A banker’s son in Hamburg
James Franck was born on August 26, 1882, in Hamburg, into a Jewish family. He was the second child and first son of Jacob Franck, a banker and a devout, observant man, and Rebecca Nachum Drucker, who came from a family of rabbis. He had an older sister, Paula, and a younger brother, Robert. The family expected the boy to follow his father into business or the law.
He attended Hamburg’s Wilhelm-Gymnasium, a boys’ school, and in 1901 went to the University of Heidelberg, ostensibly to study law and economics. He found himself drawn far more to the science lectures, and to a fellow student, Max Born, who became his lifelong friend. With Born’s help he persuaded his parents to let him switch to physics and chemistry. Because Heidelberg was weak in the physical sciences, he moved to the University of Berlin, where he heard Max Planck and Emil Warburg lecture.
Berlin and the making of an experimenter
Under Warburg, Franck wrote a doctoral thesis on the mobility of ions in gas discharges and received his doctorate in 1906. After a brief and unhappy assistantship in Frankfurt he returned to Berlin. At a concert there he met Ingrid Josefson, a Swedish pianist; they married in Gothenburg in December 1907 and had two daughters, Dagmar and Elisabeth.
Rather than write a second thesis for the right to lecture, Franck qualified through a stream of publications, 34 articles by 1914, many written with collaborators including Lise Meitner, Robert Pohl and, above all, Gustav Hertz, with whom he wrote nineteen papers. He received his habilitation in 1911. Franck’s gift was for simple, elegant apparatus and for asking exactly the question an experiment could answer.
The Franck–Hertz experiment
In 1914 Franck and Hertz sent beams of electrons through a thin vapour of mercury inside a vacuum tube and measured what happened when the electrons collided with mercury atoms. Slow electrons simply bounced off. But once an electron carried enough energy, it would lose exactly 4.9 electronvolts in a collision, never less, never more, and the mercury atoms then glowed with ultraviolet light whose wavelength corresponded precisely to that 4.9 electronvolts.
The result was direct experimental proof that atoms can absorb energy only in fixed, discrete amounts, the quantum energy levels proposed by Niels Bohr in his model of the atom a year earlier. Remarkably, Franck and Hertz did not at first see the connection; they interpreted their results differently and only in later papers accepted Bohr’s theory. Franck admitted in his Nobel lecture that it was incomprehensible to him in hindsight that they had failed to recognise its significance. In 1926 the two men were awarded the 1925 Nobel Prize in Physics “for their discovery of the laws governing the impact of an electron upon an atom.”
War, and the Göttingen years
When the First World War broke out, Franck volunteered for the German army. He served on the Western Front, rose to lieutenant and was assigned to Fritz Haber’s unit that introduced chlorine gas as a weapon, helping select sites for attacks. He later worked on gas masks at Haber’s Kaiser Wilhelm Institute in Berlin, and received the Iron Cross, Second and then First Class. After the war he headed the physics division of Haber’s institute, where his group studied excited atoms and coined the term “metastable” for atoms lingering in higher-energy states, work later important to the development of the laser.
In 1920 Max Born accepted the chair of theoretical physics at the University of Göttingen on condition that Franck lead experimental physics there. From 1920 to 1933 the two friends made Göttingen one of the world’s great centres of quantum physics. Franck was a celebrated teacher whose laboratory was fiercely competitive to enter; students and collaborators included Patrick Blackett, Edward Condon, J. Robert Oppenheimer and Eugene Rabinowitch. He actively advanced the careers of women physicists, notably Hertha Sponer and Hilde Levi. In 1925 he proposed the mechanism, developed by Condon into the Franck–Condon principle, that governs how molecules absorb and emit light, still a foundation of spectroscopy and quantum chemistry.
Resignation in protest, 1933
In April 1933 the new Nazi government enacted the Law for the Restoration of the Professional Civil Service, which removed Jews and political opponents from public posts, including university chairs. As a decorated front-line veteran, Franck was legally exempt. He resigned anyway, on April 17, 1933, the first German academic to do so in protest at the law. His resignation was reported in newspapers around the world, though no government or university formally protested on behalf of those dismissed.
