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Reference: Evolution of Physics

This paper presents Chapter III, section 8 from the book THE EVOLUTION OF PHYSICS by A. EINSTEIN and L. INFELD. The contents are from the original publication of this book by Simon and Schuster, New York (1942).

The paragraphs of the original material (in black) are accompanied by brief comments (in color) based on the present understanding.  Feedback on these comments is appreciated.

The heading below is linked to the original materials.

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Relativity and Mechanics

The relativity theory arose from necessity, from serious and deep contradictions in the old theory from which there seemed no escape. The strength of the new theory lies in the consistency and simplicity with which it solves all these difficulties, using only a few very convincing assumptions.

Although the theory arose from the field problem, it has to embrace all physical laws. A difficulty seems to appear here. The field laws on the one hand and the mechanical laws on the other are of quite different kinds. The equations of electromagnetic field are invariant with respect to the Lorentz transformation and the mechanical equations are invariant with respect to the classical transformation. But the relativity theory claims that all laws of nature must be invariant with respect to the Lorentz and not to the classical transformation. The latter is only a special, limiting case of the Lorentz transformation when the relative velocities of two CS are very small. If this is so, classical mechanics must change in order to conform with the demand of invariance with respect to the Lorentz transformation. Or, in other words, classical mechanics cannot be valid if the velocities approach that of light. Only one transformation from one CS to another can exist, namely, the Lorentz transformation.

The classical transformation is only a special, limiting case of the Lorentz transformation when the relative velocities of two CS are very small. Classical mechanics cannot be valid if the velocities approach that of light.

It was simple to change classical mechanics in such a way that it contradicted neither the relativity theory nor the wealth of material obtained by observation and explained by classical mechanics. The old mechanics is valid for small velocities and forms the limiting case of the new one.

It would be interesting to consider some instance of a change in classical mechanics introduced by the relativity theory. This might, perhaps, lead us to some conclusions which could be proved or disproved by experiment.

Let us assume a body, having a definite mass, moving along a straight line, and acted upon by an external force in the direction of the motion. The force, as we know, is proportional to the change of velocity. Or, to be more explicit, it does not matter whether a given body increases its velocity in one second from 100 to 101 feet per second, or from 100 miles to 100 miles and one foot per second or from 180,000 miles to 180,000 miles and one foot per second. The force acting upon a particular body is always the same for the same change of velocity in the same time.

We can easily modify the laws of mechanics to be consistent with the laws of field for small velocities. But the assumption (that the force acting upon a particular body is always the same for the same change of velocity in the same time) is not true. Much greater force is required when acting upon an electron for the same change of velocity in the same time, because electron’s mass (inertia) is much smaller. That assumption is valid for matter only for which inertia is very high and constant.

Is this sentence true from the point of view of the relativity theory? By no means! This law is valid only for small velocities. What, according to the relativity theory, is the law for great velocities, approaching that of light? If the velocity is great, extremely strong forces are required to increase it. It is not at all the same thing to increase by one foot per second a velocity of about 100 feet per second or a velocity approaching that of light. The nearer a velocity is to that of light the more difficult it is to increase. When a velocity is equal to that of light it is impossible to increase it further. Thus, the changes brought about by the relativity theory are not surprising. The velocity of light is the upper limit for all velocities. No finite force, no matter how great, can cause an increase in speed beyond this limit. In place of the old mechanical law connecting force and change of velocity, a more complicated one appears. From our new point of view classical mechanics is simple because in nearly all observations we deal with velocities much smaller than that of light.

Einstein is correct that extremely strong forces are required to increase a velocity approaching that of light; but he does not see that this happens because the mass has become extremely small for any particle approaching the velocity of light. You cannot push something that has no inertia or resistance because it will simply move without increasing in velocity.

A body at rest has a definite mass, called the rest mass. We know from mechanics that every body resists a change in its motion; the greater the mass, the stronger the resistance, and the weaker the mass, the weaker the resistance. But in the relativity theory, we have something more. Not only does a body resist a change more strongly if the rest mass is greater, but also if its velocity is greater. Bodies with velocities approaching that of light would offer a very strong resistance to external forces. In classical mechanics the resistance of a given body was something unchangeable, characterized by its mass alone. In the relativity theory it depends on both rest mass and velocity. The resistance becomes infinitely great as the velocity approaches that of light.

Confusion starts with the concept of “Rest Mass.” This may be the mass of a particle at rest in its inertial frame, but it still has velocity on an absolute basis, if its inertia is not infinite. Therefore, the “rest mass” may differ from one inertial frame to another even when this difference is imperceptible for matter because its inertia is very high. The greater velocity means lesser inertia (farther from infinite value). The following statement of Einstein is incorrect, “Not only does a body resist a change more strongly if the rest mass is greater, but also if its velocity is greater.” Actually, the resistance (inertia) becomes infinitely small as the velocity approaches that of light.

