Gauge symmetry and the arrow of time: How to count what counts

Abstract

This thesis addresses two major problems in the philosophy of physics. The first is how to identify the minimal physical content of a theory; that is, what features of a theory are truly needed to make predictions, and what can be removed without changing its empirical consequences. The second is the problem of time's arrow: why time seems to have a direction, even though the fundamental laws of physics treat the past and future symmetrically. I show that answering the first question leads to insights about the second. In particular, I argue that the overall size of the Universe is not used to make predictions in cosmology, and so should not count as part of the theory's minimal physical content. Describing the Universe without this feature leads to a striking result: the arrow of time becomes a local phenomenon. Observers like us who see a Universe full of matter clumped together to form structures like stars and planets are statistically much more likely to see increasing clumpiness into the future than into the past. This tendency helps explain our experience of time's direction.