Hence we conclude that if religion does indeed deal with objective truths, it ought to adopt the same criteria of truth as science. But I myself find the division of the world into an objective and a subjective side much too arbitrary. The fact that religions through the ages have spoken in images, parables, and paradoxes means simply that there are no other ways of grasping the reality to which they refer. But that does not mean that it is not a genuine reality. The whole thing started with the theory of relativity.
In the past, the statement that two events are simultaneous was considered an objective assertion, one that could be communicated quite simply and that was open to verification by any observer. However, the relativistic description is also objective inasmuch as every observer can deduce by calculation what the other observer will perceive or has perceived.
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For all that, we have come a long way from the classical ideal of objective descriptions. In quantum mechanics the departure from this ideal has been even more radical. We can still use the objectifying language of classical physics to make statements about observable facts. For instance, we can say that a photographic plate has been blackened, or that cloud droplets have formed. But we can say nothing about the atoms themselves. And what predictions we base on such findings depend on the way we pose our experimental question, and here the observer has freedom of choice.
Naturally, it still makes no difference whether the observer is a man, an animal, or a piece of apparatus, but it is no longer possible to make predictions without reference to the observer or the means of observation. To that extent, every physical process may be said to have objective and subjective features.
The objective world of nineteenth-century science was, as we know today, an ideal, limiting case, but not the whole reality. Admittedly, even in our future encounters with reality we shall have to distinguish between the objective and the subjective side, to make a division between the two. But the location of the separation may depend on the way things are looked at; to a certain extent it can be chosen at will. The fact that different religions try to express this content in quite distinct spiritual forms is no real objection.
A quarter century before mathematician Lillian Lieber demonstrated how mathematical abstractions like infinity, which have no correlate in physical reality, offer an analogue for moral questions , Bohr considers whether or not the tenets of religion can similarly offer useful abstractions, even though they are not to be taken as objective truth:. In mathematics we can take our inner distance from the content of our statements.
In the final analysis mathematics is a mental game that we can play or not play as we choose. Religion, on the other hand, deals with ourselves, with our life and death; its promises are meant to govern our actions and thus, at least indirectly, our very existence. We cannot just look at them impassively from the outside. Moreover, our attitude to religious questions cannot be separated from our attitude to society.
Atoms are neither heuristic nor logical constructions. A couple of times he emphasized this directly using arguments from experiments in a very similar way to Ian Hacking and Nancy Cartwright much later. It makes much sense to characterize Bohr in modern terms as an entity realist who opposes theory realism Folse Neither does the theory of relativity, Bohr argued, provide us with a literal representation, since the velocity of light is introduced with a factor of i in the definition of the fourth coordinate in a four-dimensional manifold CC , p.
Instead these theories can only be used symbolically to predict observations under well-defined conditions. Thus Bohr was an antirealist or an instrumentalist when it comes to theories. In general, Bohr considered the demands of complementarity in quantum mechanics to be logically on a par with the requirements of relativity in the theory of relativity. He believed that both theories were a result of novel aspects of the observation problem, namely the fact that observation in physics is context-dependent.
This again is due to the existence of a maximum velocity of propagation of all actions in the domain of relativity and a minimum of any action in the domain of quantum mechanics. And it is because of these universal limits that it is impossible in the theory of relativity to make an unambiguous separation between time and space without reference to the observer the context and impossible in quantum mechanics to make a sharp distinction between the behavior of the object and its interaction with the means of observation CC , p.
In emphasizing the necessity of classical concepts for the description of the quantum phenomena, Bohr was influenced by Kant or neo-Kantianism. But he was a naturalized or a pragmatized Kantian. The classical concepts are merely explications of common concepts that are already a result of our adaptation to the world. These concepts and the conditions of their application determine the conditions for objective knowledge.
The discovery of the quantization of action has revealed to us, however, that we cannot apply these concepts to quantum objects as we did in classical physics. Now kinematic and dynamic properties represented by conjugate variables can be meaningfully ascribed to the object only in relation to some actual experimental results, whereas classical physics attributes such properties to the object regardless of whether we actually observe them or not.
