Quantum metaphysics

What is the reality described by quantum mechanics?

On one prominent view, quantum mechanics describes a branching multiverse ontology. I discuss my research on this theory here. On another view, these multiple branches/worlds never get the opportunity to develop due to a postulated physical process called wave-function collapse. I have a significant interest in the nature of this process, whether it can be invoked to make sense of quantum mechanics, and if it can, what are the philosophical implications.

The most prominent theory of (i) what brings about wave-function collapse and (ii) the mathematical structure of the collapse process, is the so-called Ghirardi–Rimini–Weber (GRW) theory. Regarding (i) collapse is spontaneous in that every particle has a small probability per unit time for spontaneous collapse. And regarding (ii) the spontaneous collapse of a particle is modelled by the particle’s wave-function being multiplied by a Gaussian collapse function that localises the particle’s position. In a recent paper I have challenged the GRW account of (ii):

2015. ‘Four Tails Problems for Dynamical Collapse Theories’, Studies in the History & Philosophy of Modern Physics 49: 10-18. (Published version, Preprint)

Here I argue that because the GRW collapse function fails to remove macroscopic superpositions (leaving behind ‘wave function tails’), the theory is in adequate: ontologically it is committed to a branching multiverse making the collapse process superfluous. Crucially, the collapse function distorts the structure of the tails. In the paper I consider David Wallace’s account of the distortion according to which radiation-levels steadily rise. This account has recently been challenged by Lev Vaidman (‘Quantum Mechanics and Determinism’). Vaidman argues that macroscopic structures in the tails in fact rapidly disintegrate. Vadiman speculates that this may actually resolve the tails problem. I am currently working with Vaidman on these details.

I am also collaborating with David Chalmers on alterative accounts of (i) what brings about wave-function collapse. The GRW account on which collapse is brought about spontaneously is somewhat ad hoc. We are considering accounts on which collapse is triggered by pre-existing properties we call M-properties.

Another issue of interest is the dimensionality of space according to (non-relativistic) quantum mechanics: is it three-dimensional or is it 3N-dimensional where N is the number of elementary particles? In collaboration with Jon Simon (NYU) we are writing papers arguing that the arguments against the purely 3D ontology establish nothing more than the claim that objects typically spatially overlap without interacting. We argue that this is unproblematic and that the 3D ontology should be the default. We aim to develop a perspicuous account of 3D space in worlds described by (non-relativistic) quantum mechanics.

A further issue of interest concerns the traditional distinction between intrinsic and extrinsic (or relational) properties. In collaboration with René van Woudenberg (VU) I am writing a paper that evaluates a number of ‘tests for intrinsicness’, that is, procedures for determining whether a property is intrinsic or not. We argue that the only proposed test that adequately track our pre-theoretical conception of intrinsicness fail to make sense in worlds whose physics is given by quantum mechanics. Our paper is currently under review.

More to come as these projects progress!

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