In this paper I examine the connection between symmetry and modality from the perspective of `reduction' methods in geometric mechanics. I begin by setting the problem up as a choice between two opposing views: reduction and non-reduction. I then discern four views on the matter in the literature; they are distinguished by their advocation of distinct geometric spaces as representing `reality'. I come down in favour of non-reductive methods.

In the exuberance that followed Einstein’s discoveries, philosophers at one time or another have proposed that his theories support virtually every conceivable moral in ontology. I present an opinionated assessment, designed to avoid this overabundance. We learn from Einstein’s theories of novel entanglements of categories once held distinct: space with time; space and time with matter; and space and time with causality. We do not learn that all is relative, that time in the fourth dimension in any non-trivial sense, that (...) coordinate systems and even geometry are conventional or that spacetime should be reduced ontologically to causal, spatio-temporal or other relations. (shrink)

Tian Yu Cao has written a serious and scholarly book covering a great deal of physics. He ranges from classical relativity theory, both special and general, to relativistic quantum …eld theory, including non-Abelian gauge theory, renormalization theory, and symmetry-breaking, presenting a detailed and very rich picture of the mainstream developments in quantum physics; a remarkable feat. It has, moreover, a philosophical message: according to Cao, the development of these theories is inconsistent with a Kuhnian view of theory change, and supports (...) better a quali…ed realism. (shrink)

A relationist will account for the use of ‘left’ and ‘right’ in terms of relative orientations, and other properties and relations invariant under mirroring. This analysis will apply whenever mirroring is a symmetry, so it certainly applies to classical mechanics; we argue it applies to any physical theory formulated on a manifold: it is in this sense an a priori symmetry. It should apply in particular to parity violating theories in quantum mechanics; mirror symmetry is only broken in such theories (...) as a special symmetry. (shrink)