View source for Maupertuis's principle

{{short description|Principle of least length in physics}}
In [[classical mechanics]], '''Maupertuis's principle''' (named after [[Pierre Louis Maupertuis]], 1698 – 1759) states that the path followed by a physical system is the one of least length (with a suitable interpretation of ''path'' and ''length'').<ref>{{Cite book |last=Jahnke |first=Hans Niels |title=A history of analysis |date=2003 |publisher=American mathematical society |isbn=978-0-8218-2623-2 |series=History of mathematics |location=Providence (R.I.) |pages=139}}</ref>  It is a special case of the more generally stated [[principle of least action]].  Using the [[calculus of variations]], it results in an [[integral equation]] formulation of the [[equations of motion]] for the system.

== Mathematical formulation ==

Maupertuis's principle states that the true path of a system described by <math>N</math> [[generalized coordinates]] <math>\mathbf{q} = \left( q_{1}, q_{2}, \ldots, q_{N} \right)</math> between two specified states <math>\mathbf{q}_{1}</math> and <math>\mathbf{q}_{2}</math> is a minimum or a saddle point<ref>{{Cite journal |last1=Gray |first1=C. G. |last2=Taylor |first2=Edwin F. |date=May 2007 |title=When action is not least |url=https://pubs.aip.org/aapt/ajp/article/75/5/434-458/1041968 |journal=American Journal of Physics |language=en |volume=75 |issue=5 |pages=434–458 |doi=10.1119/1.2710480 |bibcode=2007AmJPh..75..434G |issn=0002-9505}}</ref> of the [[Action (physics)#Abbreviated action (functional)|abbreviated action functional]],

<math display="block">
\mathcal{S}_{0}[\mathbf{q}(t)] \ \stackrel{\mathrm{def}}{=}\  
\int \mathbf{p} \cdot d\mathbf{q},
</math>
where <math>\mathbf{p} = \left( p_{1}, p_{2}, \ldots, p_{N} \right)</math> are the conjugate momenta of the generalized coordinates, defined by the equation 
<math display="block">p_{k} \ \stackrel{\mathrm{def}}{=}\  \frac{\partial L}{\partial\dot{q}_{k}},</math>
where <math>L(\mathbf{q},\dot{\mathbf{q}},t)</math> is the [[Lagrangian mechanics|Lagrangian]] [[function (mathematics)|function]] for the system.  In other words, any ''first-order'' perturbation of the path results in (at most) ''second-order'' changes in <math>\mathcal{S}_{0}</math>.  Note that the abbreviated action <math>\mathcal{S}_{0}</math> is a [[Functional (mathematics)|functional]] (i.e. a function from a vector space into its underlying scalar field), which in this case takes as its input a function (i.e. the paths between the two specified states).

==Jacobi's formulation==

For many systems, the kinetic energy <math>T</math> is quadratic in the generalized velocities <math>\dot{\mathbf{q}}</math>
<math display="block">T = \frac{1}{2} \dot{\mathbf{q}} \ \mathbf{M} \ \dot{\mathbf{q}}^\intercal</math>
although the '''mass tensor''' <math>\mathbf{M}</math> may be a complicated function of the generalized coordinates <math>\mathbf{q}</math>.  For such systems, a simple relation relates the kinetic energy, the generalized momenta and the generalized velocities
<math display="block">2 T = \mathbf{p} \cdot \dot{\mathbf{q}}</math>
provided that the potential energy <math>V(\mathbf{q})</math> does not involve the generalized velocities.  By defining a normalized distance or [[Metric (mathematics)|metric]] <math>ds</math> in the space of generalized coordinates
<math display="block">ds^2 = d\mathbf{q} \ \mathbf{M} \ d\mathbf{q^\intercal}</math>
one may immediately recognize the mass tensor as a [[metric tensor]]. The kinetic energy may be written in a massless form
<math display="block">T = \frac{1}{2} \left( \frac{ds}{dt} \right)^{2}</math>
or,
<math display="block">2 T dt = \sqrt{2 T} \ ds.</math>

Therefore, the abbreviated action can be written
<math display="block">
\mathcal{S}_0 \ \stackrel{\mathrm{def}}{=}\  \int \mathbf{p} \cdot d\mathbf{q} = 
 \int ds \, \sqrt{2}\sqrt{E_\text{tot} - V(\mathbf{q})}
</math>
since the kinetic energy <math>T = E_\text{tot} - V(\mathbf{q})</math> equals the (constant) total energy <math>E_\text{tot}</math> minus the potential energy <math>V(\mathbf{q})</math>. In particular, if the potential energy is a constant, then Jacobi's principle reduces to minimizing the path length <math display="inline">s = \int ds</math> in the space of the generalized coordinates, which is equivalent to [[Hertz's principle of least curvature]].

