Chapter Four—Scientific Springtime (1928–1936):
Smell of Amsterdam's Canals
Ci lasciaron talune una fragranza
così tenace che per una intera
notre avemmo nel cuor la primavera
e tanto auliva la solinga stanza
che foresta d'april non piú dolce era.
(Some of them left us a fragrance
so lingering that we had spring
in our hearts for a whole night;
and the lonely room was so perfumed
that no forest in April was sweeter.)
Gabriele D'Annunzio, "Le mani," from Poema paradisiaco
(trans. Louise George Clubb)
The laurea that entitled me to call myself Dr. Segrè completed my formal scholastic career, but my study of physics was to be a lifelong occupation. In fact, most of the physics that was to form the subject of my later work did not exist when I was at university; not even in an embryonic state. The neutron and artificial radioactivity were far in the future, not to mention particle physics.
Having finished university, I was thinking of the next step, which I hoped would be an assistantship at Rome. I did not have to worry about earning money, because my father could support me without any sacrifice and would do so gladly, but I wanted a salary as an acknowledgment of my work, although not at the price of taking a job that might endanger my scientific prospects.
For the present, fulfilling my military obligations would give me time for reflection. At the officers' training school in Spoleto to which I was sent, I entered a world utterly new to me. My new comrades came mostly from the Italian bourgeoisie, from every region of Italy; most of them were lawyers, literary men, small businessmen or landowners, with very few engineers or technicians.
Discipline was strict and unreasonable. One of the main occupations was changing one's uniform at high speed many times a day. There was enough food, but it was of poor quality. We were taught mainly by noncommissioned officers, who were happy to display their authority and zeal, using methods reminiscent of the Catholic catechism, occasionally peppered with cruelty. I was assigned to the artillery section, and a major gave us theoretical gunnery courses, whose content went back to about 1890.
One of my comrades whom I vividly remember was a Marquis Lignola, who belonged to an aristocratic Neapolitan family. He considered the Bourbons, ousted from Naples in 1860, to be his legitimate rulers and regarded the House of Savoy as usurpers. Lignola was small and somewhat clumsy. The other students provoked him into expressing his extreme opinions and then scurrilously made fun of him. The victim turned his eyes to heaven and "offered his sufferings to Jesus," showing courage, extreme firmness in his opinions (the pope had erred in dealing with the usurper), and strength of character. Soon his tormentors started to respect him and left him in peace.
The strictly disciplined military life and lack of freedom were boring, but restful, because one did not have to think of anything or make any decisions; the sergeants prescribed all our activities. I had taken with me several books: Courant and Hilbert's Methoden der mathematischen Physik, Oscar Wilde's The Picture of Dorian Gray, and similar esoteric reading. I used to read them during a compulsory siesta period after lunch, lying on my field bed. The books, in foreign languages to boot, allowed me to put on airs with my comrades, but also spiritually transported me far from my military surroundings. A few weeks after the beginning of instruction, we had a free Sunday, and a comrade and I used the short furlough by going to Gubbio and other places near Spoleto. I still remember the exhilarating impression of having regained our freedom, albeit only for a short time. We manifested it by carrying our heavy army swords on our shoulders like hoes. It was a childish gesture, which could have brought unpleasant punishment if detected. A little later I was called to headquarters and given three or four days' furlough for Yom Kippur; I did not know I was entitled to this leave and was pleasantly surprised. Several of my comrades proposed converting to Judaism if that produced leaves of absence. I went to Rome by train, put myself in mufti, rejoicing in contact with the soft flannel of my elegant trousers, and, well groomed, went to court Renata J.
Quite unexpectedly one day, Fermi and his wife Laura showed up in Spoleto in their small yellow Peugeot car. The visit greatly raised my spirits because it gave me an opportunity to resume contact with physics and because it demonstrated that my Physics Institute friends remembered me.
At the officers' training school I had my dose of small adventures, such as falling off my horse right in front of an inspecting general. I immediately got to my feet, unharmed, and grabbed the horse by its bit. The good general kindly commented to me that anybody could be thrown by a horse, and that he had appreciated the way in which I had done the correct thing by preventing the animal from running away! I noted that whenever there was an inspection by some visiting bigwig, I was always called upon to aim the guns. My superiors had soon discovered that I was to be trusted to make the required simple calculations correctly, without sign errors, which occasionally pointed the guns backward.
I graduated from the officers' training school on January II, 1929, and resumed work at the laboratory until the following July 1, when I was commissioned a second lieutenant in the anti-aircraft artillery and stationed at Forte Braschi, very near Rome, as I had requested. On this assignment, I did not have much to do. I slept at home, going to the barracks early in the morning with some book on physics to study. However, in my military service, I learned many other things besides physics. My captain taught me a card game called scopone , which I greatly enjoyed, and also revealed to me novel attitudes to life. I had been brought up with the idea that I should work, that everything had to be taken seriously, and that I was expected to excel, or at least to do well. From my captain I learned that zeal was a grave fault; that many problems took care of themselves provided they were left alone; and that when one received an order, contrary to what we had been repeatedly told, prompt execution was imprudent, and that it was advisable to await its countermanding. Furthermore, contact with the soldiers gave me a chance to get to know contemporaries of very different education and from diverse social and economic conditions. I could also see firsthand the differences between soldiers from the various Italian regions. The unhomogeneousness of Italy's population was such that soldiers often did not have a common language, used as they were to speaking dialect. We had orders, moreover, to see to it that soldiers from the rice-eating north and those from the pasta-eating south did not throw away their rations on alternate days, when the food unusual for them was served.
As a commissioned officer, I bought myself secondhand a gorgeous blue cape, which, although not a very efficient protection against cold, was certainly elegant. When I finished my service I sold it to Edoardo Amaldi, then also an officer.
During my service in Rome, I managed to go once in a while to the laboratory and keep in touch, but I did not have time for experimental work. One day I was urgently called from the barracks at Forte Braschi to the Physics Institute at Via Panisperna. For some reason, all the scientists were away, and the factotum of the institute, who did not know English, was faced with an obviously important Indian visitor with whom he could not communicate. I rushed down and found that the visitor was none less than Sir Chandrasekhara Venkata Raman (1888–1970), who in 1930 received the Nobel Prize in physics for his work on the diffusion of light and the discovery of the Raman effect, on which Roman physicists had done important work. I did the honors as well I could, unexpectedly helped by being in dress uniform, with a blue sash and conspicuous gold epaulettes. Raman believed that I had dressed like this to honor him and thanked me; I did not disillusion him by revealing that the true reason was H.M. the Queen's birthday!