The results just quoted enable us to put the theory to the test of experiment. Do projectiles with a velocity approaching that of light resist the action of an external force as predicted by the theory? Since the statements of the relativity theory have, in this respect, a quantitative character, we could confirm or disprove the theory if we could realize projectiles having a speed approaching that of light.

The inertia of a photon is extremely small, and its velocity is extremely high. It simply cannot be pushed to a higher velocity, no matter the amount of force applied. This shows that the idea of relativistic mass does not apply to the moving particle. It is a misinterpretation of what is really going on.

Indeed, we find in nature projectiles with such velocities. Atoms of radioactive matter, radium for instance, act as batteries which fire projectiles with enormous velocities. Without going into detail we can quote only one of the very important views of modern physics and chemistry. All matter in the universe is made up of elementary particles of only a few kinds. It is like seeing in one town buildings of different sizes, construction and architecture, but from shack to skyscraper only very few different kinds of bricks were used, the same in all the buildings. So all known elements of our material world from hydrogen the lightest, to uranium the heaviest are built of the same kinds of bricks, that is, the same kinds of elementary particles. The heaviest elements, the most complicated buildings, are unstable and they disintegrate or, as we say, are radioactive. Some of the bricks, that is, the elementary particles of which the radioactive atoms are constructed, are sometimes thrown out with a very great velocity, approaching that of light. An atom of an element, say radium, according to our present views, confirmed by numerous experiments, is a complicated structure, and radioactive disintegration is one of those phenomena in which the composition of atoms from still simpler bricks, the elementary particles, is revealed.

By very ingenious and intricate experiments we can find out how the particles resist the action of an external force. The experiments show that the resistance offered by these particles depends on the velocity, in the way foreseen by the theory of relativity. In many other cases, where the dependence of the resistance upon the velocity could be detected, there was complete agreement between theory and experiment. We see once more the essential features of creative work in science: prediction of certain facts by theory and their confirmation by experiment.

The Standard Model of Particle Physics lists 17 different elementary particles. A naturally disintegrating heavy nucleus ejects elementary particles at very high velocity. The velocity of an elementary particle is as great as its inertia is small. The experiments referred to by Einstein are simply being misinterpreted.

This result suggests a further important generalization. A body at rest has mass but no kinetic energy, that is, energy of motion. A moving body has both mass and kinetic energy. It resists change of velocity more strongly than the resting body. It seems as though the kinetic energy of the moving body increases its resistance. If two bodies have the same rest mass, the one with the greater kinetic energy resists the action of an external force more strongly.

It is difficult to change the inertia of an elementary particle, so it is difficult to change its intrinsic velocity also. The velocity may be influenced by forces, such as, electromagnetic, gravity and friction; but the intrinsic inertia may try to restore the intrinsic velocity. Einstein’s reference to kinetic energy in the context of mass brings into play the arbitrary velocity of the inertial frame.

Imagine a box containing balls, with the box as well as the balls at rest in our CS. To move it, to increase its velocity, some force is required. But will the same force increase the velocity by the same amount in the same time with the balls moving about quickly and in all directions inside the box, like the molecules of a gas, with an average speed approaching that of light? A greater force will now be necessary because of the increased kinetic energy of the balls, strengthening the resistance of the box. Energy, at any rate kinetic energy, resists motion in the same way as ponderable masses. Is this also true of all kinds of energy?

In case of light, the photons are moving at their natural extremely high velocities. It is not the same situation as the molecules of a gas. Changing the velocities of photons is more difficult because it means changing their inertia (frequency).

The theory of relativity deduces, from its fundamental assumption, a clear and convincing answer to this question, an answer again of a quantitative character: all energy resists change of motion; all energy behaves like matter; a piece of iron weighs more when red-hot than when cool; radiation travelling through space and emitted from the sun contains energy and therefore has mass; the sun and all radiating stars lose mass by emitting radiation. This conclusion, quite general in character, is an important achievement of the theory of relativity and fits all facts upon which it has been tested.

Before one can push a particle moving at high velocity, one needs to catch up with that particle first. Therefore, it is more difficult to further accelerate a moving particle. The difficulty comes from higher velocity and not from higher inertia (mass) as Einstein thinks. But, it is true that radiation has very small inertia (mass) and very high velocity. Inertia and velocity balance each other.

Classical physics introduced two substances: matter and energy. The first had weight, but the second was weightless. In classical physics we had two conservation laws: one for matter, the other for energy. We have already asked whether modern physics still holds this view of two substances and the two conservation laws. The answer is: “No”. According to the theory of relativity, there is no essential distinction between mass and energy. Energy has mass and mass represents energy. Instead of two conservation laws we have only one, that of mass-energy. This new view proved very successful and fruitful in the further development of physics.

