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In physics, **action** is a scalar quantity that describes how the balance of kinetic versus potential energy of a physical system changes with trajectory.
Action is significant because it is an input to the principle of stationary action, an approach to classical mechanics that is simpler for multiple objects.^{[1]} Action and the variational principle are used in Feynman's quantum mechanics^{[2]} and in general relativity.^{[3]} For systems with small values of action similar to the Planck constant, quantum effects are significant.^{[4]}

Action | |
---|---|

Common symbols | S |

SI unit | joule-second |

Other units | J⋅Hz^{−1} |

In SI base units | kg⋅m^{2}⋅s^{−1} |

Dimension |

In the simple case of a single particle moving with a constant velocity (thereby undergoing uniform linear motion), the action is the momentum of the particle times the distance it moves, added up along its path; equivalently, action is the difference between the particle's kinetic energy and its potential energy, times the duration for which it has that amount of energy.

More formally, action is a mathematical functional which takes the trajectory (also called path or history) of the system as its argument and has a real number as its result. Generally, the action takes different values for different paths.^{[5]} Action has dimensions of energy × time or momentum × length, and its SI unit is joule-second (like the Planck constant *h*).^{[6]}

Introductory physics often begins with Newton's laws of motion, relating force and motion; action is part of a completely equivalent alternative approach with practical and educational advantages.^{[1]} However the concept took many decades to supplant Newtonian approaches and remains a challenge to introduce to students.^{[7]}

For a trajectory of a baseball moving in the air on Earth the **action** is defined between two points in time, and as the kinetic energy minus the potential energy, integrated over time.^{[4]}

The action balances kinetic against potential energy.^{[4]}
The kinetic energy of a baseball of mass is where is the velocity of the ball; the potential energy is where is the gravitational constant. Then the action between and is

The action value depends upon the trajectory taken by the baseball through and . This makes the action an input to the powerful stationary-action principle for classical and for quantum mechanics. Newton's equations of motion for the baseball can be derived from the action using the stationary-action principle, but the advantages of action-based mechanics only begin to appear in cases where the Newton's laws are difficult to apply. Replace the baseball by an electron: classical mechanics fails but stationary action continues to work.^{[4]} The energy difference in the simple action definition, kinetic minus potential energy, is generalized and called the Lagrangian for more complex cases.

The Planck constant, written as or when including a factor of , is called *the quantum of action*.^{[8]} Like action, this constant has unit of energy times time. It figures in all significant quantum equations, like the uncertainty principle and the de Broglie wavelength. Whenever the value of the action approaches the Planck constant, quantum effects are significant.^{[4]}

Pierre Louis Maupertuis and Leonhard Euler working in the 1740s developed early versions of the action principle. Joseph Louis Lagrange clarified the mathematics when he invented the calculus of variations. William Rowan Hamilton made the next big breakthrough, formulating Hamilton's principle in 1853.^{[9]}^{: 740 } Hamilton's principle became the cornerstone for classical work with different forms of action until Richard Feynman and Julian Schwinger developed quantum action principles.^{[10]}^{: 127 }

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 a stationary point (usually, a minimum) of the action. Action has the dimensions of [energy] × [time], and its SI unit is joule-second, which is identical to the unit of angular momentum.

Several different definitions of "the action" are in common use in physics.^{[11]}^{[12]} The action is usually an integral over time. However, when the action pertains 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 action is typically 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:^{[11]}
where the integrand *L* is called the Lagrangian. For the action integral to be well-defined, the trajectory has to be bounded in time and space.

Most commonly, the term is used for a functional which takes a function of time and (for fields) space as input and returns a scalar.^{[13]}^{[14]} In classical mechanics, the input function is the evolution **q**(*t*) of the system between two times *t*_{1} and *t*_{2}, where **q** represents the generalized coordinates. The action is defined as the integral of the Lagrangian *L* for an input evolution between the two times:
where the endpoints of the evolution are fixed and defined as and . According to Hamilton's principle, the true evolution **q**_{true}(*t*) is an evolution for which the action is stationary (a minimum, maximum, or a saddle point). This principle results in the equations of motion in Lagrangian mechanics.

