# Event Detail

Thu Dec 8, 2022 Howison Library 4–6 PM |
Graduate Research Colloquium Patrick Ryan Cosmic Topology, Conventionality, and the Constitutive
A Priori |

In classical general relativity, one models the physical universe with a *spacetime*, i.e., a 4-dimensional manifold (representing events) endowed with a particular metrical structure. Interestingly, it is possible to equip such a spacetime with a wide variety of global, qualitative properties, all of which are compatible with our best (even ideal!) observational evidence. What, then, is the epistemic status of such properties? The question is a pressing one, since some choice of global spacetime properties is required for the cogency of cosmological theorizing.

I consider, in particular, the puzzling epistemic status of the topology of space, viz., whether space is *simply* or *multiply connected*. (Extremely roughly, a multiply connected space has a “hole” or many “holes” in it, while a simply connected space does not.) A natural suggestion, building on work by Poincaré and Reichenbach, is that the topology of space is conventional. Unfortunately, though there is a sense in which calling spatial topology “conventional” is correct, conventionality of any stripe does not fully capture its epistemic status. This is because of its foundational role in cosmology: it would appear that the topology of space makes possible the application of fundamental physical concepts and subsidiary physical laws. Thus, I turn to Friedman’s work on the constitutive *a priori* and argue that our understanding of spatial topology requires the construction of a new epistemic category that incorporates aspects of both conventionalism and the constitutive *a priori*.