In other words, Bohr denied that classical concepts could be used to attribute properties to a physical world in-itself behind the phenomena, i. In contrast, classical physics rests on an idealization, he said, in the sense that it assumes that the physical world has these properties in-itself, i. Complementarity is first and foremost a semantic and epistemological reading of quantum mechanics that carries certain ontological implications. Bohr's view was, to phrase it in a modern philosophical jargon, that the truth conditions of sentences ascribing a certain kinematic or dynamic value to an atomic object are dependent on the apparatus involved, in such a way that these truth conditions have to include reference to the experimental setup as well as the actual outcome of the experiment.
This claim is called Bohr's indefinability thesis Murdoch ; Faye Hence, those physicists who accuse this interpretation of operating with a mysterious collapse of the wave function during measurements haven't got it right. Indeed, Bohr, Heisenberg, and many other physicists considered complementarity to be the only rational interpretation of the quantum world. They thought that it gave us the understanding of atomic phenomena in accordance with the conditions for any physical description and the possible objective knowledge of the world. Bohr believed that atoms are real, but it remains a much debated point in recent literature what sort of reality he believed them to have, whether or not they are something beyond and different from what they are observed to be.
Henry Folse argues that Bohr must operate with a distinction between a phenomenal and a transcendental object. The reason is that this is the only way it makes sense to talk about the physical disturbance of the atomic object by the measuring instrument as Bohr did for a while Folse , But Jan Faye has replied that Bohr gave up the disturbance metaphor in connection with his discussion of the EPR thought-experiment because he realized that it was misleading.
Moreover, there is no further evidence in Bohr's writings indicating that Bohr would attribute intrinsic and measurement-independent state properties to atomic objects though quite unintelligible and inaccessible to us in addition to the classical ones being manifested in measurement Faye Complementarity has been commonly misunderstood in several ways, some of which shall be outlined in this section.
First of all, earlier generations of philosophers and scientists have often accused Bohr's interpretation of being positivistic or subjectivistic. Today philosophers have almost reached a consensus that it is neither. There are, as many have noticed, both typically realist as well as antirealist elements involved in it, and it has affinities with Kant or neo-Kantianism. The influence of Kant or Kantian thinking on Bohr's philosophy seems to have several sources. But because Bohr's view on complementarity has wrongly been associated with positivism and subjectivism, much confusion still seems to stick to the Copenhagen interpretation.
Don Howard argues, however, that what is commonly known as the Copenhagen interpretation of quantum mechanics, regarded as representing a unitary Copenhagen point of view, differs significantly from Bohr's complementarity interpretation. More recently, Mara Beller argued that Bohr's statements are intelligible only if we presume that he was a radical operationalist or a simple-minded positivist.
In fact, complementarity was established as the orthodox interpretation of quantum mechanics in the s, a time when positivism was prevalent in philosophy of science, and some commentators have taken the two to be closely associated. Although their anti-metaphysical approach to science may have had some influence on Bohr especially around during his final discussion with Einstein about the completeness of quantum mechanics , one must recall that Bohr always saw complementarity as a necessary response to the indeterministic description of quantum mechanics due to the quantum of action.
The quantum of action was an empirical discovery, not a consequence of a certain epistemological theory, and Bohr thought that indeterminism was the price to pay to avoid paradoxes. Never did Bohr appeal to a verificationist theory of meaning; nor did he claim classical concepts to be operationally defined. But it cannot be denied that some of the logical empiricists rightly or wrongly found support for their own philosophy in Bohr's interpretation and that Bohr sometimes confirmed them in their impressions Faye Second, many physicists and philosophers see the reduction of the wave function as an important part of the Copenhagen interpretation.
