Popular Science Monthly/Volume 45/August 1894/Modern Views and Problems of Physics
By DANIEL W. HERING, C. E.,
PROFESSOR OF PHYSICS IN THE UNIVERSITY OF THE CITY OF NEW YORK.
A GOOD idea of the generally accepted views upon a science in all its branches may be obtained by inspecting standard text-books on the subject, for such works are not likely to meet with the approval of scholars, and especially of professors, if they present views that are antiquated in form or palpably erroneous in statement.
In thus approaching a modern text-book of physics, to a beginner, or one with no preconceived ideas on the subject, there would perhaps appear nothing surprising, but to an older student, say the college alumnus of fifteen years' standing who has not kept abreast of the science, the change would be striking. He would probably be impressed as much by the absence of things he had thought inseparable from the subject as by the presence of things of which he heard little or nothing in his college course. An illustration of this may be seen in a very recent book of the kind named. In its general tone it is similar to that adopted about ten years earlier in the masterly presentation of the Principles of Physics, by Prof. Daniell, but it is less conservative than that work. A glance at the headings gives the keynote of the whole treatment. After a brief discussion of kinematics and dynamics, mass physics is further divided into work and energy, attraction and potential, properties of matter, energy of mass vibration, sound. Then physics of the ether has energy of ether vibration, radiant energy, energy of ether stress, electrostatics, energy of ether vortices, magnetism, energy of ether flow, electro-kinetics, electro-magnetic character of radiation.
There is not an allusion to the old familiar "simple mechanical powers"; there is no mention of light or optics as a branch of physics; sound, heat, electricity, and magnetism are only appended as subtitles to more general terms expressing forms of energy, and appended in a way that would permit them to be dropped altogether without detriment to the treatment of their phenomena. Not that the phenomena are different from what they were in former times, but they have become much more effectually correlated in a general scheme of energy. Such a mode of presenting the subject might be ascribed to a mere desire to break away from conventional lines, but it is in strict accord with the work and conclusions of physicists generally in the last quarter of a century and especially within the last decade. Physicists accept fully the mechanical theory of heat. They regard the heat of a body as the aggregate kinetic energy of the molecules. They accept in general the kinetic theory of gases, but are not uniform in their views as to the extension of this theory to liquids and solids. In the mechanical theory of heat, however, the idea that all the molecules of all bodies are in motion is fundamental. Nowadays, instead of ascribing phenomena to the action of mysterious "forces," with perhaps a force of one kind for gravity, of another kind for thermal or electric or magnetic effects, and treating force as a real agent bringing about changes, it is the custom to recognize in any body or system of bodies a certain quantum of energy of which the form or distribution is altered by a change in the form or configuration of the body or system of bodies. Energy is the thing studied and force is merely the rate at which the energy of a body is altered in comparison with the change in the position or shape of the body. The term force is still in use for convenience and brevity, but the objectivity of force has disappeared. Force is not a real thing at all, but energy, like matter, has an objective existence. Also, when force was regarded as an agent, it was discussed as acting at a distance without regard to a medium for transmitting action from one body to another, or, as we now say, for conveying energy. But if bodies possess and exchange energy, and energy is only perceived by us in connection with matter, we find it proper not only to recognize a medium throughout space, but to discuss the forms in which energy exists in that medium, which is spoken of as ether. The idea of such a medium is not modern. After pointing out that the hypothesis of an ether was a device often resorted to for the purpose of mystification as much as explanation, Maxwell says: "Ethers were invented for the planets to swim in, to constitute electric atmospheres and magnetic effluvia, to convey sensations from one part of our bodies to another, and so on, until all space had been filled three or four times over with ethers. It is only when we remember the extensive and mischievous influence on science which hypotheses about ethers used formerly to exercise that we can appreciate the horror of ethers which sober-minded men had during the eighteenth century. . . . The onlyether which has survived is that which was invented by Huygens to explain the propagation of light."
