James Franck

James Franck: The Physicist Who Warned First

On 11 June 1945, a group of scientists working on the American atomic bomb signed a document begging their own government not to use it without warning. "If the United States would be the first to release this new means of indiscriminate destruction upon mankind," they wrote, "she would sacrifice public support throughout the world, precipitate the race of armaments." The report carries the name of the man who chaired the committee — a Nobel laureate who had already resigned a professorship on principle twelve years earlier, and who had spent his career proving that atoms accept energy only in exact amounts.

Hamburg, Heidelberg, Berlin

Franck was born on 26 August 1882 in Hamburg. He attended the Wilhelm Gymnasium, then read chemistry at Heidelberg before switching to physics at Berlin, where he studied under Emil Warburg and Paul Drude. He took his doctorate at Berlin in 1906, worked briefly as an assistant in Frankfurt, and returned to Berlin to work with Heinrich Rubens. By 1911 he was lecturing in physics at Berlin University, rising to associate professor and holding the post until 1918, with an interruption for military service in the First World War.

Electrons and Atoms

Franck's central work concerned what happens when an electron strikes an atom. With Gustav Hertz he devised experiments that measured the exchange precisely — and found that the atom would not take arbitrary amounts of energy. It accepted specific quantities and nothing in between. This was the first direct experimental demonstration of the quantised energy levels that quantum theory had proposed, and it turned an abstract postulate into a measurable laboratory fact. The 1925 Nobel Prize in Physics was awarded jointly to the two men "for their discovery of the laws governing the impact of an electron upon an atom."

After the war Franck headed the Physics Division at the Kaiser Wilhelm Institute under Fritz Haber, continuing to investigate electron kinetics and the behaviour of atoms in gases.

Göttingen

In 1920 he became Professor of Experimental Physics at Göttingen, and for thirteen years he was one half of the partnership that made the town the world capital of theoretical physics, working in close cooperation with Max Born. The students who passed through included J. Robert Oppenheimer. It was at Göttingen that Franck developed the principle now known by his name and Edward Condon's, which explains photochemical dissociation — how molecules break apart when they absorb light.

Resignation, 1933

When the Nazi regime came to power, Franck publicly opposed its racial laws and gave up his position rather than serve under them. It was an unusually early and unusually visible refusal, made by a decorated veteran who could have claimed exemption. He went first to Johns Hopkins University as Speyer Professor, spent a period in Copenhagen, and in 1938 joined the physics faculty at the University of Chicago.

Chicago and the Bomb

During the Second World War Franck directed the Chemistry Division of the Metallurgical Laboratory, the University of Chicago arm of the Manhattan Project. As the weapon neared completion, he chaired the committee that produced the Franck Report of June 1945, signed alongside Leo Szilard, Glenn Seaborg, Donald J. Hughes, J. J. Nickson, Eugene Rabinowitch and J. C. Stearns.

The report argued against an unannounced atomic attack on Japan and proposed a demonstration in an uninhabited area instead. Its reasoning was strategic as much as moral. The scientists insisted the secret could not hold, predicting that within ten years other countries would possess bombs capable of destroying cities, and urged the establishment of "an effective international control of nuclear explosives" before an arms race became unstoppable. They were overruled. They were also, on the timetable, right.

Photosynthesis

After the war Franck led Chicago's Photosynthesis Research Group until 1956, turning the tools of physical chemistry on the question of how plants convert light into chemical energy — in a sense the same problem as the Franck-Condon principle, asked of living matter. Honours accumulated: the Max Planck Medal in 1951, honorary citizenship of Göttingen in 1953, the Rumford Medal in 1955, and election to the Royal Society in 1964. He died on 21 May 1964 while visiting Göttingen, the city he had walked out of thirty-one years before.

Why James Is Called a Genius

The faculty at issue is experimental imagination: the ability to design an apparatus that forces nature to answer a question it has been avoiding. Bohr's atom, with its discrete energy states, was a theoretical proposal that many physicists regarded as a convenient fiction. Franck and Hertz built a device in which that proposal became visible as a repeating pattern in an electrical current — evidence anyone could read off an instrument. Converting a contested abstraction into an unarguable measurement is among the hardest things a physicist can do, and the Nobel committee's citation, "the laws governing the impact of an electron upon an atom," credits precisely that. The Franck-Condon principle shows the same habit of mind applied to molecules and light.

The counter-case deserves airing. The theory that gave his results their meaning was not his; it belonged to Bohr, and the quantum mechanics that followed to Born, Heisenberg and others at the very institution where Franck worked. He was a superb measurer inside a revolution that theorists led. The prize was shared, the principle is co-named, and his substantial postwar photosynthesis research produced nothing of comparable stature. It is also fair to observe that a good deal of Franck's modern reputation rests on conscience rather than physics — the resignation of 1933 and the report of 1945. That is a real distinction, and arguably a rarer one, but it is moral clarity rather than intellectual brilliance, and the two should not be quietly merged.

Legacy

The Franck-Hertz experiment is now a standard undergraduate laboratory exercise, which is the highest compliment physics pays: a result so secure that students are asked to reproduce it. The Franck Report has had the stranger afterlife. Ignored in 1945, it became the founding document of the scientists' movement for arms control and reads today less like a protest than a forecast — the moment when the people who built the weapon told their government exactly what the next fifty years would look like, and were not believed.

Achievements

• Nobel Prize in Physics — 1925
• Affiliated with Johns Hopkins University, University of Göttingen and University of Chicago
• Educated at Heidelberg University, Humboldt-Universität zu Berlin and Wilhelm-Gymnasium
• Worked as physicist, university teacher and chemist