Original Source
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The Refinement of Complementarity

Bohr's Como paper made very little impact upon its audience. It was not a paper on physics and presented no new empirically testable consequences for quantum theory. However, the Como paper did make it clear that Bohr now regarded quantum theory in a certain sense as complete. This surprised, puzzled and left unimpressed a great many of Bohr's colleagues.
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Bohr had intended his Como paper as a general outline of his new framework to be worked out in greater detail, and in the years following 1927 his attention turned to refining his concepts of complementarity. The application to quantum physics was always uppermost in Bohr's mind, but as early as 1929 it was obvious that he had a strong interest in carrying complementarity as an epistemological lesson to other fields, eventually developing it as a generalization of the classical framework over all empirical knowledge.
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His thoughts did not develop as he had originally planned, however. Instead, they were sharply influenced by the criticisms of Einstein, and a great deal of his thoughts were manifested in specific reply to challenges put forward by Einstein. Although the differences between Bohr and Einstein reflected a real disagreement about the nature of scientifically describing physical reality, there were also some very serious misunderstandings on both sides. Bohr probably weakened his own cause by not stating at the beginning that what was at stake was no less than a proposed revision of our understanding of our relationship between physical reality and the concepts we use to describe it. In a way, Einstein realized this more clearly than Bohr himself.
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The Bohr-Einstein Debates

About one month after Bohr's presentation of his Como paper, he met Albert Einstein at the Solvay Congress in Brussels. Einstein opposed Bohr's complementarity view and proposed the first of a series of thought experiments designed to get around the uncertainty principle so that the classical notion of the state of system could be retained. In 1927, Bohr had little difficulty in showing that Einstein's experiment failed to provide more empirical information about the system than permitted by the quantum postulate. But at the Solvay Congress of 1930, Einstein presented a particularly difficult thought experiment.
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Bohr spent a sleepless night pondering it, then demonstrated that Einstein had failed to take into consideration the effects of his own general relativity theory, which when considered, gave the exact degree of uncertainty that was compatible with Heisenberg's principle. Then, in 1935, in conjunction with B. Podolsky and N. Rosen, Einstein made his final attempt at a Gedanken-Experiment to show that the physical system had simultaneous properties that quantum theory could not explain, demonstrating that the theory was incomplete. This become commonly known as the EPR experiment, or paradox.
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This experiment brought to light the opposition between two conceptions of physical reality. Bohr concluded that complementarity is a "new feature of natural philosophy [which] means a radical revision of our attitude as regards physical reality...". Bohr felt that the difference between complementarity and the classical framework was such that there cannot be an "experimentum crucis" demonstrating the correctness of one view or another. In other words, from the point of view of complementarity, Bohr's answer was completely consistent, while from the point of view of the classical framework, Einstein's objections were equally consistent. Of course, Einstein's rejection allowed him to continue to use the classical framework, while Bohr's acceptance of complementarity forced him to adopt a new framework.
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I am not interested in exploring the actual experiments themselves. If the reader wishes, they may purchase "Atomic Theory and the Description of Nature; Philosophical Writings of Niels Bohr Series, Vol. 1" and read Bohr's comments personally. But I do want to stress that most of Bohr's writings after 1927 were designed primarily to win Einstein over to the framework of complementarity. This changed Bohr's original focus on complementarity as a "description of nature" into a use as a "conceptual framework" for a detailed analysis of the "physical" situation in numerous imaginary experiments. This application to quantum theory gave way to the "Copenhagen Interpretation" and Bohr's original concern with a fundamental revision of the framework of science was largely lost from sight.
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In his last interview, Bohr was asked, "Einstein was always apparently asking for a definition or a clarification, a precise formulation of what is the principle complementarity. Could you give that?" Bohr replied, "He also got it, but he did not like it." [2] Einstein seemed to think of complementarity as a principle in a physical sense, and so was frustrated by Bohr's failure (from Einstein's viewpoint) to provide a clear formulation which would have testable consequences, like Einstein's own principle of relativity.
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Einstein argued that properties corresponding to classical parameters belong to physical entities independent of observation, which he considered justified because such parameters have empirical reference to observed phenomenal properties which are used to confirm theory. Einstein felt that since, with the classical framework, it is always possible in principle to define precisely the mechanical state of the system even as it is being observed, it then follows that if these parameters used to define the state are interpreted as corresponding to properties possessed by the system, these systems must "really" always have simultaneousness of such properties as position and momentum.
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The EPR Experiment

