My family: Uncle Emilio (right) and my future brother-in-law Victor Weisskopf discuss the CERN accelerator’s progress.
CERN’s climb to success was not easy, nor was the United States standing still. With a 1968 ground-breaking for the National Accelerator Laboratory (now renamed Fermilab), U.S. physicists were planning a machine capable of accelerating protons to almost 10 times the energy reached at Brookhaven, a level competitive with anything Europe would achieve. But CERN persevered, and within a decade of my arrival, it announced its first truly major discovery. There is an old adage in physics that “yesterday’s discoveries are today’s tools and tomorrow’s background events.” In 1933 physicists were quite sure they would never be able to detect a neutrino being scattered by another particle. By 1973 CERN had a neutrino beam that made it possible to study the details of a newly identified force that acted on these particles and on electrons and protons. This was a breakthrough. It would be another decade before CERN would announce its triumphal sighting of the particle that mediated this force, commonly called the Z boson. Its discovery was another example of the back-and-forth between theory and experiment that has characterized the whole century-long arc. Theorists had predicted that the Z boson would be 90 times as massive as a proton; consequently, it would not be directly observed until machines were capable of reaching the energies necessary for its production. When the Z turned up in 1983, with the predicted mass, the discovery became one of the cornerstones in the establishment of what has come to be called the standard model of particle physics. The long journey begun in 1909 had now reached a summit. The atom’s constituents and the nature of all the forces between them finally seemed to have been identified.
By then I had long since left CERN. In the summer of 1965 I began a two-year postdoctoral appointment at Berkeley, still a power in the world of high-energy physics, even if its impact was not quite what it had once been. A side benefit of this stay in California was getting to know my uncle Emilio, now a senior professor at the university. I had seen very little of him while growing up because my father and he always seemed to be at odds. Emilio, never known as an easy person to get along with, described their relationship this way in his autobiography: “My patience and tolerance derived in part from a certain regard I felt for Angelo’s keen intellect, and in part because in several respects I felt that I to some extent resembled him.” My own view was that, despite Emilio’s claims of patience and tolerance with my father, neither of them was a paragon of such virtues. However, spending time with my uncle, now near retirement, was a great window into the evolution of 20th-century physics and, with no sibling rivalry in play, an altogether pleasurable family experience.
My father was a historian who wanted to be a scientist. I now saw Emilio turning to history, in part to examine the scientific events he had witnessed and in part to describe the extraordinary people he had met and sometimes worked with. Within several years he had written From X-Rays to Quarks, an engrossing history of a hundred years of physics as observed by a participant. I read the book (in its original Italian version) when it appeared in 1976, but I was too involved with the day-to-day events of establishing my own career to give it much thought. By then a professor at the University of Pennsylvania and deeply involved in the mysteries of the standard model, I was back at CERN for a year on a Guggenheim fellowship. Europeans were now beginning to talk about building a new kind of particle accelerator: one that would produce very high-energy electron-positron collisions, envisioned as ideal for studying Z meson decay. The Z had of course not yet been observed experimentally, but its discovery was anticipated on theoretical grounds, and planning had to start right away, since it took many years to construct a large accelerator. This was now the way high-energy physics operated: build for the expected and the unexpected. The LEP (Large Electron-Positron Collider) was formally approved in 1981. Construction began in 1983 and finished in a little over five years; by the end of the 1980s, the LEP, also known as the Z Factory, was working marvelously.
The United States now needed to act if it wished to remain competitive. In 1993, Congress canceled the U.S. physics community’s response, the Superconducting Super Collider, after a $2 billion initial expenditure. With that move, it was clear that the balance of power was shifting to Europe. Thirty years earlier, I had returned to a Europe envious of America’s success in building particle accelerators. It was now America’s turn to be envious. The completion of the Large Hadron Collider has simply underlined this shift, though I do wish to emphasize that the collider’s European location does not mean the end of U.S. participation in operating such machines. In an era of global scientific coöperation, one finds people from all countries involved in ensuring the success of experiments at the great accelerators. In fact, the field has rightly claimed to be a model of international collaboration.