1. Introduction
This volume on philosophical issues in the physics of condensed matter fills a crucial gap in the overall spectrum of philosophy of physics. Philosophers have generally focused on emergence in debates relating to the philosophy of mind, artificial life and other complex biological systems. Many physicists working in the field of condensed matter have significant interest in the philosophical problems of reduction and emergence that frequently characterise the complex systems they deal with. More than four decades after Philip W. Andersons influential paper More is Different (Anderson 1972) and his well known exchange with Steven Weinberg in the 1990s on reduction/emergence, philosophers of physics have begun to appreciate the rich and varied issues that arise in the treatment of condensed matter phenomena. It is one of the few areas where physics and philosophy have a genuine overlap in terms of the questions that inform the debates about emergence. In an effort to clarify and extend those debates the present collection brings together some well-known philosophers working in the area with physicists who share their strong philosophical interests.
The traditional definition of emergence found in much of the philosophical literature characterizes it in the following way: A phenomenon is emergent if it cannot be reduced to, explained or predicted from its constituent parts. One of the things that distinguishes emergence in physics from more traditional accounts in philosophy of mind is that there is no question about the physical nature of the emergent phenomenon, unlike the nature of, for example, consciousness. Despite these differences the common thread in all characterizations of emergence is that it depends on a hierarchical view of the world; a hierarchy that is ordered in some fundamental way. This hierarchy of levels calls into question the role of reduction in relating these levels to each other and forces us to think about the relation of parts and wholes, explanation, and prediction in novel ways.
In discussing this notion of a hierarchy of levels it is important to point out that this is not necessarily equivalent to the well known fact that phenomena at different scales may obey different fundamental laws. For instance, while general relativity is required on the cosmological scale and quantum mechanics on the atomic, these differences do not involve emergent phenomena in the sense described above. If we characterise emergence simply in terms of some appropriate level of explanation most phenomena will qualify as emergent in one context or another. Emergence then becomes a notion that is defined in a relative way, one that ceases to have any real ontological significance. In true cases of emergence we have generic, stable behaviour that cannot be explained in terms of microphysical laws and properties. The force of cannot here refers not to ease of calculation but rather to the fact that the micro-physics fails to provide the foundation for a physical explanation of emergent behaviour/phenomena. Although the hierarchical structure is certainly present in these cases of emergence the ontological status of the part/whole relation is substantially different.
What this hierarchical view suggests is that the world is ordered in some fundamental way. Sciences like physics and neurophysiology constitute the ultimate place in the hierarchy because they deal with the basic constituents of the worldfundamental entities that are not further reducible. Psychology and other social sciences generally deal with entities at a less fundamental level, entities that are sometimes, although not always, characterised as emergent. While these entities may not be reducible to their lower level constituents they are nevertheless ontologically dependent on them. However, if one typically identifies explanation with reduction, a strategy common across the sciences, then this lack of reducibility will result in an accompanying lack of explanatory power. But, as we shall see from the various contributions to this volume, emergent phenomena such as superconductivity and superfluidity, to name a few, are also prevalent in physics. The significance of this is that these phenomena call into question the reliance on reduction as the ultimate form of explanation in physics and that everything can be understood in terms of its micro-constituents and the laws that govern them.
The contributions to the collection are organized in three parts: reduction, emergence, and the part-whole-relation, respectively. These three topics are intimately connected. The reduction of a whole to its parts is typical of explanation and the practices that characterise physics; novel phenomena typically emerge in complex compound systems; and emergence puts limitations on our ability to see reduction as a theoretical goal. In order to make these relations transparent, we start by clarifying the concepts of reduction and emergence. The first part of the book deals with general issues related to reduction, its scope, concepts, formal tools, and limitations. The second part focuses on the characteristic features of emergence and their relation to reduction in condensed matter physics. The third deals with specific models of the part-whole-relation used in characterizing condensed matter phenomena.
1.1 Reduction
Part I of the book embraces four very different approaches to the scope, concepts, and formal tools of reduction in physics. It also deals with the relation between reduction and explanation, as well as the way limitations of reduction are linked with emergence. The first three papers are written by condensed matter physicists whose contributions to the collection focus largely on reduction and its limitations. The fourth paper, written by a philosopher-physicist, provides a bridge between issues related to reduction in physics and more philosophically oriented approaches to the problem.
On the Success and Limitations of Reductionism in Physics by Hildegard Meyer-Ortmanns gives an overview of the scope of reductionist methods in physics and beyond. She points out that in these contexts ontological and theoretical reduction typically go together, explaining the phenomena in terms of interactions of smaller entities. Hence, for her, ontological and theoretical reduction are simply different aspects of methodological reduction which is the main task of physics; a task that aims at explanation via part-whole relations (ontological reduction) and the construction of theories describing the dynamics of the parts of a given whole (theoretical reduction). This concept of methodological reduction closely resembles what many scientists and philosophers call mechanistic explanation (see Chap. ). The paper focuses on the underlying principles and formal tools of theoretical reduction and illustrates them with examples from different branches of physics. She shows how the same methods, in particular, the renormalization group approach, the single step approaches to pattern formation, and the formal tools of quantum field theory, are used in several distinct areas of research such as particle physics, cosmology, condensed matter physics, and biophysics. The limitations of methodological reduction in her sense are marked by the occurrence of strong emergence, i.e., non-local phenomena which arise from the local interactions of the parts of a complex system.
