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p We cited above statements by distinguished scientists on the methodological value of the principle of observability in physics. Let us add that Max Born included this principle in one of his last papers among modern physics’ most important methods of thought, because in his view the methods of thought dealt with in traditional philosophy had ceased to operate in the practice of modern physics.^^23^^ V. A. Fock also spoke about the great positive role of this principle in establishing the laws of quantum mechanics.^^24^^

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p Before turning to the theme of this section let us note (which can be seen from the above) that the term ” observability principle’ does not altogether adequately reflect its content. Max Born in particular speaks, in the paper mentioned above, not of ‘observability’ but of ‘decidability’ or ‘determinability’ (Entscheidbarkeit), and formulated the principle as follows: not to use any concept for which it is undecidable in principle whether it corresponds or not to reality.^^25^^

p From the angle of the true content of the observability principle, Heisenberg’s reminiscences about the time when what is now known as the Bohr-Heisenberg interpretation of quantum mechanics was created, present considerable interest.^^26^^ After the paper read by Heisenberg in 1926 in which the idea of describing phenomena solely by means of observable quantities played a major role, Einstein asked him: ’What did you mean by only observable quantities?’ Heisenberg’s reply was that he ’did not believe any more in electronic orbits, in spite of the tracks in a cloud chamber’. He felt it necessary to ’go back to those quantities which really can be observed’ and that this had been exactly Einstein’s view in the theory of relativity ’because he also had abandoned absolute time and introduced only the time of the special coordinate system’.

p Heisenberg then continued: ’Well, he laughed at me and then he said: "but you must realize that it is completely wrong ... it is nonsense".’

p Heisenberg gives Einstein’s explanation: ’Whether you can observe a thing or not depends on the theory which you see. It is the theory which decides what can be observed.’ ’Einstein,’ he continued, ’had pointed out tome that it is really dangerous to say that one should only speak about observable quantities. Because every reasonable theory will, besides all things which one can immediately observe, also give the possibility of observing other things more indirectly. For instance, Mach himself had believed that the concept of the atom was only a point of convenience, a point of economy in thinking, he didn’t believe in the reality of the atoms. Nowadays everybody would say that it is nonsense, that it is quite clear that the atoms really exist. These were the points [Heisenberg stressed] which Einstein raised.’^^27^^

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p Later, when Heisenberg discussed the interpretation of quantum mechanics with Bohr, he remembered Einstein’s remark: ’It is the theory which decides what can be observed.’ ’In this way,’ Heisenberg said, ’things became clear ... so finally we all agreed that now we had understood quantum theory.’^^28^^

p Perhaps we have cited Heisenberg’s remarks at too great length, but we have done so deliberately. We want to show the reader with his own eyes, so to say, that the authors of the observability principle did not associate it at all with positivist and idealist principles. That is the first point. The second point is that the ‘exclusion’ of a certain concept from a new theory as ‘unobservable’, and without meaning, is not simply a consequence of this new theory but that it helps construct the new theory. Einstein’s explanation, as a matter of fact, means that when one has to pass, and is passing, from one fundamental theory to another, more general and deeper one, the old theory must necessarily be altered at certain points: new, more meaningful concepts are developed in place of some basic concepts or other of the old theory that reflect the sphere of phenomena with which the old theory could not cope. It is this thesis that is also employed in the search for a new theory by means of the observability principle. What follows is devoted to concrete analysis of this idea.

p How and why does the question of the observability in principle or non-observability of a quantity arise? To bring out the methodological role of this principle it is essential to determine that such and such quantity precisely is not observable in principle. If, for example, it had been assumed, before the theory of relativity, that ether was not observable in principle, that assumption would not have affected the meaning of the principles of classical physics in any essential way: the concept ‘ether’ did not figure in classical mechanics, as we know, and classical electromagnetic theory would have become more rigorous and would have better reflected its object, because the concept of field would have had an appropriate place in it (as a matter of fact, that is exactly what happened in the electromagnetic theory after Maxwell). The history of the creation of relativity theory witnesses that establishment of the non- observability precisely of absolute simultaneity was the methodological starting point from which the development of 110 non-classical theories began. In general it was establishment of the non-observability of that quantity and no other, and the extraction from it of everything needed to build a new theory diverging in its fundamental content from the principles of known theories, verified by experiment, that constituted the element that was unknown earlier from the angle of the cognising mind’s approaches to the phenomena of nature, which was ‘alien’ to the ’style of thinking’ of physicists of the classical period of the development of science.

