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IN THE spring of 1749, Emilie du Châtelet realised to her horror that she was pregnant. She adored her two children, but was engrossed in her most ambitious project yet – translating and interpreting Isaac Newton’s Principia, his monumental book on mechanics and gravity. Childbirth was always a hazardous affair – one of her three babies had died – but at the age of 42 she knew that it would be particularly perilous. It was a race against time to finish her work.

More than 60 years had gone by since Newton published his revolutionary cosmology, but in France his ideas remained controversial. Convinced that reason and mathematics could unlock the mysteries of the universe, du Châtelet was determined to persuade her compatriots that a single force of gravity tied the universe together. She counted the months and then the days, working long into the nights and plunging her hands into ice-cold water to keep herself awake. A few days before giving birth, she completed her manuscript, triumphantly noting down the date. She died just days later; her baby daughter soon followed.

Until recently, if du Châtelet was mentioned at all, it was as Voltaire’s mistress rather than as a talented mathematician in her own right. At a time when women were largely excluded from scientific debates, she earned respect across Europe. In books and articles she critiqued the theory of the French national hero, René Descartes. While he envisioned the planets swirling within clouds of tiny invisible particles, Newton’s cosmos consisted largely of empty space, criss-crossed by invisible forces that obeyed a single mathematical law.

After her death, du Châtelet’s friends safeguarded her manuscript, the last testament of a life cut short. In tribute, they published it 10 years later, when Halley’s comet was predicted to return. Arriving on cue, it provided proof that du Châtelet had been right: Newton’s physics worked.

Born into the aristocracy as Emilie le Tonnelier de Breteuil, she was lucky to have a father who recognised and nurtured her intelligence, encouraging her to educate herself. By the age of 12, she was fluent in six languages and clearly preferred intellectual pursuits normally reserved for men.

But she could not escape the constraints of convention. When she was 18, her parents arranged for her to marry a wealthy nobleman, the Marquis du Châtelet. For several years, her days were filled with caring for her children, dancing and playing the part of society hostess. Du Châtelet also raised eyebrows at the gambling tables, where her mathematical aptitude gave her an advantage. But her appetite for knowledge persisted. The point of life, she believed, is to enjoy oneself – and in that she included intellectual pleasure.

Du Châtelet bitterly resented being denied the opportunities open to men. Excluded from university, in her late 20s she exploited one advantage she did have – money – and hired a mathematics tutor. And then she fell passionately in love with Voltaire, at that stage a controversial author just beginning to build his reputation as a playwright. He had recently returned from political exile in England, where he had become obsessed with Newton.

At first, the couple were discreet about their relationship. But when Voltaire ran into trouble with the authorities for writing a book that celebrated English liberty, she came to the rescue, hiding him away in the family chateau at Cirey, in the east of France. Voltaire repeatedly implored du Châtelet to join him – and soon she did. They lived together until her death 16 years later. Fortunately, her husband was cooperative: often away on military duty, he visited them from time to time.

In their country hideout, du Châtelet and Voltaire balanced research and writing with lavish entertaining. She was fastidious in all things, from decorating – coordinating even the dog basket with the blue and yellow colour scheme – to ordering experimental equipment and stocking their extensive library. Tackling discrimination head-on, she effectively converted the chateau into a private research centre, inviting scholars to visit and engaging in the extensive correspondence networks traversing Europe.

For their first shared scientific project, Voltaire and du Châtelet investigated fire. But after a few months working in their private laboratory, they found they disagreed. Voltaire insisted that fire has mass, but du Châtelet felt there was insufficient evidence. She warned against extrapolating to general rules based on a handful of observations. “One must never conclude from the particular to the general,” she insisted. Separately – and anonymously – they submitted essays to a competition run by the French Academy of Sciences. Both were published, but neither won.

Their next endeavour was much more of a success. Ever since his enforced trip, Voltaire was enamoured of all things English. Aware that many French people were, like him, incapable of following du Châtelet into the higher realms of mathematics, he set out to provide a clear and simple explanation of Newtonian gravity and optics. In 1738, the pair published Elements of Newton’s Philosophy – the text often credited with converting the French nation to English philosophy. Only Voltaire’s name was on the title page, but he paid tribute to du Châtelet’s scientific superiority in the preface, and the frontispiece shows her dressed as the Goddess of Truth (see above). “She dictated and I wrote,” he told a friend.

Voltaire returned to plays and essays, while du Châtelet continued her scientific enquiries. She studied energy, a concept not fully incorporated into physics until the 1800s. Building on earlier experiments, she dropped heavy balls from different heights onto a sheet of soft clay. After measuring the depressions they made, she could see that both the mass and the square of the velocity mattered. In modern terms, she was exploring the relationship between potential energy due to position and kinetic energy gained in motion.

Making the case for Newton

Not content with explaining physical theories, du Châtelet examined their philosophical foundations. Newtonianism, she pointed out, purports to be based solely on experimental observations, yet it inevitably entails a metaphysical assumption that the universe follows scientific laws. In her Foundations of Physics (1740), she attempted to reconcile the Cartesian and Newtonian systems, as well as the ideas proposed by German mathematician Gottfried Leibniz, known today for his rivalry with Newton over the invention of calculus.

This book boosted her international reputation, but she remained controversial. The German philosopher Immanuel Kant sneered that “a woman who… conducts learned controversies on mechanics like the Marquise de Chatelier might as well have a beard”.

She steeled herself against such criticisms to tackle the hardest task of all. In her Principia, still the only complete version in French, du Châtelet did more than simply translate from the original Latin. She converted the mathematics into words, adding her own examples for clarification. She also showed how Newton’s geometry could be expressed algebraically, and provided a detailed 180-page introduction – a basic guide to Newton’s big ideas. As one reviewer enthused, “it offers us Newton made accessible to everyone, no less than to all those who can follow philosophical reasoning”.

Frantically working to finish the mammoth task, du Châtelet couldn’t know what her legacy might be. She did not live to see her final work published, but – perhaps luckily – neither did she learn how soon she was forgotten.

During her lifetime, du Châtelet railed against the limited opportunities for her sex. As Voltaire wrote, she “was a great man whose only fault was being a woman”. More than 250 years later, perhaps it is time to revise that sentiment. Emilie du Châtelet was a great woman, whose only fault was living in a man’s world.

This article appeared in print under the headline “A woman of gravity”