Peter Debye

• 1. Introduction

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  Peter Debye was born in Maastricht in the Netherlands on the 24th of March 1884, and by the time he died on the 2nd of November 1966, his name had been attached to a unit of measurement, a lunar crater, a minor planet, and more than a dozen fundamental concepts in physics and chemistry. That is not a common fate for a scientist. It suggests someone who did not merely solve problems but reshaped the vocabulary of an entire field.

  His teacher Arnold Sommerfeld would later say that his most important discovery was Peter Debye himself. The Nobel Prize in Chemistry followed in 1936. But threaded through decades of scientific achievement was a wartime record that, in 2006, triggered an international controversy. Universities debated removing his name from institutes and prizes. Historians argued over a single letter signed with the words Heil Hitler. Intelligence researchers floated the possibility that Debye was, secretly, a British spy.

  How does a man who helped Jewish colleagues flee Nazi persecution end up the subject of a formal government investigation into whether his name should be erased from institutions that bear it? That question drives everything that follows.

• 2. The Making of a Scientist

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  Debye enrolled at the Aachen University of Technology in 1901 and completed his first degree in electrical engineering in 1905. His earliest published paper, which appeared in 1907, offered a mathematically elegant solution to a problem involving eddy currents. The elegance was not accidental. It was a signal of how his mind worked.

  Arnold Sommerfeld noticed. When Sommerfeld received an appointment at Munich in 1906, he brought Debye along as his assistant. Debye earned his doctorate with a dissertation on radiation pressure in 1908, and two years later he derived the Planck radiation formula by a method simpler than the one Max Planck himself had used. Planck agreed.

  In 1911, Albert Einstein left his professorship at the University of Zurich for Prague, and Debye stepped into that vacancy. From Zurich he moved to Utrecht in 1912, to Gottingen in 1913, to ETH Zurich in 1920, to the University of Leipzig in 1927, and finally to Berlin in 1934. Each move tracked opportunity and ambition across the map of European science. In Berlin he became director of the Kaiser Wilhelm Institute for Physics, succeeding Einstein in the role, overseeing the construction of its facilities.

• 3. Dipoles, Crystals, and Electrolytes

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  In 1912 Debye applied the concept of dipole moment to the charge distribution in asymmetric molecules, developing equations that linked dipole moments to temperature and dielectric constant. The scientific community honored this work by naming the unit of molecular dipole moment the debye. That same year he extended Einstein's theory of specific heat to lower temperatures, incorporating contributions from low-frequency phonons in what became known as the Debye model.

  In 1913 he extended Niels Bohr's theory of atomic structure by introducing elliptical orbits, a development Arnold Sommerfeld was independently pursuing. In 1914-1915, working with Paul Scherrer, he calculated how temperature affects X-ray diffraction patterns in crystalline solids. That calculation produced the Debye-Waller factor, still a standard tool in the analysis of crystal structures.

  Under his supervision at the University of Gottingen, Luise Lange measured the dipolar moment of molecules in solution for the first time in 1918. In 1923, Debye and his assistant Erich Huckel improved on Svante Arrhenius' theory of electrical conductivity in electrolyte solutions. The Debye-Huckel equation was refined further by Lars Onsager in 1926, but it remained a major step in understanding how ions behave in liquids. Also in 1923, Debye worked out a theory to explain the Compton effect, the shift in frequency that occurs when X-rays interact with electrons.

• 4. Cornell and the American Years

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  In 1939 Debye traveled to the United States to deliver the Baker Lectures at Cornell University in Ithaca, New York. He had planned his departure from Germany during a visit to his mother in Maastricht in late 1939. He boarded a ship in Genoa in January 1940 and arrived in New York in early February. Within months he accepted a permanent position at Cornell and in June of that year crossed into Canada and returned on an immigration visa.

  His wife reached the United States by December 1940. His daughter and his sister-in-law did not leave Germany at that point; they stayed in his official Berlin residence and were supported by wages Debye continued to draw through a carefully maintained leave of absence. His son had already been in the United States, having arrived on a planned two-month vacation in summer 1939 and never returning because war broke out.

  At Cornell, Debye chaired the chemistry department for ten years. He became an American citizen in 1946 and retired in 1952, though he continued research until his death. Much of his Cornell-era work focused on using light-scattering techniques, derived from his earlier X-ray scattering methods, to determine the size and molecular weight of polymer molecules. That work grew out of wartime research on synthetic rubber and later extended to proteins and other macromolecules. In April 1966 he suffered a heart attack, and a second one in November of that year proved fatal. He is buried in the Pleasant Grove Cemetery in Ithaca.

• 5. The Letter and the Controversy

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  On the 9th of December 1938, in his capacity as chairman of the Deutsche Physikalische Gesellschaft, Debye wrote to all members of the organization. The letter informed Jewish members, as defined by the Nuremberg laws, that their membership could not continue, and it closed with the words Heil Hitler.

