Philosophy of Physics Seminar (Thursday - Week 5, MT19)
Thursday 14th November 2019, 16:30
Adam Caulton (Philosophy, Oxford): Is a particle an irreducible representation of the Poincaré group?
Ever since investigations into the group representation theory of spacetime symmetries, chiefly due to Wigner and Bargmann in the 1930s and ‘40s, it has become something of a mantra in particle physics that a particle is an irreducible representation of the Poincaré group (the symmetry group of Minkowski spacetime). Call this ‘Wigner’s identification’. One may ask, in a philosophical spirit, whether Wigner’s identification could serve as something like a real definition (as opposed to a nominal definition) of ‘particle’—at least for the purposes of relativistic quantum field theory. In this talk, I aim to show that, while Wigner’s identification is materially adequate for many purposes—principally scattering theory—it does not provide a serviceable definition. The main problem, or so I shall argue, is that the regime of legitimate particle talk surpasses the constraints put on it by Wigner’s identification. I aim further to show that, at least in the case of particles with mass, a promising rival definition is available. This promising rival emerges from investigations due to Foldy in the 1950s, which I will outline. The broad upshot is that the definition of ‘particle’ may well be the same in both the relativistic and non-relativistic contexts, and draws upon not the Poincaré group (or any other spacetime symmetry group) but rather the familiar Heisenberg relations.
Philosophy of Physics Seminar Convenors for MT19: Tushar Menon, Adam Caulton and Chris Timpson