Abstract
Advocates of the New Mechanicism in philosophy of science argue that scientific explanation often consists in describing mechanisms responsible for natural phenomena. Despite its successes, one might think that this approach does not square with the ontological strictures of quantum mechanics. New Mechanists suppose that mechanisms are composed of objects with definite properties, which are interconnected via local causal interactions. Quantum mechanics calls these suppositions into question. Since mechanisms are hierarchical it appears that even macroscopic mechanisms must supervene on a set of “objects” that behave non-classically. In this paper we argue, in part by appeal to the theory of quantum decoherence, that the universal validity of quantum mechanics does not undermine neo-mechanistic ontological and explanatory claims as they occur within in classical domains. Additionally, we argue that by relaxation of certain classical assumptions, mechanistic explanatory strategies can sometimes be carried over into the quantum domain.
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Notes
While our discussion of the philosophical implications of decoherence theory focuses on the New Mechanists, much of what we say may be relevant to a broader class of views that hold that macroscopic objects are ontologically legitimate and explanatorily central.
The central argument of this paper does not depend upon the technical details of quantum mechanics and decoherence, so our aim in this paper will be to present the argument in a non-technical form, comprehensible to the non-specialist. For more precise formulations of decoherence, see Schlosshauer 2007 and Joos et al. 2003.
A notable exception here appears to be Craver (2007). Principally because of concerns about causal relevance the problem of causation by disconnection, Craver retreated from the MDC approach to activities and adopted significant parts of Woodward’s difference making approach to causality. While it is beyond the scope of this paper to argue for it, our view is that Craver’s retreat does not sit well with other of his commitments – most notably to a picture of etiological causal explanations that looks very much like Salmon’s.
A referee has suggested that this hierarchical picture seems to commit the New Mechanists to a view that gives ontological priority to parts over wholes in the sense discussed by, e.g., Morganti (2009) and Schaffer (2010). It is true that the New Mechanists hold that the causal powers (or activities) of wholes depend upon those of their parts, and this would seem to give ontological priority to their parts. But at the same time, as noted above, the New Mechanists’ are committed to a perspectivalism about part-decomposition that is holistic: what the parts are cannot be answered independently of an account of what the mechanism as a whole is doing. (This kind of holism is not inconsistent with the aforementioned anti-holism according to which it is not the case that every part is connected to every other part.)
We expect that the best way to cash out claims about the ontological priority of mechanisms versus their parts might be by looking at patterns of temporal order and causal dependence – and here the answers might not be univocal. For instance, one might argue that the parts of the lawnmower are ontologically prior to the lawnmower as a whole, because the parts pre-exist the lawnmower. Contrast this with the case of a living system (like a chipmunk) and its parts (like its muscles or veins). In the latter case the parts do not pre-exist the whole, and they cannot survive and maintain their identity except in the context of the whole.
The picture changes if one adopts one of the rival interpretations or revisions of quantum mechanics such as a many-worlds interpretation or the Bohmian approach. However, this would make the discussion far more complex and would distract from our main line of reasoning.
In fact, (A) and (C), too, are intimately connected, because in order to spell out (A) one eventually has to deal with the compound state of some quantum object and a measurement apparatus, which become entangled through the measurement interaction. Nevertheless, metaphysically (A) and (C) focus on two distinct issues. Whereas (A) concerns the ascription of properties to single quantum objects, (C) focuses on the question how composite quantum systems relate to their parts.
As Hüttemann (2005) argues, “synchronic microexplanations” fail in the realm of quantum physics. Although Hüttemann’s focus differs from that of the present investigation, his arguments are nevertheless relevant with some suitable adjustments.
See Scheibe (1973, ch. 1) for a very accurate account of Bohr’s ideas. There is considerable dispute about what exactly “the Copenhagen interpretation” is and how it relates to Bohr’s views. Howard (2004) argues that “the Copenhagen interpretation” was an invention by Heisenberg in the mid-1950s. Interestingly, Howard thinks that Bohr’s complementarity interpretation is even close to decoherence theory (private communication). However, that is a very non-standard reading of Bohr.
Since there is no need to distinguish (non-relativistic) quantum mechanics, (relativistic) quantum field theory and other parts of quantum physics, we will often use the most well-known term ‘quantum mechanics’ in a comprehensive way, as it is quite common among physicists, too.
See Tonomura et al. 1989.
Note that this common talk is somewhat misleading because superpositions do not actually consist of the superposed states. Any basis change leads to a different set of “component states”.
In the course of a decoherence process quantum states effectively lose their (quantum) coherence. The coherence that is removed by decoherence is the one present in coherent superpositions, where the adjective ‘coherent’ is usually dropped and one simply talks about ‘superpositions’. Note that the term ‘decoherence’ was introduced only in the late 1980s. This now well-established term may be a bit unfortunate because coherence sounds like a desirable property, which classical objects in particular do have. Also note that the term ‘decoherence’ may refer either to the process of decoherence and or the theory of decoherence.
