Yoav Ben-Dov

Quantum Theory - Reality and Mystery

Chapter 5

THE OLD QUANTUM THEORY

Achievements and embarrassment

In the early 1920's, the physicists found themselves in a strange situation. Undoubtedly, since the research into the quantum domain started, a substantial advance had occurred in the study of basic physical phenomena. As we saw, at the beginning of the century, the very existence of atoms was still under doubt. Now, around 1924, there existed specific and detailed rules for the calculation of atomic properties - for example, the frequencies of light emitted and absorbed in atoms. Further, with de Broglie's work, one could even discern something which vaguely looked like a theoretical structure underlying these rules. But this advance had its price: it meant that physicists gave up the possibility to give a clear account of what they were doing.

There seemed to be some fundamental paradox in this situation. On one hand, the basis for any explanation or calculation was the use of concepts borrowed from classical physics, for example the image of a small material particle which moves in a circle around the nucleus in Bohr's model of the atom. On the other hand, in order to reach the desired results (and to avoid absurd consequences, like the collapse of atoms, or the radiation loss of all the energy of material particles), one had to add at some moment new rules, which involved the mysterious constant h, and made no sense in the framework of the classical theories. Also, the question which physical concept (particle or wave, for example) should be applied to which particular objects in nature started to appear as if it had different and self-contradictory answers: the electron is a particle but also a wave, and light is a wave but also a stream of particles. In other words, a significant advance was undoubtedly achieved, but it was based on the readiness to use a conceptual system full of fundamental contradictions.

A state of uncertainty, and a feeling of a conceptual fog, are not by themselves surprising at the early stages of the development of a new theory. The scientists who are discovering a new world of phenomena and ideas have, as their departure point, the old system in whose terms they were educated, and which still serves for the communication with their social environment (that is, the community of scientists). From the old viewpoint, the considerations of these innovative discoverers are "irrational". But the new viewpoint, from which the same considerations make perfect sense, does not yet exist.

In a similar manner, Arthur Koestler once compared the pioneers of the scientific revolution in the 17th century, like Galileo and Kepler, to "sleepwalkers" (that was the title of his book), who are doing actions that they cannot justify. Indeed, the full justification to Galileo and Kepler's conceptual choices came only afterwards, and as a result of their efforts, with the full development of Newton's theory of mechanics. However, this conceptual fog of the intermediate period is normally supposed to disperse gradually, as the new theory becomes more clearly perceptible. In the new domain of quantum phenomena, it seemed as if the opposite is true: as the theoretical structure which deals with these phenomena became more solid and well-defined, the conceptual fog around it only grew thicker.

The problem concerned especially the status of classical physics. In the early stages of the quantum research, from 1900 onwards, one could still think that the old classical structure could be maintained, while maybe adding some new assumptions to it, in order to account for the quantum results. However, near the mid-1920's, there was a growing feeling that physics may have reached a real crisis, and that in order to solve it, there is a need for completely new concepts and ideas, and a new basis for physics.

The setting in which physical research takes place was favorable for such a radical transformation. The lesson of Einstein's theory of relativity, which introduced fundamental changes in the physical concepts of space and time, and perhaps also the general cultural atmosphere of the collapse of old value-systems following the first world war, enhanced the readiness to accept new systems of ideas, both inside and outside science. However, in contrast with the 17th-century scientific revolution, which explicitly relied on a total rejection of the Aristotelian framework of the middle ages, it was now clear that a wholesale rejection of the old is impossible, as the classical theories still had an important role to play in the study of the new quantum phenomena. Thus, it appeared as if something from the classical concepts should remain in the new quantum framework. The question was, of course, which of the classical concepts should be rejected, and what should take their place.

Violation of classical properties

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Meyerson's "irrationals"

Thus, we may say that among the five classical properties listed, only the quantum violation of the property of objectivity was not explicitly discussed at that period. And yet, questions concerning objectivity, and the relation between the physical object and the observing human subject, were discussed at that time from a philosophical perspective, which sometimes was related to criticism of the existing theories of physics. For example, in his 1908 book "Identity and Reality", the French philosopher of science Emile Meyerson claimed that the world-vision of classical mechanics is based on two fundamental riddles that the human mind cannot comprehend, and he called them "irrationals".

