“The Structure of Spacetime: A New Approach to the Spacetime Ontology Debate”
Abstract: I argue that the debate about spacetime ontology is a debate about the fundamental nature of the world. The substantivalist says that spacetime structure is fundamental, inhering in a fundamental physical space. The relationalist says that spacetime structure is instead grounded in—it arises from—the spatiotemporal relations among material objects. Understanding the debate in this way helps to clarify the point of contention between the two views, and to offset worries that the dispute is merely verbal. It also helps to locate the best argument for substantivalism, which I argue is the stronger position. A more general lesson: there is a fact about a world’s fundamental nature, which can differ from its non-fundamental nature. Occam’s razor and related principles apply in the first instance to the fundamental nature of a world.
University of California – San Diego
“The Problem of Space in Quantum Gravity”
Abstract: There is, of course, the well-rehearsed problem of time (and change) in canonical theories of quantum gravity. Within the strictures of these approaches, the problem of time states that if we only accept a few seemingly innocuous assumptions, then we are forced into accepting that, fundamentally, there cannot be time, or at least that there cannot be genuine change. In this talk, I will suggest that in some approaches to quantum gravity, an analogous, though perhaps less severe, problem of space arises from similarly defensible assumptions. ‘Space’ in quantum gravity, it turns out, lacks quite a bit of the structure we normally attribute to it. In particular, I will consider two programs in quantum gravity–loop quantum gravity and causal set theory–in order to make this point evident. I do this by showing how what can reasonably be interpreted as that which corresponds to, or gives rise to, ‘space’ lacks several of what are naturally taken to be essential properties of physical space.
University of California – Irvine
“Against Dogma: On Superluminal Propagation in Classical Electromagnetism”
Abstract: It is experimentally well-established that under some circumstances, classical electromagnetic radiation may propagate with superluminal group velocities and superluminal phase velocities. But it is usually claimed that these situations do not result in a superluminal “signal velocity” in the sense of Sommerfeld and Brillouin—and thus that there is no conflict with relativity theory as usually understood. In this talk, I will draw on recent work by Bob Geroch to argue that classical electromagnetic radiation in a dielectric medium may propagate superluminally, in the sense that the causal cones associated with its field equations may be wider than the metric lightcones for some parameters. These widened light cones are associated with the superluminal velocity (suitably understood) of a discontinuity in a solution, which is precisely the Sommerfeld-Brillouin notion of “signal velocity”. I will conclude by relating my argument to Sommerfeld’s no-go result for superluminal signal velocities, observing that the crucial assumption in Sommerfeld’s theorem concerns the atomic nature of matter and has nothing to do with relativistic constraints. The moral will be that the precise sense in which relativity theory prohibits superluminal propagation is poorly understood.
University of Western Ontario and the Rotman Institute of Philosophy
“On the Existence of Spacetime Structure”
I examine the debate between substantivalists and relationalists about the ontological character of spacetime and conclude it is not well posed. I argue that the so-called Hole Argument does not bear on the debate, because it provides no clear criterion to distinguish the positions. I propose two such precise criteria and construct separate arguments based on each to yield contrary conclusions, one supportive of something like relationalism and the other of something like substantivalism. The lesson is that one must fix an investigative context in order to make such criteria precise, but different investigative contexts yield inconsistent results. I examine questions of existence about spacetime structures other than the spacetime manifold itself to argue that it is more fruitful to focus on pragmatic issues of physicality, a notion that lends itself to several different explications, all of philosophical interest, none privileged a priori over any of the others. I conclude by suggesting an extension of the lessons of my arguments to the broader debate between realists and instrumentalists.
Organizer: Chris Smeenk