Schlosshauer 2007, ch. 1 and sec. 9.2, has a concise non-technical discussion of this procedure.
Note that the “environment” is not necessarily something outside of the measurement apparatus. “Environment” can also refer to macroscopic internal degrees of freedom besides those which are explicitly responsible for the measurement.
Technically speaking, the interaction with the environment has the effect that S + A is no longer in a (pure) state, whereas the superposition, in which S + A + E is, is a pure state. If we are interested in S + A alone, we need to calculate the so-called “reduced state” of S + A by removing all the information that refers to the environment. The reduced state of S + A encodes only the local measurement statistics for S + A, and no measurement statistics referring to correlations between S + A and the environment E.
But note that even if the off-diagonal terms vanish completely, an ignorance interpretation is not allowed, since this would presuppose a classical ensemble of alternatives.
This is in turn determined by how effectively the environment can unambiguously detect the state of the system. One point is that the environment must have at least as many different states as the system under observation; another one is that these environment states must have little, or ideally: no, overlap.
Mathematically, this is represented by the suppression of off-diagonal elements in the reduced density matrix of S + A.
Ultimately we need something more, either a particular interpretation or possibly a combination of decoherence with a different approach (see Landsman 2007).
Note that this notion of locality differs from the one familiar in the philosophy of quantum mechanics, which refers to the causal separation of space-like related events.
Moreover, in most, if not all, of the extant proposals for an interpretation (or modification) of quantum mechanics, classical mechanics and quantum mechanics are at least less in conflict from the very start. For instance, in the many worlds interpretation within each of the many worlds all objects have definite properties. And in Bohmian quantum mechanics, the position is classical, and thus localized, from the very start; it is only unobservable (a “hidden variable”). However, like quantum mechanics, yet in a more drastic way, Bohmian mechanics is a “non-local” theory, e.g. even space-like separated objects can influence each other.
See Hartmann (2008) for a philosophical discussion.
See Band and Avishai (2013, sec. 1.2, ch. 11, 13, 15).
See Ball (2011) for a brief survey.
Even apart from the potential problems with QM, Bechtel and Richardson (1993, part IV) as well as Kuhlmann (2011, forthcoming b) argue that there are various cases where the decomposition of a system into localized parts with specific stable functions in the whole fails while there are still good reasons for maintaining that we are dealing with mechanistic explanations. This typically happens in complex systems.
For the full story see Kuhlmann (forthcoming a).
In “semi-classical” laser theory only the atoms are described by quantum mechanics while the field in the laser cavity is treated as a classical electrical field.
As Maudlin (1998, 60) nicely puts it, “[t]he world is not just a set of separately existing localized objects, externally related only by space and time. Something deeper, and more mysterious, knits together the fabric of the world.”
Note that this essential fact is important not only for the explanatory aspect of mechanisms but also for its ontological one.
Hüttemann (2005) discusses in detail what this point implies for the issue of emergence in entangled systems.
Using the total energy one can derive differential equations for how the various parts of the laser will evolve in time. This leads to a huge system of equations, which in addition are coupled with each other. One crucial starting point for solving this almost intractable system of equations is the empirical observation that there is a hierarchy of time scales: The slowly varying quantities, namely the field modes, can be treated as constant (in time) in comparison to the other quantities that change much faster. Eventually, one particular field mode wins the competition and dominates the beat, so to say. As a consequence, only one dominant mode of the light field emerges, giving rise to laser light.
We think that this is the case for the dynamics of so-called “EPR experiments”, e.g. for measurements of spin-correlated pairs of electrons. What happens in these experiments cannot be explained mechanistically. However, this doesn’t seem to be much of a limitation for the EPR experiments may not be sufficiently explainable at all as of now. Dorato and Felline (2011) explore whether there may be at least “structural explanations” of non-local quantum correlations, beyond a commitment to ontic structural realism.
There is a recent discussion about the explanation of universal macro behavior, e.g. identical phase transition behavior in gases and liquids, in terms of renormalization group methods. Batterman (2000) and Reutlinger (forthcoming) argue that this is best construed as a non-causal and a fortiori non- mechanistic type of explanation.
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Acknowledgements
We thank Laura Felline, Helmut Fink, Thorben Petersen and Manfred Stöckler for discussions and comments on earlier drafts. We are also grateful to two anonymous referees whose feedback much improved this paper.
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Kuhlmann, M., Glennan, S. On the relation between quantum mechanical and neo-mechanistic ontologies and explanatory strategies. Euro Jnl Phil Sci 4, 337–359 (2014). https://doi.org/10.1007/s13194-014-0088-3
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DOI: https://doi.org/10.1007/s13194-014-0088-3