Maupertuis's principle explained

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).^[1] 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

N generalized coordinates q = \left( q _{1} , q _{2} , \ldots, q _{N} \right) between two specified states q _{1} and q _{2} is a minimum or a saddle point^[2] of the abbreviated action functional,

$$\backslash mathcal\_[\backslash mathbf\{q\}(t)]\; \backslash \; \backslash stackrel\backslash \; \backslash int\; \backslash mathbf\; \backslash cdot\; d\backslash mathbf,$$where

p = \left( p _{1} , p _{2} , \ldots, p _{N} \right) are the conjugate momenta of the generalized coordinates, defined by the equation $$p\_\; \backslash \; \backslash stackrel\backslash \; \backslash frac,$$where • L( q , q ,t) is the Lagrangian function for the system. In other words, any first-order perturbation of the path results in (at most) second-order changes in l{S} _{0} . Note that the abbreviated action l{S} _{0} is a 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

T is quadratic in the generalized velocities • q $$T\; =\; \backslash frac\; \backslash dot\; \backslash \; \backslash mathbf\; \backslash \; \backslash dot^\backslash intercal$$although the mass tensor M may be a complicated function of the generalized coordinates q . For such systems, a simple relation relates the kinetic energy, the generalized momenta and the generalized velocities$$2\; T\; =\; \backslash mathbf\; \backslash cdot\; \backslash dot$$provided that the potential energy V( q ) does not involve the generalized velocities. By defining a normalized distance or metric ds in the space of generalized coordinates$$ds^2\; =\; d\backslash mathbf\; \backslash \; \backslash mathbf\; \backslash \; d\backslash mathbf$$one may immediately recognize the mass tensor as a metric tensor. The kinetic energy may be written in a massless form$$T\; =\; \backslash frac\; \backslash left(\backslash frac\; \backslash right)^$$or,$$2\; T\; dt\; =\; \backslash sqrt\; \backslash \; ds.$$

Therefore, the abbreviated action can be written$$\backslash mathcal\_0\; \backslash \; \backslash stackrel\backslash \; \backslash int\; \backslash mathbf\; \backslash cdot\; d\backslash mathbf\; =\; \backslash int\; ds\; \backslash ,\; \backslash sqrt\backslash sqrt$$since the kinetic energy

T = E _{tot} - V( q ) equals the (constant) total energy E _{tot} minus the potential energy V( q ) . In particular, if the potential energy is a constant, then Jacobi's principle reduces to minimizing the path length $s\; =\; \backslash int\; ds$ 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...
• the solution that they determine...
• ...and the constraints on the variation.

History

See main article: 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.^[3] 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 $\backslash int\; v\; \backslash ,\; ds$, which was to be minimized over all paths connecting two specified points. Here

v is the velocity of light the corpuscular theory. Fermat had minimized $\backslash int\; \backslash ,ds/v$ where v 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.^[4] Subsequently, the Berlin Academy under Euler's direction declared the letter to be a forgery^[5] and that Maupertuis, could continue to claim priority for having invented the principle. Curiously Voltaire got involved in the quarrel by composing 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.^[4]

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

• Pierre Louis Maupertuis, Accord de différentes loix de la nature qui avoient jusqu'ici paru incompatibles (original 1744 French text); Accord between different laws of Nature that seemed incompatible (English translation)
• Leonhard Euler, Methodus inveniendi/Additamentum II (original 1744 Latin text); Methodus inveniendi/Appendix 2 (English translation)
• Pierre Louis Maupertuis, Les loix du mouvement et du repos déduites d'un principe metaphysique (original 1746 French text); Derivation of the laws of motion and equilibrium from a metaphysical principle (English translation)
• Leonhard Euler, Exposé concernant l'examen de la lettre de M. de Leibnitz (original 1752 French text); 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, "The Berlin Academy and forgery", (2003), at 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. 140–143. (hardcover) and (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 Œuvres complètes volume 8 at Gallica-Math from the Gallica Bibliothèque nationale de France.
• H. Hertz, (1896) Principles of Mechanics, in Miscellaneous Papers, vol. III, Macmillan.

Notes and References

1. Book: Jahnke, Hans Niels . A history of analysis . 2003 . American mathematical society . 978-0-8218-2623-2 . History of mathematics . Providence (R.I.) . 139.
2. Gray . C. G. . Taylor . Edwin F. . May 2007 . When action is not least . American Journal of Physics . en . 75 . 5 . 434–458 . 10.1119/1.2710480 . 0002-9505.
3. Book: Whittaker, Edmund T. . A history of the theories of aether & electricity. 2: The modern theories, 1900 - 1926 . 1989 . Dover Publ . 978-0-486-26126-3 . Repr . New York.
4. Fee . Jerome . 1942 . Maupertuis and the Principle of Least Action . American Scientist . 30 . 2 . 149–158 . 0003-0996.
5. Book: Euler, Leonhard . Investigation of the letter, allegedly written by Leibniz . 1752.