p Thus, one cannot answer the question of the heuristic value of the observability principle in any concrete way without a methodological analysis of the appearance of the theory of relativity and, if we pose the question on a broader scale, without a methodological analysis of the rise of nonclassical theories in physics.

p How did Einstein’s special theory of relativity come about, remembering the methodological aspect of the question? Classical mechanics (with the notions of absolute space and absolute time, characteristic of classical physics), in accordance with experience, affirmed the relativity of the uniform, rectilinear motion of bodies (Galileo’s principle of relativity)  [110•* ; this, however, contradicts the fact that the velocity of light is independent of the motion of the source. On the other hand, classical electromagnetic theory assumed ether, but this assumption contradicts Galileo’s principle of relativity although it agrees that the velocity of light is independent of the frame of reference.

p The contradictions that arose here were resolved by Einstein who extended the statement about the relativity of uniform, rectilinear motion to electromagnetic phenomena and adopted it as the first principle of a new non-classical theory, the theory of relativity. As the second principle of his theory he put forward the proposition that the velocity of light is independent of the motion of the source, expressing it as the principle of the constancy of the velocity of light. It proved—and here the dialectics in Einstein’s reasoning was revealed especially clearly—that these 111 principles did not contradict each other if the classical concepts of space and time were altered.

p It would have been possible not so much to resolve the contradiction between Galileo’s principle of relativity and the fact that the velocity of light is independent of the motion of the source (as permitted by the hypothesis of ether) as, it seemed, to eliminate it. For that some hypothesis in the spirit of classical conceptions could have been added to these statements.  [111•*  Einstein did without these arbitrary hypotheses, and this opened the way to him to create the first non-classical theory.

p The idea of changing the classical concepts of space and time was thus the turning point in the genesis of the theory of relativity, but the source of this thought in Einstein’s reasoning was his rejection of the concept of absolute simultaneity in the content and structure of the theory. Or rather, when he excluded absolute simultaneity and introduced relative simultaneity, he reached a higher synthesis of two mutually contradictory statements: namely, that of the relativity of uniform, rectilinear motion and that of the velocity of light being independent of the motion of the source. On that foundation the theory of relativity was consolidated.

p Quantum mechanics arose in a similar way, although the way it came into physics was much more complicated and confusing than the birth of the theory of relativity. We shall not dwell on this, but shall just make the following general observation. When Heisenberg assumed the nonobservability of an electron’s position and velocity on its orbit in the atom, he opened the first door, by his matrix mechanics so to speak, for Bohr’s complementarity principle (with its ’relativity with respect to the means of observation’ and the other basic concepts unknown to classical theories) which underlies modern quantum mechanics; the ‘second’ door for the complementarity principle was opened by Schrodinger’s wave mechanics.

p When one examines the process of cognition in physics, or cognition as a whole, one finds it has a very peculiar 112 dual nature. (1) In cognising something, i.e. in going beyond the limits of the already known, we extend to this something the established concepts, laws, and theory that are treated as known. (2) This process of extension does not exclude but, on the contrary, implies that in doing so one may have to alter (revise) some established basic concepts and principles or other of the theory qualitatively and therefore, as a result, to construct new concepts and principles and a new theory. These two elements of cognition, in spite of their being opposite, pass into each other and are in fact one; depending on the conditions, however (which above all include the cognised object itself with its specific features), one or another of them is pushed to the fore.  [112•*  Here we are interested only in the second element because it is exactly when a theory new in its fundamental content is born that ‘non-observables’ appear.

p From this point of view the revision of a concept ( quantity) in physics can be reduced to the following: it is assumed (on the basis of certain considerations) that this concept (quantity) is regarded as observable in principle (it can often be determined experimentally) from the standpoint of the established theory extended to the unknown (new) field of research; it is stated that experimental determination of this concept (quantity) in regard to the unknown field of research will either not yield a positive result or at least sow doubts about its objectively real existence; and a mental equivalent of the concept (quantity) so experimentally rejected is established. This mental equivalent is unobservable in principle from the angle of the theory embracing the new field of research, a theory that still has to be crystallised out from the established one.

