special relativity - Einsteins idea about solving the ether problem

I'm struggling a bit with my Special Relativity course. Most exercises are quite trivial and can be done without much effort, but when you really start to think and analyse why and how certain things are, they can get really complicated and I often find myself thinking for hours about these things.

In Moore's work Six Ideas That Shaped Physics: The Laws of Physics are Frame-independent he explains a bit about what originally sparked Einsteins work. I will explain it as far as I understand it and then underline what I do not quite get.

• Maxwell published at the end of the 19th century a set of equations that describe the phenomena of electromagnetism. From these equations it can be concluded that the speed of electromagnetic waves are well defined since it depends on certain well established universal constants. This speed happens to be $c\approx 3.00 \cdot 10^8 \text{ m/s}$.


• It was known that the speed of light was very similar (no measurable significant difference) to the speed of electromagnetic waves and also that light behaved like a wave in certain situation (from work done by for example Fraunhover). Therefore it was assumed that light must also be an electromagnetic wave.


• Since there was no known isolated "carrier" for electromagnetic waves it was conjectured that some kind of ether must exist in which an electromagnetic wave could be produced, much like when you trow a rock in a pool it also creates ripples in carrier (water).


• The concept of ether was very unsatisfying to start with, since it was not measurable and had to be virtually everywhere. Michelson and Morley tried to prove the existence of this ether, since if it were to exist, it would produce an "ether wind" which would result in the speed of light being measured differently in the direction of rotation of the earth in comparison with the opposite direction of this rotation.


• However, nor Michelson and Morley nor any other physicist did find the existence of this "ether wind". They all found that the speed of light was measured exactly the same in each direction.



• Einstein concluded from this that the conjectured ether did not exist and that electromagnetic waves must therefore be able to travel trough vacuum, at the speed of light. Since no particular reference frame can be attached to vacuum we must conclude that the measured speed of light is independent of the reference frame and therefore that the Galilean transformations are incorrect (assuming the Maxwell equation and the postulate of relativity are true).

Now what I do not understand entirely (I do feel certain points a bit intuitively, but that is not enough for me):

• How did the experiment of Michelson and Morley disprove the existence of the conjectured ether? What if the ether moves along with the direction of the rotation of the earth?


• In the last point I phrased the book by saying "no particular reference frame can be attached to vacuum". Why can't we think of vacuum as an inertial reference frame with an absolute speed of 0? Why is vacuum assumed to be part of every reference frame (so that the speed of light must be measured the same in every frame)? Basically: what is the fundamental difference between the conjectured ether and vacuum?


• Why did the assumption that light does not travel equally fast in all directions not violate the principle of relativity? Was it because electromagnetic waves were assumed to travel in an other reference frame (the ether) and not in the frame that generated the emission (for example earth)?

When I am talking about reference frames, I assume - in the context of this question - that they are inertial (also assuming earth is practically inertial).

Thanks a lot for taking the time to answer my questions. Best wishes to you all!