- Action (physics)
physics, the action is a particular quantity in a physical systemthat 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 Bernoulliand Pierre Louis Maupertuisdefined the "action" for lightas 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 .
Physical laws are most often expressed as
differential equations, 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 in Hamilton's principle, which states that the differential equations of motion for "any" physical system can be re-formulated as an equivalent integral equation. It applies not only to the classical mechanicsof a single particle, but also to classical fields such as the electromagnetic and gravitational fields.
Hamilton's principle has also been extended to
quantum mechanicsand quantum field theory.
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 an extremum(usually, a minimum) of the action.
Several different definitions of 'the action' are in common use in physics:
*The action is usually an
integralover 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 motionfor a physical system as an equivalent " integral equation". Although several variants have been defined (see below), the most commonly used action principle is Hamilton's principle.
*An earlier, less informative action principle is
Maupertuis' principle, which is sometimes called by its (less correct) historical name, the principle of least action.
Disambiguation of "action" in classical physics
classical physics, the term "action" has at least eight distinct meanings.
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. In
classical mechanics, the input function is the evolution of the system between two times and , where represent the generalized coordinates. The action is defined as the integralof the Lagrangianfor 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 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.
Abbreviated action (functional)
Usually denoted as , 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 is defined as the integral of the generalized momenta along a path in the generalized coordinates
Maupertuis' principle, the true path is a path for which the abbreviated action is stationary.
Hamilton's principal function
Hamilton's principal function is defined by the
Hamilton–Jacobi equations (HJE), another alternative formulation of classical mechanics. This function is related to the functional by fixing the initial time and endpoint and allowing the upper limits and the second endpoint to vary; these variables are the arguments of the function . In other words, the action function is the indefinite integral of the Lagrangian with respect to time.
Hamilton's characteristic function
When the total energy is conserved, the HJE can be solved with the additive separation of variables
where the time independent function is called "Hamilton's characteristic function". The physical significance of this function is understood by taking its total time derivative
This can be integrated to give
which is just the abbreviated action.
Other solutions of Hamilton–Jacobi equations
Hamilton–Jacobi equations are often solved by additive separability; in some cases, the individual terms of the solution, e.g., , are also called an "action".
Action of a generalized coordinate
This is a single variable in the
action-angle coordinates, defined by integrating a single generalized momentum around a closed path in phase space, corresponding to rotating or oscillating motion
The variable is called the "action" of the generalized coordinate ; the corresponding canonical variable conjugate to is its "angle" , for reasons described more fully under
action-angle coordinates. The integration is only over a single variable and, therefore, unlike the integrated dot product in the abbreviated action integral above. The variable equals the change in as is varied around the closed path. For several physical systems of interest, is either a constant or varies very slowly; hence, the variable is often used in perturbation calculations and in determining adiabatic invariants.
Action for a Hamiltonian flow
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 equations (called the Euler–Lagrange equations) that may be determined using the calculus of variations. We illustrate this derivation here using only one coordinate, "x"; the extension to multiple coordinates is straightforward.
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
where the initial and final times ( and ) and the final and initial positions are specified in advance as and . Let represent the true evolution that we seek, and let be a slightly perturbed version of it, albeit with the same endpoints, and . The difference between these two evolutions, which we will call , is infinitesimally small at all times
At the endpoints, the difference vanishes, i.e., .
Expanded to first order, the difference between the actions integrals for the two evolutions is
Integration by partsof the last term, together with the boundary conditions , yields the equation
The requirement that be stationary implies that the first-order change must be zero for "any" possible perturbation about the true evolution. This can be true only if
Those familiar with
functional analysiswill note that the Euler–Lagrange equations simplify to :.
The quantity 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 , then 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 coordinatesas 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.
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 the proper timeis:.
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 :
Lagrangianis:. [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 the
equations of motionfor fields, such as the electromagnetic fieldor gravity.
Einstein equationutilizes the " Einstein-Hilbert action" as constrained by a variational 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 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. In particular, it is fully appreciated and best understood within quantum mechanics. Richard Feynman's path integral formulationof quantum mechanics is based on a stationary-action principle, using path integrals. Maxwell's equationscan 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 is Noether's theorem, which states that to every continuous symmetryin a physical situation there corresponds a conservation 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 spacegiven certain features such as noncommutative geometry. However, a physical basis for these mathematical extensions remains to be established experimentally.
Calculus of variations
Path integral formulation
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 [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. Landauand 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
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