We feel the presence of substance due to force moving through a distance. Force and motion can be separated in case of matter, but not in case of field. Therefore, from matter to radiation we are looking at the conservation of a force-motion combination. A combination of force and motion is represented mathematically as momentum, and also as energy.

How is it that this fact of energy having mass and mass representing energy remained for so long obscured? Is the weight of a piece of hot iron greater than that of a cold piece? The answer to this question is now “Yes”, but on p. 43 it was “No”. The pages between these two answers are certainly not sufficient to cover this contradiction.

The difficulty confronting us here is of the same kind as we have met before. The variation of mass predicted by the theory of relativity is immeasurably small and cannot be detected by direct weighing on even the most sensitive scales. The proof that energy is not weightless can be gained in many very conclusive, but indirect, ways.

The reason for this lack of immediate evidence is the very small rate of exchange between matter and energy. Compared to mass, energy is like a depreciated currency compared to one of high value. An example will make this clear. The quantity of heat able to convert thirty thousand tons of water into steam would weigh about one gram! Energy was regarded as weightless for so long simply because the mass which it represents is so small.

The differentiation here should be between mass (inertia, centeredness) and velocity (forward motion, spreading) rather than between mass and energy. Energy is a combination of mass and velocity.

The old energy-substance is the second victim of the theory of relativity. The first was the medium through which light waves were propagated.

The influence of the theory of relativity goes far beyond the problem from which it arose. It removes the difficulties and contradictions of the field theory; it formulates more general mechanical laws; it replaces two conservation laws by one; it changes our classical concept of absolute time. Its validity is not restricted to one domain of physics; it forms a general framework embracing all phenomena of nature.

Aether should be understood as the substance of lowest inertia on the spectrum of substance. The substance of highest inertia is matter (black hole). The law of conservation applies to a combination of inertia and velocity, which is momentum or energy.

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Final Comment

Einstein’s error is not to differentiate between a “particle of high inertia” and a “particle of high velocity.” Einstein looks at a “particle of high inertia” as mass; and at a “particle of high velocity” as relativistic mass. There is difficulty in accelerating both type of particles; but the two difficulties are very different in nature. There is a balance in nature between inertia and velocity, but Einstein fails to acknowledges that fact. Einstein does not provide a theoretical model for the theory of relativity, but leaves it to mathematical interpretation.

With the advantage of developments in Atomic Physics, Postulate Mechanics now provides a theoretical model for the theory of relativity. This model looks at inertia as “centeredness” due to spin, and velocity as “spreading” due to forward motion.” It relates inertia to magnetic lines of force, and velocity to electric lines of force.

The variation in inertia provides a spectrum of substance from matter to radiation to aether. Aether is the substance of lowest inertia on the spectrum of substance. A particle has an intrinsic velocity related to its intrinsic inertia. Whenever the intrinsic velocity of the particle is changed, its intrinsic inertia tries to restore the velocity to its intrinsic value. Neither Newton nor Einstein acknowledge this role of inertia.

According to the PM model, there is a reciprocal relationship between inertia and velocity. The lesser is inertia of a particle the greater is its intrinsic velocity. The intrinsic velocity of an elementary particle depends on its inertia or mass.

No material object can reach the velocity of light unless its inertia is reduced to that of a photon. A material object approaches the condition of absolute rest only when it acquires infinite inertia or centeredness.

For radiation, its inertia is represented by its frequency; so as its frequency changes, its velocity shall  change also. The velocity of light is an average value of velocities of the visible part of the spectrum. 

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Reference: Evolution of Physics

This paper presents Chapter III, section 6 from the book THE EVOLUTION OF PHYSICS by A. EINSTEIN and L. INFELD. The contents are from the original publication of this book by Simon and Schuster, New York (1942).

The paragraphs of the original material (in black) are accompanied by brief comments (in color) based on the present understanding.  Feedback on these comments is appreciated.

The heading below is linked to the original materials.

.

Ether and Motion

The Galilean relativity principle is valid for mechanical phenomena. The same laws of mechanics apply to all inertial systems moving relative to each other. Is this principle also valid for non-mechanical phenomena, especially for those for which the field concepts proved so very important? All problems concentrated around this question immediately bring us to the starting-point of the relativity theory.

The Galilean relativity principle applies to a narrow range of velocities of matter. In this range, the velocities are small and inertia is very large (compared to light). Change in inertia corresponding to change in velocity is virtually undetectable. The laws of mechanics are based on this invariability of inertia (mass). All inertial frames exist in this narrow range of velocities of matter.

We remember that the velocity of light in vacuo, or in other words, in ether, is 186,000 miles per second and that light is an electromagnetic wave spreading through the ether. The electromagnetic field carries energy which, once emitted from its source, leads an independent existence. For the time being, we shall continue to believe that the ether is a medium through which electromagnetic waves, and thus also light waves, are propagated, even though we are fully aware of the many difficulties connected with its mechanical structure.

