METAPHYSICS

Metaphysics is concerned with human thinking about the nature of reality in the widest and most general sense. A general worldview of this kind will be influenced by what physics has to say about the character of the universe, but it will not be determined by this alone. Other forms of knowledge must also be brought into play. Physics constrains metaphysics, but it does not entail it, as the foundations of a house constrain what can be erected upon them, but they do not fix the form of the edifice. The proper relationship between physics and metaphysics is that of consonance and not one of deductive necessity.

In this kind of intellectual exchange, particle physics has been one of the most metaphysically influential branches of physics because of its ability to discuss the basic constituents out of which the variety of the physical world appears to be constructed. Its influence on general human thinking has been varied and contentious, however, just because of the ambiguity inherent in attempting to move from the particularities of physics to the generalities of metaphysics.

History

The pre-Socratic philosophers, such as Thales and Anaximander, considered that the variety of the world resulted from different states of a single kind of basic stuff. Thales assigned water to this fundamental role, while Anaximander favored air. The intuition that simplicity underlay apparent complexity was brilliant (one could think of the pre-Socratics as proto-particle physicists), but these early thinkers were unable to develop their ideas in any plausible detail. It was Leucippus and Democritus in the fifth century b.c.e. who took the further important step of proposing the existence of irreducible atoms (the Greek word means "uncut"), whose motions in the void constituted the nature of the world. The idea was developed more than a century later by Epicurus to form the basis of his atheistic philosophy. This way of thinking was fluently and fervently expounded by the Latin poet Lucretius in his influential poem De Rerum Natura (On the nature of things, 58 b.c.e.).

Atomism became of renewed interest in the seventeenth century with the rise of modern science. On this occasion, however, its principal proponents, such as Robert Boyle and Isaac Newton, were theists. It was only later, principally in eighteenth-century France, that atomism again began to be associated with atheism. Atomism in a recognizably modern form dates from the beginning of the nineteenth century, starting with John Dalton's development of his atomic theory of chemistry. This enabled Dalton to bring impressive order into a complex collection of data relating to chemical interactions. He himself was a Quaker.

Modern Developments

For the contemporary particle physicist, atoms are very large-scale systems, and they are certainly not uncuttable. Current candidates for basic constituents are at least as small as quarks and gluons and perhaps as infinitesimal as superstrings. Two aspects of the chain of discovery linking Dalton to the Standard Model of contemporary particle physics have particularly impressed themselves on the thinking of a wider intellectual public. One is simply the success of the program in accounting for many of the properties of the physical world in terms of the behavior of a small set of basic constituents. The other striking feature has been that at each level in the exploration of the structure of matter the resulting theories have been characterized by elegant economy and simplicity. Just as Dalton's atomic theory succeeded in bringing order into a bewildering welter of chemical facts, so contemporary particle physics uses the search for underlying simplicity as a successful guide to the discovery of yet more basic theories.

Such success raises the question of whether these elemental entities are not entitled to the metaphysically more profound epithet "fundamental." Are they, in fact, the stuff of reality, so that a particle physics perspective is the right way to approach metaphysics, with the other levels of human experience of the world simply being the complex corollaries of the aggregation of elementary entities? To ask the question raises the issue of reductionism (thinking in terms of constituents) versus holism (thinking in terms of totalities). If the former stance were the totally correct one, particle physics would not only influence human thought—it would be the proper basis for all fundamental human thinking. But this would be a metaphysical, rather than a scientific, conclusion.

Reductionism

Reductionism is the way of thinking that concentrates on an understanding founded in terms of the properties of constituents, rather than in terms of entities considered as a whole. The atheistic thinking of Epicurus was strongly reductionist—atoms and the void constituted the actual nature of reality. The examples of Boyle and Newton show, however, that atomistic thinking need not lead either to atheism or to the view that atomism is the all-sufficient account of reality. In the introduction the ambiguity inherent in the relation between physics and metaphysics is noted. The issues at stake for human thought can be clarified by recognizing that there are a variety of distinct forms of reductionist thinking.

