449

Unit E

Nuclear and quantum physics

Introduction

In Quantum Mechanics we can only calculate probabilities as to what the future state of a system will be. Many of the familiar concepts from classical physics break down: the energy of a system in classical mechanics, for example the kinetic energy of a ball or the total energy of an orbiting satellite are continuous quantities. This is not the case in Quantum Mechanics. The energy of the hydrogen atom is discrete. It can have a possible set of values and no other. The same applies to the energy of a nucleus. The quantum world is discrete whereas the classical world is continuous.

Einstein’s explanation for the photoelectric effect required another paradigm shift: the accepted theory of light as a wave could not explain the features of the phenomenon. Einstein suggested that light consists of a large number of bundles of energy, massless particles moving at the speed of light and behaving the way particles do. Years later de Broglie would suggest that just as light that was thought to be a wave sometimes behaved as particles, particles too sometimes behave as waves. Experiments would soon verify this hypothesis and “the duality of matter” entered Physics.

At the same time our understanding of the structure of the atom increased with the discovery of the nucleus and the phenomenon of radioactivity. Most nuclei are unstable and decay by emitting energy and particles. New fundamental forces were discovered adding to the already known electromagnetic and gravitational forces. The strong nuclear force was necessary to keep nucleons bound together and the weak nuclear force was needed to explain beta decay.

Nuclear fission provided a way to harness the energy of the atom in peace and, unfortunately, war. The laws of nuclear physics applied to fusion in stars explain the creation of the elements and, at the same time, in an incredible combination of the laws of the very small with the laws of the very large predict the correct abundances of helium 4, deuterium, helium 3 and lithium 7 providing astonishing support for the Big bang model of the Universe.