No shield can protect scientific realism from dealing with the quantum measurement problem. One may be able to erect barriers around the observable or classical, preserving a realism about tables, chairs and the like, but there is no safety zone within the quantum realm, the domain of our best physical theory. The upshot is not necessarily that scientific realism is in trouble. That conclusion demands further arguments. The lesson instead may be that scientific realists ought to stake their case on (...) particular interpretations of quantum theory. In any case, the realist can’t ignore the interpretational issues plaguing quantum mechanics. (shrink)

The paper takes up Bell's “Everett theory” and develops it further. The resulting theory is about the system of all particles in the universe, each located in ordinary, 3-dimensional space. This many-particle system as a whole performs random jumps through 3N-dimensional configuration space – hence “Tychistic Bohmian Mechanics”. The distribution of its spontaneous localisations in configuration space is given by the Born Rule probability measure for the universal wavefunction. Contra Bell, the theory is argued to satisfy the minimal desiderata for (...) a Bohmian theory within the Primitive Ontology framework. TBM's formalism is that of ordinary Bohmian Mechanics, without the postulate of continuous particle trajectories and their deterministic dynamics. This “rump formalism” receives, however, a different interpretation. We defend TBM as an empirically adequate and coherent quantum theory. Objections voiced by Bell and Maudlin are rebutted. The “for all practical purposes”-classical, Everettian worlds exist sequentially in TBM. In a temporally coarse-grained sense, they quasi-persist. By contrast, the individual particles themselves cease to persist. (shrink)

Much scholarship has claimed the physics of Emilie du Châtelet’s treatise, Institutions de physique, is Newtonian. I argue against that idea. To do so, I distinguish three strands of meaning for the category ‘Newtonian science,’ and I examine her book against them. I conclude that her physics is not Newtonian in any useful or informative sense. To capture what is specific about it, we need better interpretive categories.

Traditionally Ψ is used to stand in for both the mathematical wavefunction (the representation) and the quantum state (the thing in the world). This elision has been elevated to a metaphysical thesis by advocates of the view known as wavefunction realism. My aim in this paper is to challenge the hegemony of the wavefunction by calling attention to a little-known formulation of quantum theory that does not make use of the wavefunction in representing the quantum state. This approach, called Lagrangian (...) quantum hydrodynamics (LQH), is not an approximation scheme, but rather a full alternative formulation of quantum theory. I argue that a careful consideration of alternative formalisms is an essential part of any realist project that attempts to read the ontology of a theory off of the mathematical formalism. In particular, I show that LQH undercuts the central presumption of wavefunction realism and falsifies the claim that one must represent the many-body quantum state as living in a 3n-dimensional configuration space. I conclude by briefly sketching three different realist approaches one could take toward LQH, and argue that both models of the quantum state should be admitted. When exploring quantum realism, regaining sight of the proverbial forest of quantum representations beyond the Ψ is just the first step. (shrink)

In the science fiction novel Quarantine, Greg Egan imagines a universe where interactions with human observers collapse quantum wavefunctions. Aliens, unable to collapse wavefunctions, tire of being slaughtered by these collapses. In response they erect an impenetrable shield around the solar system, protecting the rest of the universe from human interference and locking humanity into a starless Bubble. When confronting scientific realism and the quantum, many philosophers try to do the theoretical counterpart of this fictional practical strategy. Quantum mechanics is (...) beset with many hard-to-resolve interpretational challenges. Philosophers — appealing to decoherence and coarse-graining — try to put these in a bubble and hope that they can go about their philosophizing as before. My chapter aims to burst this Bubble. (shrink)

Although the electron density can be calculated with the formal resources of quantum mechanics, in physics it does not play the leading role that the quantum state does. In contrast, the concept of electron density is central in quantum chemistry. There is no doubt about how the electron density is computed in terms of the wave function of an atom or molecule. However, when the interpretation of the concept is at stake, there is no general agreement. In this article we (...) will analyze the two main interpretations of the concept of electron density: the Born-style probability density interpretation and the Schrödinger-style charge density interpretation. In particular, we will examine their differences, their relations with quantum mechanics and the consequences that each of them entails from a strictly quantum point of view. (shrink)

