Curie' Principle and spontaneous symmetry breaking s John Earman Department of History and Philosophy of Science, University of Pittsburgh, Pittsburgh, PA, USA Abstract In 1894 Pierre Curie announced what has come to be known as Curie' Principle: the asymmetry of e¤ects must be found in their causes. s In the same publication Curie discussed a key feature of what later came to be known as spontaneous symmetry breaking: the phenomena generally do not exhibit the symmetries of the laws that govern them. Philosophers have long been interested in the meaning and status of Curie' Principle. s Only comparatively recently have they begun to delve into the mysteries of spontaneous symmetry breaking. The present paper aims to advance the discussion of both of these twin topics by tracing their interaction in classical physics, ordinary quantum mechanics, and quantum …eld theory. The features of spontaneous symmetry that are peculiar to quantum …eld theory have received scant attention in the philosophical literature. These features are highlighted here, along with a explanation of why Curie' Principle, though s valid in quantum …eld theory, is nearly vacuous in that context. 1. Introduction The statement of what is now called Curie' Principle was announced in 1894 s by Pierre Curie: (CP) When certain e¤ects show a certain asymmetry, this asymmetry must be found in the causes which gave rise to it (Curie 1894, p. 401).1 This principle is vague enough to allow some commentators to see in it profound truth while others see only falsity (compare Chalmers 1970; Radicati 1987; van Fraassen 1991, pp. 23-24; and Ismael 1997). I will give a formulation of (CP) that makes it virtually analytic. Nevertheless, I claim, the principle so understood is useful in guiding the search for explanations of observed asymmetries. Some commentators credit Curie (1894) with recognizing the importance of the phenomena that lie at the the roots of the concept of spontaneous symmetry breaking (see, for example, Cao 1997, p. 281). If this attribution 1

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