Franck was a secular Jew. He once said that science was his god and nature his religion, and his daughters grew up without religious instruction, but he was proud of his Jewish heritage and refused to accept a privileged status denied to his colleagues. Before leaving Germany in November 1933 he helped Frederick Lindemann of Oxford find posts abroad for dismissed Jewish scientists. He spent a year at Niels Bohr’s institute in Copenhagen, where his Nobel medal was left in Bohr’s care; when Germany invaded Denmark in 1940, the chemist George de Hevesy dissolved it in acid to hide it from the Nazis, and after the war the gold was recovered and the medal recast.
A new life in America
In 1935 Franck became professor of physics at Johns Hopkins University in Baltimore, and in 1938 he moved to the University of Chicago as professor of physical chemistry, partly because he needed money to help family members still in Germany escape. He succeeded in bringing his daughters and their families to the United States, as well as his elderly mother and an aunt. He became an American citizen in July 1941. His wife Ingrid died in January 1942.
In Copenhagen, under Bohr’s influence, Franck had turned to a new question: how plants use light to turn carbon dioxide and water into sugars. Photosynthesis proved vastly more complicated than collisions between atoms, and it absorbed him for the rest of his life. He led the University of Chicago’s photosynthesis research group until 1956.
Exile was not easy. The Johns Hopkins laboratory was poorly equipped compared with Göttingen’s; a $10,000 grant from the Rockefeller Foundation bought apparatus, but the university had no money for skilled staff. In Chicago, Franck’s first paper was written with another émigré, Edward Teller, and he found a place in his laboratory for his old Berlin collaborator Peter Pringsheim, who had escaped from Belgium after the German invasion. Franck also found a post in Copenhagen for his son-in-law, the physicist Arthur von Hippel, and divided his Nobel Prize money among his children to give them security.
The Manhattan Project and the Franck Report
In 1942 Arthur Compton asked Franck to direct the chemistry division of the Metallurgical Laboratory in Chicago, the Manhattan Project centre building reactors to produce plutonium. Compton hesitated because of Franck’s German origins; Franck, by Compton’s account, welcomed the invitation as a chance to fight not the German people but the Nazis.
As the bomb neared completion, Franck chaired the laboratory’s Committee on Political and Social Problems, whose members included Leo Szilard, Eugene Rabinowitch and Glenn Seaborg. Their report, completed on June 11, 1945, and known ever since as the Franck Report, argued that the bomb should not be used against Japanese cities without warning and proposed instead a demonstration of the new weapon before representatives of the United Nations on a desert or barren island. The committee’s report was largely drafted by Rabinowitch. It warned that a surprise attack would undermine trust and trigger a nuclear arms race, and urged international control of atomic energy. The government’s Interim Committee decided otherwise, and two months later the bombs fell on Hiroshima and Nagasaki. The report remains one of the earliest and most important statements of scientists’ moral responsibility for the weapons they create.
Later years
In June 1946 Franck married Hertha Sponer, his former Göttingen assistant and by then a physics professor at Duke University. He became professor emeritus at Chicago in 1947 but kept working on photosynthesis, receiving honours including the Max Planck Medal and the Rumford Medal. Lise Meitner, who knew him from his youth, observed that his research followed an almost straight line from his earliest studies of ions to his last work on plants: it was always the exchange of energy between atoms and molecules that fascinated him.
Despite everything, Franck kept ties with Germany and with Göttingen. He died suddenly of a heart attack while visiting the city on May 21, 1964, at the age of 81. He is buried in Chicago beside his first wife, Ingrid.
Legacy
The Franck–Hertz experiment is performed by physics students around the world as the classic laboratory demonstration that energy inside atoms is quantised, the same principle at work in fluorescent lamps, neon signs and lasers. The Franck–Condon principle underlies the interpretation of molecular spectra in chemistry, astrophysics and biology.
In 1967 the University of Chicago named its James Franck Institute in his honour, and a lunar crater bears his name. His papers and his recast Nobel medal are held at the University of Chicago Library. But his most enduring legacy may be moral: twice, in 1933 and in 1945, Franck chose to speak plainly against power at personal cost, first as a Jewish scientist refusing an exemption from persecution, then as an American citizen warning his new country about the weapon he had helped to build.
Sources & further reading
James Franck, Wikipedia · James Franck, Biographical, NobelPrize.org · James Franck, Jewish Virtual Library · James Franck, Atomic Heritage Foundation · Researched from the sources above and checked against them. Spotted an error? Tell us.
A life of tikkun olam. This is part of tikkun olam, the Jewish idea of repairing the world, and every one of us can add to it.