Enrico Fermi: Father of the Nuclear Age
By Minnie Soo 蘇慧音
The 20th century was a golden era for physics, with brilliant minds pushing the boundaries of scientific innovation. Prominent physicists include Albert Einstein, Werner Heisenberg, Niels Bohr, Richard Feynman, and one must not forget the legendary Enrico Fermi, a giant in the history of nuclear physics.
When we think of the word “Fermi”, a whole array of related notions comes to mind. Fermilab, a renowned scientific institution dedicated to the study of particle physics; fermions, particles with an odd half-integer spin such as electrons; Fermi’s Paradox, a perplexing conundrum which challenges the possibilities of existence of extraterrestrial life. The use of such nomenclature highlights Fermi’s contributions and status in the scientific community.
Enrico Fermi and the Atomic Bomb
Enrico Fermi is a consequential contributor to the atomic bomb launched on Hiroshima and Nagasaki. He was burdened with no less responsibility than J. Robert Oppenheimer, the leading scientist in the Manhattan Project [1].
The story started with racial discrimination in Fascist Italy. In 1938, the establishment of antisemitic policies by Mussolini [2] posed a significant threat to Fermi particularly because his wife is of Jewish heritage [3, 4]. When Fermi was awarded the 1938 Nobel Prize in Physics at the age of 37, he took this opportunity to go directly from Stockholm, the place where he received the award, to the United States and never returned to Italy [5, 6].
In the summer of 1939, Fermi met Heisenberg in a lecture tour in the States, during which he tried to convince Heisenberg to join him at the physics faculty of Columbia University [7]. However, to his bafflement, Heisenberg decided to head back and serve the Nazi’s project to build an atomic bomb [7]. Disappointment in this unsuccessful recruitment has evolved into a fear that, with the great mind of Heisenberg, the Nazis would succeed in developing the atomic bomb and win the war. While the scientists in the US were making continuous efforts to alert the government to the destructive power that uranium fission chain reactions could bring [8], they teamed up to push forward the progress of uranium research [9]. In 1942, Fermi successfully initiated the first controlled chain reaction of nuclear fission [6]. As World War II progressed, Fermi joined the Manhattan Project as an associate director in Los Alamo [1, 4]. In three years, the first atomic bomb was built and dropped on Hiroshima, causing numerous deaths and catastrophic damage.
The Chicago Pile Experiment
As for the invention of the atomic bomb, one must mention the breakthrough in the Chicago Pile experiment. On a chilly winter day in 1942, Fermi and his colleagues placed a 6.1-meter wide by 7.6-meter high pile of graphite bricks with 6 tons of uranium metal and 40 tons of uranium-235 in the squash court under the University of Chicago football field, together with cadmium rods [1, 6, 10, 11].
The theory behind the experiment is as follows. As a neutron hits a uranium-235 atom, the latter splits into two smaller atoms and releases energy [6, 12]. This fission reaction also releases neutrons as by-products to split other uranium-235 atoms, resulting in a chain reaction to unleash gargantuan amounts of energy [6, 12]. For each mole of uranium-235 that goes under fission, the resulting products weigh approximately 0.2 grams less than the reactants [13]. By the famous equation E = mc2, this loss in mass corresponds to the conversion of order of 1013 joules of energy. In fact, the fission reaction of one kilogram of uranium-235 produces energy that is 2.5 million times greater than the energy generated by burning one kilogram of coal [13]. Therefore, for the reaction to be controllable, cadmium rods were inserted to absorb some neutrons during the reaction, thereby controlling the reaction rate and the amount of energy produced [6]. If Fermi’s calculations had been wrong and the cadmium rods had been insufficient to control the reaction, catastrophic amount of energies could have been unleashed, potentially destroying half of Chicago [6]. Fortunately, the experiment turned out to be a success, creating the first controlled, self-sustaining nuclear chain reaction. The pile was later refined by a substantial reduction in size, which made the controlled nuclear chain reaction possible to be incorporated into an atomic bomb that is small enough to be carried in an airplane.
