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The Journal of Philosophy

Our Fundamental Physical Space

Eddy Keming Chen

Published Jan 1, 2017• 28 citations
Publisher

Abstract

The mathematical structure of realist quantum theories has given rise to a debate about how our ordinary 3-dimensional space is related to the 3N-dimensional configuration space on which the wave function is defined. Which of the two spaces is our (more) fundamental physical space? I review the debate between 3N-Fundamentalists and 3D-Fundamentalists and evaluate it based on three criteria. I argue that when we consider which view leads to a deeper understanding of the physical world, especially given the deeper topological explanation from the unordered configurations to the Symmetrization Postulate, we have strong reasons in favor of 3D-Fundamentalism. I conclude that our evidence favors the view that our fundamental physical space in a quantum world is 3-dimensional rather than 3N-dimensional. I outline lines of future research where the evidential balance can be restored or reversed. Finally, I draw lessons from this case study to the debate about theoretical equivalence.

DOI
10.5840/jphil2017114724

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The mathematical structure of realist quantum theories has given rise to a debate about how our ordinary 3-dimensional space is related to the 3N-dimensional configuration space on which the wave function is defined. Which of the two spaces is our (more) fundamental physical space? I review the debate between 3N-Fundamentalists and 3D-Fundamentalists and evaluate it based on three criteria. I argue that when we consider which view leads to a deeper understanding of the physical world, especially given the deeper topological explanation from the unordered configurations to the Symmetrization Postulate, we have strong reasons in favor of 3D-Fundamentalism. I conclude that our evidence favors the view that our fundamental physical space in a quantum world is 3-dimensional rather than 3N-dimensional. I outline lines of future research where the evidential balance can be restored or reversed. Finally, I draw lessons from this case study to the debate about theoretical equivalence.
The mathematical structure of realist quantum theories has given rise to a debate about how our ordinary 3-dimensional space is related to the 3N-dimensional configuration space on which the wave function is defined. Which of the two spaces is our (more) fundamental physical space? I review the debate between 3N-Fundamentalists and 3D-Fundamentalists and evaluate it based on three criteria. I argue that when we consider which view leads to a deeper understanding of the physical world, especially given the deeper topological explanation from the unordered configurations to the Symmetrization Postulate, we have strong reasons in favor of 3D-Fundamentalism. I conclude that our evidence favors the view that our fundamental physical space in a quantum world is 3-dimensional rather than 3N-dimensional. I outline lines of future research where the evidential balance can be restored or reversed. Finally, I draw lessons from this case study to the debate about theoretical equivalence.
The question concerning the dimensionality of our fundamental physical space is arguably one of the most profound cases of underdetermination in non-relativistic quantum mechanics. Those who take the wave function to represent fundamental reality think that the world fundamentally has 3N dimensions, where N is the total number of particles. Against this, various versions of quantum mechanics postulate certain entities in 3-dimensional space as fundamental. An important argument for the latter view suggests that this is not strictly a case of underdetermination of theory by evidence, because a view that does not include some entities in 3-dimensional space fails to explain how there can be empirical evidence for quantum mechanics in the first place. A critical assessment of this argument will clarify how the debate on dimensionality depends on the evaluation of non-empirical virtues which is well-known from other debates on underdetermination.
Abstract The three major interpretations of quantum mechanics—Bohm, GRW, and many worlds—all treat the wave function as a physical entity. But this entity apparently resides in a high-dimensional configuration space, with three coordinates for each particle in the universe. Some conclude on this basis that the three-dimensionality of the world of experience is an illusion. Others respond by adding three-dimensional ontology to the theory or by reinterpreting the wave function either as a law or as an indirect representation of properties in three-dimensional space. It is argued that the latter possibility makes it unnecessary to postulate either additional ontology or a fundamentally high-dimensional world.
In recent years, there has been a controversial debate in the literature regarding the choice among many interpretations of quantum mechanics. Based on certain realist desiderata, this debate has been partly addressed in the context of the choice between two opposite views about the mathematical space in which the fundamental ontology of this theory lives. Indeed, some scholars advocate a worldview in which this ontology lives in the abstract high-dimensional space, whilst others advocate a three-dimensional worldview, according to which the ontology lives in the physical space of our everyday experience. In this contribution, I will critically evaluate the tenability of solving the resulting underdetermination between the three/high-dimensional worldviews in the context of one particular quantum theory: the Bohmian approach. In so doing, I will be following two strategies: firstly, I will undermine the assumption that a set of theoretical or metaphysical virtues blocks the underdetermination problem associated with the mathematical space in which the fundamental Bohmian ontology lives; and secondly, I will propose a structural Bohmian interpretation through the use of dynamical symmetries (symplectic Lie groups), according to which the three/high-dimensional space distinction is just apparent as both spaces are grounded by the same dynamical, group-theoretic structure.
I propose a local, realist interpretation of quantum mechanics based on a physical scalar wave field in ordinary three-dimensional space. Each fundamental particle (electron, photon, proton, etc.) possesses its own wave field. The field is fundamental — it is not emitted by the particle, nor does it draw energy from the particle’s motion. The particle is guided by the phase gradient of its own wave via a Bohmian guidance law. The wave evolves according to a linear Klein-Gordon equation. For a single particle this reproduces the full empirical success of quantum mechanics, including the hydrogen spectrum and the double-slit interference pattern. For multiple particles, each particle retains its own local 3D wave; no configuration space is needed. Entanglement is rejected as a fundamental physical phenomenon: the correlations observed in Bell-type experiments are interpreted as ordinary statistical correlations arising from common past interactions, not as evidence of non-locality or of a non-separable quantum state. The quantum mechanical wave function in configuration space is regarded as a mathematical tool, not a physical field. The theory is local, realist, and fully three-dimensional.
The quantum wave-function is at the heart of quantum mechanics. It is the physical state of a quantum system before a measurement is taken. This wave-function is described mathematically by the formulation of matrix mechanics and represents the evolution of the quantum wave-function. In contemporary philosophy of physics, the wave-function’s nature of space is hotly debated. Contemporaries such as David Albert, who posits an ontology which is based off the mathematical features of the wave-function, claim that the dimensionality of our physical space is proportional to 3N, where N is equal to the number of particles in the universe. Others such as Bradley Monton claim quantum mechanics to be a false theory from the outset due to its incompatibility with general relativity and posit a much more realist property-based-physics ontology. However, these ontologies have their problems. Albert’s ontology takes the mathematics of the quantum wave-function at face value and disregards the distance properties of metric space to misconstrue the phase space nature of the 3N-configuration space as a metric space. Monton, on the other hand, questionably grounds his thesis on the incompatibility of quantum mechanics with general relativity and on common sense rather than on observation and experimentation, as required by the scientific method. In this thesis, I discuss the viewpoints of Albert and Monton in detail and explore the nature of space in the quantum wave-function in an attempt to reach a better understanding of the fundamental nature of space in the quantum wave-function.