Laura Ruetsche

Investigating the foundations of quantum field theories, I have suggested that theory specification has a pragmatic dimension: strategies for equipping physical theories with content, if sensibly pursued, eventuate in contents indexed not only (or not just) to the way the world is, but also to our aims in using our theories and the circumstances we use them in. Here I assess the “fundamentalist” response, that the apparent appeal of pragmatizing content rests on transient artifacts of the present incomplete state of physics. Fundamental physics, the response continues, can be properly understood if only it’s understood as representing the way the world is. Insofar as none of the physics we have now is genuinely fundamental, we can’t directly assess this fundamentalist response. Undertaking an indirect assessment, I develop two reasons to predict that future scientific theories, including theories of “fundamental physics,” will continue to be best understood as possessing pragmatized content.

When quantum theory is applied to systems with an infinite number of degrees of freedom, wholly new interpretative issues arise. This is due to the fact that such theories have physically inequivalent instantiations. Here the interpretative issues of quantum field theory and of the quantum theory of systems at the thermodynamic limit are discussed. In quantum mechanics the so-called commutation operators on the basic descriptive operators fix the descriptive realm of the theory. This chapter addresses the question of how we are to understand this theory that seems to allow for distinct, incompatible ways of representing the basic features of the world.

Some philosophers of science suggest that a narrow calculus of rationality—the axioms of probability calculus and the rule of conditionalization—suffices to characterize the epistemic aspect of science, including the phenomena of scientific knowledge and empirical justification. But what if a rationality constricted to a narrow calculus is a rationality inhibited in its pursuit of epistemic aims key to science? This chapter uses virtue as Aristotle understands it to develop a broader picture of rationality, a picture that likens some varieties of rationality to second-nature capacities. It discusses not only the epistemic aims that might be advanced by the exercise of epistemic second natures in the sciences but also the social conditions promoting this advancement.

Subject to techniques of perturbative renormalization, the Standard Model makes empirical predictions that are stupendously successful. But also deeply mysterious. Not every quantum field theory (qft) is renormalizable. Indeed, most aren’t. The mystery is: Why should we be so lucky, that we live in a world governed by a renormalizable qft? I explicate this Renormalizability Puzzle, and explain why Renormalization Group (RG) approaches are widely thought to resolve it. Looking under the hood of the RG resolution, I identify a load-bearing element that might not be adequate to the explanatory burden the RG resolution places upon it.

It's a mistake to afflict upon on our best theories a single, uniform interpretation meant to apply in all circumstance. It's a mistake because it impedes the capacity of those theories to function as science. To refrain from the mistake is to adopt the locavore hypothesis: the same theory can merit different interpretations in different circumstances. Using quantum mechanics as an example, I argue for the locavore hypothesis, and examine its consequences not only for the scientific realism debate but also for our notion of scientific understanding.

The mathematical centerpiece of many physical theories is a Lagrangian. So let’s imagine that there’s some Lagrangian we trust. Should that induce us to endorse an ontology? If so, what ontology, and how is it related to our trustworthy Lagrangian? I’ll examine these questions in the context of quantum field theoretic Lagrangians. When these Lagrangians are understood as “merely effective,” a variety of approximations figure in the physics they frame. So do distinctive grounds for trusting those Lagrangians, grounds recent literature has adduced in support of a novel variety of scientific realism known as _Effective Realism_. This essay attempts to undermine those grounds, and to do so without presupposing extensive prior knowledge of quantum field theories.

This article deals with the methodologies used in philosophy of physics. It begins by considering some methodological inclinations at large in the community of philosophers of physics in order to convey some sense of the plethora of methodologies, self-conscious and otherwise, to be found at the interfaces of philosophy, mathematics, and physics. It then describes and defends a methodological inclination to understand and pursue the project of interpreting physical theories in a way that runs counter to a methodological disposition prevalent among philosophers of physics. This disposition is toward “Naturalism”: the view that the only respectable metaphysics is the metaphysics that makes the best sense of our best physics.

