When the layman says ‘reality’, he usually thinks that he is talking about something self-evident and well-known; whereas to me it appears to be the most important and exceedingly difficult task of our time to establish a new idea of reality.
Pauli’s letter to Fierz, 12.8.1948.
Abstract
The present study attempts to provide a consistent and coherent account of what the world could be like, given the conceptual framework and results of contemporary quantum theory. It is suggested that standard quantum mechanics can, and indeed should, be understood as a realist theory within its domain of application. It is pointed out, however, that a viable realist interpretation of quantum theory requires the abandonment or radical revision of the classical conception of physical reality and its traditional philosophical presuppositions. It is argued, in this direction, that the conceptualization of the nature of reality, as arising out of our most basic physical theory, calls for a kind of contextual realism. Within the domain of quantum mechanics, knowledge of ‘reality in itself’, ‘the real such as it truly is’ independent of the way it is contextualized, is impossible in principle. In this connection, the meaning of objectivity in quantum mechanics is analyzed, whilst the important question concerning the nature of quantum objects is explored.
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Notes
It should be noted that this is hardly the case in the quantum theory of the measurement process (e.g., Karakostas and Dickson 1995).
The principle of value-definiteness has variously been called in the literature as, for instance, “the determined value assumption” in Auletta (2001, 21, 105).
Undoubtedly, the classical physical ontology presented here is fully compatible with the structure of classical physics. Nonetheless, it is still an interpretation of classical physics, rather than being uniquely dictated by it, as after all the historical record of Kant’s and Mach’s reinterpretation of mechanics shows.
In this work we shall not consider in any detail alternative interpretations to Hilbert-space quantum mechanics as, for instance, Bohm’s ontological or causal interpretation.
It is well known that spin-singlet correlations violate Bell’s inequalities. We note in this connection the interesting result of Gisin (1991), Popescu and Rohrlich (1992) that for any entangled state of a two-component system there is a proper choice of pairs of observables whose correlations do violate Bell’s inequality.
For instance, the entangled correlations between spatially separated systems cannot be explained by assuming a direct causal influence between the correlated events or even by presupposing the existence of a probabilistic common cause among them in Reichenbach’ s sense. Butterfield (1989) and van Fraassen (1989) have shown that such assumptions lead to Bell’s inequality, whereas, as is well-known, the latter is violated by quantum mechanics. See in addition, however, Hofer-Szabó and Rédei (2004) for the notion of a common common cause in relation to quantum correlations among spatially separated events.
It should be pointed out that Bohr already on the basis of his complementarity principle introduced the concept of a ‘quantum phenomenon’ to refer “exclusively to observations obtained under specified circumstances, including an account of the whole experiment” (Bohr 1963, 73). This feature of context-dependence is also present in Bohm’s ontological interpretation of quantum theory by clearly putting forward that “quantum properties cannot be said to belong to the observed system alone and, more generally, that such properties have no meaning apart from the total context which is relevant in any particular situation. In this sense, this includes the overall experimental arrangement so that we can say that measurement is context dependent” (Bohm and Hiley 1993, 108).
Note that the so-called invariant or state-independent, and therefore, context-independent properties—like ‘rest-mass’, ‘charge’ and ‘spin’—of elementary objects-systems can only characterize a certain class of objects; they can only specify a certain sort of particles, e.g., electrons, protons, neutrons, etc. They are not sufficient, however, for determining a member of the class as an individual object, distinct from other members within the same class, that is, from other objects having the same state-independent properties. Thus, an ‘electron’, for instance, could not be of the particle-kind of ‘electrons’ without fixed, state-independent properties of ‘mass’ and ‘charge’, but these in no way suffice for distinguishing it from other similar particles or for ‘individuating’ it in any particular physical situation. For a detailed treatment of this point, see, for example, French and Krause (2006).
In standard quantum mechanics, it is not possible to establish a causal connection between a property A(t) at time t and the same property A(t′′) at a later time t′′, both pertaining to an object-system S, if S had been subjected at a time value t′, t < t′ < t′′, to a measurement of a property B incompatible with A. Because the successive measurement of any incompatible property of this kind would provide an uncontrollable material change of the state of S. Thus, a complete causal determination of all possible properties of a quantum object, most notably, coordinates of position and their conjugate momenta, allowing the object, henceforth, to traverse well-defined trajectories in space-time is not possible.
