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p The discovery of the corpuscular properties of light and the wave properties of matter in phenomena on an atomic level made it necessary for physics to combine the corpuscular and wave conceptions of matter and field as applied to atomic processes. It proved far from simple to resolve the theoretical problems that emerged in physics in this connection, the main reason being that, from the standpoint of classical physics (with its notions of particle and wave), an object could not exist simultaneously in those processes as a particle and as a wave. The difficulties emerging, it seemed, were coped with by the Copenhagen interpretation of quantum mechanics.

p The last expression (Bohr does not use it) is often used by Heisenberg. In his view ’the Copenhagen" interpretation of quantum theory starts from a paradox’: it assumes that any experiment ’(regardless of whether it refers to the everyday phenomena or those of atomic physics) ’is to be described in 40 the terms of classical physics’, which cannot be replaced by any other concepts; at the same time it affirms that their applicability ’is limited by the relations of uncertainty’.²¹ Heisenberg goes on to explain Bohr’s idea of complementarity by the particle and wave characteristics of atomic objects being mutually exclusive, since’a certain thing cannot at the same time be a particle (i.e. substance confind to a very small volume) and a wave (i.e. a field spread out over a large space)’. These two characteristics, he says, complement each other.^^22^^

p We would like to note two aspects of the Copenhagen interpretation. The first, developed by Heisenberg, can be formulated as follows. Where it is possible, in principle, in classical physics to eliminate the effect of observation (or measurement) on the object, it is impossible, in principle, in quantum mechanics, since the latter contains an uncertainty relation that causes inaccuracies in measurements (the idea of uncontrollability in principle).

p The second aspect, developed by Bohr, can be represented as follows. Micro-objects cannot be regarded either as particles or as waves in the sense of classical physics. In some experimental conditions the most natural description of micro-objects is that based on corpuscular conceptions, in other conditions it would be most natural to describe microobjects in terms of waves. Both descriptions are complementary to each other, and this relation ensures consistent application of] the classical concepts in the realm of microphenomena.

p It is usually assumed that these aspects are equivalent. Heisenberg, for instance, frequently employed the idea of complementarity in his arguments; Bohr, too (especially in his work of the thirties and forties), often resorted to the idea of an ’uncontrollable interaction between object and instrument’. All these aspects are far from identical, which can be seen quite well in Bohr’s last works.

p In the aspect of the Copenhagen interpretation developed by Heisenberg attention is drawn not so much to the fact that the properties and behaviour of macro-objects should not be ascribed to micro-objects as to the idea of the inaccuracies associated with applying classical concepts to microobjects. These inaccuracies and the corresponding interpretation of the uncertainty principle are given (as we shall show later) a fundamental importance in the physics of 41 quanturn mechanics that is quite untypical of them. Heisenberg also resolved the paradox above in the spirit of just this interpretation.

p Why do the classical concepts by which experiments with atomic objects are described not correspond precisely to these objects? According to Heisenberg, the answer is that ’the observation plays a decisive role in the event and that the reality varies, depending upon whether we observe it or not’.^^23^^

p From this standpoint (if it is consistently held), momentum or energy, for example, are not so much objectively real as that they appear and disappear depending on the choice of one method of observation or another. The mathematical apparatus of quantum mechanics is not so much objective as symbolic in nature (needed only for agreeing the readings of the instruments). The uncertainty relation is becoming the absolute boundary of human knowledge; no new fundamental physical concepts have been developed in quantum mechanics.

p One must emphasise that Heisenberg’s view formulated above (again, if it is held consistently) on the uncontrollability principle leads to non-materialist philosophical conclusions. In fact, however, Heisenberg himself by no means employed it consistently. When he considers philosophical questions inseparable from the theoretical content of modern physics, its basic materialist and dialectical spirit is revealed, often quite clearly, in his statements. When, on the other hand, he moves away from physics into the realm of general philosophical problems, the idealist and metaphysical line gains the upper hand in his reasoning.

p Consider his standpoint relative to the cognition of nature which underlies, as could be expected, his philosophical reasoning about quantum mechanics.

p According to Heisenberg, man describes and explains ’not nature in itself but nature exposed to our method of questioning’ and our techniques of research.^^24^^ Heisenberg highly appreciated the idea of the German physicist Carl von Weizsacker that ’Nature is earlier than man, but man is earlier than natural science’. ’The first part of the sentence,’ he wrote, ’justifies classical physics, with its ideal of complete objectivity. The second part tells us why we cannot escape the paradox of quantum theory, namely, the necessity of using the classical concepts.’^^25^^

