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…


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