In an article titled �Underdetermination, Conventionalism and Realism: the Copenhagen vs,. Bohm interpretation of Quantum mechanics, James Cushing argues that although the Copenhagen interpretation of Quantum mechanics has far less in common with classical mechanics, it is a more successful interpretation as opposed to the �causal� interpretation of QM put forward by David Bohm. Cushing makes a case for underdetermination, as he tries to show how both the Copenhagen interpretation and the Bohm interpretation support radically different ontologies, yet are empirically adequate scientific theories. In this essay, I will take a different route to argue that although Cushing�s distinction is acceptable in some aspects, it might have been exaggerated in certain others.
Why I agree with Cushing: As far as the Bohm�Copenhagen distinction is concerned, there are valid points that Cushing raises � �[O]ne theory represents the fundamental physical processes in the world as being inherently and irreducibly indeterministic and another theory that is based on an absolutely deterministic behaviour of the physical universe. This would not appear to be a minor or irrelevant difference� (Cushing 1993). The first part of his statement gives the inherent indeterministic nature of the quantum theory as understood via the Copenhagen interpretation. The second part deals with the presuppositions underlying Bohm�s description of the quantum system. To understand Cushing�s argument, we need to examine what is the Copenhagen interpretation and what is the Bohm interpretation of quantum mechanics.
The Copenhagen Interpretation of quantum mechanics developed out of a series of lectures given by Niels Bohr in Copenhagen during the 1920s and 1930s. Bohr, along with his students like Heisenberg and Pauli developed a coherent system of thought called the �Copenhagen Interpretation of quantum mechanics�. Bohr�s institute in Copenhagen was the centre of all ideas in theoretical physics at this period of time. Bohr�s lectures were primarily on atomic systems and on the description of nature and reality. These ideas later developed into a philosophy based on the limitations of human knowledge. Henry Folse points out, Bohr�s real achievements were in �postulating the non-classical stability of stationary states from which it changes discontinuously when it interacts with the radiation field� (Folse, 1985). This quantum postulate as Bohr called it, advanced the Copenhagen viewpoint that the description of atomic systems requires a conceptual framework different from the classical understanding of objects and particles.
Although a few scholars have distinguished between the Copenhagen and the Bohr interpretations (Gomatam 1999) essentially the Copenhagen interpretation had developed as a consistent school of thought with Bohr as the central figure. Bohr�s disciples, Heisenberg and Pauli, the other important contributors to the Copenhagen interpretation, added strong instrumentalist dimensions to the system. Whereas Heisenberg�s uncertainty principle proposed limitations to knowledge in an experimental situation where either position or momentum could be measured with absolute accuracy but never both, Pauli�s exclusion principle also suggested that no two electrons in one atom can have identical quantum numbers. In the same vein, Bohr�s principle of complementarity suggested that descriptions of a quantum object are mutually exclusive in different experimental situations. The common element among all the proponents of the Copenhagen interpretation is a clear separation of two distinct �descriptions� of one system. In so far as physics can get one description right, the other becomes elusive. An emphasis on �descriptions� seems to have given the impression that the Copenhagen physicists advocated a strong anti-realist view. Cushing definitely stresses on Bohr�s supposed anti-realism as against Bohm�s very obvious realistic ideas.
The mathematical formalism of the Copenhagen school was matrix mechanics, although Bohr�s own philosophy included a lot more of other elements. Bohr developed his philosophy on �complementarity� and inseparability and wrote extensively on the need for a new conceptual framework. As Folse points out, although Bohr emphasized on the limits of visualizability, he did not write at length on the wave-particle duality. Instead Bohr�s emphasis is on the �description� of the system. Bohr�s complementarity means that in a particular experimental set-up, it is possible to observe only one aspect of the system, depending on the phenomena we want to observe. The position and momentum are mutually exclusive attributes of a quantum object, just like wave and particle are also mutually exclusive descriptions of the quantum object.
Heisenberg comments on this, �Bohr�s concept of complementarity resulted in the same restrictions to the applicability of classical concepts owing to the appearances of quite different simple pictures which were complementary and which could coexist without contradiction only if their range of application was restricted�(Heisenberg, 1955). Thus descriptions which are applicable at the level of the classical get severely restricted when they are brought to the quantum level. Bohr did try to distinguish between the microscopic and the macroscopic when trying to understand the limitations of quantum theory.
