Is Chemistry Reducible to Physics?
In recent philosophy of chemistry, the issue of whether chemistry is reducible to physics has been vividly debated. The debate was originally inspired by older bold claims like that of the mathematician Paul Dirac from 1929, according to whom the whole of chemistry would be reducible to quantum mechanics and thus would be part of physics.
Insofar as such claims express disciplinary chauvinism as a means to acquire social prestige and intellectual hegemony, or just the frequent disciplinary narrowmindedness that ignores everything outside one's discipline, they should not much concern philosophy. On the other hand, insofar as such claims belong to the general position of physicalism - according to which physics would be fundamental to any science, including biology, the social sciences, and psychology - they express a metaphysical worldview that, in its generality, is beyond the scope of philosophy of chemistry, although philosophers of chemistry can make specific and useful contributions to such debates. Furthermore, if the claim is about the explanatory and predictive scope of a specific theory, it is up to scientists rather than philosophers to assess the exact limits of the theory by checking the thesis against experimental findings and rejecting unfounded claims according to established scientific standards. The remaining job of philosophers - both of chemistry and physics, because the reductionist claim is about the relation between chemistry and physics - largely consists in clarifying the underlying concepts and in checking for hidden assumptions and blind spots.Because there are many different versions of reductionism, conceptual distinctions are necessary. Metaphysical or ontological reductionism claims that the supposed objects of chemistry are actually nothing other than the objects of quantum mechanics and that quantum-mechanical laws govern their relations.
In its strong, eliminative, version, metaphysical reductionism even states that there are no chemical objects proper. Microstructural essentialism reformulates eliminative metaphysical reduc- tionism in semantic terms by employing a certain theory of meaning and reference to claim that the proper meaning of chemical substance terms, such as “water,” is nothing other than the (quantum-mechanical) microstructure of the substance. However, as was shown above, it makes a difference if the objects of chemistry are substances or interatomic structures, such that giving up substances, as eliminative reductionism and its semantic twin claim, would be giving up chemistry as we know it. Even if substances have an interatomic structure, the fact that a theory can be used to describe the structure and to develop useful explanations does not mean that it “owns” interatomic structures. There are other important theories to describe interatomic structures, such as classical chemical structure theory which is much more useful to explain chemical properties, as we will see below. Moreover, anti-reductionists argue that theoretical entities are determined by their corresponding theory, such that theoretical entities of different theories cannot simply be identified. For instance, from the different meanings of the term ‘electron’ in quantum electrodynamics and in chemical reaction mechanisms one might conclude that the term ‘electron’ has different references, which rules out ontological reductionism.Epistemological or theory reductionism claims that all theories, laws, and fundamental concepts of chemistry can be derived from first principle quantum mechanics as the more basic and more comprehensive theory. That claim has prompted many technical studies on the difficulties of quantum mechanics to derive the classical concept of molecular structure and the chemical law that underlies the periodic system of elements. Moreover, because most of the successful applications of quantum mechanics to chemical problems include model assumptions and concepts taken from chemistry rather than only first principles, their success can hardly support epistemological reductionism.
Apart from such technical matters, quantum mechanics cannot derive chemistry’s classificatory concepts of substances and reactions, and it cannot explain, cannot even compete with, chemical structure theory, which has been developed since the midnineteenth century in organic chemistry to classify, explain, predict, and synthesize substances.Methodological reductionism, while acknowledging the current failure of epistemological reductionism, recommends applying quantum-mechanical methods to all chemical problems, because that would be the most successful approach in the long run (approximate reductionism). However, the mere promise of future success is hardly convincing unless a comparative assessment of different methods is provided.
