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 is not like the material wave. The electromagnetic wave is “transverse” in the sense that its “density” does not change as it propagates. The optical and electromagnetic phenomena have about the same velocity because both are massless.
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.
A fast moving particle behaves like a wave that does not change in “density”. For all practical purposes, it behaves like a wave but it is called a “particle” because it has density. This description satisfies both the corpuscular and the wave theory of light. The difference between optical and electromagnetic wave phenomenon is primarily in terms of their densities.
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.
I shall differentiate between the field theory of Faraday, and the electromagnetic theory of Maxwell. The electromagnetic theory is a special case of the field theory. A field wave is very different from a material wave. The concept of material wave-length may not apply to a field wave the same way. A better concept shall be field density. The field density is many degrees of magnitude smaller than material density.
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.
The concept of field filling the space does not exist in the mechanical view.
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.
The field is the new substance that is quite real.
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 charge now belongs to the old theory. The electrostatic field originates from the unbalanced particle-void interface.
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.
This paragraph is quite a poetic expression by Einstein.
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.
The field theory of Faraday and Maxwell has finally overcome the mechanical dogma.
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.
Aether no longer stands for a mechanical medium built up by particles. It now fills the space with the physical property of transmitting field waves.
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FINAL COMMENTS
A fast moving particle behaves like a wave. The lighter it becomes, the faster it moves, and the more wave-like properties it acquires. After shrinking down in size to the nucleus of an atom, the particle starts to lessen in density and becomes diffused; and so we have the quantum particles like electron. Such particles diffuse further in density to become a field and the wave-like xsdc disturbance in it.
The property of a fast moving quantum particle is that it does not change in its density; and so it appears like a transverse wave. Its wave characteristics are very different from that of a material wave. It is diffused in its density and spread out in space.
An electromagnetic wave that exists among the nuclei of atoms is much denser than the light that exists in space. Both are field waves. The “electromagnetic spectrum” is more properly characterized as “field spectrum”.
The electromagnetic theory of Maxwell applies more specifically to the electromagnetic wave that has a certain uniform density among the nuclei of atoms. It does not apply to all the field waves because their specific characteristics are based on their densities, which differ throughout the field spectrum.
The concept of field filling the space is very similar to the concept of aether filling the space, except that field, unlike the aether, is not mechanical. The mechanical view applies to solid particles, whereas the field view applies to diffused quanta.
The field is the new substance that is quite real. It now fills the space with the physical property of transmitting field waves.
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