==Comparison with Hamilton's principle==

[[Hamilton's principle]] and Maupertuis's principle are occasionally confused with each other and both have been called the [[principle of least action]].  They differ from each other in three important ways: 
* ''their definition of the [[action (physics)|action]]...'' {{block indent | Hamilton's principle uses <math>\mathcal{S} \ \stackrel{\mathrm{def}}{=}\  \int L \, dt</math>, the integral of the [[Lagrangian mechanics|Lagrangian]] over [[time]], varied between two fixed end times <math>t_1</math>, <math>t_2</math> and endpoints <math>q_1</math>, <math>q_2</math>.  By contrast, Maupertuis's principle uses the abbreviated action integral over the [[generalized coordinates]], varied along all constant energy paths ending at <math>\mathbf{q}_1</math> and <math>\mathbf{q}_2</math>.}}
*''the solution that they determine...'' {{block indent | Hamilton's principle determines the trajectory <math>\mathbf{q}(t)</math> as a function of time, whereas Maupertuis's principle determines only the shape of the trajectory in the generalized coordinates.  For example, Maupertuis's principle determines the shape of the ellipse on which a particle moves under the influence of an inverse-square central force such as [[gravitation|gravity]], but does not describe ''per se'' how the particle moves along that trajectory.  (However, this time parameterization may be determined from the trajectory itself in subsequent calculations using the conservation of energy.)  By contrast, Hamilton's principle directly specifies the motion along the ellipse as a function of time.}}
*''...and the constraints on the variation.'' {{block indent | Maupertuis's principle requires that the two endpoint states <math>q_1</math> and <math>q_2</math> be given and that energy be conserved along every trajectory.  By contrast, Hamilton's principle does not require the conservation of energy, but does require that the endpoint times <math>t_1</math> and <math>t_2</math> be  specified as well as the endpoint states <math>q_1</math> and <math>q_2</math>.}}

==History==
{{Main | History of variational principles in physics}}
Maupertuis was the first to publish a ''principle of least action'', as a way of adapting [[Fermat's principle]] for waves to a corpuscular (particle) theory of light.<ref>{{Cite book |last=Whittaker |first=Edmund T. |title=A history of the theories of aether & electricity. 2: The modern theories, 1900 - 1926 |date=1989 |publisher=Dover Publ |isbn=978-0-486-26126-3 |edition=Repr |location=New York}}</ref>{{rp|96}} [[Pierre de Fermat]] had explained [[Snell's law]] for the [[refraction]] of [[light]] by assuming light follows the path of shortest ''time'', not distance.  This troubled Maupertuis, since he felt that time and distance should be on an equal footing: "why should light prefer the path of shortest time over that of distance?"  Maupertuis defined his ''action'' as <math display="inline">\int v \, ds</math>, which was to be minimized over all paths connecting two specified points. Here <math>v</math> is the velocity of light the corpuscular theory. Fermat had minimized <math display="inline">\int \,ds/v</math> where <math>v</math> is wave velocity; the two velocities are reciprocal so the two forms are equivalent.

=== Koenig's claim ===

In 1751, Maupertuis's priority for the principle of least action was challenged in print (''Nova Acta Eruditorum'' of Leipzig) by an old acquaintance, [[Johann Samuel Koenig]], who quoted a 1707 letter purportedly from [[Gottfried Wilhelm Leibniz]] to [[Jakob Hermann]] that described results similar to those derived by [[Leonhard Euler]] in 1744.

