Einstein by Philipp Frank

definite measurement could be made even if the velocity of the earth through the ether were only a small fraction of that due to its revolution around the sun. The result, however, was entirely negative. It was impossible to find any relative motion of the earth through the ether.
    Thus the mechanistic theory of light led to a dilemma. The
aberration
showed that the earth moved through the ether without disturbing it, but the
Michelson experiment
showed that it was not possible to find the velocity with which the earth traveled through the ether.
     
6.
Remnants of Medieval Concepts in Mechanistic Physics
    In medieval physics the characteristic feature concerning the motion of objects had been the revolution of the heavenly bodies around the earth taken as the fixed center. This system represented a kind of a universal framework within which everything had its proper place, and motion within this system meant motion relative to this framework. The problem of absolute motion hardly appeared. Also a natural measure of time was given by the period of revolutions of the heavenly bodies.
    It may seem at first that the Copernican theory and the mechanics of Galileo and Newton had disrupted this “closed world” of the Middle Ages, but a careful examination shows that a similar concept was still retained in mechanistic physics. Newton’s law of inertia implied that freely moving objects can travel beyond all spatial limits, but it was in relation to “absolute space.” Since the connection between absolute space and the empirical content of physical laws was difficult to demonstrate, the auxiliary concept of “inertial system” was introduced. Itwas not possible, however, to explain why the law of inertia should be valid in certain systems and not in others. This characteristic was not related to any other physical property of the system. Thus the inertial system still retained something of the character of the medieval universal framework. Furthermore, in extending the laws of mechanics to optical phenomena, it had been found necessary to “materialize” space with ether. This ether was a genuine universal framework. The motion of a laboratory relative to it should be observable by means of optical experiments.
    The physicists of the mechanistic period always felt uneasy in using the expressions “absolute space,” “absolute time,” “absolute motion,” “inertial system,” and “universal ether.” Newton himself did not succeed in explaining how one recognized the motion of a body in “absolute space” by actual observation, and he wrote: “It is indeed a matter of great difficulty to discover, and effectually to distinguish, the true motion of particular bodies from the apparent; because the parts of that immovable space, in which those motions are performed, do by no means come under the observation of our senses.” Consequently, if one remains within the bounds of physics, one cannot give a satisfactory definition of “absolute motion.” The theory becomes completely and logically unobjectionable only if, as was self-evident for Newton, God and his consciousness are added to the physical facts.
    For a long time no one had realized precisely what was the actual link between Newton’s theological reflections and his scientific work. It was often asserted that they had no logical connection and that his reflections were significant only from a purely emotional standpoint or as a concession to the theological spirit of his time. But this is certainly not so. Although there might have been some doubt about this point earlier, yet since the discovery of the diary of David Gregory, a friend and student of Newton’s, we know definitely that Newton introduced the theological hypothesis in order to give his theory of empty and absolute space a logically unobjectionable form. Gregory’s diary for 1705 contains an entry concerning a conversation with Newton on this topic. It says: “What the space that is empty