By about 1640 the work of Brahe, Kepler, and Galileo had been largely accepted by the scientific community despite opposition from religious leaders. The old Aristotelian astronomy and physics were in ruins, and several fundamental breakthroughs had been made. But the new findings failed to explain what forces controlled the movement of the planets and objects on earth. That challenge was taken up by English scientist Isaac Newton (1642–1727), a genius who spectacularly united the experimental and theoretical-mathematical sides of modern science.

Newton was born into the lower English gentry, and he enrolled at Cambridge University in 1661. He arrived at some of his most basic ideas about physics in 1666 at age twenty-four, but was unable to prove them mathematically. In 1684, after years of studying optics, Newton returned to physics for eighteen intensive months. The result was his towering accomplishment, a single explanatory system that could integrate the astronomy of Copernicus, as corrected by Kepler’s laws, with the physics of Galileo and his predecessors. Newton did this through a set of mathematical laws that explain motion and mechanics. These laws were published in 1687 in Newton’s Mathematical Principles of Natural Philosophy (also known as the Principia). Because of their complexity, it took scientists and engineers two hundred years to work out all their implications.

The key feature of the Newtonian synthesis was the law of universal gravitation. According to this law, every body in the universe attracts every other body in the universe in a precise mathematical relationship, whereby the force of attraction is proportional to the quantity of matter of the objects and inversely proportional to the square of the distance between them. The whole universe — from Kepler’s elliptical orbits to Galileo’s rolling balls — was unified in one majestic system. Newton’s synthesis of mathematics with physics and astronomy prevailed until the twentieth century and established him as one of the most important figures in the history of science. Yet, near the end of his life, this acclaimed figure declared: “I do not know what I may appear to the world; but to myself I seem to have been only like a boy, playing on the seashore, and diverting myself, in now and then finding a smoother pebble or a prettier shell than ordinary, whilst the great ocean of truth lay all undiscovered before me.”3