Those ethers were working hypotheses which might be expected to give way wholly or in part to better ones constructed for working purposes under fuller knowledge. So, too, at first, was the luminiferous ether, which, as a hypothesis, had to be endowed arbitrarily with properties suited to the phenomena it was to account for, but the ether of modern science is accepted as beyond question. For example, Lord Kelvin says: ". . . This thing we call the luminiferous ether. That is the only substance we are confident of in dynamics. One thing we are sure of, and that is the reality and substantiality of the luminiferous ether." It is not necessary here to go into the evidences of its reality, but our belief in it rests upon exactly the same kind of evidence and just as strong evidence as does our belief in the existence of any kind of matter. For we only infer the existence of any form of matter from its phenomena, and the phenomena of light, heat, magnetism, and electricity to the extent of a very large group are not only explainable but are best explainable by the assumption of the ether. The defect as yet in such an assumption lies in the fact that the ether is a substance of an unfamiliar kind. It is this want of familiarity that physicists to-day are doing their utmost to overcome, and the more it is examined the more are they impressed by the multiplicity of purposes which this one medium is competent to serve and which it seems to be serving. The time for doubting its existence is past—it is now only a question as to its nature and properties; and it is accepted as a fact, not merely a hypothesis, that the same medium is concerned, if not a principal factor, in the phenomena of light, heat, magnetism, electricity, and gravitation. Radiant heat and light are wave motion in the ether, and are similar forms of energy, the only difference being in the period of vibration. Their manifestation as energy only occurs when the vibrations affect matter, and this, the most difficult part of the subject, involves the relation between ether and ordinary forms of matter. We say "ordinary forms" of matter, because ether may or may not be considered a form of matter.
One of the great, the primary questions of science is. What is ether? The question. What is light? has found its answer, so too has the query as to heat and as to sound; as to electricity, not so assuredly or so definitely, but both it and magnetism are to find their explanation through this same medium in some way or other. There is no longer any doubt about that, and Maxwell's theory, which rests upon this idea fundamentally, has a strong hold upon modern science and a hold that is growing stronger as research advances.
We know the ether as a vehicle of energy in several forms, and when various agencies are collected into a group of forms of energy there is still the question, "What is energy?" These general problems now engaging the attention of the physicist—viz., the ultimate nature of matter, by which the properties of matter may be accounted for; the nature of the ether and its properties; the mutual relations subsisting between matter and ether, if they are different things; the nature of energy, and whence it arises, and whether it is primarily potential or kinetic—these, in part at least, are not new problems, but they are now approached from new directions, along new ways, and by the aid of new light. Under each of these heads appear numerous special questions, and along all these lines investigators are working earnestly.
The attempt to explain the nature of ether or of matter at once raises the question whether ether is matter. Now, of course, a great deal depends upon the definition of terms, and it is perhaps best to confine our attention at first to the structure of matter rather than its nature. The properties and behavior of matter as it is ordinarily recognized are largely known, the actions and functions of the ether are largely known, and it is only a question of the propriety or possibility of including both in one general view. Clerk Maxwell regards as a proper test of a material substance its ability to contain and transmit energy. He then points out that energy can not exist except in connection with matter; that in the space between the sun and the earth, the luminous and thermal radiations which have left the sun and which have not reached the earth possess energy in definitely measurable amount, and therefore this energy must belong to matter in the interplanetary spaces. On the other hand. Prof. Dolbear stands as an exponent of the views of others who decline so to class the ether when he says: "If, then, the ether fills all space, is not atomic in structure, presents no friction to bodies moving through it, and is not subject to the law of gravitation, it does not seem proper to call it matter." But Prof. Dolbear has previously announced as his criterion of matter, the possession of the property of gravitative attraction. On such grounds we may concede each view to be correct, but we are brought at once to the old question, "What is matter?" It is the view of some that, with the present limitations of intellect, it is beyond our powers ever to conceive of the ultimate nature of matter. Of the structure of matter this is not the case. Various hypotheses have been offered regarding the structure of matter—all, save one, have been charged with some fatal objection and have broken down. This one, the suggestion of that powerful mind. Lord Kelvin's, is known as the vortex-ring theory. We can not give it here in any detail, but the gist of it is that the ether is universal and for the most part formless, but that some parts are differentiated from the remainder by being in motion in the shape of vortex rings. These parts in such rotational motion are matter in the ordinary forms. A remarkable thing about it, and one which exhibits the very spirit of modern physics, is that those properties of ordinary matter which emphasize its stability of form and position, especially inertia, elasticity, and rigidity, can be a result of motion. Yet Lord Kelvin has shown that with ordinary matter a limp system of bodies could be made a rigid system by merely putting them into gyroscopic rotation, and also that elasticity itself might properly be regarded as a mode of motion. The vortex-ring theory is as yet only a speculation, but when its adaptability to occult as well as to plainer properties of matter are considered, we need not wonder that it has been thought so beautiful that "it deserves to be true." At any rate it stands in such an attitude toward modern views concerning the structure of matter that "it is either that theory or nothing. There is no other one that has any degree of probability at all" (Dolbear). We can see how such a theory might reconcile conflicting views such as those above given concerning matter and ether separately.
