Because Pierre-Simon de Laplace was secretive about his background, little is known of his early life; he may have been ashamed of his past and kept the details to himself. He was born on March 23, 1749, at Beaumont-en-Auge, Normandy, France. It has been suggested that he came from a poor family and affluent neighbors helped him get an education. Other evidence, however, supports the contention that he came from a middle-class family. At any rate, Laplace was extremely intelligent; when only 18 years old he went to Paris and obtained a professorship in mathematics, based on a paper he had written on mechanics.
Laplace, working with Antoine-Laurent Lavoisier, conducted research on the heats of various substances. The two gave birth to the science of thermochemistry when they demonstrated in 1780 that the amount of heat necessary to decompose an object was equal to the heat evolved when the object had been originally formed. This concept would later resurface as the conservation of energy.
But the topic that interested Laplace most was the stability of the solar system and how to account for observed variations in the orbits of its members. Joseph-Louis Lagrange had been studying this subject as well. One of Laplace's greatest discoveries concerning orbits was his observation that the Moon was accelerating a bit faster than it was supposed to be according to existing calculations. He suggested that certain changes in Earth's orbit, caused by the gravitational tug of the other planets upon Earth, were generating an increase in the Moon's motion. Expanding on Lagrange's work, he suggested that gravitational tugging was the cause of variation in the orbits of Jupiter and Saturn as well.
Laplace realized that to maintain a constant equilibrium in the solar system, any change in one member had to produce a change in another. If one planet's eccentricity increased, there had to be a decrease in that of another. The solar system would remain the same indefinitely, so long as there was no change in the nature of the Sun. This supposition extended the work of Isaac Newton, earning Laplace the nickname of the "French Newton."
Laplace's gravitational theory was summed up in Celestial Mechanics, a five-volume book that appeared over the years from 1799 through 1825. An egotistic and boastful individual, Laplace was reluctant to give credit to the contributions of others such as Lagrange. To his credit, Lagrange did not protest his exclusion.
In 1704 Newton had suggested that short-range forces between particles could explain most chemical and physical phenomena. Laplace adopted this concept in 1796. He and his colleagues applied Newton's calculus to forces acting between particles of ordinary matter, light, heat, and electricity. By doing so, they determined equations for the refraction of light, the conduction of heat, capillary action, the elasticity of solids and the static distribution of electricity on conductors. There were a number of critics to the "Laplacian school of thought." It began to lose influence after 1810 and fell out of favor after 1820. The criticism was justified; by 1853 Jean-Bernard-L(on Foucault showed that light moved as a wave form and did not behave like a stream of particles.
Laplace also developed a hypothesis regarding the formation of the planets. All the planets orbit the Sun in the same direction and in about the same plane. (At least that was true for the seven Laplace knew about; Uranus had been discovered by William Herschel in 1781.) Laplace suggested that the Sun had formed from a rotating nebula, or cloud of gas. As the nebula contracted, rings of gas thrown off by centrifugal force were left behind. These rings condensed into the planets, which continued to orbit in the same direction as the original cloud.
This nebular hypothesis became very popular with some astronomers of the day, even though Laplace himself did not take it all that seriously. It had actually been suggested earlier by German philosopher Immanuel Kant, but that may have been unknown to Laplace. The hypothesis eventually fell by the wayside. Later, geologist Thomas Chrowder Chamberlin (1843-1928) and astronomer Forest Ray Moulton (1872-1952) put forward an alternative hypothesis, suggesting that the planets had formed by accretion of particles. The nebular hypothesis was resurrected by Carl von Weizäcker in the 1940s, and it became more popular than ever.
Laplace died in Paris on March 5, 1827. His last words were "That which we know is mere trifle, that we are ignorant of is immense."
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