The Newtonian view of the universe may be described as a mechanistic interpretation. All components of the universe, small or large, obey the laws of mechanics, and all phenomena are in the last analysis based on matter in motion. A conceptual difficulty in Newtonian mechanics, however, is the way in which the gravitational force between two massive objects acts over a distance across empty space or in electromagnetism how a magnetic force operates between two charged particles. Newton did not address this question, but many of his contemporaries hypothesized that the forces were mediated through an invisible and frictionless medium which Aristotle had called the ether. The problem is that everyday experience of natural phenomena shows mechanical things to be moved by forces which make contact. Any cause and effect without a discernible contact, or action at a distance, contradicts common sense and has been an unacceptable notion since antiquity. Whenever the nature of the transmission of certain actions and effects over a distance was not yet understood, the ether was resorted to as a conceptual solution of the transmitting medium. By necessity, any description of how the ether functioned remained vague, but its existence was required by common sense and thus not questioned.

After 1916 Einstein strove to produce what is now called the theory of relativity into a formulation that includes gravitation, which was still being expressed in the form imparted to it by Newton; i.e., that of a theory of action at a distance. Einstein did succeed in the case of gravitation in reducing it to a local-action theory, but, in so doing, he increased the mathematical complexity considerably, as Maxwell, too, had done when he transformed electrodynamics from a theory of action at a distance to a local-action theory.

Excerpt from the Encyclopedia Britannica without permission.

Excerpt from the Encyclopedia Britannica without permission.