In physics, Hamilton's principle is William Rowan Hamilton's formulation of the principle of stationary action. It states that the dynamics of a physical system are determined by a variational problem for a functional based on a single function, the Lagrangian, which may contain all physical information concerning the system and the forces acting on it. The variational problem is equivalent to and allows for the derivation of the differential equations of motion of the physical system. Although formulated originally for classical mechanics, Hamilton's principle also applies to classical fields such as the electromagnetic and gravitational fields, and plays an important role in quantum mechanics, quantum field theory and criticality theories. Mathematical formulation Hamilton's principle states that the true evolution of a system described by generalized coordinates between two specified states and at two specified times and is a stationary point (a point where the variation is zero) of the action functional where is the Lagrangian function for the system. In other words, any first-order perturbation of the true evolution results in (at most) second-order changes in . The action is a functional, i.e., something that takes as its input a function and returns a single number, a scalar. In terms of functional analysis, Hamilton's principle states that the true evolution of a physical system is a solution of the functional equation That is, the system takes a path in configuration space for which the action is stationary, with fixed boundary conditions at the beginning and the end of the path. Euler–Lagrange equations derived from the action integral See also more rigorous derivation Euler–Lagrange equation Requiring that the true trajectory be a stationary point of the action functional is equivalent to a set of differential equations for (the Euler–Lagrange equations), which may be derived as follows. Let represent the true evolution of the system between two specified states and at two specified times and , and let be a small perturbation that is zero at the endpoints of the trajectory To first order in the perturbation , the change in the action functional would be where we have expanded the Lagrangian L to first order in the perturbation . Applying integration by parts to the last term results in The boundary conditions causes the first term to vanish Hamilton's principle requires that this first-order change is zero for all possible perturbations , i.e., the true path is a stationary point of