These vibrations which give rise to propagation of waves require a certain medium in which to oscillate. Sound is due to vibrations of the molecules in the air; there is no sound in a vacuum. Seismic waves, by which earthquakes are recorded, are due to the vibrations of the interior matter of the earth. Water waves are due to the motion of the water on the surface. But light from distant stars reaches us even though there is apparently no material medium in interstellar space. Nevertheless, according to mechanistic physics, it is absolutely essential that the oscillations that give rise to propagation of light have some medium in which to oscillate. This medium was called the ether.
Two questions arise when we consider the analogy between sound waves in air and light waves in ether. When any object such as an airplane or a projectile moves through air, there is a certain resisting force due to the friction, and a certain amount of air is dragged along with the object in its progress through the air. Hence the first question: Is it possible to detect motion of objects through the ether, say that of the earth as it revolves around the sun? And the second: Does the ether impede the progress of objects that move through it, and is there any dragging effect?
In order to answer these questions it is necessary to consider the properties of the propagation of light through the ether, since it is only by means of light that ether manifests itself. Now, if a flash of light is just like the spread of ripples on a stagnant pond, its velocity of propagation will have a fixed value with respect to the ether; and to any observer who is moving with respect to it, the velocity will be greater or less depending on whether the direction of the propagation and the motion of the observer are in opposite directions or in the same direction. Thus if the earth moves through the ether without dragging it along in its revolutions around the sun, its velocity relative to the ether should be observable by measuring the velocity of light relative to the earth in different directions.
The fact that the earth moves through the ether without affecting it is known by the aberration of starlight. The way in which the spread of light from a star is seen by an observer on the earth, which revolves around the sun, is like that in which a person watches a performance on a stage from a platform that revolves around it. It will appear to him that everything on the stage exhibits periodic annual changes. Astronomers have long known that the fixed stars undergo such annual apparent motions. Thus the phenomenon of aberration shows that the ether is not influenced by the motion of the earth.
The decisive experiment to find the relative motion of the earth through the ether was first prepared at the United States Naval Academy in 1879 by A. A. Michelson. It was carried out afterward at the Astrophysical Observatory in Potsdam, where he spent a year of research, and repeated later in the United States. Michelson, who was the outstanding expert on precise optical measurements, had arranged the experimental conditions so that a 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. It was 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 of body is filled with, the plain truth is that he [Newton] believes God to be omnipresent in the literal sense; and that as we are sensible of objects when their images are brought home within the brain, so God must be sensible of everything, being intimately present with everything: for he [Newton] supposes that as God is present in space where there is no body, he is present in space when a body is also present.”
E. A. Burtt in The Metaphysical Foundations of Modern Physical Science, published in 1925, interprets correctly:
“Certainly, at least, God must know whether any given motion is absolute or relative. The divine consciousness furnishes the ultimate center of reference for absolute motion. Moreover, the animism in Newton’s conception of force plays a part in the premise of the position. God is the ultimate originator of motion. Thus in the last analysis all relative or absolute motion is the resultant of an expenditure of the divine energy. Whenever the divine intelligence is cognizant of such an expenditure, the motion so added to the system of the world must be absolute.”
By means of this anthropomorphic conception of God, a scientific, almost physical definition of absolute motion is obtained. It is linked with the energy expended by a being called “God,” but to which properties of a physical system are ascribed. Otherwise the concept of energy could not be applied to the system. Fundamentally the definition means that one assumes the existence in the world of a real source of energy that is distinguished from all others. Motion produced by the energy
expenditure of mechanical systems in general is described as only “relative” motion, while motion produced by this select being is characterized as “absolute.” It should never be forgotten, however, that the logical admissibility of this definition of absolute motion is bound up with the existence of the energy-producing being. During the eighteenth century, in the age of the Enlightenment, men no longer liked to ascribe to God a part in the laws of physics. But it was forgotten that Newton’s concept of “absolute motion” was thereby deprived of any content. Burtt in his aforementioned book says very aptly: “When, in the eighteenth century, Newton’s conception of the world was gradually shorn of its religious relations, the ultimate justification for absolute space and time as he had portrayed them disappeared and the entities were left empty.”
7. Critics of the Mechanistic Philosophy
Toward the end of the nineteenth century more and more physical phenomena were discovered that could be explained only with great difficulty and in a very involved way by the principles of Newtonian mechanics. As a consequence new thories appeared in which it was not clear whether they could be derived from Newtonian mechanics, but which were accepted as temporary representations of the observed phenomena. Was this true knowledge of nature or only a “mathematical description,” as the Copernican system was considered in medieval physics? These doubts could not be resolved so long as it was believed that there were philosophical proofs according to which reduction to Newtonian mechanics provided the only possibility for the true understanding of nature.
During the last quarter of the nineteenth century a critical attitude toward this mechanistic philosophy became more and more evident. An understanding of this criticism is an essential prerequisite for the understanding of Einstein’s theory and its position in the development of our knowledge of nature. As long as it was believed that Newtonian mechanics was based ultimately on human reason and could not be shaken by scientific advance, every attempt such as that of Einstein, to establish a theory of motion not founded on Newton’s theory necessarily appeared absurd. The critics of mechanistic philosophy plowed the soil in which Einstein was then able to plant his seeds.
As the first of these critics, we may mention Gustav Kirchhoff, the discoverer of spectral analysis. In 1876 he stated that the task of mechanics was “to describe completely and as simply as possible motions occurring in nature.” This meant that Newtonian mechanics is itself only a convenient scheme for a simple presentation of the phenomena of motion that we observe in daily experience. It does not give us an “understanding” of these occurrences in any other philosophical sense. By thus contravening the general opinion that Newton’s principles of mechanics are self-evident to the human mind, he created something of a sensation among natural scientists and philosophers.
