THE BACKGROUND: CLASSICAL PHYSICS

The first unified program of modern physics was created by Isaac Newton. He showed how the motion of diverse objects, including the planets, the tides, falling bodies and projectiles, could be derived from a single force ruling them all: universal gravitation. Newton's successes were generalized into a mechanical world view based on the assumption that, in principle, all phenomena could be explained in a similar fashion, though perhaps requiring other forces besides gravitation. Some of those additional forces were evident, such as the force that accounts for the elasticity in the collisions of billiard balls. Whatever the range of qualitatively different forces still to be discovered, they were all conceived to operate within a general formal framework, originally set up by Newton and further developed and perfected by d'Alembert, Euler, Lagrange, Laplace, and others, so that by 1800 a well-established scheme for the explanation of natural phenomena was available. Special assumptions about the forces proved in the end to be less crucial for this scheme than a certain mathematical structure. The basic physical properties of a system were explained using differential equations of second order in time, and later also mathematically equivalent tools such as variation principles.