Week 4 (March 4)

From the Discovery of Nuclear Fission to the Atomic Bomb (1938 - 1945)

Atomic bomb – note that the term is an unfortunate misnomer since it really has to do with fission of the nucleus rather than of the atom. 

Points to ponder

• Was it inevitable that the mechanism of nuclear fission would be discovered?

• Once discovered, was it inevitable that it would be used for a massive weapon? Could the scientists have withheld that knowledge?

• The Allies were fearful that Heisenberg's brilliance as physicist would enable Germany to develop an atomic bomb. Was this fear misplaced? 

• By 1944, the U.S. knew that Germany was not developing an atomic bomb. Should the Manhattan Project have been suspended or canceled at that point?

• The new film "Oppenheimer" gave a (rather lengthy) portrayal of J. Robert Oppenheimer, some of his physics and his essential role in the Manhattan Project.  In what ways do Heisenberg and Oppenheimer appear to be different or alike?   

[Personal opinion: For a number of reasons, I prefer the 1981 documentary "The Day After Trinity"] 

• Heisenberg traveled and gave lectures in other countries, including occupied ones, as a kind of scientific and cultural ambassador. At times he was heard to say that wanted Germany to win the war, and that the Soviets were the greater threat.  Was this love of country, naîvete, or insensitivity on his part?

The roadmap to the discovery of nuclear fission:

• The nuclear atom (Ernest Rutherford, 1909)

• Discovery of the neutron (James Chadwick, 1932)

• Use of neutrons as projectiles on heavy elements like uranium (Enrico Fermi, 1934)

• Experiments with neutrons on uranium indicating the presence of barium  (Otto Hahn and Fritz Strassmann, 1938)

• Explanation of barium as a product of nuclear fission, with generation of energy (Meitner and Frisch, 1939)

• Research and theoretical developments continued, with publication, during 1939. 

The Discovery of Nuclear Fission and After

 Disintegration of Uranium by Neutrons: a New Type of Nuclear Reaction  (for the pdf, click here) (3 pages)

The ground-breaking Feb. 1939 paper by Lise Meitner and Otto Frisch explaining the result of the Hahn-Strassmann experiment of neutrons on uranium as nuclear fission. 

It included an estimate of the energy released in this  fission as about 200 million electron volts, compared to chemical reactions where the energy is typically of the order of electron volts. 

British reports on the feasibility of developing a bomb 

• Frisch-Peierls Memorandum March 1940  (for the pdf, click here)   (7 pages)
  Two articles laying out the feasibility of developing a bomb based on U-235, by two British physicists; the second is somewhat technical and so, less readable. It includes a calculation of an estimate of the amount needed, as well as the effect that such a bomb would have on civilians. 

• MAUD Report 1941  (for the pdf, click here)   (4 pages)

A further British analysis of the use of uranium for a bomb.

"Hitler's Uranium Club: The Secret Recordings at Farm Hall" by Jeremy Bernstein – texts extracted from the Introduction.  Farm Hall is the site in England where ten German scientists, including Heisenberg, were taken directly after the war had ended.  Their conversations were monitored and recorded. 

The Uranium Club: the start of the German work on the uses of nuclear fission, and a detour on the discovery of fission  (for the pdf, click here)  (11 pages)

Heisenberg and the German Project  (for the pdf, click here)  (8 pages)

German wartime nuclear research: a short synopsis  (for the pdf, click here)  (7 pages)

The German Project: more on the project (for the pdf, click here)  (11 pages)

Heisenberg and German bomb development:

• Sept. 19, 1939: Heisenberg was conscripted by Army Ordnance for military research on nuclear fission, for a Sept. 10 meeting with other scientists. Research programs in nine different groups began at various institutes. Within three months they produced a comprehensive, secret report "The possibility of technical acquisition of energy from uranium fission." 

