Giulini, Dominico: Electron Spin or "klassisch nicht beschreibbare Art von Zweideutigkeit"

One of the central phenomena which led to the conviction that theoretical decriptions based on mechanical models were fundamentally incorrect at atomic scales was the anomalous Zeeman effect. In 1925 Pauli argued that it must be due to a magnetic moment carried by the outer electron (rather than the inner body of the atom, as thought previously) with an anomalous g-factor of 2. He did not, however, attempt to interpret the magnetic moment and angular momentum as being produced by a rotational motion of the electron. Rather, he spoke of a "eigentuemliche, klassisch nicht beschreibbare Art von Zweideutigkeit". After Goudsmit and Uhlenbeck introduced the idea of a rotating (spinning) electron, Pauli urged to avoid falling back on mechanical analogies. Without going into details he claimed that this idea fails since on had to accept superluminal velocities (presumably of the electron's circumference). In my talk I will discuss whether this statement, which is still much around today, is actually true. This will show interesting connections to the history of Relativity.