War on Heisenberg

M.F. Perutz

  • Heisenberg’s War: The Secret History of the German Bomb by Thomas Powers
    Cape, 610 pp, £20.00, April 1993, ISBN 0 224 03641 6
  • Operation Epsilon: The Farm Hall Transcripts introduced by Charles Frank
    Institute of Physics, 515 pp, £14.95, May 1993, ISBN 0 7503 0274 7

Did the German physicists make no atomic bombs during the Second World War because they wouldn’t or because they couldn’t? This is the question which Powers addresses in his extensive study of German atomic research: a question finally answered by the recent publication of the secretly recorded conversations between Heisenberg and the other German atomic physicists interned at Farm Hall, near Huntingdon, in the summer of 1945.

Heisenberg’s leading role among German physicists stems from the revolutionary mathematical theory which he formulated at the age of 24. Heisenberg was born in 1901 in Würzburg, the university town where Röntgen had discovered X-rays a few years earlier and where Heisenberg’s father was professor of Greek philology. Heisenberg shone at school, especially in mathematics and physics. True to the geneticist André Lwoff’s dictum, ‘L’art du chercheur, c’est d’abord de se trouver un bon patron,’ he began his career in physics as the pupil of Germany’s greatest teacher, Arnold Sommerfeld.

In the early Twenties, atomic physics was dominated by Niels Bohr’s model of electrons circling like planets around the sun-like nucleus, their concentric orbits governed by Newtonian mechanics combined with Max Planck’s quantum theory. Bohr’s theory accounted for the spectra of the simplest atom with only one electron, hydrogen, but it ran into difficulties with the spectra of larger atoms and left many other observations unexplained. Heisenberg broke away from Newtonian mechanics and substituted a new kind of ‘quantum mechanics’, based only on observable quantities, which correctly predicted many hitherto uninterpretable phenomena and was immediately acclaimed by most physicists as a tremendous advance. Refinements of his theory led him to the formulation of the Uncertainty Principle, which says that it is impossible to measure simultaneously the position and momentum of an atomic particle. Ironically, it was the philosophical implications of this principle, based largely on its misconceived application to the macroscopic world, that brought Heisenberg fame comparable to Einstein’s. For his discovery he was awarded the Nobel Prize for Physics for 1932. Quantum mechanics still forms the theoretical basis for much of present-day physics and chemistry: without it, for example, there would be no microchips, no computer technology and no electronics industry.

I first saw Heisenberg soon after he received the Nobel Prize, when he lectured in Vienna, where I was a chemistry student. Knowing nothing about him, I expected to see a portly professor: in came a slim young man who looked like one of us students and was quite without pomp. We were enormously impressed. I next encountered him in Cambridge in 1947, when he told my wife and me that he had never wanted to build an atomic bomb for Hitler. Fortunately, he did not say to us, as he did to a physicist in Oxford, a German refugee some of whose family had been murdered by the Nazis: ‘The Nazis should have been left in power longer, then they would have become quite decent.’ We believed Heisenberg until we read Samuel Goudsmit’s book Alsos, which argued that his talk of not wanting to build an atomic bomb was merely an excuse invented after the war to explain Germany’s atomic failure. (Shortly before his death, however, Goudsmit wrote to Heisenberg to apologise for having maligned him.) Powers quotes extensive documentary evidence to refute Goudsmit’s accusation, confirming the conclusion already reached in David Irving’s The German Atomic Bomb, that the German physicists wanted to build a reactor, but not a bomb. Their secretly recorded comments on Hiroshima have now provided further evidence of their reluctance; Heisenberg apparently regarded it as impracticable.

The story of the atomic bomb began in 1932 at the Cavendish Laboratory in Cambridge, with James Chadwick’s discovery of the neutron, a particle the size of the nucleus of the lightest atom, hydrogen, but without its positive charge, thus making it electrically neutral. Chadwick’s discovery led Enrico Fermi in Rome to irradiate many different elements with neutrons in the hope that absorption of neutrons by their atomic nuclei would generate new radioactive elements. On irradiating the heaviest known elements, uranium and thorium, Fermi did indeed find radioactivities not ascribable to any known elements; he attributed some of these to the formation of new elements heavier than uranium. In 1935, Lise Meitner, an Austrian physicist working at the Kaiser Wilhelm Institute in Berlin, persuaded her colleague Otto Hahn, a radiochemist, to join her in a further study of these ‘transuranic’ elements. They collaborated until Austria’s occupation by German troops in March 1938 robbed Lise Meitner of her protection from anti-Jewish persecution and caused her to flee to Sweden. Continuing the work in Berlin, Hahn and his associate Fritz Strassmann found, to their initial disbelief, that one of the elements formed on irradiating uranium could not be separated from barium, an atom only slightly more than half the weight of uranium. Just before Christmas 1938, Hahn wrote a letter to Meitner reporting this puzzling result. She and her nephew Otto Robert Frisch, who later became professor of what the local Cambridge paper called Unclear Physics, realised that the uranium nucleus had split in two with the release of a prodigious amount of energy, and first coined the term ‘nuclear fission’. In April 1939, Hans von Halban, Lev Kowarski and Frédéric Joliot in Paris discovered that fission of one uranium atom, induced by the absorption of one neutron, led to the emission of more than two neutrons.

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