Egidio Landi: a Life in the Science and Teaching of Polarimetry

Egidio Landi: A Life in the Science and Teaching of Polarimetry R. Casini High Altitude Observatory, National Center for Atmospheric Research,1 P.O. Box 3000, Boulder, CO 80307-3000, U.S.A. Abstract. This is inevitably a very personal and perhaps even biased account of the work of Prof. Egidio Landi Degl’Innocenti, during his nearly 45 years long scientific career, in the field of polarimetry as it applies to the investigation of solar processes, as well as in the broader context of astrophysics. Despite the breadth of Egidio’s contributions to scientific research and teaching, I will not be providing (nor would I have been able to) a complete account of his work. I instead made the choice to emphasize Egidio’s style in approaching new science challenges as well as revisiting older prob- lems. This style is first of all a product of both personal discipline and deeply rooted curiosity, but also a legacy of the cultural and academic ambient where Egidio spent his formative years, that is, Florence and its University. Egidio Landi Degl’Innocenti was born in Florence, on January 25th, 1945, only a few months after the liberation of the city from the occupation of the German army achieved by the local Resistance groups and the Anglo-American armed forces. Despite the dramatic pillaging and destruction of the city during one year of Nazi occupation, the cultural spirit of the place had remained unscathed, and this certainly contributed to a relatively speedy recovery in the years of the “reconstruction” that followed the end of World War II. Indeed, one could claim that those were the years of a second “renais- sance” of Florence, when local intellectual and authority figures, motivated by a healthy measure of pride in the cultural and spiritual legacy of the city, helped maintain high the hopes and vision of the post-war years. Men of culture, politics, and religion, such as Pietro Annigoni, Piero Calamandrei, Elia Card. Dalla Costa, Luigi Dallapiccola, Fr. Giulio Facibeni, Eugenio Garin, Giorgio La Pira, Mario Luzi, Fioretta Mazzei, Vasco Pratolini, Vasco Ronchi, Gaetano Salvemini, Giuliano Toraldo di Francia, provided a deep and fertile intellectual soil and social fabric, which allowed the rebirth of Florence as a true center of “integral humanism”. In the mathematical and natural sciences, particularly in Physics and Astronomy, Florence had already witnessed an important revolution in the 1920s (which sadly also coincided with the advent of Fascism in Italy), while Antonio Garbasso was the director of the Institute of Physics, and Giorgio Abetti was the director of the Arcetri Observatory, both located on the hill of Arcetri, at the city’s south end. Garbasso was a physicist, who had also spent a period of study in Germany under Hertz and von Hel- moltz before returning to Italy at the turn of the 20th century, first in Genoa, and then, in 1913, in Florence. Abetti was an astronomer, and the son of Antonio Abetti, who had in turn been the director of the Arcetri Observatory for almost thirty years, when 1The National Center for Atmospheric Research is sponsored by the National Science Foundation. 1 2 Figure 1. Fermi, Carrara, and Rasetti in Florence, posing in front of the “pozzo di Arcetri”, in the cloister of the Institute of Physics. Sitting in the back is Rita Brunetti, a research assistant of Garbasso along with Rasetti. in 1922 Giorgio inherited the incumbency of running the observatory after his father. Both Garbasso and Abetti were sensitive to the recent dramatic changes of the physical sciences—at a time when Einstein’s general theory of relativity had been given solid experimental evidence (by the results of Eddington’s 1919 solar eclipse expedition), and the new quantum view of the microscopic world was still being shaped—and they both took bold steps to secure a thriving scientific environment at their respective institutes in Florence. The enthusiasm of the times are probably best demonstrated by the array of “students” and “assistants” that Garbasso was able to gather around himself during the years of his tenure as the director of the Institute of Physics in Arcetri; names such as Franco Rasetti, Bruno Rossi, Giuseppe (Beppo) Occhialini, Vasco Ronchi, and Giulio Racah, each of whom would make important contributions to the scientific research of the 20th century (Rasetti in nuclear physics, Rossi and Occhialini in cosmic rays, Ronchi in op- tics, Racah in the quantum theory of spectroscopy). However, a most important turn of events happened near the end of 1924, when Garbasso succeeded in bringing Enrico Fermi to Florence. Fermi had earned his Physics degree two years earlier at the Scuola Normale Su- periore in Pisa, where he had been a fellow student with Rasetti and Nello Carrara (see Figure 1). The latter, despite being born in Florence, did not actively participate to