Yoav Ben-Dov

Quantum Theory - Reality and Mystery

Foreword:

WHY QUANTUM MECHANICS?

In the late 1920's, a new theory called quantum mechanics was accepted in physics. In the following years, it quickly became the central and most fundamental theory, with which the physicists study and describe the behavior of matter. Quantum mechanics had, and continues to have, impressive successes and achievements in many domains, and no alternative theory has yet appeared which can challenge its position. But strangely, from the 1920's until today, the debates around quantum mechanics have not ceased.

In fact, some of the most prominent physicists who created quantum mechanics turned against it, and tried to find something better instead. Others have tried to modify or correct it, so that it would become "more reasonable". There are also many who tried to draw from quantum mechanics far-reaching conclusions, which would replace the ways of thinking that prevailed in western science since the 17th century. Among these proposals were the claim that human consciousness can influence material objects, or that there is some basic flaw in western thinking, for which the remedy should be imported from systems of eastern mysticism.

How come that a successful theory leads to such self-negating debates and such strange ideas? All of these suggestions, and others which may appear no less "bizarre" to the traditional scientist, are motivated by the fact that quantum mechanics raises some very difficult conceptual problems, which concern the very definition of physical reality, and the ways of thinking that we use for its study. In fact, the implications of these conceptual problems seem to go beyond the domain of pure physical research, and concern the basic principles on which modern scientific society is founded. This is the reason why these problems continue to intrigue and to interest both scientists in other domains, and the general public, so many years after the theory itself was securely accepted in physics itself.

To give a hint of what we discuss in this book, the main question of these debates can be formulated as follows. As we said, quantum mechanics seems to contradict some traditionally accepted beliefs on science and scientific thinking. Thus, following its acceptance, should we abandon the traditional principles of physical and scientific thinking, and adopt new ways of thinking in their place? Or rather, we should suppose the whole thing to be a temporary misunderstanding, hoping that with further progress, some way will be found to reconcile quantum mechanics within the traditional principles of science?

In this book, intended to the general reader who does not necessarily have a background in science, we shall explore quantum mechanics itself, the main conceptual problems that it raises, and some of the most important approaches suggested to deal with these problems. The first part (Chaps. 1-2) serves as an introduction, and starts with a short review of the current status of quantum mechanics. To focus our discussion, we define five "classical properties", which characterized the physical theories that preceded quantum mechanics (called "classical theories"), but apparently are not satisfied by quantum mechanics. These properties are: visuality, causality, locality, self-identity and objectivity. As we shall see later, most of the debates around quantum mechanics concern the violation of these properties, so that we can use them to compare the different approaches to quantum mechanics and its conceptual problems.

In the second part (Chaps. 3-7), we review the basic principles of quantum mechanics, as they developed from 1900 until the acceptance of this theory by the physical community in the late 1920's. Then (Chaps. 8-9) we focus on what came to be considered, from the 1930's onward, as the central conceptual problem of quantum mechanics. This is usually called "the measurement problem", and in our terms, it is related to the violation of the property of objectivity in quantum mechanics. As we shall see, the source of the problem is the fact that in quantum mechanics, it is impossible to separate between the physical object that the theory studies - for example, a particle like the electron - and the measurement action, by which the human subject studies it: it is as if the electron modified its behavior when the human observer looks at it. We shall also discuss some famous examples, that illustrate how this dependence of the measured object on the measuring act clashes with traditional ways of thinking, namely the examples of the double slit experiment and Schroedinger's cat.

At this point, we shall proceed to discuss the two main approaches to quantum mechanics and its conceptual problems. These were represented, from the late 1920's onwards, by two physicists that made important contributions to the development of quantum mechanics: Albert Einstein and Niels Bohr. As we shall see, they both agreed that quantum mechanics, at least in its currently accepted version, violates some fundamental principles, on which the scientific ways of thinking were traditionally based. In the terminology used in this book, quantum mechanics violates the classical properties listed above. However, they disagreed on the meaning of this fact.

Einstein thought that the classical properties (or at least some of them, especially objectivity) should be retained at all cost, because the very notion of "science" is based on them. Therefore, Einstein rejected quantum mechanics, and tried to find an alternative to it. The original direction followed by Einstein did not lead to any substantial results. But other physicists, mostly from the 1950's onwards, adopted in principle Einstein's approach, and tried to suggest different "interpretations" to quantum mechanics. Each of these interpretations is actually a proposal to modify the existing theory, so that the classical properties, or at least some of them, will again be satisfied. We review the main interpretations in Chaps. 10-13, and also try to understand the reasons why none of them was accepted by the physical community as an alternative to the existing theory.

After reviewing these developments in Einstein's position, we present Bohr's opposing view. Bohr thought that quantum mechanics should be accepted as it is, and in its current version. However, as it clashes with ways of thought that were hitherto accepted in scientific thinking, this means that we should change our ways of thought. In particular, Bohr claimed that one should give up the idea of an objective reality, which can be given a complete and unique description in a language accessible to humans. Instead, Bohr suggested an idea that he called "complementarity". this means that different descriptions of reality can all be valid, each in its appropriate context, although apparently they contradict each other. It is as if these descriptions "complement" each other to give a complete picture of reality, but this picture cannot be represented in any single non-contradictory system of concepts.

The idea of an objective reality which can be given one unique description, complete and non-contradictory, was an important presupposition of the modern scientific culture, as it developed from the 17th century onwards. In fact, this idea not only set the basic rules of physical research, but also influenced many domains of study and action, both within and outside science. Therefore, giving up this idea also influences all these domains, and it can have important consequences for philosophical questions from "what is reality that we can know" to "what is the nature of human being". Indeed, Bohr did not refrain from extending the idea of complementarity to such domains. In Chaps. 14-15 we examine Bohr's position, and its possible implications in science and other domains.

The last part of this book treats additional conceptual problems that quantum mechanics raises. First we discuss a question which originally was presented in the mid-1930's, in a famous paper published by Einstein and two collaborators (the "EPR paper"). This question received much attention later, following some theoretical developments in the mid-1960's, and experiments conducted in the early 1980's. In a nutshell, this question concerns the fact that under some specific conditions, quantum particles (like electrons) seem to influence each other at a distance and instantaneously. In our terms, quantum mechanics apparently violates the classical property of locality. If such influences, which pass instantly from one place to another, really exist, this stands in direct contradiction to Einstein's 1905 theory of special relativity, by which no physical influences can propagate faster than light. We shall discuss this question in Chaps. 16-19.

Finally, in Chap. 20, we shall discuss the violation of the property of self-identity in quantum mechanics. By this property, any object should have a well defined identity, which remains the same through time, and distinguishes this object from all the rest of the world. As we shall see, this could be just another case in which a fundamental principle, which prevailed in western thought, should be given up. If this is true, then maybe in this point one can also find some resemblance between quantum mechanics and non-western traditions of thinking, such as, for example, those that were developed in the Buddhist civilizations of east Asia.

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