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| A '''variational principle''' in physics is an alternative method for determining the state or dynamics of a physical system, by identifying it as an extremum (minimum, maximum or saddle point) of a function or functional. This article describes the historical development of such principles.
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| ==Variational principles before the [[Modern Times]]==
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| Variational principles are found among earlier ideas in [[surveying]] and [[optics]]. The [[rope stretchers]] of [[ancient Egypt]] stretched corded ropes between two points to measure the path which minimized the distance of separation, and [[Claudius Ptolemy]], in his [[Geographia (Ptolemy)|Geographia]] (Bk 1, Ch 2), emphasized that one must correct for "deviations from a straight course"; in [[ancient Greece]] [[Euclid]] states in his ''Catoptrica'' that, for the path of light reflecting from a mirror, the [[angle of incidence]] equals the [[angle of reflection]]; and [[Hero of Alexandria]] later showed that this path was the shortest length and least time.<ref>{{cite book|last=Kline|first=Morris|title=Mathematical Thought from Ancient to Modern Times|publisher=Oxford University Press|location=New York|year=1972|pages= 167–168|isbn=0-19-501496-0}}</ref>
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| This was generalized to [[refraction]] by [[Pierre de Fermat]], who, in the 17th century, refined the principle to "light travels between two given points along the path of shortest ''time''"; now known as the '''[[principle of least time]]''' or '''[[Fermat's principle]]'''.
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| ==Principle of extremal action==
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| Credit for the formulation of the '''principle of least action''' is commonly given to [[Pierre Louis Maupertuis]], who wrote about it in 1744{{ref|Mau44}} and 1746{{ref|Mau46}}, although the true priority is less clear, as discussed below.
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| Maupertuis felt that "Nature is thrifty in all its actions", and applied the principle broadly: "The laws of movement and of rest deduced from this principle being precisely the same as those observed in nature, we can admire the application of it to all phenomena. The movement of animals, the vegetative growth of plants ... are only its consequences; and the spectacle of the universe becomes so much the grander, so much more beautiful, the worthier of its Author, when one knows that a small number of laws, most wisely established, suffice for all movements." {{ref|Dav98}}
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| In application to physics, Maupertuis suggested that the quantity to be minimized was the product of the duration (time) of movement within a system by the "[[vis viva]]", twice what we now call the kinetic energy of the system.
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| [[Leonhard Euler]] gave a formulation of the action principle in 1744, in very recognizable terms, in the ''Additamentum 2'' to his "Methodus Inveniendi Lineas Curvas Maximi Minive Proprietate Gaudentes"{{ref|Eul44}}. He begins the second paragraph {{ref|Adit}}:
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| :"Sit massa corporis projecti ==''M'', ejusque, dum spatiolum == ''ds'' emetitur, celeritas debita altitudini == ''v''; erit quantitas motus corporis in hoc loco == <math>M\sqrt{v}</math> ; quae per ipsum spatiolum ''ds'' multiplicata, dabit <math>M\,ds\sqrt{v}</math> motum corporis collectivum per spatiolum ''ds''. Iam dico lineam a corpore descriptam ita fore comparatam, ut, inter omnes alias lineas iisdem terminis contentas, sit <math>\int M ds \sqrt{v}</math>, seu, ob M constans, <math>\int ds \sqrt{v}</math> minimum."
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| A translation of this passage reads:
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| :"Let the mass of the projectile be ''M'', and let its squared velocity resulting from its height be ''v'' while being moved over a distance ''ds''. The body will have a momentum <math>M \sqrt{v}</math> that, when multiplied by the distance ''ds'', will give <math>M ds \sqrt{v}</math>, the momentum of the body integrated over the distance ''ds''. Now I assert that the curve thus described by the body to be the curve (from among all other curves connecting the same endpoints) that minimizes <math>\int M ds \sqrt{v}</math> or, provided that ''M'' is constant, <math>\int ds \sqrt{v}</math>."
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| As Euler states, <math>\int M ds \sqrt{v}</math> is the integral of the momentum over distance traveled which, in modern notation, equals the [[reduced action]] <math>\int p\,dq</math>. Thus, Euler made an equivalent and (apparently) independent statement of the variational principle in the same year as Maupertuis, albeit slightly later. In rather general terms he wrote that "Since the fabric of the Universe is most perfect and is the work of a most wise Creator, nothing whatsoever takes place in the Universe in which some relation of maximum and minimum does not appear."
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| However, Euler did not claim any priority, as the following episode shows.
