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Seeing the Unseen

1.

Every atom is almost entirely made of empty space, with a tiny object called the nucleus and even tinier objects called electrons flying around inside it. Ernest Rutherford, a young New Zealander working in Manchester, England, discovered this fact about atoms in 1909. He shot fast particles at a thin film of gold and observed the way the particles bounced back. The pattern of the recoiling particles showed directly the internal structure of the atoms in the film. The discovery of the tiny nucleus came as a big surprise to Rutherford as well as to everybody else. The phrase “the fly in the cathedral” described what Rutherford discovered. The fly is the nucleus; the cathedral is the atom. Rutherford’s experiment showed that almost all the mass and almost all the energy of the atom was in the nucleus, although the nucleus occupied less than a trillionth part of the volume.

Rutherford’s discovery was the beginning of the science that came to be called nuclear physics. After discovering that nuclei of atoms exist, he continued to study their properties by bombarding them with fast particles and observing the results. The projectiles that he used to explore the nucleus were particles produced in the disintegration of radium. Radium is a naturally occurring radioactive metal that was discovered by Marie Curie in 1898. The particles are helium nuclei that are emitted at high speed when radium atoms decay. These particles made good probes for exploring the properties of nuclei because they were all alike and came with known energies. For twenty years, first in Manchester and later in Cambridge, Rutherford and his students and colleagues used natural particles with great success to learn how nuclei behave. They found that it was possible on rare occasions to change one kind of nucleus into another by adding or subtracting a particle. The twenty years between 1909 and 1929 were the era of table-top nuclear physics. Experiments were small enough to fit onto the tops of tables. Small and simple experiments were sufficient to establish the basic laws of nuclear physics.

Toward the end of the 1920s, nuclear physics got stuck. Major mysteries remained to be solved. Nobody knew what nuclei were made of or how their component parts were put together. But it was hard to think of exciting new experiments that could be done with the existing tools. The next round of experiments were minor variations of experiments that had already been done. It seemed unlikely that the mysteries of nuclear structure could be solved by such experiments. Rutherford announced in a public lecture in London in 1927 that new tools were needed if nuclear physics were to move ahead. Without new tools, research in nuclear physics would stagnate and bright young people would no longer be attracted to it. The most promising new tool would be a particle accelerator, an electrical machine that could produce a beam of artificially accelerated particles to replace the natural particles produced by radium. Artificially accelerated particles would be better than natural particles in three ways. They could be produced in greater quantities, they could have higher energies, and they would allow experiments to be designed more flexibly. The switch from natural sources of particles to accelerators would start a new era in the history of science, the era of accelerator physics.

The Fly in the Cathedral tells how the era of accelerator physics started. It is a dramatic story and Brian Cathcart tells it well. He is a journalist and not a scientist, but he understands enough of the science to get the details right. He has made a thorough study of the primary literature, he has read the papers and letters written by the participants, and he interviewed those of them who were still alive. The story begins with Rutherford’s decision in 1927 to explore the possibility of accelerating particles, and ends with the building of the first accelerator and its triumphant success as an atom-splitter in 1932. The era of accelerator physics began in 1932 and is not yet ended. The enormously powerful accelerators that are now exploring the fundamental forces of nature in Illinois and California and Switzerland and Japan are the direct descendants of machines that were built in 1932.

The story of the first accelerator was not only an important chapter in the history of science. It was also an international sporting event, driven by national pride as well as by scientific curiosity. Rutherford had competitors in many countries. The most formidable competitors were in the United States: Merle Tuve at the Carnegie Institution in Washington, Robert Van de Graaff at the Massachusetts Institute of Technology, and Ernest Lawrence at the University of California in Berkeley. Rutherford knew and respected his competitors but was determined to beat them. As a scientist he was a member of an international community, but as an old-fashioned New Zealander he was fiercely loyal to Britain and the Empire. He understood that science is an international enterprise that flourishes best when nations compete for the prizes.

The two men who actually built the first accelerator were John Cockcroft and Ernest Walton, graduate students working in the Cavendish Laboratory in Cambridge under the supervision of Rutherford. Cockcroft came to the Cavendish from Yorkshire in 1924, Walton from Ireland in 1927. When Walton arrived from Dublin, he proposed to Rutherford for his student project that he should start building an accelerator, not knowing that Rutherford had already announced his intention to do so. Rutherford was happy to give him the green light. Cockcroft had worked at Metropolitan-Vickers, the leading electric engineering company in Britain, before coming to Cambridge, so that he had some experience in working with heavy equipment and high voltages. Rutherford arranged for Walton to work full-time on the accelerator project, with Cockcroft working on it part-time to help with the engineering. For five years they struggled to create a technology of big machines in a laboratory of table-top experiments, just as the Wright brothers had struggled to create a technology of flying machines in a bicycle shop.