p Thus, when a new theory (which still has to take shape) may and does grow from an already formulated (old) theory on a new foundation, the introduction of something ’ unobservable in principle’ is inevitable in certain circumstances. In this respect one has to agree with Heisenberg, to whom ’it is more advisable initially to introduce a great wealth of concepts into a physical theory [he does not specify whether he means a developing theory or one already 113 developed—M.O.] without consideration of their rigorous justification by experiment [he does without the needed addition: by a new experiment—M.0.\ and to leave the decision to nature, in each case of any theory, whether and at what points a revision of the basic principles is necessary’.^^29^^

p Every step in extending an established theory to an unknown field of research should, of course, be subjected to experimental verification. That applies equally to our first and second elements (above) of the expansion of physical knowledge, and before this verification these elements should therefore be considered as hypotheses. Here Engels’ words that the form of development of science, in so far as it thinks, is the hypothesis are particularly appropriate^^30^^: without a hypothesis, this necessary element of scientific knowledge, there would have been no progress in either classical or modern physics; the development of the latter completely disproves the inventions of positivists, who reject the scientific hypothesis and consider the physical theory only as systematisation of the ‘observable’ and not as the ever more accurate and complete reflection of the material world.

p In modern physics it is not so much the hypotheses which, being confirmed by experiment, reinforce already established theories as the assumptions that lead to the creation of new theories and a radical restructuring of science that are most important. The assumption of the non- observability of a certain quantity as the starting point of a theory being created is just such a hypothesis.

p This assumption is neither a descriptive nor an explanatory hypothesis. Unlike the latter, it does not see the explanation of new facts as its task; rather it leads to an operational definition of new concepts in the developing new theory. Like descriptive and explanatory hypotheses, the assumption of the non-observability of such and such a quantity is also ‘evoked’ by an experiment. In that lies the source of its cognitive force. Thus, the exclusion^ absolute simultaneity and introduction of relative simultaneity in studies of electromagnetic phenomena in the moving bodies are a schematised, idealised expression of the negative result of the Michelson-Morley experiment. The same has to be said also about the exclusion of the classical trajectory and introduction of the concept of relativity to the means of observation into studies of phenomena on the atomic scale: 114 they were ’evoked’ by the experimental data on the particle and wave properties of one and the same micro-objects.

p The non-observability of a quantity in principle is thus not revealed as a result of elucidating the fact that the corresponding statements about the quantity are incompatible? with the principles of the theory; it had been assumed before these principles (and therefore the theory itself) received the right to exist and their explicit formulation. The process of excluding the non-observable quantity, however, is at the same time, in its developed form, the process of crystallising the theory’s principles and concepts on the basis of certain experimental observations. To put it more definitely, the establishment of non-observability is an indication that the old theory is no longer effective in some respect (as regards the new sphere of phenomena) and that a new theory needs to be created.

p It is exactly these fundamental features that determine the heuristic value of the principle of observability. As a method of finding the laws of nature the principle not only does not reject other methods of theoretical and practical research, but, on the contrary, presupposes their use. Only then can one expect fruitful results from it.

p In order to picture more concretely what this last remark means, let us consider the creation of wave mechanics and discovery of positron from the angle of our present theme, and also the proposition about the non- observability of the details of elementary particles’ behaviour when the distances between them become ultra-small.

p Schrodinger created his wave mechanics independently of Heisenberg’s matrix mechanics, as we know, and, as he demonstrated, it was mathematically equivalent to the matrix mechanics. Schrodinger arrived at wave mechanics by analysing the connection that he had found between de Broglie’s idea of ’waves of matter’ and Hamilton’s work on dynamics and geometrical optics. Thus, it was not the principle of observability that played the methodological role in the formulation of the wave mechanics, but an explanatory hypothesis, concretely the hypothesis of ’waves of matter’. To put it more accurately, however, the mathematical hypothesis served as the method here,^^31^^ while the ’waves of matter’ helped represent the matter more ’ visually’ rather than determined the quest.

p That visual models played no decisive role in the creation 115 of wave mechanics stands out particularly clearly in its further development, which led (along with the development of matrix mechanics) to modern quantum mechanics in which the concept ’waves of matter’ is not preserved literally, while the concept of a wave function in its probability interpretation is a basic one. The enormous heuristic significance of the method of mathematical hypothesis came out even more clearly in Dirac’s brilliant prediction of the positron, which was not only not governed by visual models of any kind but was rather made in defiance of them.  [115•* 

p In the literature ’waves of matter’, ’wave function’, and Dirac’s ‘holes’ are frequently called ‘unobservables’, and attempts are made to draw conclusions against the principle of observability from the corresponding discoveries made by applying the technique of mathematical hypothesis. In fact, however, the principle of observability and the mathematical hypothesis mutually mediate and complement each other. Here are some considerations apropos of that.

