Leiden was a brief and consequential stop in J. Robert Oppenheimer’s postdoctoral year in Europe. He arrived hoping to deepen his mathematical training under Paul Ehrenfest, but their difficult encounter—marked by Oppenheimer’s speed and appetite for formal complexity, and Ehrenfest’s insistence on clarity—helped redirect his studies to Zurich.

A Planned Year of Further Study

In 1928, after accepting a joint appointment in physics at the University of California, Berkeley, and the California Institute of Technology, Oppenheimer asked for a fellowship to spend another year in Europe. He believed he still needed further preparation, particularly in mathematics. His plan was to study first with Ehrenfest at Leiden University and then perhaps continue to Copenhagen, where he hoped to meet Niels Bohr.

A Difficult Apprenticeship

The intended apprenticeship did not unfold as Oppenheimer had hoped. Ehrenfest was distracted and in one of his recurrent periods of depression; Oppenheimer later remembered an atmosphere of quiet and gloom and felt that he had wasted the term, in part through his own shortcomings. He recognized that he had been drawn to formalism and complication rather than to the simplicity and clarity Ehrenfest valued. Ehrenfest, for his part, thought Oppenheimer answered too quickly, with errors sometimes concealed by his speed.

The strain was apparent to their colleagues. Max Born described Oppenheimer as exceptionally gifted but lacking mental discipline, outwardly modest yet inwardly arrogant, and admitted that working with him had been unusually trying. Still, Born also called him fine and decent. The tension at Leiden was therefore not a simple rejection: it reflected the distance between Oppenheimer’s evident ability and the habits of thought his mentor believed he needed to develop.

Language, Affection, and Departure

Leiden also became the scene of a striking display of Oppenheimer’s linguistic facility. Only six weeks after arriving, he delivered a lecture in Dutch, a language he had taught himself. His Dutch friends took to calling him Opje, an affectionate contraction of his surname that stayed with him for life. The account also records a brief affair with a young Dutch woman named Suus, or Susan.

Oppenheimer left Leiden sooner than planned. Ehrenfest persuaded him that Wolfgang Pauli in Switzerland, rather than Bohr in Copenhagen, was the more suitable teacher for the work he needed: a professional calculating physicist who would impose greater rigor. In retrospect, Leiden’s importance lies less in a successful period of study than in the redirection it produced. The encounter made the gap between Oppenheimer’s brilliance and his mathematical discipline harder to ignore, and pointed him toward the demanding training he sought next.