Interestingly enough what I did manage to find is a lecture given by Einstein in 1920 where he argues that the ether is in fact essential towards the understanding of general relativity, and that it could be through the ether that gravity and electromagnetism are unified:
https://www.researchgate.net/publication/358617464_Ether_and...
Our confidence in SR is so strong now that c is defined and length unit defined as the distance light travels during a set time.
>The next position which it was possible to take up in face of this state of things appeared to be the following. The ether does not exist at all...
>More careful reflection teaches us however, that the special theory of relativity does not compel us to deny ether. We may assume the existence of an ether; only we must give up ascribing a definite state of motion to it
This is about half way through the lecture before Einstein touches on general relativity. Towards the end he is quite adamant that a theory of the ether is necessary to fully appreciate general relativity.
With that said I do not want to fall into an argument from authority, certainly much of what we understand about relativity today along with its implications differs from its original formulation, but I present the lecture because I think a lot of people don't quite have the appreciation or historical understanding of what the ether was or wasn't, they just read about how the Michelson-Morley experiment proved that it can't exist along with sensational views that the experiment represented some kind of embarrassment or catastrophe in physics and the ether became a fall-guy of sorts that we must entirely rid ourselves of.
But if you read through the actual primary sources you get a very different picture of how physics progressed bit by bit.
It is precise enough for our purpose: ether is a hypothetical medium for light waves to propagate. Moreover it would need to have no interaction with ordinary matter, or else it would cause planets' orbits to decay.
only we must give up ascribing a definite state of motion to it - Einstein
This is a "No True Scotsman" fallacy wherein one redefines the assertion to deal with specific objections. I hesitate to criticize Einstein, of course, but in this case it's not clear that "ether" minus motion means anything. One can be generous and say he had an intuition about fields, however fields aren't ether, either.
TL:DR the aether has a reference frame. This is exactly what it's inventors wanted and exactly what modern things don't have.
Here is the long version:
When you put together a couple of the constants of classical electromagnetic theory (specifically the quantities called the permitivity and permeability of free space) you get out a quantity which has the units of a speed. You get this thing which is measured in metres per second.
Now if you're a Victorian era scientist, and you have fully internalised Gallilean relativity and Newtonian mechanics then this is absolutely, completely, insane. There is no way in their worldview for a speed to exist in isolation, without a reference frame for it to be measured with respect to.
If I measure a guy on a bike going at 10 miles per hour, and a guy in a car going at 30 miles per hour past him then the guy on the bike sees the car going at 20 miles per hour relative to him. If I sit opposite you on a train I measure your speed to be 0, even though we're both moving at 100+ km/hour. Speeds are (for Victorian scientists) completely relative.
So they have the theory of electromagnetism, which seems to be giving amazingly accurate predictions, except that it also gives you this apparently absolute speed, which makes no sense. Someone realises pretty fast that it's about the speed that light goes. So what do they do? They propose the existence of this "aether" stuff which is everywhere at all times and critically which has a reference frame. The aether provides a reference frame for the speed of light and the crazy meaningless absolute speed they didn't know what to do with now makes sense, it's relative like any other speed, but the magic quantity they got is the speed in the aether's reference frame.
Of course a few decades later Michelson and Morley show that this idea doesn't work, in an incredibly beautiful experiment, and the aether theory starts to look shaky. A few years after that Einstein (with input from people like Lorentz) cooks up special relativity which is almost like Gallilean relativity in that almost all speeds are relative, except specifically the speed of light is not. The speed of light is absolute, just as it has to be because of the way it pops out of electromagnetism.