Why are quantum probabilities encoded in measures corresponding to wave functions, rather than by a more general class of measures? Call this question WHY BORN? I argue that orthodox quantum mechanics has a compelling answer to WHY BORN? but Bohmian mechanics might not. I trace Bohmian difficulties with WHY BORN? to its antistructuralism, its denial of physical significance to the algebraic structure of quantum observables, and I propose other cases where Bohmian antistructuralism might have an explanatory cost.

Effective Realism marshals ideologies and technologies of our best current physics— the interacting quantum field theories making up the Standard Model—to articulate a selective realism resistant to skeptical affronts such as the Pessimistic Metainduction. Chapter 15 attempts an empiricist re-appropriation of the putatively realist commitments Effective Realists stake out. It also argues that resisting empiricist reappropriation entangles selective realists in something alarmingly similar to the very project of ‘Standard Interpretation’ they regard as misguided.

Philosophers of quantum mechanics have generally addressed exceedingly simple systems. Laura Ruetsche offers a much-needed study of the interpretation of more complicated systems, and an underexplored family of physical theories, such as quantum field theory and quantum statistical mechanics, showing why they repay philosophical attention. She guides those familiar with the philosophy of ordinary QM into the philosophy of 'QM infinity', by presenting accessible introductions to relevant technical notions and the foundational questions they frame--and then develops and defends answers to some of those questions. Finally, Ruetsche highlights ties between the foundational investigation of QM infinity and philosophy more broadly construed, in particular by using the interpretive problems discussed to motivate new ways to think about the nature of physical possibility and the problem of scientific realism.

One realist response to the pessimistic meta-induction distinguishes idle theoretical wheels from aspects of successful theories we can expect to persist and espouses realism about the latter. Implementing the response requires a strategy for identifying the distinguished aspects. The strategy I will call renormalization group realism has the virtue of directly engaging the gears of our best current physics—perturbative quantum field theories. I argue that the strategy, rather than disarming the skeptical possibilities evinced by the pessimistic meta-induction, forces them to retreat a level. I also suggest that those skeptical possibilities continue to carry force.

We discuss the intertwined topics of Fulling non‐uniqueness and the Unruh effect. The Fulling quantization, which is in some sense the natural one for an observer uniformly accelerated through Minkowski spacetime to adopt, is often heralded as a quantization of the Klein‐Gordon field which is both physically relevant and unitarily inequivalent to the standard Minkowski quantization. We argue that the Fulling and Minkowski quantizations do not constitute a satisfactory example of physically relevant, unitarily inequivalent quantizations, and indicate what it would take to settle the open question of whether a satisfactory example exists. A popular gloss on the Unruh effect has it that an observer uniformly accelerated through the Minkowski vacuum experiences a thermal flux of Rindler quanta. Taking the Unruh effect, so glossed, to establish that the notion of particle must be relativized to a reference frame, some would use it to demote the particle concept from fundamental status. We explain why technical results do not support the popular gloss and why the attempted demotion of the particle concept is both unsuccessful and unnecessary. Fulling non‐uniqueness and the Unruh effect merit attention despite these negative verdicts because they provide excellent vehicles for illustrating key concepts of quantum field theory and for probing foundational issues of considerable philosophical interest.

Philosophical accounts of quantum theory commonly suppose that the observables of a quantum system form a Type-I factor von Neumann algebra. Such algebras always have atoms, which are minimal projection operators in the case of quantum mechanics. However, relativistic quantum field theory and the thermodynamic limit of quantum statistical mechanics make extensive use of von Neumann algebras of more general types. This chapter addresses the question whether interpretations of quantum probability devised in the usual manner continue to apply in the more general setting. Features of non-type I factor von Neumann algebras are cataloged. It is shown that these novel features do not cause the familiar formalism of quantum probability to falter, since Gleason's Theorem and the Lüders Rule can be generalized. However, the features render the problem of the interpretation of quantum probability more intricate.

On Itamar Pitowsky’s subjective interpretation of quantum mechanics, “the Hilbert space formalism of quantum mechanics [QM] is just a new kind of probability theory”, one whose probabilities correspond to odds rational agents would accept on the outcomes of gambles concerning quantum event structures. Our aim here is to ask whether Pitowsky’s approach can be extended from its original context, of quantum theories for systems with an finite number of degrees of freedom, to systems with an infinite number of degrees of freedom, such as quantum field theory and quantum statistical mechanics in the thermodynamic limit. An impediment to generalization is that Pitowsky adopts the framework of event structures encoded by atomic algebras, whereas the algebras typical of QM for infinitely many degrees of freedom are usually non-atomic. We describe challenges to Pitowsky’s approach deriving from this impediment, and sketch and assess strategies Pitowsky might use to meet those challenges. Although we offer no final verdict about the eventual success of those strategies, a testament to the worth of Pitowsky’s approach is that attempting to extend it engages us in provocative foundational issues.

