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# The Undivided Universe

## An Ontological Interpretation of Quantum Theory

## David Bohm, Basil J. Hiley

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# The Undivided Universe

## An Ontological Interpretation of Quantum Theory

## David Bohm, Basil J. Hiley

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## About This Book

First published in 1995. Bohm, one of the foremost scientific thinkers of our time, and Hiley present a completely original approach to quantum theory which will alter our understanding of the world and reveal that a century of modern physics needs to be reconsidered.

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# Chapter 1

Introduction

**1.1 Why an ontological interpretation is called for**

The formalism of the quantum theory leads to results that agree with experiment with great accuracy and covers an extremely wide range of phenomena. As yet there are no experimental indications of any domain in which it might break down. Nevertheless, there still remain a number of basic questions concerning its fundamental significance which are obscure and confused. Thus for example one of the leading physicists of our time, M.Gell-Mann [1], has said âQuantum mechanics, that mysterious, confusing discipline, which none of us really understands but which we know how to useâ.

Just what the points are that are not clear will be specified in detail throughout this book, especially in chapters 6, 7, 8 and 14. We can however outline a few of them here in a preliminary way.

- Though the quantum theory treats statistical ensembles in a satisfactory way, we are unable to describe individual quantum processes without bringing in unsatisfactory assumptions, such as the collapse of the wave function.
- There is by now the well-known nonlocality that has been brought out by Bell [2] in connection with the EPR experiment,
- There is the mysterious âwave-particle dualityâ in the properties of matter that is demonstrated in a quantum interference experiment.
- Above all, there is the inability to give a clear notion of what the reality of a quantum system could be.`

All that is clear about the quantum theory is that it contains an algorithm for computing the probabilites of experimental results. But it gives no physical account of individual quantum processes. Indeed, without the measuring instruments in which the predicted results appear, the equations of the quantum theory would be just pure mathematics that would have no physical meaning at all. And thus quantum theory merely gives us (generally statistical) knowledge of how our instruments will function. And from this we can make inferences that contribute to our knowledge, for example, of how to carry out various technical processes. That is to say, it seems, as indeed Bohr [3] and Heisenberg [4] have implied, that quantum theory is concerned only with our

*knowledge*of reality and especially of how to predict and control the behaviour of this reality, at least as far as this may be possible. Or to put it in more philosophical terms, it may be said that quantum theory is primarily directed towards*epistemology*which is the study that focuses on the question of how we obtain our knowledge (and possibly on what we can do with it).It follows from this that quantum mechanics can say little or nothing about reality itself. In philosophical terminology, it does not give what can be called an

*ontology*for a quantum system. Ontology is concerned primarily with that which*is*and only secondarily with how we obtain our knowledge about this (in the sense, for example, that the process of observation would be treated as an interaction between the observed system and the observing apparatus regarded as existing together in a way that does not depend significantly on whether these are known or not).We have chosen as the subtitle of our book âAn Ontological Interpretation of Quantum Theoryâ because it gives the clearest and most accurate description of what the book is about. The original papers in which the ideas were first proposed were entitled âAn Interpretation in Terms of Hidden Variablesâ [5] and later they were referred to as a âCausal Interpretationâ [6]. However, we now feel that these terms are too restrictive. First of all, our variables are not actually hidden. For example, we introduce the concept that the electron

*is*a particle with well-defined position and momentum that is, however, profoundly affected by a wave that always accompanies it (see chapter 3). Far from being hidden, this particle is generally what is most directly manifested in an observation. The only point is that its properties cannot be observed with complete precision (within the limits set by the uncertainty principle). Nor is this sort of theory necessarily causal. For, as shown in chapter 9, we can also have a stochastic version of our ontological interpretation. The question of determinism is therefore a secondary one, while the primary question is whether we can have an adequate conception of the reality of a quantum system, be this causal or be it stochastic or be it of any other nature.In chapter 14 section 14.2 we explain our general attitude to determinism in more detail, but the main point that is relevant here is that we regard all theories as approximations with limited domains of validity. Some theories may be more nearly determinate, while others are less so. The way is open for the constant discovery of new theories, but ultimately these must be related coherently. However, there is no reason to suppose that physical theory is steadily approaching some final truth. It is always open (as has indeed generally been the case) that new theories will have a qualitatively different content within which the older theories may be seen to fit together, perhaps in some approximate way. Since there is no final theory, it cannot be said that the universe is either ultimately deterministic or ultimately in-deterministic. Therefore we cannot from physical theories alone draw any conclusions, for example, about the ultimate limits of human freedom.

It will be shown throughout this book that our interpretation gives a coherent treatment of the entire domain covered by the quantum theory. This means that it is able to lead to the same statistical results as do other generally accepted interpretations. In particular these include the Bohr interpretation and variations on this which we shall discuss in chapter 2 (e.g. the interpretations of von Neumann and Wigner). For the sake of convenience we shall put these altogether and call them the conventional interpretation.

