Carl Friedrich Freiherr von Weizsäcker (; 28 June 1912 – 28 April 2007) was a German physicist and philosopher. He was the longest-living member of the team which performed nuclear research in Nazi Germany during the Second World War, under Werner Heisenberg's leadership. There is ongoing debate as to whether or not he and the other members of the team actively and willingly pursued the development of a nuclear bomb for Germany during this time.
A member of the prominent Weizsäcker family, he was son of the diplomat Ernst von Weizsäcker, elder brother of the former German President Richard von Weizsäcker, father of the physicist and environmental researcher Ernst Ulrich von Weizsäcker and father-in-law of the former General Secretary of the World Council of Churches Konrad Raiser.
Weizsäcker made important theoretical discoveries regarding energy production in stars from nuclear fusion processes. He also did influential theoretical work on planetary formation in the early Solar System.
In his late career, he focused more on philosophical, ethical and historical issues, and was awarded several international honours for his work in those areas.
Early life and education Born in Kiel, Schleswig-Holstein, he was the grandson of Karl Hugo von Weizsäcker, the Prime Minister of the Kingdom of Württemberg. His grandfather was ennobled in 1897 and raised to the hereditary nobility with the title of Baron (Freiherr) in 1916. As such, the four-year-old Carl Friedrich Weizsäcker became Baron Carl Friedrich von Weizsäcker. Since 1919, noble titles have legally been considered parts of the family name.
Weizsäcker was raised in Stuttgart, Basel and Copenhagen. Between 1929 and 1933 Weizsäcker studied physics, mathematics and astronomy in Berlin, Göttingen and Leipzig, supervised by and working with Heisenberg and Niels Bohr, among others. The supervisor of his doctoral thesis was Friedrich Hund.
Work on nuclear fusion in the Sun Weizsäcker's special interests as a young researcher were the nuclear processes in stars and the binding energy of atomic nuclei. Together with Hans Bethe he found a mechanism or pathway for the cyclic process of fusion in stars (Bethe–Weizsäcker process, published 1937–1939).C.F. von Weizsäcker (1937) "Über Elementumwandlungen im Innern der Sterne. I" (On transformations of elements in the interiors of stars. I), Physikalische Zeitschrift (Physics Journal), vol. 38, pages 176–191.C.F. von Weizsäcker (1938) "Über Elementumwandlungen im Innern der Sterne. II" (On transformations of elements in the interiors of stars. II), Physikalische Zeitschrift, vol. 39, pages 633–646.Hans A. Bethe (1939) "Energy production in stars", Physical Review, vol. 55, pages 434–456. This discovery should not be confused with his earlier (1935) development of the Bethe–Weizsäcker formula, or Semi-Empirical Mass Formula (SEMF), which was a theoretical formula relating the curve of nuclear binding energy, nuclear masses, and certain other nuclear properties.Zur Theorie der Kernmassen (On the theory of nuclear masses); in: Zeitschrift für Physik (Journal of Physics) 96 (1935) pages 431–458
Work on planetary formation In 1938, Weizsäcker developed a theory on the formation of the Solar System, based on considerations regarding the unequal share of lighter and heavier elements in the Sun and the Solar System's terrestrial planets. His views were later generally acknowledged and refined by a large number of other physicists and astronomers. According to the theory, the Sun and its planets evolved from a gas cloud made up of 99% hydrogen and helium, and 1% of heavier elements. Some 10% of the cloud remained around the Sun as an extensive atmosphere during an initial phase, and the 1% of heavier elements within this 10% of the total mass of the cloud would tally with the fraction of roughly 1% that the planets contribute to the mass of the Solar System today.
The theory also helped to explain the empirically observed regular pattern of increase in the diameters of the orbits of the planets of the Solar System, from inward to outward. This result was a natural outcome of the increasing size of "planetary eddies" of gas and dust farther from the centre of the early solar system.
A further implication of his theory was that many stars out in the universe, with characteristics similar to the Sun, would have to be expected to possess planetary systems similar to our own.Heinz Haber: Unser blauer Planet [Our blue planet] (1965; in German); "Die Entstehung der Erde" [The formation of the Earth], Rororo pocket edition at Rowohlt, Reinbek near Hamburg 1967, pp. 19–23
Shortly after the end of the Second World War in Europe, the Russian émigré American physicist George Gamow co-authored an influential paper supporting Weizsäcker's work on planetary formation in the early solar system.
Work on atomic weapons After nuclear fission became known in early 1939 through the work of Otto Hahn and Lise Meitner, Weizsäcker (and by his own estimate, 200 other physicists) quickly recognised that nuclear weapons could potentially be built. He discussed the upsetting implications in February 1939 with philosopher friend Georg Max Friedrich Valentin Picht (1913–1982).
During the Second World War, Weizsäcker joined the German nuclear weapons program, participating in efforts to construct an atomic bomb, while based at the Kaiser Wilhelm Institute in Berlin. As early as August 1939, Albert Einstein warned U.S. President Franklin D. Roosevelt about this research and highlighted that "the son of the German Under-Secretary of State, von Weizsäcker, is attached to the Kaiser-Wilhelm-Institut in Berlin where some of the American work on uranium is now being repeated."