During my military service, I was once unjustly placed under arrest for something I had not done. By chance, I mentioned this to a friend of mine, who without my knowledge spoke about it to his father, a powerful general. With surprising speed, my punishment was commuted to a much lighter penalty, and the colonel who had condemned me without even talking to me must have found himself in serious trouble. I was discharged as a second lieutenant on February 15, 1930, and placed in the reserve.
In 1928 I had published my first physics paper jointly with Edoardo Amaldi, a short note in the Rendiconti of the Accademia dei Lincei, introduced by Corbino, summarizing my doctoral thesis.[1] The following year, Amaldi and I published a second paper, dealing with the Raman effect.[2] I did much early work with Amaldi, but because the rules then prevailing in university competitions penalized collaborative efforts, we often divided the work in a friendly fashion after we had finished it together.
Next I wrote a paper on anomalous dispersion in molecular band spectra. The ensemble of the absorption lines near the head of a band gives a peculiar variation of the refractive index, which I explained with Fermi's help.[3] In about the same period, Amaldi and I produced a couple of papers in the wake of Fermi's study on quantum electrodynamics. They contained results similar to those of a famous and often quoted paper by Eugene Wigner and Victor Weisskopf. Our work was done earlier, although less detailed.[4]
Fermi observed strict rules concerning the publication of his work and that of his students. He did not permit publication of completely insignificant results. Results of little importance appeared only in Italian. He allowed publication in the Zeitschrift für Physik, or as a letter to Nature, only of papers he considered important. This wise policy was motivated by his desire to establish an international reputation for our Rome group. He did not want any foreign reader ever to be disappointed in reading one of our papers; there should always be something interesting in them. He applied this rule strictly, and his judgment on the quality of an investigation rarely erred.
Furthermore, when Fermi developed a theory capable of many applications, such as his quantum theory of radiation, he explained the principles to us, gave some examples, and then left to us the satisfaction of finding further applications. Not a little of the work of this period started out like this.
"You live off Fermi's crumbs," my father, who knew no physics, but was a shrewd observer and knew men, once told me. This was quite true, and I did not forget it. Only later was I able to do something truly my own. My mother, who wanted to know whether my studying physics would lead anywhere, once asked Fermi what he thought of my ability. The ever-truthful Fermi answered, correctly and objectively, that it was too early to pass judgment and make predictions. I was not present at this conversation, but knowing both parties, I am sure that my mother continued worrying about the perfectly honest answer she received.
In the late 1920s and early 1930s, we worked intensely, but in a relaxed way. We used to read the most important journals, such as the Zeitschrift für Physik, Nature, and the Proceedings of the Royal Society, eclectically, hunting for experimental inspiration. My official job—a relic of times past—was that of "Conservator of the Tuning Fork," a sinecure that amounted to an assistantship. When students came looking for research subjects, even in theoretical physics, they were often referred to me, because I usually had a good supply of ideas. I would suggest a problem and explain it to the student in detail. However, when it came to technical details, I often had to send the student to Fermi, who instantly gave the key to the solution. The problems I set were usually in atomic physics or connected with it. I asked Fermi why he did not give out the problems himself; he answered that those he thought of were usually too difficult for students, and that those at their level did not interest him. When Fermi and I co-authored papers, I was often entrusted with the writing. I did not mind writing, and I was proud of the assignment. "It is clear that since you will never get the Nobel Prize for Physics, you are preparing yourself for the Literature Prize," my friends teased.
The study of theoretical physics at Rome progressed under full sail. Fermi was obviously not only a first-class theoretician but a superb teacher; one could not ask for more. He had contacts through conferences and visits with the principal theoreticians of his own age, as well as with Arnold Sommerfeld, Paul Ehrenfest, and some others of the previous generation. Soon postdoctoral fellows from abroad started to come to him, to learn and work under his inspiration. Among the first to arrive were Hans Bethe, Rudolf Peierls, and George Placzek, who acclimatized himself to Rome better than the others, learning Italian and striking up a solid personal friendship with Amaldi and myself. Others who followed included Edward Teller, Fritz London, Felix Bloch, D. R. Inglis, and Eugene Feenberg, who became Majorana's particular friend.
The situation in experimentation was different. One cannot learn the experimental art from books, and we felt the need to go see what happened elsewhere and to learn techniques on the spot where they were practiced. Franco Rasetti was the first to take off, in 1929, going to the California Institute of Technology at Pasadena, then dominated by R. A. Millikan. I do not know what influenced his choice. The Rockefeller Foundation granted him a fellowship.
The Rockefeller Foundation was a great benefactor of physics in that period, helping it through the general economic depression and, later, Hitler's persecutions. A shrewd and farsighted choice of Fellows was at the base of the Foundation's success. Looking today at a roster of Fellows of that period, one wonders at the sagacity of the selection, and the wonder grows when one considers that Fellows were appointed at an early age, often before the work that later distinguished most of them. The selection occurred on the basis of recommendations by two or three established professors whom the Foundation trusted, mostly in the applicant's country of origin. In Italy, it seems that these advisors were Volterra, Levi-Civita, and Corbino, a choice that in itself shows the Foundation's sagacity. All three were honest, experts on their subjects, and well informed about the local situation. They were not in the good graces of the Fascist government, but if this was resented in Italian high places, it did not matter to the Foundation.
Rasetti did very well scientifically and personally at Caltech. He accomplished important work on the Raman effect in gases and fostered the good reputation of the Rome group. He also visited Berkeley, from where he sent me a postcard. At the time he had the impression that Caltech was way ahead of Berkeley.
When he returned home, he spoke only of California's wonders: of Mount Whitney, which he had climbed in winter, of Pasadena's orange groves, of the wealth of American laboratories, of the attractiveness for him of the American way of life. He also proudly showed off a toothbrush he had bought in Hawaii. Needless to say, he would drive only an American car and bought a Ford.