The view that the quantum state vector refers to ‘possibilities’ or ‘tendencies’, as a certain extension of the Aristotelian concept of ‘potentia’, has been advocated by Heisenberg (1958, 42, 53) in his later writings on the interpretation of quantum mechanics, and especially by Fock (1957, 646). Margenau (1950, 335–337, 452–454) too has used the concept of ‘latency’ to characterize the indefinite quantities of a quantum mechanical state that take on specified values when an act of measurement forces them out of indetermination. Analogous is Popper’s (1990, Chap. 1) understanding of attributing properties to quantum systems in terms of objective ‘propensities’. Today one of the most eloquent defenders of the appropriateness of the concept of potentiality in interpreting quantum mechanics is Shimony (1993, Vol. 2, Chap. 11), while a systematic development of the dialectical scheme ‘potentiality-contextuality’ for interpreting quantum mechanics is given in Karakostas (2007).
This claim should not be conflated with the thesis of ontic structural realism set out notably by French and Ladyman (2003), and, according to which, only structures in the sense of relations that are instantiated in the world are real; on this view, objects standing in the relations are simply non-existent (ibid. 41f). Our main objection against the thesis of ontic structural realism is that it dispenses altogether with physical objects. For, concrete relations that are instantiated in the natural world presuppose relata, that is, objects among which the relations obtain and of which they are predicated. What is challenging about quantum physics is not that there are no objects, but that the properties of quantum objects are remarkably different from the properties that classical physics considers. For instance, in any case of quantum entanglement, conceived as a relation among quantum objects, there are no intrinsic properties of the objects concerned on which the relation of entanglement obtains (see Sect. 3.1). The fact, however, that quantum objects cannot be individuated, in the classical sense, does not imply their inexistence. In other words, the non-individuality of quantum objects is not and cannot be tantamount to pronouncing their non-existence.
It is tempting to think that a similar sort of context-dependence already arises in relativity theory. For instance, if we attempt to make context-independent attributions of simultaneity to spatially distant events—where the context is now determined by the observer’s frame of reference—then we will come into conflict with the experimental record. However, given the relativization of simultaneity—or the relativization of properties like length, time duration, mass, etc.—to a reference frame of motion, there is nothing in relativity theory that precludes a complete description of the way nature is. Within the domain of relativity theory, the whole of physical reality can be described from the viewpoint of any reference frame, whereas, in quantum mechanics such a description is inherently incomplete.
On this perspective, the insurmountable difficulties encountered in a complete description of the measuring process in quantum mechanics may not be just a flaw of quantum theory, but they may arise as a logical necessity in any theory which contains self-referential aspects, as it attempts to describe its own means of verification. Whereas the measuring process in quantum mechanics serves to provide operational definitions of the mathematical symbols of the theory, at the same time, the measurement concept features in the axiomatic structure of the theory, and the requirement of it’s being described in terms of the theory itself induces a logical situation of semantical completeness which is reminiscent of Gödel’s (1931/1962) undecidability theorem; the consistency of a system of axioms cannot be verified because there are mathematical statements that can neither be proved nor disproved by the formal rules of the theory; nonetheless, they may be verified by meta-theoretical reasoning.
The sense of holism appearing in quantum mechanics, as a consequence of non-separability, should not be regarded as the opposite to methodological reductionism. Holism is not an injunction to block distinctions. A successful holistic research program has to account apart from non-separability, wholeness and unity also for part-whole differentiation, particularity and diversity. In this respect, holism and methodological reductionism appear as complementary viewpoints, none can replace the other, both are necessary, none of them is sufficient.
Einstein’s letter to Schrödinger, 19 June 1935, cited in Howard (1989, 224).
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I wish to thank two anonymous referees of this journal for helpful comments and suggestions.
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Karakostas, V. Realism and Objectivism in Quantum Mechanics. J Gen Philos Sci 43, 45–65 (2012). https://doi.org/10.1007/s10838-012-9173-5
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DOI: https://doi.org/10.1007/s10838-012-9173-5