Although our main objective in this book is to show that we can give an ontological explanation of the same domain that is covered by the conventional interpretation, we do show in the last two chapters how it is possible in our approach to extend the theory in new ways implying new experimental consequences that go beyond the current quantum theory. Such new theories could be tested only if we could find some domain in which the quantum theory actually breaks down. In the last two chapters we sketch some new theories of this kind and indicate some areas in which one may expect the quantum theory to break down in a way that will allow for a test.

Partly because it has not generally been realised that our interpretation has such new possibilities, the objection has been raised that it has no real content of its own and that it merely recasts the content of the conventional interpretation in a different language. Critics therefore ask: âIf this is the case, why should we consider this interpretation at all?â

We can answer this objection on several levels. Firstly we make the general point that the above argument could be turned the other way round. Thus de Broglie proposed very early what is, in essence, the germ of our approach. But this met intense opposition from leading physicists of the day. This was especially manifest at the Solvay Congress of 1927 [7]. This opposition was continued later when in 1952 one of us [5] proposed an extension of the theory which answered all the objections and indeed encouraged de Broglie to take up his ideas again. (For a discussion of the history of this development and the sociological factors behind it, see Cushing [8] and also Pinch [9].)

Let us suppose however that the Solvay Congress had gone the other way and that de Broglieâs ideas had eventually been adopted and developed. What then would have happened, if 25 years later some physicists had come along and had proposed the current interpretation (which is at present the conventional one)? Clearly by then there would be a large number of physicists trained in the de Broglie interpretation and these would have found it difficult to change. They would naturally have asked: âWhat do we concretely gain if we do change, if after all the results are the same?â The proponents of the suggested ânewâ approach would then probably have argued that there were nevertheless some subtle gains that it is difficult to weigh concretely. This is the kind of answer that we are giving now to this particular criticism of our own interpretation. To fail to consider such an answer seriously is equivalent to the evidently specious argument that the interpretation that âgets in there firstâ is the one that should always prevail.

Let us then consider what we regard as the main advantages of our interpretation. Firstly, as we shall explain in more detail throughout the book but especially in chapters 13, 14 and 15, it provides an intuitive grasp of the whole process. This makes the theory much more intelligible than one that is restricted to mathematical equations and statistical rules for using these equations to determine the probable outcomes of experiments. Even though many physicists feel that making such calculations is basically what physics is all about, it is our view that the intuitive and imaginative side which makes the whole theory intelligible is as important in the long run as is the side of mathematical calculation.

Secondly, as we shall see in chapter 8, our interpretation can be shown to contain a classical limit within it which follows in a natural way from the theory itself without the need for any special assumptions. On the other hand, in the conventional interpretation, it is necessary to presuppose a classical level before the quantum theory can have any meaning (see Bohm [10]). The correspondence principle then demonstrates the consistency of the quantum theory with this presupposition. But this does not change the fact that without presupposing a classical level there is no way even to talk about the measuring instruments that are essential in this interpretation to give the quantum theory a meaning.

Because of the need to presuppose the classical level (and perhaps eventually an observer), there is no way in the conventional interpretation to give a consistent account of quantum cosmology. For, as this interpretation now stands, it is always necessary to assume an observer (or his proxy in the form of an instrument) which is not contained in the theory itself. If this theory is intended to apply cosmologically, it is evidently necessary that we should not, from the very outset, assume essential elements that are not capable of being included in the theory. Our interpretation does not suffer from this difficulty because the classical level flows out of the theory itself and does not have to be presupposed from outside.

Finally as we have already pointed out our approach has the potentiality for extension to new theories with new experimental consequences that go beyond the quantum theory.

However, because our interpretation and the many others that have been proposed lead, at least for the present, to the same predictions for the experimental results, there is no way experimentally to decide between them. Arguments may be made in favour or against any of them on various bases, which include not only those that we have given here, but also questions of beauty, elegance, simplicity and economy of hypotheses. However, these latter are somewhat subjective and depend not only on the particular tastes of the individual, but also on socially adopted conventions, consensual opinions and many other such factors which are ultimately imponderable and which can be argued many ways (as we shall indeed point out in more detail especially in chapters 14 and 15).

There does not seem to be any valid reason at this point to decide finally what would be the accepted interpretation. But is there a valid reason why we need to make such a decision at all? Would it not be better to keep all options open and to consider the meaning of each of the interpretations on its own merits, as well as in comparison with others? This implies that there should be a kind of dialogue between different interpretations rather than a struggle to establish the primacy of any one of them. (This point is discussed more fully in Bohm and Peat [11].)

**1.2 Brief summary of contents of the book**

We complete this chapter by giving a brief summary of the contents of this book.

The book may be divided roughly into four parts. The first part is concerned with the basic formulation of our interpretation in terms of particles. We begin in chapter 2 by discussing something of the historical background of the conventional interpretation, going into the problems and paradoxes that it has raised. In chapter 3 ...