawn from an examination of DCES experiments. (shrink)

David Wallace's `Dennett's Criterion' plays a key part in establishing realist claims about the existence of a multiverse emerging from the mathematical formalism of quantum physics, even after decoherence is fully appreciated. Although the philosophical preconditions of this criterion are not neutral, they are rarely explicitly addressed conceptually. I tease apart three: (I) a rejection of conceptual bridge laws even in cases of inhomogeneous reduction; (II) a reliance on the pragmatic notion of usefulness to highlight quasi-classical patterns, as seen in (...) a decoherence basis, over others; and (III) a structural realist or `functional realist' point of view that leads to individuating those patterns as real macroscopic objects at the coarse-grained level, as they are seen from the Classical Stance (analogous to Dennett's Intentional Stance). I conclude that the justification of Dennett's Criterion will be intimately tied up with the fate of strong forms of naturalism, and in particular that Wallacian quantum mechanics is a key case study for concretely evaluating his `math-first' structural realism (Wallace 2022). (shrink)