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p From that point of view it would appear that the transition of science from classical physics to quantum physics, instead of strengthening the bonds between man and science, separated them from each other. The point is that, according to Heisenberg, experiments with atomic processes are as real as any phenomena of daily life to describe which classical concepts are employed. Atomic or elementary particles, however, are not as real. They form a world of potential possibilities rather than a world of observed things or facts and can only be represented symbolically by mathematical signs. In other words, according to him, atomic and other micro-particles prove to be in a sort of realm of things- inthemselves. ’The “thing-in-itself” is for the atomic physicist, if he uses this concept at all, finally a mathematical structure; but this structure is—contrary to Kant—- indirectly deduced from experience.’^^26^^ On the other hand, the classical concepts (and they include, according to Heisenberg, space, time, and causality) have to be interpreted in a certain sense as a priori in respect to the theory of relativity and quantum mechanics.

p Is it true that there are adequate grounds in modern physics to revise the concept of objective reality?

p Quantum physics appeared and developed, of course, through acceptance of the objective reality of the physical world, including the atom and the elementary particles; Heisenberg, for example, noted that ’quantum theory does not contain genuine subjective features, it does not introduce the mind of the physicist as a part of the atomic event’.^^27^^ On the other hand, quantum theory has to take into account the conditions of observation (recorded by instruments) in which the objects of its research are found, because of their dual particle-wave nature (relativity to the conditions of observation),^^28^^ whereas classical theory had every right to ignore them. The description of physical phenomena by the idea of relativity to the conditions (or means) of observation means that quantum theory has made a new advance in cognition of the objective reality of nature and not that this objective reality is limited by the boundaries of classical physics, as Heisenberg asserts.

p One also cannot agree with Heisenberg when he says that only classical theory idealised nature as such, while the rise of quantum theory was accompanied with the establishment of another point of view, namely, that science describes 43 nature not as it is in itself but as subjected to human methods of research. In classical physics the picture of nature is not completely adequate to nature; it is a rough approximation and simplification, as is proved by both the theory of relativity and quantum theory. But then the statement to the effect that classical physics describes and explains nature without taking us ourselves into consideration is wrong. Much the same thought is to be found in Heisenberg’s remarks about the limited character of classical concepts. He does not, however, follow up these remarks, narrowing the philosophical concept of objective reality down in his argument to its presentation in classical theory.

p If one bears in mind that the reflection of nature through observation and thinking idealises, simplifies, and roughly approximates the reflected object, and that at the same time the progress of cognition, theory, and science on the whole overcomes this simplification, which is inevitable in each individual cognitive act, it becomes clear that the development from classical to relativistic and quantum theories reflects nature more completely in the system of concepts and at a deeper level, without exhausting it. The forward movement of physical knowledge is thus accompanied with the introduction of new methods of describing the phenomena of nature, developing new fundamental concepts and principles, constructing new theories, and creating the scientific picture of the world. It is understandable that this development of knowledge, which implies ever newer alteration of nature by the person cognising it, in no way resembles a onesided increase of subjective elements in science through cognition of its objective content; but Heisenberg holds the opposite view.^^29^^

p Thus, in the process of cognising (objectively real) nature the objective and the subjective should not be opposed and separated from one another, as has been done in his own way by Heisenberg, in whose view the difference between the subjective and objective in classical physics is only absolute, and in quantum physics only relative. The constant advance of science, which reflects the material world, reduces the one-sided elements of the objective and the subjective to nothing. In Lenin’s words, the ’logical concepts are subjective as long as they remain “abstract”, in their abstract form, but at the same time they express also the Things- inthemselves. Nature is both concrete and abstract, both 44 phenomenon and essence, both moment and relation. Human concepts are subjective in their abstractness, separateness, but objective as a whole, in the process, in the sum-total, in the tendency, in the source’.^^80^^ The development of physics—from classical science to that of today—confirms’these ideas of Lenin’s in a remarkable way.

p One can suppose that Heisenberg’s statements above are not so much a revision of the concept of objective reality in modern physics as the fact that the subject plays an active role in cognition. According to him, the passive role of the subject in classical physics, and its contemplative attitude to cognised nature, are quite natural: physics studies the objective world without violating its state in the course of observation. In modern physics (in quantum theory it is expressed with extreme clarity) it is accepted to speak of the observer and the object, as we know, as opposites. Hence it would seem to follow that Heisenberg’s argumentation deserves the closest attention and support from the standpoint of dialectical materialism, which rejects contemplative materialism.

