The disconnect between the theoretical necessity of the ether in Lorentz's electron theory and its practical irrelevance was not lost on his contemporaries. Among those who gave attention to the problem was Albert Einstein. In 1905 he addressed the issue with the publication of his paper, On the Electrodynamics of Moving Bodies. (Available here.) In the introduction, we learn that the theory is simple and consistent; it contains one set of electrodynamic laws of moving bodies based on Maxwell's theory for stationary bodies; it is founded on a new postulate for the propagation of light. And, we learn that the ether is excluded, being superfluous.
The exclusion of the ether is of interest to us, because the explanation of why light behaves as it does was thought to be in the properties of the ether. True, the extent of the ether's explanatory power was limited due to lack of experimental evidence--but the ether was all the explanation we had. We are therefore very interested in the new postulate for the propagation of light on which Einstein has built his theory.
This is the postulate:
Light is always propagated in empty space with a definite velocity c which is independent of the state of motion of the emitting body.
The statement simply asserts the propagation of light in empty space, without attempt at explanation, nor is any explanation attempted in the exposition of the theory. All previous attempts to discover how the thing is done had failed, but investigations had continued. Einstein chose to abandon the effort.
Without doubt, Einstein had good reason to eliminate the ether from electrodynamic theory. As we saw in the previous article, the ether was prominent in Lorentz's theory yet physically absent, so far as could be determined from experiment. Furthermore, the ether theory required that the fixed frame of the ether be taken into account, while the experimental results depended only on the relative motion of bodies. Einstein's etherless theory is indeed simple and consistent. Still, without the ether we are left with only a functional description of electrodynamic processes; there is no way to account for their behavior.
The question of whether it is important to account for the propagation of light in empty space is more a matter of philosophy than of science. Born, for example, considers Einstein's abandonment of the ether to be a "step toward higher abstraction", releasing us from "superfluous pictures and analogies originating from more primitive and unrefined experience." (See the end of Chapter V in his Einstein's Theory of Relativity.)
With respect to the understanding of the function of the universe, Born is certainly correct, judging by the results since the step was taken. Surely, however, there is a place for a sense of loss--not of the ether, but of what the ether represents. There is something within us that is not satisfied with knowing what happens; we want to know how and why it happens. The abandonment of the ether is an admission--or better, a recognition--that we cannot know, and must remain forever unsatisfied.
Some implications of the postulated propagation of light will be discussed in the next two articles.
For a step-by-step presentation of the principles of special relativity for non-technical readers, see Einstein's book, Relativity: The Special and General Theory.
For a discussion of the simplicity and consistency of Einstein's theory, see section 4.2.2 of Michel Janssen's A Comparison Between Lorentz's Ether Theory And Special Relativity In The Light Of The Experiments Of Trouton And Noble. The climax of the argument is in the final paragraphs of the section; its force can be appreciated even apart from the background discussion and a knowledge of the mathematics on which the argument is based. To view the complete dissertation, follow these links: [Title/TOC], [Intro], [Intro (Part I)], [Chapter 1], [Chapter 2], [Intro (Part 2)], [Chapter 3], [Chapter 4], [References].
In spite of the simplicity and consistency of his theory, Einstein's assertion that the ether is superfluous cannot be proven by experiment. With regard to predicting the outcome of any experiment, Lorentz's ether theory is just as good as Einstein's etherless theory. The two theories agree as to the function of the universe; they disagree as to its design. (See Janssen's dissertation for more information on the empirical equivalence of the two theories.)