Dmitry, Einstein and the ether is a topic of the book by the same name by Ludwik Kostro. It's a great read. Here's a brief history of Einstein and the ether, from my book:
Einstein’s views on the ether changed dramatically throughout the course of his career.1 His 1905 paper on special relativity boldly proclaimed the ether to be a “superfluous” concept. It was in fact his entire objective, in developing special relativity, to extend Galilean (mechanical) relativity to electromagnetism, thereby eliminating the need for a “privileged” frame of reference. In other words, we needn’t worry about what is the “correct” or truly stationary frame of reference in relativity because all uniformly-moving (inertial) frames of reference are equally valid, as is the case for inertial frames in Newtonian/Galilean mechanics. While this concept, if it is true, obviously has much utility, Einstein’s own thinking evolved to the point that he realized that some type of ether was theoretically necessary after all.
In 1916, Einstein published his general theory of relativity, which asserted a very different conception of space and time (as discussed in the last chapter). In general relativity, space has no independent existence; rather, it is a consequence of the various fields that are ontologically fundamental. Shortly after his momentous general relativity paper was published, he exchanged letters with Lorentz, one of his mentors, on the topic of the ether. Lorentz argued consistently that some notion of the ether was necessary. Einstein conceded eventually that indeed a non-material ether was necessary to explain inertia and acceleration. Einstein first described his “new ether” in a 1916 letter to Lorentz:
I agree with you that the general theory of relativity is closer to the ether hypothesis than the special theory. This new ether theory, however, would not violate the principle of relativity, because the state of this … ether would not be that of a rigid body in an independent state of motion, but every state of motion would be a function of position determined by material processes.2
Einstein also wrote in a 1919 letter to Lorentz:
It would have been more correct if I had limited myself, in my earlier publications, to emphasizing only the non-existence of an ether velocity, instead of arguing the total non-existence 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.3
From 1916 to 1918, Einstein was in the thick of discussions with a number of colleagues about the nature of space and the ether, with respect to general relativity. As Isaacson recounts in his wonderful biography, Einstein’s thinking changed dramatically during this period. In 1918, he published a response to critics of special and general relativity and attempted a refutation of the Twin Paradox (also discussed in the last chapter). In this dialogue, Einstein writes, with the Critic posing the question and a hypothetical “relativist” responding:
Critic:
[H]ow does the diseased man of theoretical physics fare, the Aether, that many of you have declared to be definitely dead?
Relativist:
Its fortunes have taken some turns, and overall one cannot say that it is dead now. Prior to Lorentz it existed as an all-pervasive fluid, as a gas-like fluid, and other than that in the most diverse forms of being, different from author to author. With Lorentz it became rigid, and embodied the resting coordinate system, respectively a privileged state of motion in the world. According to the special theory of relativity there was no longer a privileged state of motion, this meant a denial of the Aether in this sense of the preceding theories. For if there would be an Aether, then in each space-time point there would have to be a particular state of motion, that would have to play a part in optics. There is no such privileged state of motion, as has been taught to us by the special theory of relativity, and that is why there is no Aether in the old sense. The general theory of relativity also does not know a privileged state of motion in a point, that one could vaguely interpret as velocity of an Aether. However, while according to the special theory of relativity a part of space without matter and without electromagnetic field seems to be characterized as absolutely empty, e. g. not characterized by any physical quantities, empty space in this sense has according to the general theory of relativity physical qualities which are mathematically characterized by the components of the gravitational potential, that determine the metrical behavior of this part of space as well as its gravitational field. One can quite well construe this circumstance in such a way that one speaks of an Aether, whose state of being is different from point to point. Only one must take care not to attribute to this Aether properties similar to properties of matter (for example every point a certain velocity).
In 1920, Einstein became more emphatic regarding the ether, recognizing explicitly that the ether was a necessary medium by which acceleration and rotation may be judged, independently of any particular frame of reference:
To deny ether is ultimately to assume that empty space has no physical qualities whatever. The fundamental facts of mechanics do not harmonize with this view... Besides observable objects, another thing, which is not perceptible, must be looked upon as real, to enable acceleration or rotation to be looked upon as something real ... The conception of the ether has again acquired an intelligible content, although this content differs widely from that of the ether of the mechanical wave theory of light ... According to the general theory of relativity, space is endowed with physical qualities; in this sense, there exists an ether. Space without ether is unthinkable; for in such space there not only would be no propagation of light, but also no possibility of existence for standards of space and time (measuring-rods and clocks), nor therefore any spacetime intervals in the physical sense. But this ether may not be thought of as endowed with the qualities of ponderable media, as consisting of parts which may be tracked through time. The idea of motion may not be applied to it.4
Einstein struggled with these ideas for much of his career, converting from a positivist in the Machian tradition early in his career – avoiding discussion of things that cannot be seen or measured, and thus to be considered outside the purview of physics – into a true realist. As a realist, Einstein argued during the middle and latter parts of his career that physics must attempt to describe what is truly real and not avoid discussion of logically necessary concepts, even if they cannot be detected – such as the ether. Isaacson writes:
To a pure proponent of Mach, or for that matter of Hume, the whole phrase “really to exist in nature” lacked clear meaning. In his special relativity theory, Einstein had avoided assuming the existence of such things as absolute time and absolute distance, because it seemed meaningless to say that they “really” existed in nature when they couldn’t be observed. But henceforth, during the more than four decades in which he would express his discomfort with quantum theory, he increasingly sounded like a scientific realist, someone who believed that an underlying reality existed in nature that was independent of our ability to observe or measure it.5
Einstein made this explicit in his 1920 speech in Leiden: “Besides observable objects, another thing, which is not perceptible, must be looked upon as real….” Einstein was, as we read in the more complete quote, referring to the ether. So for Einstein, even though the ether was considered to be undetectable, he deduced its existence because of its effects on observable matter through inertia, acceleration and rotation.