The main point here is that this separation of wave and particle or position and momentum descriptions in Bohr�s philosophy is essentially epistemic and makes him stand far away from any realist or causal interpretations. Yet was Bohr an anti-realist as Cushing suggests? We will return to this issue later along with the notion of separability. Let us now turn to Bohm�s interpretation of QM.
In Bohm�s theory electrons are always �particles� and real objects guided by a wave pattern that evolves deterministically. Bohm�s interpretation thus presupposes an underlying causal deterministic nature of the particles and the waves. As Cushing rightly points out in Bohm�s system, electrons exist as particles �and� waves, whereas in the Copenhagen system electrons are either waves �or� particles. Bohm�s interpretation although consistent with the standard Schr�dinger wave equation which proposes a deterministic evolution of the wave pattern, brings in the problem of hidden variables. de-Broglie, in one of his 1927 papers also came out with a similar concept of a �pilot-wave� guiding the electron. Schr�dinger, Bohm, de-Broglie and even Einstein were all in agreement for a realist construal of quantum mechanics.
In the realism � antirealism debate, Bohr and the Copenhagen scholars are considered as a coherent group of the anti-realist camp. The Copenhagen system of philosophy is a school of thought developed around the ideas of Bohr, Heisenberg and Pauli who worked in mutual harmony to counteract any contradictory claims. The realist camp was less organized and people like Bohm, de-Broglie, Schodinger or Einstein did not work together on a single unified philosophy. This strong distinction of the scientists was primarily due to this �organization� that also made the Copenhagen interpretation a success in theoretical physics.
We have seen the reasons why Bohm and Bohr differed on theoretical grounds and why one is considered an anti-realist whereas the other is seen as a staunch realist. As Heisenberg wrote in one his papers �Bohm tries to connect particle orbits with the waves in configuration space�for Bohm the particles are �objectively real� structures like the point masses of classical mechanics. The waves in configuration space are objectively real fields like electric fields�. For Bohm, quantum objects are real and they exist along with the �real� wave and both the wave and the particle exist together in a quantum mechanical situation.� As we have seen, for Bohr or Heisenberg, this is not possible. Heisenberg goes on to write about Bohr �[I]n speaking of particle objects, we are concerned in a superfluous �ideological superstructure�, it must be particularly mentioned here that Bohr�s language destroys the symmetry between p and q which is implicit in quantum theory�. Thus Heisenberg emphasizes the distinction of the Bohm interpretation and the Copenhagen interpretation as essentially one of language suggesting that Bohm�s introduction of hidden parameters does not solve any problems but adds to it. Bohm and de-Broglie, believed in a form of wave mechanics different from the matrix mechanics that was followed in the Copenhagen school (Cushing, 1993).
Thus I agree with Cushing, somewhat. In so far as the wave-particle descriptions are concerned, Bohr and the Copenhagen school emphasize complementarity or mutual exclusiveness of the descriptions of the same system in two different experimental set-ups. To Bohm - deBroglie, the wave and the particle existed together. Bohr emphasized the limits of knowledge and visualizability which the Bohm interpretation did not entail. And as we have shown, although the mathematical formalism as Cushing says are entirely compatible for these two different viewpoints there was a difference in language, as Heisenberg pointed out.
Here it is important to point out that the mathematical formalism is generally universally accepted and there is a general agreement between theories. One formalism can be compatible with two extremely diverse theories (a fact pointed out by Cushing in his account on underdetermination), the divergence of theories seems to be more in the interpretation or the language (as we have discussed) rather than the mathematics which generally agrees.
The other very important difference between the Copenhagen school and the Bohm interpretation, according to philosophers of science is on the contentious issue of realism. Bohr and the entire Copenhagen school has been generally regarded as anti-realist as opposed to Bohm who in believing the existence of quantum objects irrespective of any measuring apparatus was a strong realist. Bohm was never comfortable with the apparent breakdown in causality that quantum theory seemed to suggest. In the debate of realism-antirealism in QM, the Bohr-Einstein debates are well known. Einstein searched for a �realist� view of quantum mechanics and disagreed with Bohr�s emphasis on the experimental situation or �observation�.