Back in Rome, Rasetti continued to do fruitful work on the Raman effect. I too tried something on the subject without obtaining anything of importance. Amaldi and Placzek studied ammonia with better results. All this work on the Raman effect lasted into 1931.[5]
In November 1930, in the form of a very short letter, I sent Nature the first paper of any importance thought out and executed entirely by myself.[6] Atomic theory gives the laws according to which the electron jumps from one atomic energy level to another, emitting photons. Sometime these rules are violated and there are so-called forbidden transitions. My little discovery concerned certain (S-D) forbidden transitions in the spectra of the alkaline metals, and the paper shows that they are owing to electric quadrupole radiation, neglected in the usual first approximation calculations. The proof is obtained by observing the Zeeman effect of these lines. I examined absorption lines because emission lines are too weak. The experiment was very simple, although at the limit of the resolving power of the instruments available to me in Rome.
The best instrument I could use was a large Hilger prism spectrograph bought by Professor Lo Surdo, and located in his rooms. Lo Surdo very kindly and generously gave me permission to use this instrument. After a few days of work, I succeeded in seeing with my own eyes the potassium absorption lines delineated on a violet continuum produced by a hydrogen discharge. When I energized the magnet in which I had placed my absorption tube, the lines broadened and almost disappeared. With a little more work, I adjusted the instrument to obtain its maximum resolving power. I still remember my great elation in recognizing that the Zeeman pattern I was seeing was the one I expected for quadrupole radiation. This was my first small discovery, and it made a permanent impression on me. If my previous work could be called "Fermi's crumbs," this was my own. Furthermore, it had been obtained rapidly, which enhanced its impact.
My friends at the Physics Institute bestowed upon me the title of "Lord Quadrupole," and, more important, Fermi told me to publish the work in the Zeitschrift für Physik .[7] The self-confidence of young scientists is a delicate plant. Even Fermi, who looked so self-assured in later years, and had performed extraordinary feats when very young, was not sure of himself until he went to Holland at about the age of twenty-one.
What I had seen in my study of the Zeeman effect of the S-D combinations in potassium was conclusive, but did not reveal all the details one would have liked to know. Unfortunately, the instruments at my disposal in Rome could not give more; I had squeezed them to their limit. I was thinking about what to do next when the great Dutch physicist and chemist Peter Debye visited Rome. There was a reception in his honor at Enriques's house and I was invited. Debye inquired in a friendly way about what I was doing, and I told him about my quadrupole work, adding that I was at a dead end for lack of adequate instruments. To my surprise, Debye sternly answered that my complaints were mere excuses; only lazy people were stopped by so-called lack of means. At the time, I was hurt, but the lesson sank in and was highly beneficial then and later. Debye himself suggested that I try going to some foreign laboratory. Four laboratories seemed likely to have a diffraction grating (a device consisting of narrowly spaced slits used for measurement of wavelengths) adequate to my project: E. Back's in Tübingen, H. Cohnen's in Bonn, F. Paschen's in Berlin, and Pieter Zeeman's in Amsterdam. I wrote four letters explaining what I wanted and asking for hospitality. (My father was more than happy to pay my expenses, so I did not need financial help.) Back did not reply; Cohnen said that his grating was at the moment out of commission, because his institute was being rebuilt; Paschen told me that he liked my idea, and that he had just put one of his doctoral candidates to work on it. I was furious at this unexpected answer, but the project must have come to naught, because I never heard any further news of it. Zeeman, a Nobel Prize winner and the discoverer of the celebrated Zeeman effect,[8] told me to catch a train and come to Holland.
I did so without delay, and arrived in Amsterdam at the beginning of the summer of 1931. After finding lodgings at a pension, I introduced myself to Zeeman and told him my precise work plan. Personally, Zeeman was most courteous, benevolent, and affable. He was then sixty-six years old and had ceased active laboratory work. I had the impression that he was not conversant with modern theory, and in particular with quantum mechanics, which was then still a relatively new field. On the other hand, he was a superb experimenter and a master of optics. In any case, talking to him was most instructive. His way of considering an experiment was new and unexpected to me. He had a refreshing diffidence about theory, and while he did not underestimate its power, he knew that nature had more imagination than we did. He thus pushed for thoroughness in experiments, saying that something unexpected was likely to happen. He was right, even in my simple case, when everything seemed predictable.
Zeeman immediately told me that his diffraction grating was the greatest treasure of his laboratory, and that he could not entrust it to me alone, since I did not have any experience in its use. He suggested that I collaborate with Cornelius J. Bakker, a doctoral candidate of his, who was familiar with the grating and with other delicate instruments in the laboratory. I found the proposal reasonable and fair and accepted it at once. Fortunately, it turned out that Bakker was a very nice person and soon we struck up a close friendship that lasted until his untimely death. I still have his portrait in my study.[9]
I immediately started preparing an absorption tube containing potassium. For this purpose, I cut off a piece of potassium and replaced the rest of it in what I believed was the bottle from which it had come. I had overlooked the presence, next to it, of an open bottle of acid. Without looking, I put the residual piece of potassium in the wrong bottle. For about half a minute nothing happened; then a tremendous explosion shook the laboratory. Everybody ran to see what had happened, and I can hardly say how I felt, although I was, fortunately, bodily unhurt. I deeply admired the calm Dutch, who gave no sign of commotion and did not ask me to leave.
The work, which started so dramatically, continued smoothly and successfully. We rapidly obtained all the expected results, as well as several more that rounded off and completed the picture. Zeeman took a liking to me, and one day he asked me about my plans for the near future. I told him that I had applied for a Rockefeller fellowship, but that nothing seemed to be happening. Zeeman remarked to me that he knew somebody in the Paris office of the Foundation, but said no more. By strange coincidence, about a week later I received a letter announcing the grant of the fellowship. Although he never told me so, I suspect that Zeeman may have had a hand in it. When I left Amsterdam to return to Rome, Zeeman told me that whenever I wanted to come back to work in his laboratory, I would be welcome. I subsequently took advantage of this cordial invitation, returning to study different types of forbidden lines, always together with my original co-worker and friend, Bakker. Zeeman also gave me a picture of himself with a warm inscription. He was not far from retirement and possibly thought of me as one of his last disciples.