In addition to the action functional, there is another functional called the *abbreviated action*. In the abbreviated action, 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 (sometime written as ) is defined as the integral of the generalized momenta, for a system Lagrangian along a path in the generalized coordinates : where and are the starting and ending coordinates. According to Maupertuis' principle, the true path of the system is a path for which the abbreviated action is stationary.

When the total energy *E* is conserved, the Hamilton–Jacobi equation can be solved with the additive separation of variables:^{[11]}^{: 225 }
where the time-independent function *W*(*q*_{1}, *q*_{2}, ..., *q _{N}*) is called

This can be integrated to give

which is just the abbreviated action.^{[15]}^{: 434 }

A variable *J _{k}* in the action-angle coordinates, called the "action" of the generalized coordinate

The corresponding canonical variable conjugate to *J _{k}* is its "angle"

When relativistic effects are significant, the action of a point particle of mass *m* travelling a world line *C* parametrized by the proper time is

If instead, the particle is parametrized by the coordinate time *t* of the particle and the coordinate time ranges from *t*_{1} to *t*_{2}, then the action becomes
where the Lagrangian is^{[16]}

Physical laws are frequently expressed as differential equations, which describe how physical quantities such as position and momentum change continuously with time, space or a generalization thereof. Given the initial and boundary conditions for the situation, the "solution" to these empirical equations is one or more functions that describe the behavior of the system and are called *equations of motion*.

*Action* is a part of an alternative approach to finding such equations of motion. Classical mechanics postulates that the path actually followed by a physical system is that for which the *action is minimized*, or more generally, is stationary. In other words, the action satisfies a variational principle: the principle of stationary action (see also below). The action is defined by an integral, and the classical equations of motion of a system can be derived by minimizing the value of that integral.

The action principle provides deep insights into physics, and is an important concept in modern theoretical physics. Various action principles and related concepts are summarized below.

In classical mechanics, Maupertuis's principle (named after Pierre Louis Maupertuis) states that the path followed by a physical system is the one of least length (with a suitable interpretation of path and length). Maupertuis's principle uses the abbreviated action between two generalized points on a path.

Hamilton's principle states that the differential equations of motion for *any* physical system can be re-formulated as an equivalent integral equation. Thus, there are two distinct approaches for formulating dynamical models.

Hamilton's principle applies not only to the classical 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 and quantum field theory—in particular the path integral formulation of quantum mechanics makes use of the concept—where a physical system explores all possible paths, with the phase of the probability amplitude for each path being determined by the action for the path; the final probability amplitude adds all paths using their complex amplitude and phase.^{[17]}

Hamilton's principal function is obtained from the action functional by fixing the initial time and the initial endpoint while allowing the upper time limit and the second endpoint to vary. The Hamilton's principal function satisfies the Hamilton–Jacobi equation, a formulation of classical mechanics. Due to a similarity with the Schrödinger equation, the Hamilton–Jacobi equation provides, arguably, the most direct link with quantum mechanics.

In Lagrangian mechanics, the requirement that the action integral be stationary under small perturbations is equivalent to a set of differential equations (called the Euler–Lagrange equations) that may be obtained using the calculus of variations.

The **action principle** can be extended to obtain the equations of motion for fields, such as the electromagnetic field or gravitational field.
Maxwell's equations can be derived as conditions of stationary action.

The Einstein equation utilizes the *Einstein–Hilbert action* as constrained by a variational principle. The trajectory (path in spacetime) of a body in a gravitational field can be found using the action principle. For a free falling body, this trajectory is a geodesic.

Implications of symmetries in a physical situation can be found with the action principle, together with the Euler–Lagrange equations, which are derived from the action principle. An example is Noether's theorem, which states that to every continuous symmetry in a physical situation there corresponds a conservation law (and conversely). This deep connection requires that the action principle be assumed.^{[17]}

In quantum mechanics, the system does not follow a single path whose action is stationary, but the behavior of the system depends on all permitted paths and the value of their action. The action corresponding to the various paths is used to calculate the path integral, which gives the probability amplitudes 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. It is best understood within quantum mechanics, particularly in Richard Feynman's path integral formulation, where it arises out of destructive interference of quantum amplitudes.

