I. The year 1927 is a landmark in the evolution of physics — the year which saw the obsequies of the notion of causality. To avoid misconceptions, it should not be thought that the concept fell a victim to the unbridled antipathy of certain physicists or their indulgence in fancies. The truth is that men of science came, very reluctantly and almost against their will, to recognize the impossibility of giving a coherent causal description of the happenings on the atomic scale, though some of them — curiously enough, amongst them Planck, Einstein, de Broglie, Schrodinger — could never bring themselves to accept wholeheartedly so drastic a renunciation of classical ideals. These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.
I. The year 1927 is a landmark in the evolution of physics — the year which saw the obsequies of the notion of causality. To avoid misconceptions, it should not be thought that the concept fell a victim to the unbridled antipathy of certain physicists or their indulgence in fancies. The truth is that men of science came, very reluctantly and almost against their will, to recognize the impossibility of giving a coherent causal description of the happenings on the atomic scale, though some of them — curiously enough, amongst them Planck, Einstein, de Broglie, Schrodinger — could never bring themselves to accept wholeheartedly so drastic a renunciation of classical ideals. These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.
The measurement problem of quantum mechanics was probably born in 1926: ‘Thus Schrödinger’s quantum mechanics gives a very definite answer to the question of the outcome of a collision; however, this does not involve any causal relationship. One obtains no answer to the question “what is the state after the collision,” but only to the question “how probable is a specific outcome of the collision” (in which the quantum-mechanical law of [conservation of] energy must of course be satisfied). This raises the entire problem of determinism. From the standpoint of our quantum mechanics, there is no quantity that could causally establish the outcome of a collision in each individual case; however, so far we are not aware of any experimental clue to the effect that there are internal properties of atoms that enforce some particular outcome. Should we hope to discover such properties that determine individual outcomes later (perhaps phases of the internal atomic motions)? Or should we believe that the agreement between theory and experiment concerning our inability to give conditions for a causal course of events is some pre-established harmony that is based on the non-existence of such conditions? I myself tend to relinquish determinism in the atomic world. But this is [also] a philosophical question, for which physical arguments alone are not decisive.’ (Born, 1926a, p. 866; translation by the author) These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.
Although many of the solitons described in previous chapters were discovered and studied in the 20th century, they could have been discovered and understood by physicists of the previous century. What I have in mind, of course, is the distinction between classical (19th century) and quantum physics (20th century). It happened that the quantum revolution in physics started at the frontier of the two centuries. At the end of 1900 the German physicist Max Planck (1858–1947) boldly introduced the notion of quanta of energy in the processes of emission of light by atoms. He also introduced a new fundamental constant in physics, the Planck constant h . The significance of this revolutionary step was not recognized until much later. Planck himself did not realize that he had started one of the greatest revolutions in physics. These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.
In a recent number of Blackfriars (July, 1949) Dr A. W. Gledhill has discussed very competently some aspects of the statistical method as used in modern physics. He has tried to show there ‘that modern physical theory does not contradict the principle of causality’. Having made a study of this subject over quite a number of years, the writer, while agreeing with some of Dr Gledhill’s conclusions, feels nevertheless that there are a few points which have not been covered as adequately as the importance of the subject for modern philosophy seems to warrant. In order to understand the controversy about the validity or non-validity of the principle of causality, which has been initiated by Heisenberg’s uncertainty principle, it is necessary to realise, first of all, that when a physicist speaks about causality, he usually does not mean the same thing as a scholastic philosopher. In scholastic philosophy one distinguishes different kinds of causes, such as the material, the efficient, the final, as well as the formal and exemplary causes. The classical physicist, on the other hand, since the time of Galileo, admits only efficient causes. Let us therefore give a brief sketch of the development of this concept of scientific causality and show the reasons for this limitation. The purpose of a scientific theory is the prediction of events among the natural objects which form the field of enquiry of the particular science concerned. Evidently, the more certain the prediction, the better the theory on which it is based.
Quantum mechanics has played a significant role in the development of various disciplines of physics. It was propounded in 1925 and has reigned supreme ever since, extending its domain over the years; of course, in its own domain it has always been in excellent agreement with experiments. It has long since become the language of physics and anyone who tries to understand the basic principles of physics without having a grasp of this subject is doomed to fade away in the darkness of ignorance. In this book an attempt has been made to provide a logical, lucid and user-friendly treatment of this elegant and fascinating subject.
In 1945, Schrödinger was 58, Bohr was 60, and Einstein was past retirement age. Heisenberg, Pauli, Jordan, and Dirac were in their 40s. With the end of the war, another crop of bright young physicists would soon be emerging from the universities. Would they accept the revolutionaries’ radical positivism — or follow Einstein and seek to restore realism in physics? These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.
Where did we get that [equation] from? Nowhere. It is not possible to derive it from anything you know. It came out of the mind of Schrödinger. Richard Feynman De Broglie's matter waves The early struggles of physicists towards a quantum theory were mostly concerned with attempts to understand the nature of light. The traditional picture of light as a wave motion had been challenged by Planck and Einstein. They had shown that certain experimental results that were impossible to understand in terms of a wave picture could be easily explained if light was thought of as a stream of particles, now called photons. William Bragg, who, with his son, won the 1915 Nobel Prize for studies of crystal structure using X-rays, summarized this dilemma for physics by exclaiming in despair that he was teaching the corpuscular theory of light on Mondays, Wednesdays and Fridays, and the undulatory theory on Tuesdays, Thursdays and Saturdays! Physicists were still wrestling with these apparently contradictory properties of light when, in 1924, Prince Louis de Broglie (pronounced ‘de Broy’) suggested that all matter, even objects that we usually think of as particles – such as electrons – should also display wavelike behaviour! This revolutionary idea was completely unexpected and, what is more, was included by de Broglie in his Ph. D. thesis. Like most people, physicists are generally reluctant to accept any wild new idea, especially if there is not a shred of evidence to support it.