This book deals with the problems of space, time, and motion; which, as the author says, are not merely academic questions, but interesting to man as contributing to an understanding of his place in the universe. He remarks, as others have done, that the work of Einstein marks a revolution in ideas similar to that brought about by Copernicus. And we would add here that it is possible that people are a bit one-sided in their criticism of the opponents of Copernicus, Galileo, and others. Ideas which are so familiar to us that we take them for granted, came upon the contemporaries of Copernicus like a vast earthquake, overwhelming not merely their ideas of celestial mechanics (a matter of little or no concern to the majority), but their entire outlook upon human life; uprooting every holdfast, and providing nothing instead except what looked like shifting sand. No wonder there was resistance; and, as to the form it took, those were rough days; when they roasted and pricked people they did it in gross physical form, not in subtle genteel refinement, as we do today.
The world picture which had existed for so many centuries before Copernicus and his compeers, was not so wrong as we are accustomed to think, says Mr. Reichenbach; for both pictures are but convenient representations of unattained realities; and the new views now being developed under the guidance of Einstein will make us hesitate before pronouncing Ptolemy wrong and Copernicus right. Rather we shall vision a pair of alternative theories superseded, and therein reconciled, by the new and more inclusive theory of Einstein, which shows the advantages of both the old theories. The Ptolemaic system, though kinematically workable, was not so simple as the Copernican; but the chief objection was the alleged dynamical inadequacy of the former. This objection, however, as we shall see later, has now been overruled by Einstein.
The first part of this book deals with the Copernican view of the world; and though this subject may seem trite to some readers, yet, in view of prevailing ignorance among the many, it is by no means superfluous. It is so well and clearly expounded, that it will doubtless serve to provide for such readers a clearer idea of "what it is all about" than they have previously had. With Copernicus go Tycho, Kepler, Galileo, and Newton, each one contributing some element to the development of the theory. It would serve no useful purpose for us to linger over this branch of the subject, as we should merely be offering an inadequate summary in place of what is much better done in the book itself; to which accordingly readers are hereby referred. Of special interest is the section on the newest theories. This begins with a chapter on the (alleged?) Ether.
The theory of relativity, we are here told, has sprung from ideas of motion and gravitation; but also from ideas of electricity and light. The attempt to make light a mechanical phenomenon has failed: instead of explaining light by mechanics, we find that mechanics have to be explained by optics. Exposition of the work of Roemer and others, as to the velocity of light, follows; as also Newton's emission theory, and the wave theory of Huyghens and others. The phenomena of interference are explained, and adduced in support of the wave theory. But what is so strongly insisted on here is that we must not transfer familiar physical experience to regions where it does not necessarily apply. Our notion of a wave is founded on our experience of various kinds of waves on the physical plane, such as water waves, and sound waves in air or denser bodies. This makes us imagine that a wave must necessitate a "medium." But can there be a wave without a medium? We may find a better answer to this question if we call the wave something else. We may call it an alternation, an oscillation, a rapid and incessant change of polarity. And after all that is all that optical phenomena justify us in assuming. Light has been first suspected and then proved to be an electrical phenomenon; and electricity is characterized by these changes of polarity. Thus we do not really need any ether; and in fact we have already replaced the words "medium" or "matter" or "fluid" by the word "field." A field is considered as the arena of forces, not as a container of physical substances: thus we speak of the magnetic field around a magnet, and the field of wireless waves spreading at right angles to the direction of a current. Two electrical fields can occupy the same space at the same time, points out the writer, as an instance of the fact that the properties of fields are not necessarily those of physical spaces or bodies. Electrical waves, he adds, are advancing fields, which should not be regarded as bound to a material medium. Light is only a small fraction of the total radiation of electricity from the sun. Light is not related to water waves or sound waves; the process is an electrical rather than a mechanical one.
The often quoted Michelson experiment, devised to detect, by observing interference, any relative motion between the earth and the (alleged) ether, is here described at length. Its negative result has been explained away by various devices; but Einstein's simple and direct inference is that there is no such thing as the ether; leastways, there is no fixed framework out in space, to which motions may be referred. Newton had assumed such a fixity as a basis for his mechanical system; wherefore he could speak of absolute motion. But in its absence all motion becomes relative, and we can only speak of bodies as moving relatively to each other. This always gives us a choice of two alternative explanations of a phenomenon, according as we may choose to regard either one of the two bodies as fixed, and the other as moving. The importance of this will appear in connexion with the relativity of force.
Another conception of Einstein's is the relativity of simultaneity. In order to determine the simultaneity of events separated by a distance, we must know a velocity (that of light); and in order to measure this velocity, we must be able to judge the simultaneity of events separated by a distance. So we move in a vicious circle; we must assume one thing in order to determine the other. We have alternative explanations, both equally valid. Simultaneity is relative. Light may be used as a measurement of time, and even of space; a geometry of light may be constructed.
As to the relativity of the force concept, Ernst Mach is quoted. When we ride in a merry-go-round, the room seems to revolve, while we seem to be at rest. Yet we feel the effect of centrifugal force, pressing us against the outer arm of the seat. Newton had used this as a proof of his doctrine of absolute motion; for, said he, the centrifugal effect shows that it is the merry-go-round which is in motion, and not the room. But Mach showed that the two cases are not opposite; for, if they are opposite, we must regard not merely the room but the whole earth and even the stars as revolving around us. In this case the centrifugal force might just as well be attributed to a gravitational force. So that gravitation and centrifugal force are seen to be alternative explanations. What appears as the effect of inertia when we conceive the machine as rotating, becomes the action of gravitation when we conceive the machine as resting while the earth rotates around it.
What is known as Einstein's "box experiment" is described; by means of this he showed that a particular effect may be equally well explained as due to accelerated motion or to gravitation. It had been shown that curvature of light rays occurs in a case of accelerated motion; hence we must infer that such curvature would occur in a gravitational field.
One important point is here stressed: the misunderstanding which many people, without scientific training, have as regards the time dimension. They imagine it to be a sort of fourth dimension of spatial extension, and worry themselves in striving to achieve an impossible visualization of such a thing. But the time dimension is still time, not space; it is not interchangeable with the three spatial dimensions. All that is meant is that, in order to fully specify an event, we must know its position in space and also its position in time.
In speaking of alternative explanations of certain phenomena, we do not mean to suggest that it is immaterial which we adopt. And it is common knowledge that Einstein's way of interpreting Nature has proved its pre-eminent value, not only by explaining things that older views could not explain, but also by enabling him to predict things which have since been verified by observation. As the writer says, Einstein, by pointing out arbitrary additions made by us in our description of Nature, has made objective truth stand out more clearly than ever. Instead of confusing he has clarified the situation. But it will take us some time to grow accustomed. Views of Nature have always to be based on certain postulates assumed as foundation for the erection of workable theories. When scientific investigation reaches a point where these postulates are no longer found adequate, we must dig deeper and assume a new set of postulates. We must strive to realize that the former familiar postulates were not universal laws, but only temporary conveniences. We must be ready to admit that customary ideas about space, time, matter, motion, etc., do not necessarily apply to those regions of the very great and the very small into which the scientific eye has penetrated. Experiment has proved that they do not, and all attempts to explain the phenomena by the old postulates result in what we call "paradoxes." We can recommend this book to the attention of those wishing to clear up their ideas as to celestial mechanics and Einstein's work in particular.
1. From Copernicus to Einstein. By Hans Reichenbach. New York: Philosophical Library, Inc., 1942. $2.00. (return to text)
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