Without waiting for a decisive answer as to the nature of ether or the structure of matter, attention is being concentrated on the relations of one to the other, the extent to which and the manner in which any change in either substance affects the other; and this examination may throw light upon the greater question regarding the nature of the substances. Do material bodies moving in the ether of space—for example, the earth and its atmosphere—move through the ether, or carry with them the ether that is distributed throughout the matter that is moving? Experiments of extraordinary precision by Prof. Michelson have led him to conclude that most probably the earth carries with it all the ether in its immediate neighborhood; that certainly the relative motion of the earth and the ether in it is exceedingly small. If he can repeat his experiments and get a different result on the top of a mountain, that conclusion may be considered established. Those conclusions were drawn from experiments in which the earth's velocity in its orbit is involved. Prof. Lodge has experimented for effects due to slower motion of bodies upon the earth. He says: "I do not believe the ether moves. It does not move at a five-hundredth part of the speed of the steel disks" (used in the experiment). "I hope to go further, but my conclusion so far is that such things as circular saws, fly wheels, railway trains, and all ordinary masses of matter do not appreciably carry the ether with them. Their motion does not seem to disturb it in the least."
Among the more special questions undergoing investigation at present by the application of physical principles is the determination of the relative motion of the heavenly bodies by spectroscopic methods. It is done by applying to light-waves what is known in acoustics as Doppler's principle. The position of any line of the spectrum depends upon the wave length, or, what comes to the same thing in this case, the period of vibration for the particular set of waves making the light at that line in the spectrum. By increasing the number of waves per second that fall upon the prism (or grating) of the spectrometer, the period is correspondingly decreased, and conversely. Therefore, while the rate of vibration remains constant, if the grating is moving toward the source of vibration, the number of waves per second falling upon the grating will be greater, and their period smaller, than if the source and the grating are stationary relatively to each other. If they are separating, the period of vibration is increased. In the former case the line of the spectrum will be more refracted, in the latter less refracted, than in a normal case. When a spectrum line of any of the heavenly bodies has been identified with that of any substance known to us, the spectrometer gives the means of determining the motion of such heavenly bodies as compared with the motion of the earth, by observing the displacement of the spectrum line. That is, it is possible to determine whether the earth is approaching the star or nebula or receding from it, and at what rate. This method was proposed and attempts were made to apply it very early in the history of the spectroscope, but the means of observation were not then sufficiently fine, and only negative results were obtained. Within the last few years, however. Prof. Huggins, Prof. Vogel, and others in Europe have made many successful measurements of this character, and Prof. Keeler, of the Alleghany Observatory, has greatly extended them. These relative motions are usually reduced to the sun, the results indicating the relative motion of the sun and the heavenly body observed. As instances. Prof. Keeler finds that the great nebula in Orion is receding from the sun at the rate of eleven miles per second; and by observations between April and August, 1890, the sun was at that time approaching the bright star Arcturus at the rate of four miles and three tenths per second. These serve as a fine illustration of modern methods of research and the degree of precision attainable. The trustworthiness of the method is shown by the close agreement between its results when applied to the other planets, and the velocities computed from the known astronomical motions of the same bodies.
