Reduction of chemistry to physics

In this article I hope to introduce chemical educators to some of the work carried out in the philosophy of chemistry. The relevance of such work and especially that carried out on the reduction of chemistry to physics is considerable, and especially so in the case of physical chemistry. As the very name of the discipline implies, physical chemistry juxtaposes aspects of chemistry with aspects of physics. The relationship between these two classical areas of science needs to be considered in order to ascertain the extent to which chemistry should be taught as applied physics or to inform the teaching of physical chemistry per se. 

How philosophers of science have approached the reduction of chemistry to physics... 

Responses to this further question appear to fall into two camps. One prominent metaphysician believes that the question needs to be approached independently of any theories of chemistry and of physics. Robin Le Poidevin has published an extensive article in which he argues in favor of the ontological reduction of chemistry to physics. He does this through what he has termed a combinatorial approach. 

The characteristic differences between the laws and theories of chemistry and physics largely arise from differences in complexity between the systems studied in the two disciplines and can be related to specific problems in the reduction of chemistry to physics. 

This acceptance leads to an apparent reduction of chemistry to physics, in which the science of chemistry may continue to have some practical importance as a specialized subdiscipline but ceases to have any fundamental significance as an independent... 

So, looked at as a whole, the reduction of chemistry to physics fails, not because of any problems of principle (though these may remain Scerri McIntyre, 1997), but because of impracticality. The application of the equations of physics to the questions raised in chemistry is ultimately intractable. 

It is our contention, then, that in some sense the program of reduction of chemistry to physics fails one of the grounds for this failure lies in practical issues of intractability rather than in any conceptual incompleteness or incompatibility. 

It is relatively easy to talk and gesture about how chemistry either does or does not reduce to physics. It is much harder to spell out exactly what is required to make good on the claim that chemistry does (or does not) reduce to physics. Philosophers have a concept of supervenience. In the case we are focused on here chemistry putatively reducing to physics—supervenience requires that every chemical change be accompanied by a physical change. This is nearly universally held, for example, if two molecules are identical in all physical respects, they will not differ chemically. However, supervenience is not sufficient for the reduction of chemistry to physics. There could be downward causation, where it is the chemical facts and laws that drive the physical facts and laws, not the other way around. Robin Hendry (Chapter 9) argues that those committed to the reducibility of chemistry to physics have not ruled out the possibility of downward causation, and moreover, he presents substantial evidence from the manner in which quantum mechanical descriptions for molecules are constructed and deployed by chemists in favor of downward causation. Quantum mechanical descriptions of molecules that have explanatory and descriptive power are constructed from chemical—not physical—considerations and evidence. Here in precise terms, we see chemistry supervenient on physics, but still autonomous, not reducible to physics. 

Many explanations have been advanced for the fact that philosophers have so stubbornly neglected chemistry as if it were virtually non-existent. Is it the lack of big questions in chemistry, its close relationship to technology, or the historically rooted pragmatism of chemists and their lack of interest in metaphysical issues Or, is the alleged reduction of chemistry to physics (quantum mechanics) the main obstacle, so that, if chemistry were only an applied branch of physics, there would be no genuine philosophical issue of chemistry ... 

Since the advent of quantum mechanics, in addition to Kant s chemistry is not an eigentliche Wissenschaft, an obligatory reference to Dirac s authority has been used to justify the reduction of chemistry to physics, i.e., to the proper mathematics of quantum mechanics.17 Dirac (1929) said ... 

There is another area in philosophy of chemistry where I have been urging a naturalistic approach ever since the beginning of my work. This concerns the question of the reduction of chemistry to physics or more specifically quantum mechanics or relativistic quantum mechanics if one insists on being very precise. 

Although chemistry has typically been presented as a branch of physics and has not captured sufficient attention within philosophy of science, it is important to note that chemistry demands a particular link to philosophy (Scerri, 1997). In posing questions of reduction of one science, such as biology to another, namely physics, one cannot ignore the question of whether or not chemistry can be reduced to physics. If reduction of chemistry to physics fails, then reduction of biology to physics is even more unlikely since chemistry is the intermediary science between physics and biology. 

Scerri McIntyre (1997) make a distinction between ontological and epistemological reduction of chemistry to physics. The authors argue that ontological reduction of chemistry to physics is a foregone conclusion. The question then becomes whether or not chemistry can be reduced to our current descriptions of physics, in other words epistemological reduction of chemistry to physics. 

In this context, by chemical laws I do not mean exceptionless and timeless universal truths, of the kind that occur in fundamental physics (or maybe not even there). Rather, I mean the kind of regularities chemists use on a daily basis, and which chemistry students find circled in chemistry textbooks. For example, the statement that Acids in reaction with metals generate hydrogen gas would count as a chemical law. If one does not accept this charitable reading of what a law should mean, then the Nagelian reduction of chemistry to physics cannot even begin to be discussed. 

For almost a century Quantum Mechanics is the physical theory which describes the interaction between particles, such as electrons and nuclei in molecules, and therefore enables the quantitative exact predictions of observables. The reduction of chemistry to physics corresponds to the mechanistic working program of Dirac [37] ... 

Perhaps the most appropriate place to begin our analysis is with the issue of reductionism, due to the unique ontological relationship that exists between chemistry and physics. Indeed, it is the closeness of this relationship that has probably led many philosophers of science to assume that the reduction of chemistry to physics is both trivial and inevitable. But does chemistry provide such a paradigm case for reductionism And, if so, why have so many chemists (and even physicists) been reluctant to eclipse the concerns of chemistry with those of physics Or, does the relationship between chemistry and physics instead highlight a case where despite ontological dependency, we wish to preserve the epistemological and explanatory autonomy of our original subject ... 

Of course, we must here begin by stating what we mean by the blanket term reduction , and what we take to be some of the problems it faces. First of all, we will not be primarily concerned with the ontological dependence of chemistry upon physics. As stated, we believe the ontological dependence of chemistry on physics to be almost a foregone conclusion. Rather, our concern will be with the epistemological reduction of chemistry to physics - with the question of whether our current description of chemistry can be reduced to our most fundamental current description of physics, namely quantum mechanics - and with its explanatory consequences. ... 

Does the computational complexity of quantum chemistry raise any special problems for the reduction of chemistry to physics The use of these approximate models in the explanation and prediction of chemical behaviom- clearly does raise a number of issues in the epistemology of idealisation and approximation (see [Hendry, 1998b]). The model Hamiltonians differ from the intractable exact Hamiltonians in that they neglect electronic interactions, and hold nuclei fixed, but electrons do interact and nuclei do move. Reductionists will respond that approximation and idealisation are ever-present features of physics think of frictionless planes in mechanics, or ideal gases. But caution is required if some serious mistakes are to be avoided ... 

This fear of the reduction of chemistry to physics has recently resurfaced in the form of a revival of the philosophy of chemistry in the United States. In 1999, the chemist Eric Scerri founded a new journal. Foundations of Chemistry, addressing precisely these kinds of issues. Indeed, the tide echoed that of the famous series of publications by the logical positivists in the US, Foundations of the Unity of Science, which, symptomatically, had nothing to say about chemistry. Eric Scerri himself sees the philosophy of chemistry as the major weapon in the fight against the reduction of chemistry to quantum mechanics, the goal being to demonstrate that the foundations of chemistry are to be found in chemistry itself and not in physics. 

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