In physics, the action principle is an assertion about the nature of motion, from which the trajectory of an object subject to forces can be determined. The path of an object is the one that yields a stationary value for a quantity called the action. Thus, instead of thinking about an object accelerating in response to applied forces, one might think of them picking out the path with a stationary action.
The principle is also called the principle of stationary action and also Hamilton's principle. Other statements of the principle include the less general and in fact incorrect principle of least action and the principle of minimal action.
The action is a scalar (a number) with the unit of measure for Action as Energy × Time. The principle is a simple, general, and powerful theory for predicting motion in classical mechanics. Extensions of the action principle describe relativistic mechanics, quantum mechanics, electricity and magnetism.
Some applications of the action principle
Although equivalent in classical mechanics with Newton's laws, the action principle is better suited for generalizations and plays an important role in modern physics. Indeed, this principle is one of the great generalizations in physical science. In particular, it is fully appreciated and best understood within quantum mechanics. Richard Feynman's path integral formulation of quantum mechanics is based on a stationary-action principle, using path integrals. Maxwell's equations can be derived as conditions of stationary action.
Many problems in physics can be represented and solved in the form of an action principle, such as finding the quickest way to run down the beach for reaching a drowning person. Water running downhill seeks the steepest descent, the quickest way down, and water running into a basin distributes itself so that its surface is as low as possible. Light finds the quickest trajectory through an optical system (Fermat's principle of least time). The path of a body in a gravitational field (i.e. free fall in space time, a so called geodesic) can be found using the action principle.
Symmetries in a physical situation can better be treated with the action principle, together with the Euler-Lagrange equations which are derived from the action principle. For example, Noether's theorem which states that with every continuous symmetry in a physical situation there corresponds a conservation law (and also the converse). This deep connection, however, requires that the action principle is assumed.
In classical mechanics (non-relativistic, non-quantum mechanics), the correct choice of the action can be proven from Newton's laws of motion. Conversely, the action principle proves Newton's equation of motion given the correct choice of action. So in classical mechanics the action principle is equivalent to Newton's equation of motion. The use of the action principle often is simpler than the direct application of Newton's equation of motion. The action principle is a scalar theory, with derivations and applications that employ elementary calculus.
The principle of least action was first formulated by Maupertuis  in 1746 and further developed (from 1748 onwards) by the mathematicians Euler, Lagrange, and Hamilton. Maupertuis arrived at this principle from a feeling that the very perfection of the universe demands a certain economy in nature and is opposed to any needless expenditure of energy. Natural motions must be such as to make some quantity a minimum. It was only necessary to find that quantity, and this he proceeded to do. It was the product of the duration (time) of movement within a system by the "vis viva" or twice what we now call the kinetic energy of the system.
Euler (in "Reflexions sur quelques loix generales de la nature", 1748) adopts the least-action principle, calling the quantity "effort". His expression corresponds to what we would now call potential energy, so that his statement of least action in statics is equivalent to the principle that a system of bodies at rest will adopt a configuration that minimizes total potential energy.
Action principle in classical mechanics
Newton's laws of motion can be stated in various ways. One of them is the Lagrangian formalism, also called Lagrangian mechanics. If we denote the trajectory of a particle as a function of time t as x(t), with a velocity x′(t), then the Lagrangian is a function dependent on these quantities and possibly also explicitly on time:
In Lagrangian mechanics, the trajectory of an object is derived by finding the path for which the action integral S is stationary (a minimum or a saddle point). The action integral is a functional (a function depending on a function, in this case x(t)). For a system with conservative forces (forces that can be described in terms of a potential, like the gravitational force and unlike friction forces), the choice of a Lagrangian as the kinetic energy minus the potential energy results in the correct laws of Newtonian mechanics (Note that the sum of kinetic and potential energy is the total energy of the system).
Euler-Lagrange equations for the action integral
The stationary point of an integral along a path is equivalent to a set of differential-equations, called the Euler-Lagrange equations. This can be seen as follows where we restrict ourselves to one coordinate only. The extension to more coordinates is straightforward.
Suppose we have an action integral S of an integrand L which depends on coordinates x(t) and dx(t)/dt, its derivative with respect to t:
Consider a second curve x1(t) which starts and ends at the same points as the first curve, and assume that the distance between the two curves is small everywhere: ε(t) = x1(t) - x(t) is small. At the beginning and endpoint we have ε(t1) = ε(t2) = 0.
