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Although best known in popular history as Voltaire’s mistress, the French genius accomplished far more than wooing the Enlightenment thinker. Born Gabrielle-Émilie Le Tonnelier de Breteuil in 1706, she took advantage of her family’s wealth to pay for private mathematics and linguistics instructors. In her adulthood, the married du Châtelet focused her mathematical explorations on the concept of energy and what comprises it. In the late 17th century, Isaac Newton had proposed that an object’s energy equaled the product of its mass and velocity, or speed. One of du Châtelet’s best-known accomplishments was translating Newton’s hefty tome “Philosophiæ Naturalis Principia Mathematica” from Latin into her native French. Studying the revolutionary text alongside Voltaire, du Châtelet confirmed that the velocity in the energy equation should be squared.

Her research proved instrumental in 1905 when Albert Einstein derived the mass-energy equivalence formula, e=mc². By the time Einstein began tinkering with his signature formula, physicists had already adopted the square of speed when calculating an object’s moving energy, thanks in large part to the theoretical groundwork established by du Châtelet. Therefore, in that landmark equation, though “c” stands for the speed of light, the understanding to square the speed of light traces directly back to du Châtelet’s prior work [source: Bodanis]. No wonder Voltaire wrote of his mistress, who would die after childbirth at the age of 40, “She has a genius that is rare/ Worthy of Newton, I do swear” [source: Weingarten].

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