- Action (physics)
In

physics , the**action**is a particular quantity in aphysical system that can be used to describe its operation. Action is an alternative to differential equations. The action is not necessarily the same for different types of systems.The action yields the same results as using differential equations. Action only requires the states of the physical variable to be specified at two points, called the initial and final states. The values of the physical variable at all intermediate points may then be determined by "minimizing" the action.

**History of term 'action'**The term "action" was defined in several (now obsolete) ways during its development.

*Gottfried Leibniz ,Johann Bernoulli andPierre Louis Maupertuis defined the "action" forlight as the integral of its speed (or inverse speed) along its path lengthFact|date=November 2007 .

*Leonhard Euler (and, possibly, Leibniz) defined it for a material particle as the integral of the particle speed along its path through spaceFact|date=November 2007 .

*Maupertuis introduced several "ad hoc" and contradictory definitions of "action" within a single , defining action as potential energy, as virtual kinetic energy, and as a strange hybrid that ensured conservation of momentum in collisionsFact|date=November 2007 .**Concepts**Physical laws are most often expressed as

differential equation s, which specify how a physical variable "changes" from its present value with infinitesimally small changes in time, position, or some other variable. By adding up these small changes, a differential equation provides a recipe for determining the value of the physical variable at any point, given only its starting value at one point and possibly some initial derivatives. The equivalence of these two approaches is contained inHamilton's principle , which states that the differential equations of motion for "any" physical system can be re-formulated as an equivalentintegral equation . It applies not only to theclassical mechanics of a single particle, but also to classical fields such as the electromagnetic and gravitational fields.Hamilton's principle has also been extended to

quantum mechanics andquantum field theory .**Mathematical definition**Expressed in mathematical language, using the

calculus of variations , the evolution of a physical system (i.e., how the system actually progresses from one state to another) corresponds to anextremum (usually, a minimum) of the**action**.Several different definitions of 'the action' are in common use in physics:

*The

**action**is usually anintegral over time. But for action pertaining to fields, it may be integrated over spatial variables as well. In some cases, the action is integrated along the path followed by the physical system.*The evolution of a physical system between two states is determined by requiring the

**action**be minimized or, more generally, be stationary for small perturbations about the true evolution. This requirement leads to differential equations that describe the true evolution.*Conversely, an

**action principle**is a method for reformulating "differential"equations of motion for a physical system as an equivalent "integral equation ". Although several variants have been defined (see below), the most commonly used action principle isHamilton's principle .*An earlier, less informative action principle is

Maupertuis' principle , which is sometimes called by its (less correct) historical name, theprinciple of least action .**Disambiguation of "action" in classical physics**In

classical physics , the term "action" has at least eight distinct meanings.**Action (functional)**Most commonly, the term is used for a functional $mathcal\{S\}$ which takes a function of time and (for fields) space as input and returns a scalar. In

classical mechanics , the input function is the evolution $mathbf\{q\}(t)$ of the system between two times $t\_\{1\}$ and $t\_\{2\}$, where $mathbf\{q\}$ represent thegeneralized coordinate s. The action $mathcal\{S\}\; [mathbf\{q\}(t)]$ is defined as theintegral of theLagrangian $L$ for an input evolution between the two times:$mathcal\{S\}\; [mathbf\{q\}(t)]\; =\; int\_\{t\_1\}^\{t\_2\}\; L\; [mathbf\{q\}(t),dot\{mathbf\{q(t),t]\; ,\; mathrm\{d\}t$

where the endpoints of the evolution are fixed and defined as $mathbf\{q\}\_\{1\}\; =\; mathbf\{q\}(t\_\{1\})$ and $mathbf\{q\}\_\{2\}\; =\; mathbf\{q\}(t\_\{2\})$. According to

