LAPLACE, PIERRE-SIMON, MARQUIS DE

born March 23, 1749, Beaumount-en-Auge, Normandy, France died March 5, 1827, Paris also called (180617) Comte De Laplace French mathematician, astronomer, and physicist who is best known for his investigations into the stability of the solar system. Laplace successfully applied the Newtonian theory of gravitation to the solar system by accounting for all the observed deviations of the planets from their theoretical orbits and developed a conceptual view of evolutionary change in the physical universe. He also demonstrated the usefulness of the probabilistic interpretation of scientific data. Laplace was the son of a peasant farmer. Little is known of his early life except that he quickly showed his mathematical ability at the military academy at Beaumont. At age 18 he left his humble surroundings for Paris, determined to make his way in mathematics. He then composed a letter on principles of mechanics for the mathematician Jean d'Alembert, who recommended him to a professorship at the cole Militaire. In 1773 he began his major lifeworkapplying Newtonian gravitation to the entire solar systemby taking up a particularly troublesome problem: why Jupiter's orbit appeared to be continuously shrinking while Saturn's continually expanded. The mutual gravitational interactions within the solar system were so complex that mathematical solution seemed impossible; indeed, Newton had concluded that divine intervention was periodically required to preserve the system in equilibrium. Laplace announced the invariability of planetary mean motions, carrying his proof to the cubes of the eccentricities and inclinations. This discovery in 1773, the first and most important step in establishing the stability of the solar system, was the most important advance in physical astronomy since Newton. It won him associate membership in the Academy of Sciences the same year. Applying quantitative methods to a comparison of living and nonliving systems, Laplace and the chemist Antoine Lavoisier in 1780, with the aid of an ice calorimeter that they had invented, showed respiration to be a form of combustion. Returning to his astronomical investigations with an examination of the entire subject of planetary perturbationsmutual gravitational effectsLaplace in 1786 proved that the eccentricities and inclinations of planetary orbits to each other will always remain small, constant, and self-correcting. The effects of perturbations were therefore conservative and periodic, not cumulative and disruptive. The opposite and secular inequalities of Jupiter and Saturn (acceleration and deceleration, respectively), for example, were due to a changing effect with a period of 929 years. Their inequalities were therefore not cumulative but periodic. Turning to the subject of the attraction between spheroids, Laplace in 178485 proved that his theorem concerning spheroids of revolution is true for any spheroids with common focuses, and he explored the problem of the attraction of any spheroid upon a particle situated outside or upon its surface. Through his discovery that the attractive force of a mass upon a particle, regardless of direction, could be obtained directly by differentiating a single function, Laplace laid the mathematical foundation for the scientific study of heat, magnetism, and electricity. Laplace removed the last apparent anomaly from the theoretical description of the solar system in 1787 with the announcement that lunar acceleration depends on the eccentricity of the Earth's orbit. Although the mean motion (average angular velocity) of the Moon around the Earth depends mainly on the gravitational attraction between them, it is slightly diminished by the pull of the Sun on the Moon. This solar action depends, however, on changes in the eccentricity of the Earth's orbit resulting from perturbations by the other planets. As a result, the Moon's mean motion is accelerated as long as the Earth's orbit tends to become more circular; but, when the reverse occurs, this motion is retarded. The inequality is therefore not truly cumulative, Laplace concluded, but is of a period running into millions of years. The last threat of instability thus disappeared from the theoretical description of the solar system. In 1796 Laplace published Exposition du systme du monde (The System of the World), a semipopular treatment of his work in celestial mechanics and a model of French prose. The book included his nebular hypothesisattributing the origin of the solar system to cooling and contracting of a gaseous nebulawhich strongly influenced future thought on planetary origin. His Trait de mcanique cleste (Celestial Mechanics), appearing in five volumes between 1798 and 1827, summarized the results obtained by his mathematical development and application of the law of gravitation. He offered a complete mechanical interpretation of the solar system by devising methods for calculating the motions of the planets and their satellites and their perturbations, including the resolution of tidal problems. The book made him a celebrity. In 1814 Laplace published a popular work for the general reader, Essai philosophique sur les probabilits (A Philosophical Essay on Probability). This work was the introduction to the second edition of his comprehensive and important Thorie analytique des probabilits (Analytic Theory of Probability), first published in 1812, in which he described many of the tools he invented for mathematically predicting the probabilities that particular events will occur in nature. He applied his theory not only to the ordinary problems of chance but also to the inquiry into the causes of phenomena, vital statistics, and future events, while emphasizing its importance for physics and astronomy. Probably because he did not hold strong political views, he escaped imprisonment and execution during the Revolution. Laplace was president of the Bureau des Longitudes (Board of Longitude), aided in the organization of the metric system, helped found the Society of Arcueil, a scientific society, and was created a marquis. He served for six weeks as minister of the interior under Napoleon, who thought his record as an administrator was undistinguished. Gerald James Whitrow Additional reading Laplace's collected works were published as OEuvres compltes de Laplace, 14 vol. in 15 (18781912). Translations of Laplace's works include Mcanique cleste, trans. by Nathaniel Bowditch, 4 vol. (182939, reprinted as Celestial mechanics, 196669), with a wealth of commentary; The System of the World, 2 vol. (1830), a classic, semipopular treatment of his work on celestial mechanics; and A Philosophical Essay of Probabilities (1902, reprinted 1951), also a classic. An excellent account of Laplace's work is by Edmund T. Whittaker in The Mathematical Gazette, 33:112 (1949). E.T. Bell, Men of Mathematics (1937, reissued 1986), chapter 11, also contains a lively biographical essay. An important source of information is M.P. Crosland, The Society of Arcueil: A View of French Science at the Time of Napoleon I (1967).