As a protégé of Werner Heisenberg, Weizsäcker was present at a crucial meeting at the Army Ordnance headquarters in Berlin on 17 September 1939, at which the German atomic weapons program was launched.John Cornwell, Hitler's Scientists (Viking 2003), 232 Early in the war — possibly until 1942 — he hoped a successful nuclear weapons project would earn him political influence. In July 1940 he was co-author of a report to the army on the possibility of "energy production" from refined uranium. The report also predicted the possibility of using plutonium for the same purpose including the production of a new type of explosives.Cornwell, Hitler's Scientists, 235 During summer 1942 Weizsäcker filed a patent on a transportable "process to generate energy and neutrons by an explosion... e.g. a bomb". The patent application was found in the 1990s in Moscow.
Historians have been divided as to whether Heisenberg and his team were sincerely trying to construct a nuclear weapon, or whether their failure reflected a desire not to succeed because they did not want the Nazi regime to have such a weapon. This latter view, largely based on postwar interviews with Heisenberg and Weizsäcker, was put forward by Robert Jungk in his 1957 book Brighter Than a Thousand Suns. In a 1957 interview with the German weekly Der Spiegel, Weizsäcker frankly admitted to the scientific ambitions of those years "We wanted to know if chain reactions were possible. No matter what we would end up doing with our knowledge – we wanted to know."Der Spiegel, "...und führe uns nicht in Versuchung: Vom gespaltenen Atom zum gespaltenen Gewissen – Die Geschichte einer menschheitsgefährdenden Waffe (...and do not lead us into temptation: From the split atom to the split conscience – the history of a mankind-endangering weapon)", vol. 11(19) (Mai 8, 1957), p. 52 Only by "divine grace", Weizsäcker said, were they spared the temptation to build the bomb as the German war economy was unable to mobilize the necessary resources.
Original sources about this question were not revealed until 1993, when transcripts of secretly recorded conversations among ten top German physicists, including Heisenberg and Weizsäcker, detained under Operation Epsilon at Farm Hall, near Cambridge in late 1945, were published. In the conversation after the group of detainees had listened to the BBC Radio news on dropping of the atomic bomb on 6 August 1945, Weizsäcker said: "I believe the reason we didn't do it was because all the physicists didn't want to do it, on principle. If we had wanted Germany to win the war we would have succeeded!"
But the "Farm Hall Transcripts" also revealed that Weizsäcker had taken the lead in arguing for an agreement among the scientists that they would claim that they had never wanted to develop a German nuclear weapon. It was this version of events which was given to Jungk as the basis of his book. This story was untrue at least to the extent that the detainees also included scientists actively engaged in eager attempts to build a nuclear bomb, namely Kurt Diebner and Walther Gerlach. Max von Laue later called this agreement "die Lesart" (the Version). Although the memorandum which the scientists drew up was drafted by Heisenberg, von Laue wrote: "The leader in all these discussions was Weizsäcker. I did not hear any mention of any ethical point of view."Cornwell, Hitler's Scientists, 398
Weizsäcker himself stated that Heisenberg, Karl Wirtz and he had a private agreement to study nuclear fission to the fullest possible extent in order to "decide" themselves how to proceed with its technical application. "There was no conspiracy, not even in our small three-man-circle, with certainty not to make the bomb. Just as little, there was no passion to make the bomb..."CFvW August 5, 1990, Letter to Mark Walker in: CFvW, Lieber Freund, lieber Gegner (Dear friend, dear opponent). München (Hanser) 2002, pp 277–283 In a recent report based on additional documents from Russian archives, historian Mark Walker concludes that "in comparison with Diebner [and] Gerlach ... Heisenberg and finally Weizsäcker did obviously not use all power they commanded to provide the National Socialists with nuclear weapons".
However, historian of science and technology Wolf Schäfer has concluded, that Weizsäcker did want to build the bomb for Hitler. In a detailed study about Weizsäcker’s contributions to both Nazi Germany and West Germany, he distinguished between the young and the older (pacifistic) Weizsäcker, that is, the person he was from 1939 to 1945, and the person he became thereafter.Wolf Schäfer: "Plutoniumbombe und zivile Atomkraft: Carl Friedrich von Weizsäckers Beiträge zum Dritten Reich und zur Bundesrepublik." In: Leviathan. Berliner Zeitschrift für Sozialwissenschaft, vol. 41, no. 3, 2013, p. 383–421 (PDF). The following quote from p. 408 is translated from German.
Ivan Supek (one of Heisenberg's students and friends) claimed that Weizsäcker was the main figure behind the famous and controversial Heisenberg–Bohr meeting in Copenhagen in September 1941. Allegedly, he tried to persuade Bohr to mediate for peace between Germany and Great Britain. According to Weizsäcker's own account, he had persuaded Heisenberg to meet Bohr in order to broker an accord of the international nuclear physicist "community" not to build the bomb.see also: Michael Schaaf "Heisenberg wollte Bohr helfen. Ein neues Dokument zum Treffen der beiden Physiker in Kopenhagen 1941." ("Heisenberg wanted to help Bohr. A new document about the meeting of the two scientists in Copenhagen in 1941.") Berliner Zeitung, 5 April 2002 However, according to Bohr's (posthumously published) account of the events, Heisenberg enthusiastically promoted the prospect of German victory and wanted Bohr and his colleagues to assist in the German atomic program.The Bohr Letters: No more uncertainty https://journals.sagepub.com/doi/pdf/10.2968/058003007
Later during the war Weizsäcker worked as a professor at the Reichsuniversität Straßburg. The American capture of his laboratory and papers there in December 1944 revealed to the Western Allies that the Germans had not come close to developing a nuclear weapon.Cornwell, Hitler's Scientists, 335