Hamilton's principle , the true evolution $mathbf\{q\}\_\{mathrm\{true(t)$ is an evolution for which the action $mathcal\{S\}\; [mathbf\{q\}(t)]$ is stationary (a minimum, maximum, or a saddle point). This principle results in the equations of motion inLagrangian mechanics .**Abbreviated action (functional)**Usually denoted as $mathcal\{S\}\_\{0\}$, this is also a

functional . Here the input function is the "path" followed by the physical system without regard to its parameterization by time. For example, the path of a planetary orbit is an ellipse, and the path of a particle in a uniform gravitational field is a parabola; in both cases, the path does not depend on how fast the particle traverses the path. The abbreviated action $mathcal\{S\}\_\{0\}$ is defined as the integral of the generalized momenta along a path in thegeneralized coordinates :$mathcal\{S\}\_\{0\}\; =\; int\; mathbf\{p\}\; cdot\; mathrm\{d\}mathbf\{q\}\; =\; int\; p\_i\; ,dq\_i$

According to

Maupertuis' principle , the true path is a path for which the abbreviated action $mathcal\{S\}\_\{0\}$ is stationary.**Hamilton's principal function**Hamilton's principal function is defined by the

Hamilton–Jacobi equation s (HJE), another alternative formulation ofclassical mechanics . This function $S$ is related to the functional $mathcal\{S\}$ by fixing the initial time $t\_\{1\}$ and endpoint $mathbf\{q\}\_\{1\}$ and allowing the upper limits $t\_\{2\}$ and the second endpoint $mathbf\{q\}\_\{2\}$ to vary; these variables are the arguments of the function $S$. In other words, the action function $S$ is the indefinite integral of the Lagrangian with respect to time.**Hamilton's characteristic function**When the total energy $E$ is conserved, the HJE can be solved with the additive separation of variables

:$S(q\_\{1\},dots,q\_\{N\},t)=\; W(q\_\{1\},dots,q\_\{N\})\; -\; Ecdot\; t$,

where the time independent function $W(q\_\{1\},dots,q\_\{N\})$ is called "Hamilton's characteristic function". The physical significance of this function is understood by taking its total time derivative

:$frac\{d\; W\}\{d\; t\}=\; frac\{partial\; W\}\{partial\; q\_i\}dot\; q\_i=p\_idot\; q\_i$.

This can be integrated to give

:$W(q\_\{1\},dots,q\_\{N\})\; =\; int\; p\_idot\; q\_i\; ,dt\; =\; int\; p\_i,dq\_i$,

which is just the abbreviated action.

**Other solutions of Hamilton–Jacobi equations**The

Hamilton–Jacobi equation s are often solved by additive separability; in some cases, the individual terms of the solution, e.g., $S\_\{k\}(q\_\{k\})$, are also called an "action".**Action of a generalized coordinate**This is a single variable $J\_\{k\}$ in the

action-angle coordinates , defined by integrating a single generalized momentum around a closed path inphase space , corresponding to rotating or oscillating motion:$J\_\{k\}\; =\; oint\; p\_\{k\}\; mathrm\{d\}q\_\{k\}$

The variable $J\_\{k\}$ is called the "action" of the generalized coordinate $q\_\{k\}$; the corresponding canonical variable conjugate to $J\_\{k\}$ is its "angle" $w\_\{k\}$, for reasons described more fully under

action-angle coordinates . The integration is only over a single variable $q\_\{k\}$ and, therefore, unlike the integrated dot product in the abbreviated action integral above. The $J\_\{k\}$ variable equals the change in $S\_\{k\}(q\_\{k\})$ as $q\_\{k\}$ is varied around the closed path. For several physical systems of interest, $J\_\{k\}$ is either a constant or varies very slowly; hence, the variable $J\_\{k\}$ is often used in perturbation calculations and in determiningadiabatic invariant s.**Action for a Hamiltonian flow**See

tautological one-form .**Euler–Lagrange equations for the action integral**As noted above, the requirement that the action integral be stationary under small perturbations of the evolution is equivalent to a set of

differential equation s (called theEuler–Lagrange equations ) that may be determined using thecalculus of variations . We illustrate this derivation here using only one coordinate, "x"; the extension to multiple coordinates is straightforward.Adopting

