Chapter One Reality in the Quantum World 1.1 The quantum revolutions Quantum mechanics, created early this century in response to certain experimental facts which were inexplicable according to previously held ideas (conveniently summarised by the title ‘classical physics’), caused three great revolutions. In the first place it opened up a completely new range of phenomena to which the methods of physics could be applied: the properties of atoms and molecules, the complex world of chemical interactions, previously regarded as things given from outside science, became calculable in terms of a few fixed parameters. The effect of this revolution has continued successfully through the physics of atomic nuclei, of radioactivity and nuclear reactions, of solid-state properties, to recent spectacular progress in the study of elementary particles. In consequence all sciences, from cosmology to biology, are, at their most fundamental level, branches of physics.
Through physics they can, at least in principle, be understood. Indeed, on contemplating the success of physics, it is easy to be seduced into the belief that ‘everything’ is physics-a belief that, if it is intended to imply that everything is understood, is certainly false, since, as we shall see, the very foundation of contemporary theoretical physics is mysterious and incomprehensible. The second revolution was the apparent breakdown of deter- minism, which had always been an unquestioned ingredient and an inescapable prediction of classical physics. Note that we are using www.com 2 Reality in the quantum world the word ‘determinism’ solely with regard to physical systems, without at this stage worrying about which systems can be so described; that is, we are not here concerned with such concepts as free will.
In a deterministic theory the future behaviour of an isolated physical system is uniquely determined by its present state. If, however, the world is correctly described by quantum theory, then, even for simple systems, this deterministic property is not valid. The outcome of any particular experiment is not, even in principle, predictable, but is chosen at random from a set of possibilities; all that can be predicted is the probability of particular results when the experiment is repeated many times. It is important to realise that the probability aspects that enter here do so for a dif- ferent reason than, for example, in the tossing of a coin, or throw of a dice, or a horse race; in these cases they enter because of our lack of precise knowledge of the orginal state of the system, whereas in quantum theory, even if we had complete knowledge of the initial state, the outcome would still only be given as a probability.
Naturally, physicists were reluctant to accept this breakdown of a cherished dogma-Einstein’s objection to the idea of God playing dice with the universe is the most familiar expression of this reluctance-and it was suggested that the apparent failure of deter- minism in the theory was due to an incompleteness in the descrip- tion of the system. Many attempts to remedy this incompleteness, by introducing what are referred to as ‘hidden variables’, have been made. These attempts will form an important part of our later discussion. We are accustomed to regarding the behaviour, at least of simple mechanical systems, as being completely deterministic, so if the breakdown of determinism implied by quantum mechanics is genuine, it is an important discovery which must affect our view of the physical world.
Nevertheless, our belief in determinism arises from experience rather than logic, and it is quite possible to con- ceive of a certain degree of randomness entering into mechanics; no obvious violation of ‘common sense’ is involved. Such is not the case with the third revolution brought about by quantum mechanics. This challenged the basic belief, implicit in all science and indeed in almost the whole of human thinking, that there exists an objective reality, a reality that does not depend for its existence on its being observed. It is because of this challenge that all who www.com The quantum revolutions 3 endeavour to study, or even take an interest in, reality, the nature of ‘what is’, be they philosophers or theologians or scientists, unless they are content to study a phantom world of their own creation, should know about this third revolution.
To provide such knowledge, in a form accessible to non- scientists, is the aim of this book. It is not intended for those who wish to learn the practical aspects of quantum mechanics. Many excellent books exist to cover such topics; they convincingly demonstrate the power and success of the theory to make correct predictions of a wide range of observed phenomena. Normally these books make little reference to this third revolution; they omit to mention that, at its very heart, quantum mechanics is totally inexplicable.
For their purpose this omission is reasonable because such considerations are not relevant to the success of quantum mechanics and do not necessarily cast doubt on its validity. In 1912, Einstein wrote to a friend, ‘The more success the quantum theory has, the sillier it looks.] If it is true that quantum mechanics is ‘silly’, then it is so because, in the terms with which we are capable of thinking, the world appears to be silly. Indeed the recent upsurge of interest in the topic of this book has arisen from the results of recent experiments; results which, though they beautifully confirm the predictions of quantum mechanics, are themselves, quite independent of any specific theory, at variance with what an apparently convincing, common-sense, argument would predict (see Chapter 5 , especially $85.5, for a complete discussion of these results). We can emphasise the essentially observational nature of the problem we are discussing by returning to the experimental facts we mentioned at the start of this section, and which gave birth to quan- tum mechanics.