p We would like, however, to stress that the subject-object relation (the objective and the subjective) in classical physics is the same in principle as in quantum theory. Man cognises nature only when he changes it, i.e. when he isolates some phenomena of nature from others (such isolation occurs already in the act of observation); when he conducts experiments in which he alters and controls the conditions under which these events take place; when he reconstructs the object in thought and approximates or simplifies concepts of it, etc., etc. The alteration of nature by cognising man is both inevitable and necessary since it is only it that creates opportunities for man to cognise nature as it is and to understand it as a unity of the diverse.

p This character of the alteration of nature, without which there is no cognition of it, has an essential special feature. The more man changes nature, the deeper and more completely he cognises it. Nature then appears more and more diversified and united to him, more ‘wonderful’ and richer in its manifestations and patterns, more and more different from the nature to which he was accustomed in the conditions of the ‘conventional’ experience.

p From the ordinary point of view, however, man’s ’ stronger’ alteration of nature in order to cognise it or, as some 45 authors put it, the subject’s greater activity in cognition, is only the reverse of cognising man’s movement forward from ignorance to knowledge, from shallow knowledge to deeper knowledge. On the other hand, the relation itself between cognising man (the subject) and cognised nature (orrather, its fragment, i.e. the object) does not change in principle: both at the level of shallow knowledge (in physics, say, classical mechanics) and at the level of deeper knowledge (e.g. quantum mechanics) nature exists before it is cognised by man, to whom it tells its secrets when he alters it and to the extent that he alters it.

p There are statements in the literature that at the level of physics described by quantum mechanics the subject of cognition is taken, so to say, beyond the bounds of its ‘classical’ passive state, since the interaction between the atomic object and the instrument is regarded as ’disturbance of phenomena by observation’ or as the ’creation of physical attributes of objects by measurements’.^^31^^

p Such statements now lack conviction. They were used to be common in the literature on quantum theory; now, if they are found at all, it is rather as relics. In his last works Bohr (and many other outstanding scientists) began to oppose the use of such expressions, which led to erroneous philosophical conclusions. One must remember that from the time when quantum mechanics was created to the end of his life Bohr kept returning to the philosophical questions of this theory, refining his terminology and perfecting his argumentation; this, as we shall show later, yielded significant positive results, one of which was his idea that the words ‘phenomenon’ and ‘measurement’ should be used in matters of quantum mechanics in the direct sense in which they are employed in classical theory.^^32^^

p The ideas and concepts developed and refined in recent years in quantum mechanics: viz. the proposition that ’the interaction between object and apparatus ... forms an inseparable part of the phenomenon’^^33^^; the notion of relativity to the means of observation; the idea that a particle is a relative concept in respect to phenomena on the atomic level, or that the concepts of particle and wave make sense in atomic physics not so much as the concept of a particle in itself and the concept of a wave in itself, as in internal interconnection, and the idea of potential possibility and probability in quantum mechanics—are all convincing 46 evidence of how far human thought has advanced by virtue of abstraction, its analytical and synthetic power since the times when classical mechanics was constructed and other classical theories arose.

p At the same time, the laws of cognition operating in classical physics and in quantum physics, or in any other scientific discipline and theory, are the same; otherwise there could be no unified science, and in general no united human knowledge reflecting the objectively real world. It is for that reason that science has followed the path of materialism, still does, and will, and why there is no place in its system of concepts for idealism and religion.

p To conclude our discussion of certain epistemological issues of quantum mechanics, let us dwell briefly on the fact that classical mechanics is much closer to it epistemologically than certain authors think. In classical mechanics, for example, the inertial motion of a particle cannot be thought of independently of the inertial frame of reference. This ’relativity^^1^^ very much resembles the ‘relativity’ of the particle in quantum mechanics, although the content of these ‘relativities’ by no means coincides. We shall return to this point, and to other topics discussed above, in the next sections of this chapter and in Chapters V and IX on dialectical contradictoriness in modern physics and philosophical aspects of the theory of mensuration.

p Let us now turn to the conception of complementarity, which we shall take as presented in Niels Bohr’s essay on quantum physics and philosophy.^^34^^ There is no concept ’uncontrollable interaction’ in it. The term ’ complementarity’ used by Bohr denotes a novel kind of relationship between the different experimental data about atomic objects obtained by means of various experimental apparatus. Although these data, says Bohr, appear contradictory when combination into a single picture is attempted they in fact exhaust all conceivable knowledge about the object.^^35^^

p The description of atomic phenomena, Bohr stresses, ’has in these respects a perfectly objective character, in the sense that no explicit reference is made to any individual observer’.^^36^^ In quantum mechanics, in his view, we are not concerned ’with a restriction as to the accuracy of measurements, but with a limitation of the well-defined application of space-time concepts and dynamical conservation laws’.^^37^^