Hamilton's principle , we assume that the Lagrangian "L" (the integrand of the action integral) depends only on the coordinate "x"("t") and its time derivative "dx"("t")/"dt", and does not depend on time explicitly. In that case, the action integral can be written:$mathcal\{S\}\; =\; int\_\{t\_1\}^\{t\_2\};\; L(x,dot\{x\}),mathrm\{d\}t$

where the initial and final times ($t\_\{1\}$ and $t\_\{2\}$) and the final and initial positions are specified in advance as $x\_\{1\}\; =\; x(t\_\{1\})$ and $x\_\{2\}\; =\; x(t\_\{2\})$. Let $x\_\{mathrm\{true(t)$ represent the true evolution that we seek, and let $x\_\{mathrm\{per(t)$ be a slightly perturbed version of it, albeit with the same endpoints, $x\_\{mathrm\{per(t\_\{1\})=x\_\{1\}$ and $x\_\{mathrm\{per(t\_\{2\})=x\_\{2\}$. The difference between these two evolutions, which we will call $varepsilon(t)$, is infinitesimally small at all times

:$varepsilon(t)\; =\; x\_\{mathrm\{per(t)\; -\; x\_\{mathrm\{true(t)$

At the endpoints, the difference vanishes, i.e., $varepsilon(t\_\{1\})\; =\; varepsilon(t\_\{2\})\; =\; 0$.

Expanded to first order, the difference between the actions integrals for the two evolutions is

:$egin\{align\}delta\; mathcal\{S\}\; =\; int\_\{t\_1\}^\{t\_2\};\; left\; [\; L(x\_\{mathrm\{true+varepsilon,dot\; x\_\{mathrm\{true\; +dotvarepsilon)-\; L(x\_\{mathrm\{true,dot\; x\_\{mathrm\{true)\; ight]\; dt\; \backslash =\; int\_\{t\_1\}^\{t\_2\};\; left(varepsilon\{partial\; Loverpartial\; x\}\; +\; dotvarepsilon\{partial\; Loverpartial\; dot\; x\}\; ight),mathrm\{d\}t\; end\{align\}$

Integration by parts of the last term, together with the boundary conditions $varepsilon(t\_\{1\})\; =\; varepsilon(t\_\{2\})\; =\; 0$, yields the equation:$delta\; mathcal\{S\}\; =\; int\_\{t\_1\}^\{t\_2\};\; left(varepsilon\{partial\; Lover\; partial\; x\}\; -varepsilon\{dover\; dt\; \}\{partial\; Loverpartial\; dot\; x\}\; ight),mathrm\{d\}t.$

The requirement that $mathcal\{S\}$ be stationary implies that the first-order change $deltamathcal\{S\}$ must be zero for "any" possible perturbation $varepsilon(t)$ about the true evolution. This can be true only if

:$\{partial\; Loverpartial\; x\}\; -\; \{mathrm\{d\}over\; mathrm\{d\}t\; \}\{partial\; Loverpartialdot\{x\; =\; 0$ Euler–Lagrange equationThose familiar with

functional analysis will note that the Euler–Lagrange equations simplify to :$frac\{delta\; mathcal\{S\{delta\; x(t)\}=0$.The quantity $frac\{partial\; L\}\{partialdot\; x\}$ is called the "conjugate momentum" for the coordinate "x". An important consequence of the Euler–Lagrange eqations is that if "L" does not explicitly contain coordinate "x", i.e.

: if $frac\{partial\; L\}\{partial\; x\}=0$, then $frac\{partial\; L\}\{partialdot\; x\}$ is constant.

In such cases, the coordinate "x" is called a "cyclic" coordinate,and its conjugate momentum is conserved.

