Symmetry considerations dominate modern fundamental physics, both in quantum theory and in relativity. Philosophers are now beginning to devote increasing attention to such issues as the significance of gauge symmetry, the role of symmetry breaking, the empirical status of symmetry principles, and so forth. These issues relate directly to traditional problems in the philosophy of science, including the status of the laws of nature, the relationships between mathematics, physical theory, and the world, and the extent to which mathematics dictates physics.

In January 2001 the first philosophy of physics workshop on symmetries in physics was held in Oxford. It became clear from the success of the workshop, the enthusiasm and sense of shared work-in-progress, that the time is right for a collection of papers in philosophy of physics on the subject of symmetry. As the organizers of the workshop, we decided to bring together in one book the current philosophical discussions of symmetry in physics, and to do so in a format that would provide a point of entry into the subject for non-experts, including students and philosophers of science in general. As such, the book is intended to be accessible and of interest to a wide audience of physicists and philosophers. It is appropriat for courses in foundations of physics, philosophy of physics, and advanced courses in philosophy of science. Some of the papers in this collection originated from papers presented at the Oxford workshop, but most have been written expressly for this book.

This book is about the various symmetries at the heart of modern physics. How should we understand them and the different roles that they play? Before embarking on this investigation, a few words of introduction may be helpful. We begin with a brief description of the historical roots and emergence of the concept of symmetry that is at work in modern physics (section 1). Then, in section 2, we mention the different varieties of symmetry that fall under this general umbrella, outlining the ways in which they were introduced into physics. We also distinguish between two different uses of symmetry: symmetry principles versus symmetry arguments. In section 3 we change tack, stepping back from the details of the various symmetries to make some remarks of a general nature concerning the status and significance of symmetries in physics. Finally, in section 4, we outline the structure of the book and the contents of each part.

The meanings of symmetry

Symmetry is an ancient concept. Its history starts with the Greeks, the term συμμετρíα deriving from σύν (with, together) and μέτρоν (measure) and originally indicating a relation of commensurability (such is the meaning codified in Euclid's Elements, for example). But symmetry immediately acquired a further, more general meaning, with commensurability representing a particular case: that of a proportion relation, grounded on (integer) numbers, and with the function of harmonizing the different elements into a unitary whole (Plato, Timaeus, 31c):

Introduction

Emmy Noether's greatest contributions to science were in algebra, but for physicists her name will always be remembered for her paper of 1918 on an invariance problem in the calculus of variations. The most celebrated part of this work, associated with her ‘first theorem’, has to do with the connection between continuous (global) symmetries in Lagrangian dynamics and conservation principles, though the main focus of the paper was the relationship between this and the second part of her paper, where she gives a systematic treatment of the more subtle and general case of continuous local symmetries (symmetries depending on arbitrary functions of the spacetime coordinates).

The connection between global or ‘rigid’ symmetries and conservation principles in classical mechanics was hardly news in 1918. As Kastrup (1987) discusses in his historical review, it had been appreciated in the previous century by Lagrange, Hamilton, Jacobi, and Poincaré, and an anticipation of Noether's first theorem in the special cases of the 10-parameter Lorentz and Galilean groups had been given by Herglotz in 1911 and Engel in 1916, respectively. Noether's own contribution is often praised for its degree of generality, and not without reason. But interestingly it does not cover the cases in which the symmetry transformation preserves the Lagrangian or Lagrangian density only up to a divergence term. It does not therefore cover such cases as the boost symmetry in classical pre-relativistic dynamics, although modern treatments of Noether's first theorem commonly rectify this defect.