During my stay in Holland, I became acquainted with its cigars. Zeeman invited me to a couple of very formal and elegant dinners at his home, on the occasion of which he offered his guests exquisite cigars, which I liked. I noted the brand and kept buying it, although I later switched to Otto Stern's favorite brand, which was available in Germany and was equally good.
Many months after publication of our papers on quadrupole radiation, I received Back's response to my application to work in his laboratory; in reply I sent him a reprint of our work.
In the summer vacation of 1931, I went to England for the first time. Between the day Zeeman's lab closed for the summer vacation and a date I had with Amaldi and Rasetti for a hiking tour in Norway, there was time for a short visit to London. I boarded a ship and saw a young man, about my age, with skis. It occurred to me that he was possibly a student who had spent the winter in Germany at some university and was now returning home. Hazarding a wild guess, I asked him whether he had been studying with Sommerfeld. Incredibly, my surmise turned out to be right, and the young man was absolutely flabbergasted. He was very friendly, found me a suitable hotel in Russell Square, and offered to help me to orient myself in London. In exchange, he asked that I join him for my first English breakfast, because he wanted to see my reactions. Undeterred, I ate porridge and haddock and drank tea, all of which I liked, much to my new friend's surprise.
My first impression of England in 1931 was of a great imperial power at sunset. As I wrote home, one saw a thousand signs that the country had passed its zenith. My parents had witnessed the coronation of Edward VII in a very different, splendid period. While in England, I visited Cambridge, where I saw J. J. Thomson but did not talk to him.
Soon thereafter, I met Amaldi and Rasetti in Oslo. We had planned a long hiking trip on Norway's glaciers. We left Oslo by train and alighted at 2 A.M. at Finsoe in daylight. We started walking on the Hardanger Fjell until we reached the sea. Later we explored other fjords by boat and on foot.
On my return to Rome, I kept working on forbidden lines and found other interesting features of their Zeeman effect, revealing their origin, in cases where they could not be the result of quadrupole radiation. Furthermore, theory indicated that there should also be forbidden quadrupole lines in X-ray spectra. I made a systematic search of the literature to see if by chance somebody had observed them without understanding their origin. To my joy, I found that that was indeed the case.[10] All this work was noted, and I had the satisfaction of seeing myself quoted in a new edition of Sommerfeld's famous treatise, Atombau und Spektrallinien .[11] Sommerfeld was always ready to help young scientists and had excellent relations with the Rome group.
In line with our program of learning new experimental techniques abroad, Fermi suggested I spend my Rockefeller fellowship in Hamburg, where I would be able to study vacuum technique (one of our weaknesses) and molecular beams under Otto Stern.[12] He made the necessary arrangements with Stern, and at the end of 1931, I set out.
Fermi and other friends had described Hamburg's wretched climate and dark, wet winters to me, and I found that they had not exaggerated (they did not, however, know what came after—that is, the long, beautiful northern spring, unknown in Italy). I installed myself satisfactorily in a rented room in a private house. The Rockefeller fellowship stipend of $150 per month made me rich and, to satisfy the usual pride of spoiled children (figli di papà ), I did not want any supplement from home. Soon after my arrival at Hamburg, following Fermi's advice, and as an act of courtesy, I visited the Italian consul. This gentleman opened my eyes to the German politics of the time, and even more so to Italian foreign policy, giving me a well-reasoned lecture on anti-Fascism. It seems that among Italian officials there were some who thought independently and had the courage of their own convictions.
Stern was then in the process of making important discoveries and was entirely submerged in his work. He used to arrive at the laboratory every morning at about 10 o'clock; at noon he had lunch with his assistants and guest workers, then returned to the lab, if necessary until late in the evening. The schedule depended considerably on the behavior of the instruments and the vagaries of the vacuum.
Stern suggested that I finish an experiment on the dynamics of space quantization and explained the motivation and theory behind it to me, as well as the details of the existing apparatus. This had been built by my predecessor, the American T. E. Phipps, whose fellowship had expired before he could obtain results. I asked Stern to teach me some physico-chemistry, and in several conversations he gave me interesting illuminations of thermodynamics.
After a while he left me to myself. I tried to learn techniques by watching Stern, as well as younger people such as F. Knauer, Otto Frisch, R. Schnurmann, and B. Josephy. Stern's institute was small in size and in number of scientists, but in spite of this there were not many exchanges between its workers. Schnurmann was the most open, and he introduced me a little to German life. Others had their girlfriends or other concerns, and as soon as the day's work was done, they left on their own private business. Frisch served as Stern's personal assistant at that time and was involved in two major experiments, a demonstration of de Broglie's waves with helium atoms, and the measurement of the magnetic moment of the proton.[13]
Stern taught me a way of experimenting that I had not seen before. He calculated everything possible about his apparatus, such as the shape and intensity of the molecular beams he expected to generate, and did not proceed until preliminary experiments were in complete quantitative agreement with his calculations. This modus operandi slowed down the preliminary work, but it shortened the total time by making it possible to avoid errors and was absolutely necessary for the extremely difficult experiments Stern was conducting. The method allowed him to localize sources of misbehavior in his apparatus and of failures, and to come to a firm decision as to whether there were new and unexpected results, which occurred repeatedly. It was a rigorous and most useful schooling, very different from Zeeman's, but just as valid. I learned much from both, more in the philosophy of experimentation than in technical details. Years later, I saw the totally different, much more pragmatic and empirical, approach taken by Ernest Lawrence. Tutte le strade portano a Roma.
There was also an active theoretical seminar at Hamburg. Pauli had been there until recently; William E. Gordon and later H. D. Jensen followed him. Hermann Minkowski, who later became a noted astronomer, was a lively member of the company. W. Lenz was older, and he seemed to me less interested in current problems.
As to my own work, having thoroughly studied the apparatus I had inherited, I concluded that to make it work, there needed to be a radical change in the method used for producing certain magnetic fields, although I had no idea how to achieve this. I had long admired James Clerk Maxwell's Treatise on Electricity and Magnetism (1873), however, and one day, while looking at an illustration in it of the magnetic field produced by a rectilinear current in a homogeneous magnetic field, I immediately saw the solution to my problem with molecular beams. The theoretical analysis had to be changed, but that seemed to me more feasible than following Stern and Phipps's original experimental plan.
