Abstract
This paper combines naturalized metaphysics and a philosophical reflection on a recently evolving interdisciplinary branch of quantum chemistry, ab initio molecular dynamics. Bridging the gaps among chemistry, physics, and computer science, this cutting-edge research field explores the structure and dynamics of complex molecular many-body systems through computer simulations. These simulations are allegedly crafted solely by the laws of fundamental physics, and are explicitly designed to capture nature as closely as possible. The models and algorithms employed, however, involve many approximations and significant degrees of idealization of their target systems. Therefore, for philosophers of science the pivotal question of whether relying only on the fundamental laws of physics supports a reductionist or realist stance arises. One conceivable answer to this question is that the irreducible approximations and idealizations support rather anti-realist positions. After reviewing an influential attitude in the philosophy of computer simulations and the debate concerning scientific realism, I offer a fair interpretation of such ab initio modelling in quantum chemistry within a naturalistic metaphysical framework that gives rise to a specific type of ontic structural realism.
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Notes
Cf. for instance the literature review in Heymann (2010). Heymann emphasises that this ‘deep epistemic and cultural rupture’ is characterized by the fact that computers transformed ‘knowledge practices like representation, communication, visualization, computation and simulation’ and ‘fundamentally changed and reshaped processes of knowledge production and forms of knowledge and uncertainty’ (2010, 193).
Emphatic on the philosophical relevance and novelty of this revolution are (to mention only a few): Fritz Rohrlich: ‘[…] computer simulation provides […] a qualitatively new and different methodology for physical sciences […]’ (1991, 507); Peter Galison: ‘[…] the Monte Carlo ushered physics […] create[d] a marginalized netherland that was at once nowhere and everywhere on the usual methodological map’ (1996, 120); Eric Winsberg: ‘[…] the techniques that simulationists use to attempt to justify simulation are unlike anything that usually passes for epistemology in the philosophy of science literature’ (2001, 447), cf. also Winsberg (1999, 275); Paul Humphreys (2004, vii): ‘The influence of computational methods on science today is far more striking […] and many parts of science have, as a result, a distinctively different phase of their development’, cf. also Humphreys (1994, s103), (2004, 49–55) and, for a consideration of this ‘distinctively different phase’ in terms of the broader realm of the philosophy of science, Humphreys (2004, 57–60). Roman Frigg and Julian Reiss (2009) hold a more restrained view which is, at least on the main lines, in agreement with, for instance, Manfred Stöckler: ‘Computer simulations enable tremendous progress on a pragmatic level, as a matter of degree in terms of speed and quantity, but not a revolution in the principles of methodology’ (2000, 356). For a discussion of the ‘common view’ (Winsberg (2003)) concerning the methodological significance of computer simulations, see, for instance, Küppers and Lenhard (2005, 307–309) and Winsberg (2010, 25).
This line of thinking is already predominant in Humphreys monograph titled ‘Extending Ourselves’. Cf. for instance the statement given subsequently to an analysis of the increasing automatization (e.g. automated DNA sequencing, automated data collection in some areas of astronomy) in recent sciences: ‘In all of the cases mentioned thus far, the common feature has been a significant shift of emphasis in the scientific enterprise away from humans because of enhancements without which modern science would be impossible. For convenience, I shall separate these enhancements into the computational and the non-computational […]. This division of convenience should not conceal the fact that in extending ourselves, scientific epistemology is no longer human epistemology’ (2004, 8).
Generally, I endorse the arguments in Scerri and McIntyre (1997) against an epistemological reduction of chemistry to physics. Kostas Gavroglu’s view on chemistry is directly in line with Scerri and McIntyre stating that confining ‘the study of realism predominantly to the problems of physics is more a matter of convenience rather than something which has serious justification’ (Gavroglu (1997, 287)). That attitude neglect, according to Gavroglu, the theoretical particularity of chemistry and supposes an absolute reductionism of chemistry to physics.
It should already be clear that the decisive feature of AIMD is not that only quantum mechanics is involved, but that ‘no experimental data whatsoever are admitted into the computation’.
Even the hydrogen case is highly idealized: a non-relativistic quantum mechanics is applied, etc.
I will not bring out the details of classical molecular dynamics here. For a concise overview, see Griebel et al. (2004, 17–31).
For a concise overview, see Bailer-Jones (2004).
For a concise overview see French and Ladyman (2011).
Note that metaphysical claims are understood as strictly hypothetical. If future fundamental physics resurrects self-subsistent individuals, naturalized metaphysics will have to adopt these findings.
It is indeed not possible to do justice to (or be sufficiently precise about) the proposed conceptual framework in just four or five pages.
Cf. Ladyman et al. (2007).
The resolution R depends on, for instance, the quality of the used telescope.
Cf. Ladyman et al. (2007).
Note that the optimality criterion is necessary but not sufficient. Not every method that provides an optimal data compression gives rise to a real pattern representation. Only an optimal empirical substructure passes this quality on to a higher-level theory or model.
Do not confuse the Ehrenfest approximation and Ehrenfest molecular dynamics. The former is the general rationale for taking the nuclei as classical particles, whereas the latter is an AIMD model.
For details, see Marx and Hutter (2009).
Because the constraints of the classical motion of the nuclei are holonomic, there should be a conserved energy quantity over the simulated period. The actual energy conservation can, therefore, serve as a way to check the numerical quality of the simulation.
For convenience, I abstract away from the individual molecule cluster.
In fact, this figure is quite realistic with regard to the current ‘molecular dynamics community’ (cf. Marx and Hutter (2009)).
This is because in such cases even larger time steps can be chosen without a significant loss of numerical quality.
Note that this optimality is relative to the current scientific practice. Although there are a priori reasons for conceiving CP models as supplying optimal algorithms for a class of systems, future methods may be more efficient, so CP models would no longer be optimal. In that case, a naturalized metaphysics would consequently need to adjust its ontological furnishing of the world.
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Seck, C. Metaphysics within Chemical Physics: The Case of Ab Initio Molecular Dynamics. J Gen Philos Sci 43, 361–375 (2012). https://doi.org/10.1007/s10838-012-9198-9
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DOI: https://doi.org/10.1007/s10838-012-9198-9