Dear Israel
"As I understood, your argument is that only objects are observable and therefore what mediates between them can be disregarded in the formulation. I wished you had answered my second question, and I'd like to insist on it: what is the physical thing that mediates between bodies?"
Why there should be a physical thing that mediates between bodies? If you have never observed it, why do you we feel the necessity to introduce it? The only possible reason would be to solve paradoxes that exist and are otherwise unsolvable, according to you. Any discussion against relational conception of motion should focus there.
" Can you see the epistemological difference between "empty container" and "substance" (Newton's view of space is not to be conceived as an "ethereal container" because aether is a substance and it cannot be a container)?"
What substance? In fact, what do you mean by substance? Why have you introduced it? Maybe you could cite the CMB and say that this is the substance which one should identify with absolute space, since it is, indeed, detectable. But even so, the relational conception of motion may be a better description (it leads to GR!) and could reproduce the apparent "absolute space" stage. I have shown an argument by Barbour last year which shows that the existence of a quantum field does not entail the existence of absolute space.
"Because if we reject this substance as the absolute frame of reference we arrive at logical inconsistencies. Theories should be internally consistent and free of paradoxes."
This is the central point. You should focus here. And I think we had this discussion last year. As far as I remeber, we have reached a conclusion. I have reasons to believe that there are no inconsistencies.
"In this sense, the second postulate of SR DEPARTS FROM EMPIRICAL EVIDENCE but despite this, the isotropy of the OWSL has to be ASSUMED because of LOGICAL CONSISTENCY in the theory."
Let´s go by parts. You claim only the two way speed of light can be measured. Let´s take this as given. OK. Then we must impose the isotropy of space to calculate the OWSL.Ok. Now you claim that because of that, the existence of absolute space is a matter of pure logic. I cannot see that. I ask you to show me exactly that logical contradictions will exist if we don´t assume an absolute space in this picture.
Also, by saying that, you´re saying that SR has contradictions. In GR, the world is mapped in a 4-d manifold called space-time. Different observers may assign different coordinates to the same event, yet the events ARE THE SAME. This mathematical formalsim does not lead to contradictions. You can make the problem arbitrarily difficult with words, creating an apparent paradox in SR is something rather simple, but since the theory is based on the usage of tensors and differential manifolds, it is supposed to be free of any contradictions. However, to show that a particular paradox is not a paradox in SR sometimes requires a lot of thought.
I think you´re thinking SR just in terms of the speed of light postulate, but a better look is simply to state Lorentz invariance. The invariance of the interval is something directly accesible once we have clocks and rulers precise enough. Now once you state that Lorentz invariance is one of the symmetries of the universe, we may get untestable predictions, like maybe the value of OWSL as you defend. You can either reject Lorentz invariance and try another theory without untestable predictions or keep it. In face of decades of work of high energy and particle physics I prefer to keep it.
Finally, Barbour has shown (see the references) that GR (and with it, Lorentz invariance) coupled to electromagnetism can be seen as a direct consequence of imposing a relational conception of motion. This includes the OWSL (which you say is not measurable) and without any backgound absolute space.
" So you have to admit that this field is static and that it can be considered as preferred frame of reference to judge the motion of the charges, do you agree?"
No. As I said, Barbour derives all electromagnetism without any absolute structure.
"Finally, with respect to gamma, I see no difference since in your formulation you require a parameter denoting change and in Newton's theory t, plays this role with the same meaning."
The difference, as I said before, is that we must have reparametrization invariance with such a gamma. We don´t reparametrize t arbitrarily in Newtonian mechanics.
Finally, I would not like to have long arguments about paradoxes of SR here. As I said, this sometimes becomes very hard to solve. I prefer to keep this space to discuss my proposal on the measurement problem. We´ve had a discussion on relational x absolute conceptions of motion last year, and we´ve reached a conclusion, so I feel it is unecessary to repeat it.
Best regards,
Daniel