Leiden University

Leiden University was the setting for a short but revealing episode in J. Robert Oppenheimer’s scientific formation. He went there in 1928 hoping to deepen his mathematics with Paul Ehrenfest; instead, their difficult intellectual encounter helped redirect him from the Netherlands to Wolfgang Pauli in Switzerland.

Intended Study at Leiden

After accepting a joint appointment at the University of California, Berkeley, and Caltech, Oppenheimer requested a fellowship that would let him return to Europe for another year of postdoctoral work. He believed he still needed further mathematical preparation and chose Leiden University to study under Ehrenfest, a physicist he greatly admired. His initial plan was to spend a term at Leiden and then go to Copenhagen in the hope of meeting Niels Bohr.

A Difficult Intellectual Encounter

The intended period of study began under strained circumstances. Ehrenfest was suffering from a recurrent bout of depression, and Oppenheimer later remembered the atmosphere as quiet and gloomy. He felt he had wasted the term, judging that he was still drawn to formalism and complication rather than to the simplicity and clarity Ehrenfest valued. In Oppenheimer’s later account, he had failed to appreciate how useful it would be to have problems put in clear order.

Ehrenfest, too, found working with the young physicist emotionally draining. Max Born described Oppenheimer as highly gifted but lacking mental discipline, outwardly modest yet inwardly arrogant; Born also acknowledged that Oppenheimer was a fine and decent man. The exchange captures the mixed judgments surrounding Oppenheimer at Leiden: exceptional ability did not ensure that he was easy to teach or work with. Ehrenfest thought Oppenheimer answered too quickly, and that errors could hide behind that speed.

Language, Reputation, and Personal Life

Oppenheimer showed a different kind of quickness six weeks after arriving: he delivered a lecture in Dutch, a language he had taught himself. The spirited performance impressed his Dutch friends, who gave him the affectionate nickname Opje, a 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, reported by physicist Abraham Pais; it does not establish that the affair caused his facility with Dutch.

Departure and Significance

Oppenheimer soon decided to leave Leiden rather than continue toward the Copenhagen plan he had originally imagined. Ehrenfest recommended a different next step, writing to Pauli that Oppenheimer needed to be disciplined into shape. He believed that Pauli, as a professional calculating physicist, would suit the young man better than Bohr, whom he regarded as too broad and imprecise for what Oppenheimer then needed. Oppenheimer later understood the recommendation as an effort to set his scientific habits right. Leiden’s importance in this episode lies less in a long period of study than in the contrast it made visible: Oppenheimer’s formidable fluency and ambition alongside a need for greater discipline and clarity.