" [...] in 1905 I was of the opinion that it was no longer allowed
to speak about the ether in physics. This opinion, however, was too
radical, as we will see later when we discuss the general theory of
relativity. It does remain allowed, as always, to introduce a medium
filling all space and to assume that the electromagnetic fields (and
matter as well) are its states. [...] once again 'empty' space
appears as endowed with physical properties, i.e., no longer as
physically empty, as seemed to be the case according to special
relativity [...] ".

And again:

"This word ether has changed its meaning many times in the
development if science [...] Its story, by no means finished, is
continued by relativity theory."

Then, at last, the essential questions remain open: is really
general relativity's formalistic and abstract approach a
satisfactory answer to the fundamental questions regarding the
nature of physical space? Or was all the XXth century theoretical
physics on the wrong track, and we should completely change
direction, resurrecting perhaps some old suggestions, too hastily
considered as dead??

Einstein and the Ether

Ludwik Kostro

(Apeiron, Montreal, 2000)

"Whether gravitational, electrical, and nuclear

interactions can be encompassed within a unified

theoretical structure, and whether such a structure

will be conceived as a plenary space with physical

properties, remains to be seen. But if the history of

the successive dynasties of aether is any guide,

we can eventually proclaim:

The luminiferous aether is dead!

Long live the aether!" (Owen Gingerich)

Nowadays, nobody talks any longer about the ether in scientific
ortohodox books, in higher school or university classes, etc., yet
this concept has been one of the corner stones of many rational
interpretation of natural phaenomena for a great long time - to such
an extent that a good physicist recently wrote to us that all XIXth
century physics tried to "prove the existence of the ether which was
later proved not to exist".

If we ask why the ether has disappeared from the major scenes of our
knowledge of Nature, everybody will answer that Einstein has proved,
with his celebrated theory of relativity, that the ether does not
exist. This was one of those concepts that old physicists were
accustomed to use in their "primitive" speculations, but today,
luckily, it has been completely overthrown, together with other
similar relics of "superstition", by XXth century scientists. It was
in that time that mankind has realized the greatest achievements of
ever in science and technology, which can be interpreted as the goal
of a long walk, that began thanks to such men like Copernic,
Galilei, Descartes, Newton,... just sprung out from the darkness of
Middle Ages.

Thus, "common people", and even the "common scientist", would be
surprised in reading this book (about 240 pp.), written by the
physicist and philosopher Ludwik Kostro, and intended for physicists
as well as for historians of science, philosophers, or in general
for any people interested in the development of scientific culture.
As a matter of fact, it is entirely dedicated to the troublesome
relationships between the greatest scientist of all times - or at
least many people think so! - and the elusive ether.

Let us see the question with the author's own words (Introduction):

"In the eyes of most physicists and philosophers, Albert Einstein
has acquired a reputation for abolishing the concept of the ether as
a medium filling space (or identified with it), which was
responsible for carrying electromagnetic, gravitational and other
interactions. Today, this notion is echoed in textbooks,
encyclopaedias, and scientific reviews. However, it does not fully
reflect the historical truth, and in a sense even represents a
distortion [...] Einstein denied the existence of the ether for only
11 years - from 1905 to 1916. Thereafter, he recognized that his
attitude was too radical and even regretted that his works published
before 1916 had so definitely and absolutely rejected the existence
of the ether."

The author proves this assertion directly referring to the opinions
which Einstein himself expressed during his life, in a book which is
therefore full of quotations and precise bibliographical references
(up to the point of quoting even the original Deutsch passages in a
special appendix). Here they are some examples of Einstein's
thoughts:

"It would have been more correct if I had limited myself, in my
earlier publications, to emphasizing only the nonexistence of an
ether velocity, instead of arguing the total nonexistence of the
ether, for I can see that with the word ether we say nothing else
than that space has to be viewed as a carrier of physical
qualities."

Moreover:

" [...] in 1905 I was of the opinion that it was no longer allowed
to speak about the ether in physics. This opinion, however, was too
radical, as we will see later when we discuss the general theory of
relativity. It does remain allowed, as always, to introduce a medium
filling all space and to assume that the electromagnetic fields (and
matter as well) are its states. [...] once again 'empty' space
appears as endowed with physical properties, i.e., no longer as
physically empty, as seemed to be the case according to special
relativity [...] ".

And again:

"This word ether has changed its meaning many times in the
development if science [...] Its story, by no means finished, is
continued by relativity theory."

It seems interesting to quote even the following passages by
Eistein, where he somehow admits the rational necessity of the
ether, that is to say, the necessity of conceiving a space which
cannot be thought of but endowed with physical properties:

"There is an important argument in favour of the hypothesis of the
ether. To deny the existence of the ether means, in the last
analysis, denying all physical properties to empty space."

"The ether hypothesis was bound always to play a part even if it is
mostly a latent one at first in the thinking of physicists."

