A baby left on church steps grew up to write the wave equation
A few days after his birth in 1717, a Paris baby was abandoned on the steps of the church of Saint-Jean-le-Rond, and named after it. He became Jean le Rond d'Alembert, co-editor of the Encyclopédie and author of the equation that describes a vibrating string, still central to physics today.
His mother was the writer Claudine Guérin de Tencin; his father, the artillery officer Louis-Camus Destouches, was abroad at the birth. Destouches tracked the child down in a foundling home and placed him with a glazier's wife, Madame Rousseau, and quietly paid for his schooling without ever acknowledging him. D'Alembert lived with her for almost 50 years. She was unimpressed by his work, telling him a philosopher was merely an ass who tormented himself so people would talk about him after death.
He trained in law and qualified as an advocate in 1738, but mathematics won. In 1739 he sent the Academy of Sciences a list of errors in a standard textbook he had learned from. His Traité de dynamique of 1743 set out his own laws of motion, and his analysis of vibrating strings produced the operator physicists still use. He devised the ratio test for whether an infinite series converges. In 1752 he showed that, in an idealised fluid with no viscosity, a moving body should feel no drag at all, a result now called d'Alembert's paradox. Not everything went well: he argued that a coin grows more likely to land heads after each tails, and a losing betting system now carries his name.
From the late 1740s he edited the Encyclopédie's mathematics and science with Diderot and wrote more than a thousand entries, including its Preliminary Discourse. A 1757 article claiming Geneva's pastors had drifted into Socinianism, based on Voltaire's information, provoked such outrage that he stepped back from the project. He also championed the composer Rameau's view of music as a mathematical science, until the two fell out bitterly.
Frederick the Great offered to name a supposed moon of Venus after him; the moon did not exist, and d'Alembert declined anyway. A regular of Paris salons, he eventually moved in with Julie de Lespinasse. Known as an unbeliever, he died in 1783 and was buried in an unmarked common grave.
Source: Jean Le Rond d'Alembert
