Toward the end of the nineteenth century, experimental evidence summarized in this Topic showed that classical mechanics, which was very successful at explaining the motion of everyday objects such as pendulums and projectiles, failed when it was applied to individual atoms, nuclei, and electrons. Quantum mechanics was devised to take its place and became the basis for a broader framework—quantum theory—for describing radiation and matter.
Toward the end of the nineteenth century, experimental evidence summarized in this Topic showed that classical mechanics, which was very successful at explaining the motion of everyday objects such as pendulums and projectiles, failed when it was applied to individual atoms, nuclei, and electrons. Quantum mechanics was devised to take its place and became the basis for a broader framework—quantum theory—for describing radiation and matter.
Abstract A scientist in the late nineteenth century could be forgiven for thinking that the major elements of physics were built on unshakeable foundations and effectively established for all time. The inspirational work of Galileo Galilei and Isaac Newton in the seventeenth century had been shaped by a further 200 years of theoretical and, in particular, experimental science into a marvellous construction which we now call classical physics. This physics appeared to explain almost every aspect of the physical world: the dynamics of moving objects, thermodynamics, optics, electricity, magnetism, gravitation, etc. So closely did theory agree with and explain experimental observations of the everyday world that there could be no doubt about its basic correctness- its essential ‘truth’. Admittedly, there were a few remaining problems but these seemed to be trivial compared with the fundamentals - a matter of dotting a few is and crossing some ts.
Toward the end of the nineteenth century, it became clear that some experiments cannot be explained on the basis of known physics, now referred to as classical physics. In 1900, Max Planck was able to correctly describe the distribution of blackbody radiation energy as a function of wavelength λ or frequency ν = c/λ. He had to accept the existence of energy packets (quanta) with energy E = hν. A few years later, Albert Einstein found that this unexpected relation between energy and frequency is also necessary in order to understand photoemission from metallic surfaces. A decade later, Niels Bohr arrived at the correct equation for the electronic spectrum of the hydrogen atom by applying “quantum conditions” on the motion of the electron around the nucleus and the Planck’s relation E = hν on the energy difference between the energy levels (1916). A decade later, Heisenberg, Born, and Jordan introduced “quantum mechanics” and de Broglie and Schrödinger (1925-1926) introduced “wave mechanics.”
Quantum mechanics was developed as a response to the inability of the classical theories of mechanics and electromagnetism to provide a satisfactory explanation of some of the properties of electromagnetic radiation and of atomic structure. As a result, a theory has emerged whose basic principles can be used to explain not only the structure and properties of atoms, molecules and solids, but also those of nuclei and of “elementary” particles such as the proton and neutron. Although there are still many features of the physics of such systems that are not fully understood, there are presently no indications that the fundamental ideas of quantum mechanics are incorrect. In order to achieve this success, quantum mechanics has been built on a foundation that contains a number of concepts that are fundamentally different from those of classical physics and which have radically altered our view of the way the physical universe operates. This book aims to elucidate and discuss the conceptual basis of the subject as well as explaining its success in describing the behaviour of atomic and subatomic systems.
This chapter looks at the experimental evidence towards the end of the nineteenth century that showed that classical mechanics failed when it was applied to very small particles and when the transfers of energy were very small. It discusses classical mechanics and shows how it predicts a precise trajectory for particles, with specified locations and moments at each instant. It also describes experiments which have showed that energy was transferred only in discrete amounts and showed that electromagnetic radiation behaves like a stream of particles. The chapter analyzes how electrons, which had been supposed to be particles since their discovery in 1897, can behave like waves. It reviews the translational, rotational, and vibrational modes of motion that are excited to any energy simply by controlling the forces that are applied.
Ever since the advent of quantum mechanics, it has been clear that the atoms composing matter do not obey Newton's laws. Instead, their behavior is described by the Schroedinger equation. Surprisingly though, until recently, no clear explanation was given for why everyday objects, which are merely collections of atoms, are observed to obey Newton's laws. It would seem that, if quantum mechanics explains all the properties of atoms accurately, they, too, should obey quantum mechanics. This reasoning led some scientists to believe in a distinct macroscopic, or ``big and complicated,'' world in which quantum mechanics fails and classical mechanics takes over, although there has never been experimental evidence for such a failure. Even those who insisted that Newtonian mechanics would somehow emerge from the underlying quantum mechanics as the system became increasingly macroscopic were hindered by the lack of adequate experimental and theoretical tools. In the last decade, however, this quantum-to-classical transition has become accessible to experimental study and quantitative description, and the resulting insights are the subject of this article.
Science has developed through a variety of investigations more or less over the time scale of human existence. On this scale, quantum mechanics is an extremely young field, existing essentially for only just over a century. Even our understanding of classical mechanics has existed for a comparatively long period—roughly having been formalized with Newton's equations published in his Principia Mathematica , in April 1686 (1687). Up until the very end of the nineteenth century, mechanics in the form of Newton's laws were comfortable, being understood and accepted by all who dealt with the motion of reasonable bodies. With the turn of the new century, however, mechanics would be shaken to its roots. The change began already on December 14, 1900, when Planck (1900) presented his new theory of black-body radiation to the German Physical Society, and this change would affect mechanics in physically small structures (and lead to quantum mechanics). Mechanics of large bodies would similarly be changed in 1905 with the arrival of Albert Einstein's (1905a) relativity theory. In this book, we will deal with only the first of these two great breakthroughs.