Bodies in motion: Enrico Fermi and Niels Bohr stroll along Rome’s Appian Way discussing, perhaps, the mysteries of the nucleus.
Fermi’s most notable work as an experimentalist also began around late 1933, when he realized that the recently discovered neutron provided the means for a new kind of projectile in Rutherford-type experiments. Protons or alpha particles, used heretofore, had to have relatively high energies to penetrate a nucleus, since the electrically charged target and projectile repelled each other. A neutron, on the other hand–even a slow one–could freely make its way into a nucleus, because it was without electric charge. Fermi, his old friend Franco Rasetti, and a few assistants–including, of course, Amaldi and my uncle–quickly began a multiyear study of such reactions, yielding many new and important discoveries about how nuclei behave.
The enterprise came to an end when Italy adopted racial laws in 1938. Fermi, realizing that his family was endangered (his wife, Laura, was Jewish), left for the United States in December of 1938–departing from Stockholm, where he had just received a Nobel Prize in recognition of his pioneering work with neutron scattering. His group at the University of Rome dissolved.
As Fermi was leaving Italy, two German chemists, Otto Hahn and Fritz Strassmann, found a curious result when they bombarded uranium with neutrons. Lise Meitner, a longtime collaborator of Hahn’s who had been forced to flee Germany a few months earlier because she was Jewish, helped explain what would prove to be a crucial discovery. During a walk on Christmas Eve of that year, she and her physicist nephew Otto Frisch (family again) realized that uranium nuclei had probably been split into two pieces as they absorbed a neutron, a process that would necessarily lead to a large release of energy. Two weeks later, Frisch coined the term nuclear fission to describe what had happened in the Hahn-Strassmann experiment. It also became clear that if additional neutrons were released during fission, a chain reaction could occur. The first controlled such event, guided by Fermi, took place in a squash court at the University of Chicago in December 1942. Soon after that, Fermi and many others, including my uncle, moved to Los Alamos to work on developing a much bigger chain reaction: the atom bomb.
My family left Italy soon after the Fermis did–in our case, supposedly to visit the 1939 World’s Fair in New York. I was then only seven months old. My seven-year-old brother might benefit from the experience, the U.S. consul in Florence politely suggested when we applied for visas, but wasn’t I a little young? My father replied that Jewish children were now becoming interested in such events at a very early age. Fortunately, the consul–knowing full well our intention of staying in the United States if at all possible–had a sense of humor and an abundant dose of charity.
Emilio left Italy in the summer of 1938. His exodus took him to Berkeley, CA, a place he had visited in the summer two years earlier. He was gratefully discovering that the physics family was growing rapidly across the Atlantic and welcoming refugees from Europe. Ernest Lawrence’s Radiation Laboratory at Berkeley was in the process of replacing Cambridge’s Cavendish as the world’s great nuclear-physics lab. In some ways Lawrence was just as much a pioneer as Rutherford. One grew up in New Zealand as the child of immigrants and attended local Canterbury College. The other, a grandchild of immigrants, was raised in South Dakota and studied at his state university. Both were forceful and effective leaders in later life, but their aims and styles were different. Rutherford liked inexpensive experiments that could fit on a bench. Lawrence, an enthusiastic fund-raiser and entrepreneur, was interested in building bigger and better cyclotrons, machines capable of accelerating particles to much higher energies than anything that could be achieved at Rutherford’s laboratory. In realizing his dream, Lawrence made ample use of America’s ingenuity and its new economic power.
Rutherford believed in building the apparatus that the physics required. But Lawrence’s philosophy was different: build machines, he thought, and the physics would follow. This was a decisive point in the arc of 20th-century physics. It would no longer be possible for a few individuals to simply set out and collect the necessary tools for an experiment they planned. The era of big physics had begun.