By modifying the popular notion that “the whole is nothing but the sum of its parts,” two further versions of reductionism have been developed. Emergentism acknowledges that new properties of wholes (say, of water) emerge when the parts (say, oxygen and hydrogen) are combined, but concedes that the properties of the whole can be explained or derived from the relations between the parts (i.e., epistemological reductionism). Supervenience, in a simple version, means that, although epistemological reductionism might be wrong, the properties of a whole asymmetrically depend on the properties of the parts, such that every change of the properties of the whole is based on changes of the properties of or the relations between the parts, but not the other way round. If applied to the reduction of chemistry to quantum mechanics (i.e., to chemical entities as wholes and quantum-mechanical entities as parts), emergentism and supervenience presuppose elements of epistemological or ontological reductionism, such that the criticism of these positions applies accordingly.
The discussion of reductionism distracts from the fact that chemistry and physics have historically closely developed with many fruitful interdisciplinary exchanges without giving up their specific disciplinary focus.
For instance, chemistry greatly benefits from quantum mechanics, because that is the only theory we have to explain electromagnetic, mechanical, and thermodynamic properties of materials. However, when it comes to chemical properties, the properties that define chemical substances and which chemists are mostly interested in, quantum mechanics is extremely poor, such that chemists here rely almost exclusively on chemical structure theory. Rather than focusing on reductionism, with its underlying notion of a Theory of Everything, it seems more useful to discuss the strengths and weaknesses of different theories for different purposes. For instance, quantum mechanics helps analyze the optical properties that chemists routinely use in all kinds of spectroscopies to understand the kind of time- averaged interatomic structures that chemists are interested in. If these structures can successfully be translated into chemical structure theory, however, it is chemical structure theory rather than quantum mechanics that provides information about chemical properties.Chemical structure theory, which has been continuously developed since the mid-nineteenth century, is more like a rich sign language than a depiction of individual physical structures. It is one of the hidden assumptions of reductionism that both kinds of structures are the same. However, chemical structure theory encodes types of chemical reactivities according to chemical similarities in characteristic groups of atoms and it has numerous general rules for how these groups can interact and be reconfigured to describe chemical reactions. The important difference to physical structures, which are described in terms of individual space coordinates, is that it describes both the structures and their reconfigurations in general concepts that are chemically meaningful. Despite its recourse to general concepts, the language is rich enough to distinguish clearly between hundreds of millions of substances and their chemical properties.
Once the chemical structure of a substance is known, chemical structure theory allows both identifying the substance and predicting its chemical properties. Moreover, because chemical properties describe radical change of substances, these predictions enable one to make new, unknown substances in the laboratory, such that predictions guide the production of novelty. This is nowadays successfully performed several million times per year, which makes chemical structure theory one of the most powerful predictive tools of science.One of the blind spots of reductionism, or physicalism for that matter, is that sciences other than physics deal with different issues and subject matters that require entirely different kinds of methodologies, concepts, and theories. In chemistry, which deals with substances and radical change, classification and synthesis are at least as important as analysis, or its physics counterpart of a quantitatively accurate and true description of the world as it is. Classification is not only a matter of building useful empirical or operational concepts. It also requires theoretical approaches that include or can deal with classificatory concepts and substantial change, otherwise the theories cannot address the issues that are to be explained or predicted. Chemical theories need to deal with hundreds of millions of different substances and hundreds of thousands of kinds of reactions. Theoretical physics, on the other hand, stands out among the sciences because, apart from particle physics, it intentionally lacks classificatory concepts.
Furthermore, because radical change is essential to chemistry, synthesis is an integral part of chemistry both on the experimental and theoretical level. That is not simply because synthesis can provide useful compounds, although this option has historically shaped much of chemistry. Chemical properties are revealed only through synthesis (i.e., by chemical reactions that change one substance into another under controlled laboratory conditions). Accordingly, a chemical theory that is expected to make predictions must be able to predict syntheses, and the only way to test the predictions is, of course, by way of synthesis. Again, synthesis is not part of the methodology of physics, at least as mainstream philosophers of physics conceive it, so that the model of physics would miss a central part of chemical concepts, theories, and methods. However, since many physicists along with chemists engage in materials science to produce new useful materials, the methodology of experimental physics might approach that of chemistry.
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