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| Maupertuis' priority was disputed in 1751 by the mathematician [[Samuel König]], who claimed that it had been invented by [[Gottfried Leibniz]] in 1707. Although similar to many of Leibniz's arguments, the principle itself has not been documented in Leibniz's works. König himself showed a ''copy'' of a 1707 letter from Leibniz to [[Jacob Hermann (mathematician)|Jacob Hermann]] with the principle, but the ''original'' letter has been lost. In contentious proceedings, König was accused of forgery{{ref|Oco03}}, and even the King of Prussia entered the debate, defending Maupertuis, while [[Voltaire]] defended König. Euler, rather than claiming priority, was a staunch defender of Maupertuis, and Euler himself prosecuted König for forgery before the Berlin Academy on 13 April 1752.{{ref|Oco03}}. The claims of forgery were re-examined 150 years later, and archival work by [[C.I. Gerhardt]] in 1898{{ref|Ger98}} and [[W. Kabitz]] in 1913{{ref|Kab13}} uncovered other copies of the letter, and three others cited by König, in the [[Bernoulli]] archives.
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| ==Further developments of the extremal-action principle==
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| Euler continued to write on the topic; in his ''Reflexions sur quelques loix generales de la nature'' (1748), he called 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.
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| The full importance of the principle to mechanics was stated by [[Joseph Louis Lagrange]] in 1760 (need ref), although the variational principle was not used to derive the equations of motion until almost 75 years later, when [[William Rowan Hamilton]] in 1834 and 1835 {{ref|Ham34}}applied the variational principle to the function <math>L=T-V</math> to obtain what are now called the '''[[Lagrangian equations of motion]]'''.
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| ==Other formulations of the extremal-action principle==
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| In 1842, [[Carl Gustav Jacobi]] tackled the problem of whether the variational principle found minima or other extrema (e.g. a [[saddle point]]); most of his work focused on geodesics on two-dimensional surfaces {{ref|Jac42}}. The first clear general statements were given by [[Marston Morse]] in the 1920s and 1930s, {{ref|Mor34}} leading to what is now known as [[Morse theory]]. For example, Morse showed that the number of conjugate points in a trajectory equaled the number of negative eigenvalues in the second variation of the Lagrangian.
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| Other extremal principles of [[classical mechanics]] have been formulated, such as [[Gauss' principle of least constraint]] and its corollary, [[Gauss' principle of least constraint|Hertz's principle of least curvature]].
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| ==Variational principles in electromagnetism==
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| The action for electromagnetism is:
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| :<math> \mathcal{S} = -\int \frac{1}{4 \mu_0} \, \mathrm{d}^4x \, F^{\alpha\beta} F_{\alpha\beta} - \int \mathrm{d}^4x \, j^{\alpha}A_{\alpha} </math>
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| ==Variational principles in relativity theory==
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| The [[Einstein-Hilbert action]] which gives rise to the vacuum [[Einstein field equations]] is
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| :<math>\mathcal{S}[g] =\frac{c^4}{16 \pi G}\int_{\mathcal{M}} R \sqrt{-g} \, \mathrm{d}^4 x</math>,
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| where <math>g=\det(g_{\alpha\beta})</math> is the determinant of a spacetime [[Lorentz metric]] and <math>R</math> is the [[scalar curvature]].
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| ==Variational principles in quantum mechanics==
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| Sum over possible paths, Feynman approach. See [[Path integral formulation]]
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| ==Apparent teleology?==
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| Although equivalent mathematically, there is an important ''philosophical'' difference between the [[differential equation|differential]] [[equations of motion]] and their [[integral equation|integral]] counterpart. The differential equations are statements about quantities localized to a single point in space or single moment of time. For example, [[Newton's laws of motion|Newton's second law]] <math>F=ma</math> states that the ''instantaneous'' force <math>F</math> applied to a mass <math>m</math> produces an acceleration <math>a</math> at the same ''instant''. By contrast, the action principle is not localized to a point; rather, it involves integrals over an interval of time and (for fields) extended region of space. Moreover, in the usual formulation of [[classical physics|classical]] action principles, the initial and final states of the system are fixed, e.g.,
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| :''Given that the particle begins at position <math>x_{1}</math> at time <math>t_{1}</math> and ends at position <math>x_{2}</math> at time <math>t_{2}</math>, the physical trajectory that connects these two endpoints is an extremum of the action integral.''
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| In particular, the fixing of the ''final'' state appears to give the action principle a [[teleology|teleological character]] which has been controversial historically. This apparent [[teleology]] is eliminated in the [[quantum mechanics|quantum mechanical]] version of the action principle.
