Lessons from realistic physics for the metaphysics of quantum theory

Tim Maudlin

Eddy Keming Chen

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.

Hernán Bruno Pringe

Matthias Egg

The existence of ontologically different but empirically indistinguishable versions of non-relativistic quantum mechanics is one of the most serious and widely discussed cases of metaphysical underdetermination. I argue that this problem is unlikely to disappear when we turn to the next more fundamental theory, namely relativistic quantum field theory. The problem is merely overshadowed by yet another interpretational controversy, but it persists regardless of the outcome of this latter debate. I further argue that one should not be too optimistic about transferring ontological lessons learned in the context of one physical theory to the context of a more fundamental theory.

Richard Healey

Quantum theory does not describe the world and so contributes little to natural philosophy: it implies neither that a particle can be in two places at once, that a cat can be neither dead nor alive, that there is instantaneous action at a distance, nor that our observations create the world they reveal. Quantum entanglement does not say that the world is radically holist or non-separable, that the world is indeterministic or deterministic, that mind influences matter, or that consciousness plays a special role in the natural world. But the theory does have lessons to teach about how philosophy should approach topics including causation, probability, laws, composition, and ontology that traditionally fall within metaphysics. Here the quantum revolution reinforces the pragmatist lesson that such topics are best approached by asking why agents like us should have developed the concepts we have when physically situated in a world like this.

Juha T. Saatsi