Einstein, Ehrenfest, and the quantum measurement problem

A joint paper by Albert Einstein and Paul Ehrenfest, published just weeks after the results of the Stern-Gerlach experiment became known, shows with remarkable clarity and prescience the unsurmountable difficulties that the experiment posed for any classical interpretation. With a focus on the measurement process, rather than on the underlying theoretical alternatives for this experimentum crucis, the authors almost anticipate what would later be recognized as a central conceptual difficulty of quantum mechanics, i.e., the quantum measurement problem.