Stern approved my idea as soon as he heard it, got me the few extra parts I needed, and told me to rebuild the apparatus in the way I proposed. As to theory, I knew whom to look to for help. I wrote a letter submitting my problem to Ettore Majorana in Rome, and soon received the answer I needed. By then it was spring, and over the Easter vacation I went to Rome, where I reported all I had seen and discussed at Hamburg. There was enough to keep my theoretical friends busy, and Majorana, Gian Carlo Wick, and Ugo Fano were soon struggling with three different problems strictly connected with the Hamburg experiments.
Europe now seemed to me to be destined for catastrophe. Naturally I did not acquire this foreboding in one day; it grew slowly. Already in 1929, during a trip to Germany with Angelo and Rasetti, I was dismayed by the fanatic enthusiasm with which a group of children on a boat on the Rhine were singing "Deutschland, Deutschland, über Alles." Later events, my life in Hamburg in the last years of the Weimar Republic, and conversations with colleagues convinced me of the deadly seriousness of the Nazi menace; they were a band of fanatics ready for anything. I could not anticipate what "anything" meant, but I included in it a major war—that is, a world war, or at least a European war.
Italy's position seemed to me ambiguous, and I did not know on which side of the fence she would come down. I realized that the Duce's big talk was mostly empty bombast. My profession fostered cosmopolitan attitudes and relations. Both Fermi's and Rasetti's horizons consistently extended beyond Italy, and they often considered emigration. It was fundamental for me that my wife share my mobility, because I might find emigration from Italy desirable or, God forbid, necessary. Any future wife must agree with me on this in advance. I sometimes touched on this issue with my physicist friends, and I spoke freely about it to Riccardo Rimini. At that stage, I was not thinking of the dangers of anti-Semitism, but rather of the situation as a whole.
I did not expect too much from Italian girls. Indeed, perhaps I expected too little of them. My parents on the other hand were eager to see me married to an Italian Jewish girl and suggested I meet some young women who looked suitable to them. One was in Ferrara, where Riccardo was then working in a hospital. On the pretext of visiting him, I met the girl, but I barely remember her. After the war, I learned that she had been murdered by the Nazis.
Later it was the turn of a Neapolitan beauty, who had many advantages. We went for a walk together and I said something like: "Neither of us is a child, and we both know perfectly well why we are on this walk together. I will speak openly; because of my profession and other circumstances, it is possible, or even probable, that I shall end by emigrating. What do you think of it? Could you adapt to life outside of Italy?" The girl was somewhat taken aback and haltingly said that she could not live far from her mother and from Naples. That ended the conversation.
There was another Italian girl, whom I had loved for over ten years, and about whom I had thought seriously many times. The trouble was that I did not succeed in conquering her heart in spite of many efforts and great sufferings on my part, and she married somebody else. I have remained her constant friend.
The first university competition I entered was sponsored by the University of Ferrara and decided on October 31, 1932. There were many competitors older and more ignorant than I. Among my contemporaries, Bruno Rossi already had a reputation for his work on cosmic rays. The judges were Quirino Majorana, Alfredo Pochettino, Carlo Somigliana, Luigi Puccianti, and Fermi, who was the only one who understood contemporary physics. Fermi, preserving the secrecy demanded of the panel, never told me what happened, but I learned about it from other sources. It seems that the majority of the committee said something like: "We have the votes to do whatever we want, no matter what you may say, Professor Fermi. We are, however, considerate, and we shall permit you to choose one candidate. We shall choose the other two." And they selected O. Specchia and C. Valle. Fermi pondered the situation and chose Rossi. I was thus not one of the three winners. Fermi did not know it, but by not selecting me, he had conferred upon me an inestimable benefit. It was a true blessing in disguise, not the last of my life.
This did not prevent me from sulking for a while. Rossi and I were virtually the same age, and our papers, although in very different fields, might have been seen as of comparable quality. It was noted that Fermi had preferred a Florentine to a Roman, and to his pupil. I am sure that Fermi's vote reflected his deeply considered evaluation of merit, and today, so many years later, I think he was probably right. At the time, however, Fermi saw that I was angry and unhappy. With a rare show of solicitude and affection, he told me that I should not be angry, that there would be other competitions, that I was young, and that what counted above all was to do good physics.
He proposed that we do some research together, which quickly cured my sullenness. We investigated hyperfine atomic structures with a view to showing that they could be completely explained by the nuclear magnetic moment, and that there were no other nuclear forces at play. The paper we subsequently co-authored contains the standard Fermi-Segrè formula.[16] During this work we labored together for hours on end, and on a couple of occasions I fell asleep out of exhaustion while Fermi was talking to me.
In the summer of 1933 I visited America for the first time. After his 1929 experiences, Rasetti had stuffed our heads with fabulous descriptions of the United States—the promised land, according to him. He declared he could not drive Italian cars anymore and imported a Ford Model A, in which we traveled extensively. Rasetti's tales persuaded Fermi to go and see for himself, taking advantage of an invitation in 1930 from the University of Michigan at Ann Arbor, which held famous summer schools on theoretical physics. Invitations were extended to young European luminaries, as well as to about thirty American students and postdoctoral fellows, and Fermi's old friend G. E. Uhlenbeck, who was on the Michigan faculty, always attended. In Rome, Fermi had reformulated P. A.M. Dirac's quantum theory of radiation in a form much easier to understand than the original papers and had made many illuminating applications of the theory. In his course at Ann Arbor, he reported on this investigation with extraordinary success. He was reinvited many times later, and whenever he could, he accepted. Fermi loved the American atmosphere, and in particular Ann Arbor and its stimulating school. In fact, he became one of its mainstays.
In 1933, Fermi suggested that I accompany him, and I gladly accepted. I first stopped at the Long Island home of G. M. Giannini, a contemporary of mine who had also studied physics in Rome. Then I moved to Ann Arbor, where I shared a room in a filthy fraternity building that had been vacated for the summer.
