The two centuries from 1687 to 1887 may appropriately be described as the mechanical age of physics. Science seemed to have found that we lived in a mechanical world, a world of particles which moved as the forces from other particles made them move, a world in which the future is completely determined by the past. In 1687 Newton's Principia had interpreted the astronomical universe very successfully in this way. Before 1887 Maxwell had interpreted radiation in an essentially similar way, teaching that it consisted of disturbances travelling through an ether under the direction of mechanical laws. Finally, in 1887 Hertz produced radiation of Maxwellian type from electric sources in the laboratory, and demonstrated its similarity to ordinary light. This seemed to fit a final keystone into the structure which had been built up in the preceding two centuries.

Most physicists now thought of this structure as standing foursquare, complete and unshakable. It was hard to imagine the physicists of the future finding any more exciting occupation than dotting the i's and crossing the t's of the mechanical explanation of the universe, and carrying the measurement of physical quantities to further decimal places.

Little did anyone imagine how completely different the actual course of events would be. Yet the year 1887, which had provided a keystone to the structure, also saw the structure begin visibly to totter; it was the year of the famous Michelson-Morley experiment, which first showed that there was something wrong with the foundations.