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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 .

The action principle can be extended to obtain the Equations Of Motion for Fields , such as the Electromagnetic Field or Gravity .

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.


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. 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.

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.


HISTORY


The principle of least action was first formulated by , 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 \dot{x}(t), then the Lagrangian is a function dependent on these quantities and possibly also explicitly on time:

:L {Link without Title}

The action ''S'' is the Integral of the Lagrangian over time between a ''given'' starting point ''x''(''t1'') at time ''t1'' and a ''given'' end point ''x''(''t2'') at time ''t2''

: S=\int_{t_1}^{t_2} L {Link without Title} \, dt.

In Lagrangian mechanics, the trajectory of an object is derived by finding the path for which the action ''S'' is Stationary (a minimum, maximum, or a saddle point). The action 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).

Also a curious (approximate) identity given by Hamilton relating the Wave function of optics and the action S is:

: \Psi=e^{iS/\hbar}

this was first discovered by Hamilton (although he put a constant different from Planck,s h) as an approximation relating optics and mechanics, unfortunately nor Hamilton neither other scientist could see far beyond this at their time, Schrödinger used this relation with h to obtain his non-relativistic wave function of quantum mechanics.


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'':

: S = \int_{t_1}^{t_2}\; L(x,\dot{x})\,dt.

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:

: \delta S = \int_{t_1}^{t_2}\; \left L(x+ arepsilon,\dot x+\dot arepsilon)- L(x,\dot x) ight dt = \int_{t_1}^{t_2}\; \left(
arepsilon{\partial L\over\partial x} +
\dot arepsilon{\partial L\over\partial \dot x} ight)\,dt


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:

:
\delta S = \int_{t_1}^{t_2}\;
\left(
arepsilon{\partial L\over \partial x}
- arepsilon{d\over dt }{\partial L\over\partial \dot x}
ight)\,dt.


''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


:
{\partial L\over\partial x_{a}} - {d\over dt }{\partial L\over\partial
\dot{x}_{a}} = 0
   Euler-Lagrange equations


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 rac{\partial L}{\partial x}=0, then rac{\partial L}{\partial\dot x} is constant.

Then the coordinate ''x'' is called a ''cyclic'' coordinate,
and rac{\partial L}{\partial\dot x} 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
: rac{\delta S}{\delta x_{i}(t)}=0.


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
: rac{1}{2} mv^2= rac{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 = rac{1}{2}m \left( \dot{r}^2 + r^2\dot arphi^2 ight).


The radial ''r'' and φ components of the Euler-Lagrangian equations become, respectively

:
rac{d}{dt} \left( rac{\partial L}{\partial \dot{r}} ight)
- rac{\partial L}{\partial r}
= 0 \qquad
\Rightarrow \qquad
\ddot{r} - r\dot{ arphi}^2 = 0

:
rac{d}{dt} \left( rac{\partial L}{\partial \dot{ arphi}} ight)
- rac{\partial L}{\partial arphi}
= 0 \qquad
\Rightarrow \qquad
\ddot{ arphi} + rac{2}{r}\dot{r}\dot{ arphi} = 0.


The solution of these two equations is given by

: r\cos arphi = a t + b
: r\sin arphi = c t + d

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 Number s and in general, it needn't even necessarily be an Integral because Nonlocal Action s are possible. The configuration space needn't even necessarily be a Functional Space because we could have things like Noncommutative Geometry .


EINSTEIN-HILBERT ACTION

The Einstein Equation utilizes the '' Einstein-Hilbert Action '' as constrained by a Variational Principle .


SEE ALSO



LITERATURE


For an annotated bibliography, see Edwin F. Taylor
{Link without Title}
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 {Link without Title}
#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.