THE HUNT FOR VULCAN

Levenson (Science Writing/MIT; Newton and the Counterfeiter: The Unknown Detective Career of the World's Greatest Scientist, 2009, etc.) connects Albert Einstein’s theory of general relativity to Isaac Newton's Mathematical Principles of Natural Philosophy. In their day, each provided "a radical new picture of gravity" that ultimately depended on astronomical confirmation.

For Newton, his moment of truth occurred in 1687, when he established the universality of the inverse-square law of gravitation that governed the elliptical orbits of the planets. He showed that it also applied to the path of the major comet of 1680. "It was cosmic proof,” writes Levenson, “that the same laws that governed ordinary experience—the apple's fall, an arrow’s flight, the moon's constant path—ruled all experience, to the limits of the universe.” Newton based his theory on the estimated distance from the sun to the then-known planets. Pierre-Simon Laplace extended Newton's theory to account for the orbital perturbations caused by interactions between neighboring planets such as Jupiter and Saturn. Similar calculations allowed astronomers to predict the existence of Neptune based on discrepancies in the elliptical orbit of Uranus. The case of Mercury was more puzzling because its divergence from an ellipse could not be accounted for by the gravitational pull of neighboring Venus. Scientists entertained the spurious hypothesis of the existence of a heretofore-unobserved planet orbiting the sun, which they named Vulcan. Einstein solved the dilemma by replacing Newton's inverse-square law with his theory of general relativity, a complicated mathematical theory based on a simple geometrical image of "the sun with its great mass, creat[ing] a bulge in space time." Rather than action-at-a-distance, he introduced the curvature of space-time as a medium for the propagation of gravity. This allowed him to make a more precise prediction of Mercury's orbit, which was verified in 1917 by observations made during a solar eclipse. Though brief, Levenson’s narrative is a well-structured, fast-paced example of exemplary science writing.

A scintillating popular account of the interplay between mathematical physics and astronomical observations.