A Mistake in Fermi’s Nobel Prize
The Nobel Prize is widely regarded as one of the most prestigious awards in the scientific community. Consequently, Fermi was rightfully honored with this esteemed recognition. However, there was a critical mistake concerning the scientific discovery for which Fermi received the award [14].
This is actually a story about Fermi discovering nuclear fission without realizing it. In 1938, Fermi was granted the Nobel Prize "for his demonstrations of the existence of new radioactive elements produced by neutron irradiation, and for his related discovery of nuclear reactions brought about by slow neutrons [15]." At that time, the known heaviest element was uranium with an atomic number of 92. It was believed that Fermi had successfully produced transuranic (beyond uranium) elements with atomic numbers 93 and 94 by bombarding uranium with slow moving neutrons. They called the new elements Ausenium and Heperium, respectively. However, German chemists Otto Hahn and Fritz Strassmann subsequently made a pivotal discovery that the elements produced through uranium bombardment were not novel entities, but lighter elements like barium with an atomic number of 56 [14]. In fact, uranium split into two lighter elements upon neutron bombardment, through a reaction later known as nuclear fission. Based on the discovery of nuclear fission, Hahn was awarded the Nobel Prize in Chemistry in 1944 [16]. As for the actual elements 93 and 94, they were eventually created in 1940, and named neptunium and plutonium [17].
Fermi’s Paradox
One question that bugged scientists, including an intelligent physicist like Fermi, is the possible existence of extraterrestrial creatures, a.k.a. aliens. From geocentrism to realizing that we are not the center of the universe, from thinking that the Milky Way was all that there was to discovering that there are billions of galaxies [18], we should be wise enough to know that we are not special in the vast universe. Fermi proposed that if we are not unique and the Earth is young compared to the copious stars and planets out there, extraterrestrial civilizations should have evolved and colonized nearby galaxies by now [19]. Yet, where did everyone go? Before making further discoveries on this subject, Fermi died in 1954, and the question fell to other scientists. Despite ongoing research and numerous papers being published on this topic, the question remains unresolved and highly debatable.
Fermi Problem
During the Trinity Test, the first detonation of the atomic bomb in history, Fermi tore paper into scraps and threw them from a height of 1.83 meters [20]. While standing 16 kilometers away from the explosion site [21], he utilized their displacement shift of 2.5 meters to estimate the energy produced by the detonation through a series of deductions and calculations. Fermi’s estimate (10 kilotons of T.N.T.) was within the same order of magnitude of the true value (21 kilotons of T.N.T.) despite employing what initially seemed like an unrelated method. This led to the emergence of a new class of problems known as the Fermi problems, to be solved using this estimation method to approximate the order of magnitude of values when our knowledge is limited. It involves making educated guesses by breaking down complex problems to simpler components, with reasonable assumptions.
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Physics Books in HKUST Library To get a sense of this estimation method, let’s try to solve this problem: What is the order of magnitude of the number of physics books (including e-books) in the HKUST Library?
This is an open-ended question. You are free to tackle the problem in your own way, and make any useful assumptions, e.g. science books can be generally divided into four subjects. A basic fact: The six-story HKUST Library has a collection of 2,442,592 books (including e-books).
