Particles, Fields, and the Measurement of Electron Spin

In this talk, I will compare treatments of the Stern-Gerlach experiment across different physical theories, building up to a novel analysis of electron spin measurement in the context of classical Dirac field theory. Analyzing the experiment in this context is helpful for clarifying the nature of electron spin and the relationship between classical and quantum field theories. In classical Dirac field theory, the electron can be modeled as a wave packet of rotating charge that (depending on the axis of rotation) might simply be deflected in a Stern-Gerlach experiment or might split into two pieces. In this classical context, we can explain a feature of electron spin that is often presented as distinctively quantum (that there are only two locations where electrons hit the detector) but we cannot explain another important feature (that each electron hits the detector at just one location). Modeling the electron as a classical rigid body or point particle, we can explain the second feature but not the first. Of course, both features can be explained within quantum field theory, relativistic quantum mechanics, or non-relativistic quantum mechanics.