Einstein (9 page)

But what if the second room performs a uniform motion in a straight line? The ball can then travel parallel to an edge without the exertion of any force in the “moving” room. In fact, all motion that occurs with uniform velocity in a straight line in the first system will also take place with uniform velocity in a straight line in the second system. Consequently the law of inertia also holds in the “moving system,” and it is true whatever the velocity of the system is with respect to the “resting” room as long as it takes place in the same direction with constant magnitude.

When forces operate on the ball and its velocity does not remain constant but acceleration is introduced, the acceleration will be the same in both systems. Hence the law of force, which determines acceleration and is independent of the initial velocity, is the same for both systems. Thus we cannot determine the velocity with which the room moves in relation to the original inertial system by means of experiments on the motion of particles performed in this room; and conversely, with the law of force and the initial velocities, we can predict the future motions of particles without knowing anything about the uniform velocity with which the room may be moving. All systems that move uniformly along a straight line relative to an inertial system are likewise inertial systems. But Newton’s laws do not say which material body is an inertial system.

For most practical purposes, the effect of the rotation and revolution
of the earth is very small, and its motion can be regarded as a uniform motion in a straight line. Within this range the earth is approximately an inertial system and we can predict the motion of particles on the earth by means of Newton’s laws. The same can be done on railway trains, in elevators, and in ships as long as their motion relative to the earth is in a straight line with constant velocity. It is a common experience that we can play with a ball in exactly the same way whether we are on board a train or in a ship so long as it does not jerk or roll.

This law concerning the possibility of predicting future motions from the initial velocities and the laws of force may be called the
relativity principle of mechanistic physics
. It is a deduction from the Newtonian laws of motion and deals only with relative motions and not, as Newton’s laws proper, with absolute motion. In this form it is a positive assertion, but it can also be formulated in a negative way, thus: It is impossible by means of experiments such as those described above to differentiate one inertial system from another.

Thus the relativity principle first appeared as a characteristic feature of Newtonian mechanics. As we shall see, it was Einstein’s greatest achievement to have discovered that this principle still applies even when Newtonian mechanics is no longer valid. He saw that the relativity principle is more suitable than the Newtonian laws to serve as a basis for a general theory of physical phenomena. It continues to remain valid even when mechanistic physics becomes untenable.

The explanation of optical phenomena such as reflection and refraction of light was first given in terms of two opposing theories. Newton had propounded the corpuscular theory, in which light was considered as a stream of particles that behaved according to the laws of motion, while a contemporary of his named Huygens had proposed the wave theory, in which light was considered as a vibration in a certain medium in the same way that sound is a vibration in air. The controversy was settled about 1850 in favor of the wave theory by the French physicists Arago and Foucault. Then the theoretical calculations of Maxwell and the experimental work of Hertz established the result that these vibrations associated with light are electromagnetic
in nature; that is, that light is due to very rapid oscillations of electric and magnetic fields.

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.

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.”

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.