Introduction: The Metaphysics of Quantum Mechanics

Abstract

The study and interpretation of quantum phenomena have generated a lively debate not only among physicists but also among philosophers, from a multiplicity of perspectives. There seems to be broad agreement among physicists that the worldview depicted by quantum mechanics is radically different from the one of classical mechanics. Because of this, many philosophers have identified the transition from classical to quantum mechanics as a prototypical example of a paradigm shift, and identified the ‘discovery’ of quantum mechanics as a scientific revolution, as famously described by Thomas Kuhn, whereby there is no possible way of understanding the quantum phenomena with the allegedly obsolete concepts and metaphysical underpinnings of classical mechanics. In the first part of this special issue, Allori’s and Rosaler’s essays address the question of how irreducibly novel the quantum paradigm is. Related to Rosaler’s essay, Zalamea discusses how the defining features of a physical system can be best captured via mathematical formalisms. These three essays are briefly introduced in what follows. In her essay ‘Quantum Mechanics and Paradigm Shifts’, Valia Allori engages critically with the widespread way of thinking of quantum mechanics as necessitating a paradigm shift in science and philosophy. Allori identifies the origin of this line of thought in Bohr’s (1949) argument that quantum object cannot be described with our ‘old’ concepts, and that all science can do for us is to predict the results of measurements derived in terms of a mathematical object that evolves in time according to an equation typical of a wave, and therefore has been interpreted as a wave, called ‘‘the wave function.’’ Allori argues that the alleged necessity of taking the wave function to describe physical objects is what motivates the alleged necessity of a paradigm shift. But recently it has been acknowledged that we do not have to interpret quantum theories as theories of the wave function. Various proposals have been made, whereby, as in classical theories, the world is described by trajectories of microscopic stuff in space–time that compose macroscopic objects. In this way, Allori argues, we can develop a new but clear explanatory scheme, on the lines of the classical one, to account for the macroscopic world in terms of its microscopic constituents. The particles in Bohmian mechanics, the mass density in GRWm and Sm, and the flashes in GRW are the so called ‘‘primitive ontology’’ of the quantum theory. If we take this route, there is no quantum revolution, or at least not the one that has been advertised so far by many as a necessary transition to a new scheme. Also on the topic of the relationship between quantum and classical theories is Joshua Rosaler’s essay ‘‘‘Formal’’ versus ‘‘Physical’’ Approaches to the Quantum–Classical Correspondence’. Rosaler identifies two approaches used in the literature to address the issue of the relationship between classical and quantum mechanics; he calls the two approaches the ‘‘formal’’ and the ‘‘physical’’ one. The formal approach investigates whether there are abstract correspondences between the mathematical frameworks of quantum and classical mechanics. The physical approach by contrast is more directly concerned with the manner in which structures characteristic of classical and quantum models are exhibited in the behaviour of real physical systems. Correspondingly, Rosaler discusses two different types of reduction of classical physics to quantum mechanics that have been explored in the literature. A formal reduction of one to the other would require the & Anna Marmodoro anna.marmodoro@philosophy.ox.ac.uk