But Bohr never talked about the collapse of the wave packet. Nor did it make sense for him to do so because this would mean that one must understand the wave function as referring to something physically real. Bohr spoke of the mathematical formalism of quantum mechanics, including the state vector or the wave function, as a symbolic representation. Bohr associated the use of a pictorial representation with what can be visualized in space and time. Quantum systems are not vizualizable because their states cannot be tracked down in space and time as can classical systems.
The reason is, according to Bohr, that a quantum system has no definite kinematical or dynamical state prior to any measurement. Also the fact that the mathematical formulation of quantum states consists of imaginary numbers tells us that the state vector is not susceptible to a pictorial interpretation CC , p. Thus, the state vector is symbolic. Third, Bohr flatly denied the ontological thesis that the subject has any direct impact on the outcome of a measurement.
Hence, when he occasionally mentioned the subjective character of quantum phenomena and the difficulties of distinguishing the object from the subject in quantum mechanics, he did not think of it as a problem confined to the observation of atoms alone. Rather, by referring to the subjective character of quantum phenomena he was expressing the epistemological thesis that all observations in physics are in fact context-dependent.
There exists, according to Bohr, no view from nowhere in virtue of which quantum objects can be described.
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Later he always talked about the interaction between the object and the measurement apparatus which was taken to be completely objective. The cat would be dead or alive long before we open the box to find out. What Bohr claimed was, however, that the state of the object and the state of the instrument are dynamically inseparable during the interaction. Moreover, the atomic object does not posses any state separate from the one it manifests at the end of the interaction because the measuring instrument establishes the necessary conditions under which it makes sense to use the state concept.
It was the same analysis that Bohr applied in answering the challenge of the EPR-paper. Bohr's reply was that we cannot separate the dynamical and kinematical properties of a joint system of two particles until we actually have made a measurement and thereby set the experimental conditions for the ascription of a certain state value CC , p. Bohr's way of addressing the puzzle was to point out that individual states of a pair of coupled particles cannot be considered in isolation, in the same way as the state of the object and the state of the instrument are dynamically inseparable during measurements.
Thus, based on our knowledge of a particular state value of the auxiliary body A, being an atomic object or an instrument, we may then infer the state value of the object B with which A once interacted Faye , pp. It therefore makes sense when Howard , p. Finally, when Bohr insisted on the use of classical concepts for understanding quantum phenomena, he did not believe, as it is sometimes suggested, that macroscopic objects or the measuring apparatus always have to be described in terms of the dynamical laws of classical physics. The use of the classical concepts is necessary, according to Bohr, because by these we have learned to communicate to others about our physical experience.
The classical concepts are merely a refinement of everyday concepts of position and action in space and time. However, the use of the classical concepts is not the same in quantum mechanics as in classical physics. The Copenhagen interpretation is not a homogenous view. This insight has begun to emerge among historians and philosophers of science over the last ten to fifteen years. Both James Cushing and Mara Beller take for granted the existence of a unitary Copenhagen interpretation in their social and institutional explanation of the once total dominance of the Copenhagen orthodoxy; a view they personally find unconvincing and outdated partly because they read Bohr's view on quantum mechanics through Heisenberg's exposition.
But historians and philosophers of science have gradually realized that Bohr's and Heisenberg's pictures of complementarity on the surface may appear similar but beneath the surface diverge significantly. Don Howard , p.
In addition, Howard also argues that it was Heisenberg's exposition of complementarity, and not Bohr's, with its emphasis on a privileged role for the observer and observer-induced wave packet collapse that became identical with that interpretation. This audience included people like Bohm, Feyerabend, Hanson, and Popper who used Heisenberg's presentation of complementarity as the target for their criticism of the orthodox view.
Following up on Don Howard's research, Kristian Camilleri , points to the fact that complementarity was originally thought by Bohr in his Como-paper to exist between the space-time description and the causal description of the stationary states of atoms — and not between different experimental outcomes of the free electron. So the formulation of complementarity was restricted to the concept of stationary states because only there does the system have a well-defined energy state independent of any measurement.