The principle of field applies to a broad spectrum of electromagnetic waves. In this spectrum, the inertia (represented by frequencies) is extremely small, but the velocity is very large. Change in velocity corresponding to change in inertia (frequency) is virtually undetectable. The electromagnetic field carries energy which, once emitted from its source, leads an independent existence.

We are sitting in a closed room so isolated from the external world that no air can enter or escape. If we sit still and talk we are, from the physical point of view, creating sound waves, which spread from their resting source with the velocity of sound in air. If there were no air or other material medium between the mouth and the ear, we could not detect a sound. Experiment has shown that the velocity of sound in air is the same in all directions, if there is no wind and the air is at rest in the chosen c.s.

Let us now imagine that our room moves uniformly through space. A man outside sees, through the glass walls of the moving room (or train if you prefer), everything which is going on inside. From the measurements of the inside observer he can deduce the velocity of sound relative to his c.s. connected with his surroundings, relative to which the room moves. Here again is the old, much discussed, problem of determining the velocity in one c.s. if it is already known in another.

The observer in the room claims: the velocity of sound is, for me, the same in all directions.

The outside observer claims: the velocity of sound, spreading in the moving room and determined in my c.s., is not the same in all directions. It is greater than the standard velocity of sound in the direction of the motion of the room and smaller in the opposite direction.

These conclusions are drawn from the classical transformation and can be confirmed by experiment. The room carries within it the material medium, the air through which sound waves are propagated, and the velocities of sound will, therefore, be different for the inside and outside observer.

We can draw some further conclusions from the theory of sound as a wave propagated through a material medium. One way, though by no means the simplest, of not hearing what someone is saying, is to run, with a velocity greater than that of sound, relative to the air surrounding the speaker. The sound waves produced will then never be able to reach our ears. On the other hand, if we missed an important word which will never be repeated, we must run with a speed greater than that of sound to reach the produced wave and to catch the word. There is nothing irrational in either of these examples except that in both cases we should have to run with a speed of about four hundred yards per second, and we can very well imagine that further technical development will make such speeds possible. A bullet fired from a gun actually moves with a speed greater than that of sound and a man placed on such a bullet would never hear the sound of the shot.

Sound lies in the narrow range of velocities of matter where inertia is very large. Because of high inertia, the wind is substantial. Sound is carried with the wind, so its velocity is higher in the direction of the wind.

All these examples are of a purely mechanical character and we can now formulate the important questions: could we repeat what has just been said of a sound wave, in the case of a light wave? Do the Galilean relativity principle and the classical transformation apply to optical and electrical phenomena as well as to mechanical? It would be risky to answer these questions by a simple “yes” or “no” without going more deeply into their meaning.

In the case of the sound wave in the room moving uniformly, relative to the outside observer, the following intermediate steps are very essential for our conclusion:

1. The moving room carries the air in which the sound wave is propagated.
2. The velocities observed in two c.s. moving uniformly, relative to each other, are connected by the classical transformation.

The corresponding problem for light must be formulated a little differently. The observers in the room are no longer talking, but are sending light signals, or light waves in every direction. Let us further assume that the sources emitting the light signals are permanently resting in the room. The light waves move through the ether just as the sound waves moved through the air.

Is the ether carried with the room as the air was? Since we have no mechanical picture of the ether, it is extremely difficult to answer this question. If the room is closed, the air inside is forced to move with it. There is obviously no sense in thinking of ether in this way, since all matter is immersed in it and it penetrates everywhere. No doors are closed to ether. The “moving room” now means only a moving c.s. to which the source of light is rigidly connected. It is, however, not beyond us to imagine that the room moving with its light source carries the ether along with it just as the sound source and air were carried along in the closed room. But we can equally well imagine the opposite: that the room travels through the ether as a ship through a perfectly smooth sea, not carrying any part of the medium along but moving through it. In our first picture, the room moving with its light source carries the ether. An analogy with a sound wave is possible and quite similar conclusions can be drawn. In the second, the room moving with its light source does not carry the ether. No analogy with a sound wave is possible and the conclusions drawn in the case of a sound wave do not hold for a light wave. These are the two limiting possibilities. We could imagine the still more complicated possibility that the ether is only partially carried by the room moving with its light source. But there is no reason to discuss the more complicated assumptions before finding out which of the two simpler limiting cases experiment favours.

We shall begin with our first picture and assume, for the present: the ether is carried along by the room moving with its rigidly connected light source. If we believe in the simple transformation principle for the velocities of sound waves, we can now apply our conclusions to light waves as well. There is no reason for doubting the simple mechanical transformation law which only states that the velocities have to be added in certain cases and subtracted in others. For the moment, therefore, we shall assume both the carrying of the ether by the room moving with its light source and the classical transformation.

