In 2014 I discovered a small folder in the Fermi Archives of the University of Chicago’s Regenstein Library labeled “Columbia, Geophysics, 1941.” At the time I was writing a book about Enrico Fermi, The Pope of Physics, with my wife, Bettina Hoerlin. Fermi had wide interests and expertise in physics, so I shouldn’t have been surprised that he was fluent in the study of geophysics. But the anomaly stuck with me. A year later I consulted a Columbia University archivist who informed me that Fermi had offered a geophysics course in the spring semesters of 1939–41. It met once a week on Fridays, from 4:30pm to 6:00pm.
Given Fermi’s accomplishments in theoretical and experimental physics, teaching geophysics may seem like an unusual choice. Yet it was the first course that Fermi taught at Columbia when he and his family immigrated to the US, via Stockholm, in January 1939. In Stockholm Fermi was awarded the 1938 Nobel Prize in Physics for his study of nuclear reactions brought about by slow neutrons and for discovering new radioactive elements. He and his family had fled Fascist Italy, where recently passed racial laws threatened his wife, Laura, who was Jewish. He had little doubt that the laws would discriminate against his two children and quash the future of Italian physics.
At Columbia Fermi immediately began a research program studying nuclear fission, a phenomenon discovered only weeks before his immigration to the US. As part of his appointment, he also began teaching.
Fermi had previous experience building a physics curriculum from scratch. The Italian university system in the 1920s treated physics as a purely experimental subject, so the course offerings were limited. When Fermi became Italy’s first professor of theoretical physics in 1926 at the Sapienza University of Rome and was asked to teach something in addition to a quantum physics course, which itself was a first in Italy, there were few courses to choose from. He opted for Fisica terrestre. Fermi’s syllabus for that course bears a marked resemblance to the geophysics course he taught at Columbia little more than a decade later.
Fermi was a famously lucid master of exposition; his lengthy 1932 Reviews of Modern Physics article “Quantum theory of radiation”—a notoriously difficult subject—has been considered an exemplary model of clarity and thoroughness. Luminaries in the field such as Hans Bethe and Richard Feynman would later say that it was from this article that they learned the subject. Student notes from Fermi’s thermodynamics course taught at Columbia in the summer of 1936 and his nuclear physics course taught at the University of Chicago after World War II were developed into popular textbooks.
A good example of the contrast between how the notes appear and how the classes must have been conducted is seen in the treatment of the variation of atmospheric pressure with altitude. One page of Fermi’s notes, reproduced below, begins by simply presenting the equations of state for isothermal and adiabatic atmospheres, which were likely to be part of a longer classroom discussion. Fermi then quickly derives how pressure declines relative to altitude in the two cases, exponentially for an isothermal atmosphere and linearly for an adiabatic one.
Perhaps an astute student would have noted that, although Fermi had said the adiabatic approximation was more realistic, the derived equations implied that both pressure and temperature fell to zero for a sufficiently high altitude. At that point, if he had not done so already, Fermi would have explained that the perceived discrepancy was an excellent example of the need to understand the limits of the conditions for which a formula is valid, a topic he surely would have been emphasizing throughout the course.
Fermi was particularly fond of challenging students and friends to make numerical estimates of physical quantities. These challenges were so frequent that this type of inquiry became known in the physics community as a “Fermi Question.” His former students have reported that during the shared lunches he regularly had with them, Fermi asked questions such as “How thick can the dirt on the window be before it begins to fall off?” He may have felt that developing the skill to answer such questions would give them the confidence to attack any problem.
Geophysics has seen great advances since 1941. Some topics, plate tectonics being an outstanding example, are not mentioned in Fermi’s notes. That omission is understandable given that the theory was not widely accepted until the 1960s, although its roots date back to Alfred Wegener’s 1915 work on continental drift. The fields of geomagnetism and computer sciences have also rapidly advanced. Fermi certainly would have followed the latter in great detail given that, among his many interests, he was also a pioneer in computational physics.
But most of geophysics is still based on the laws of physics already known in 1941. The techniques Fermi used are therefore not out of date, and his approach to teaching geophysics is of more than historical interest. Young and old scientists can enjoy and profit from seeing how a giant of physics dealt with teaching the continually evolving and fascinating field.
Gino Segrè is an emeritus professor of physics at the University of Pennsylvania. Along with University of Illinois emeritus physics professor John Stack, he wrote the book Unearthing Fermi’s Geophysics, which will be published by the University of Chicago Press in December.