The first irrational of Meyerson's concerns the relation between the qualitative features of the world, which are perceived by the mind, and its quantitative features, which govern its interaction with the body. For example, one can understand how a motion of light rays induces a motion of the eye cells, and how this motion induces, in its turn, a motion of the brain nerves (a present-day biologist would say "electrochemical signals" instead of "motion", but this is irrelevant to the argument). Still, how can these quantitative motions of nerves result in the qualitative feeling of vision in the mind? In other words, how come that a collection of physical atoms can develop the subjective experience of "now I perceive this"?

This question is undoubtedly one of the most complex and entangled problems in philosophy. In these terms, it arose in the 17th century, following the works of the French philosopher Rene Descartes, who distinguished for the first time between the concepts of the physical body and the thinking mind, and saw them as two distinct substances. Following his terminology, the problem is therefore called "the mind-body problem". Descartes himself believed in the real existence of both mind and body, but he failed to give a satisfactory account of the relation between them. Therefore, some of his followers tried to solve the problem by assuming that only the body exists as a real object, while the mind does not have an independent existence, but rather arises somehow from the physical processes occurring in the material brain.

This position, which came to be called "materialism", became the ruling view in the scientific culture near the end of the 19th century. But still, materialism could not explain how the quality of subjective experiences comes about from a collection of quantitative physical processes. At most, one can give a description of some physiological mechanisms, by which some parts of the brain represent the external world, in terms of the structure of sense organs and nerve cells in the brain. For example, the perception of "seeing red" would be represented by a specific physical state of brain cells. Yet, representation is different from experience. For example, we can think of an accurate map which represents a certain geographical territory, and assume some mechanism which updates the map with each change of the terrain. Still, we would not say that this map experiences the territory, in a way similar to the subjective experience that arises in the human mind.

Debates on the mind-body problem have been going on for at least 300 years, without reaching any satisfactory conclusion. It is therefore difficult to expect that a solution can be found at all, at least in the framework in which this problem has been discussed so far. Thus, in the Cartesian terms which clearly separate matter from spirit, it can be regarded as a problem that the human reason is incapable of understanding. However, classical mechanics, when viewed as a description of physical reality, can be formulated only in such Cartesian terms, because it refers explicitly to the world of matter, and says nothing of the world of spirit. Therefore, in Meyerson's view, from a viewpoint of reality based on classical mechanics, the relation between mind and body appears as something "irrational".

Meyerson's second "irrational" concerns the nature of atoms. According to the vision of classical mechanics accepted at that time, atoms (or at least their fundamental parts) were supposed to be completely simple, and have no internal structure (otherwise they would not be the real "atoms", which originally means the indivisible parts of matter). Suppose now that two such atoms, completely simple and with equal masses, are moving towards each other with equal velocities, and then collide and recoil in opposite directions (Meyerson's original example is a little different, but this is inconsequential).

At the exact moment of collision, the two atoms are at rest, because they are no longer approaching each other, and not yet receding from each other. According to the law of conservation of energy, the energy of motion that the atoms possessed before the collision must remain in the system, and keep its constant value. Indeed, it is exactly equal to the energy of motion that the atoms have after the collision. But where is this energy at the exact moment of collision?

On one hand, if we say that the atoms are elastic, this means that they are compressed at the moment of collision, and then the energy can appear as elastic energy, like the energy contained in a compressed spring. But for this to be true, the atoms must have internal parts that can be squeezed together, so that they are not simple atoms (and we may ask the same question on their internal parts). On the other hand, if we say that the atoms are hard, so that they have no internal parts that can be squeezed together, then at the moment of collision they have no energy of motion, and they cannot have any other form of energy. Therefore, we cannot explain where the energy is at the moment of collision, and yet it should be somewhere, as its total amount must always remain constant. Thus, says Meyerson, the concept of a completely simple atom, moving in continuous space and time, cannot be clearly perceived by reason: it is also "irrational".

In both cases we encounter something which lies beyond the capacities of human understanding, and Meyerson suggests the possibility that the two irrationals have a common origin. One can view this suggestion as a groundless speculation: as it is a question of irrationals, it is difficult to see how any claim on the relation between them can be supported rationally. Still, it is interesting to note that the new quantum mechanics, which was meant to correct the failure of the classical theories in describing the structure of atoms, also differs from them in its violation of the property of objectivity, as we mentioned in Chap. 2 and will discuss further in later chapters. Thus, quantum mechanics draws our attention back to the relation between the material body, which can be given an objective description, and the perceiving mind, which experiences itself subjectively. In this sense, quantum mechanics may give some justification to Meyerson's intuitions, according to which the basic mystery of atomic theory is somehow related to the basic mystery of the mind-body problem.

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