A non-relativistic quantum mechanical theory is proposed that describes the universe as a continuum of worlds whose mutual interference gives rise to quantum phenomena. A logical framework is introduced to properly deal with propositions about objects in a multiplicity of worlds. In this logical framework, the continuum of worlds is treated in analogy to the continuum of time points; both “time” and “world” are considered as mutually independent modes of existence. The theory combines elements of Bohmian mechanics and of Everett’s (...) many-worlds interpretation; it has a clear ontology and a set of precisely defined postulates from where the predictions of standard quantum mechanics can be derived. Probability as given by the Born rule emerges as a consequence of insufficient knowledge of observers about which world it is that they live in. The theory describes a continuum of worlds rather than a single world or a discrete set of worlds, so it is similar in spirit to many-worlds interpretations based on Everett’s approach, without being actually reducible to these. In particular, there is no splitting of worlds, which is a typical feature of Everett-type theories. Altogether, the theory explains (1) the subjective occurrence of probabilities, (2) their quantitative value as given by the Born rule, and (3) the apparently random “collapse of the wavefunction” caused by the measurement, while still being an objectively deterministic theory. (shrink)

Why does time reversal involve two operations, a temporal reflection and the operation of complex conjugation? Why is it that time reversal preserves position and reverses momentum and spin? This puzzle of time reversal in quantum mechanics has been with us since Wigner’s first presentation. In this paper, I propose a new solution to this puzzle. First, it is shown that the standard account of time reversal can be derived based on the assumption that the probability current is reversed by (...) the time reversal transformation. Next, this assumption is justified and the meaning of time reversal is clarified by analyzing the relationship between the rates of change and the instantaneous quantities which determine them. Finally, I explain how the new analysis help solve the puzzle of time reversal in quantum mechanics. (shrink)

It’s generally taken to be established that no local hidden-variable theory is possible. That conclusion applies if our world is a _thread_, where a thread is a world where particles follow trajectories, as in Pilot-Wave theory. But if our world is taken to be a _set_ of threads locality can be recovered. Our world can be described by a _many-threads_ theory, as defined by Jeffrey Barrett in the opening quote. Particles don’t follow trajectories because a particle in our world is (...) a set of _elemental_ particles following different trajectories, each in a thread. The “elements” of a superposition are construed as subsets in such a way that a particle in our world only has definite position if all its set-theoretic elements are at corresponding positions in each thread. Wavefunction becomes a 3D density distribution of particles’ subset measures, the stuff of an electron’s “probability cloud”. Current Pilot-Wave theory provides a non-relativistic dynamics for the elemental particles (approximated by Many Interacting Worlds theory). EPR-Bell nonlocality doesn’t apply because the relevant measurement outcomes in the absolute elsewhere of an observer are always in superposition. (shrink)

Within quantum chemistry, the electron clouds that surround nuclei in atoms and molecules are sometimes treated as clouds of probability and sometimes as clouds of charge. These two roles, tracing back to Schrödinger and Born, are in tension with one another but are not incompatible. Schrödinger’s idea that the nucleus of an atom is surrounded by a spread-out electron charge density is supported by a variety of evidence from quantum chemistry, including two methods that are used to determine atomic and (...) molecular structure: the Hartree-Fock method and density functional theory. Taking this evidence as a clue to the foundations of quantum physics, Schrödinger’s electron charge density can be incorporated into many different interpretations of quantum mechanics. (shrink)

Historically, appearance of the quantum theory led to a prevailing view that Nature is indeterministic. The arguments for the indeterminism and proposals for indeterministic and deterministic approaches are reviewed. These include collapse theories, Bohmian Mechanics and the many-worlds interpretation. It is argued that ontic interpretations of the quantum wave function provide simpler and clearer physical explanation and that the many-worlds interpretation is the most attractive since it provides a deterministic and local theory for our physical Universe explaining the illusion of (...) randomness and nonlocality in the world we experience. (shrink)

Primitive ontology is a program which seeks to make explicit the ontological commitments of physical theories in terms of a distribution of matter in ordinary space-time. This program targets wave-function realism, which interprets the high-dimensional configuration space on which wave-functions are defined as our fundamental physical space. Wave-function realism allegedly fails to account for a correspondence between the ontology it postulates and the ‘manifest image’ of the world in which experimental tests of the theory are performed, and therefore the wave-function (...) must be completed with an additional structure which describes what fundamentally exists in ordinary space-time: the ‘primitive ontology’. However primitive ontologies face some difficulties, in particular concerning the ontological status of the wave-function. In this paper, I defend a realist interpretation of the wave-function as describing objects in ordinary space-time, which does not require supplementing the theory with additional structure. The main difference between this proposal and other primitive ontology proposals is that the fundamental constituents of reality are not conceived of as localised objects, but as primitive relations. This interpretation purports to share the advantages of both wave-function realism and primitive ontologies without facing the same difficulties. I argue that the need for an additional structure stems from a commitment to the locality of the fundamental constituents of reality, and that such commitment is unnecessary for recovering the manifest image of the world. (shrink)

In recent years, the non-relativistic quantum dynamics derived from three assumptions; probability current conservation, average energy conservation, and an epistemic momentum uncertainty. Here we show that, these assumptions can be derived from a natural extension of classical statistical mechanics.