If I turn on the light and its source is rigidly connected with my room, then the velocity of the light signal has the well-known experimental value 186,000 miles per second. But the outside observer will notice the motion of the room, and, therefore, that of the source—and, since the ether is carried along, his conclusion must be: the velocity of light in my outside c.s. is different in different directions. It is greater than the standard velocity of light in the direction of the motion of the room and smaller in the opposite direction. Our conclusion is: if ether is carried with the room moving with its light source and if the mechanical laws are valid, then the velocity of light must depend on the velocity of the light source. Light reaching our eyes from a moving light source would have a greater velocity if the motion is toward us and smaller if it is away from us.

If our speed were greater than that of light, we should be able to run away from a light signal. We could see occurrences from the past by reaching previously sent light waves. We should catch them in a reverse order to that in which they were sent, and the train of happenings on our earth would appear like a film shown backward, beginning with a happy ending. These conclusions all follow from the assumption that the moving c.s. carries along the ether and the mechanical transformation laws are valid. If this is so, the analogy between light and sound is perfect.

But there is no indication as to the truth of these conclusions. On the contrary, they are contradicted by all observations made with the intention of proving them. There is not the slightest doubt as to the clarity of this verdict, although it is obtained through rather indirect experiments in view of the great technical difficulties caused by the enormous value of the velocity of light. The velocity of light is always the same in all c.s. independent of whether or not the emitting source moves, or how it moves.

Light lies in the broad spectrum of electromagnetic waves where inertia is extremely small. The “wind” due to moving light source is not substantial at all, and the velocity is extremely large. Therefore, no change in velocity is detected.

We shall not go into detailed description of the many experiments from which this important conclusion can be drawn. We can, however, use some very simple arguments which, though they do not prove that the velocity of light is independent of the motion of the source, nevertheless make this fact convincing and understandable.

In our planetary system the earth and other planets move around the sun. We do not know of the existence of other planetary systems similar to ours. There are, however, very many double-star systems, consisting of two stars moving around a point, called their centre of gravity. Observation of the motion of these double stars reveals the validity of Newton’s gravitational law. Now suppose that the speed of light depends on the velocity of the emitting body. Then the message, that is, the light ray from the star, will travel more quickly or more slowly, according to the velocity of the star at the moment the ray is emitted. In this case the whole motion would be muddled and it would be impossible to confirm, in the case of distant double stars, the validity of the same gravitational law which rules over our planetary system.

Let us consider another experiment based upon a very simple idea. Imagine a wheel rotating very quickly. According to our assumption, the ether is carried by the motion and takes a part in it. A light wave passing near the wheel would have a different speed when the wheel is at rest than when it is in motion. The velocity of light in ether at rest should differ from that in ether which is being quickly dragged round by the motion of the wheel, just as the velocity of a sound wave varies on calm and windy days. But no such difference is detected! No matter from which angle we approach the subject, or what crucial experiment we may devise, the verdict is always against the assumption of the ether carried by motion. Thus, the result of our considerations, supported by more detailed and technical argument, is:

1. The velocity of light does not depend on the motion of the emitting source.
2. It must not be assumed that the moving body carries the surrounding ether along.

We must, therefore, give up the analogy between sound and light waves and turn to the second possibility: that all matter moves through the ether, which takes no part whatever in the motion. This means that we assume the existence of a sea of ether with all c.s. resting in it, or moving relative to it. Suppose we leave, for a while, the question as to whether experiment proved or disproved this theory. It will be better to become more familiar with the meaning of this new assumption and with the conclusions which can be drawn from it.

The analogy between sound and light in terms of wind and velocity does not exist. If there is any analogy, it is in terms of a balance between inertia and velocity.

There exists a c.s. resting relative to the ether-sea. In mechanics, not one of the many c.s. moving uniformly, relative to each other, could be distinguished. All such c.s. were equally “good” or “bad”. If we have two c.s. moving uniformly, relative to each other, it is meaningless, in mechanics, to ask which of them is in motion and which at rest. Only relative uniform motion can be observed. We cannot talk about absolute uniform motion because of the Galilean relativity principle. What is meant by the statement that absolute and not only relative uniform motion exists? Simply that there exists one c.s. in which some of the laws of nature are different from those in all others. Also that every observer can detect whether his c.s. is at rest or in motion by comparing the laws valid in it with those valid in the only one which has the absolute monopoly of serving as the standard c.s. Here is a different state of affairs from classical mechanics, where absolute uniform motion is quite meaningless because of Galileo’s law of inertia.

What conclusions can be drawn in the domain of field phenomena if motion through ether is assumed? This would mean that there exists one c.s. distinct from all others, at rest relative to the ether-sea. It is quite clear that some of the laws of nature must be different in this c.s., otherwise the phrase “motion through ether” would be meaningless. If the Galilean relativity principle is valid, then motion through ether makes no sense at all. It is impossible to reconcile these two ideas. If, however, there exists one special c.s. fixed by the ether, then to speak of “absolute motion” or “absolute rest” has a definite meaning.