This observation deserves general recognition. But when Bohr rather soon thereafter began analysing the double slit experiment in his discussion with Einstein , he had to extend his interpretation to cover the electron in interaction with the measuring apparatus.
Camilleri then shows how Heisenberg's view of complementarity, in spite of Heisenberg's own testimony, radically differs from Bohr's. As Heisenberg understood complementarity between the space-time description and causal description, it holds between the classical description of experimental phenomena and the description of the state of the system in terms of the wave function. A quotation from Heisenberg , p. In other words, Heisenberg, in contrast to Bohr, believed that the wave equation gave a causal, albeit probabilistic description of the free electron in configuration space.
It also explains why so many philosophers and physicists have identified the Copenhagen interpretation with the mysterious collapse of the wave packet. According to Heisenberg, these two modes of description are complementary. In another study Ravi Gomatam agrees with Howard's exposition in arguing that Bohr's interpretation of complementarity and the textbook Copenhagen interpretation i. Recently, Henderson has come to a similar conclusion.
He makes a distinction between different versions of Copenhagen interpretations based on statements from some of the main characters. On one side of the spectrum there is Bohr who did not think of quantum measurement in terms of a collapse of the wave function for a contrasting view see Jens Hebor ; in the middle we find Heisenberg talking about the collapse as an objective physical process but thinking that this couldn't be analyzed any further because of its indeterministic nature, and at the opposite side Johann von Neumann and Eugene Wigner argued that the human mind has a direct influence on the reduction of the wave packet.
Unfortunately, von Neumann's dualistic view has become part of the Copenhagen mythodology by people opposing this interpretation. Apparently, we are living in a quantum world since everything is constituted by atomic and subatomic particles. Hence classical physics seems merely to be a useful approximation to a world which is quantum mechanical on all scales. Such a view, which many modern physicists support, can be called quantum fundamentalism Zinkernagel forthcoming. It can be defined as a position containing two components: 1 everything in the Universe is fundamentally of quantum nature the ontological component ; and 2 everything in the Universe is ultimately describable in quantum mechanical terms the epistemological component.
Thus, we may define quantum fundamentalism to be the position holding that everything in the world is essentially quantized and that the quantum theory gives us a literal description of this nature. The basic assumption behind quantum fundamentalism is that the structure of the formalism, in this case the wave function, corresponds to how the world is structured. For instance, according to the wave function description every quantum system may be in a superposition of different states because a combination of state vectors is also a state vector.
Now, assuming that both the quantum object and the measuring apparatus are quantum systems that each can be described by a wave function, it follows that their entangled state would likewise be represented by a state vector. Then the challenge is, of course, how we can explain why the pointer of a measuring instrument enters a definite and not a superposition position, as experience tells us, whenever the apparatus interacts with the object. In a nutshell this is the measurement problem.
The Copenhagen interpretation is often taken to subscribe to a solution to the measurement problem that has been offered in terms of John von Neumann's projection postulate. In he suggested that the entangled state of the object and the instrument collapses to a determinate state whenever a measurement takes place. This measurement process a type 1-process as he called it could not be described by quantum mechanics; quantum mechanics could only described type-2 processes i.
According to von Neumann, the shift from a type 2-process to a type 1-process takes place only in the presence of the observer's consciousness. So what causes such a collapse seems to be the mind of the observer. But von Neumann never explained how it was possible for something mental to produce a material effect like the collapse of a quantum system. Although von Neumann's position is usually associated with the Copenhagen Interpretation, such a view was definitely not Bohr's as we shall see in a moment.
Quantum fundamentalists must indeed be ready to explain why the macroscopic world appears classical. An alternative to von Neumann's projection postulate is the many-world-interpretation which claims that the formalism should be read literally and that measurements classical outcomes do not describe the world as it really is.
The cost is ontologically horrendous. In one interpretation the world divides into as many worlds as there are possible measurement outcomes each time a system is observed or interacts with another system. Other fundamentalists had hoped that the decoherence program might come up with an appropriate explanation. The decoherence theory sees entanglement to exist not only between object and the measurement but also as something which includes the environment.