Although, by abandoning some of the principles of classical physics, quantum theory predicted these facts, it did not explain them. The search for an explanation has continued and we shall endeavour in this book to outline the various possibilities. All involve radical departures from our normal ways of thinking about reality. On almost all the topics which we shall discuss below there is a large literature.
However, since this book is intended to be a popular introduction rather than a technical treatise, I have given www.com 4 Reality in the quantum world very few references in the text but have, instead, added a detailed bibliography. For the same reason various ifs and buts and qualifying clauses, that experts might have wished to see inserted at various stages, have been omitted. I hope that these omissions do not significantly distort the argument. I have tried to keep the discussion simple and non-technical, partly because only in this way can the ideas be communicated to non-experts, but also because of a belief that the basic issues are simple and that highly elaborate and symbolic treatments only serve to confuse them, or, even worse, give the impression that problems have been solved when, in fact, they have merely been hidden.
The appendices, most of which require a little more knowledge of mathematics and physics than the main text, give further details of certain interesting topics. Finally, I conclude this section with a confession. For over thirty years I have used quantum mechanics in the belief that the prob- lems discussed in this book were of no great interest and could, in any case, be sorted out with a few hours careful thought. I think this attitude is shared by most who learned the subject when I did, or later.
Maybe we were influenced by remarks like that with which Max Born concluded his marvellous book on modern physics [Atomic Physics (London: Blackie 1935)]: ‘For what lies within the limits is knowable, and will become known; it is the world of experience, wide, rich enough in changing hues and patterns to allure us to explore it in all directions. What lies beyond, the dry tracts of metaphysics, we willingly leave to speculative philosophy.’ It was only when, in the course of writing a book on elementary particles, I found it necessary to do this sorting out, that I discovered how far from the truth such an attitude really is. The present book has arisen from my attempts to understand things that I mistakenly thought I already understood, to venture, if you like, into ‘speculative philosophy’, and to discover what progress has been made in the task of incorporating the strange phenomena of the quantum world into a rational and convincing picture of reality.2 External reality As I look around the room where I am now sitting I see various www.com External reality 5 objects. That is, through the lenses in my eyes, through the struc- ture of the retina, through assorted electrical impulses received in my brain, etc, I experience sensations of colour and shape which I interpret as being caused by objects outside myself.
These objects form part of what I call the ‘real world’ or the ‘external reality’. That such a reality exists, independent from my observation of it, is an assumption. The only reality that I know is the sensations of which I am conscious, so I make an assumption when I introduce the concept that there are real external objects that cause these sen- sations. Logically there is no need for me to do this; my conscious mind could be all that there is.
Many philosophers and schools of philosophy have, indeed, tried to take this point very seriously either by denying the existence of an external reality, or by claiming that, since the concept cannot be properly defined, proved to exist, or proved not to exist, then it is useless and should not be discussed. Such views, which as philosophic theories are referred to by words such as ‘idealism’ or ‘positivism’, are logically tenable, but are surely unacceptable on aesthetic grounds. It is much easier for me to understand my observations if they refer to a real world, which exist even when not observed, than if the observations are in fact everything. Thus, we all have an intuitive feeling that ‘out there’ a real world exists and that its existence does not depend upon us.
We can observe it, interact with it, even change it, but we cannot make it go away by not looking at it. Although we can give no proof, we do not really doubt that ‘full many a flower is born to blush unseen, and waste its sweetness on the desert air’. It is important that we should try to understand why we have this confidence in the existence of an external reality. Presumably one reason lies in selective evolution which has built into our genetic make-up a predisposition towards this view.
It is easy to see why a tendency to think in terms of an external reality is favourable to survival. The man who sees a tree, and goes on to the idea that there is a tree, is more likely to avoid running into it, and thereby killing himself, than the man who merely regards the sensation of seeing as something wholly contained within his mind. The fact of the built-in prejudice is evidence that the idea is at least ‘useful’. However, since we are, to some extent, thinking beings, we should be able to find rational arguments which justify our belief, and indeed there are several.
These depend on those aspects of our www.com 6 Reality in the quantum world experience which are naturally understood by the existence of an external reality and which do not have any natural explanation without it. If, for example, I close my eyes and, for a time, cease to observe the objects in the room, then, on reopening them, I see, in general, the same objects. This is exactly what would be expected on the assumption that the objects exist and are present even when I do not actually look at them.