So we have seen the description of one of the main themes of
Einstein's research, going on through a complex route which Kostro
aims to carefully reconstruct, as the Index of his book clearly
shows:

Introduction

Chap. 1 - Einstein's views on the ether before 1905

Chap. 2 - Einstein denies the existence of the ether (1905-1916)

Chap. 3 - Einstein introduces his new concept of the ether
(1916-1924)

Chap. 4 - Development of Einstein's ether concept (1925-1955)

Chap. 5 - Physical meaning of Einstein's relativistic ether

Appendix - Original quotations

Bibliography

[the Bibliography lists more than 250 items!]

We could stop here this presentation, but we feel necessary to add
some more words discussing the opinion, shared by the author, that
general relativity could be truly conceived of as a theory of a
dynamical ether. Indeed, it is perhaps appropriate to emphasize that
the general opinion about Einstein discarding the ether is in some
sense not absolutely groundless, since a relativistic ether - as we
must theorize the physical space according to the relativistic
conceptions of space and time - is quite different for instance from
the Descartes's ether. This relativistic ether cannot have the same
meaning, and role, of the "old" mechanical ether in a rational
description of the universe: otherwise, it cannot be properly
defined a "true ether". Einstein's 4-dimensional ether, if we wish
to call it so, cannot provide the same natural causal explanations
which, on the contrary, could be supplied by the introduction of a
physical "fluid", filling up the whole 3-dimensional space. The
action, and properties, of this universal medium could, in
principle, rationally explain all physical phaenomena by means of a
simple mechanical analogy (in which, for instance, a force can be
interpreted only as a vis a tergo, a field as a perturbation of the
space, etc.). Einstein's ether, instead, cannot be thought of but in
four dimensions, which means that time must be included in the
structure of "space" itself (which is in fact more properly called
space-time). This circumstance implies that it is absolutely
impossible for the human mind to make an intuitive image of it, and
to give any simple meaning for instance to expressions like:
"dynamical ether", which would have, vice versa, an easy
interpretation with respect to a 3-dimensional fluid ether. As a
matter of fact, an ether "moving" with respect to what time? (modern
cosmological models are indeed able to introduce a kind of universal
time, but it is by no means a simple concept to talk about).

Relativity definitively denies any possibility of human
understanding of natural phaenomena, that is to say of any real
explanation - it destroys for instance the tomistic proposal of
describing science as adaequatio intellectus et rei. It seems more
realistic, as far as the widespread attitude of contemporary natural
philosophy is concerned, the opinion expressed by Richard P. Feynman
(see Episteme N. 1, Umberto Bartocci, "Della natura 'ambigua' della
luce"), which exhorts men to resign to the idea that Nature is
absurd for their minds:

"The theory of quantum Electrodynamics describes Nature as absurd
from the point of view of common sense. And it agrees full with
experiment. So I hope you can accept Nature as She is - absurd"
(QED - The strange theory of light and matter, Princeton University
Press, 1985, p. 10).

Summing up, this would seem one of those cases1 in which the "public
opinion" is more concrete, in accepting a lesson, than the teacher
himself, with all his doubts, after thoughts, hesitations. As a
matter of fact, it does not seem extreme to notice that, after all,
ether disappeared as a possible protagonist in the very moment when
the european physicist élite dealt with the study of quantum
properties of microphysical world, and that all counterintuitive
"complications" of quantum mechanics seem due exactly to this
absence. Ether could have been, in principle, very well assumed for
being the responsible of quantum fluctuations, mysterious
interference patterns, etc., but this possibility has never been
thoroughly investigated, exactly because of the negative
relativistic influence2.

Then, at last, the essential questions remain open: is really
general relativity's formalistic and abstract approach a
satisfactory answer to the fundamental questions regarding the
nature of physical space? Or was all the XXth century theoretical
physics on the wrong track, and we should completely change
direction, resurrecting perhaps some old suggestions, too hastily
considered as dead??

1 Another one is concerning Newton's own attitude towards the ether,
which was rather multifarious (perhaps even a bit ambiguous?!), and
which generated accordingly multiple theories of light: but people
(like the "newtonian" Bradley himself, the discoverer of
astronomical annual aberration) generally remember him only for the
corpuscular theory, which is more coherent with the hypothesis of an
empty space.

2 For instance, according to the physicists B.H. Lavenda and E.
Santamato: "Quantum indeterminism is explainable in terms of the
random interactions between quantum particles and the underlying
medium in which they supposedly move" ("The Underlying Brownian
Motion of Nonrelativistic Quantum Mechanics", Foundations of
Physics, Vol. 11, N. 9/10, 1981, p. 654); "It might perhaps be
possible to develop a completely classical formulation of quantum
mechanics based upon the irregular motion of a single Brownian
particle immersed in a suspension of lighter particles" ("Stochastic
Interpretations of Nonrelativistic Quantum Theory", Int. J. of Th.
Physics, Vol. 23, N. 7, 1984).

http://itis.volta.alessandria.it/episteme/ep3-24.htm