Henry Folse takes an interesting turn in this game, opining that since Bohr believed in �real objects� and real experimental situation, he was a realist. Folse mentions in several occasions Bohr�s idea of a �physical reality� and this is an idea that seems to need a detailed analysis. So, here we move on to our arguments.
Why I disagree with Cushing : The general agreement with Cushing has been in several areas. Bohm�s views differ significantly from the entire Copenhagen philosophy, but I argued in the beginning that Cushing�s total distinction between these two schools of thought requires a closer examination.
Bohr�s philosophy of QM is mainly a �context-dependent� philosophy. But as Folse rightly points out, Bohr never denies the existence of atoms. In fact in several occasions he voices a definite realist notion of QM. Einstein did not accept Bohr�s explanation as he felt Bohr was denying reality to unobservable property and Folse writes this is Einstein�s �correct� understanding of Bohr. We would like to differ here. What Bohr was getting at was not an importance on the phenomena of observation but that of the experimental situation. Bohr�s interpretation is �context-dependent�. Yet he never denied the existence of objects. A quantum object is a wave or a particle according to the experimental set-up. Thus we might call Bohr a �context-dependent realist�. Although Einstein�s own theory of relativity is context-dependent (Bohr in fact mentioned that he was inspired by the theory of relativity while developing the principle of complementarity), the theory of relativity is essentially objective because it is observer independent.
Can this �context-dependence� in both the theories be an essential point of similarity between Bohr and Einstein? Apparently the differences between them seemed to have outweighed any similarity they might have had.
Yet this dependence of context in Bohr�s philosophy might look like an attempt to have a realist interpretation of QM. Bohr never stresses on the observer, but rather on the experimental situation. However the Copenhagen school denied the existence of �independent� objects and were thus considered as significantly different from traditional, classical realists like Bohm or Schr�dinger. That is why the Copenhagen philosophers have been severely misunderstood. They are not traditional realists. Their realism depended on the context or experimental set-up. So why do we claim they are realists?
To quote Bohr�s realist inclinations, "we have here a direct connection with the customary ideas of motion, since, owing to the great energy of the fragments expelled by the atomic nuclei, the paths of these particles may be directly observed." (Bohr ATDN, pg. 12)
"As is well known, the marvelous development of the art of physical experimentation not only has removed the last traces of the old belief that the coarseness of our senses would forever prevent us from obtaining direct information about individual atoms, but has even shown us that the atoms themselves consist of still smaller corpuscles which can be isolated and the properties of which can be investigated separately. (Bohr APHK, p. 24, emphasis added.)
These quotations clearly show Bohr�s strong belief in the existence of atoms.
We would like to suggest that Bohr, Heisenberg, Einstein, Bohm are all ontological realists in the sense that they believed that objects exist. The only difference is for Bohr-Heisenberg and the Copenhagen school is that objects exist in a context and maybe even in an observational context. For Bohm � deBroglie, objects do not have a context, they exist independently. Einstein�s realism seems to say �objects may or may not have a context, but objects definitely do not have an observational context.�
The essential separation of realist-antirealist philosophies has added to a lot of confusion to the interpretations of quantum theory. The Copenhagen school of thought did not propound a traditional realism and so they are categorized as �anti-realists�, but it is the definition of realism that needs to be widened if we are to understand the implications of quantum theory and the interpretations of the Copenhagen scientists.
This brings us to the subject of separability. For Bohr, in any experimental situation, the quantum objects should be considered along with the measuring apparatus, so unlike classical realism, Bohr denies separability or separate knowledge of objects outside the experimental situation. The difference between Bohr and Bohm is as Bohm pointed out �Quantum theory had no place in it for an adequate notion of an independent actuality � i.e. of an actual movement or activity by which one physical state could pass over into another� (Bohm, 1987� emphasis added)
To quote Bohm again on where he differed, unlike what Cushing claims , �My main difficulty was not that the wave function was interpreted only in terms of probabilities, so that the theory was not deterministic; rather it was that it could only be discussed in terms of the results of an experiment or an observation,
which has to be treated as a set of phenomena that are ultimately not further analyzable or explainable in any terms at all. So the theory could not go beyond the phenomena or appearances.� (Bohm, 1987, emphasis added)
But Cushing is partially right that there are differences between the Bohm and the Copenhagen interpretations, but we have tried to show there are more reasons for which they are similar. So let�s move on.