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 as noncommutative geometry. However, a physical basis for these mathematical extensions remains to be established experimentally.^{[13]}

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^{a}^{b}Neuenschwander, Dwight E.; Taylor, Edwin F.; Tuleja, Slavomir (2006-03-01). "Action: Forcing Energy to Predict Motion".*The Physics Teacher*.**44**(3): 146–152. doi:10.1119/1.2173320. ISSN 0031-921X. **^**Ogborn, Jon; Taylor, Edwin F (2005-01-01). "Quantum physics explains Newtons laws of motion" (PDF).*Physics Education*.**40**(1): 26–34. doi:10.1088/0031-9120/40/1/001. ISSN 0031-9120. S2CID 250809103.**^**Taylor, Edwin F. (2003-05-01). "A call to action".*American Journal of Physics*.**71**(5): 423–425. doi:10.1119/1.1555874. ISSN 0002-9505.- ^
^{a}^{b}^{c}^{d}^{e}"The Feynman Lectures on Physics Vol. II Ch. 19: The Principle of Least Action".*www.feynmanlectures.caltech.edu*. Retrieved 2023-11-03. **^**Goodman, Bernard (1993). "Action". In Parker, S. P. (ed.).*McGraw-Hill Encyclopaedia of Physics*(2nd ed.). New York: McGraw-Hill. p. 22. ISBN 0-07-051400-3.**^**Stehle, Philip M. (1993). "Least-action principle". In Parker, S. P. (ed.).*McGraw-Hill Encyclopaedia of Physics*(2nd ed.). New York: McGraw-Hill. p. 670. ISBN 0-07-051400-3.**^**Fee, Jerome (1942). "Maupertuis and the Principle of Least Action".*American Scientist*.**30**(2): 149–158. ISSN 0003-0996. JSTOR 27825934.**^**"Max Planck Nobel Lecture". Archived from the original on 2023-07-14. Retrieved 2023-07-14.**^**Kline, Morris (1972).*Mathematical Thought from Ancient to Modern Times*. New York: Oxford University Press. pp. 167–168. ISBN 0-19-501496-0.**^**Yourgrau, Wolfgang; Mandelstam, Stanley (1979).*Variational principles in dynamics and quantum theory*. Dover books on physics and chemistry (Republ. of the 3rd ed., publ. in 1968 ed.). New York, NY: Dover Publ. ISBN 978-0-486-63773-0.- ^
^{a}^{b}^{c}Analytical Mechanics, L.N. Hand, J.D. Finch, Cambridge University Press, 2008, ISBN 978-0-521-57572-0 **^**Encyclopaedia of Physics (2nd Edition), R.G. Lerner, G.L. Trigg, VHC publishers, 1991, ISBN 3-527-26954-1 (Verlagsgesellschaft), ISBN 0-89573-752-3 (VHC Inc.)- ^
^{a}^{b}The Road to Reality, Roger Penrose, Vintage books, 2007, ISBN 0-679-77631-1 **^**T. W. B. Kibble,*Classical Mechanics*, European Physics Series, McGraw-Hill (UK), 1973, ISBN 0-07-084018-0- ^
^{a}^{b}^{c}Goldstein, Herbert; Poole, Charles P.; Safko, John L. (2008).*Classical mechanics*(3, [Nachdr.] ed.). San Francisco Munich: Addison Wesley. ISBN 978-0-201-65702-9. **^**L. D. Landau and E. M. Lifshitz (1971).*The Classical Theory of Fields*. Addison-Wesley. Sec. 8. p. 24–25.- ^
^{a}^{b}Quantum Mechanics, E. Abers, Pearson Ed., Addison Wesley, Prentice Hall Inc, 2004, ISBN 978-0-13-146100-0

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