Einstein finally gave up trying to disprove quantum mechanics by finding a way to simultaneously make observational determinations, and instead focused on demonstrating the incompleteness of quantum theory. And to argue that a theory is incomplete makes it necessary to stipulate what constitutes completeness. As a criterion of completeness, Einstein suggested "every element of physical reality must have a counterpart in physical theory. If without in any way disturbing a system, we can predict with certainty (i.e., with probability equal to unity) the value of a physical quantity, then there exists an element of physical reality corresponding to this physical quantity." [3]
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In the EPR experiment, the authors propose a situation in which two physical systems, the initial states of which are known, are allowed to interact for a finite period of time. As noted already, quantum theory says that once the systems interact, it becomes theoretically impossible to define the states of each system separately. However, given the representations of the initial states as defined in quantum formalism, it is possible to define the state of the systems combined, treated in this sense as an individual interacting whole. What cannot be defined by quantum theory is the state of each system considered separately either during the interaction or after that interaction. For that, another observation is needed. As the uncertainty principle demands, observation cannot determine both parameters necessary to define precisely the state of either system. However, from the observation of the first system and theoretical definition of the combined states of the two systems, by means of unambiguous communication, it is possible to theoretically predict the interaction of the second system without in any way interacting with that system.
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Bohr did not dispute this conclusion. And indeed, using the assumption that "position" and "momentum" refer to properties possessed by systems independent of an observational interaction, then what the observing system records is the causal effects of said properties. The claim that measuring the value of a phenomenal observable which is in no way physically interacting with the observer would seem to demand a mysterious, non-physical communication between the two systems. This claim does indeed seem unreasonable, as the EPR experiment was designed to show.
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However, from Bohr's point of view, these same theoretical parameters do not refer to properties of an independent reality, but instead refer to phenomenal properties. From this point of view, if there is no interaction to observe, say the position of the system, there is no phenomenal property to which the position parameter can refer. From this point of view, Einstein's conclusion seems equally unreasonable.
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Bohr believed that although we have "free choice" of which experiment to observe, that freedom merely indicated a freedom in which phenomenon we choose to bring about. Bohr writes: ...in the phenomena concerned we are not dealing with an incomplete description characterized by the arbitrary picking out of different elements of physical reality at the cost of sacrificing other such elements, but with a rational discrimination between essentially different experimental arrangements and procedures which are suited either for an unambiguous use of the idea of space location, or for a legitimate application of the conservation theorem of momentum. Any remaining appearance of arbitrariness concerns merely our freedom of handling the measuring instruments, characteristic of the very idea of experiment.
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In fact, the renunciation in each experimental arrangement of the one or the other of two aspects of the description of physical phenomena - the combination of which characterizes the method of classical physics, and which therefore in this sense may be considered complementary to one another - depends essentially on the impossibility, in the field of quantum theory, of accurately controlling the reaction of the object on the measuring instruments, i.e., the transfer of momentum in the case of position measurements and the displacement in case of momentum measurements...we are, in the "freedom of choice" offered by the...[EPR] arrangement, just concerned with a discrimination between different experimental procedures which allow of the unambiguous use of complementary classical concepts. [4]
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Bohr's alternative interpretation of the EPR experiment revealed the difference between two conceptions of physical reality. The ambiguity that Bohr focused on was hidden in Einstein's use of "state of the system". Bohr believed that when we determine the state of the system, the "system" to which we refer is necessarily a phenomenal object. Einstein, on the other hand, believed that observable properties of the state of the system is presumed to be existing independent from the observation.
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In a paper written in 1939, four years after the EPR experiment was proposed, Bohr writes: ...the very fact that in quantum phenomena no sharp separation can be made between an independent behavior of the objects and their interaction with the measurement instruments, lends indeed to any such phenomena a novel feature of individuality which evades all attempts at analysis on classical lines, because every imaginable experimental arrangement aiming at a subdivision of the phenomena will be incompatible with its appearance and give rise, within the latitude indicated by the uncertainty relations, to other phenomena of similar individual character.
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In fact the [EPR] paradox finds its complete solution within the framework of the quantum mechanical formalism, according to which no well-defined use of the concept of "state" can be made as referring to the object separate from the body with which it has been in contact, until the external conditions involved in the definition of this concept are unambiguously fixed by a further suitable control of the auxiliary body. [5]
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Bohr's point is that there can be no empirical reason for assuming classical parameters as referring to an "independent reality", and if one does make such an assumption, quantum theory becomes involved in the contradiction of asserting the atomic domain system as having incompatible properties of both particles and waves. As Bohr saw it, these descriptive concepts are well-defined only in reference to observed phenomenal objects, or abstractions.
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From Einstein's point of view, it seemed the "free choice" we have of which of the two possible measurements to make involves one and the same system. But from Bohr's point of view, in order to give terms like "position" and "momentum" empirical significance, "system" must be interpreted in the sense of that which is observed. And since the experimental arrangement necessary for alternative observations of position and momentum are mutually exclusive, he concluded that the two observations refer to the properties of two distinct phenomenal objects. There was simply no "same system" in a sense that both position and momentum could be observed.
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In short, the EPR experiment, as advanced by Einstein, assumes that in the interaction necessary to determine the parameter representing the phenomenal observation, it remains possible to describe the system independently of the observational interaction. And if the observing apparatus is changed, for example, if measurement involves momentum instead of position, Einstein assumed that it will not influence the "property" of the "system". In fact, Bohr was disheartened by Einstein's reasoning, for he felt it missed the core of his framework of complementarity and the quantum postulate. And unfortunately, since all Bohr's arguments were erected on the quantum postulate, it all by-passed Einstein.
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Bohr's Concept of "Phenomenon"

