Pierre-Simon Laplace
1749-1827
French Astronomer and Mathematician
Pierre-Simon Laplace's Celestial Mechanics is generally regarded as the crowning achievement of Newtonian gravitation. Laplace also made fundamental contributions to mathematics introducing the Laplace transform method, Laplace coefficients, and establishing probability theory.
Laplace was born at Beaumont-en-Auge, Normandy, France, on March 23, 1749. From anearly age his education was directed towards an ecclesiastical vocation; he attended the Benedictine-run Collège in Beaumont-en-Auge (1756-66) before matriculating at the University of Caen (1766). At Caen his interest in mathematics was sparked, and in 1768 he left for Paris with a letter of introduction to Jean d'Alembert (1717-1773). Impressed, d'Alembert secured for Laplace an appointment at the Ecole Militaire.
Pierre-Simon Laplace. (Library of Congress. Reproduced with permission.)Laplace quickly established his mathematical reputation and was elected to the Académie des Sciences (1773). Though he made many fundamental contributions to mathematics, Laplace was primarily interested in understanding the natural world. His collaborations with Antoine Lavoisier (1743-1794) initiated thermo-chemistry. They designed an ice calorimeter, measured the specific heats of selected substances, and determined that the heat required to decompose compounds into their elements equals the heat evolved when they are formed from their elements (1784). Laplace also investigated the cohesive properties of liquids, explained capillary action, and contributed to tidal theory by taking into account Earth's rotation. However, Laplace devoted most of his energies to celestial mechanics.
One of the most vexing problems for eighteenth-century astronomy was the solar system's stability. In addition to the cyclic variations oflunar and planetary motions, there also exist cumulative orbital deviations. If these went uncorrected, it was generally believed the solar system would fly apart. Isaac Newton (1642-1727) was willing to accept Divine intervention to prevent such a state of affairs. Laplace was intent on proving such anomalies were not cumulative but periodic effects governed by universal gravitation.
In five papers between 1785 and 1788, Laplace demonstrated that these anomalies were indeed periodic. He first showed that Jupiter's orbital acceleration and Saturn's deceleration over time resulted from their mutual, gravitational-perturbing effect on each, which caused their orbits to oscillate with a 929-year cycle. He then showed that the Moon's increasing acceleration was also periodic, arguing that the gravitational influence of the Sun and planets on Earth's eccentricity resulted in a lunar cyclic effect on the order of several hundred-thousand years. Generalizing these results, Laplace showed the total eccentricity of planetary orbits had to remain constant, implying the solar system would remain stable indefinitely. Laplace summarized his research on gravitational theory in his five-volume Celestial Mechanics (1799-1825). Presented with a copy of the volume, Napoleon said he had been told there was no mention of God, whereupon Laplace supposedly replied, "I had no need of that hypothesis."
Laplace is best known for his version of the nebular hypothesis (1796), according to which the solar system originated from a rotating disk of gas. As the gas contracted, centrifugal forces exceeded gravitational forces, causing rings of material to be shed. These rings then accumulated into gaseous balls that later condensed into planets, all rotating in the same direction about the core, which condensed into the Sun. Accepted throughout the nineteenth century, the theory fell into dis-repute when Venus's retrograde motion and the eccentricity of Pluto's orbit were discovered.
During the French Revolution (1787-99) Laplace served on the Commission on Weights and Measures but was later dismissed. After Napoleon became first consul, he appointed Laplace, successively, as Minister of the Interior, Senator, and Senate Chancellor. Laplace eventually turned on Napoleon and rallied to Louis XVIII (1814). Elected to the French Academy (1816), Laplace became president of that body (1817). His last words were reported to have been, "What we know is minute, what we are ignorant of is vast."
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