It is usually thought necessary to caution students of electricity against regarding either of the hypotheses, known respectively as the two-fluid and the one-fluid hypothesis, in the light of an assured thing, and the lecturer commonly hastens to declare that no one knows what electricity is. The declaration is as just as the caution; but it is not in human nature to allow such a declaration long to stand unchallenged. The very fact that it is possibly correct is a stimulus to investigation. Recent research has not conclusively shown what electricity is, but it has considerably shaken the foundations of the above assertion regarding it, and some singular views have been developed that indicate light ahead. We are learning that although the terms "electrification" and "electric" may continue in service to express a condition of matter or to characterize particular phenomena, yet the very name "electricity" may probably become useless and vanish from the vocabulary of physics, for the reason that, instead of electricity being any object, it is probably only a mode in which the ether makes itself manifest. One of the latest views, strongly advocated, is that ether may be analyzed into two constituents, equal and opposite, each endowed with inertia and each connected with the other by elastic ties which are weakened or dissolved by the presence of gross matter. The two constituents are called positive and negative electricity respectively, and of these two electricities the ether is composed. Electric currents which are obtained in such diversity and magnitude for commercial purposes are in almost every case the result of electro-magnetic induction, and are not due to the action of a battery. Yet there is no difference electrically between the currents obtained in the two ways. Maxwell's theory, which treats electro-magnetic action as a variation of ether stress in the medium in which the conductor is situated, may be applied to the conductors of battery currents also, and the medium surrounding the conductor in all cases is the home of the energy transmitted (as we are in the habit of saying) along the wire. But the energy is not transmitted by the wire; on the contrary, the wire, in just so far as it is a good conductor, fails to transmit the energy (the strain) which the action of the generator has sent out into the surrounding medium, and which breaks down or gives way in the conductor. "The energy of a dynamo does not, therefore, travel to a distant motor through the wires, but through the air. The energy of an Atlantic cable does not travel through the wire strands, but through the insulating sheath. This is a singular and apparently paradoxical view, but it is well founded" (Lodge). And even as to the power of a wire to conduct whatever it does conduct, a special feature has risen into considerable prominence. The most important principle for many years in the study of electricity has been Ohm's law, which states that the resistance of a conductor may be measured by the ratio of the electro-motive force to the current strength. This law when first enunciated was scrutinized closely, demurred against by some experimenters, and shown mathematically to be impossible if carried to extreme applications; it was re-established and experimentally and mathematically proved correct, chiefly by Kirchhoff's work; and is now known to be inaccurate as an expression of the effect transmitted (or resisted) by a conductor under rapid alternations of current, so that to express the energy transmitted under such circumstances another factor has to be taken into account besides what is usually regarded the resistance. This additional quality is called the impedance, and the total resistance of a circuit carrying periodic currents is made up of the ohmic resistance and the impedance. The latter has no value when the current is steady, but has reference only to the time while the current is rising from zero to its maximum strength. The principle of impedance was known a good while ago, but it has only demanded the attention of electricians since the alternating currents have begun to be employed on any considerable scale. Ohm's law is just as true as it ever was, but the limitations of its applicability are now better recognized than formerly.
A rapid succession of electric discharges sets up strains and relaxations in a non-conducting medium, which result in the propagation of waves of electro-magnetic induction through it. With oscillations of great frequency, the waves become short enough to be observed and measured readily, and the recent experiments of Hertz show so many features of similarity in the laws and phenomena of reflection, refraction, and speed of transmission of these waves and of light as to sustain Maxwell's theory of the electromagnetic character of light.