We really have no choice. We tried to save the Galilean relativity principle by assuming that systems carry the ether along in their motion, but this led to a contradiction with experiment. The only way out is to abandon the Galilean relativity principle and try out the assumption that all bodies move through the calm ether-sea.

The laws of mechanics based on Galileo’s Relativity principle apply to a very small range of velocities for near constant inertia. Absolute velocities do not exist because the condition of absolute rest is not recognized in this range. But in the wider range, there can be absolute rest for a body of infinite inertia. An example would be a black hole at the center of the galaxy.

The next step is to consider some conclusions contradicting the Galilean relativity principle and supporting the view of motion through ether, and to put them to the test of an experiment. Such experiments are easy enough to imagine, but very difficult to perform. As we are concerned here only with ideas, we need not bother with technical difficulties.

Again we return to our moving room with two observers, one inside and one outside. The outside observer will represent the standard c.s., designated by the ether-sea. It is the distinguished c.s. in which the velocity of light always has the same standard value. All light sources, whether moving or at rest in the calm ether-sea, propagate light with the same velocity. The room and its observer move through the ether. Imagine that a light in the centre of the room is flashed on and off and, furthermore, that the walls of the room are transparent so that the observers, both inside and outside, can measure the velocity of the light. If we ask the two observers what results they expect to obtain, their answers would run something like this:

The outside observer: My c.s. is designated by the ether-sea. Light in my c.s. always has the standard value. I need not care whether or not the source of light or other bodies are moving, for they never carry my ether-sea with them. My c.s. is distinguished from all others and the velocity of light must have its standard value in this c.s., independent of the direction of the light beam or the motion of its source.

The inside observer: My room moves through the ether-sea. One of the walls runs away from the light and the other approaches it. If my room travelled, relative to the ether-sea, with the velocity of light, then the light emitted from the centre of the room would never reach the wall running away with the velocity of light. If the room travelled with a velocity smaller than that of light, then a wave sent from the centre of the room would reach one of the walls before the other. The wall moving toward the light wave would be reached before the one retreating from the light wave. Therefore, although the source of light is rigidly connected with my C.S., the velocity of light will not be the same in all directions. It will be smaller in the direction of the motion relative to the ether-sea as the wall runs away, and greater in the opposite direction as the wall moves toward the wave and tries to meet it sooner.

Thus, only in the one c.s. distinguished by the ether-sea should the velocity of light be equal in all directions. For other c.s. moving relatively to the ether-sea it should depend on the direction in which we are measuring.

The crucial experiment just considered enables us to test the theory of motion through the ether-sea. Nature, in fact, places at our disposal a system moving with a fairly high velocity: the earth in its yearly motion around the sun. If our assumption is correct, then the velocity of light in the direction of the motion of the earth should differ from the velocity of light in an opposite direction. The differences can be calculated and a suitable experimental test devised. In view of the small time-differences following from the theory, very ingenious experimental arrangements have to be thought out. This was done in the famous Michelson-Morley experiment. The result was a verdict of “death” to the theory of a calm ether-sea through which all matter moves. No dependence of the speed of light upon direction could be found. Not only the speed of light, but also other field phenomena would show a dependence on the direction in the moving c.s., if the theory of the ether-sea were assumed. Every experiment has given the same negative result as the Michelson-Morley one, and never revealed any dependence upon the direction of the motion of the earth.

The situation grows more and more serious. Two assumptions have been tried. The first, that moving bodies carry ether along. The fact that the velocity of light does not depend on the motion of the source contradicts this assumption. The second, that there exists one distinguished c.s. and that moving bodies do not carry the ether but travel through an ever calm ether-sea. If this is so, then the Galilean relativity principle is not valid and the speed of light cannot be the same in every c.s. Again we are in contradiction with experiment.

Michelson-Morley’s experiment simply demonstrated that the motion of Earth through space does not affect the speed of visible light. This result may be interpreted in different ways. But it consistent with almost zero inertia of light and aether. Light is a slightly condensed form of aether. Matter is an extremely condensed form of aether.

More artificial theories have been tried out, assuming that the real truth lies somewhere between these two limiting cases: that the ether is only partially carried by the moving bodies. But they all failed! Every attempt to explain the electromagnetic phenomena in moving c.s. with the help of the motion of the ether, motion through the ether, or both these motions, proved unsuccessful.

Thus arose one of the most dramatic situations in the history of science. All assumptions concerning ether led nowhere! The experimental verdict was always negative. Looking back over the development of physics we see that the ether, soon after its birth, became the enfant terrible of the family of physical substances. First, the construction of a simple mechanical picture of the ether proved to be impossible and was discarded. This caused, to a great extent, the breakdown of the mechanical point of view. Second, we had to give up hope that through the presence of the ether-sea one c.s. would be distinguished and lead to the recognition of absolute, and not only relative, motion. This would have been the only way, besides carrying the waves, in which ether could mark and justify its existence. All our attempts to make ether real failed. It revealed neither its mechanical construction nor absolute motion. Nothing remained of all the properties of the ether except that for which it was invented, i.e. its ability to transmit electromagnetic waves. Our attempts to discover the properties of the ether led to difficulties and contradictions. After such bad experiences, this is the moment to forget the ether completely and to try never to mention its name. We shall say: our space has the physical property of transmitting waves, and so omit the use of a word we have decided to avoid.

