Yoav Ben-Dov

Quantum Theory - Reality and Mystery

Chapter 2

THE CLASSICAL PROPERTIES

Intuitions and properties

In the previous chapter we mentioned the theories of classical physics, which prevailed before the appearance of quantum mechanics - namely, Newton's theory of mechanics from the 17th century, and the theories of thermodynamics and electrodynamics from the 19th century. In many popular science books, one can find the claim that quantum mechanics is "strange", because it clashes with our every-day common sense intuitions. In contrast, these authors claim, the theories of classical physics correspond to these intuitions, and therefore they are much more acceptable to common sense.

This claim may look intriguing to many readers, that might have had difficulties learning physics in high school, or that have tried to teach physics at school. The core of high school physics studies are the classical theories, and especially Newtonian mechanics. However, it is a well-known fact that for most school students, physics is considered as one of the most difficult subjects. Now, if indeed the theories of classical physics are so intuitive and corresponding to common sense, why is it that so many students have difficulties in learning and comprehending them?

The answer to this question is that apparently, the theories of classical physics are not at all intuitive and corresponding to the common sense intuitions of everyday life. If they were - that is, if the classical theories were really intuitive - then one could expect that they would appear and develop at a much earlier stage in human history, without the need to wait until the 17th century and the scientific revolution of western Europe. In fact, studies conducted in recent years have shown clearly, that the physical intuitions of most students that begin their school physics studies (and in many cases, also later) flatly contradict the basic principles of classical physics, and thus they appear to teachers as "student misconceptions".

For example, everyday intuitions may lead us to expect that any object on which no force is applied will be at rest. But classical mechanics is based on the Newtonian principle of inertia, which states explicitly that an object on which no forces are applied may move indefinitely in a straight line and constant velocity. A success in high school physics studies means, among else, that the student's intuitions were transformed from "normal", every-day intuitions, to a new set of physical intuitions, which enable him or her to perceive physical situations in the abstract and mathematical terms of the classical theory. This transformation seems to be so difficult, that historically it appeared only at a very developed stage of scientific inquiry, and even today, only a small minority of high school students undergo it successfully.

Still, we can understand the claim that the classical theories are intuitive if we remember that popular science books about quantum mechanics are usually written by physicists, and that in order to become a physicist, one usually has to show success in high school physics. The popular science books thus represent the point of view of the small minority, that as high school students did succeed in undergoing the transformation from "natural" to "classical physics" intuitions. For these physicists, the classical theories are indeed intuitive and corresponding to common sense. In fact, the classical theories of physics have become for them so intuitive, that they sometimes have difficulties in appreciating that for most people, this is not the case.

What we can conclude from here is that expressions like "common sense" or "everyday intuitions" are far from being self-evident. Therefore, the attempt to present the conceptual problems of quantum mechanics in terms of a violation of "common-sense intuitions" is also problematic. The same idea may look intuitive an self-evident to one person, and counter-intuitive for another person. For example, in the previous chapter we mentioned briefly Niels Bohr's "comlementarity" approach, according to which one should use different and mutually incompatible descriptions, in order to give a full account of all the perceptible features of a system. We shall discuss it in fuller detail later, but already we can note that it clashes with the habitual ways of thinking of classical mechanics, according to which any system has only a single "true" description, for example in terms of locations and motions in space. In this sense, Bohr's approach is counter-intuitive for the classical physicist. On the other hand, one could claim that in the changing circumstances of everyday life, we may be accustomed to use different sets of considerations, and therefore descriptions in different sets of terms, even if it's not always clear whether they are mutually compatible. Thus, from this viewpoint, Bohr's approach may seem much more natural and intuitive (in everyday terms) than classical physics.

To avoid these ambiguities, in this book we shall not rely on terms like common sense and everyday intuitions. Instead, we shall try to characterize the conceptual problems of quantum mechanics in terms of the differences between this theory and classical physics, without qualifying any of them as more intuitive per se. For this purpose, we shall define some basic properties which were satisfied by the classical theories, but are violated by quantum mechanics in its current version. These we call "classical properties".

For people whose physical intuitions became accustomed to the classical theories, any such violation of a classical property may appear counter-intuitive, and thus as a conceptual problem. To some of these people, it may appear that the relevant classical properties are essential requirements from any acceptable physical theory - and therefore, that their violation makes quantum mechanics not only counterintuitive, but actually an unacceptable theory. As we shall see, this was the point of view adopted by Einstein and some other critics of the current version of quantum mechanics.

