Einstein 1938: Ether and Motion

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.

Field concepts add non-mechanical elements that lead to the theory of relativity.

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 electromagnetic field carries energy which, once emitted from its source, leads an independent existence. The identity of ether as an independent medium is speculative.

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.

In the case of sound, the classical transformation applies to the medium in which sound travels.

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.

A material body can move faster than the speed of sound and catch up with a particular sound wave.

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.

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.

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 remaining possibility is that all matter moves through the ether, which takes no part whatever in the motion.

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 concept of “motion through ether” is essentially the case of no inertia, which would be a unique c.s. The Galilean relativity principle applies only when there is inertia.

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.

For the c.s. moving relatively to the ether-sea the velocity of light 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.

Earth is not moving fast enough on an absolute scale of motion for Michelson-Morley’s experiment to differentiate between the speeds of light in the two different directions. If Michelson-Morley’s experiment were conducted on the surface of a speeding electron, it would show that speed of light differs in the two different directions.

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 is questionable because it simply is not able to detect the difference, because the speeds in the material domain are very small on absolute scale.

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.

Neither the mechanical, nor the non-mechanical point of view is universal. But both are part of the picture. Einstein replaces the concept of aether by the concept of mathematical space.

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.

The fundamental assumptions (1), (2) and (3), do not take the magnitude of inertia into account. The classical viewpoint does not differentiate among inertia. All c.s. are assumed to have the same inertia even when their uniform velocities are different. The facts seem to be as follows:

(1) The velocity of not just light, but the natural velocity of any particle is constant because it depends on its inertia.
(2) Two c.s., moving uniformly relative to each other, will have uniform inertia and, also, identical laws consistent with that level of inertia.
(3) Absolute motion can be distinguished by distinguishing corresponding inertia.



The mechanical view is described by unalterable objects moving in space with simple forces acting between them. If the laws of mechanics are valid in one c.s. (coordinate system), then they are valid in any other c.s. moving uniformly relative to the first. The field concept of electromagnetic phenomenon adds non-mechanical component of variable forces in space.

Light is considered a field phenomenon. It is seen as carrying energy, but it is made up of extremely fast moving particles called photons. These particles have infinitesimal inertia and infinite velocity. They are string-like rather than ball-like. The motion imparts wave nature to them. Being particles, they do not require a medium to travel in. These fast moving infinitesimal string-like particles form what has long been postulated as the aether.

The intensity of light, is determined by the concentration of its quanta—photons. As light spreads out in all directions, its intensity decreases but not its frequency. Decreasing intensity means that the swarm of photons is separating and becoming thinner. A photon can become vanishingly thin as long as it can hold on to its frequency. A photon is described by its frequency, just like a particle is described by its mass.

A source emits light through conversion of its substance. During this conversion, the inertia of the substance reduces, and the velocity increases, by many degrees of magnitude. The velocity of light is independent of the velocity of its source, as observed through experiments. The velocity of light is, therefore, determined by its own nature. Since inertia is the universal property of all substance, the velocity of light is somehow in balance internally with the inertia of light.

Newton’s mechanics states that a body may be given any velocity by pushing it. This arbitrary velocity is then maintained by inertia. Many observations contradict this assertion. The center of a galaxy that keeps it fixed in space is a black hole of infinite inertia. On the other hand, light that moves faster than anything else in this universe has infinitesimal inertia. There appears to be a relationship between velocity and inertia.  A body, after being pushed, must return to its natural velocity corresponding to its inertia. This modifies Newton’s laws of motion. The natural uniform velocity decreases as inertia increases, and vice versa.

The Galilean relativity principle appears only on the plateau of MATTER. Different rules apply on the plateau of FIELD (AETHER). Between the two plateaus we have the steep gradient of electromagnetic phenomena.

In the domain of MATTER, we can differentiate among velocities but not among inertia, because all gradients of inertia appear to be infinite.In the domain FIELD (AETHER), we can differentiate among inertia (frequencies) but not among velocities, because all velocities appear to be infinite.

The problem all along has been to consider aether at rest. Only a body of infinite inertia can be at rest. Aether is the manifestation of extremely light particles moving at extremely high speeds and thus creating the medium for their wavelike properties.

A man standing on the ship has the same uniform velocity as that of the ship because his inertia is combined with the inertia of the ship. The situation with inertia is not different even when the man is walking with a velocity relative to the ship because he is in contact with the ship. Here the relative velocities are not independent.

The facts seem to be as follows:

  1. The velocity of not just light, but the natural velocity of any particle is constant because it depends on its inertia.
  2. Two c.s., moving uniformly relative to each other, will have uniform inertia and, also, identical laws consistent with that level of inertia.
  3. Absolute motion can be distinguished by distinguishing corresponding inertia.


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