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p In quantum mechanics, Bohr writes, the word ’ measurement’ should be used in its direct sense of quantitative comparison (comparison with a standard). He is against using expressions like ’disturbance of phenomena by observation’ or ’creation of physical attributes of objects by measurements. ’^3s

p Summarising, Bohr concluded that ’far from involving any arbitrary renunciation of the ideal of causality, the wider frame of complementarity directly expresses our position as regards the account of fundamental properties of matter presupposed in classical physical description, but outside its scope’.^^39^^

p Thus, one can find explicit expression in Bohr’s essay Quantum Physics and Philosophy of a position that is basically materialist and dialectical. By linking the mathematical apparatus of quantum mechanics with visualisable notions and classical concepts he reveals, as a philosopher would say, the antithetical character of the corpuscular and wave conceptions. Contrasting these ideas as a certain antinomy always played a decisive role in Bohr’s notion of complementarity. In his early work on quantum mechanics, however, this antithesis was masked by the idea of’ uncontrollable interaction’; in Quantum Physics and Philosophy, however, this drawback is eliminated.

p The considerable philosophical significance of the idea of complementarity for physical theory is that, according to it, the application of opposite concepts to the same objects under study is not simply possible but even necessary in certain conditions. As Bohr demonstrated (especially in his discussions with Einstein), this does not lead to any formal logical contradictions in physical theory, but enables the mathematical apparatus of quantum mechanics to be interpreted in accordance with the experimental data and a whole picture to be given of atomic phenomena that classical theories could not cope with.

p We limit ourselves here to a brief outline of complementarity, allowing for the fact that Bohr’s ideas will be discussed almost throughout the present book.

p The complementarity principle, as a conception of quantum mechanics (taking it in its mature form), is distinguished by a harmonious fusion of its philosophical and physical content. Although we have not touched on the other concepts of quantum mechanics in our presentation we shall 48 make an exception for one of them, viz. Reichenbach’s conception, which seems to us as a matter of fact to contain almost no physics but a great deal of philosophy. In his analysis of quantum mechanics and its philosophical aspect Reichenbach employed his theory of equivalent descriptions (the essence of which will be brought out below).

p In discussing quantum mechanics, Reichenbach speaks of phenomena and interphenomena. The physicist draws inferences about phenomena (e.g. about the electrons striking photographic film) from macroscopic events (tracks in emulsion; instrument readings), relying on a theory that contains no quantum laws. His conclusions about interphenomena (e.g. about the motion of an ^electron before it strikes a screen) are deduced from phenomena relying on quantum theory. Inferences about interphenomena, Reichenbach notes, assume certain definitions or rules^that make it possible, not being either true or false, ’to extend the language of phenomena to that of interphenomena’.^^40^^

p For a description of interphenomena in terms of ‘particle’ and ‘wave’, Reichenbach says, the following definitions are adopted. ’When we lay down the rule that the quantity had the same value before the measurement, we have introduced the particle interpretation.’ But if we assume that the quantity has all possible values simultaneously before the measurement this introduces the wave interpretation.^^41^^

p In certain conditions the particle and wave interpretations, Reichenbach notes further, are ’equivalent descriptions; both are admissible, and they say the same thing, merely using different languages.’^^42^^

p If certain definitions are postulated and the question of the physical properties of interphenomena is posed, Reichenbach states, one finds that ’the behaviour of interphenomena violates the principle of causality’, i.e. the principle of shortrange interaction fails and an anomaly arises. Reichenbach thinks that the existence of such anomalies is an inevitable feature of quantum mechanics, and therefore introduces a ’principle of anomaly’.^^43^^

p The following examples will help explain these ideas of Reichenbach’s and to draw_fccertain inferences. If the particle interpretation is applied to the diffraction of electrons by a single aperture, the principle of causality is satisfied, since the electron-particles hitting the screen produce scintillations on it. When, on the other hand, one employs the 49 wave interpretation, an anomaly arises, in other words the principle of short-range effect is violated, since the electronwaves contract incomprehensibly into points on hitting the screen.

p When electron diffraction by two apertures is observed and the particle interpretation is used, an anomaly arises (we omit the appropriate argument). When, on the other hand, this phenomenon is subjected to the wave interpretation, the principle of causality is satisfied, i.e. the principle of short-range interaction is not violated.

It follows from Reichenbach’s argumentation about the various interpretations or equivalent descriptions applied to the atomic world that for him a physical theory is simply a means of systematising of the observed in one way or another, and that the question of objective reality’s being reflected by the theory is thought to be deprived of meaning. This idealist interpretation of physical theory by Reichenbach is closely related to his neglect of the real dialectical unity of the particle-wave properties of matter.