# Einstein's Leyden lecture in comparison with the ether interpretation

In a lecture titled "Ether and the Theory of Relativity" held 1920 at the University of Leyden Albert Einstein revived the notion of an "ether" in the context of general relativity. This proposal was not supported by the scientific community, and not mentioned later by Einstein again, but the lecture itself is often quoted by various defenders of the ether.

The concept of the ether proposed by Einstein is nicely summarized at the end of the lecture (emphasis mine):

Recapitulating, we may say that according to the general theory of relativity space is endowed with physical qualities; in this sense, therefore, there exists an ether.

According to the general theory of relativity 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 space-time intervals in the physical sense. But this ether may not be thought of as endowed with the quality characteristic of ponderable media, as consisting of parts which may be tracked through time.

The difference between the ether proposed by Einstein and the ether interpretation proposed here is, therefore, a quite fundamental one: What according to Einstein "may not be thought of" is done in the ether interpretation given here, namely the ether consists of parts which may be tracked through time, given their velocity $$v^i(\mathfrak{x},\mathfrak{t}) = g^{0i}(\mathfrak{x},\mathfrak{t})/g^{00}(\mathfrak{x},\mathfrak{t})$$.

One can say that it is simply an unfortunate fact that Einstein has not known this ether interpretation of the GR equations. Possibly, the knowledge of this interpretation would have changed his position. Probably the Gödel solution of the rotating universe, which has a causal loop, and the fact that such solutions with causal loops are incompatible with the ether interpretation, because they have necessarily regions where the ether interpretation would have to have a negative density, would have provided a sufficiently strong argument.

But Einstein has known very well the Lorentz ether interpretation of SR, and argued in the same lecture against this interpretation:

For if K be a system of co-ordinates relatively to which the Lorentzian ether is at rest, the Maxwell-Lorentz equations are valid permanently with reference to K. But by the special theory of relativity the same equations without any change of meaning also hold in relation to any new system of co-ordinates K' which is moving in uniform translation relatively to K. Now comes the anxious question: - Why must I in the theory distinguish the K system above all K' systems, which are physically equivalent to it in all respects, by assuming that the ether is at rest relatively to the K system?

For the theoretician such an asymmetry in the theoretical structure, with no corresponding asymmetry in the system of experience, is intolerable. If we assume the ether to be at rest relatively to K, but in motion relatively to K', the physical equivalence of K and K' seems to me from the logical standpoint, not indeed downright incorrect, but nevertheless inacceptable.

This argument would remain valid against the ether interpretation given here too.

There is no doubt that this argument, in itself, has some value. Why I have to assume that there is some distinguished rest frame of the ether if I cannot distinguish it from other frames by any observation?

But the problem is not a really big one, worth to be named inacceptable. Here, all I have to assume is that there exist, objectively, some differences which I'm unable to see. Is this really problematic? Certainly not if I do not believe that the theory we actually have is already the most fundamental one. But if I believe into an ether theory, I do not believe this almost by definition - instead, I believe that the ether has some microscopic structure, far to small to be observable now, but it would be observable in a yet unknown atomic ether theory. And in this atomic ether theory the question if we can distinguish absolute rest from movement would be different.

What is, instead, the alternative? It seems much worse. Once I'm unable to measure absolute rest, in relativity I'm also unable to measure absolute contemporaneity. So, the alternative would be that there is no such thing as absolute contemporaneity, there is no global "now", that various different moments of time on some star of Andromeda, which differ from each other by whole days, would have equal rights to be classified as "now". So, or none of them exists, leaving us with solipsism, or they have to exist all, have all the same status of existence. So, not only one moment of Andromeda's history exists now, but a whole range of history, including some past as well as some future of every event happening on that star. And, consequently, without introducing artificial boundaries of that existence, the whole past and the whole future of that star, and, consequently, also my own whole past and whole future, has to exist in the same sense as the "now" exists.

So, I can choose between solipsism, there nothing exists beyond me now, and fatalism, where the whole history of the whole universe, including all of the future, exists in the same way as I exist now.

And such a choice between fatalism and solipsism will be philosophically more satisfactory than the sceptical hypothesis that our best theories are simply not yet complete?