• Heisenberg gave a lecture in Feb. 1942 to the House of German Research titled "The Theoretical Foundations for Obtaining Energy from Fission of Uranium" (annotated by Jeremy Bernstein). To read or download the translation and original, click here (11 pages total). Of particular interest are two excerpts:

"So if one just assembles a certain amount of U-235, so that neutron loss through the surface stays small compared with internal multiplication, then the number of neutrons will increase enormously in a very short time and the whole fission energy of 15 trillion kilocalories per ton is released in a fraction of a second. The pure isotope U-235 undoubtedly represents, then, an explosive material of unimaginable force. Granted, this explosive is very hard to obtain."

"As soon as such a machine is in operation, the question of how to obtain explosive material, according to an idea of von Weizsäcker, takes a new turn. In the transmutation of the uranium in the machine, a new substance comes into existence, element 94, which very probably—just like U-235 — is an explosive of equally unimaginable force. This substance is much easier to obtain from uranium than 235U92, however, since it can be separated from uranium by chemical means.
"

The first excerpt alludes to the difficulties of extracting the very small fraction (about 1/140) of U-235 from natural uranium.  The second excerpt is about generating element 94 (plutonium) from natural uranium; this requires a working reactor (referenced as "a machine"), and chemical separation.  For a number of reasons, neither proved to be feasible in the German war effort.

In its massive effort, the U.S. Manhattan Project achieved both: a bomb using all available U-235 that was dropped on Hiroshima, and plutonium-based  bombs with one first tested at Trinity and then another dropped on Nagasaki. 

Foreign lectures

Some Heisenberg foreign lectures during wartime. They were on scientific subjects, but still needed multiple approvals by the bureaucracy, and required a report afterward:

• • • Copenhagen, Sept. 1941 (including the famous visit with Bohr) 

    • German-occupied Budapest in 1941 and 1942.


    • Switzerland (Zürich) in 1942 and 1944.
  The latter was attended by Moe Berg, former baseball catcher and now OSS agent, with orders to assassinate Heisenberg if he was engaged in successful development of a German atomic bomb  (for a bio of Moe Berg, click here). 

    • Occupied Netherlands in October 1943. 

    • Krakow, Poland, in December 1943 as a guest of the infamous Dr. Hans Frank, general governor of Poland.  A lecture visit in 1941 had been turned down by the bureaucracy.

    • Copenhagen twice in 1944 after Bohr had fled to England and America.
 Heisenberg used his influence to preserve the Bohr Institute. 

    • Königsberg in East Prussia (now Kaliningrad, Russia) in February 1944.

Heisenberg as a member of the Mittwochgesellschaft  (Wednesday Society)

In December 1942 Heisenberg is invited by the Prussian finance minister Popitz to the Wednesday Society (Mittwochsgesellschaft), an elite Berlin men’s club of distinguished intellectuals, academics, statesmen, and military, meeting - since the time of Kaiser Wilhelm – once every several weeks (onWednesdays) in the home of one of its members. Before the refreshments the host used to provide a general lecture on a topic in his field. The 28 members of the Society have been chosen among the German cultural elite that had a critical attitude towards the Nazi regime. By 1944 the idea arose of an assassination attempt on Hitler as an interlude to a military coup. Heisenberg hosts the last meeting of the Society21 on July 12, at his Institute house in the Berlin suburb of Dahlem (his lecture on the constitution of stars includes an explanation of nuclear fission). Four of the conspirators are among the 10 members attending the meeting. Heisenberg hands the minutes of the meeting and a copy of his lecture to Popitz on July 19 and leaves for Southern Germany stopping for a couple of days at his family house in Urfeld. There they hear the next day, July 20, 1944, on the radio about the failed assassination attempt.

– Ivan Todorov

"Goudsmit, Bohr, and Heisenberg"

 A talk by Rudy Paul Lidner at the Michigan Center for Theoretical Physics, November 27, 2001, describing the interactions between Goudsmit and Heisenberg, particularly relating to the famous September 16, 1941, meeting between Heisenberg and Bohr in Copenhagen.

(For the pdf, click here)

Updated Nov. 16, 2023