The Embarrassing Managerie – Part Two
At the dawn of the 20th century, scientists believed that they stood at last on the threshold of uncovering the secrets of the atom. They were convinced that once they had discovered the fundamental constituents of the atom, they would be able to validate their conviction that the universe existed as an objective reality composed of indivisible units of matter. Yet this conviction was to be shattered within a few decades.
We have seen how Ernest Rutherford had demonstrated that the atom was like a miniature solar system, with negatively charged electrons orbiting around positively charged protons. The differences between different elements were explained as mere differences between the number and orbit of these two elemental particles. But Rutherford’s elegant theory was soon to give way to an atomic model of increasing complexity.
Rutherford himself was unable to obtain the correct results for the various atomic masses that his theory demanded. He found that the combined mass of electrons and protons did not add up to the total mass of the atom itself. To overcome this problem, he proposed that an entirely new particle was responsible for this difference. This particle would have a mass similar to the proton, but would carry a neutral electrical charge.
In 1932, an English physicist by the name of James Chadwick found that when certain light elements were bombarded with alpha particles, neutral particles similar to those proposed by Rutherford were found to be emitted. Chadwick christened these new particles “neutrons”, and for this discovery was awarded the Nobel prize for physics in 1935. The new model of the atom now consisted of protons, electrons and neutrons.
By the early 1930’s, scientists felt confident that they were at last close to a complete understanding of the universe. The only major question that remained to be answered was the way in which the nucleons (protons, electrons and neutrons) were held together inside the nucleus of the atom. Unfortunately, scientists found that the force that held these nucleons together was so strong, that the alpha particles generated by radioactivity were not sufficient to break these bonds. They needed to find other particles that exerted greater energy.
Rutherford was again instrumental in solving this problem. He foresaw that any device that was capable of accelerating the speed of known particles would be as useful a tool for the nuclear physicist, as the telescope had become for the astronomer. The larger the accelerator, and the faster the particles that could be derived, the more powerful would be the beam of charged particles that could be used to split the atom. The challenge was to find some way in which protons could be made to act like projectiles.
in 1932 two British physicists Cockroft and Walton designed the world’s first artificial accelerator. Their work was soon matched by a Princeton scientist named Van de Graaff, who constructed an accelerator based on the principle of electrostatics. As the assault on the atom gained momentum, demands were made for even faster particles, leading to the construction of new improved accelerators. It was the California physicist E.O. Lawrence who devised an accelerator based on circular motion known as a cyclotron.
The purpose of all these accelerators was identical. It was to enable scientists to increase the speed, and therefore the energy, of sub-nuclear particles. These energized particles could then be aimed at atoms or other particles, and from these collisions, scientists could study the energy and nature of the resulting particles. But before they could study these results, scientists first needed to find a way to observe and record these experiments.
The problem of detection was solved by an English physicist named Wilson, who invented a device which became known as a cloud chamber. The Wilson cloud chamber made use of the fact that electrically-charged particles moving through the air at high speed produced a phenomenon known as ionization in their wake. When these particles passed through a chamber saturated with water vapor, they ionized the air along their paths.
Although it was not possible to see the sub-nuclear particles themselves, it was possible to observe the thin foggy tracks which told of their passing. It was found that if an intense magnetic field was created within the cloud chamber itself, then particles carrying an electrical charge would be deflected into curving paths. By analyzing the curvature of these paths, it became possible for scientists to calculate the energy and charge of the particles passing through the cloud chamber.
The development of these powerful atomic accelerators at last provided scientists with the means to penetrate the nucleus of the atom, and to investigate the force that held these nucleons together. But far from clarifying their understanding, scientists found that the results of their atom smashing experiments led to more and more confusing results.
Whereas they began these experiments believing that the atom contained just three nucleons, they were soon to discover an embarrassing managerie of particles that exceeded all their expectations. Not only were more and more new particles being discovered, but they seemed to act in ways that defied all understanding, causing the Danish pioneer of atomic structure Niels Bohr, himself a Nobel prize winner, to remark:
“Those who are not shocked when they first come across quantum theory, cannot possibly have understood it.”