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| ==References==
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| <references/>
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| * {{note|Mau44}} P.L.N. de Maupertuis, ''Accord de différentes lois de la nature qui avaient jusqu'ici paru incompatibles.'' (1744) Mém. As. Sc. Paris p. 417.
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| * {{note|Mau46}} P.L.N. de Maupertuis, ''Le lois de mouvement et du repos, déduites d'un principe de métaphysique.'' (1746) Mém. Ac. Berlin, p. 267.
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| * {{Note|Eul44}} Leonhard Euler, ''Methodus Inveniendi Lineas Curvas Maximi Minive Proprietate Gaudentes.'' (1744) Bousquet, Lausanne & Geneva. 320 pages. Reprinted in ''Leonhardi Euleri Opera Omnia: Series I vol 24.'' (1952) C. Cartheodory (ed.) Orell Fuessli, Zurich. [http://math.dartmouth.edu/~euler/pages/E065.html scanned copy of complete text] at ''[http://math.dartmouth.edu/~euler/ The Euler Archive]'', Dartmouth.
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| * {{note|Ham34}} W.R. Hamilton, "On a General Method in Dynamics.", ''Philosophical Transaction of the Royal Society'' [http://www.emis.de/classics/Hamilton/GenMeth.pdf Part I (1834) p.247-308]; [http://www.emis.de/classics/Hamilton/SecEssay.pdf Part II (1835) p. 95-144]. (''From the collection [http://www.emis.de/classics/Hamilton/ Sir William Rowan Hamilton (1805-1865): Mathematical Papers] edited by David R. Wilkins, School of Mathematics, Trinity College, Dublin 2, Ireland. (2000); also reviewed as [http://www.maths.tcd.ie/pub/HistMath/People/Hamilton/Dynamics/ On a General Method in Dynamics]'')
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| * {{note|Jac42}} G.C.J. Jacobi, ''Vorlesungen über Dynamik, gehalten an der Universität Königsberg im Wintersemester 1842-1843''. A. Clebsch (ed.) (1866); Reimer; Berlin. 290 pages, available online [http://math-doc.ujf-grenoble.fr/cgi-bin/oeitem?id=OE_JACOBI__8_1_0 Œuvres complètes volume '''8'''] at [http://math-doc.ujf-grenoble.fr/OEUVRES/ Gallica-Math] from the [http://gallica.bnf.fr/ Gallica Bibliothèque nationale de France].
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| * {{note|Ger98}} Gerhardt CI. (1898) "Über die vier Briefe von Leibniz, die Samuel König in dem Appel au public, Leide MDCCLIII, veröffentlicht hat", ''Sitzungsberichte der Königlich Preussischen Akademie der Wissenschaften'', '''I''', 419-427.
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| * {{note|Kab13}} Kabitz W. (1913) "Über eine in Gotha aufgefundene Abschrift des von S. König in seinem Streite mit Maupertuis und der Akademie veröffentlichten, seinerzeit für unecht erklärten Leibnizbriefes", ''Sitzungsberichte der Königlich Preussischen Akademie der Wissenschaften'', '''II''', 632-638.
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| * {{Note|Mor34}} Marston Morse (1934). "The Calculus of Variations in the Large", ''American Mathematical Society Colloquium Publication'' '''18'''; New York.
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| * {{Note|Dav98}} Chris Davis. ''[http://www.idlex.freeserve.co.uk/idle/evolution/ref/leastact.html Idle theory]'' (1998)
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| * {{Note|Adit}} Euler, ''Methodus Inveniendi Lineas Curvas Maximi Minive Proprietate Gaudentes: [http://math.dartmouth.edu/~euler/docs/originals/E065h Additamentum II]'', Ibid.
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| * {{Note|Oco03}} J J O'Connor and E F Robertson, "[http://www-history.mcs.st-andrews.ac.uk/history/HistTopics/Forgery_2.html The Berlin Academy and forgery]", (2003), at ''[http://www-history.mcs.st-andrews.ac.uk/history/ The MacTutor History of Mathematics archive]''.
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| {{Use dmy dates|date=September 2010}}
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| * Cassel, Kevin W.: Variational Methods with Applications in Science and Engineering, Cambridge University Press, 2013.
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| {{DEFAULTSORT:History Of Variational Principles In Physics}}
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| [[Category:Calculus of variations]]
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| [[Category:History of physics|Variational principles in physics]]
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