The Quantum WorldThe Quantum World
John Polkinghorne

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This compact volume presents quantum mechanics for the genreal reader. It offers a lucid description fo the intellectual challenges and disagreements in the study of the behavior of atomic and sub-atomic particles-a field that has completely changed our view of the physical world, but that is still the subject of unresolved debate about its fundamental interpretation.

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From the preface:
The physics of the twentieth century is divided from all that which came before by two great discoveries which have transformed our view of the natural world. One is Einstein's theory of special relativity; the other is quantum mechanics. There is no doubt in my mind that quantum mechanics is much the more revolutionary of the two. Although Einstein profoundly changed our understanding of the nature of time and the meaning of simultaneity, there is a sense in which his work is the last great flowering of the classic tradition in physics. It preserved the clarity of description and the inexorable determinism which had been the hallmarks of mechanics since Newton. Quantum mechanics, on the other hand, abolished clear-cut trajectories and introduced a probabilistic fitfulness into nature. The resulting elusive quality bestowed on physical reality has been the subject of much confident assertion. 'Modern physics' has been called in aid of all sorts of mutually inconsistent philosophical positions. To tell the truth, even the professionals have not been immune from confusion. Unresolved matters of disputed interpretation remain in quantum theory even after almost sixty years of successful exploitation. This book is not written to convey hard and fast answers to all such questions. Its purpose is to try to indicate what is agreed and to circumscribe the areas of debate in which men of experience and ability still take differing views.

Quantum theory is arguably the great cultural achievement of our century. It is too important to be the preserve and pleasure of professionals alone. I have tried to write in a way accessible to any person of reasonable intelligence and persistence who is prepared to take the trouble to look into the matter. Accordingly I have eschewed the use of mathematics beyond the ability to read a simple equation (though I have provided some meatier fare in an Appendix for those with stronger stomachs). To do so has only made things easier in one sense. In another sense I have made things harder by denying myself of the use of that natural language which is perfectly adapted to the discussion of such matters. This has to be compensated for by a willingness to simplify at times. I believe that what I write is accurate for the purposes intended but I do not suppose that a professional could not sometimes tighten up its expression, even if his pedantry would be likely to war against the general accessibility of the argument.
A greater hazard is provided by the fact that one can scarcely write about such matters of import for an understanding of physical reality without venturing at times to make remarks of a kind which might charitably be termed 'philosophical'. I am aware that my offerings of this sort can scarcely hope to be viewed by a metaphysician as other than jejune. I am not wholly apologetic, however, as I feel that when leather is under discussion, cobblers deserve a hearing. I write, not as a philosopher of science, but as a specimen physicist. To change the metaphor, I am a dinosaur and not a palaeontologist. I suppose it is possible that a live brontosaur might have a thing or two to tell the students of the fossil record.

A word is necessary about the way I have decided to approach the subject. After what I trust is a gentle historical introduction, I have chosen in Chapter 3 to take the plunge into deep waters. An account, inescapably demanding if mathematically innocent, is given of the principles of quantum mechanics as the professionals use them every day. Only after that do I deal with topics like the uncertainty principle, the two-slits experiment, Schrödinger's cat and the like, which others might have chosen to use as the means of dabbling their readers' toes in the shallow end. My defence of this procedure is that the initial effort required is more than adequately compensated for by the enhanced insight with which the reader will then be able to approach these subsequent illustrations of the odd character of the quantum world. Understanding, rather than mere intellectual titillation, is what we are aiming for and I believe that this is best attained by tackling the difficult basic material first. This book may be a 'physics without calculus' but it is not intended to be just a gee-whiz tour of the Strange But True.

I am grateful to Dr J. S. Bell, FRS., Professor T. W. B. Kibble, FRS., and to my son Michael, for reading the manuscript and making useful suggestions. I am also grateful to Dr Michael Rodgers for his skill as an editor and for his help in improving the manuscript.