In this book, we shall be interested in those classical properties that can thus be presented as conditions for acceptability. As most of the debates around quantum mechanics concern these properties, their list will serve us for comparing the different approaches to the theory and its conceptual problems. A detailed list of such "essential" classical properties may be a matter of personal preference. For example, one may claim that the list should include the property of continuity, by which "nature does not make jumps". The property of continuity was satisfied not only in the classical theories. In fact, it was one of the basic principles of Aristotelian physics, which prevailed in the western world from the late middle ages until the 17th century. As we shall see in later chapters, quantum mechanics supposes the existence of discontinuous jumps. For example, an electron may jump from one trajectory to another in Bohr's model of the atom, to be discussed in chapter 4. However, the issue of continuity disappeared from the quantum mechanical debates at a relatively early stage, so that from the 1930's onwards, it is difficult to find somebody who claims seriously that the violation of continuity is a fundamental conceptual problem. Therefore, we shall not include continuity in our list of classical properties.

Our list of classical properties that are satisfied by classical physics, but violated by quantum mechanics, includes five items, namely: visuality, causality, locality, self-identity and objectivity. To focus the discussion, we define briefly these properties here, and continue to elaborate more on each of them in the corresponding chapters.

Visuality

(---)

Causality

(---)

Locality

(---)

Self-identity

(---)

Objectivity

The classical theories are formulated in terms which may be perceived as referring to an objective reality that exists "by itself" - that is, independently of the question whether or not we (or any other intelligent beings) observe it. This does not mean that there is no physical influences that depend on human existence. If we were not here, the physical condition on earth would surely be different from what they are today. But basically, the nature of physical reality would have remained the same. For example, the classical theory of mechanics saw the world as a huge collection of material particles moving through space according to some definite mathematical laws. In this viewpoint, the nature of these particles, and the form of the mathematical laws, would have been the same, even if humankind had never appeared on earth. Similar things can be said on classical electromagnetics, only that instead of particles, it presented an objective reality made of electromagnetic fields and their vibrations: for this description of reality, human existence or non-existence is irrelevant.

In this sense, the classical physics description of the world is objective, and independent of the observing human subject. In fact, at least since the advent of Darwin's theory of evolution in the 19th century, the commonly accepted scientific doctrine is that it was the basic constituents of objective physical reality which gave rise to the existence of the human subject, and therefore the knowledge and the observation of the human subject have no fundamental significance in physics. Thus, we can define objectivity as follows: a theory satisfies the property of objectivity if it can provide a description of physical reality, whose basic features remain the same whether or not they are observed by someone.

Quantum mechanics does not provide such an objective description of reality. For example, in the mathematical formulation of quantum mechanics, a physical system is supposed to obey one set of rules as long as nobody observes it, and another set of rules when a human observer measures it. As long as no measurement is performed, the state of a physical system is described by a set of equations whose solution may correspond to different states of affairs, all coexisting one besides the other. For example, one can conceive a situation in which the equations of quantum mechanics describe a door as both open and closed simultaneously. As long as nobody observes the door, the equations should maintain both possibilities. But when somebody makes a measurement (that is, when he or she looks at the door to determine its state), then the laws change. Now we must use a completely different equation, and consider the physical situation as if the act of measurement forced a choice between the two possibilities. In other words, once somebody looks at it, the door becomes either "open" or "closed", and remains so subsequently.

This use of a double set of equations - one set when nobody is looking on the system, and another set when somebody observes it - enables the physicists to give accurate predictions of the results of laboratory experiments, and for the moment, there seems to be no acceptable alternative to it. But such a "double-set" formulation of physics cannot be regarded as a description of objective reality, because on the face of it, it appears as if this formulation assumes that the laws of physics depend on the decision of the human observer, whether or not to look on the system. Is it possible to believe that the physical world behaves in one way when we look at it, and in another way when we are not looking? And if so - why should the observation, and the very presence of the human observer, have such an influence on the laws of matter? We shall discuss in detail this problem in Chap. 9, and have more to say on the property of objectivity in other places.

An important note: in the philosophical literature, this classical property is sometimes called not "objectivity" but "realism". However, although both terms have wide and varied meanings in different contexts, it seems that the term "realism" is particularly prone to different interpretations. We shall not enter here a discussion about terms. Instead, we shall use the term "objectivity", assuming it is understood that in this book, it is employed in the sense defined here.

As we shall see further on, many of the conceptual debates around quantum mechanics concern the classical properties and their status. The question that often arises is which of them - if at all - should be abandoned, and which - if at all - should be retained at all costs. But before we enter in detail these arguments, we shall review in the next chapters (3-7) the basic principle of quantum mechanics itself, as they developed from the early 1900's to the late 1920's.

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