Both Fermi and I were eager to improve our English pronunciation, and we asked some of the students to point out where we were weakest. It seems that our pronunciation of the letter r was particularly bad. I accordingly invented the exercise "Rear Admiral Byrd wrote a report concerning his travels in the southern part of the Earth," which we declaimed at least twelve times a day, if possible in the presence of somebody who could correct us. Fermi and I bought a secondhand car from D. R. Inglis, which we named "The Flying Turtle" because of its performance, and toured the state of Michigan in it, eating very well as the paying guests of local farmers. We thus discovered delicious rural American dishes, which are difficult to obtain in the cities because they require very fresh vegetables.
To justify my presence at Ann Arbor, I tried doing some experimental work, but the humid heat of the place prevented me from working efficiently and I did not get anywhere. In trying to fit a rubber tube onto a glass tube, I badly cut my left middle finger. I went to the hospital, and as soon as he saw the wound, before I could open my mouth, the doctor said "You have been trying to fit a rubber tube to a glass one." Later I saw other victims of the same accident.
On my return to Italy, I felt that although I had spent a year learning about molecular beams, and had even performed a creditable experiment with them, I was more interested in forbidden lines, which had a special attraction for me. After the study of quadrupole radiation, it was clear to me that there were also other mechanisms producing forbidden transitions, among them the presence in a discharge of ions that created random electric fields. I devised a simple theory to explain this effect and verified it with Bakker's help by studying the Zeeman effect in a suitable case.[17]
Besides using the field produced by ions in the discharge, one could think of using an external electric field, under more controllable conditions. Bakker and a colleague named Kuhn performed this experiment, while G. C. Wick and I developed the theory. There were some reasons for thinking that the behavior of potassium and sodium would be different. Bakker had studied potassium. Amaldi and I experimented on sodium. During these experiments we observed high quantum states, corresponding to enormous orbits. I called them "swollen atoms"; today more scientifically, but less pictorially, they are called "Rydberg states."
We noted then that the foreign gases we had introduced into our absorption tubes to prevent distillation did not broaden the lines as much as we had feared, but rather, to our surprise, shifted them. We mentioned this unexpected phenomenon to Fermi, who thought about it a little and then said that it was probably because of the dielectric constant of the gas we had added to the alkali vapor. This effect had to be reckoned with, and he calculated it at once. However, for some gases the observed effect had the sign opposite to that expected. Surprised, we went back to Fermi with the puzzle. This time it took Fermi several days to come up with an additional cause of level shifts, and he wrote an important paper on the subject, which for the first time introduced the idea of what is now called a "pseudopotential." Subsequently I noted that swollen atoms should show a term in their Zeeman effect, quadratic in the applied field. In the usual theory, this term is justifiably neglected, but I experimentally showed its importance in suitable cases.[18]
The quadratic Zeeman effect, the shift of the lines, and the effect of the electric field on lines near the series limit have been extensively investigated and now form a subdivision of spectroscopy. Amaldi and I summarized our investigations in a review article that appeared in a festschrift for Zeeman's retirement.[19]
At about this time my father made an important decision that was to affect the future of everyone in our family. He was over seventy years old and was increasingly inclined to leave the management of the paper mill to Marco, by now his undisputed successor, because Angelo was busy with completely different matters and I was committed to physics. In his business, my father carefully observed developments, gave general directions, and as the sole stockholder had the final say on everything, but he left the day-to-day management to Marco, whom he tried to groom as his successor.
The paper mill had prospered substantially, but especially in later years, Father had chosen to invest profits in real estate rather than in enlarging and modernizing the mill. I do not know why; possibly there was a certain amount of tacit mistrust of Fascism and its economic policies. He also planned the disposition of his estate, which he wanted to divide into three parts of equal monetary value. He thought the mill should go to Marco, who worked there, and that Angelo and I should receive assets we could easily administer independently of Marco. In recent years I had been abroad for extended periods and had often hinted that I might emigrate. Furthermore, I had seen Nazism with my own eyes and had no illusions on the subject.
Up to 1933 I had not paid any attention to financial problems. Papà handled them with much greater ability than I could hope to muster, and I had no money of my own. I had occasionally intervened in personnel questions at the paper mill, in particular defending Bindo Rimini, who had been attacked by Marco. In this connection I once made a trip to Florence to investigate certain paper sales, and proved that Marco had blamed Bindo unjustly. Bindo remained deeply grateful to me for my help. I also introduced my friend Giovanni Ferro-Luzzi at the paper mill, hoping he might keep an eye on what was going on.
In years past, his foreign business had produced some assets outside of Italy, and my father had left them there, as was legally permitted at the time. About 1933, however, the Fascist government promulgated new laws demanding the repatriation of foreign money, with severe penalties for transgressors. These laws worried my father, and one day in the fall of that year he invited a prominent banker who was his friend and advisor to his office, along with Marco and myself, and asked for our opinions on the subject. The banker at once said something along the lines of: "You did not leave this nest egg overseas for times in which the export of assets was permitted. You left it there for times like the present. The new laws are the best proof of the importance and prudence of keeping such a reserve." He also pointed out that there was currently a flurry of capital exports on the part of industrialists, professionals, and people of means. Why should my father give up his own safety net? The argument convinced me at once. Our friend had hit the nail on the head.
Father said that at his age he wanted a quiet life. Marco signaled his own importance, saying what I expected of him: he was in the limelight and could easily be subjected to an investigation; he had a family to protect, and being such a prominent industrialist, he had to be prudent.
It was my turn to speak, and I said something like: "I am a physicist who often works abroad, and I might emigrate. I am not a person in the public eye. You may transfer the funds to my name, and I shall keep the money for the benefit of the whole family. I insist however in being the only one with access to the account." Papà agreed on the spot and told me to make the necessary provisions for disposal of the money in the event of my death. Angelo was not consulted; my mother was not present, but I am sure that my father had informed her.
To make the necessary arrangements, I needed to go to Switzerland without attracting attention, so over the Christmas vacation I went skiing near the Italian border with some friends, including the physicist Giulio Racah. We encountered foul weather, and, what with storms and avalanches, I had several close shaves. Escaping with nothing worse than a broken ski, however, I was able to enter Switzerland without seeing any police or custom official. There I transferred the money to my name and deposited with a notary a letter, to be delivered only upon presentation of my death certificate, giving access to my account. In this way we created a secret fund that was to be providential five years later when Italy started its racial persecutions.
