The behavior of a quantum system whose classical motion could involve chaotic behavior is called quantum chaos. Chaos can arise in a classical system—a system of objects which are large compared to atomic scale—when the equations governing its behavior are nonlinear. Chaos in this classical sense generally cannot be found in a system fully described by quantum mechanics, since the Schrödinger equation, which describes quantum systems, is linear. The correspondence principle requires that a result obtained from quantum mechanics approach the classical result in the classical limit, that is, the limit of large size. The primary issues in quantum chaos in a three-body system are what are the signs of chaos, and how large the quantum system has to be to display chaotic behavior.