- Laplace , Marquis Pierre Simon de
*(1749–1827) French mathematician, astronomer, and physicist*Laplace, born the son of a small estate owner in Beaumont-en-Auge, France, was educated at the University of Caen. Jean D'Alembert, impressed by a letter on mechanics sent to him by Laplace, arranged for him to become professor of mathematics in the Ecole Militaire in Paris. He became a full member of the French Academy of Sciences in 1785. Laplace prospered during the reign of Napoleon who had a genuine interest in mathematics, taking pleasure in rewarding the eminent mathematicians of his day. Laplace served briefly as Napoleon's minister of the interior in 1799 and much longer as one of his senators. He was also made a count by the emperor. Laplace was clearly one of nature's survivors, for with the Bourbon restoration in 1814 he found no embarrassmant in signing the decree banishing his patron. He was made a marquis by Louis XVIII.Laplace's greatest work was the*Traité de mécanique céleste*(Celestial Mechanics) published in five volumes between 1799 and 1825. Although Newton had derived the laws that govern the movements of the heavenly bodies it was clear to him that there were certain irregularities or perturbations in the movements of the planets that would lead to the end of the universe if not corrected. Newton was willing to accept God's intervention in the system to prevent its collapse. What Laplace showed was that while it was true that there were irregularities in the system they were periodic and not cumulative. The system was basically stable. When Napoleon pointed out to him that he had not mentioned God in his book Laplace proudly retorted that he had no need of that hypothesis. A popular account,*Exposition du système du monde*(Exposition of the System of the World), was published in 1796. It is in this work that he proposed the nebular hypothesis. Here he argued that the solar system had evolved from a rotating mass of gas that had condensed to form the Sun. From this had been thrown off the planets which in turn threw off their various satellites. The theory had in fact been proposed earlier by Immanuel Kant (in 1755) and was soon to run into trouble with the discovery of retrograde orbits in the system. However, a new form of the nebular hypothesis has been introduced into astronomy by Carl von Weizsäcker.Laplace's second great achievement was the establishment of probability theory on a rigorous basis in his*Théorie analytique des probabilités*(1812; Analytic Theory of Probabilities) and*Essai philosophique sur les probabilités*(1814; Philosophical Essay on Probabilities). His third great achievement was his development of the concept of a ‘potential’ and its description by the*Laplace equation*:δ^{2}*u*/δ*x*^{2}+ δ^{2}*u*/δ*y*^{2}+ δ^{2}*u*/δ*z*^{2}= 0For Laplace*u*was a ‘velocity potential’ but it was soon seen to have an enormous number of applications in all kinds of areas and became an essential means of dealing with motion in any field be it electromagnetic, gravitational, or hydrodynamic.Although Laplace has the reputation among mathematicians of being both a careerist and a plagiarizer he did much to help others and to encourage the growth of French science. His home at Arcueil, Paris, was the center for the greatest concentration of scientific talent of his or any other time. He died there uttering what sound like some well-prepared last words: “What we know is very slight; what we don't know is immense.”

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