The difference between the integrals along curve one and along curve two is:
where we have used the first order expansion of L in ε and ε′. Now use integration by parts on the last term and use the conditions ε(t1) = ε(t2) = 0 to find:
S reaches a stationary point, i.e. δ S = 0 for each ε. Note that the point could either be a minimum, saddle-point or formally even a maximum. δ S = 0 for each ε if and only if
Where we have replaced xa, a = 0,1,2,3 for x, since this must hold for every coordinate. This set of equations is called the Euler-Lagrange equations for the variational problem. An important simple consequence of these equations is that if L does not explicitly contain coordinate x, i.e.
- if , then is constant.
Then the coordinate x is called a cyclic coordinate, and is called the conjugate momentum, which is conserved. For example if L does not depend on time, the associated constant of motion (the conjugate momentum) is called the energy. If we use spherical coordinates t, r, φ, θ and L does not depend on φ, the conjugate momentum is the conserved angular momentum.
Those familiar with functional analysis will note that the Euler-Lagrange equations simplify to
Example: Free particle in polar coordinates
Trivial examples help to appreciate the use of the action principle via the Euler-Lagrangian equations. A free particle (mass m and velocity v) in Euclidean space moves in a straight line. Using the Euler-Lagrange equations, this can be shown in polar coordinates as follows. In the absence of a potential, the Lagrangian is simply equal to the kinetic energy
in orthonormal (x,y) coordinates, where the dot represents differentiation with respect to the curve parameter (usually the time, t). In polar coordinates (r, φ) the kinetic energy and hence the Lagrangian becomes
The radial r and φ components of the Euler-Lagrangian equations become, respectively
The solution of these two equations is given by
for a set of constants a, b, c, d determined by initial conditions. Thus, indeed, the solution is a straight line given in polar coordinates.
The formalisms above are valid in classical mechanics in a very restrictive sense of the term. More generally, an action is a functional from the configuration space to the real numbers and in general, it needn't even necessarily be an integral because nonlocal actions are possible. The configuration space needn't even necessarily be a functional space because we could have things like noncommutative geometry.
- Lagrangian mechanics
- Noether's theorem
- Hamiltonian mechanics
- Calculus of variations
- Functional derivative
- Functional integral
- Path integral formulation
- Quantum physics
- Entropy (the least Action Principle and the Principle of Maximum Probability or Entropy could be seen analogous)
For an annotated bibliography, see Edwin F. Taylor  who lists, among other things, the following books
- Cornelius Lanczos, The Variational Principles of Mechanics (Dover Publications, New York, 1986). ISBN 0-486-65067-7. The reference most quoted by all those who explore this field.
- L. D. Landau and E. M. Lifshitz, Mechanics, Course of Theoretical Physics (Butterworth-Heinenann, 1976), 3rd ed., Vol. 1. ISBN 0-7506-2896-0. Begins with the principle of least action.
- Thomas A. Moore "Least-Action Principle" in Macmillan Encyclopedia of Physics (Simon & Schuster Macmillan, 1996), Volume 2, ISBN 0-0286457-1, pages 840 – 842.
- David Morin introduces Lagrange's equations in Chapter 5 of his honors introductory physics text. Concludes with a wonderful set of 27 problems with solutions. A draft of is available at 
- Gerald Jay Sussman and Jack Wisdom, Structure and Interpretation of Classical Mechanics (MIT Press, 2001). Begins with the principle of least action, uses modern mathematical notation, and checks the clarity and consistency of procedures by programming them in computer language.
- Dare A. Wells, Lagrangian Dynamics, Schaum's Outline Series (McGraw-Hill, 1967) ISBN 007-069258-0, A 350 page comprehensive "outline" of the subject.
- Robert Weinstock, Calculus of Variations, with Applications to Physics and Engineering (Dover Publications, 1974). ISBN 0-486-63069-2. An oldie but goodie, with the formalism carefully defined before use in physics and engineering.
- Wolfgang Yourgrau and Stanley Mandelstam, Variational Principles in Dynamics and Quantum Theory (Dover Publications, 1979). A nice treatment that does not avoid the philosophical implications of the theory and lauds the Feynman treatment of quantum mechanics that reduces to the principle of least action in the limit of large mass.
- Template:Fnb p.1052, Stephen Wolfram, A New Kind of Science. May 2002. ISBN 1579550088.
- Edwin F. Taylor's page 
- Historical notes:
- WILLIAM ROWAN HAMILTON (1805 - 1865) Here is a website from which you can download "the mathematical papers of Sir William Rowan Hamilton published during his lifetime, transcribed and edited by David R. Wilkins" in TeX, DVI, PDF, or PostScript 
- Hamilton's papers on the action principle are separately available at de:Wirkung (Physik)