Einstein 1938: Ether and the Mechanical View

Reference: Evolution of Physics

This paper presents Chapter II, section 10 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 the Mechanical View

The discussion of all the various attempts to understand the mechanical nature of the ether as a medium for transmitting light would make a long story. A mechanical construction means, as we know, that the substance is built up of particles with forces acting along lines connecting them and depending only on the distance. In order to construct the ether as a jelly-like mechanical substance physicists had to make some highly artificial and unnatural assumptions. We shall not quote them here; they belong to the almost forgotten past. But the result was significant and important. The artificial character of all these assumptions, the necessity for introducing so many of them all quite independent of each other, was enough to shatter the belief in the mechanical point of view.

The idea of aether was used in an effort to make the mechanical view apply universally. But it has been impossible to do so consistently.

But there are other and simpler objections to ether than the difficulty of constructing it. Ether must be assumed to exist everywhere, if we wish to explain optical phenomena mechanically. There can be no empty space if light travels only in a medium.

Yet we know from mechanics that interstellar space does not resist the motion of material bodies. The planets, for example, travel through the ether-jelly without encountering any resistance such as a material medium would offer to their motion. If ether does not disturb matter in its motion, there can be no interaction between particles of ether and particles of matter. Light passes through ether and also through glass and water, but its velocity is changed in the latter substances. How can this fact be explained mechanically? Apparently only by assuming some interaction between ether particles and matter particles. We have just seen that in the case of freely moving bodies such interactions must be assumed not to exist. In other words, there is interaction between ether and matter in optical phenomena, but none in mechanical phenomena! This is certainly a very paradoxical conclusion!

There is interaction between aether and matter in optical phenomena, but none in mechanical phenomena! This is certainly a very paradoxical conclusion!

There seems to be only one way out of all these difficulties. In the attempt to understand the phenomena of nature from the mechanical point of view, throughout the whole development of science up to the twentieth century, it was necessary to introduce artificial substances like electric and magnetic fluids, light corpuscles, or ether. The result was merely the concentration of all the difficulties in a few essential points, such as ether in the case of optical phenomena. Here all the fruitless attempts to construct an ether in some simple way, as well as the other objections, seem to indicate that the fault lies in the fundamental assumption that it is possible to explain all events in nature from a mechanical point of view. Science did not succeed in carrying out the mechanical programme convincingly, and today no physicist believes in the possibility of its fulfilment.

The inconsistencies seem to indicate that the fault lies in the fundamental assumption that it is possible to explain all events in nature from a mechanical point of view.

In our short review of the principal physical ideas we have met some unsolved problems, have come upon difficulties and obstacles which discouraged the attempts to formulate a uniform and consistent view of all the phenomena of the external world. There was the unnoticed clue in classical mechanics of the equality of gravitational and inertial mass. There was the artificial character of the electric and magnetic fluids. There was, in the interaction between electric current and magnetic needle, an unsolved difficulty. It will be remembered that this force did not act in the line connecting the wire and the magnetic pole, and depended on the velocity of the moving charge. The law expressing its direction and magnitude was extremely complicated. And finally, there was the great difficulty with the ether.

The mechanical view could not explain the consistency between the effect of a push (inertial mass) and that of continuously acting force (gravitational mass). It could not explain the consistency between the directions of electric current and magnetic force.

Modern physics has attacked all these problems and solved them. But in the struggle for these solutions new and deeper problems have been created. Our knowledge is now wider and more profound than that of the physicist of the nineteenth century, but so are our doubts and difficulties.

The effort to solve these inconsistencies has led to new difficulties.


In the old theories of electric fluids, in the corpuscular and wave theories of light, we witness the further attempts to apply the mechanical view. But in the realm of electric and optical phenomena we meet grave difficulties in this application.

A moving charge acts upon a magnetic needle. But the force, instead of depending only upon distance, depends also upon the velocity of the charge. The force neither repels not attracts but acts perpendicular to the line connecting the needle and the charge.

In optics we have to decide in favour of the wave theory against the corpuscular theory of light. Waves spreading in a medium consisting of particles, with mechanical forces acting between them, are certainly a mechanical concept. But what is the medium through which light spreads and what are its mechanical properties? There is no hope of reducing the optical phenomena to the mechanical ones before this question is answered. But the difficulties in solving this problem are so great that we have to give it up and thus give up the mechanical views as well.



We hear of aether in the context of light but not so much in the context of electromagnetism. Faraday didn’t find any use for the mechanical aether. He found the concept of “lines of force” more useful because it provided flexibility. Maxwell followed the mechanical point of view and based his electromagnetic theory on aether. Maxwell’s aether filled the empty space with particles that acted as dielectrics. This made the electromagnetic theory quite inflexible. Einstein’s later discovery of quanta then brought about some flexibility.

Electric current has a direction of motion. This is the direction of electrical lines of force. Perpendicular to it, are the magnetic lines of force. The existence of a force perpendicular to the line of motion contradicts the mechanical view. This limitation of the mechanical view came to the forefront.

Light also has a direction of motion. It parallels the idea of electric current, but the two have little in common. Perpendicular to the direction of motion there are two planes of polarization. They parallel the idea of magnetic lines of force, but, again, the two have little in common. Light is considered to be electromagnetic simply because its speed matches the speed of the electrical disturbance predicted by the electromagnetic theory. This is like saying that planet-A matches planet-B in speed, so they are similar in construction.

Electromagnetism is a phenomenon confined to the interface between particle and void. Light is a phenomenon of the void. These two phenomena are very different, and light should not be placed under the category of electromagnetism. Light is simply a radiating substance.

Aether has been considered as the medium in which light travels as a wave. Once again, this is a mechanical viewpoint that distinguishes between the movements of the substance and a disturbance traveling through that substance. In case of light it is the light particle itself moving at the speed of light, so no medium is required. A fast moving massless particle happens to appear as a wave.

Mechanical properties seem to accompany substance with solid particles. The properties are less mechanical in electricity because its particles (electrons) have a less solid structure. The light particles (photons) have no solid structure at all and they hardly display mechanical properties. For still lighter particles we may postulate the following.

The lighter is the particle; the greater shall be its speed and wave length. It would appear like a long, thin string. If aether is the lightest and flimsiest substance that there is, then it would be farthest from being fixed in space. Its speed would be infinite. The aether current is much lighter than even light. In the direction perpendicular to its motion it may have infinite number of “polarization planes”. This may tell us something not only about the true form of aether, but about the line of force.


We postulate that aether is made of lines of force that are massless. A line of force forms the “particle” of aether, which appears as a long, thin string. This “particle” is moving in a circle of infinite radius at infinite velocity. These particles (lines of force) have a direction of propagation, perpendicular to which there are infinite polarization planes.

A line of force becomes more substantial (thickens up) as its wavelength shortens and the frequency increases. The directions of its polarization also shrink in numbers. It may become slightly slower in its speed as a result.

By the time the lines of force thickens up as light it is left with only two polarization planes and a finite speed. And, by the time the lines of force thicken up to become electromagnetic current, it has no polarization planes but a single magnetic field, which is perpendicular to its direction of propagation.


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