Spin: All is not what it seems

Spin is typically thought to be a fundamental property of the electron and other elementary particles. Although it is defined as an internal angular momentum much of our understanding of it is bound up with the mathematics of group theory. This paper traces the development of the concept of spin paying particular attention to the way that quantum mechanics has influenced its interpretation in both theoretical and experimental contexts. The received view is that electron spin was discovered experimentally by Stern and Gerlach in 1921, 5 years prior to its theoretical formulation by Goudsmit and Uhlenbeck. However, neither Goudsmit nor Uhlenbeck, nor any others involved in the debate about spin cited the Stern–Gerlach experiment as corroborating evidence. In fact, Bohr and Pauli were emphatic that the spin of a single electron could not be measured in classical experiments. In recent years experiments designed to refute the Bohr–Pauli thesis and measure electron spin have been carried out. However, a number of ambiguities surround these results—ambiguities that relate not only to the measurements themselves but to the interpretation of the experiments. After discussing these various issues I raise some philosophical questions about the ontological and epistemic status of spin. Because it is a curious hybrid of the mathematical and the physical these questions are relatively complex, and while I do not pretend to have answered them here, the goal of the paper is to uncover and isolate how spin presents problems for traditional realism and to illustrate the power that theories like quantum mechanics have for shaping both philosophical questions and answers.