The obstacles that Cockcroft and Walton had to overcome were cultural as well as technical. If they were to build big machines, they would obviously need more space, but the idea of building a new building to house a new machine was not thinkable in the Cavendish culture. Rutherford was notoriously stingy and kept all expenditures to a minimum. The Cavendish was a historic building and could not be remodeled. So everything that Cockcroft and Walton built had to fit into existing rooms and pass through existing doors. This had the consequence that they could not use any commercial power equipment that could not be squeezed through the historic Gothic gateway of the Cavendish. They had to spend long months designing and testing their own equipment.

The culture of the Cavendish was strongly paternalistic. Rutherford took fatherly care of his students and imposed strict limits on their hours of work. Every evening at six o’clock the laboratory was closed and all work had to stop. Four times every year, the laboratory was closed for two weeks of vacation. Rutherford believed that scientists were more creative if they spent evenings relaxing with their families and enjoyed frequent holidays. He was probably right. Working under his rules, an astonishingly high proportion of his students, including Cockcroft and Walton, won Nobel Prizes.

They were maintaining the cul-ture of nineteenth-century gentlemen-scientists, who were supposed to pursue scholarly leisure-time activities in addition to their science. But this culture of short hours and plentiful leisure was hard to combine with heavy machine–building. Years went by while Cockcroft and Walton put together improvised vacuum systems, laboriously sealed the leaks, tried out various devices for handling high voltages, and found them to be inadequate.

It took Cockcroft and Walton five years to build a machine that worked. Finally, in April 1932, they had a machine that produced a steady stream of hydrogen nuclei with an energy of about half a million volts. They were carefully measuring the quality of the stream, in no hurry to begin doing nuclear experiments. On the morning of April 13, Rutherford came into their room, saw what they were doing, and lost his temper. He told them to stop wasting their time and do some science. The next day, Cockcroft was occupied with other business. Walton was alone in the laboratory, ready to do the first experiment, using his hydrogen nuclei to bombard a target made of the light metal lithium. The result was spectacular. The lithium nuclei were split in two and fell apart into pairs of helium nuclei. The helium nuclei came out with thirty times as much energy as the hydrogen nuclei going in. Walton ran to Rutherford’s office to tell him the news, and Rutherford happily spent the rest of the day serving as Walton’s assistant, checking the result and tidying up the details. That day, the era of table-top nuclear physics ended and the era of big machines and big projects began.

The American competitors were running close behind Rutherford and were beaten only by a few weeks. Van de Graaff had invented an electrostatic accelerator that was in many ways superior to the Cockcroft-Walton machine, and Lawrence had invented the cyclotron, which was in many ways better still. They were not forbidden to work after six o’clock in the evening, but they, too, had to struggle with cultural obstacles similar to those that hampered Cockcroft and Walton. American academic administrators, hard hit by the economic depression of the 1930s, were almost as stingy as Rutherford. Tuve and Van de Graaff had built a splendid machine at the Carnegie Institution in Washington, but it was standing on the grass out-of-doors, where dust and insects interfered with its operation. They were forced to dismantle it because the laboratory had no room large enough to house it.

Even Lawrence, who became in later years the main driving force of big-machine physics, had similar problems when he started to build cyclotrons. In 1931 he had acquired for his newest cyclotron a huge magnet that weighed eighty-five tons, but he had no building big enough for it and so the cyclotron remained unfinished. Van de Graaff and Lawrence were the hares, Rutherford was the tortoise, and the tortoise won the race.

In the few years that remained to him after 1932, Rutherford enjoyed the rebirth of nuclear physics brought about by the new machines. His students in the Cavendish continued to explore the universe of nuclei, in friendly competition with explorers in America and Europe. He died a year before his German friend Otto Hahn discovered the fission of the uranium nucleus in 1938. Walton returned to Dublin in 1934 and spent the rest of his life there peacefully as a professor of physics. Cockcroft stayed at the Cavendish until 1939, then moved into war research, and then became director of the British Atomic Energy Research Establishment at Harwell when it was established after the war. Harwell was mainly concerned with the development of reactors, for scientific research and for the nuclear power industry. Cockcroft showed me around Harwell when I visited there in the 1950s. The most conspicuous feature of Harwell at that time was a huge array of high-voltage cables connecting the laboratory to the national electricity grid. “The main reason the public supports us,” Cockcroft said with a smile, “is that they think the electricity is flowing out of the lab. Actually, of course, it is flowing in.”

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