Complex Systems: Science for the 21st Century
A new world awaits our exploration. Things act differently in this world, but that is its attraction. It is a world of small things and of complex things. When we understand them and can control them, they will have an enormous impact in our lives. Executive Summary 2 Complex Systems 2 A s we look to the next century, we find science and technology at yet another threshold: the study of simplicity will give way to the study of "complexity" as the unifying theme. The triumphs of science in the
... of science in the past century, which improved our lives immeasurably, can be described as elegant solutions to problems reduced to their ultimate simplicity. We discovered and characterized the fundamental particles and the elementary excitations in matter and used them to form the foundation for interpreting the world around us and for building devices to work for us. We learned to design, synthesize, and characterize small, simple molecules and to use them as components of, for example, materials, catalysts, and pharmaceuticals. We developed tools to examine and describe these "simple" phenomena and structures. The new millennium will take us into the world of complexity. Here, simple structures interact to create new phenomena and assemble themselves into devices. Here also, large complicated structures can be designed atom by atom for desired characteristics. With new tools, new understanding, and a developing convergence of the disciplines of physics, chemistry, materials science, and biology, we will build on our 20th century successes and begin to ask and solve questions that were, until the 21st century, the stuff of science fiction. Complexity takes several forms. The workshop participants identified five emerging themes around which research could be organized. Collective Phenomena-Can we achieve an understanding of collective phenomena to create materials with novel, useful properties? We already see the first examples of materials with properties dominated by collective phenomena-phenomena that emerge from the interactions of the components of the material and whose behavior thus differs significantly from the behavior of those individual components. In some cases collective phenomena can bring about a large response to a small stimulus-as seen with colossal magnetoresistance, the basis of a new generation of recording memory media. Collective phenomena are also at the core of the mysteries of such materials as the hightemperature superconductors. Materials by Design-Can we design materials having predictable, and yet often unusual properties? In the past century we discovered materials, frequently by chance, determined their properties, and then discarded those materials that did not meet our needs. Now we will see the advent of structural and compositional freedoms that will allow the design of materials having specific desired characteristics directly from our knowledge of atomic structure. Of particular interest are "nanostructured" materials, with length scales between 1 and 100 nanometers. In this regime, dimensions "disappear," with zero-dimensional dots or nanocrystals, one-dimensional wires, and two-dimensional films, each with unusual properties distinctly different from those of the same material with "bulk" dimensions. We could design materials for lightweight batteries with high storage densities, for turbine blades that can operate at 2500˚C, and perhaps even for quantum computing.