p The appearance of the ‘unobservables’ to which mathematical hypothesis leads is nothing other than the process of creating (or rather one of the elements of creating) a new theory in which these ‘unobservables’ (as they are regarded from the angle of the old theory) become observables. The exclusion of an ‘unobservable’ (from the angle of the new theory), however, as we have already made clear, is also a process of creating (or rather an element of the creating) of the new theory. Without going into details of the relevant argument here, we may note that if, let us say, the wave function in its probability interpretation and the positron are observables from the standpoint of quantum mechanics and quantum electrodynamics, respectively, that only confirms the idea of an inner connection between the principle of observability and the method of mathematical hypothesis. Bearing all these circumstances in mind, we must stress the great progressive significance for the development of new theories of the introduction of ’ unobservables’ in this sense of the term into science.

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p Finally, let us briefly comment on the proposition about the non-observability in principle of the details of elementary particles’ behaviour when they come within ultrasmall distances of one another (i.e. high energy particles^^32^^), a proposition being employed in the theory of elementary particles that is now taking shape.

p The fact that this theory treats elementary particles (and there are the necessary experimental grounds for doing it) as transformable into each other according to the conservation laws and the principles of symmetry (we would be justified in saying that mutual transformability is the mode of existence of elementary particles) makes this proposition very plausible. In particular, it accords with the accepted view that interactions between high energy particles cannot be described by such quantummechanical concepts as wave functions and operators.

p There is no closed theory of elementary particles, however, that would resolve the contradiction between quantum mechanics and the theory of relativity, one of the crucial contradictions of the modern physics, in a higher synthesis. This situation in the modern physics of elementary particles resembles that which built up when quantum mechanics was taking shape soon after the creation of matrix and wave mechanics, but with quite serious differences. (1) The conception of the theory of elementary particles (represented by G. F. Chew) that expresses the principle of observability in its pure form (rejecting the idea of a space-time continuum)  [116•*  does not yet have a developed mathematical formalism. (2) The other conception of the development of the modern theory of elementary particles (represented by Heisenberg), i.e. the theory of quantised fields based on the idea of a space-time continuum, is not sufficiently ‘crazy’ for a new theory, to use Bohr’s expression. In any event, Chew’s and Heisenberg’s different conceptions are evolving and perhaps, in coming closer together, may lead to the formulation of a closed theory of elementary particles.

p There is the possibility, of course, of a theory of elementary particles being formed in another concrete way. 117 Problems of this kind, it seems to us, can hardly be solved in general if one does not take into account the profound meaning of Niels Bohr’s observation ’that the reason why no progress was being made in the theory of transformations of matter occurring at very high energies is that we have not so far found among these processes any one exhibiting a sufficiently violent contradiction with what could be expected from current ideas to give us a clear and unambiguous indication of how we have to modify these ideas’.³³ In examining the heuristic value of the principle of observability we have tried to indicate clearly that the method based on this principle presupposes a necessary connection with the other methods of physics and that it is employed not according to a known scheme, given once and for all, but concretely, in various ways, developing new schemes of application each time during the study of new spheres of phenomena.

p REFERENCES

p  ^^1^^ See, for instance, Max Born. Symbol und Wirklichkeit. Physikalische Blatter, 1965, 21, 2: 57-59; R. P. Feynman, R. B. Leighton, and Matthew Sands. The Feynman Lectures on Physics, Vol. I ( Addison-Wesley Publishing Co., Reading, Mass., 1963), pp 38-39; I. V. Kuznetsov. The Structure of a Physical Theory. Voprosy filosofii, 1967, 11: 89; P. S. Dyshlevy and V. M. Sviridenko. On the Principle of Observability and the Conception of Complementarity. In: P. S. Dyshlevy (Ed.). Metodologicheskie problemy teorii izmereniya (Naukova Dumka Publishers, Kiev, 1966); M. Bunge. Physics and Reality. Dialectica, 1966, 20, 2.

p  ^^2^^ See V. I. Lenin. Materialism and Empiric-criticism. Collected Works, Vol. 14 (Progress Publishers, Moscow), p 260.

p  ^^3^^ Albert Einstein. Relativity. The Special and General Theory ( Hartsdale House, New York, 1947), p 26.