the establishment of the early Florentine school of Physics, although he would later on become the founder and first director of the local Istituto di Ricerca sulle Onde Elet- tromagnetiche (IROE), which is today named after him. After his degree, Fermi had spent a brief period abroad, first in Göttingen with Max Born, and then in Leiden with 3 Figure 2. Mandò and Franchetti (on the right) discuss with I. I. Rabi (center) on the rooftop of the Institute of Physics. The Arcetri Observatory is in the background, on the top of the hill. Paul Ehrenfest. He had returned to his native Rome, and briefly worked at the Physics institute directed by Orso Maria Corbino, when he got the opportunity to join the newly established Università degli Studi of Florence.2 The two years Fermi worked in Florence were extremely productive. He was charged with lecturing classes on Analytic Mechanics and Mathematical Physics, but at the same time he was also able to pursue his own scientific interests, both in theoretical research and experimental physics. While in Florence, he produced one of his most important works, which is the derivation of the properties of the Fermi-Dirac statistics describing identical particles with half-integer spin numbers (Fermi 1926). Quite rele- vant to the subject area of this polarization workshop is the theoretical and experimental work that Fermi and Rasetti pursued about the effects of an alternating magnetic field on the polarization of resonance radiation (Fermi & Rasetti 1925). Unfortunately, Fermi and Rasetti did not stay in Florence very long. Fermi went back to Rome in 1927, to fill the first Italian professorship in Theoretical Physics, while his childhood friend and colleague Enrico Persico moved in the opposite direction, from Rome to Florence, to fill the position left vacant by Fermi. Shortly after, Rasetti rejoined with Fermi in Rome, to fill the position left vacant by Persico. This amusing shuffling of personnel between the Florentine and Roman institutions was in fact instrumental in cementing the professional and personal interactions between the two research groups for years to come. 2October 1st, 1924 is the official date of the institution of the modern university by royal decree. However, from a cultural standpoint, we can trace its origins back to the Studium generale et Universitas scholarium of Florence, which was founded in 1321, the same year of Dante Alighieri’s death. 4 Persico carried on the teaching of Analytic Mechanics and Mathematical Physics in Florence, and began establishing a strong school on the new “wave mechanics” within the Italian academia, first in Florence, and later in Turin, to where he moved in 1931. Two of his students were Racah in Florence and Ugo Fano in Turin (who also happened to be a first-degree cousin of Racah), who were to make remarkable and lasting contributions to the field of spectroscopy and polarimetry. In addition, some of Fermi’s students in Florence were there to stay (such as Giulio Calamai, who had attended Fermi’s course on Analytic Mechanics, and passed the exam with a very hon- orable 28/30 grade3), and, providentially, a few of them had also diligently transcribed the lecture notes of his courses. To some extent, this also helped carry on the baton of Fermi’s teaching and research styles at the University. The physicists that formed in Florence during those times, or who had been called to fill available positions there, such as Gilberto Bernardini, left a lasting legacy in the Florentine atheneum, characterized by the breadth, depth, and rigor of scientific research and teaching. As Manlio Mandò had put it (see Bonetti & Mazzoni 2007), the “echo” of the passage of Rasetti, Fermi, and Persico was still resounding loudly when in 1930 he started attending the university courses in Arcetri. Beppo Occhialini, who had been an assistant professor in Arcetri before moving to Cambridge to work with Blackett, likened the triad Garbasso–Abetti–Persico to a “Trinity”, or more figuratively to a “three-legged stool” upon which the Arcetri school of Physics had been solidly established, and was able to thrive in the years ahead. Abetti, in particular, had established a cycle of scientific “seminaries”, through which many rising personalities of modern physics (among those, Hans Bethe) were able to exchange their experiences and scientific accomplishments with their Florentine colleagues. Beside the people already mentioned, in the decade between 1930 and 1940, Arcetri produced several other graduates who will become stable and critically important figures of the local academic circle: Simone Franchetti, Guglielmo Righini, and Giuliano Toraldo di Francia. Sadly, Franchetti (visible in Figure 2 on the right, next to Mandò) was banned from teaching in 1938, as a consequence of the newly imposed national racial laws of the Fascist regime.

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