According to Einstein, aether is an invented concept. All we can say is that our space has the physical property of transmitting waves. Einstein then expresses the idea of space mathematically only.

The omission of a word from our vocabulary is, of course, no remedy. Our troubles are indeed much too profound to be solved in this way!

Let us now write down the facts which have been sufficiently confirmed by experiment without bothering any more about the “e——- r ” problem.

(1) The velocity of light in empty space always has its standard value, independent of the motion of the source or receiver of light.

(2) In two c.s. moving uniformly, relative to each other, all laws of nature are exactly identical and there is no way of distinguishing absolute uniform motion.

There are many experiments to confirm these two statements and not a single one to contradict either of them. The first statement expresses the constant character of the velocity of light, the second generalizes the Galilean relativity principle, formulated for mechanical phenomena, to all happenings in nature.

In mechanics, we have seen: If the velocity of a material point is so and so, relative to one c.s., then it will be different in another c.s. moving uniformly, relative to the first. This follows from the simple mechanical transformation principles. They are immediately given by our intuition (man moving relative to ship and shore) and apparently nothing can be wrong here! But this transformation law is in contradiction to the constant character of the velocity of light. Or, in other words, we add a third principle:

(3) Positions and velocities are transformed from one inertial system to another according to the classical transformation.

The contradiction is then evident. We cannot combine (1), (2), and (3).

The classical transformation seems too obvious and simple for any attempt to change it. We have already tried to change (1) and (2) and came to a disagreement with experiment. All theories concerning the motion of “e——- r” required an alteration of (1) and (2). This was no good. Once more we realize the serious character of our difficulties. A new clue is needed. It is supplied by accepting the fundamental assumptions (1) and (2), and, strange though it seems, giving up (3). The new clue starts from an analysis of the most fundamental and primitive concepts; we shall show how this analysis forces us to change our old views and removes all our difficulties.

Einstein’s assumptions may be criticized as follows: (1) Light is a substance of almost zero inertia; whereas, the source of light is considered to be a material object of extremely high inertia. The velocity of light is so high that variations in it due to changes in frequency cannot be detected in the framework of matter. (2) The idea of c.s. is limited to a narrow range of velocities for which inertia is so large that changes in it corresponding to changes in velocity cannot be detected. Based on inertia we can say that the velocity of light intuitively applies to “sunlight relative to the sun” and not to “sun relative to the sunlight.” (3) The laws of classical mechanics are a very small subset of the laws of nature under conditions as described above. The classical viewpoint does not consider the balance in nature between velocity and inertia.

Velocities can be differentiated as being greater or lesser on an absolute basis by comparing the inertia of the particles.

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Final Comment

According to the Galilean relativity principle of mechanical phenomena, it is impossible to say which particle is at rest relative to the other. But we have no such difficulty in intuitively recognizing that the Sun is at rest relative to the sunlight and not the opposite. This intuition has its basis in the concept of inertia.

The problem with the laws of classical mechanics is that they are limited to the assumption that the only substance is matter that has a fixed level of inertia (mass). Classical mechanics does not recognize that from matter to aether, there is a spectrum of substance with a gradation of inertia, and that the relative velocity is a manifestation of that inertia. The lesser is the inertia, the greater is the inherent velocity of substance.

Science assumes that all material systems have the same inertia (mass) for different velocities. It then makes another assumption that all electromagnetic systems have the same velocity for different levels of inertia (frequencies). Both assumptions are approximations for two extremes. Einstein’s theory predicts velocities for situations close to these extremes by extrapolating between these two extremes. But that extrapolation does not predict velocities correctly for situations farther away from these extremes.

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Reference: Evolution of Physics

This paper presents Chapter III, section 4 from the book THE EVOLUTION OF PHYSICS by A. EINSTEIN and L. INFELD. The contents are from the original publication of this book by Simon and Schuster, New York (1942).

The paragraphs of the original material (in black) are accompanied by brief comments (in color) based on the present understanding.  Feedback on these comments is appreciated.

The heading below is linked to the original materials.

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Field and Ether

The electromagnetic wave is a transverse one and is propagated with the velocity of light in empty space. The fact that their velocities are the same suggests a close relationship between optical and electromagnetic phenomena.

The electromagnetic wave may appear as “transverse” as it propagates in its domain marked as “spherical surface.” Light may be described as an electromagnetic wave.