p  ^^4^^ Paul Langevin. La Notion de corpuscules et d”atom.es (Hermann et C*e, Paris, 1934), p 40.

p  ^^8^^ R. P. Feynman, et al. Op. cit., pp 38-39.

p  ^^6^^ F. A. Kaempffer. Concepts in Quantum Mechanics (Academic Press, New York, London, 1965), p 1.

p  ^^7^^ Sir Arthur Eddington. The Philosophy of Physical Science (CUP, Cambridge, 1949), p 37.

p  ^^8^^ Ibid., p 104.

p  ^^9^^ Ibid.

p  ^^10^^ P. A. M. Dirac. The Principles of Quantum Mechanics (Clarendon Press, Oxford, 1958), p 3.

p  ^^11^^ W. Heisenberg, E. Schrodinger, and P. A. M. Dirac. Die modern* Atomtheorie (S. Hirzel Verlag, Leipzig, 1934), p 35.

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p  ^^12^^ P. A. M. Dirac. Op. cit., p 3.

p  ^^13^^ Werner Heisenberg. Das Naturbild der heutigen Physik (Rowohlt, Hamburg, 1955), p 21.

p  ^^14^^ For a brief review of this topic, see Dyshlevy and Sviridenko’s paper cited above.

p  ^^16^^ S. G. Suvorov. Max Born and His Philosophical Views. In: Max Born. Fizika v zhizni moyego pokoleniya (Nauka Publishers, Moscow, 1963), p 505.

p  ^^18^^ Werner Heisenberg. Die physikalischen Prinzipien der Quantentheorie (Hirzel Verlag, Stuttgart, 1958), p 1.

p  ^^17^^ Max Born. Experiment and Theory in Physics (CUP, Cambridge, 1944), pp 15-18.

p  ^^18^^ Max Planck. The Universe in the Light of Modern Physics (Norton & Co., New York, 1931), pp 49, 50.

p  ^^18^^ Max Born. Op. cit., pp 38-39.

p  ^^20^^ Karl Marx and Frederick Engels. Collected Works, Vol. 3 (Progress Publishers, Moscow, 1973), pp 335-376.

p  ^^21^^ Arnold Sommerfeld. Atombau und Spektrallinien, Vol. II (Vieweg & Sohn, Brunswick, 1951), pp 195-196.

p  ^^22^^ Werner Heisenberg. Physics and Philosophy (George Allen & Unwin, London, 1959), pp 76-77.

p  ^^23^^ Max Born. Symbol und Wirklichkeit. Physikalische Blatter, 1965, 21, 2: 57-58.

p  ^^24^^ V. A. Fock. The Basic Laws of Physics in the Light of Dialectical Materialism. Vestnik Leningradskogo universiteta, 1949, 4: 44.

p  ^^25^^ Max Born. Op. cit., p 57.

p 26 Werner Heisenberg and Niels Bohr. Die Kopenhagener Deutung der Quantentheorie (Ernst Battenberg Verlag, Stuttgart, 1963).

p  ^^27^^ Werner Heisenberg. From a Life of Physics. Evening Lectures at the International Centre for Theoretical Physics, Trieste, Italy. IAEA Bulletin (Special Supplement), 1968, pp 36-37. See also: Werner Heisenberg. Der Tell und das Ganze (Munich, 1969), pp 91-92.

p  ^^28^^ Werner Heisenberg. From a Life of Physics. Evening Lectures at the International Centre for Theoretical Physics, Trieste, Italy. IAEA Bulletin (Special Supplement), 1968, p 41.

p  ^^29^^ Werner Heisenberg. Die physikalischen Prinzipien der Quantentheorie, pp 1-2.

p  ^^30^^ Frederick Engels. Dialectics of Nature (Lawrence & Wishart, London, 1940), p 158.

p  ^^31^^ On the method of mathematical hypothesis in physics see: S. I. Vavilov. Lenin and Modern Physics. Sobranie sochinenii, Vol. 3 (AN SSSR, Moscow, 1956), p 79.

p  ^^32^^ This assumption was stressed by I. E. Tamm in his paper on elementary particles in Glazami uchenogo (Through the Scientist’s Eyes) (AN SSSR, Moscow, 1963), p 188.

 ^^33^^ Leon Rosenfeld. Consolidation and Extension of the Conception of Complementarity. In: S. Rosental (Ed.). Niels Bohr in the Thirties (North-Holland Publishing Co., Amsterdam, 1967), p 118.