If Bohr had known the idea of decoherence, he would probably have had no objection to it. However, it is generally agreed that decoherence does not solve the measurement problem Zinkernagel Nevertheless, the intent of quantum fundamentalism seems to overrule Bohr's claim that classical concepts are necessary for the description of measurements in quantum mechanics.
In spite of that there is no general agreement to what extend Bohr opposed quantum fundamentalism. Time and again Bohr emphasized that the epistemological distinction between the instrument and the object is necessary because this is the only way one can make sense of a measurement.
It is also generally agreed that he didn't treat the classical world of the measuring instrument as separated from quantum object along the line of a microscopic and macroscopic division. He sometimes included parts of the measuring instrument to which the quantum mechanical description should be applied. Don Howard therefore concludes that Bohr was not only an ontological quantum fundamentalist but in fact also a sort of an epistemological one.
He believes that one can make Bohr's requirement that measuring apparatus and the experimental results have to be described in ordinary language supplemented with the terminology of classical physics consistent with ontological quantum fundamentalism.
According to him, Bohr never considered the measuring instrument as a classical object. Moreover, he thinks that this implies that Bohr had to understand the use of classical concepts differently from what scholars usually think. Howard believes that with respect to an experimental context in which an instrument interacts with an object Bohr didn't understand them as being in an entangled state but being separated in a mixture state.
The consequence would be that the instrument and the object exist in a definite quantum state since such a state could be represented as a product of the wave function for the instrument and for the object. But, as Maximilian Schlosshauer and Kristian Camilleri Other Internet Resources , have pointed out, this does not solve the measurement problem.
Howard does not explain under which circumstances one can move from a quantum system being in a non-separable state to a mixture of separated states. Therefore one cannot be sure that the measuring apparatus is in a definite state and its pointer in a definite place. Some philosophers seem to argue that Bohr was an ontological but not an epistemological quantum fundamentalist. So Klaas Landsman , accepts Howard's suggestion that Bohr is an ontological quantum fundamentalist but he rejects that Bohr should be considered an epistemological quantum fundamentalist.
Landsman argues that Bohr held that the measuring instrument should be described in classical terms since the results of any measurement like in classical physics would always have a definite value. However, Landsman agrees to the point that Bohr understood all objects as essentially quantum mechanical objects. However, it seems as if both Howard and Landsman miss the pragmatic nature of Bohr's view on ontological issues.
Bohr mentioned more than once that physics was not about finding the essence of nature but about describing the phenomena in an unambiguous way. In the foreground of Bohr's thinking was the 1 the need of classical concepts for the description of measuring results; 2 non-separability due to the entanglement of the system and the measuring instrument; 3 the contextual nature of the measurements of complementary properties; and 4 the symbolic character of the quantum formalism. One has to take all four components into consideration if one wants to understand Bohr's solution to the classical-quantum problem.
This is definitely a non-classical feature which is described by quantum mechanics alone. In his response to EPR-paper Bohr strongly rejected that this form of interaction could be regarded as a mechanical influence. The influence was on the conditions of description, i. But during a measurement we need to separate the system from the measuring instrument and the environment for pragmatic reasons. The pragmatic reasons seem to be reasonably clear. The outcomes of whatever experiment always yield a definite value, so the entanglement of object and the measurement instrument described by the quantum formalism only lasts until the interaction between object and instrument stops.
The quantum formalism can predict the statistical outcome of these interactions but it cannot say anything about the trajectory of objects.
The Creation of Quantum Mechanics and the Bohr-Pauli Dialogue
Bohr's firmness about the use of classical concepts for the descriptions of measurement can be seen as his response to the measurement problem. This problem arises from the fact that quantum mechanics itself cannot account for why experiments on objects in a state of superposition always produce a definite outcome. Hence if one does not argue for spontanuous collapse of the wave function, hidden variables, or many worlds, one needs to supplement quantum mechanics with a classical description of measuring instruments in terms of clocks and rods.
