IS NEWTON's LAW PERFECT?

We left Chapter 2 with the impression that Newton's law of gravitation gave a successful account of the diverse nature of phenomena in which gravity is believed to play a leading role. Not only is this law able to account for motions of such celestial bodies as planets, comets, and satellites, it also helps us in understanding the complex problem of the structure and evolution of the Sun and other stars. Modern scientists use the same law in determining the rocket thrusts, spacecraft trajectories, and the timing of space encounters. That a good scientific law should be basically simple but universal in application is epitomized in Newton's law of gravitation. What more could one ask for?

Yet science by nature is perfectionist. The laws and theories of science are accepted as long as they are able to fulfil its primary purpose of explaining natural phenomena. Any law of science, despite a history of past successes, is inevitably discarded if it fails in even one particular instance. To the scientist, such an event brings mixed feelings. Disappointment and confusion that an old, well established idea has to be given up or modified are coupled with excitement and expectation that nature is about to reveal a new mystery.

Newton's law of gravitation was no exception to this rule. By the beginning of the present century, cracks were beginning to appear in the impressive facade of physics erected on the Newtonian ideas of motion and gravitation.