  In January 2006, a Dutch-language book by Sybe Rispens entitled Einstein in the Netherlands brought this letter to wide attention. Rispens argued that documents he had found showed Debye was actively involved in purging Jewish scientists from German institutions. The University of Maastricht removed Debye's name from its science prize. Utrecht University briefly considered renaming the Debye Institute.

  Cornell University's department of Chemistry and Chemical Biology published a report on the 31st of May 2006 stating that it had found no evidence supporting the accusations that Debye was a Nazi sympathizer or collaborator or that he held anti-Semitic views. In May 2006, Dutch Nobel laureate Martinus Veltman, who had written the foreword to the Rispens book, withdrew that foreword and asked the board of Utrecht University to reverse its decision to rename the institute.

  Historians Dieter Hoffmann and Mark Walker wrote that Debye was not a Nazi activist and noted that the DPG was one of the last scientific societies to remove Jewish members, doing so only very reluctantly. The Reich University Teachers League, a National Socialist body, complained publicly that the DPG was still clinging to its Jewish members and had acted only reluctantly under outside pressure. Scholars defending Debye also pointed out that when Debye received the Max Planck Medal from the DPG in 1950, neither Max von Laue, Einstein, Lise Meitner, nor James Franck objected. Einstein, who had long abstained from voting for Max Planck Medal nominees, rejoined the voting process specifically to cast a vote for Debye.

• 6. The NIOD and Terlouw Verdicts

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  The Netherlands Institute for War Documentation, authorized by the Dutch Education Ministry, published its official report in 2007. The NIOD described Rispens' portrayal of Debye as a caricature. Its finding was more layered: Debye was an opportunist who showed loyalty to the dominant political system, first in the Third Reich and then in the United States, while keeping an escape route available in each case. In Germany, that escape route was his retained Dutch nationality. In the United States, it was his attempt to secretly maintain contact with Nazi Germany through the Foreign Office, apparently to preserve the Berlin house and wages his daughter depended on.

  The NIOD report concluded that Debye had not been coerced into writing the Heil Hitler letter and had not simply followed the example of other societies; other societies followed his lead. It also concluded that Debye felt obliged to write the letter and viewed it as a confirmation of circumstances already in effect. The report attributed his conduct to a nineteenth-century positivist view of science as generating blessings for humankind, a worldview that allowed him to compartmentalize political realities from his research.

  In January 2008, the Terlouw Commission, appointed by the universities of Utrecht and Maastricht and headed by Jan Terlouw, advised both institutions to keep Debye's name. The Commission concluded that Debye was not a party member, not an anti-Semite, not a propagandist, and not a collaborator, but also not a resistance hero. It called him pragmatic, flexible, and brilliant, and idealistic about science while only superficially engaged with politics. The Commission characterized the DPG letter as an extraordinarily unpleasant fact and a dark page in his life but stated that, seventy years later, no judgment could be made about what a person in those exceptionally difficult circumstances ought to have done. Utrecht University accepted the recommendation and retained the institute's name. Maastricht University did not, though the Hustinx Foundation, which originated and sponsored the Peter Debye Prize, announced it would continue awarding it. The city of Maastricht, Debye's birthplace, declared it saw no reason to rename Debye Street or Debye Square.

• 7. Spy, Fisherman, Legacy

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  In a 2010 publication, Jurrie Reiding raised the possibility that Debye was an MI6 agent. The basis was Debye's friendship with Paul Rosbaud, a well-documented British spy, whom Debye first met around 1930 when both were working as editors for scientific journals. The two later collaborated in helping Lise Meitner cross the Dutch-German border in 1938-1939 to escape Nazi persecution. Reiding noted that Debye, as a board member of the German Academy for Aviation Research, had access to figures including Hermann Goring, giving him potential intelligence value. Reiding also proposed that Debye's hasty departure on the 16th of January 1940 coincided with the planned, though later delayed, German invasion of the Netherlands set for the following day.

  The historian Philip Ball contested this hypothesis, noting that friendship with Rosbaud was no measure of a person's politics; Rosbaud was broadly connected, and Debye also showed regard for Friedrich Drescher-Kaden, a committed Nazi.

  Apart from the controversies, the man his son remembered was apolitical in private, someone for whom politics were never discussed at home. Debye was described by colleagues as demanding on scientific principles but approachable and generous with students. He was an avid trout fisherman, a gardener who cultivated roses with his wife Mathilde late into his life, a collector of cacti, and someone always known to enjoy a nice cigar. His son Peter P. Debye, born in 1916, became a physicist and collaborated on some of his father's research. His grandson also became a chemist. The minor planet 30852 Debye, originally designated 1991 TR6, still carries his name through the solar system.