Stephen Hawking has argued that universes containing evaporating black holes can evolve from pure initial states to mixed final ones. Such evolution is non-unitary and so contravenes fundamental quantum principles on which Hawking's analysis was based. It disables the retrodiction of the universe's initial state from its final one, and portends the time-asymmetry of quantum gravity. Small wonder that Hawking's paradox has met with considerable resistance. Here we use a simple result for C*-algebras to offer an argument for pure-to-mixed state evolution in black hole evaporation, and review responses to the Hawking paradox with respect to how effectively they rebut this argument.

A number of arguments have been given to show that the modal interpretation of ordinary nonrelativistic quantum mechanics cannot be consistently extended to the relativistic setting. We find these arguments inconclusive. However, there is a prima facie reason to think that a tension exists between the modal interpretation and relativistic invariance; namely, the best candidate for a modal interpretation adapted to relativistic quantum field theory, a prescription due to Rob Clifton, comes out trivial when applied to a number of systems of physical interest. However, it is far from clear whether this difficulty for the modal interpretation is traceable to relativistic invariance per se or to the infinite number of degrees of freedom involved. In any case, the proponents of the modal interpretation have work to do.

An “intrinsically mixed” state is a mixed state of a system that is ‘orthogonal’ to every pure state of that system. Although the presence of such states in the quantum theories of infinite systems is well known to those who work with such theories, intrinsically mixed states are virtually unheralded in the philosophical literature. Rob Clifton was thoroughly familiar with intrinsically mixed states. I aim here to introduce them to a wider audience—and to encourage that audience to cultivate their acquaintance by suggesting that intrinsically mixed states undermine assumptions framing standard discussions of the quantum measurement problem

To most laypersons and scientists, science and progress appear to go hand in hand, yet philosophers and historians of science have long questioned the inevitability of this pairing. As we take leave of a century acclaimed for scientific advances and progress, Science at Century's End, the eighth volume of the Pittsburgh-Konstanz Series in the Philosophy and History of Science, takes the reader to the heart of this important matter. Subtitled Philosophical Questions on the Progress and Limits of Science, this timely volume contains twenty penetrating essays by prominent philosophers and historians who explore and debate the limits of scientific inquiry and their presumed consequences for science in the 21st century.

Van Fraassen's 1991 modal interpretation of Quantum Mechanics offers accounts of measurement and state preparation. I argue that both accounts overlook a class of interactions I call General Unitary Measurements, or GUMs. Ironically, GUMs are significant for van Fraassen's account of measurement because they challenge it, and significant for his account of preparation because they simplify it. Van Fraassen's oversight prompts a question about modal interpretations: developed to account for ideal measurement outcomes, can they consistently account as well for the whole horizon of laboratory practices by which we investigate QM?

It has been suggested that the Modal Interpretation of Quantum Mechanics (QM) is "incomplete" if it lacks a dynamics for possessed values. I argue that this is only one of two possible attitudes one might adopt toward a Modal Interpretation without dynamics. According to the other attitude, such an interpretation is a complete interpretation of QM as standardly formulated, an interpretation whose innovation is to attempt to make sense of the quantum realm without the expedient of novel physics. Then I explain why this attitude, though available, is unattractive. Without dynamics, the Modal Interpretation vanquishes the measurement problem only, it seems, to succumb to the problem of state preparation. On this view, the Modal Interpretation needs dynamics not to be an interpretation at all, but to be an adequate one. I review reasons to suspect that the dynamics which would best suit the Modal Interpretation--a dynamics equivalent to a set of two time transition probabilities of the sort used to solve the preparation problem--is not a dynamics the interpretation can have. I close with a brief discussion of versions of the Modal Interpretation that may not succumb to the considerations presented here.