First, there is a strategy of enquiry that can be called methodological reductionism. The complex character of totalities means that if they are decomposed into their component parts, these constituents will often be much easier to understand. Much can be learned by this technique of intellectual "divide and rule," and particle physics is the ultimate reductionist subject in this sense. Its successes indicate that this is indeed a very fruitful procedure to follow. It by no means follows, however, that one can learn all that is knowable and worth knowing by following this reductive tactic alone. The success of regarding sub-atomic matter as composed of quarks and gluons does not at all imply that this is all that needs to be said about physical reality.

A second form of reductionism corresponds to what may be called ontological reductionism. This asserts that when the decomposition of systems into their smallest parts is actually carried out, the entities that will be found among the fragments are indeed those that particle physics describes. Adopting this point of view implies, for example, that there is no "extra ingredient" necessary for living entities of the kind that vitalism supposed to be required. This form of reductionism is very widely accepted, and it is reinforced by the successes of molecular biology in giving an account of the processes taking place within living cells. However, it does not follow from this acceptance that higher-level entities are "nothing but" collections of quarks and gluons. That this caveat is necessary can be seen by considering the truth or falsehood of a third form of reductionism.

This is conceptual reductionism. This would claim that the concepts used in physics are all reducible to being expressed in terms of elementary particle physics; that all of chemistry is reducible to physics; and so on. Many thinkers reject this view. Its implausibility seems clear enough within physics itself. Subjects such as condensed matter physics and fluid mechanics make use of concepts that cannot be expressed just in terms of elementary entities. Single atoms have neither temperature nor viscosity. Although there has been a tendency among some particle physicists to call the currently unknown Grand Unified Theory a "Theory of Everything," that claim is overblown. More persuasive is the remark made by the condensed matter theorist, Philip Anderson, that "more is different." New properties emerge at higher levels of complexity, and they have to be treated on their own terms.

Some emergent properties are intricate but, in principle, unproblematic. An example is wetness. H₂O molecules individually do not exhibit this property, but it is scarcely surprising that when large numbers of them are brought together, the resulting redistribution of energy due to intermolecular interactions results in the collective effect that we call surface tension. This insight is an example of causal reductionism. The phenomenon of wetness is capable of being understood as due to effects operating at the atomic, and ultimately at the particle, level of causation. It is not at all clear, however, that all emergences are capable of being satisfactorily understood in this causally reductive way. There are emergences that appear to differ qualitatively from the properties of the substrate that sustains them. The most striking example is consciousness. Many see a yawning gap between neuroscience's talk of neural interactions, however interesting and sophisticated such talk may be, and the simplest mental experience, such as that of seeing pink. The execution of a willed intention obviously involves lower-order processes, such as muscle contractions, but it is not at all obvious that there is not also a new kind of mental causality that has also been brought into play. The issue here is essentially metaphysical, and it certainly cannot be settled by particle physics.

The most significant issue concerning the relationship of particle physics to human thought concerns the status of the entities that it describes. Are they simply an interesting and significant level in the structure of reality, or are they to be considered as "more fundamental" than other entities, such as cells or human beings? The question is a metaphysical one. Strong opinions can be expressed on either side of the issue, but it is not one that particle physics itself can settle.

See also:Benefits of Particle Physics to Society; Culture and Particle Physics; Influence on Science; Philosophy and Particle Physics; Universe

Bibliography

Barrow, J. D. Theories of Everything: The Quest for Ultimate Explanation (Oxford University Press, Oxford, UK, 1991).

Cushing, J. T. Philosophical Concepts in Physics: The Historical Relation between Philosophy and Scientific Theories (Cambridge University Press, Cambridge, UK, 1998).

Penrose, R. The Large, the Small and the Human Mind (Cam-bridge University Press, Cambridge, UK, 1997).

Polkinghorne, J. C. Beyond Science: The Wider Human Context (Cambridge University Press, Cambridge, UK, 1996).

Weinberg, S. Dreams of a Final Theory: The Search for the Fundamental Laws of Nature (Pantheon Books, New York, 1992).

John Polkinghorne