BB theory and preferred frames

[Dale]

a simple way to obtain absolute motion or velocity is to have as assumption that the spatial distribution of matter be homogeneous or uniform and the same in all directions (isotropic),

That is not what is meant by "absolute velocity" in relativity. What is meant by "absolute velocity" is that the principle of relativity is violated, or in other words, that the laws of physics are different in different frames.

Mount Everest is tautologically at rest in Mount Everest's rest frame, but the laws of physics are not different in a frame where Mount Everest is moving. Therefore Mount Everests' rest frame does not constitute an absolute rest frame.

The CMB is tautologically at rest in the FRW coordinates, but the laws of physics are not different in a frame where the CMB is moving. Therefore the CMB frame does not constitute an absolute frame.

If you want to show that the CMB represents an absolute rest frame it is not sufficient to show that the CMB is at rest in some coordinate system nor even that some class of observers is at rest in that same frame, it is necessary to show that the laws of physics are different in that rest frame. That is impossible, I refer you again to post 3.

 

[TrickyDicky]

That is not what is meant by "absolute velocity" in relativity. What is meant by "absolute velocity" is that the principle of relativity is violated, or in other words, that the laws of physics are different in different frames.

Just what I said in the last line of my post.
The laws of physics are generally considered as absolute and universal, and in that case no possible violation of the principle of relativity as you quote it s permitted by definition, turning the principle of relativity into a true tautology. To avoid ambiguities you should define clearly what a physical law is and what constitutes something that in your opinion counts as a difference in a physical law.

Mount Everest is tautologically at rest in Mount Everest's rest frame, but the laws of physics are not different in a frame where Mount Everest is moving. Therefore Mount Everests' rest frame does not constitute an absolute rest frame.

The CMB is tautologically at rest in the FRW coordinates, but the laws of physics are not different in a frame where the CMB is moving. Therefore the CMB frame does not constitute an absolute frame.

If you want to show that the CMB represents an absolute rest frame it is not sufficient to show that the CMB is at rest in some coordinate system nor even that some class of observers is at rest in that same frame, it is necessary to show that the laws of physics are different in that rest frame. That is impossible, I refer you again to post 3.

You have some kind of obsession with tautologies, I wish you well wrt that. Also, you need to address what is actually said in the post you reply to, not what you imagine was said.

 

[Dale]

To avoid ambiguities you should define clearly what a physical law is and what constitutes something that in your opinion counts as a difference in a physical law.

Fair enough. What would count as a difference in a physical law in different frames would be a term in the physical law which depends on the reference frame.

In the case of GR the physical law is the EFE, which contains no term depending on the reference frame. Therefore, anything which is a solution to the EFE (e.g. the FRW metric) cannot depend on the reference frame.

 

[TrickyDicky]

Fair enough. What would count as a difference in a physical law in different frames would be a term in the physical law which depends on the reference frame.

In the case of GR the physical law is the EFE, which contains no term depending on the reference frame. Therefore, anything which is a solution to the EFE (e.g. the FRW metric) cannot depend on the reference frame.

That's my point, no absolute frame is possible for the EFE solutions. Therefore no event can be assigned to a certain "absolute" date.

 

[Dale]

That's my point, no absolute frame is possible for the EFE solutions. Therefore no event can be assigned to a certain "absolute" date.

I agree.

You can, however, adopt any convention that is convenient and use it to assign dates to events. That is all the time coordinate in the FRW metric is.