Following 1935, Bohr distinguished his view by emphasizing that the description of nature is free from ambiguity and metaphysical dogma only if it is realized that the observational basis of science depends on devising ways to describe unambiguously an individual interaction between systems. This point of view was already implicit in his Como paper, however after 1935 there was a real change in Bohr's emphasis and manner of speaking. In his first philosophical writing after the EPR experiment, in 1937, Bohr is careful to point out that what we must renounce is the classical attempt to "visualize" objects of quantum description as possessing "such inherent attributes as the idealizations of classical physics [i.e., "waves" and "particles"] would ascribe to the object. ..
...the fundamental postulate of the indivisibility of the quantum of action is itself, from the classical point of view, an irrational element which inevitably requires us to forgo a causal mode of description and which, because of the coupling between phenomena and their observation, forces us to adopt a new mode of description designated as complementary in the sense that any given application of classical concepts precludes the simultaneous use of other classical concepts which in a different connection are equally necessary for the elucidation of the phenomena...
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...the finite magnitude of the quantum of action prevents altogether a sharp distinction being made between a phenomenon and the agency by which it is observed, a distinction which underlies the customary concept of observation and, therefore, forms the basis of the classical ideas of motion. [6]

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From 1939 on, Bohr always used the word "phenomenon" to refer to the whole observational interaction. Bohr emphasized that the two complementary measurements of position and momentum are different phenomena and hence determine the properties of different phenomenal objects. Eventually he adopted a way of speaking which referred to the complementarity of different phenomena or complementary evidence from different observations.
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For this reason, some physicists have seen Bohr's complementarity as having undergone a radical change during this time and that therefore there are really two distinct notions of complementarity. Bohr agreed that complementarity presents a many faceted viewpoint, yet he resolutely maintained that the complementarity of modes of description and the complementarity of phenomena are simply two consequences of the quantum postulate. The first indication of a change in Bohr's way of describing complementarity came in 1938 when he wrote: Information regarding the behavior of an atomic object obtained under definite experimental conditions may...be adequately characterized as complementary to any information about the same object by some other experimental arrangement excluding the fulfillment of the first conditions. [7] ..
Later, Bohr began to de-emphasize the complementarity between space-time description and applying the conservation principle in favor of defining the complementary relationship as holding between phenomena. He writes: Although the phenomena in quantum physics can no longer be combined in the customary manner, they can be said to be complementary in a sense that only together do they exhaust the evidence regarding the objects, which is unambiguously definable. [8] ..
This sentence, or variations of it, became a standard formula for Bohr's later work and is repeated over and over again in later essays. He does continue to use other formulations as well however, for example: ...we are faced with the contrast revealed by the comparison between observations regarding an atomic system, obtained by means of different experimental arrangements. Such empirical evidence exhibits a novel relationship, which has no analogue in classical physics and which may be conveniently termed "complementarity" in order to stress that in the contrasting phenomena we have to do with equally essential aspects of all well-defined knowledge about the objects. [9] ..
Once it is recognized that Bohr's talk of complementarity phenomena refers to the effects observed under different experimental conditions, then the confusion between complementary aspects of nature and complementary descriptive modes disappears. All of Bohr's analyses of experiments in his debates with Einstein were intended to show that determining the spatio-temporal properties of the observed object is possible only in experimental interactions which preclude any interaction determining momentum or energy properties of the object. Thus, to give empirical reference to terms like spatio-temporal co-ordination and causal descriptions require physical interactions, "phenomena" in Bohr's later usage, which exclude each other but are complementary.
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The Object of Description