When we had to choose between the corpuscular and the wave theory, we decided in favour of the wave theory. The diffraction of light was the strongest argument influencing our decision. But we shall not contradict any of the explanations of the optical facts by also assuming that the light wave is an electromagnetic one. On the contrary, still other conclusions can be drawn. If this is really so, then there must exist some connection between the optical and electrical properties of matter that can be deduced from the theory. The fact that conclusions of this kind can really be drawn and that they stand the test of experiment is an essential argument in favour of the electromagnetic theory of light.

Diffraction of light is an optical property. But, if light is an electromagnetic wave, diffraction may also have an explanation as an electromagnetic property.

This great result is due to the field theory. Two apparently unrelated branches of science are covered by the same theory. The same Maxwell’s equations describe both electric induction and optical refraction. If it is our aim to describe everything that ever happened or may happen with the help of one theory, then the union of optics and electricity is, undoubtedly, a very great step forward. From the physical point of view, the only difference between an ordinary electromagnetic wave and a light wave is the wave-length: this is very small for light waves, detected by the human eye, and great for ordinary electromagnetic waves, detected by a radio receiver.

From the physical point of view, the only difference between an ordinary electromagnetic wave and a light wave is the wave-length: this is very small for light waves, detected by the human eye, and great for ordinary electromagnetic waves, detected by a radio receiver.

The old mechanical view attempted to reduce all events in nature to forces acting between material particles. Upon this mechanical view was based the first naive theory of the electric fluids. The field did not exist for the physicist of the early years of the nineteenth century. For him only substance and its changes were real. He tried to describe the action of two electric charges only by concepts referring directly to the two charges.

In the beginning, the field concept was no more than a means of facilitating the understanding of phenomena from the mechanical point of view. In the new field language it is the description of the field between the two charges, and not the charges themselves, which is essential for an understanding of their action. The recognition of the new concepts grew steadily, until substance was overshadowed by the field. It was realized that something of great importance had happened in physics. A new reality was created, a new concept for which there was no place in the mechanical description. Slowly and by a struggle the field concept established for itself a leading place in physics and has remained one of the basic physical concepts. The electromagnetic field is, for the modern physicist, as real as the chair on which he sits.

In the new field language it is the description of the field between the two charges, and not the charges themselves, which is essential for an understanding of their action.

But it would be unjust to consider that the new field view freed science from the errors of the old theory of electric fluids or that the new theory destroys the achievements of the old. The new theory shows the merits as well as the limitations of the old theory and allows us to regain our old concepts from a higher level. This is true not only for the theories of electric fluids and field, but for all changes in physical theories, however revolutionary they may seem. In our case, we still find, for example, the concept of the electric charge in Maxwell’s theory, though the charge is understood only as a source of the electric field. Coulomb’s law is still valid and is contained in Maxwell’s equations from which it can be deduced as one of the many consequences. We can still apply the old theory, whenever facts within the region of its validity are investigated. But we may as well apply the new theory, since all the known facts are contained in the realm of its validity.

The new theory shows the merits as well as the limitations of the old theory and allows us to regain our old concepts from a higher level. The concept of the electric charge is understood only as a source of the electric field in Maxwell’s theory.

To use a comparison, we could say that creating a new theory is not like destroying an old barn and erecting a skyscraper in its place. It is rather like climbing a mountain, gaining new and wider views, discovering unexpected connections between our starting-point and its rich environment. But the point from which we started out still exists and can be seen, although it appears smaller and forms a tiny part of our broad view gained by the mastery of the obstacles on our adventurous way up.

It was, indeed, a long time before the full content of Maxwell’s theory was recognized. The field was at first considered as something which might later be interpreted mechanically with the help of ether. By the time it was realized that this programme could not be carried out, the achievements of the field theory had already become too striking and important for it to be exchanged for a mechanical dogma. On the other hand, the problem of devising the mechanical model of ether seemed to become less and less interesting and the result, in view of the forced and artificial character of the assumptions, more and more discouraging.

Our only way out seems to be to take for granted the fact that space has the physical property of transmitting electromagnetic waves, and not to bother too much about the meaning of this statement. We may still use the word ether, but only to express some physical property of space. This word ether has changed its meaning many times in the development of science. At the moment it no longer stands for a medium built up of particles. Its story, by no means finished, is continued by the relativity theory.

A new theory provides new and wider view that helps discover unexpected connections. The old view of aether has been a medium built up of particles. Maxwell’s theory now provides a new view of aether as the physical property of transmitting electromagnetic waves in space.

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Final Comment

In the model of an atom, the field may best describe the electronic region of very low mass and the radiation envelope of no mass. At the extreme outer boundary of the radiation envelope shall lie the layer of substance of the least  density. This layer may be referred to as aether. This aether would act as the background for spherical surfaces of all different densities. In other words, this aether will be able to contain all possible fields, and transmit electromagnetic waves of all energy densities.

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