Isaacson states regarding Einstein’s late-career ruminations on the ether:
[Einstein] had to face the possibility that general relativity did not necessarily eliminate the concept of absolute motion, at least with respect to the metric of spacetime.
It was not exactly a retreat, nor was it a return to the nineteenth-century concept of the ether. But it was a more conservative way of looking at the universe, and it represented a break from the radicalism of Mach that Einstein had once embraced.
This clearly made Einstein uncomfortable.6
Einstein was most explicit in his thinking on the ether in a 1924 paper, in which he distinguished the ether of Newtonian mechanics and special relativity (contrary to the statement in his 1905 paper on special relativity which dismissed the mechanical ether theory as “superfluous”) with the ether of general relativity:
The inertia-producing property of [the Newtonian] ether, in accordance with classical mechanics, is precisely not to be influenced, either by the configuration of matter, or by anything else. For this reason, one may call it “absolute.” That something real has to be conceived as the cause for the preference of an inertial system over a non-inertial system is a fact that physicists have only come to understand in recent years …. Also, following the special theory of relativity, the ether was absolute, because its influence on inertia and light propagation was thought to be independent of physical influences of any kind … The ether of the general theory of relativity differs from that of classical mechanics of the special theory of relativity respectively, insofar as it is not “absolute,” but is determined in its locally variable properties by ponderable matter.7
In other words, ether as absolute space (or rest) is uninfluenced by matter (ponderable bodies). The ether of general relativity, in contrast, is influenced by matter. The now famous depiction of general relativity’s spacetime as akin to a trampoline with bowling balls or other heavy objects deforming its surface illustrates this mutual interaction between matter and the ether (spacetime) in general relativity.
Einstein labored mightily in the 1920s and 1930s to develop a unified field theory that would re-cast everything as a manifestation of fields. In 1934, he shifted his language explicitly away from “ether” to describing the properties of “physical space.” He stated: “Physical space and ether are only different terms for the same thing; fields are physical states of space. If no particular state of motion can be ascribed to the ether, there do not seem to be any grounds for introducing it as an entity of a special sort alongside space.”8
Einstein stated in his 1938 book, The Evolution of Physics: “This word ether has changed its meaning many times in the development of science. … Its story, by no means finished, is continued by the relativity theory.”9 The book itself is somewhat contradictory, however, with respect to the term, stating in an earlier section, with respect to the 19th Century concept of the ether as a mechanical medium: “[T]his is the moment to forget the ether completely and to try never to mention its name…. 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!”10
His last published statements on the ether are contained in the 1952 additions to his 1916 popular book on relativity. Confusingly, Einstein returns to his earlier approach and describes the field concept as an irreducible element of the universe that does not require any material carrier such as the ether, without clarifying that he is using the term here to refer to the 19th Century concept of the ether, not the “new ether” that he long advocated as a necessary concept:
Max Jammer, in a Foreword to Kostro’s book, summarizes Einstein’s interesting and complicated history vis a vis the ether:
Einstein … acknowledged only three kinds of ether. But what Einstein called “ether” is no longer a rarified material medium that permeates all space, but rather the much more abstract geometrodynamic constituent of spacetime which determines the inertio-gravitational behavior of matter. … efore Einstein, space and time had played the role of merely a passive background in which events take place, but Einstein’s theories transformed them into active participants in the dynamics of the cosmos. We should also recall that Einstein created not only the special and general theory of relativity: during the last decades of his life he was preoccupied, albeit without success, with establishing a third theory, a unified theory that unites the gravitational with the electromagnetic forces. With each of these three theories he associated an “ether” in the above-mentioned sense. The distinction between the three kinds of “ether” finds its mathematical expression in the different properties of the corresponding gravitational potentials gμν of the fundamental metric tensor: the ether of the special theory of relativity is characterized by the condition that gμν = ŋμν, where the latter is the Minkowski metric, the ether of the general theory by gμν = gνμ of the Riemann metric, and the ether of the unified theory by the fact that gμν ≠ gνμ 11