Personnes
Pierre-Simon de Laplace
23 Mar 1749 - 5 Mar 1827

Laplace was born in Calvados of peasant stock, a fact which he later did his best to conceal. His talent for mathematics appeared early, and a wealthy neighbor made it possible for him to attend the military academy of Beaumont as a day student. It is said that he even taught mathematics there for a time. At eighteen, he set out for Paris with a letter of introduction to D'Alembert, who refused to see him. Laplace returned to his lodgings and composed a letter on the general principles of mechanics. On receipt of this more appropriate calling card, D'Alembert not only welcomed him, but within a few days got him appointed Professor of Mathematics at the military school in Paris.

Laplace, not shy about the sort of problem that his genius deserved, proceeded to extend the recently discovered Newtonian law of gravitation to the entire solar system, a matter of mind-boggling conceptual and computational complexity. As a first step, he produced a memoir on the mean distances of the planets from the sun in 1773, when he was still only twenty-four. The following year, at the unheard of age of twenty-five, he was granted associate membership in the Academy of Sciences. He continued to pursue his gigantic goal with remarkable singlemindedness. As Fourier later said of him, at this period in his life,

"Laplace gave to all his works a fixed direction from which he never deviated: the imperturbable constancy of his views was always the principal feature of his geniue. He was already at the extreme of mathematical analysis, knowing all that is most ingenious in this, and no one was more competent than he to extend its domain. He had solved a capital problem of astronomy, and he decided to devote all his talents to mathematical astronomy, which he was destined to perfect. he meditated profoundly on his great project, and passed his whole life perfecting it with a perseverence unique in the history of science. The vastness of the subject flattered the pride of his genius. He undertook to compose the Almagest of his age - the Mécanique Céleste, and his immortal work carries him as far beyond that of Ptolemy as the analytical science of the moderns surpasses the Elements of Euclid."

Years elapsed before the final masterpiece began to appear, but they were not without further recognition. In 1785, Laplace was promoted to full membership in the Academy in 1785. In that same year, it fell to him to examine a sixteen-year-old candidate for the Military School: Napoleon Bonaparte.

The French Revolution began in 1789. Laplace was fortunately situated for avoiding its dangers, in part because, like Lagrange, his talents were found useful in calculating artillery trajectories. Napoleon esteemed Laplace, and after the Revolution showered him with honors. The first edition of his summary work, the largely nonmathematical Exposition du Système du Monde, appeared in 1796 with a dedication to the Council of Five Hundred; it concluded with a suitably republican peroration on truth and justice.

The great work itself, undiluted, appeared in five parts: two volumes of 1799 describing the motions and shapes of the planets, two further ones in 1802 and 1805, and the final part in 1823-25.

They preliminary Exposition of 1796 has been wryly described by Bell as "Laplace's masterpiece with all the mathematics left out." The mathematics is also often left out of the Mécanique Céleste itself; Laplace often omits the argument for a result with the remark "Il est aise a voir." It was not so easy for most of his readers to see. Laplace himself, later on, was often unable to recover his reasoning at a particular point, and would need hours or days to fill in the gap. There is a time to abbreviate, but key steps in a new and complex argument are not that time.

By no means all of his work was original: it included, without acknowledgement, discoveries by Lagrange and Legengre; Newton he could not entirely avoid mentioning in the eventual masterpiece, the Mécanique Céleste, but otherwise the reader is left to infer that Laplace did it all. Posterity's disapproval of this petty ungenerosity should not cancel out the fact that in this work, Laplace did a great deal.

He had his share of courage also. On being presented with a copy of the 1796 Exposition, Napoleon remarked to the author, "You have written this huge book on the system of the universe without once mentioning the creator of the universe." Laplace gained a share of instant immortality with his answer,

"Sire, I had no need of that hypothesis."

When the political tide turned, Laplace willingly enough signed the decree which banished Napoleon. The monarchy was restored, and Laplace, now the Marquis de Laplace, took a place in the Chamber of Peers; in 1816 Louis XVIII appointed him president of the committee to reorganize the Ecole Polytechnique. In 1824, when the final part of the Mécanique at last appeared, he prefaced it not with the republican sentiments of years before, but with something more appropriately monarchical. He died, in retirement at his estate at Arceuil, on 5 March 1827. His last words, "What do know is not much; what we do not know is immense," are a studied attempt to outdo Newton's famous remark about the boy playing on the seashore. Newton's is better. But in mathematics, Laplace has a place alongside Newton. The new procedures which he developed in the course preparing his masterwork are the real masterwork: the theory of potential, without which it would be impossible to understand electromagnetism, and which generated the intensely important new subject of boundary value problems, the zone in which the infinite touches hands with the things on which engineers work.

Laplace concluded that the solar system was stable. We may concede that this was true of the idealized model of the solar system which he addressed. As for the real one which the rest of us inhabit, with such disturbing elements as tidal friction (a factor which Laplace omitted), to mention nothing nearer home, that question may still be open.

 

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