A number of arguments purport to show that quantum field theory cannot be given an interpretation in terms of localizable particles. We show, in light of such arguments, that the classical → 0 limit can aid our understanding of the particle content of quantum field theories. In particular, we demonstrate that for the massive Klein-Gordon field, the classical limits of number operators can be understood to encode local information about particles in the corresponding classical field theory.
Keywords quantum field theory • particle interpretation • classical limit • deformation quantization 1 IntroductionRelativistic quantum field theory underlies the modern discipline of particle physics. Practitioners use the theory to conceptualize interactions between particles and make quantitative predictions about scattering experiments. Yet a number of arguments purport to show that various features of our particle concept are incompatible with the constraints of relativistic quantum physics. 1 Building on results of Malament (1996), Halvorson and Clifton (2002) argue that in relativistic quantum theory, particles cannot be localized in spatial regions. The conclusions of such arguments leave interpreters of relativistic quantum field theory with a puzzle. How can an underlying theory that does not allow for localized particles support descriptive and explanatory practices that appear to involve localized particles?Previous investigations have focused on the issue of recovering the phenomenology of particle physics from relativistic quantum field theory. For example, Buchholz (1995) provides a way of recovering scattering theory at asymptotic times. 2 In contrast, the goal of this paper is to make a small contribution toward our understanding of the theoretical role of particles in quantum field theory. We aim to make precise a sense in which a theoretical description of particles emerges from quantum field theory through the behavior of number operators.