From Switzerland I returned to Italy by train and then went to the Val Gardena to join some physicist friends who were there for a Christmas skiing vacation. Several days had passed since my adventurous crossing of the Alps, and as a result of my falls on the frozen snow, my buttocks had acquired impressive green and black spots, of hues rarely seen. I could not deprive my friends of the fun of such a sight. Later, Fermi, sitting on a bed in the small hotel room, explained to us his new theory of beta decay, as yet unpublished.
Physics in the meantime was taking an important new turn for us. For some time Fermi, and we as a consequence, had been making longrange scientific plans. Fermi felt that the golden age of atomic physics was coming to an end, and that the future lay with nuclear physics. In a letter dated September 9, 1932, he wrote to me: "I have no program for next year's work: I do not even know whether I shall start fooling around with the Wilson Cloud Chamber again, or if I shall again become a theoretician. . . . The problem of equipping the Institute for nuclear work is certainly becoming ever more urgent if we do not want to fall into a state of intellectual slumber."
My personal reaction was that we had just learned spectroscopic techniques, with which we were reaping good results, and that we might persist in that field a little longer. I was, however, open to Fermi's arguments. Amaldi and Rasetti also had their points of view, and we had long, lively discussions on the subject. As was to be expected, Fermi's ideas prevailed, although everybody was left free to do what he liked best. Thus I continued to work experimentally on spectroscopy until we started our neutron work. However, we all increased our reading on nuclear subjects. As a bridge between spectroscopy and nuclear physics, Fermi and I actively investigated hyperfine structure, as already mentioned.
Even my work in Hamburg on the dynamics of space quantization had an unexpected nuclear ramification. In the last weeks of my experiments, Otto Frisch had helped me, and Stern suggested that he sign the paper with me, to which I consented. In spite of all our efforts, we found that the experimental results did not agree with the theoretical expectation, but the experiment had been difficult, for its time, and we were able to find experimental excuses for the discrepancy. A few months later, however, we received a letter from I. I. Rabi inquiring about some experimental details we had not published. I sent them to Rabi, and he answered that the reason for the apparent disagreement between the theory and our results was that in the theoretical calculation, we had neglected the effect of nuclear spin! Had we included it, attributing to potassium a spin of 3/2, theory and experiment would have agreed. In other words, we had without knowing it measured the nuclear spin of potassium. Rabi most generously published all this in the Physical Review .[20]
By 1933 Fermi had started an intensive investigation of nuclear subjects. Amaldi organized a seminar to study Ernest Rutherford, James Chadwick, and C. D. Ellis's recent book Radiations from Radioactive Substances.[21] Soon Rasetti and Fermi started learning experimental nuclear techniques. Together they built a gamma-ray spectrograph using a bismuth crystal; then, following a model used by Lise Meitner, they designed a cloud chamber, which was built in a machine shop in Rome. All of us together built some Geiger-Müller counters that more or less functioned. Rasetti was the moving spirit in this preparatory work; he had also been working in Meitner's laboratory to learn some radiochemistry. In particular, he had learned how to prepare Po + Be neutron sources and had taught me that art. Fortunately, G. C. Trabacchi, director of the Istituto fisico della sanità pubblica, which was located in the same building as the Physics Institute, had a gram of radium for medical purposes and benevolently lent us a fraction of it, making our neutron work possible. We thus laid a respectable experimental foundation for nuclear studies.
To further enhance our readiness, Fermi used his clout as a member of the Accademia d'Italia to promote a small international nuclear physics conference, which was held in Rome in October 1931 and attended by about thirty well-chosen physicists. At the conference I had the privilege of cleaning the blackboard for Marie Curie. Regrettably, I did not do it to her satisfaction, and she told me so in no uncertain terms. The timing of the conference was unfortunate, because it was a few months before the discovery of the neutron, which opened a new era in nuclear physics.
By 1932 Fermi had already accepted an invitation to report on nuclear physics to a large international conference in Paris, and he also participated in the famous 1933 Solvay conference devoted to nuclear physics. Shortly thereafter, on his return at Rome, he invented the beta-ray theory—in his own opinion, his theoretical masterpiece. In it, developing Pauli's neutrino hypothesis, he formulated a quantitative theory of beta decay. This theory introduced the so-called weak interaction, which turned out to be a new "force of Nature," as Faraday would have said. After Fermi's death, the weak interaction revealed startling properties, such as the nonconservation of parity and, ultimately, deep relationships with electromagnetism.
Great events, however, were incubating in a different field. In February of 1934 we were stunned by the announcement of Irène and Frédéric Joliot-Curie's discovery of artificial radioactivity. By bombarding light elements with alpha particles, they had obtained new radioactive isotopes of common elements that decayed by positron emission. Fermi thought at once of the advantage of using neutrons as projectiles. Although the available neutron sources emitted many fewer neutrons than the alpha particle sources emitted alphas, the much superior efficiency of neutrons overcompensates this handicap. This is because the alphas are repelled by the nuclear charge and do not penetrate the nucleus. The neutrons on the other hand always end by penetrating a nucleus.
Thanks to the previous year's work, we had all the tools ready for testing these ideas. Rasetti, who had contributed so much to it, was in Morocco, where the king was decorating him with some order. Fermi recalled him by telegram, but he answered that he did not want to be disturbed. Fermi proceeded alone and, using a Rn + Be source, tried to form new radioactive isotopes in all elements, in order of increasing atomic number. He first succeeded with fluorine (Z = 9).
The next step was to try to activate all the elements, and to study all the radioactive isotopes formed. This formidable task was beyond the capabilities of a single person, even of a Fermi. Having struck scientific gold, he most generously invited Rasetti, Amaldi, and me to take part in its exploitation.
We were talking about the need for professional chemical help when Oscar D'Agostino showed up at the Physics Institute. He was a graduate of the department of chemistry at the University of Rome and was spending a postdoctoral fellowship in Marie Curie's laboratory in Paris learning radiochemistry. He had returned to Rome for the Easter vacation. We told him our problems, and Fermi invited him to help us. He postponed his return to Paris, and the delay extended indefinitely.
