# Many-Worlds: Why Is It Not the Consensus?

## Abstract

**:**

## 1. Introduction

## 2. Setting the Stage

#### 2.1. The Measurement Problem

#### 2.2. Constructive Explanations, Principle Theories, Interactions, and Frameworks

## 3. The IT Approach: Standard Quantum Mechanics as a Principle Theory

#### 3.1. Motivation: Empirical Adequacy

#### 3.2. Explanations as Kinematic Top-Down Systematizations

## 4. Primitive Ontology: The Pilot-Wave Theory as a Constructive Quantum Theory

#### 4.1. Motivation: Constructive Explanation

“…in the classical framework we have a clear and straightforward scheme of explanation: given the primitive ontology at the microscopic level, one can employ standard methods to determine the properties of familiar macroscopic objects. Since in classical theories this is possible because the theories have a primitive ontology, for any other fundamental physical theory with a primitive ontology, like the quantum theories we just discussed, we could employ an explanatory scheme derived along the lines of the classical one”.[16]

“Should one, […] completely abandon any attempt to explain the Lorentz contraction atomistically? We think the answer to this question should be No. The contraction of a measuring rod is not an elementary but a very complicated process. It would not take place except for the covariance with respect to the Lorentz group of the basic equations of the electron theory, as well as of those laws, as yet unknown to us, which determine the cohesion of the electron itself. We can only postulate that this is so, knowing that then the theory will be capable of explaining atomistically the behaviour of moving rods and clocks”.[17]

“In a theory of principle, one starts from some general, well-confirmed empirical regularities that are raised to the status of postulates (e.g., the impossibility of perpetual motion of the first and the second kind, which became the first and second laws of thermodynamics). With such a theory, one explains the phenomena by showing that they necessarily occur in a world in accordance with the postulates. Whereas theories of principle are about the phenomena, constructive theories aim to get at the underlying reality. In a constructive theory one proposes a (set of) model(s) for some part of physical reality (e.g., the kinetic theory modeling a gas as a swarm of tiny billiard balls bouncing around in a box). One explains the phenomena by showing that the theory provides a model that gives an empirically adequate description of the salient features of reality”.

#### 4.2. Explanations as Dynamical Bottom-Up Constructions

## 5. Everettians: Unitary Quantum Mechanics as a Framework

#### 5.1. Motivation: Practice in Physics

“This, in short, is the Everett interpretation. It consists of two very different parts: a contingent physical postulate, that the state of the Universe is faithfully represented by a unitarily evolving quantum state; and an a priori claim about that quantum state, that if it is interpreted realistically it must be understood as describing a multiplicity of approximately classical, approximately non-interacting regions which look very much like the ‘classical world’”.[47]

#### 5.2. Explanations as Dynamically Emerging Structures

## 6. Wavefunction Realism: Quantum Theories as Interaction Theories

#### 6.1. Motivation: Locality and Separability

#### 6.2. Explanations as Non-Constructive Dynamical Hybrids

## 7. The Disagreement in a Table

## Funding

## Data Availability Statement

## Acknowledgments

## Conflicts of Interest

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View | Motivation | Ontology | Explanation | Theory |
---|---|---|---|---|

IT approach QBism Pragmatists | Empirical adequacy | Macroscopic Ontology | Principle explanation: Macroscopic phenomena are explained in terms of principles constraining the phenomena. | Standard QM (unitary dynamics and collapse rule) |

Primitive Ontology | Compositionality and dynamical reduction | Spatiotemporal & microscopic ontology | Constructive, dynamical explanation: macroscopic objects are composed of the fundamental microscopic entities, and macroscopic behavior is explained in terms of the microscopic dynamics. | Pilot-wave theory (GRW-x) x = some spatiotemporal microscopic ontology |

Spacetime State Realism | Coherence with physical practice | Spatiotemporal ontology | Structuralist explanation: macroscopic phenomena are accounted for in terms of structures dynamically emerge from a spatiotemporal fundamental ontology. Functionalist explanation: objects are what they are because of what they do. | Many-worlds |

Wavefunction Realism | Keep locality and separability | Local & separable (not necessarily in spacetime) | Non-constructive/ dynamical explanation: principles are used to recover the nonfundamental spatiotemporal microscopic ontology from the fundamental high dimensional space, then constructive explanation is used to account for macroscopic objects and their behavior. | Many-worlds (bare GRW) |

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**MDPI and ACS Style**

Allori, V.
Many-Worlds: Why Is It Not the Consensus? *Quantum Rep.* **2023**, *5*, 80-101.
https://doi.org/10.3390/quantum5010007

**AMA Style**

Allori V.
Many-Worlds: Why Is It Not the Consensus? *Quantum Reports*. 2023; 5(1):80-101.
https://doi.org/10.3390/quantum5010007

**Chicago/Turabian Style**

Allori, Valia.
2023. "Many-Worlds: Why Is It Not the Consensus?" *Quantum Reports* 5, no. 1: 80-101.
https://doi.org/10.3390/quantum5010007