In physics, action is an attribute of the dynamics of a physical system. It is a functional which takes the trajectory (also called path or history) of the system as its argument and returns a real number as the result. It has units of energy × time (joule-seconds in SI units). Planck's constant is the quantum of action. Generally, the action takes different values for different paths. Classical mechanics postulates that the path actually followed by a real physical system is that for which the action is minimized (or, more strictly, is stationary). The classical (differential) equations of motion of a system can be derived from this principle of least action. The stationary action formulation of classical mechanics extends readily to quantum mechanics in the Feynman path integral formulation, where a physical system follows simultaneously all possible paths with amplitudes determined by the action. It also provides a basis for the development of string theory. If the action is represented as an integral over time, taken along the path of the system between the initial time and the final time of the development of the system, the integrand,, is called the Lagrangian. For the action integral to be well defined the trajectory has to be bounded in time and space.

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