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References
[1] Badash, L. (2024, April 12). American career of Enrico Fermi. Encyclopædia Britannica. https://www.britannica.com/biography/Enrico-Fermi/American-career
[2] Burgwyn, H. J. (2018). Persecution of the Jews. In Mussolini and the Salò Republic, 1943–1945 (pp. 141-164). Palgrave Macmillan Cham. https://doi.org/10.1007/978-3-319-76189-3_8
[3] Goodchild, P. (1986, August 10). Time Bomb!FERMI, HEISENBERG, AND THE RACE FOR THE ATOMIC BOMB by Malcolm C. MacPherson (Dutton: $18.95; 293 pp., illustrated). Los Angeles Times. https://www.latimes.com/archives/la-xpm-1986-08-10-bk-2027-story.html
[4] Whitacre, M., & Belotti, A. (2022, January 28). How science earned Enrico Fermi a Nobel Prize (and saved his Jewish wife and children). Los Alamos National Laboratory. https://discover.lanl.gov/news/0127-enrico-fermi/
[5] U.S. Department of Energy. (n.d.). The Life of Enrico Fermi. https://science.osti.gov/fermi/The-Life-of-Enrico-Fermi
[6] PBS. (n.d.). A science odyssey: People and discoveries: Fermi creates controlled nuclear reaction. https://www.pbs.org/wgbh/aso/databank/entries/dp42fe.html
[7] MacPherson, M. (1987). Time Bomb: Fermi, Heisenberg, and the race for the Atomic Bomb. Berkley Books.
[8] U.S. Department of Energy. (n.d.). EINSTEIN'S LETTER. The Manhattan Project–an interactive history. https://www.osti.gov/opennet/manhattan-project-history/Events/1939-1942/einstein_letter.htm
[9] U.S. Department of Energy. (n.d.). EARLY URANIUM RESEARCH. The Manhattan Project–an interactive history. https://www.osti.gov/opennet/manhattan-project-history/Events/1939-1942/uranium_research.htm
[10] Allardice, C., & Trapnell, E. R. (1946). The First Pile. https://www.iaea.org/sites/default/files/publications/magazines/bulletin/bull4-0/04005004147su.pdf
[11] Dean, K. M. (2022, December 1). Chicago Pile 1: A bold nuclear physics experiment with enduring impact. Argonne National Laboratory. https://www.anl.gov/article/chicago-pile-1-a-bold-nuclear-physics-experiment-with-enduring-impact#:~:text=An%20unassuming%20pile%20of%20black,to%20deliver%20on%20its%20promise
[12] Hellman, M. E. (2012). The Physics of Nuclear Weapons [Class handout]. https://ee.stanford.edu/~hellman/sts152_03/handout02.pdf
[13] Flowers, P., Theopold, K., & Langley, R. (n.d.). 21.6: Nuclear Fission. LibreTexts Chemistry. https://chem.libretexts.org/Bookshelves/General_Chemistry/Map%3A_Chemistry_-_The_Central_Science_(Brown_et_al.)/21%3A_Nuclear_Chemistry/21.06%3A_Nuclear_Fission
[14] Galison, P. (2005). Author of Error. Social Research: An International Quarterly, 72(1), 63-76. https://doi.org/10.1353/sor.2005.0032
[15] The Nobel Prize in Physics 1938. (n.d.). The Nobel Prize. https://www.nobelprize.org/prizes/physics/1938/summary/
[16] Spence, R. (2024, March 11). Otto Hahn. Encyclopædia Britannica. https://www.britannica.com/biography/Otto-Hahn
[17] American Chemical Society. (n.d.). Discovery of Transuranium Elements at Berkeley Lab. https://www.acs.org/education/whatischemistry/landmarks/transuranium-elements-at-berkeley-lab.html
[18] Gunn, A. (2024, March 18). How many galaxies are there in the universe? BBC Sky at Night Magazine. https://www.skyatnightmagazine.com/space-science/how-many-galaxies-in-universe
[19] Howell, E. (2023, August 24). Fermi paradox: Where are the aliens? Space.com. https://www.space.com/25325-fermi-paradox.html
[20] Tanner, B. (2022, January 10). Fermi Problems Part 1: Envelopes at the Ready! Tom Rocks Maths. https://tomrocksmaths.com/2022/01/10/fermi-problems-part-1-envelopes-at-the-ready/
[21] Fermi, E. (n.d.). My Observations During the Explosion at Trinity on July 16, 1945. Atomarchive.com. https://www.atomicarchive.com/resources/documents/trinity/fermi.html