The debate about chemistry’s relation to quantum physics has been a persisting one. Is chemistry reduced to quantum physics or is there something about chemistry that renders it in some way autonomous or independent of physics? Such questions have intrigued philosophers for quite some time.
Initially, it was thought that chemistry is a straightforward example of a special science that will ultimately be replaced and fully explained by a more fundamental physical theory (this view is often referred to as reduction). However, with the advent of philosophy of chemistry, this view was challenged. Reduction didn’t seem to work out the way philosophers initially thought it would for chemistry. Instead, philosophers of chemistry argued that there is something about chemistry that makes it separate and autonomous from quantum physics.
While various examples from chemistry were invoked to support such views, there is one case that stands out in the literature: the case of molecular structure. The way quantum physics identifies a molecule’s structure is taken to show that there is something about chemical properties that cannot (and possibly may never) be captured by the more fundamental physical theory.
More precisely, quantum mechanics describes molecules and their properties by solving the Schrödinger equation. Solutions to the equation provide a way to identify the molecule’s chemical properties, including its structure. However, quantum mechanics only identifies structure if certain assumptions are applied to the Schrödinger equation. Without making these assumptions, the Schrödinger equation is unable to differentiate between molecules that differ only in terms of their structure (these are called isomers).
This situation is regarded by many philosophers of chemistry as problematic for reduction. That quantum mechanics cannot- on its own and without making presuppositions - identify a molecule’s structure, (putatively) shows that the reduction of chemistry to quantum physics is not possible. Not only that, some also take it to indicate that chemical entities and properties are in a way over and above the physical stuff that make them up.
This ‘over and above the physical’ is where all the mystery and interest lies! In general, the status of the special sciences (that is, not only of chemistry, but also of biology, etc.) has been extensively discussed, and philosophers are trying to work out if and in what way these sciences are over and above physics.
Unfortunately I cannot cover here the vast literature on the relations between the special sciences and physics in order to spell this out in detail. Instead, I focus on how certain philosophers have spelled out this ‘over and aboveness’ for the case of chemistry and in particular for molecular structure.
One of the most detailed and influential views of how chemical properties are over and above physical ones has been offered by Robin Hendry and his account of strong emergence. In particular, Hendry claims that molecular structure is over and above the physical in the sense that it strongly emerges from its quantum mechanical entities. As he puts it:
“the emergent behaviour of complex systems must be viewed as determining, but not being fully determined by, the behaviour of their constituent parts.” (Hendry 2006: 180)
There are three important features of Hendry’s account. First, this account adheres to a hierarchy of levels; namely “an ontology that is divided into an ordered hierarchy of levels, with the more fundamental entities occurring toward the bottom of that hierarchy” (Humphreys 2016: 8). Secondly, the chemical properties of molecules depend on their underlying physical entities and properties (namely the subatomic particles that make them up). This dependence relation is understood in terms of supervenience according to which, whenever any chemical property of a molecule changes, then it is always the case that one or more of its quantum mechanical properties have changed as well (Hendry 1999: 120).
Thirdly, there are chemical properties which- while dependent on the underlying physical interactions (as supervenience requires)- are to some extent causally independent from those interactions. This idea is spelled out via the notion of downward causation. Put simply, there is downward causation when “the special science properties sometimes push their physical supervenience bases around.” (Hendry 2010b: 185). For the case at hand this can be expressed like this: the structure of a molecule is not fully determined by the interactions of the physical entities that make it up- instead, molecular structure partially determines how the subatomic particles interact with each other.
Hendry’s account has been regarded as an exemplary account of emergence. However, I believe that there are certain features of this account that are not sufficiently clear in their present form and that therefore undermine the tenability of this position.
First, I believe that there is an inconsistency in the way this account is putatively supported by empirical evidence. Recall that the Schrödinger equation does not identify the different isomeric structures unless ad hoc assumptions are made about the examined system. This is taken to show that structure strongly emerges. However, this also seems to undermine supervenience, which is one key features of this account (for a full explanation of why this is the case see Seifert 2020).
Secondly, Hendry does not spell out in what way exactly structure partially determines the underlying physical interactions. This is particularly important if one takes into account the extensive philosophical literature about the notions of determination and causation. In fact, if we apply particular understandings of causation then the postulation of a downward causal relation between a molecule’s structure and its quantum mechanical entities is untenable. So, a lot more has to be said about what is meant by downward causation.
Thirdly- and perhaps more importantly- it is ambiguous how exactly the use of assumptions in the molecular Schrödinger equation purportedly supports strong emergence. Is it the fact that such assumptions are ad hoc? Namely, that they are not derived by quantum mechanics and are instead added to the description by hand? Or, does the issue lie in the content of these assumptions? These assumptions import information about the structure of the molecule, so quantum mechanics only identifies molecular structure after one has already presupposed such structure. In light of this, one could argue that the use of these assumptions shows strong emergence because there is no structure at the level which is describable by quantum mechanics.
All this is quite complicated and requires a thorough analysis. However, the way one interprets the use of assumptions for the support of strong emergence significantly affects the tenability of such a position.
In conclusion, strong emergence is a unique way of understanding how quantum mechanics identifies molecular structure and thus how chemistry is related to quantum physics. Nevertheless, there are aspects of Hendry’s account that should be explored further before adhering to the view that molecular structure strongly emerges.
So, the question of how chemistry is related to quantum physics is not settled yet. Nevertheless, it is clear that the case of molecular structure and how it is identified in quantum mechanics play a central role in illuminating this relation.
References
1. Hendry, R. F. (1999). Molecular models and the question of physicalism. HYLE, 5, 117–134.
2. Hendry, Robin F.. (2006). Is there downwards causation in chemistry?’, in Philosophy of chemistry: Synthesis of a new discipline. Boston studies in the philosophy of science, ed. by Davis Baird, Eric Scerri and Lee McIntyre, , Vol. 242 (Dordrecht: Springer) pp. 173–189.
3. Hendry, R. F. (2010b). Ontological reduction and molecular structure. Studies in History and Philosophy of Modern Physics, 41, 183–191.
4. Seifert, V.A. The strong emergence of molecular structure. Euro Jnl Phil Sci 10, 45 (2020). https://doi.org/10.1007/s13194-020-00308-7
* This article is a short summary of my recently published paper ‘The Strong Emergence of Molecular Structure’ (ref. below).
** Vanessa is a postdoctoral researcher in the University of Bristol, working for the European Research Council Project ‘The Metaphysical Unity of Science’ (grant no. 771509).
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