Henrik Zinkernagel forthcoming seems to get close to Bohr's view when he argues that Bohr not so much solved the measuring problem as he dissolved it.
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One can say with Zinkernagel that according to Bohr all objects can be treated as quantum objects but they cannot all be treated as quantum objects at the same time. Depending on the context objects cannot be treated as quantum objects in those situations in which they acts as measuring apparatuses.
Because in these situations their classical description guarantees a frame of reference of space and time with respect to which an atomic object has a position, and, mutatis mutandis, with respect to which it has energy and momentum. Such a frame of reference is necessary for our ability to define and measure a particular property. The implication is that Bohr didn't not exclude the application of quantum theory to any system. Every system can in principle be treated quantum mechanically, but since we always need a frame of reference to describe experimental outcomes, not all systems can be treated quantum mechanically at once.
After the s a number of alternative interpretations to Bohr's complementarity were articulated and they all found their proponents among physicists and philosophers of science. The Copenhagen interpretation started to lose ground to other interpretations such as Bohm's interpretation, the many worlds interpretation, the modal interpretation and the decoherence interpretation, which have been more in vogue the last couple of decades. But parallel with the growing awareness of the essential differences between Bohr's and Heisenberg's understanding of quantum mechanics several philosophers of science have revitalised Bohr's view on complementarity.
Around the millennium a new recognition of the Copenhagen interpretation has emerged. Rob Clifton and Hans Halvorson , argue that Bohm's interpretation of quantum mechanics can be seen as the special case of Bohr's complementarity interpretation if it is assumed that all measurements ultimately reduce to positions measurement.
Originally Jeffrey Bub and Clifton were able to demonstrate given some idealized conditions that Bohr's complementarity and Bohm's mechanics fall under their uniqueness theorem for no-collapse interpretations. It turns out that either position or momentum are dynamically significant, but it is not permissible to assume that position and momentum are both dynamically significant in any single context.
From these assumptions they deduced Bohm' mechanics by adding the metaphysical postulate that position measurement is always dynamically significant, but this metaphysical restriction requires, as they emphasize, that positions have a dubious priviledged ontological status. Rather, Clifton and Halvorson and Halvorson believe that complementarity may give us a realist interpretation of quantum field theory.
The Creation of Quantum Mechanics and the Bohr Pauli Dialogue | John Hendry
Another insight into Bohr's view of complementarity is due to Michael Dickson , By using the contemporary theory of reference frames in quantum theory, he proves that Bohr's reponse to the EPR thought-experiment was in fact the correct one. Moreover, he also maintains that Bohr's discussions of spin, a property much less frame dependent than position and momentum, were very different from his discussions of the latter, and based on these differences he offers a Bohrian account of Bell's theorem and its significance.
A re-assessment of Bohr's philosophy of quantum mechanics is made by Whitaker on the basis of Clifton and Halvorson's and Dickson's works and in the light of quantum information theory. Besides these attempts to apply Bohr's notion of complementarity to the contemporary discussions of the interpretation of quantum mehanics and quantum field theory there is an ongoing attempt to understand Bohr's idea of symbolic representation Tanona, a, b and his notion of complementarity with respect to trends in philosophy and general epistemology Plotnitsky, , and Katsumori, The Background 2.
Classical Physics 3. The Correspondence Rule 4. Complementarity 5. Misunderstandings of Complementarity 6. The Divergent Views 7. The theory was based on two postulates: An atomic system is only stable in a certain set of states, called stationary states, each state being associated with a discrete energy, and every change of energy corresponds to a complete transition from one state to another.
It introduced an element of discontinuity and indeterminism foreign to classical mechanics: Apparently not every point in space was accessible to an electron moving around a hydrogen nucleus. An electron moved in classical orbits, but during its transition from one orbit to another it was at no definite place between these orbits.