**Example: Free particle in polar coordinates**Simple 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 :$frac\{1\}\{2\}\; mv^2=\; frac\{1\}\{2\}m\; left(\; dot\{x\}^2\; +\; dot\{y\}^2\; ight)$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:$L\; =\; frac\{1\}\{2\}m\; left(\; dot\{r\}^2\; +\; r^2dotvarphi^2\; ight).$

The radial "r" and φ components of the Euler–Lagrangian equations become, respectively

:$egin\{align\}frac\{mathrm\{d\{mathrm\{d\}t\}\; left(\; frac\{partial\; L\}\{partial\; dot\{r\; ight)\; -\; frac\{partial\; L\}\{partial\; r\}\; =\; 0\; qquad\; Rightarrow\; qquad\; ddot\{r\}\; -\; rdot\{varphi\}^2\; =\; 0\; \backslash frac\{mathrm\{d\{mathrm\{d\}t\}\; left(\; frac\{partial\; L\}\{partial\; dot\{varphi\; ight)\; -\; frac\{partial\; L\}\{partial\; varphi\}\; =\; 0\; qquad\; Rightarrow\; qquad\; ddot\{varphi\}\; +\; frac\{2\}\{r\}dot\{r\}dot\{varphi\}\; =\; 0end\{align\}$

The solution of these two equations is given by

:$egin\{align\}rcosvarphi\; =\; a\; t\; +\; b\; \backslash rsinvarphi\; =\; c\; t\; +\; dend\{align\}$

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.

**Action principle for single relativistic particle**When relativistic effects are significant, the action of a point particle of mass "m" traveling a

world line "C" parameterized by theproper time $au$ is:$S\; =\; -\; m\; c^2\; int\_\{C\}\; ,\; d\; au$.If instead, the particle is parameterized by the coordinate time "t" of the particle and the coordinate time ranges from "t"

_{1}to "t"_{2}, then the action becomes :$int\_\{t1\}^\{t2\}\; L\; ,\; dt$where the

Lagrangian is:$L\; =\; -\; m\; c^2\; sqrt\; \{1\; -\; frac\{v^2\}\{c^2$. [*L.D. Landau and E.M. Lifshitz "The Classical Theory of Fields" Addison-Wesley 1971 sec 8.p.24-25*]**Action principle for classical fields**The

**action principle**can be extended to obtain theequations of motion for fields, such as theelectromagnetic field or gravity.The

Einstein equation utilizes the "Einstein-Hilbert action " as constrained by avariational principle .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.

**Action principle in quantum mechanics and quantum field theory**In quantum mechanics, the system does not follow a single path whose action is stationary, but the behavior of the system depends on all imaginable paths and the value of their action. The action corresponding to the various paths is used to calculate the path integral, that gives the

probability amplitude s of the various outcomes.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 'spath 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.**Action principle and conservation laws**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. An example isNoether's theorem , which states that to everycontinuous symmetry in a physical situation there corresponds aconservation law (and conversely). This deep connection requires that the action principle be assumed.**Modern extensions of the action principle**The action principle can be generalized still further. For example, the action need not be an integral because nonlocal actions are possible. The configuration space need not even be a

functional space given certain features such asnoncommutative geometry . However, a physical basis for these mathematical extensions remains to be established experimentally.**ee also**

*Lagrangian

*Lagrangian mechanics

*Noether's theorem

*Hamiltonian mechanics

*Calculus of variations

*Functional derivative

*Functional integral

*Path integral formulation

*Quantum physics

*Planck's constant

*Entropy (the least Action Principle and the Principle of Maximum Probability or Entropy could be seen analogous)**References**For an annotated bibliography, see Edwin F. Taylor [

*http://www.eftaylor.com/pub/BibliogLeastAction12.pdf*] 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 0-02-897359-3, OCLC|35269891, 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 [*http://www.courses.fas.harvard.edu/~phys16/Textbook/ch5.pdf*]

#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 0-07-069258-0, 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.

#Edwin F. Taylor's page [*http://www.eftaylor.com/leastaction.html*]

# [*http://www.eftaylor.com/software/ActionApplets/LeastAction.html Principle of least action interactive*] Excellent interactive explanation/webpage**External links**

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