Laplace, Pierre Simon (1749–1827)

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LAPLACE, PIERRE SIMON (1749–1827)

Pierre Simon Laplace, the most influential of the French mathematician-scientists of his time, made many important contributions to celestial mechanics, the theory of heat, the mathematical theory of probability, and other branches of pure and applied mathematics. He was born into a Normandy family of well-to-do peasant farmers and merchants. Intended for an ecclesiastical career, he matriculated at the University of Caen in theology but, discovering his aptitude for mathematics, departed for Paris at age nineteen bearing a letter from his instructors to the mathematician d'Alembert, a leading intellectual figure in prerevolutionary France.

D'Alembert, impressed by Laplace, took him on as his protégé and obtained an appointment for him as a professor of mathematics at the École Militaire, where he taught mathematics to teenage cadets. In this position, Laplace was also expected to present his work to the Académie Royal des Sciences, which was under royal patronage. He presented thirteen papers to the Academy in his first three years in Paris. In 1773, his fifth year in Paris, he was elected an adjunct member of the Academy. By the 1780s he was regarded as one of its leading figures and was appointed to important royal committees, largely based on his published papers on probability, astronomy, and geophysics. In 1785 he was promoted to the rank of senior pensioner in the Academy. However, Laplace was far from a popular figure. He rightly considered himself to be the leading mathematician in France and presumed upon it, pronouncing judgment on matters in other sciences. Even when he was correct, his abrasive manner created enemies. Nevertheless, he played a preeminent role in the Academy committee to reform the system of weights and measures. During this period (1782–1793) he also collaborated with Lavoisier in a series of investigations on the nature of heat and the phenomena of combustion and respiration. The collaboration terminated during the Reign of Terror, when the Academy was suppressed. Toward the end of 1793, probably in fear for his safety, Laplace left Paris until after the fall of Robespierre in the following year.

The functions of the Académie Royal des Sciences were assumed in 1795 by a branch of the newly formed National Institute. Laplace was elected vice president of this reincarnated Academy and then elected president a few months later, in 1796. The duties of this position put him in contact with Napoleon Bonaparte. Three weeks after Napoleon seized power in 1799, Laplace presented him with copies of his work on celestial mechanics. Bonaparte quipped that he would read it "in the first six weeks I have free" and invited Laplace and his wife to dinner. Three weeks later, Napoleon named Laplace his minister of the interior. After six weeks, however, he was replaced; Napoleon thought him a complete failure as an administrator. However, Napoleon continued to heap honors and rewards upon him, regarding him as a decoration of the state. He made Laplace a chancellor of the Senate with a salary that made him wealthy, named him to the Legion of Honor, and raised him to the rank of count of the empire. Laplace's wife was appointed a lady-in-waiting to the Italian court of Napoleon's sister. Laplace responded with adulatory dedications of his works to Napoleon. When Napoleon fell from power, however, Laplace carefully dissociated himself from the emperor. In the Senate, Laplace voted for the return of the Bourbon monarchy and absented himself from Paris in 1815 during Napoleon's brief hundred-day return from Elba. In 1817 Louis XVIII raised Laplace to the rank of marquis. Laplace remained loyal to the Bourbons for the rest of his life, and his 1826 refusal to sign a petition supporting freedom of the press condemned him as far as the liberals in the Academy were concerned.

Laplace's lifelong work was the successful application of Newtonian gravitation to the entire solar system, accounting for all the observed deviations of the planets and satellites from their theoretical orbits. He began this work in 1773. His five-volume work on celestial mechanics (1798–1827) provided a complete mechanical description of the solar system. During the course of his investigations, Laplace discovered that the force on a body in a gravitational field could always be derived as the gradient of a potential function, a discovery that had profound implications in other branches of physics.

In mathematics, Laplace's name is most often associated with the "Laplace transform," a technique for solving differential equations. Laplace transforms are an often-used mathematical tool of engineers and scientists. In probability theory he invented many techniques for calculating the probabilities of events, and he applied them not only to the usual problems of games but also to problems of civic interest such as population statistics, mortality, and annuities, as well as testimony and verdicts.

In the first and second decades of the nineteenth century, Laplace exercised a powerful influence on physics in France, not only through his publications but also through the power to direct the research of others by virtue of his prestige and his position in the Academy. The genius of Laplace lay in his skill in surmounting mathematical difficulties. However, unlike other great scientists, he never attempted to go beyond the view of the world that existed when his career began. Laplace was an apostle of Newtonian mechanics. He believed, for example, that phenomena such as the behavior of light in material media could be understood on the basis of Newton's corpuscular theory of light and short-range molecular forces acting on the corpuscles of light; he directed research to strengthen this approach. As the weight of evidence in favor of the wave nature of light grew larger, Laplace's influence and power waned. In his last years he developed an elaborate caloric theory of heat, again based on a theory of short-range mechanical forces, but the theory was greeted with indifference and generated no further research.

Leonard S. Taylor

See also: Gravitational Energy; Heat and Heating.