His debate with Einstein forced Bohr to realize that there was a fundamental ambiguity hidden in the notion of "physical reality" as that which is described in natural science. In order for observation to determine unambiguously the values of phenomenal objects, the description of the observation must indicate exactly what part of the whole interaction is considered observing instruments and what part is considered phenomenal object being observed. For this reason, Bohr introduced an "inherent arbitrariness" into the description of any phenomenal object, making what is now part of the observing system part of the observed or visa versa. Only by making such an arbitrary separation between what is treated as the means of observation and what the phenomenal object is can an unambiguous description of what is observed be communicated.
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"Observing system" and "observed object" are terms which are well defined only in the context of a particular description of interaction. They must be regarded as descriptive categories invoked for an unambiguous communication of the results of an observation rather than as referring to different constituents of nature. The requirement that descriptive terms must have some empirical reference if they are to describe phenomenal objects make their attempted use to describe an atomic system apart from its phenomenal appearances in observational interactions descriptively ambiguous.
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Bohr never argued that the notion of object as an independent reality is "meaningless", but he did argue that because of a certain physical fact, the individuality of atomic interactions, the classical description of particles and waves become restricted in their reference to the phenomenal object. Therefore, the use of these concepts to picture the state of an isolated system does not refer to an independent reality but to an abstraction. Bohr's point is that we cannot describe an independent reality in the manner which was thought possible in the classical framework, because that framework makes different presuppositions about the nature of observation.
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An example involving the concept of temperature may help clarify Bohr's position here. Since it has been empirically discovered that material bodies can be represented as composed of many molecules, and that the property of temperature observed in many-molecule bodies can be represented as the causal effects of the motion of these molecules, these physical facts make the concept of temperature inapplicable below the molecular level.

In Bohr's epistemological lesson we are taught that at the atomic level, spatio-temporal concepts are not defined in an unambiguous sense for an objective description of an independent physical reality. This ends Part 5 of this review. Thanks for reading!
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Footnotes

[1] Niels Bohr, Can Quantum Mechanical Description of Reality Be Considered Complete? Review 47
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[2] AHQP, Bohr, Last Interview, page 4
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[3] A. Einstein, B. Podolsky, N. Rosen, Can Quantum Mechanical Description of Reality Be Considered Complete? Review 48
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[4] Niels Bohr, Can Quantum Mechanical Description of Reality Be Considered Complete? Review 48
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[5] Niels Bohr, The Causality Problem in Atomic Physics
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[6] Niels Bohr, Atomic Theory and the Description of Nature
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[7] Niels Bohr, Natural Philosophy and Human Culture
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[8] Niels Bohr, Newton's Principles and Modern Atomic Physics
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[9] Niels Bohr, On the Notions of Causality and Complementarity



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The Framework of Complementarity

Part 1 - Overview Early Years Bohr Formulates Complementarity
Part 2 - Argument for Complementarity
Part 3 - Comments on Complementarity
Part 4 - Complementarity and the Uncertainty Principle
Part 5 - Refinement of Complementarity
Part 6 - Extension of Complementarity
Part 7 - The Nature of Empirical Knowledge
Part 8 - Complementarity and the Metaphysics of Quality