Thread by @mcnees

Astronomer and mathematician Pierre-Louis Moreau de Maupertuis, exact date of birth unknown, was born around this time in 1698 and baptized #OTD. He was among the first to articulate the principle of least action, one of the most beautiful ideas in physics.
Image: Wellcome Trust

Maupertuis originally formulated his principle to explain the motion of light, arguing that the integral of its velocity along the path it follows is minimized. This allowed him to recover Snell's Law for the refraction of light as it passes between different materials.

By 1744, Maupertuis extended this idea to the motion of material objects. He defined an object's “action" as the integral of momentum along its path, or the time integral of its "vis viva" (kinetic energy) from start to finish. He proposed that Nature acts to minimize the action.

(Leonhard Euler proposed a very similar idea around the same time, but ceded priority to Maupertuis. Konig claimed they both swiped it from Leibniz, who probably came up with the same principle decades earlier, but only discussed it in private letters.)

In 1746 Maupertuis presented his paper “Laws of Movement and Rest” to the Berlin Academy of Sciences, applying his principle to the motion of point masses. It is probably the oldest work that I have cited in one of my own publications.
arxiv.org/abs/0803.1485

In his essay “The Laws of Rest and Motion Deduced from the Attributes of God,” Maupertuis gave a statement of the principle of least action that is often quoted or paraphrased:

He tried to apply his principle in other fields, including biology, psychology (he proposed a principle of pleasure and pain), and theology. Those efforts didn't meet with quite as much success.

Anyway, Lagrange and later Hamilton would generalize and refine the principle in important ways, but we usually credit Maupertuis with originating the principle of least action.