(See CV for a complete list of talks and presentations.)
The classical spacetime manifold of general relativity disappears in quantum gravity, with different research programs suggesting a variety of different alternatives in its place. As an illustration of how philosophers might contribute to an interdisciplinary project in quantum gravity, I will give an overview of recent philosophical debates regarding how classical spacetime "emerges." I will criticize some philosophers as granting too much weight to the intuition that a coherent physical theory must describe objects as located in space and time. I will further argue, based in part on historical episodes, that an account of emergence needs to recover the structural features of classical GR responsible for its empirical success. This is more demanding than it might at first appear, although the details of recovery will differ significantly among different approaches to quantum gravity.
Are physicists using machine learning (ML) techniques like a hero consulting an oracle? Although the oracle speaks the truth, that is not sufficient to guide action. The oracle’s cryptic statements cannot be interpreted properly until, tragically, the hero’s fate has already been decided. There seems to be a similar tension between the capacities of some ML techniques and the goals of fundamental physics. As with the oracle, in many cases ML techniques have achieved extremely high accuracy. Yet the workings of these models are often a "black box" – accuracy is accompanied, again, by inscrutability or opacity. An uninterpretable answer to a fundamental problem threatens to be as useless as the oracle’s pronouncements. I will pursue two related lines of argument in response to this pessimistic view. First, in many applications the opacity of an ML technique is not an obstacle to guiding further research. For example, a "black box" technique can discover new physical quantities that are relevant to explaining patterns in the data, perhaps spurring different types of physical models. There are several ways opaque ML results can enrich our picture of a physical system. Second, philosophers have recently proposed that we should think of scientific understanding as a form of mastery – an ability to grasp how a system will respond in a variety of situations. Understanding in this sense requires an ability to extend models to new domains, but it does not require full transparency. ML methods may provide understanding of novel domains, in this sense, without also providing a "solution" (in the form of a simple, easily interpreted model).
Inflationary cosmology has been widely accepted for decades. Yet there are persistent debates about inflation which raise central questions in philosophy of science. Skeptics have often expressed doubt regarding whether inflation is "testable" or "falsifiable," due to the flexibility of inflationary models. This is an instance of a general question in philosophy of science: to what extent does phenomenological success support the claim that a theory gets the physics right? How does one answer a skeptical worry, that the theory "fits the data" because it is flexible? My aim in this talk is reframe this debate, drawing on ideas from George Smith’s historical and philosophical assessment of celestial mechanics. Smith answers the skeptic by looking at the role a theory plays in guiding inquiry. Astronomers "closed the loop" by starting with an initial description of motions; using discrepancies with observations to identify sub-dominant physical details; incorporating these details into a more refined description; and then starting the process over again. Through this process astronomers discovered hundreds of new details about the solar system, based on assuming the theory of gravity, that could be checked independently. Considering this case helps to characterize one challenge facing theories of the early universe: our lack of clarity about the underlying physics driving inflation has blocked pursuit of a similar process of iterative refinement. I will close by considering several different responses to this challenge.
Contemporary cosmology pursues several ambitious aims, including uncovering new aspects of fundamental physics based on their role in the very early universe. The success cosmologists have had in pursuing these aims is particularly striking in light of evidential challenges they face. To overcome these challenges, cosmologists have revisited basic questions about what constitutes an acceptable scientific theory, what explanatory demands a theory should meet, and how to understand theory confirmation in a domain to which we have such limited access. Contemporary cosmology reflects a set of distinctive and interesting answers to these questions – an implicit philosophy of science, so to speak – that has guided research. Articulating and assessing this set of views is the primary goal of a monograph I am co-writing with Jim Weatherall. In this talk, I will elucidate two aspects of the philosophical views guiding research in early universe cosmology, and then assess recent debates about the testability of inflation in light of them.
Several philosophers have advocated an eliminativist position regarding gravitational energy and conservation principles applied to it. We cannot directly characterize the energy carried by the gravitational field with a local quantity analogous to what is used in other field theories: we cannot define a gravitational energy-momentum tensor that assigns local properties to spacetime points and can be integrated over volumes to characterize energy-momentum flows. Because of the equivalence principle, we can always choose a locally freely falling frame, and by so doing locally transform away the gravitational field. The eliminitavists take these features to imply that there is no such thing as "gravitational energy" or integral conservation laws governing it, and that efforts to resurrect such a notion illustrate how misleading it can be to treat general relativity as analogous to other field theories. In this talk I will consider how quasi-local definitions of energy and conservation laws based on them support a response to the eliminativists, and in particular concerns about whether such proposals depend on "background structure" in a problematic sense. Quasi-local energy and conservation laws depend on background structure — we need a way to designate some motions as "freely falling," so that energy-momentum transfers can be measured via departures from these trajectories. But I will argue that these background structures can justifiably be introduced within particular modeling contexts. The challenge regarding gravitational energy then has a different character: namely that there are many conflicting proposals for how to define quasi-local energy, and it is not clear whether they deliver consistent verdicts.
My main aim is to articulate Newton’s distinctive brand of empiricist metaphysics, exemplified in his discussion of the nature of time. On this Newtonian approach, questions about the nature of time become intricately linked to empirical enquiry. Newtonian time is a refinement of some aspects of everyday thinking about time, under the pressure of new problems brought to the fore by advances in the study of motion. A secondary aim is to respond to recent discussions of the relationship between the account of time required for Newton’s empirical project, and for his theological views. Recently Schliesser and Janiak have both argued that some aspects of Newtonian time are justified based on theological considerations. I will argue against this reading of Newton, but my main interest is in what the debate reveals regarding Newton’s approach and implicit assumptions.
Newton’s natural philosophy broke with the mechanical philosophy dominant earlier in the seventeenth century in several ways. This talk will focus on the status of the sensible qualities of objects in Newton’s work. The is a pressing problem because Newton’s innovations in natural philosophy undermine the dominant view of sensible qualities among his contemporaries in (at least) two ways. Mechanical philosophers, from Galileo onwards, held that the constituents of matter (whether atomistic or corpuscular) share the sensible qualities of dry-goods sized objects given in ordinary experience. A preferred set of these, primary qualities such as size, shape, and motion, then provide a sufficient basis for explanatory accounts of our experience. By contrast, Newton’s work directly undercuts the explanatory accounts of experience offered by the mechanical philosophers. The geometrical properties favored by the mechanical philosophers play almost no role within his system of natural philosophy. Second, and more importantly, the Principia treats the experience of bodies in terms of theoretical quantities that are not directly manifested in experience. For example, the mass of an object is a quantity defined within the theoretical framework provided by the laws of motion. One can infer the mass from a body’s motions granted an inertial reference frame and identification of the relevant forces responsible for the body’s motion. I will argue that in light of these two points, the logical character of the contribution sense experience makes to natural philosophy must be different that that assumed by the mechanical philosophers.