Maupertuis and others demanded that Koenig produce the original of the letter to authenticate its having been written by Leibniz. Leibniz died in 1716 and Hermann in 1733, so neither could vouch for Koenig. Koenig claimed to have the letter copied from the original owned by [[Samuel Henzi]], and no clue as to the whereabouts of the original, as Henzi had been executed in 1749 for organizing the Henzi conspiracy for overthrowing the aristocratic government of [[Bern]].<ref name=Fee1942>{{Cite journal |last=Fee |first=Jerome |date=1942 |title=Maupertuis and the Principle of Least Action |url=https://www.jstor.org/stable/27825934 |journal=American Scientist |volume=30 |issue=2 |pages=149–158 |jstor=27825934 |issn=0003-0996}}</ref> Subsequently, the [[Prussian Academy of Sciences|Berlin Academy]] under Euler's direction declared the letter to be a forgery<ref>{{Cite book |last=Euler |first=Leonhard |url=https://en.wikisource.org/wiki/Translation:Investigation_of_the_letter_of_Leibniz |title=Investigation of the letter, allegedly written by Leibniz |year=1752}}</ref> and that Maupertuis, could continue to claim priority for having invented the principle. Curiously [[Voltaire]] got involved in the quarrel by composing ''[[Doctor Akakia|Diatribe du docteur Akakia]]'' ("Diatribe of Doctor Akakia") to satirize Maupertuis' scientific theories (not limited to the principle of least action). While this work damaged Maupertuis's reputation, his claim to priority for least action remains secure.<ref name=Fee1942/>

==See also==
* [[Analytical mechanics]]
* [[Hamilton's principle]]
* [[Gauss's principle of least constraint]] (also describes '''Hertz's principle of least curvature''')
* [[Hamilton–Jacobi equation]]

==References==
{{Reflist}}

* [[Pierre Louis Maupertuis]], [[s:fr:Accord de différentes loix de la nature qui avoient jusqu'ici paru incompatibles|Accord de différentes loix de la nature qui avoient jusqu'ici paru incompatibles]] ''(original 1744 French text)''; [[s:Translation:Accord between different laws of Nature that seemed incompatible|Accord between different laws of Nature that seemed incompatible]] ''(English translation)''
* [[Leonhard Euler]], [[s:la:Methodus inveniendi/Additamentum II|Methodus inveniendi/Additamentum II]] ''(original 1744 Latin text)''; [[s:Methodus inveniendi/Additamentum II|Methodus inveniendi/Appendix 2]] ''(English translation)''
* [[Pierre Louis Maupertuis]], [[s:fr:Les loix du mouvement et du repos déduites d'un principe metaphysique|Les loix du mouvement et du repos déduites d'un principe metaphysique]] ''(original 1746 French text)''; [[s:Translation:Derivation of the laws of motion and equilibrium from a metaphysical principle|Derivation of the laws of motion and equilibrium from a metaphysical principle]] ''(English translation)''
* [[Leonhard Euler]], [[s:fr:Exposé concernant l'examen de la lettre de M. de Leibnitz|Exposé concernant l'examen de la lettre de M. de Leibnitz]] ''(original 1752 French text)''; [[s:Investigation of the letter of Leibniz|Investigation of the letter of Leibniz]] ''(English translation)''
* König J. S. "De universali principio aequilibrii et motus",  ''Nova Acta Eruditorum'', '''1751''', 125–135, 162–176.
* 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]''.
* C. I. Gerhardt, (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.
* W. Kabitz, (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.
* L. D. Landau and E. M. Lifshitz, (1976) ''Mechanics'', 3rd. ed., Pergamon Press, pp.&nbsp;140–143.  {{ISBN|0-08-021022-8}} (hardcover) and {{ISBN|0-08-029141-4}} (softcover)
* 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 [https://web.archive.org/web/20071122083808/http://math-doc.ujf-grenoble.fr/cgi-bin/oeitem?id=OE_JACOBI__8_1_0  Œuvres complètes volume '''8'''] at [https://web.archive.org/web/20081123234823/http://math-doc.ujf-grenoble.fr/OEUVRES/ Gallica-Math] from the [http://gallica.bnf.fr/ Gallica Bibliothèque nationale de France].
* H. Hertz, (1896) ''Principles of Mechanics'', in ''Miscellaneous Papers'', vol. III, Macmillan.
* {{springer|id=H/h047140|title=Hertz's principle of least curvature|author=V.V. Rumyantsev}}

[[Category:Calculus of variations]]
[[Category:Hamiltonian mechanics]]
[[Category:Mathematical principles]]