Émilie du Châtelet was a mathematician and physicist who lived in France in the Age of Enlightenment. She predicted that the spectrum of light extended past the visible and proposed an experiment to prove it. She reviewed the available evidence (mathematical and experimental) on kinetic energy and proved correctly that the energy of a moving body is proportional to the square of its velocity (not the velocity, as Newton proposed). She translated Newton’s masterwork, the Principia Mathematica from Latin into French, with commentary and extra mathematical derivations of key formulae, and her version is still the standard French version used today.
Émilie du Châtelet was born in 1706 into a minor noble family. Her father was the Principal Secretary and Introducer of Ambassadors to King Louis XIV, and held a salon for intellectuals in Paris. Her family recognised her early brilliance, and unusually for the time, she was educated widely. By the age of 12, she was fluent in Latin, Italian, Greek and German, and was being educated in philosophy, science and mathematics.
In a standard move for the times, at 19 her marriage was arranged to a 30 year old aristocrat, the Marquis Florent-Claude du Chastellet-Lomont. She had three children, and after the third was born, resumed her mathematical studies, studying algebra and calculus with Moreau de Maupertuis and Alexis Clairaut.
The relationship which did most for her scientific studies, but also has tended to hide her from history, was her relationship with Voltaire. They met when she was 22, but began a close relationship when she was 26, in 1733. De Châtelet invited Voltaire to come and live with her and her husband on their country estate in Cirey-sur-Blaise, in North eastern France. They lived and worked there together for fifteen years, working on mathematics and physics together, and editing each others’ publications. Voltaire declared in a letter to his friend King Frederick II of Prussia that du Châtelet was “a great man whose only fault was being a woman”.
Her first major work was on heat on light, in 1737. She published a paper on the nature of light and heat in which she predicted the existence of infrared radiation beyond the visible spectrum of light. Three years later, she published a major book on Physics, called Institutions de Physique, which was ostensibly a review of the major ideas of Physics at the time for a teenager, but was more of a synthesis of the state of knowledge at the time, arguing strongly for Newton’s theories, and explaining them and comparing them with the latest ideas from the continent.
In this book, she looked hard at the energy of moving bodies, and examined the latest theories, before coming down on the side of energy being proportional to the square of velocity. It wasn’t unusual to have a view on the question, but what du Châtelet did was systematically investigate all the theoretical and experimental evidence to reach the correct conclusion.
Her masterwork, which she is most recognised for, was her translation of Newton’s Principia Mathematica. She worked systematically to understand the whole of Newton’s ideas, providing explanations and side notes as well as the basic translations.
She had already started work on the translation, working with Voltaire, when she became pregnant, at the age of 42, with a new lover, poet Jean François de Saint-Lambert. She was very worried about the pregnancy at her age, and wrote to a friend that she did not expect to survive the birth. So she worked systematically at her translation, rising early, and doing very little else until the end of her pregnancy.
She gave birth to a daughter in 1749, at the age of 42, and died five days later, as she feared. Her daughter died around 18 months later.
During her life Émilie du Châtelet had quite a number of lovers, including the most famous, Voltaire. But it is clear that her first love was science, and she was a key player in the development of theories of energy and motion during the Age of Enlightenment.
This is part of a serious of notable women from where we are as we travel the world. I’d love suggestions for future subjects – our itinerary is here.