Jean Le Rond d'Alembert ( DAL-əm-BAIR; French: [ʒɑ̃ lə ʁɔ̃ dalɑ̃bɛʁ]; 16 November 1717 – 29 October 1783) was a French mathematician, physicist, philosopher, and music theorist. Until 1759 he was, together with Denis Diderot, a co-editor of the Encyclopédie.
His most famous achievements include the wave equation, also known as d'Alembert's equation, and D'Alembert's formula for solving said equation. In French, the fundamental theorem of algebra is named in his honour.
Born in Paris, d'Alembert was the natural son of the writer Claudine Guérin de Tencin and the chevalier Louis-Camus Destouches, an artillery officer. Destouches was abroad at the time of d'Alembert's birth. Days after birth his mother left him on the steps of the Saint-Jean-le-Rond de Paris church. According to custom, he was named after the patron saint of the church. D'Alembert was placed in an orphanage for foundling children, but his father found him and placed him with the wife of a glazier, Madame Rousseau, with whom he lived for nearly 50 years. She gave him little encouragement. When he told her of some discovery he had made or something he had written she generally replied,
You will never be anything but a philosopher—and what is that but an ass who plagues himself all his life, that he may be talked about after he is dead.
Destouches secretly paid for the education of Jean Le Rond, but did not want his paternity officially recognised.
D'Alembert first attended a private school. The chevalier Destouches left d'Alembert an annuity of 1,200 livres on his death in 1726. Under the influence of the Destouches family, at the age of 12 d'Alembert entered the Jansenist Collège des Quatre-Nations (the institution was also known under the name "Collège Mazarin"). Here he studied philosophy, law, and the arts, graduating as baccalauréat en arts in 1735.
In his later life, d'Alembert scorned the Cartesian principles he had been taught by the Jansenists: "physical promotion, innate ideas and the vortices". The Jansenists steered d'Alembert toward an ecclesiastical career, attempting to deter him from pursuits such as poetry and mathematics. Theology was, however, "rather unsubstantial fodder" for d'Alembert. He entered law school for two years, and was nominated avocat in 1738.
He was also interested in medicine and mathematics. Jean enrolled first as Jean-Baptiste Daremberg and subsequently changed his name, perhaps for reasons of euphony, to d’Alembert.
Later, in recognition of d'Alembert's achievements, Frederick the Great of Prussia proposed the name "d'Alembert" for a suspected (but non-existent) moon of Venus, however d'Alembert refused the honor.
In July 1739 he made his first contribution to the field of mathematics, pointing out the errors he had detected in Analyse démontrée (published 1708 by Charles-René Reynaud) in a communication addressed to the Académie des Sciences. At the time L'analyse démontrée was a standard work, which d'Alembert himself had used to study the foundations of mathematics. D'Alembert was also a Latin scholar of some note and worked in the latter part of his life on a translation of Tacitus, for which he received wide praise including that of Denis Diderot.
In 1740, he submitted his second scientific work from the field of fluid mechanics Mémoire sur la réfraction des corps solides, which was recognised by Clairaut. In this work d'Alembert theoretically explained refraction.
In 1741, after several failed attempts, d'Alembert was elected into the Académie des Sciences. He was later elected to the Berlin Academy in 1746 and a Fellow of the Royal Society in 1748.
In 1743, he published his most famous work, Traité de dynamique, in which he developed his own laws of motion.
When the Encyclopédie was organised in the late 1740s, d'Alembert was engaged as co-editor (for mathematics and science) with Diderot, and served until a series of crises temporarily interrupted the publication in 1757. He authored over a thousand articles for it, including the famous Preliminary Discourse. D'Alembert "abandoned the foundation of Materialism" when he "doubted whether there exists outside us anything corresponding to what we suppose we see." In this way, d'Alembert agreed with the Idealist Berkeley and anticipated the transcendental idealism of Kant.
In 1752, he wrote about what is now called d'Alembert's paradox: that the drag on a body immersed in an inviscid, incompressible fluid is zero.
In 1754, d'Alembert was elected a member of the Académie Française, of which he became Permanent Secretary on 9 April 1772.
In 1757, an article by d'Alembert in the seventh volume of the Encyclopedia suggested that the Geneva clergymen had moved from Calvinism to pure Socinianism, basing this on information provided by Voltaire. The Pastors of Geneva were indignant, and appointed a committee to answer these charges. Under pressure from Jacob Vernes, Jean-Jacques Rousseau and others, d'Alembert eventually made the excuse that he considered anyone who did not accept the Church of Rome to be a Socinianist, and that was all he meant, and he abstained from further work on the encyclopaedia following his response to the critique.
He was elected a Foreign Honorary Member of the American Academy of Arts and Sciences in 1781.