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Many-body theory (or many-body physics) is an area of physics which provides the framework for understanding the collective behavior of large numbers of interacting particles, often on the order of Avogadro's number. In general terms, many-body theory deals with effects that manifest themselves only in systems containing large numbers of constituents. While the underlying physical laws that govern the motion of each individual particle may (or may not) be simple, the study of the collection of particles can be extremely complex. In some cases emergent phenomena may arise which bear little resemblance to the underlying elementary laws.
Many-body theory plays a central role in condensed matter physics.