Nuclear Physics and Professor Ernest Rutherford

Abstract

At the end of the nineteenth century, physics had some serious problems. Scientists differed on whether atoms existed or not. Some believed that matter could be divided indefinitely, as seems to be the situation with a jelly, in contrast to a bag of ball bearings. In part, this belief of the possible division of a piece of matter repeatedly and without limit arose because of the success of calculus, where a distance can apparently be divided for ever into smaller and smaller and smaller intervals. The relation between the then current theory of motion (part of the “classical theory”) and recent discoveries about electricity and magnetism was uncertain. It was unknown how all sorts of radiation passed through space. Some mysterious and invisible substance called the “aether” was believed to fill the universe to allow passage of all this radiation, for example to allow the sun’s light and heat to reach the earth. But the aether’s existence gave rise to many difficulties. Another serious problem was picturesquely called the ultra-violet catastrophe. It concerned the heat and other sorts of radiation which came from a hot black body. The then current theory (the classical theory) predicted that the amount of radiation emitted increased rapidly in quantity as it varied between heat (infra-red) visible light and ultra-violet. But measurements showed this was not true. The observed amounts of radiation did not increase steadily in this way. They first increased then decreased, as the radiation varied between these different types. Many of these problems were solved by some revolutionary advances. These advances arose from extremely original thinking which jettisoned the classical ideas that had held sway for centuries. They were parts of the new theories of Planck, Bohr, Einstein and others. Max Planck in 1900 suggested that radiation was emitted in little packets (called quanta, from the plural of the Latin word quantum, meaning “how much”) and not as a continuous wave. This was the beginning of the quantum theory. Albert Einstein proposed his special theory of relativity and other novel mechanisms in 1905, which managed to clarify difficulties in the understanding of space and time and especially the aether, which then became a totally unnecessary concept. Planck showed that if quanta were emitted by a hot body, the amounts produced of different colours of light and other radiations could be explained and predicted, without any ultra-violet catastrophe. In a way that correlated with observations, Einstein explained how certain quanta could also be absorbed, for the previous classical theory was inadequate for this. So the introduction of quanta allowed both the emissions from hot bodies and some absorptions of ultra-violet (and the photoelectric effect) to be understood. Otherwise, both were inexplicable. Niels Bohr applied quantum ideas to certain atoms and could explain the light emitted when an atom was excited. For a while Rutherford resisted accepting the new quantum theory, but experiments performed by one of his colleagues, and outlined below, finally convinced him of its value.