The ideas about magnetism and electricity that began to be widely discussed by natural philosophers at the outset of the seventeenth century take us deep into the mysteries of the fundamental forces of nature. Even at the end of the twentieth century, this part of modern physics had many unanswered questions, including just how to think about the four basic forces of nature: strong, weak, gravitational, and electromagnetic. Today, perhaps electric and magnetic forces seem the simplest to comprehend, but in 1600, no one had even imagined the existence of “electricity.” William Gilbert stumbled onto it while divining the nature of magnetism. Only that innovation paved the way for the continuous study of electric forces throughout the seventeenth century. In the meantime, astronomy was about to be transformed from mere mathematical model-building to philosophical speculation about just what holds our universe together. But before we can approach that great intellectual struggle, we need to consider the discovery of the more subtle forces that bind our world, and that began to be glimpsed in the early seventeenth century.

Holding the World Together

The question of what holds the planets in their orbits was abruptly brought into focus in the late sixteenth century. In 1577, a comet appeared in Europe, seen by many observers, but especially Tycho Brahe. He was then the most accomplished European astronomer. He noticed that the path of the comet was such that it would have crashed through the crystalline spheres that were supposed to hold the planets and fixed stars in their orbits. If this comet on a path through a crystalline sphere did not cause a crash, then those spheres vanished. If the crystalline heavenly spheres were gone from the universe and therefore could not explain why the planets and fixed stars continued in their daily and yearly paths, then cosmological thinkers had to ask themselves if there is not some intrinsic force in nature that attracts objects to each other. This was the deeper background to Kepler's thinking in 1605.