From a historical point of view, a distinction can be drawn between ‘old quantum theory’ and the full quantum theory that we now use. Old quantum theory was in essence a patchwork, cobbled together by incorporating some quantum ideas into essentially classical physics. The work of Planck, Einstein, Bohr, and Sommerfeld can be bracketed as old quantum theory. This classification is not meant to undermine the tremendous achievement that old quantum theory represents, but simply to underline its standing as a stage on the way to a fully formed quantum theory, which emerged from the work of Heisenberg, Schrodinger, Born, and Dirac. The development of the photon concept is a key example of a radically new idea gradually gaining acceptance. The process works via a growing realization that more and more things can be explained in the context of the photon model.
From a historical point of view, a distinction can be drawn between ‘old quantum theory’ and the full quantum theory that we now use. Old quantum theory was in essence a patchwork, cobbled together by incorporating some quantum ideas into essentially classical physics. The work of Planck, Einstein, Bohr, and Sommerfeld can be bracketed as old quantum theory. This classification is not meant to undermine the tremendous achievement that old quantum theory represents, but simply to underline its standing as a stage on the way to a fully formed quantum theory, which emerged from the work of Heisenberg, Schrodinger, Born, and Dirac. The development of the photon concept is a key example of a radically new idea gradually gaining acceptance. The process works via a growing realization that more and more things can be explained in the context of the photon model.
Now that we have reached the end of our brief tour through the early history of quantum theory, we need to stop and consider some important themes. From a historical point of view, a distinction can be drawn between ‘old quantum theory’ and the full quantum theory that we now use. Old quantum theory was in essence a patchwork cobbled together from classical physics with a few quantum ideas thrown in. e works of Planck, Einstein, Bohr, and Sommerfeld can be classed as old quantum theory. is is not meant to undermine the tremendous achievements that old quantum theory represents, but simply to underline its standing as a stage on the way to a fully formed quantum theory, which slowly emerged from the works of Heisenberg, Schrödinger, Born, and Dirac.
Quantum theory has passed through three distinct stages during its evolution. The first is generally referred to as ‘old quantum theory’. During a period roughly between 1900 and 1925 Planck, Bohr and Einstein, among others, grappled with various experimental and theoretical crises by patching up classical mechanics with some new quantum ideas applied as band-aids. This is not to disparage the work; it was vitally important, and you can only do what is possible at the time. The quantum world is very different to the picture painted by classical physics. That being the case and given the extreme nature of some of these differences, we might question the wisdom of accepting the quantum view. Scientists can be quite disparaging about philosophy, sometimes while in the process of espousing, quite stridently, a philosophical position of their own. The chapter also presents an overview on the key concepts discussed in this book.
As expected when I started out on this project, this has proved to be a complex and, at times, difficult story. After all, what was involved was tearing up the foundations of classical physics, which had been extraordinarily successful in explaining the macroscopic world about us, and replacing it by something radically different and non-intuitive in terms of our everyday experience. But the effort involved has been more than repaid by the very much deeper appreciation I have gained of the extraordinary works of the pioneers of quantum mechanics, both the theorists and the experimenters. If the brilliant theoretical researches of Planck, Einstein, Bohr, Heisenberg, Born, Jordan, Schrödinger, Pauli, Dirac and many others form the central core of this story, it should be remembered that their researches were inspired by the equally brilliant achievements of experimental physics. Another huge bonus has been a deepened understanding of quantum mechanics itself – if only I had these insights more than 50 years ago when I first encountered the subject. There is a great deal more that could be said. I must reiterate that I have presented a somewhat streamlined version of the story in order to ensure that there is some continuous pathway, however tortuous, to the way in which the new understandings came about. For a full appreciation of the complexity of the story and the numerous blind alleys and diversions which took place, there is no substitute for in-depth absorption in Mehra and Rechenberg's magisterial exposition of the history of quantum theory.
The founding fathers of quantum mechanics had already perceived the essence of the difficulties of quantum mechanics; today, after almost a century, the discussions are still lively and, if some very interesting new aspects have emerged, at a deeper level the questions have not changed so much. What is more recent, nevertheless, is a general change of attitude among physicists: until about 1970 or 1980, most physicists thought that the essential questions had been settled, and that “Bohr was right and proved his opponents to be wrong”. This was probably a consequence of the famous discussions between Bohr, Einstein, Schrödinger, Heisenberg, Pauli, de Broglie, and others (in particular at the Solvay meetings [1–3], where Bohr's point of view had successfully resisted Einstein's extremely clever attacks). The majority of physicists did not know the details of the arguments. They nevertheless thought that the standard “Copenhagen interpretation” had clearly emerged from the infancy of quantum mechanics as the only sensible attitude for good scientists. This interpretation includes the idea that modern physics must contain indeterminacy as an essential ingredient: it is fundamentally impossible to predict the outcome of single microscopical events; it is impossible to go beyond the formalism of the wave function (or its generalization, the state vector ∣ψ) and complete it.
‘Quantum mechanics is very impressive. But an inner voice tells me that it is not yet the real thing. The theory produces a good deal but hardly brings us closer to the secret of the Old One’. These views were contained in a letter written by Albert Einstein to Max Born in December 1926. Though the following year would see the completion of work on the theoretical foundation of the modern physics of atoms and particles, Einstein was never to change his judgement and remained firmly convinced that ‘this business […] contains some unreasonableness’. He was unwilling to sacrifice his own ideas as to the cognitive scope of science even in the face of the important results being achieved by the new theory. To the undeterministic findings of quantum mechanics he opposed his belief in the ‘possibility of giving a model of reality, a theory, that is to say, which shall represent events themselves and not merely the probability of their occurrence’. For this reason he chose to live with the isolation of his scepticism and dissent, ironically accepting the reputation as a obstinate heretic that he had won over the years among his colleagues, and worked to the end on his own research programme to develop a unified field theory of rigorously causal nature. As is generally known, Einstein carried on a strong scientific and philosophical dispute with the defenders of the so-called official interpretation of quantum mechanics.