  Despite the apparent cross-disciplinary interactions among scientific fields,
a formal description of their evolution is lacking. Here we describe a novel
approach to study the dynamics and evolution of scientific fields using a
network-based analysis. We build an idea network consisting of American
Physical Society Physics and Astronomy Classification Scheme (PACS) numbers as
nodes representing scientific concepts. Two PACS numbers are linked if there
exist publications that reference them simultaneously. We locate scientific
fields using a community finding algorithm, and describe the time evolution of
these fields over the course of 1985-2006. The communities we identify map to
known scientific fields, and their age depends on their size and activity. We
expect our approach to quantifying the evolution of ideas to be relevant for
making predictions about the future of science and thus help to guide its
development.
