Complex adaptive matter: Emergent phenomena in materials

In the study of matter, both living and inanimate, the breakthrough discoveries and most scientists' intellectual obsessions often flow from what we call emergent behavior: phenomena not readily predictable from a detailed knowledge of the material subunits alone. We call systems that display emergent behavior complex adaptive matter, and their relevant organizing principles are unique to their scales of length and time. This issue of MRS Bulletin provides an overview of the aggregate of research on complex adaptive matter through a survey of five examples, ranging from intrinsically disordered electron matter in high-temperature superconductors to protein aggregates in amyloid diseases like Alzheimer's. We explain the philosophy and motivation for this research, noting that the study of emergent phenomena complements a globally reductionist scientific approach by seeking to identify, with intellectual precision, the relevant organizing principles governing the behavior. Our authors focus on the character of emergence for their particular systems, the role of materials research approaches to the problems, and the efforts to identify the organizing principles at work.