We have already started the pattern that this essay aimed to follow. Bohm and Bohr are both realists, yet different sorts of realists � Bohm is a traditional realist and Bohr is a non-classical, context dependent realist. Bohm tried to maintain causality and Bohr rejected it without hesitation even proposing a radical change of concepts instead. He emphasized on the need for new conceptual framework not just in physics but across all branches of knowledge.
Folse describes complementarity as �the fact that a position measurement excludes a momentum measurement implies that when a position is determined, the phenomenon in which momentum could be defined cannot occur, hence the appropriate property cannot be accorded physical reality.�(Folse 1985) Folse says Bohr refused to ascribe physical reality to unobserved property. We would say Bohr really refused to ascribe physical reality to any context-independent property as we have mentioned and this context may or may not be observation. For Einstein it could never be observation, for Bohm it could never be context dependence.
So the Bohr school did not radically differ from Bohm�s line of thought, but they differ is some aspects. Heisenberg sarcastically mentions �Bohm�s language says nothing about physics that is different from what the Copenhagen language says� (Heisenberg, 1955)
So if we pursue the lines of Bohr-Bohm interpretations, we really get at the wider philosophical implications they were both getting at and at this point, their roads definitely merge. Both Bohm and Bohr and also Einstein were all talking about �wholeness�. Bohr was emphasizing on an interconnectedness, an epistemic whole; his wholeness of the experimental set-up has broad implications in philosophy of science - the wholeness of all situations or contexts. What he pointed out was not just the importance of the experimental set-up but the experimental set-up and the observation as a whole.
Bohm proposed a very advanced philosophy of wholeness. It was a wholeness in an undivided universe with layers of an implicate order that connects all things. Einstein also talked of an ensemble of particles and also mention a kind of wholeness . As Basil Hiley points out, Bohm was immensely influenced by Bohr�s emphasis of wholeness of the experimental set-up. As for Bohr�s influence on Bohm, Hiley writes �Bohr�s deepest perception was not wave-particle duality, nor complementarity, but wholeness��when Bohm found Bohr was advocating an extreme form of interconnectedness, he became ever fascinated with this notion and explored it with much more energy�. Bohm developed a mathematical structure for his implicate order which is perfectly compatible with QM. Folse writes �Bohr takes �physical reality� to refer to phenomena which he explicitly defines to mean the whole observational interaction in which either position or momentum is measured�. Even Bohr himself writes �[E]vidence obtained under different experimental conditions cannot be comprehended within a single picture, but must be regarded as complementary in the sense that only the �totality� of the phenomena extract the possible information about the objects.�(APHK, emphasis added)
Bohm wrote on the form of the wave �the quantum potential is capable of constituting a non-local connection, depending directly on the state of the whole in a way that is not reducible to a pre-assigned relationship among parts.�
In a way, Bohm�s wholeness is subtler than Bohr�s and probably even with wider implications. Yet both Bohm and Bohr tried to extrapolate their idea of �interconnectedness� and nonseparability and wholeness to other subjects. Bohr mentions complementarity in biology and chemistry. He writes �the integrity of living organisms and the characteristics of conscious individuals and human cultures present features of wholeness, the account of which implies a typical complementary mode of description�.
To account for limitations of knowledge in biology, Bohr mentions that any examination to attain complete knowledge of a living organism which will involve an examination of the organism in its atomic level will preclude the possibility of the organism alive.
In this essay we have tried to break the line that separates the realists and the anti-realists in QM.
We tried to show that Bohm and Bohr are essentially similar in that they are both realists and they both believed in a form of wholeness. Although there are differences between the two interpretations, a strict �Copenhagen vs Bohm interpretation� set as contrasting or contradictory interpretations as proposed by Cushing might be just too hasty and overstated.
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