Physics

The Newtonian synthesis was, first and foremost, a unification of celestial and terrestrial physics. Newton's famous story of seeing an apple fall in his mother's garden does a good job in summarizing this achievement. According to the story, the falling apple made Newton consider that the gravitational force that influences the apple (a projectile in terrestrial motion) might also act on the moon (a satellite in celestial motion). He concluded that "the power of gravity … was not limited to a certain distance from the earth but that this power must extend much farther than was usually thought" (Westfall, 1980, p. 154). This idea is displayed in a famous diagram in the Principia, depicting a projectile being thrown from a mountain peak, which rests on a small planet; as the projectile is thrown with greater and greater speed, it eventually goes into orbit around the planet and becomes a satellite. Consideration of the moon's motion led Newton to the force law for universal gravitation. Simply by virtue of having mass, any two objects exert mutually attractive forces on each other (in accordance with the third law of motion). The inverse-square force law made the gravitational force quantified and calculable but regarding the cause of gravity itself, Newton famously claimed, "I feign no hypotheses."

As much as his specific scientific achievements, Newton's method of working became a touch-stone for scientists of the eighteenth century and defined a general scientific culture of "Newtonianism." In this regard, the Newtonian synthesis can be seen as a combination of three broad traditions: experiment, mathematics, and mechanism. Newton's Opticks (1704) exemplified the empirical, inductive approach recommended by Francis Bacon. There, Newton reports on careful experiments with light, including a series showing that when light passes through a prism, the prism does not modify the color of the light but rather separates it into its component colors (see Fig. 2). He also did experiments in which he shone monochromatic light on thin plates and films, to produce patterns of light and dark regions; these later became known as "Newton's rings."

The effort to describe physical events with mathematics, which was so evident in the work of Kepler, Galileo, and Descartes, reached its full expression in Newton's dynamics. The universal gravitation law, along with the three laws of motion and the calculus, presented a complete Newtonian approach to quantifying and calculating the motion of everything from the smallest atom to the largest planet. Closely related to this mathematical tendency is the mechanical philosophy pursued by Descartes, Gassendi, and Robert Boyle (1627–1691). Although Newton rejected Descartes's plenum, he retained a modified idea of mechanical causality. For Newton, gravity was an action at a distance; two masses acted on one another despite the fact that empty space lay between them. Defined as a change in motion, Newton's conception of force was a mechanical, causal agent that acted either through contact or through action at a distance.