Advances in science are often the outcome of efforts to apply its principles in the arts. A great problem of physics which engineers have to solve is to find economical means of utilizing the energy that Nature is ready to furnish in place of the present wasteful ones. The inefficiency of the best steam engine is a standing reproach to an inventive age. The reproach is to be removed not by the improvement of the steam engine—for its limitations are such that, in the nature of things, it can not be highly efficient—but by the substitution of a better type of machine. Ether vibrations bring us energy in the form of heat, light, or electricity, according to their periods and amplitudes; but these, instead of being available in any particular form, are always more or less complex. If we could produce waves of just the rate and amplitude we desire, without any others in combination, a great step would be gained. Then we could produce light without wasting at the same time a great amount of energy in producing heat which we do not want. This is one of the subordinate problems awaiting solution. If to the production of such waves as are wanted we could add a means of recording and fixing them in their true relative proportion, we would have the solution of another great and fascinating subordinate problem—the exact reproduction of natural scenes in color. A long step has been taken toward accomplishing the first of these achievements in the remarkable experiments by Mr. Tesla with alternating electrical currents of high frequency and high potential. Among the startling facts brought out in these experiments is that although a current of electricity, either direct or alternating, from ordinary dynamos under fifteen hundred or two thousand volts electro-motive 'force will kill, yet under alternations of a million to a million and a half per second a voltage of fifty thousand produces no shock or injury. Electric lamps light with but a single wire leading to them. Vacuum tubes become luminous in a properly prepared room with no wires, and it is not extravagant, in view of what has already appeared, to predict a future when unlimited power will be available at every man's hand. That will be when, as Mr. Tesla says, we are able to "hook our machinery to the machinery of Nature." In the conclusion of his lecture before the Institution of Electrical Engineers, London, after describing a plan by which he thinks it would be practicable to telephone across the Atlantic, he adds: "But such cables will not be constructed, for, ere long, intelligence—transmitted without wires—will throb through the earth like a pulse through a living organism. The wonder is that, with the present state of knowledge and the experience gained, no attempt is being made to disturb the electrostatic or magnetic condition of the earth, and transmit, if nothing else, intelligence." It is probable that this wonder will give place to a still greater at no distant period, by reason of successful attempts of just the kind here mentioned. The problem is already in course of solution, the distinguished electrician, Mr. Preece, having recently succeeded in sending telephonic messages over a circuit which was wholly disconnected from that in which the generator was placed, and at a distance of three miles from it.
Unquestionably one of the most powerful aids to investigation of late has been photography. Both as a science and as an art it has grown in precision, speed, and availability, until now it has become a weapon of attack as well as a means of record. While owing more itself to chemistry than to physics, in the latter especially has it been of assistance to the spectroscope, so that the experimenter is not dependent upon the observations of the moment to make his comparisons. The most considerable work of this kind has been done by Prof. Rowland within the last half-dozen years, in making remarkably large and detailed photographs of the solar spectrum, the spectrum itself, in its perfection and beauty, being due to the matchless gratings constructed under Rowland's directions. Photography has proved to be an unassailable recorder for all the natural sciences, and is likely to become more and more firmly established as such. Disputes over priority in discovery will become less frequent since investigations made in solitude will appeal to their photographic record as a safe witness, impartial and indisputable.
Another subordinate problem is to determine the intensity of sound in absolute measure. Acoustics has been studied with reference to the energy involved less than other branches of physics, although we easily recognize some transformations of such energy into mechanical in the phonograph and electrical in the telephone. But most determinations of the intensity of sound have been relative, by comparison of different sounds, or else the same sound at different distances or in different media. They have not been expressed in absolute units. Absolute values of radiant energy, in the form of heat and light, have been determined, but the methods have not been sufficiently simplified to make them readily applicable in experimenting. Temperatures are still given in arbitrary degrees, and intensity of illumination has no acceptable basis expressible in terms of the fundamental quantities mass, time, and distance, although several methods have been suggested in which the direct, subjective estimate of it by the eye plays no part.
This brings us to a consideration of the great service rendered to scientific investigation by an absolute system of units and measurements. Such systems were instituted by Gauss and Weber between the years 1834 and 1850, and their introduction was especially fruitful in the study of electricity. The mechanic was enabled by that means for the first time to compare the electric forces produced with the mechanical ones employed, and gained thereby for the first time a just estimate of the former. The adoption throughout the scientific world of the centimetre-gramme-second absolute system for all branches of science is by no means the least valuable outcome of the development which electrical science has undergone since 1850, for in the possibility of tracing back all natural phenomena to the three mechanical units of space, mass, and time, science received new evidence for the inherent unity and the mechanical character of all forces of Nature. Energy as considered in physics, apart from chemistry has been classified in various forms, viz., energy of motion (translation or rotation), strain, vibration, beat, radiation, electrification, electricity in motion, magnetization, and gravitative separation. Those forms which are represented directly by bodies (whether extended masses or molecules) in motion or deformation, and which do not appeal to our special senses for recognition, constitute mechanical energy. The first two named above are plainly such, and all the others except the last have been shown to be such indirectly; it is generally believed that the last will be found to be reducible to the same form, so that probably all are essentially mechanical, and physicists are hoping to reduce them all to the mechanical as the ultimate form of energy. The importance to the physicist, therefore, of an acquaintance with the principles of mechanics can not be overestimated: without such an acquaintance his efforts to unravel the mysteries of physical science or to gain possession of its secrets will be futile.