Furthermore, with Kirchhoff’s conception that mechanics is only a description of the phenomena of motion, the mechanical explanations of the phenomena in optics, electricity, heat, etc. — the aim of mechanistic physics — became simply descriptions of these results in terms of a pattern that had been found to be most suitable for mechanics. Why should one describe by this roundabout method of using mechanics instead of trying to find directly the most suitable scheme for the description of various phenomena? Newtonian mechanics was thus deprived of its special philosophical status.
In 1888 Heinrich Hertz discovered the electromagnetic waves, which form the basis of our modern wireless telegraphy and radio, and he then set out to explain these phenomena in terms of a physical theory. He took as his starting-point Maxwell’s theory of electromagnetic fields. James Clerk Maxwell had derived his fundamental equation from mechanistic physics by assuming that electromagnetic phenomena are actually mechanical oscillations in the ether. Hertz noticed that in doing this Maxwell had been compelled to invent mechanisms that were very difficult to calculate, and found it was simpler to represent electromagnetic phenomena directly by means of Maxwell’s equation between electric and magnetic fields and charges. Since it was also evident to him, however, that these relations could not be derived directly from experience, he was led to a consideration of the logical character of these equations. In 1889 he made a remark that can be regarded as the program for the new approach to physics, a conception that was eventually to replace the mechanistic view. Hertz said:
“But in no way can a direct proof of Maxwell’s equations be deduced from experience. It appears most logical, therefore, to regard them independently of the way in which they had been arrived at, and consider them as hypothetical assumptions and let their plausibility depend upon the very large number of natural laws which they embrace. If we take up this point of view we can dispense with a number of auxiliary ideas which render the understanding of Maxwell’s theory more difficult.”
Thus Hertz consciously abandoned that which during both the organismic and the mechanistic period was described as the “philosophical” foundation of physics. He maintained that it was sufficient to have a knowledge of laws from which phenomena could be calculated and predicted without raising any question of whether these laws were intrinsically evident to the human mind.
8. Ernst Mach: The General Laws of Physics Are Summaries of Observations Organized in Simple Forms
The criticisms of the mechanistic philosophy by physicists such as Kirchhoff and Hertz were only occasional and aphoristic. There were others, however, whose criticisms were based on a very precise conception of nature and of the task of science. The French philosopher Auguste Comte advanced the sociological theory that the “metaphysical” stage in the development of a science is already succeeded by a “positivistic” one. This means that the demand for the use of a specific analogy such as the organismic and mechanistic views is abandoned and after that a theory is judged only as to whether it presents “positive” experience in a simple, logically unobjectionable form.
This approach was most widely and profoundly developed by the Austrian physicist Ernst Mach, who became one of Einstein’s immediate forerunners. Mach carried out a thorough historical, and logical analysis of Newtonian mechanics and showed that it contains no principle that is in any way self-evident to the human mind. All that Newton did was to organize his observations of motion under several simple principles from which movements in individual cases can be predicted. But all these predictions are correct only so long as the experiences upon which Newton based his principles are true.
Mach emphasized, in particular, the demand for simplicity and economy of thought in a physical theory: the greatest possible number of observable facts should be organized under the fewest possible principles. Mach compared this requirement to the demand for economy in practical life and spoke of the “economic” nature of scientific theories. Thus Mach, instead of demanding the use of a specified analogy, insisted that science be “economical.”
Furthermore, not only did Mach criticize the attempts of philosophers to make a philosophical system out of Newton’s mechanics, but he also criticized the remains of medieval physics that it still retained. He pointed out that Newton’s theory contained such expressions as “absolute space” and “absolute time,” which cannot be defined in terms of observable quantities or processes. In order to eliminate such expressions from the fundamental laws of mechanics, Mach raised the demand which is now frequently described as the positivistic criterion of science: namely, that only those propositions should be employed from which statements regarding observable phenomena can be deduced.
This demand is very aptly elucidated by his criticism of Newton’s law of inertia. If we wish to test this law experimentally, we can never formulate a question such as this: Does a body tend to maintain the direction of its initial velocity relative to absolute space? The question is meaningless since absolute space is unobservable. If we perform, say, Foucault’s pendulum experiment, which gives an experimental proof of the rotation of the earth, we observe actually that the pendulum maintains its plane of oscillation relative, not to absolute space, but rather to the fi
xed stars in the sky.
Consequently, according to Mach, all mention of absolute space should be removed from the law of inertia, and it would then be expressed as follows: Every body maintains its velocity, both in magnitude and in direction, relative to the fixed stars as long as no forces act upon it. This means that the fixed stars exert an observable influence on every moving body, an effect that is in addition to and independent of the law of gravitation. For the motion of terrestrial objects this latter influence is hardly observable in practice, since the force of gravity decreases with the square of the distance between the attracting bodies, but the laws of inertia will determine all terrestrial motion if the framework of the fixed stars is declared as an inertial system.
9. Henri Poincaré: The General Laws of Physics Are Free Creations of the Human Mind
In consequence of the criticisms of Mach and others, it had become clear that the laws of Newtonian mechanics and the understanding of all physical phenomena in terms of it are not demanded by human reason. However, Mach’s assertion that the general laws of physics are only simple economical summaries of observed facts was not satisfactory to many scientists. Particularly for physicists who thought along mathematical lines and had a greater formal imagination, the assertion, for example, that Newton’s law of gravitation is only a simple summary of observation on the positions of the planets did not seem adequate. Between the actual observation of the position of the planets by a telescope and the statement that the gravitational force between two bodies is inversely proportional to the square of the distance there seemed to be a wide gap.
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