In the neutron work, each of us assumed special duties, although we collaborated on all the phases of the investigation. Fermi was the natural chief, not in the sense that he told us what to do on a detailed basis, but rather in that he set the general guidelines. If there was any problem, we talked it over together, and it is not difficult to guess whose words carried most weight. Once the program was established, each of us took responsibility for some part of it.
From the very beginning of the experiments, we saw that we needed a minimum amount of money beyond the Physics Institute's regular endowment. Fermi had good relations with the Consiglio nazionale delle ricerche. He had been its secretary for physics, and I his assistant secretary. At the present juncture, Fermi asked for help from the CNR and immediately obtained 20,000 lire (then about U.S. $1,000). I doubt whether any scientific grant has ever been more fruitful.
I was charged with procuring what we needed for our work. Luckily, there was no bureaucracy. I could carry our money in my pocket; it was not much, but I could pay cash on the barrel. With this freedom, money multiplied its purchasing power in an astounding way. For chemicals, I turned to a Signor Troccoli, an old and experienced merchant who took pride in stocking a most extensive supply of chemicals. In his youth he had studied in a seminary, and he liked to speak Latin, once in a while offering me some chemical that had been on his shelves for years "gratis et amore Dei." After I explained to him what we were doing, the worthy gentleman helped me in any way he could. However, when, in my ignorance, I asked him for a sample of masurium, he answered, "Nunquam vidi" (I have never seen it). A few years later I realized why. Masurium did not exist.
For some absorption measurements, we needed a gold ingot. I went to the firm of Staccioli, who were dealers in precious metals, and without any difficulty, on the basis of a simple receipt, they loaned me the ingot; I returned to the institute loaded with gold. I purchased necessities that could not be found at Rome through my friend Bakker in Holland.
Work proceeded rapidly. Our group reminded me of a wellrehearsed orchestra, and its conductor, Fermi, got superb music from it. We all outdid ourselves, each achieving more than any of us could have done on his own. The whole was definitely greater than the sum of its parts, Fermi included.
Our communications were published in La ricerca scientifica, the bulletin of the CNR, where Amaldi's wife worked, and it is easy to follow the daily progress of our work in it.[22] We sent reprints to wellchosen, strategically located correspondents who could read Italian, and our reports soon attracted the universal attention of nuclear physicists. Corbino kept in close touch with us through frequent visits to our laboratory.
We systematically proceeded to irradiate all the elements we could find, trying to use our sources as efficiently as possible. We prepared them once a week, because the radon they contained had a half-life of 3.82 days. The operation was delicate, but we proceeded cautiously, and nobody got hurt.
Soon we identified the two reactions, neutron capture followed by proton or alpha-particle emission, or, in the usual notation, (n,p) and (n,a ). We also found that frequently neutron bombardment produced a radioactive isotope of the target, but we did not know whether this was owing to neutron capture followed by gamma-ray emission or by emission of two neutrons: (n,g) or (n,2n). At the time we believed that the more energetic the bombarding neutron, the more efficient it would be in producing nuclear reactions. Only months later did we find out how erroneous this assumption was.
Continuing our bombardments by increasing the atomic number of the targets, we arrived at thorium and uranium.[23] The activities we could produce were weak compared with the natural activity of the targets; hence it was necessary before bombardment to remove from them the different radioactive substances they contained in radioactive equilibrium with the primary substance. This was a long and delicate operation, and the substances removed grew again after some time. We thought that in capturing a neutron, uranium and thorium would form a beta emitter that decayed into transuranic elements, for which we anticipated chemical properties similar to those of rhenium, osmium, and iridium. The nuclear processes occur, but the supposed chemical resemblance is false. We erred in the way we extrapolated the periodic system of the elements. Hahn and Meitner fell into the same trap, as did the Joliot-Curies. Only after several years was it realized that transuranic elements form a family similar to the rare earths.[24]
In 1934, at Rome, we proved that some of the activities formed in uranium bombardment were not isotopic with elements between lead and uranium, but we drew the wrong conclusion that they were transuranic. Like other investigators of this period, we noticed that the total activity produced was much larger than that of the products we were isolating, and we should have further investigated its nature. We did not seriously entertain the possibility of nuclear fission, although it had been mentioned by Ida Noddack, who sent us a reprint of her work.[25] The reason for our blindness, shared by Hahn and Meitner, the Joliot-Curies, and everybody else working on the subject, is not clear to me even today.
Transuranic elements presented a difficult experimental problem, full of pitfalls unless one had the right ideas or impeccable techniques. In fact, in December 1938, Otto Hahn and Fritz Strassmann found the solution to the puzzle through an ironclad experiment proving the formation of radioactive barium in neutron bombardment of uranium.[26]
The outpouring of work mentioned above occupied the spring of 1934. During the summer, the Physics Institute was closed, and Fermi went to South America on a lecture tour. Amaldi, his wife, and I went to the Cavendish Laboratory in Cambridge. On the way, we stopped in London to meet Fritz Paneth, from whom I wanted to learn some chemical techniques, as well as Leo Szilard, with whom we had had some correspondence and who seemed to be, in more than one way, an interesting fellow. We made an appointment for a certain time in the afternoon, but nobody showed up. Around 10 P.M. , we found Paneth with tears in his eyes and Szilard obviously shaken. We did not talk science. Hitler had attempted a coup in Austria; Chancellor Engelbert Dollfuss had been murdered, and no one knew what would happen next. Paneth was Austrian and Szilard Hungarian; the blow struck close to home. Italian mobilization, ordered by Mussolini, foiled Hitler's plan. The attempted putsch should have forced French, British, and other European politicians to open their eyes. Perhaps what they did not want to see was too ugly to contemplate. They hesitated and missed another opportunity of putting an end to Hitler.
Amaldi and I brought with us the manuscript of a paper summarizing the neutron work done in Rome, which we delivered to Lord Rutherford at the Cavendish Laboratory, begging him to communicate it to the Royal Society. He showed keen interest in our paper, took it home, and returned it the next day with some corrections to the English, saying that he was fo
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